37?/67531/metadc332323/...2-silahex-3-ene. these silenes were also trapped as their [4+2]...

37? /V 9 / d

M O , 3 0

THE STEREOCHEMISTRY OF SILENES AND

ALPHA-LITHIO SILANES

DISSERTATION

Presented to the Graduate Council of the

North Texas State University in Partial

Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

By

Tim Frank Bates, B.S,

Denton, Texas

May, 1987

Bates, Tim Frank, The Stereochemistry of Silenes and

Alpha-Lithio Silanes. Doctor of Philosophy (Chemistry),

March, 1987, 105 pp., 16 tables, bibliography, 91 titles.

When E- or Z-l-methyl-l-phenyl-2-neopentylsilene was

generated by the retro-Diels-Alder vacuum-sealed tube

thermolysis of its corresponding anthracene adduct, in the

presence of various alkoxysilanes, only one diastereomeric

adduct was formed in each case, showing that the reactions

are stereospecific. An x-ray crystal structure of the

methoxytriphenylsilane adduct of the E-silene confirmed its

relative configuration as (R,S) or (S,R). This

demonstrated that the addition of alkoxysilanes to silenes

is stereospecific and syn.

The relative configurations of similar alkoxysilane

and alkoxystannane adducts to E- and Z-l-methyl-l-phenyl-2-

neopentylsilene were assigned based on a combination of x-

ray structures and *3C NMR data. A strong, nonbonded

oxygen-metal interaction is apparent in all of those

compounds studied. Treatment of the alkoxystannane adducts

with alkyl lithium reagents results in tin-lithium exchange

in some cases. The results indicate that the resulting

<x-lithio alkoxysilanes are not configurationally stable in

either THF or hydrocarbon solvents.

The reaction of tert butyl lithium with a-trimethyl-

silylvinylmethylphenylchlorosilane in hydrocarbon solvents

yields E- and Z-l-methyl-l-phenyl-2-neopentyl-2-

trimethylsilylsilene. In the absence of any traps these

silenes undergo a novel tert butyl lithium catalyzed

rearrangement to 2-phenyl-3-trimethylsilyl-5,5-dimethyl-

2-silahex-3-ene. These silenes were also trapped as their

[4+2] cycloadducts with anthracene. The Z-isomer of the

anthracene adduct was separated and its stereochemistry

confirmed by an x-ray crystal structure.

The anthracene adducts of both E- and Z-l-methyl-1-

phenyl-2-neopentyl-2-trimethylsilylsilene undergo a facile,

stereospecific decomposition at temperatures as low as 190°

C to regenerate their respective silenes, the mildest

stereospecific route to a silene yet reported. The E- and

Z-silenes react stereospecifically with methanol under

vacuum-sealed tube conditions. The stereochemistry of the

addition is syn and a common mechanism is proposed for the

addition of alcohols and the addition of alkoxysilanes to

silenes.

TABLE OF CONTENTS

page

LIST OF TABLES iv

LIST OF ILLUSTRATIONS v

Chapter

I. INTRODUCTION 1

II. STEREOCHEMISTRY AND MECHANISM OF THE ADDITION OF ALKOXYSILANES TO SILENES . . . 21

III. THE CONFIGURATIONAL STABILITY OF ALPHA-LITHIO ALKOXYSILANES 52

IV. STEREOCHEMISTRY AND MECHANISM OF THE ADDITION OF ALCOHOLS TO SILENES 73

ill

LIST OF TABLES

Table Pa<?e

I. Conditions and Yields for Synthesis

of Chromium-Diastereomer Complex 25

II. The Conditions of the Analytical GLC . . . . 47

III. Response Factors on the Perkin Elmer Sigma-3 48

IV. Cross Reference of Experiment Numbers

and Notebook Numbers • 49

V. Trapping Reactions in THF 54

VI. Selected 13C NMR Data for Diastereomers . . . 57 VII. Relative Configurations of the Diastereomeric

Adducts to E- and Z-l-Methyl-l-Phenyl-2-Neopentyl Silene 59

VIII. Tin-Lithium Exchange Reactions 62

IX. The Conditions of the Analytical GLC . . . . 69

X. Response Factors on the Perkin Elmer Sigma-3 70

XI. Cross Reference of Experiment Numbers

and Notebook Numbers 71

XII. VSTT Experiments with Methanol 80

XIII. Attempts to Trap Silallylic Anion with D20. . 89

XIV. The Conditions of the Analytical GLC . . . . 97 XV. Response Factors on the Perkin Elmer

Sigma-3 98

XVI. Cross Reference of Experiment Numbers and Notebook Numbers 99

IV

LIST OF ILLUSTRATIONS

Figure Page

1. Molecular Structure of (R,S)-or (S,R)-2-methoxy-2-phenyl-3-triphenylsilyl-5,5-dimethyl-2-silahexane • • • • 33

2. Molecular Structure of (Z)-2-methyl-2-phenyl-3-neopentyl-3-trimethylsilyl [5,6:7,8] dibenzo-2-silabicyclo [2.2.2] octane 78

3. Scheme of Orbital Interactions for the Addition of Alcohols to Silenes 83

CHAPTER I

INTRODUCTION

The study of pi-bonded silicon species, which began in

earnest less than twenty years ago, has become an area of

considerable theoretical and experimental interest (1).

Since that time, substantiated claims for the existence of

compounds containing Si=C, Si=Si, Si=0, Si=N and Si=P bonds

have appeared in the chemical literature (2). Without

exception, all of these pi-bonded silicon species have been

found to be very reactive and it is only very recently that

any have been isolated and characterized (3).

From the first half of the twentieth century up to the

mid-1960's intermittent reports appeared which claimed the

existence of compounds containing a silicon-carbon double

bond: a silene. In 1912 diphenylsilene, 1, was postulated

as the major product in the reaction of silicon

tetrachloride with phenylmagnesium bromide and

methylmagnesium bromide followed by hydrolysis with water

(4). However, compound 1 did not react with bromine or

aqueous permanganate and was later found to be a mixture of

dimethyldiphenylsilane, dimethylsilanol and biphenyl (5).

1)PhMgBr,MeMgBr

SiCl4< ->MePh2SiOH -> Ph2Si=CH2

2 )H20

Other reports of silenes appeared intermittently over the

next fifty or so years but, in general, they have all been

shown to be incorrect.

The first indirect evidence for the existence of a

silene appeared in 1966 (6). Gusel'nikov and co-workers

reported a study of the gas phase pyrolysis of 1,1-dimethyl

silacyclobutane, 2. Ethylene and

1,1,3,3-tetramethyldisilacyclobutane were the products of

this reaction. It was postulated that this latter compound

arose through the dimerization of 1,1-dimethylsilene.

Me2C Me2C=CH2 + CH2=CH2

k (sec-1) = 1 0 15.68 exp(-61000/RT)

Me2Si- Me3Si=CH2 + CH2=CH2

k (sec-' ) = 10 15*60 exp( - 62300/RT)

Kinetic data were obtained for this reaction and compared

with those from the analogous reaction of a substituted

cyclobutane. The Arrhenius parameters obtained for the two

reactions were very similar and therefore the same

mechanism for the two reactions was suggested (7).

Since the tame of their report, the study of silicon-

carbon pi-bonded compounds has become one of the most

rapidly advancing areas in organometallic chemistry. While

the original method of Gusel'nikov is still used for

producing transient silenes, it suffers from several

disadvantages. Perhaps the most serious of these is the

high temperature required for the decomposition of

silacyclobutanes. These reactions are generally run under

flash vacuum pyrolysis conditions at temperatures in excess

of 700 degrees Kelvin. Under these high temperature

conditions, isomerization of the simple silenes to

silylenes can occur and can lead to complications in the

interpretation of results (8,9,10,11).

• •

H2Si- ^ H2Si=CH2 > / S i \

A H CH-

Other thermal rearrangements of silenes have been

shown to occur under milder conditions. A number of [1,3]

shifts of alkyl groups from a saturated silicon atom to an

unsaturated silicon atom in 2-silyl substituted silenes

have been reported (2). For example, the occurrence of

degenerate 1,3 methyl shifts in

l,l-dimethyl-2,2-bis(trimethylsilyl)silene, 3, has been

demonstrated by isotopic labeling (12).

SiMe3 SiMe2 SiMe3

/ > // — . / Me2Si=C <— Me3Si-C^ < Me 3Si-C^

SiMe-, SiMe-, SiMe,

The half-life is about 30 minutes at 120° C.

The only documented example of a thermal silene

isomerization involving a hydrogen shift to an unsaturated

silicon atom is the rearrangement of 1-silacyclopentadiene

to 1-methylsilole, which then dimerizes (13).

SiHMe >

'Si' A ^Si'

I A / \ Me Me H

Shechter has reported the formation of dimethylvinylsilane

in low yield from the pyrolysis of

trimethylsilyldiazomethane, along with the head-to-tail

dimer of 1,1,2-trimethylsilene, 5. As Shechter recognized,

the vinylsilane could arise either from the isomerization

of 5 or from rearrangement of 1,1-dimethylsilacyclopropane,

derived directly from carbene 4 (14).

Me3SiCH

Me2Si=CR ?e

M e 2 S i ^< ^

H

Among the other reported thermal routes to silenes is

the "ene" reaction of allyl silanes as shown below (15,16)

Si

In addition, both silylenes (10) and alpha-silylcarbenes

(17) have been reported to isomerize in part to silenes. A

relatively mild thermal route to silenes, which appears to

avoid the complications of isomerization, is the retro

Diels-Alder reaction of bicyclo[2.2.2]octadienes

(10,11,18,19,20).

Me

j d i _ H

4- h n CF3 \ 1 Si=CH2

Me

There have been a number of reported photochemical

routes to silenes. One of the more successful photochemical

routes involves photolysis of polysilylacylsilanes as

reported by Brook (21).

O 0SiMe3

II / (Me3Si)3Si-C-R (Me3Si)2Si=C^

R

R = CMe3, CEt3, i-Pr, Ph, bicyclo [2.2.2] octyl,

benzyl, 1-methyl cyclohexyl, Et, adamantyl

These silenes show some peculiar reactivity. In particular,

they are the only silenes reported to dimerize in a head-

to-head fashion to yield 1,2-disilacyclobutanes (22). When

R is made very bulky these silenes survive long enough for

NMR data to be gathered, and in the case of R = CEt3 and R

= adamantyl, have been isolated as crystalline solids; a

crystal structure of the latter compound has been reported

(3). The unusual reactivity of these silenes is perhaps

due to their electronically perturbing ligands. Brook has

suggested resonance forms such as those shown below for

this special class of silenes (3).

OSi OSi +OSi

\ / V J v / Si=C « > Si-C < > Si-C

/ \ / \ / \

These resonance forms would act to negate, at least

partially, the "normal" polarity of the Si=C bond (23).

Si=C / X

8

Another major method of generating silenes is the salt

elimination method. Jones and Lim demonstrated that the

vinyl chlorosilane, 6, in hydrocarbon solvents produces the

silene intermediate, 7, when treated with tert butyl

lithium (24,25).

Me2SiCH=CH2

I

CI

6

t-BuLi

hydrocarbon

-78°

Me2Si-CHCH2t-Bu I I Cl Li

-LiCl

Me2Si=CHCH2t-Bu

This intermediate dimerized in the head-to-tail fashion

which is typical of silenes (26). In 1980 they reported

further evidence for the formation of silenes in this

system (27). When the reaction between

dimethylvinylchlorosilane and tert butyl lithium was

carried out in the presence of conjugated dienes, such as

1,3-butadiene, 2,3-dimethyl-l,3-butadiene, cyclopentadiene

and anthracene, the expected [4+2] cycloadducts were

formed. The [2+2] cycloadducts of silene 4 were also

formed when 1,3-butadiene was present and are evidence of

the high reactivity of silenes. None of these cycloadducts

were obtained when the reactions were carried out in THF

(27) .

Si=C o

o O

10

When Jones and co-workers ran similar experiments

using methylphenylvinylchlorosilane, 8, two isomeric

silenes, 9a and 9b, were produced (28,29).

Ph t-BuLi Me I Ph

I „ \ . / ~ T ~ \ . / " Me-Si-CH=CH2 * Si=C + , Si=C

I / \ / \ CI Ph H Me H

9a 9b

In the absence of any trapping agent all five possible

isomeric 1,3-disilacyclobutanes were obtained in ratios

consistent with a stepwise mechanism for their formation

(29). In contrast to this result, when the reaction was run

in the presence of conjugated dienes such as

2,3-dimethyl-l,3-butadiene, cyclopentadiene, or anthracene,

the expected Z and E [4+2] cycloadducts were recovered in a

consistent ratio of 30 to 70 (29). Apparently this was the

Z to E ratio of the silene which was being produced.

Wiberg and Preiner have also observed the elimination

of lithium salts to give silene intermediates (30). The

thermal stability of the alpha-lithio silanes was found to

be dependent on the nature of the leaving group attached to

silicon. The rate constants for the decompostion of

compound 10, substituted with various leaving groups, X,

11

were determined in diethyl ether. In general the poorer

leaving groups necessitated higher temperatures for the

elimination to take place (31).

Me2Si-C(SiMe3)2 t-BuLi Me2Si-C(SiMe3)2

X Br X Li

10

-LiX

Me2Si=C(SiMe3)2

11

The intermediacy of silenes in these reactions was

confirmed by trapping reactions with 2,3-dimethyl-

butadiene, bis(trimethylsilyl)diazene and

trimethylsilylazide, where [4+2], [2+2] and [3+2]

cycloadducts were obtained, respectively. Silene 11 also

reacted readily with various alcohols and amines (12).

12

Me2Si

(Me3Si)2

(Me3Si)^T SiMe

Me-,SiN=NSiMe3 Me3Si

N — N

/ \ . SiMe.

Me2Si=C(SiMe 3)2 Me3SiN3

ROH

RNH-

(Me3Si)

SiMe.

V N-SiMe-

/ NZTN

Me2Si-C(SiMe3)2

I I OR H

Me2Si-C(SiMe3)2

I I RNH H

13

Although silenes have been shown to undergo a wide

variety of addition reactions involving sigma bond cleavage

of the trapping agent, very little is known about the

mechanisms or stereochemistries of these reactions. They

all appear to involve nucleophilic attack at the pi-bonded

silicon atom, and the nucleophilic reagents include

alcohols, amines, silyl ethers, hydrogen halides and water

(2). In the addition of alcohols, Wiberg has observed the

following relative rates: MeOH (96), EtOH (62), i-PrOH

(48), t-BuOH (32), n-pentanol (8), cyclohexanol (4) and

phenol (1). These were determined for

l,l-dimethyl-2,2-bis(trimethylsilyl)silene in ether at 100

degrees C. Similarly, a series of amines was found to

react with the relative rates: i-PrNH2 (97), t-BuNH2 (48),

PhNH2 (2.2). From these data it would appear that the

nucleophilicity of the reagents as well as their steric

bulk are the major factors affecting their rates of

reactivity towards silenes (12).

In one of the very few stereochemical studies of

silenes, Bertrand et al. used a series of chiral alcohols

to trap a prochiral silene generated by photolysis of

silacyclobutanes and observed substantial asymmetric

induction (32).

14

The authors concluded that the transition state for this

asymmetric induction reaction involved a pi-bonded silicon

component.

r Rj-Si

k V \ Si=CH-

/

ROH

• Me

\ .4 Si*

/ \ OR

+ CH2=CH2

Further experiments by Jones and Lee showed that

either silene 9a or 9b could be generated in isomerically

pure form by the sealed tube thermolysis of the

corresponding anthracene adduct (33). When the E silane

12a, for example, was heated to 300°C in a sealed tube in

the presence of trimethylmethoxysilane, a known silene

trapping agent (34), only one diastereomeric product was

produced.

Si-Ph

O OMe SiMe.

Me3SiOMe^ -Si-

I Ph

13a

13b

15

Thermolysis of the Z adduct produced the other possible

diastereomer. No appreciable isomerization of the silene

precursors occured in these experiments. These results

showed not only that silenes 9a and 9b are

configurationally stable up to 300°C but also that they

react stereospecifically with trimethylmethoxysilane.

However, lack of stereochemical information concerning the

isomeric products, 13a and 13b, precluded any determination

of the stereochemistry of this reaction. This led to a

more thorough investigation of this reacton, with the goal

of determining its stereochemistry. The results of this

investigation are reported in Chapter II.

As mentioned earlier, the silenes 9a and 9b are

configurationally stable up to 300°C. Beyond this

temperature they begin to isomerize, as demonstrated by

trapping experiments with trimethylmethoxysilane (35). On

the other hand, the precursor molecules 12a and 12b do not

undergo decomposition at a significant rate much below this

temperature. Therefore, it became desirable to find a

lower temperature stereospecific route to silenes. Although

the salt elimination method of Jones and Lee to yield the

silenes 6a and 6b did show significant stereochemical

induction, I wished to find an even more selective and

possibly stereospecific low temperature route in order to

carry out further stereochemical studies. The ready

availability in these laboratories of isomerically pure

16

alpha-stannyl alkoxysilanes, 14, of known stereochemistry

(36), offered a promising starting point.

OMe SnR 3 I

- S i -

I R

R = Ph, Bu

14

These compounds contain a moderately good leaving group on

silicon, which could theoretically be replaced by an even

better one by reactions of known stereochemistry (37).

They also offer the potential for stereospecific

introduction of lithium in the alpha position via the well

documented tin-lithium exchange reaction (38,39,40). The

results of these studies are presented in Chapter III.

Also, of much interest was the study of the

stereochemistry and mechanism of the addition reaction of

alcohols with silenes. The reaction of silenes with

alcohols, to yield alkoxysilanes, is among the most widely

cited reactions of transient silenes (1,2). As the

absolute configurations of the silanes 13a and 13b had

recently been determined, it was desirable to find a

stereospecific route to one or both of the isomers of the

previously unknown silene 15. Trapping of either isomer of

15 with methanol should give either one or both of the

diastereomeric silanes 13 and allow some insight into the

17

into the mechanism of this reaction. The results of this

investigation are reported in the final chapter of this

dissertation.

Me

Ph

V / Si=C

/ \

SiMe-

15

MeOH •»

OMe SiMe.

I. —Si-

I Ph

13a, 13b

18

CHAPTER BIBLIOGRAPHY

1. For a recent survey of silicon-carbon multiply bonded species see Brook, A.G. and Baines, K.M., Advances in Organometallic Chemistry, Stone, F.G.A. and West, R., eds.; Academic Press, New York, Vol. 25 (1985) pp 1-38.

2. For a comprehensive survey of all multiply bonded silicon species see Raabe, G. and Michl, J., Chemical Reviews, 85 (1985) 419-509.

3. Brook, A.G., Ryburg, S.C., Abdesaken, F., Gutekunst, B., Gutekunst, G., Kalbury, R.K.M.R., Poon, Y.C., Chang, Y.-M., and Wong-Ng, W., J. Am. Chem. Soc., 104 (1982) 5667-5672.

4. Schlenk, W. and Renning, J., Justus Liebigs Ann. Chem., 394 (1912) 221.

5. (a) Kipping, F.S., J. Chem. Soc., (1927) 104-107. (b) Cusa, N.W. and Kipping, F.S., J. Chem. Soc., (1932) 2205-2209.

6. (a) Nametkin, N.S., Vdovin, V.M., Gusel'nikov, L.E. and Zab'yalov, V.I., Izv. Akad. Nauk SSSR, Ser. Khim., (1966) 589. (b) Nametkin, N.., Gusel'nikov L.E., Vdovin, V.M., Grinberg, P.L., Zav'yalov V.I. and Oppergeim, V.D., Dokl. Akad. Nauk SSSR, 171 (1966) 630.

7. (a) Gusel'nikov, L.E., and Flowers, M.C., Chem. Comm., (1967) 864-865. (b) Flowers, M.C. and Gusel'nikov, L.E., J. Chem. Soc. B, (1968) 419-423.

8. Conlin, R.T. and Wood, D.L., J. Am. Chem. Soc., 103 (1981) 1843-1844.

9. Conlin, R.T. and Gill, R.S., J. Am. Chem. Soc., 105 (1983) 618-619.

10. Burns, S.A., Burns, G.T. and Barton, T.J., J. Am. Chem. Soc., 104 (1982) 6140-6142.

11. Barton, T.J., Burns, S.A. and Burns, G.T., Orqanometallics, 1 (1982) 210-212.

12. For a review of this work see Wiberg, N., J. Organomet. Chem., 273 (1984) 141-177.

19

13. Barton, T.J. and Burns, G.T., J. Organomet. Chem., 179 (1979) C17-C20.

14. Kreeger, R.L. and Shechter, H., Tett. Lett., 25 (1975) 2061-2064.

15. Barton, T.J. and Burns, G.T., J. Am. Chem. Soc., 100 (1978) 5246.

16. Block, E. and Revelle, L.K., J. Am. Chem. Soc., 100 (1978) 1630-1632.

17. (a) Ando, W., Sekiguchi, A. and Sato, T.J., J. Am. Chem. Soc., 103 (1981) 5573-5574. (b) Ando, W., Sekiguchi, A. and Sato, T.J., J. Am. Chem. Soc., 104 (1982) 6830-6831.

18. Maier, G., Mihm, G. and Reisenauer, H.P., Angew. Chem., 93 (1981) 615.

19. Jones, P.R. said Lee, M.E., J. Am. Chem. Soc., 105 (1983) 6725-6726.

20. Conlin, R.T. and Kwak, Y.W., Organometallics, 3 (1984) 918-922.

21. For a review of this work see Brook, A.G. J. Organomet• Chem., 300 (1986) 21-37.

22. Brook, A.G., Harris, J.W., Lennon, J. and El Sheikh, M., J. Am. Chem. Soc., 101 (1979) 83-95.

23. Trinqiuer, G., and Mahieu, J.-P., J. Am. Chem. Soc., 103 (1981) 6313-6319.

24. Jones, P.R. and Lim, T.F.O., J. Am. Chem. Soc., 99 (1977) 2013-2015.


26. Gusel'nikov, L.E., Nametkin, N.S. and Vdovin, V. , Acc. Chem. Res., 8 (1975) 18-25.

27. Jones, P.R., Lim, T.F.O. and Pierce, R.A., J. Am. Chem. Soc., 102 (1980) 4970-4973.

28. Jones, P.R. and Lee, M.E., J. Organomet. Chem., 232 (1982) 33-39.

29. Jones, P.R., Lee, M.E. and Lin, L.T., Organometallics, 2 (1983) 1039-1042.

20

30. Wiberg, N. and Priener, G., Angew. Chem. Int. Ed. Eng., 16 (1977) 328-330.

31. Wiberg, N., Priener, G., Schieder, 0. and Fischer, G. Chem. Ber., 14 (1981) 3505.

32. Bertrand, G., Duboc, J., Magerolles, P. and Ancelle, J., J. Chem. Soc., Chem. Comm., (1980) 382-383.

33. Jones, P.R. and Lee, M.E., J. Am. Chem. Soc., 105 (1983) 6725-6726.

34. John, P., Gowenlock, B.G. and Groome, P., J. Chem. Soc., Chem. Comm., (1981) 806-807.


36. Cheng, A.H-B., Ph.D. Dissertation, North Texas State University, 1985.

37. Cheng, A.H-B., Jones, P.R., Lee, M.E. and Roussi, P., Organometallics, 3 (1985) 581-584.

38. Seyferth, D. and Weiner, M.A., J. Am. Chem. Soc., 83 (1961) 3583-3586.

39. Seyferth, D. and Weiner, M.A., Org. Synthesis, 41 (1961) 30.

40. Peterson, D.J., J. Am. Chem. Soc., 93 (1971) 4027-4031.

CHAPTER II

STEREOCHEMISTRY AND MECHANISM OF THE

ADDITION OF ALKOXYSILANES TO SILENES

In 1983, Jones and Lee reported that the E- and Z-

isomers of l-methyl-l-phenyl-2-neopentylsilene, 9, are

configurationally stable up to 300° C and that they can be

trapped stereospecifically by methoxytrimethylsilane to

give separable diastereomeric adducts (1). Unfortunately,

their inability to determine the absolute configurations of

the isomeric products, 13a and 13b, precluded determination

of the stereochemistry of this reaction. This led to

investigation of the possibility of producing an

isomerically pure derivative of 13a or 13b suitable for x-

ray crystallographic study and therby derermining the

configuration at the two chiral centers. One possibility

considered was the use of an alkoxysilane trapping agent of

higher molecular weight. Although the adducts of silenes

9a and 9b with methoxytrimethylsilane were viscous oils, it

was hoped that their adducts with some other alkoxysilane

might be crystalline solids. The other possibility was the

synthesis of a transition metal-arene complex of one of the

isomers of compound 13.

21

22

For the latter possibility I chose to investigate the

synthesis of the arene-tricarbonylchromium complex of 13.

Results and Discussion

To synthesize the diastereomeric arene ligands, 13a

and 13b, for (<n6-2-phenyl-2-methoxy-3-trimethylsilyl-5, 5-

dimethyl-2-silahexane) chromium tricarbonyl, 16, I chose

the reaction of tert butyl lithium with

methylphenylmethoxyvinylsilane, 17, to produce the a-lithio

compound, 18, which can be efficiently trapped by

trimethylchlorosilane in THF to give the alkoxysilanes 13a

and 13b, in high yield, in a ratio of 33 to 67

respectively.

Ph I

Me-Si-CH=CH„ +

I

OMe

17

THF

•Li

-78o

18

Me^iCl

OMe I •Si-I Ph

SiMe.

13a : 13b

(33 : 67)

These enantiomeric pairs of diastereomeric adducts can be

separated analytically by using capillary GLC

techniques.(2).

23

Diastereomerically pure 13a was produced by trapping

the E-silene, 9a, produced by the retro Diels-Alder

cycloreversion of E-2-phenyl-3-neopentyl[5,6:7,8]

dibenzo-2-silabicyclo[2.2.2]octane, 12a, with

methoxytrimethylsilane (1). These and other vacuum-sealed

tube thermolysis experiments discussed in this chapter were

carried out in cyclohexane in evacuated, degassed, sealed

tubes at 300° C.

• D

Me3SiOMe

VSTT

3000 C

OMe SiMe-

I -Si-

Ph

13a The conversion of the mixed diastereomers to compounds

16a and 16b was first accomplished by boiling a mixture of

13a, 13b, (33:67) and Cr(CO)6 in an n-butylether:THF

solvent mixture (10:1).

13a : 13b

(33 : 67)

N-butylether/THF

reflux, Cr(CO),

<] >>-5!

Me

OMe SiMe-I

.Cr

/l\ c c c

in hi % 0 0 0

16a,b (33:67)

24

The reaction was followed by GLC and after four hours the

reaction appeared to have stopped. GLC analysis indicated

that the products were present in the same ratio (33:67) as

the remaining starting material. This indicated that both

isomers of 13 were reacting at essentially the same rate

and that no isomerization at the chiral centers was

occuring. However, the conversion was a disappointing

fifty-seven percent.

In order to improve the yield of this reaction I

wished to obtain a starting chromium compound which

contained more labile ligands. Tris(acetonitrile)chromium

tricarbonyl was chosen and was readily prepared by

refluxing chromium hexacarbonyl in acetonitrile (3).

CH3CN

Cr (CO) & (CH3CN) 3Cr(CO)

reflux

The reaction between the tris(acetonitrile) chromium

tricarbonyl and the mixed diastereomers was run both in

hexane and in dioxane. The results are shown in Table I.

The low yields in hexane may be due to the fact that the

boiling point of hexane is below that of acetonitrile,

whereas dioxane has a higher boiling point and may drive

25

reaction towards completion by boiling off the acetonitrile

as it is released from the chromium center.

TABLE I

CONDITIONS AND YIELDS FOR SYNTHESIS OF

CHROMIUM-DIASTEREOMER COMPLEX

Reactants Solvent Time Yieldi

13a:13b Cr(CO)6 butyl ether/ 48h 57%

(33:67) THF (10:1)

(CH3CN)3Cr(CO)3 hexane 3h 0%

ii N 24h 3%

H M 72h 24%

N dioxane 14h 72%

LYields determined by GLC.

26

The compounds 16a and 16b were obtained as bright yellow

crystals in solution under argon, but all attempts at

further isolation led to their decomposition. They were

extremely sensitive to the atmosphere.

As the reaction between silenes and alkoxysilanes is

not limited to methoxytrimethylsilane (4,5), I synthesized

some alkoxysilanes of higher molecular weight in order to

test their reactivity with silenes 9a and 9b and to obtain

crystalline adducts suitable for x-ray crystallography.

When para-bromophenol was refluxed with

hexamethyldisilazane, the expected product, para-

bromophenoxytrimethylsilane, 19, was obtained in good

yield.

SiMe.

O (Me3Si)2NH

reflux

When compound 19 was heated in a sealed tube with the E-

silene precursor, 12a, at 300° C for ten hours, only traces

27

of the expected product, 20, could be detected by GC-mass

spectrometry.

Br

\ Si—Ph

o o

19

VSTT, 300 °C-

10 h.

SiMe.

The synthesis of methoxydimethyl(«-naphthyl)silane,

22, was then undertaken. The necessary precursor,

dimethyl (<x-naphthyl)chlorosilane, 21, was made by adding an

ether solution of a-naphthyl lithium slowly to excess

dimethyldichlorosilane (6). The naphthyl lithium was

prepared by a lithium-halogen exchange reaction between

a-bromonaphthalene and n-butyl lithium in hexane. The

insoluble naphthyl lithium was then filtered and dissolved

in ether before addition to the dichlorodimethylsilane.

After purification, methanolysis of 21 with added pyridine

to trap the liberated hydrochloric acid, yielded the

desired compound 22.

Li

MeOH >

Me2SiCl2 Me2SiCl

DIQ Me2SiOMe

ether/hexane Py.

21 22

28

When compound 22 was heated in a sealed tube with the

E-silene precursor, 12a, at 300° C for ten hours, GLC and

GC/MS analysis indicated that the silene adduct of

alkoxysilane 22, 2-methoxy-2-phenyl-3-(dimethyl

<x-naphthylsilyl) -5, 5-dimethyl-2-silahexane, 23, was the

major volatile product.

Si—Ph

D 22

VSTT, 300°C. — >

10 h.

23b (37%)

Compound 23 appeared as a single sharp peak on a capillary

GC column, indicating that probably only one diastereomer

was present. In order to produce larger amounts of 23 and

also to check the separability of its isomeric forms under

our GC conditions, I trapped the a-lithio silane 18 with

the chlorosilane 21 in THF. This yielded both

diastereomers, 23a and 23b, in a 28:72 ratio in good yield.

These isomers were separable by use of analytical GLC

techniques. Comparison of retention times of this mixture

of diastereomers with the retention time of the isomer

produced in the sealed tube reaction indicated that 23b was

the only isomer produced from the VSTT of silene precursor

12a in the presence of alkoxysilane 22.

29

After purification of this mixture from other minor

impurities a pale yellow oil was obtained. All attempts at

crystallization were unsuccessful.

Methoxytriphenylsilane, 24, another potential

stereospecific silene trapping agent, was readily prepared

from methanol and triphenylchlorosilane in the presence of

pyridine. When compound 24 was heated in a sealed tube

with the E-silene precursor, 12a, at 300° C for ten hours,

GLC analysis indicated a 61% yield of 2-methoxy-2-phenyl-3-

(triphenylsilyl)-5,5-dimethyl-2-silahexane, 25.

The success of methoxytriphenylsilane as a silene

trapping agent under sealed tube thermolysis conditions led

to synthesis of its adducts with silenes 9a and 9b in

larger quantities by trapping the a-lithio silane, 18, with

triphenylchlorosilane in THF. This produced the

alkoxysilane, 25, in good yield.

OMe

I -Si-I Ph

18

Li Ph3SiCl

THF

OMe SiPh-

I •Si-I

Ph

25a, 25b (77%)

(45 : 55)

After purification by column chromatography, proton NMR

indicated that a 45:55 mixture of isomers was present in

30

the viscous oily material. Crystals of pure 25a and 25b

were separated and purified by recrystallization from

absolute ethanol.

OMe SiPh3 Absolute EtOH

•Si-

Ph

25a : 25b

(45 : 55)

-> 25b

mp 100.0-100.5

25a

mp 102.8-103.2

Proton NMR analysis of the compound 25 produced in the

sealed-tube thermolysis experiments indicated that 25b was

the only isomer of this compound present. To further

verify that the reaction between methoxytriphenylsilane and

silenes is stereospecific, I produced the silenes 9a and 9b

at low temperature by the method of Jones and Lee (7).

They have reported that the ratio of 9a to 9b from this

reaction is consistently 70 to 30, respectively, based on

trapping experiments with various dienes (8).

31

In the presence of methoxytriphenylsilane this reaction

yielded 25b and 25a in a ratio of 69 to 31 (9).

• Ph3SiOMe

VSTT, 300°

10 h.

OMe SiPh-

I -Si-

I

Ph

25a : 25b

(0 : 100)

Ph I

Me-Si-CH=CH2 4- Ph3SiOMe I CI

Li

> 25a : 25b

hexane, -78° (31 : 69)

These results show that methoxytriphenylsilane, like

methoxytrimethylsilane (1), reacts stereospecifically with

silenes. There are, of course, two possible modes of

addition which would yield a single diastereomeric silane

from the reaction of a single isomer of silene 9 and

methoxytriphenylsilane. In the reaction above where the E-

silene 9a was generated, a syn addition would yield the

32

[R,S],[S,R] enantiomers, wheras an anti-addition would

yield the [R,R],[S,S] set of enantiomers.

syn

Me Ph3Si 0 0 SiPh

H Ph

\ / Si=C

/ \ -I- Ph3SiOMe

Me

anti

[S,R] [R/ S]

SiPh

[S,S] [R/R]

The crystal structure of 25b was determined and is

shown on the next page (9). It can be seen that the

diastereomer 25b consists of the [R,S] and [S,R]

enantiomers. Clearly then, it can be said that the

reaction between alkoxysilanes and silenes is a

stereospecific and syn addition. This would suggest a

33

Figure 1. Molecular Structure of (R,S) or (S,R)-2-methoxy-2-phenyl-

3-triphenylsilyl-5,5-dimethyl-2-silahexane•

34

concerted reaction and a transition state involving six

electrons in a linearly conjugated array of five orbitals

can be imagined (10).

\0 9/ Si C

:g--Gv Q.

/ .0^^ ->SiR 3

Me

It has been stated that the concerted addition of a polar

sigma bond to a pi-bond is symmetry-forbidden (11).

However, a mechanism involving a lone pair of electrons on

the nucleophilic reactant is not a pericyclic process to

which the Woodward-Hoffman selection rules apply (12). The

lone pair electrons and the electrons of the polar sigma

bond are in orthogonal molecular orbitals and have a

resonance integral of essentially zero.

Silenes are electrophilic by nature (10). In fact,

Wiberg has isolated a THF-silene adduct in which the oxygen

atom of the THF molecule is coordinated to the silicon of

the silene (13). It therefore appears likely that silicon-

oxygen bond formation is running ahead of carbon-silicon

bond formation in the reaction of silenes with

alkoxysilanes. However, the stereospecific nature of these

reactions would seem to rule out a two-step mechanism

involving a bipolar intermediate such as 27. If such an

intermediate does exist on the potential surface for the

35

addition of alkoxysilanes to silenes, it is not chemically

significant and for all intents and purposes this reaction

should be considered concerted.

^'Si=C

4-

R O — S i =

^Si=C

RO—Si = +

. . Si-C

1- ^ 1 _ RO-+ •Si=

I I — S i - C —

' I I RO S i =

27

36

Experimental Section

All reactions were carried out under an inert

atmosphere of dry nitrogen or argon in glassware that was

either oven dried or flame dried. Solvents were distilled

from lithium aluminum hydride or sodium/potassium alloy (n-

butyl ether under reduced pressure) immediately prior to

use. Chromium hexacarbonyl was purchased from Pressure

Chemicals Company and was used as received. Solutions of

tert butyl lithium and n-butyl lithium were obtained from

Aldrich Chemical Company and were standardized by the

method of Kofran (14). Unless otherwise noted, yields and

percent conversions were determined by GC analysis, using

an appropriate internal standard, on a Perkin-Elmer Sigma-3

FID gas chromatograph with a 25 m fused silica capillary

column containing methylphenylsilicone stationary phase,

SE-54, equipped with a Hewlett-Packard 3390A recording

integrator. In determining response factors it was assumed

that all isomers of a particular compound have the same

response factor.

GC/MS analysis of some of the reaction mixtures was

performed using an HP 5970A GC/MS and data system. NMR

spectra were determined on a Perkin-Elmer R24B 60 MHz or a

JEOL FX-90Q 90-MHz spectrometer with CDCl3 or D20 as the

lock solvents. Chemical shifts are reported in parts per

37

million downfield from tetramethylsilane. Elemental

analysis was performed by Galbraith Laboratories,

Knoxville, TN.

1. 2-methoxy-2-pheny1-5,5-dimethyl-3-(trimethylsilyl)-

2-silahexane. (13a,b).

To a stirred solution of 4.45 g (25.0 mmol) of

methoxymethylphenylvinylsilane (15) in 50 mL of dry THF

cooled to -78° C was added dropwise 13.9 mL of a 1.8 M

solution of tert butyl lithium (25.0 mmol) with magnetic

stirring. The temperature was maintained at -78° C for 1.5

hours before 3.24 g (30.0 mmol) of trimethylchlorosilane

was added. The mixture was allowed to warm slowly to room

temperature and stirred overnight prior to hydrolysis with

saturated ammonium chloride solution. The organic layer

was separated, washed twice with water, combined with

petroleum ether extractions of the aqueous layers and dried

over magnesium sulfate. The solvent was removed under

reduced pressure to give 7.1 g (92%) of 13a and 13b in a

ratio of 33:67 (>99% pure by GC). The compounds were

identified by comparison of their GC retention times and

mass spectra with authentic samples (2).

38

2. (Ti6-2-phenyl-2-methoxy-3- (trimethylsilyl) - 5 , 5-dimethyl

2-silahexane)Cr (CO) ->. (16a,b) . Method A.

To a mixture of 75 mL of dry n-butyl ether and 7.5 mL

of dry THF was added 4.62 g (15.0 mmol) of a 33:67 mixture

of 13a:13b and 3.30 g (15.0 mmol) of chromium hexacarbonyl.

This mixture was heated to reflux for 48 hours. Removal of

the volatiles under reduced pressure left an orange-red

oil. GC analysis indicated two closely spaced peaks in a

33:67 ratio with much longer retention times than the

remaining starting material (43%) which also gave two

closely spaced peaks in a 33:67 ratio. No other products

could be detected by GC. Attempts at crystallization of

the new product or separation from the remaining starting

material by column chromatography were unsuccessful. The

IR spectra of this mixture showed strong CO stetching bands

at 1963 and 1885 cm-i.

3. Method B.

To 70 mL of freshly distilled acetonitrile (dried over

4A molecular sieves) was added 2.20 g (10.0 mmol) of

chromium hexacarbonyl. This mixture was refluxed for 16

hours. The subliming chromium hexacarbonyl was returned to

the flask manually at intervals. The volatiles were

removed from the yellow solution to yield

tris(acetonitrile) chromium tricarbonyl as a bright yellow

powder. This solid was dissolved in 75 mL of dry dioxane

and refluxed with 1.85 g (6.0 mmol) of 13a, 13b (33:67).

39

After 14 hours the greenish reaction mixture was filtered

through celite in a nitrogen filled glovebag and the

volatiles removed under vacuum. GC analysis indicated a

72% conversion. A small amount of dry hexane was added and

the solution cooled over dry ice. A large amount of yellow

crystals formed in the yellow solution but all attempts to

isolate them led to their decomposition. This reaction was

also run in hexane instead of dioxane. This gave poorer

conversions and the results are summarized in Table I.

A. p-Bromophenoxytrimethylsilane. (19).

p-Bromophenol, 9.65 g (50.0 mmol) was dissolved in

8.07 g (50.0 mmol) of hexamethyldisilazane and the mixture

refluxed for four hours. Distillation at 1.8 torr yielded

9.73 g (79%) of the title compound, BP 80-81°C, (lit.

126°/25 torr (16)).

5. Dimethyl(oc-naphthyl)chlorosilane. (21).

A solution of 19.6 g (94.5 mmol) of a-bromonaphthalene

in 50 mL of dry hexane was added to a solution of 35 mL of

a 2.7 M solution of n-butyl lithium (94.5 mmol) in 35 mL of

additional hexane. This mixture was stirred for two hours

and the insoluble blue-green naphthyl lithium was then

filtered and washed twice with dry hexane before being

dissolved in 350 mL of dry ethyl ether.

This solution was added dropwise to 18.2 mL (150 mmol)

of dimethyldichlorosilane in 50 mL of dry hexane. After

40

stirring for two hours the liquid portion was decanted from

the precipitated salt and the solvents removed under

reduced pressure. Distillation at 1.7 torr yielded 8.6 g

(41% based on a-bromonaphthalene) of the title compound, BP

137-139" C, (lit. 108-110/0.15 torr (6)).

6. Dimethyl( ot-naphthyl)methoxysilane. (22) .

To a solution of 0.320 mL of methanol (7.8 mmol) and

0.630 mL of pyridine (7.8 mmol) at 0° C was added 1.00 g

(4.5 mmol) of dimethyl(a-naphthyl)chlorosilane. This

mixture was allowed to warm to room temperature and stirred

overnight before hydrolysis with water. The organic layer

was washed twice more with water and the combined aqueous

portions extracted with hexane. The combined organic

layers were dried over magnesium sulfate and the volatiles

removed under reduced pressure. The residue was vacuum

distilled at 0.5 torr to yield 0.79 g (81%) of the title

compound, BP 107-108° C. (17). MS of 22, m/e (relative

intensity) 216 (29) P, 201 (100) P-15, 171 (30), 169 (16),

141 (22), 59 (22).

7. Methoxytriphenylsilane. (24).

To a solution of 0.971 mL of methanol (24.0 mmol) and

1.94 mL of pyridine (24.0 mmol) at 0° C was added 5.9 g

(20.0 mmol) of powdered triphenylchlorosilane dissolved in

40 mL of dry ethyl ether. After stirring for two hours the

mixture was hydrolyzed with water. The organic layer was

41

washed twice more with water and the combined aqueous

layers extracted with hexane. The combined organic layers

were dried over magnesium sulfate and the volatiles removed

to yield a light colored solid material. This was

dissolved in hexane and passed through a short column of

silica gel with hexane as the elutant and the solvent

removed to yield 4.87 g (84%) of the title compound as a

white powder, MP 53-55° C, (lit. 54.5-55° C (18)).

8. 2-methoxy-2-phenyl-3-(dimethyl((X-naphthyl)silyl)

5,5-dimethyl-2-silahexane. (23a,b).

To a solution of 3.56 g (20.0 mmol) of

methylphenylvinylmethoxysilane in 40.0 mL of dry THF

cooled to -78° C was added 13.3 mL of a 1.5 M solution of

tert butyl lithium (20.0 mmol) in pentane. After stirring

for two hours 5.28 g (24.0 mmol) of

dimethyl(a-naphthyl)chlorosilane was added to the reaction

mixture which was then allowed to warm slowly to room

temperature while being stirred overnight. Following

aqueous workup with saturated ammonium chloride solution

and removal of the solvent, GLC analysis indicated a 70%

yield of the title compound. The ratio of 23a to 23b as

indicated by GLC was 28:72.

42

9. 2-methoxy-2-phenyl-3-(triphenylsilyl)-5,5-dimethyl

2-silahexane. (25a,b). Method A.


methylphenylvinylmethoxysilane in 40 mL of dry THF cooled

to -78° C was added 11.1 mL of a 1.8 M solution of tert-

butyl lithium (20.0 mmol) in pentane. After stirring for

one hour, a solution of 8.85 g (30.0 mmol) of

chlorotriphenylsilane in 25 mL of dry THF was added

dropwise, and the mixture allowed to warm to room

temperature overnight with stirring. Following hydrolytic

workup with saturated ammonium chloride solution and

removal of the solvent, GLC analysis indicated a 77% yield

of the title compounds. Proton NME. indicated that the

ratio of 25a to 25b was 45:55. The diastereomers were

purified by column chromatography on silica gel using a

10:90 mixture of ethyl acetate to hexane as the solvent.

Trituration of the oily mixture of 25a and 25b with

absolute ethanol at 0° C yielded crystals of approximately

90% pure 25b. Recrystallization from absolute ethanol gave

pure 25b, mp 100:0-100.5° C. After a period of time pure

25a crystallized from the mother liquor, mp 102.8-103.2° C.

43

10. Method B.


methylphenylvinylchlorosilane and 4.15 g (14.3 mmol) of

methoxytriphenylsilane in 125 mL of dry hexane at 0° C was

added 8.82 mL of a 1.7 M solution of tert butyl lithium

(10.0 mmol) in hexane. The mixture was allowed to warm to

room temperature overnight with stirring. Following

hydrolytic workup with saturated ammonium chloride solution

and removal of the solvent, GLC analysis indicated a 42%

yield of the title compounds. Proton NMR showed a ratio of

25a to 25b of 31:69.

General Procedure for Vacuum Sealed Tube Thermolysis (VSTT)

Experiments.

All of the thermolyses were carried out in sealed 6.3

mm (OD) x 120 mm Pyrex tubes (thick wall, 1.2 mm). The

reaction mixture of the silene precursor dissolved in

cyclohexane solvent, trapping reagents and hexadecane as

internal standard was transferred to the thermolysis tube

through a capillary tube. After degassing under vacuum

(0.05 torr) the thermolysis tube was sealed under vacuum.

The entire tube was then placed in a vertical pyrolysis

oven that was preheated to the temperature indicated.

After the indicated time the tube was removed, allowed to

cool and opened. If any solid compounds were present,

sufficient benzene was added until a homogeneous solution

44

was obtained. Analysis of the product mixtures and

determination of yields was accomplished by GLC as

described earlier.

11. VSTT with p-bromophenoxytrimethylsilane.

The reaction mixture was prepared with the silene

precursor, 12a, (0.0275 g), p-bromophenoxytrimethylsilane

(0.199 g), hexadecane (0.0305 g) and cyclohexane (0.300 mL)

as solvent. After ten hours at 300° C the thermolysate was

analyzed by GLC and GC/MS. A very small yield of a product

which had a mass spectrum consistent with the structure of

compound 20 was detected.

12. VSTT with methoxydimethyl(<x-naphthyl)silane.


precursor, 12a (0.0488 g),

methoxydimethyl(a-naphthyl)silane (0.330 g), hexadecane

(0.0320 g) and cyclohexane (0.300 mL) as solvent. The

degassed, sealed tube was held at 300° C for ten hours.

GLC analysis indicated a 37% yield of compound 23b.

13. VSTT with methoxytriphenylsilane.


precursor, 12a (0.200 g), methoxytriphenylsilane (0.754 g),

hexadecane (0.0272 g) and cyclohexane (0.300 mL) as

solvent. The degassed, sealed tube was held at 300° C for

ten hours. GLC analysis indicated a 61% yield of compound

45

25. Proton NMR analysis indicated that the only isomer

present was 25b.

Characterization of New Compounds

23a. 2-methoxy-2-phenyl-3-(dimethyl(«-naphthy1)sily1)

5,5-dimethyl-2-silahexane.

NMR : iH-NMR : 0.31 (s,3H) CH3Si; 0.49 (s,overlapping)

(CH3)2Si; 0.5 (s,overlapping) (CH3)3C; 0.5-1.0

(m,lH) methine proton; 1.5-1.8 (m,2H) methylene

protons; 3.32 (s,3H) CH30; 7.24-7.8 (m,12H) aryl

protons. 13C-NMR : -3.18 (q,CH3SiOCH3); 0.27

(q,(CH3)2Si); 9.18 (d,CH=); 29.33 (q,(CH3)3C); 31.54

(s,C(CH3)3); 37.40 (t,CH2); 50.27 (q,SiOCH3);

124.8-134.3 (complex,arylcarbons).

MS : Both isomers show very similar mass spectral

fragmentation patterns; m/e (relative intensity),

420 (1) P, 405 (100) P-15, 185 (53), 151 (34), 121

(35) .

23b.

NMR : *H-NMR : 0.25 (s,3H) CH3Si; 0.5 (s,overlapping)

(CH3)2Si; 0.5 (s,overlapping) (CH3)3C; 0.5-1.0

(m,lH) methine proton; 1.5-1.8 (m,2H) methylene

protons; 3.24 (s,3H) CH30; 7.24-7.8 (m,12H) aryl

protons. 13C-NMR : -3.20 (q,CH3SiOMe); -0.38

(q,(CH3)2Si); 9.37 (d,CH=); 29.33 (q,(CH3)3C); 31.54

46

( S , C ( C H 3 ) 3 ) ; 37.40 (T,CH 2); 50.27 (q,CH 3OSi);

124.8-134.3 (complex,aryl carbons).

25b. 2-methoxy-2-phenyl-3-(triphenylsilyl) -5,5-dimethyl

2-silahexane.

NMR : iH-NMR : 0.20 (s,3H); 0.45 (s,9H); 1.73 (m,9H); 2.93

(s,9H); 0.1-0.5 (m,lH); 7.25-7.69 (m,20H). i^C-NMR

; -3.18 (q,SiCH3); 7.42 (d,CH=); 29.33 (q,(CH3)3C);

31.87 (s,C(CH3)3); 37.79 (t,CH2); 50.01 (q,CH30);

127-138 (complex,aryl carbons).

MS : Both isomers show very similar mass spectral

fragmentation patterns; m/e 479 (2) P-15, 417 (32),

259 (100), 181 (36), 147 (44), 121 (53), 105 (35).

Anal: Calcd. for C 3 2H 3 8Si 20 : C, 77.67; H, 7.74. Found

for a mixture of 25a and 25b: C, 77.37; H, 7.58.

25a.

NMR iH-NMR : 0.14 (s,3H); 0.40 (s,9H); 1.73 (m,2H); 3.12

(s,3H); 0.1-0.5 (m,lH); 7.25-7.69 (m,20H). i3C-NMR

: -1.49 (q,SiCH3); 8.05 (d,CH=); 9.33 (q,(CH3)3C);

31.74 (s,C(CH3)3); 37.07 (t,CH2); 50.27 (q,OCH3);

127-138 (complex,aryl carbons).

47

TABLE II

THE CONDITIONS OF ANALYTICAL GLC

Conditions

Parameters A B

Column 25m Fused Silica Capillary with SE-54

25m Fused Silica Capillary with SE-54

Initial Temp. 100" c 150° C

Initial Time 2 min. 5 min.

Ramp Rate 5° C/min. 10° C/min.

Final Temp. 250° C 2500 C

Inj. Temp. 250° C 250° C

Det. Temp. 250° C 2500 C

Chart Speed 0.5 cm/min. 0.5 cm/min.

Attenuation -2 -2

Threshold -3 -3

A: The conditions for all compounds.

B: The conditions for compounds 25a,b,

48

TABLE III

RESPONSE FACTORS ON PERKIN ELMER SIGMA-31

Compounds GLC Conditions Response Factors

23a,b. a 0.43

25a,b. b 0.27

^Response factors given in units:

g.(std) area(si) RF= x

g.(si) area(std)

where (si) is the known compound and (std) is the standard, hexadecane.

49

TABLE IV

CROSS REFERENCE OF EXPERIMENT NUMBERS AND NOTEBOOK NUMBERS

Experiment Number Notebook Number

1 II TFB-23,31,41.

2 II TFB-9,11,25.

3 II TFB-27,33,37,43.

4 I TFB-15,35.

5 I TFB-55,87,107.

6 I TFB-61,71,95.

7 II TFB-67,71,85.

8 I TFB-91.

9 II TFB-21.

10 II TFB-83,87.

11 I TFB-17,19,31,33.

12 I TFB-65,103.

13 II TFB-69,73.

50




3. Tate, D.P., Knipple, W.R. and Augl, J.M., Inorg. Chem., 1 (1962) 433-434.

4. Elsheikh, M., Pearson, N.R. and Sommer, L.H., J. Am. Chem. Soc., 101 (1979) 2491-2492.

5. Lee, M.E., Ph.D. Dissertation, North Texas State University, 1984.

6. Fritz, D.F., Sahil, A., Keller, H-P. and Kovats, E., Anal. Chem. 51 (1979) 7-12.



9. Jones, P.R., Bates, T.F., Cowley, A.H. and Arif, A.M., J. Am. Chem. Soc., 108 (1986) 3122-3123.

10. Raabe, G. and Michl, J., Chem. Rev. 85 (1985) 419-509.

11. Wiberg, N., J. Organomet. Chem., 273 (1984) 141-177.

12. Woodward, R.B. and Hoffman, R., " The Conservation of Orbital Symmetry Verlag Chemie: Weinheim/Bergstrasse, 1970.

13. Wiberg, N., Wagner, G., Muller, G. and Riede, J., J. Organomet. Chem., 271 (1984) 381-391.

14. Kofran, W.G. and Baclauski, L.B., J. Org. Chem., 41 (1976) 1879.

15. Cheng, H-B., Ph.D. Dissertation, North Texas State University, 1985.

16. Fedotov, N.S., Kozlikov, V.L. and Mironov, V.F., Zh. Khim., 40 (1970) 2589.

51

17. Oullette, R.J., Pang, J.M. and Williams, S.H., J. Organomet. Chem., 39 (1972) 267-272.

18. Benkeser, R.A., Londesman, H. and Foster, D.J., J. Am. Chem. Soc., 74 (1952) 648-653.

CHAPTER III

THE CONFIGURATIONAL STABILITY OF

ALPHA-LITHIO ALKOXYSILANES

The addition of alkyl lithium reagents, especially

tert butyl lithium, across the carbon-carbon double bond of

vinyl silanes bearing a functional group on silicon has

been studied in some detail by Jones et al. (1,2,3,4,5,6).

Perhaps the most striking observation from these studies is

the pronounced effect of the solvent on the reaction

products. Although addition of tert butyl lithium to the

vinyl group occurs readily at low temperatures in either

hydrocarbon or THF solvents the susceptibility of silicon

to nucleophilic attack by alkyl lithium reagents in THF

causes a complete change in the course of further reactions

as compared with the analogous reaction in hydrocarbon

(2,3,4,5). With a good leaving group on silicon (i.e.

chloride), elimination of the lithium salt occurs in hydro-

carbon to give good yields of silene intermediates (2,4,6).

In contrast, silene formation is not observed in THF (4).

Instead the intermediate a-lithio halo-silane (or silenoid)

undergoes intermolecular coupling reactions (4,5).

If the halogen is replaced by an alkoxy group the

intermolecular coupling reactions in THF are suppressed and

52

53

the silenoid can be trapped efficiently by group 14

trialkylmetallohalides to give good to excellent yields of

the corresponding <*-metallo alkoxysilanes (7). A series of

a-metallo alkoxysilanes, 28, has been prepared by this

method by trapping the «-lithio silane, 18, produced by the

addition of tert butyl lithium to methylphenylmethoxyvinyl

silane.

Ph

4.

OMe

THF

Li

Ph

I Me-Si-

I

OMe Li

18

R-.MC1

OMe I

-Si I Ph

28a, b

MR. MR3 = SiMe 3

SiPh3

SiNapMe2

SnBu 3

SnPh3

In each case two diastereomeric isomers are formed. The

13C NMR chemical shifts combined with x-ray crystal

structures in two cases allow assignment of the absolute

configurations of all of these compounds as well as their

derivatives.

In an attempt to produce a single diastereomer of

compound 18 from the diastereomerically pure

trialkylstannyl derivatives, I have studied the tin-lithium

exchange reaction between compounds 28 (MR3 = SnPh3, SnBu3)

54

and some commercially available alkyl lithium reagents in

both THF and hydrocarbon solvents. These results are also

reported in this chapter.


When tert butyl lithium is allowed to react with a solution

of methylphenylmethoxyvinylsilane in THF at -78° C the

a-lithio silane, 18, is formed. This alkyl lithium can be

efficiently trapped at this temperature by various

trialkylchlorosilanes and trialkylchlorostannanes (7) to

give diastereomeric mixtures of the corresponding

a-metallosilanes, 28. The results of these reactions are

summarized in Table V.

TABLE V

TRAPPING REACTIONS IN THF

Ph MeSi-""^

OMe

t-BuLi,THF Trapping Rgnt.

-78°C,2h. R3MC1,16h. 28a,b

R,MC1 A:B (yield)

Me3SiCl NapMe2SiCl

Ph3SiCl Bu3SnCl Ph3SnCl

67:33 72:28 55:45 69:31 30:70

(92%) (70%) (77%) (70%)7

(43%) 7

55

These reactions were all run under similar conditions and

the yields ranged from good to excellant. m each case two

diastereomeric isomers, A and B, were produced.

0 MR Me 0 MR.

x4 Me \ Ph

H

B

The corresponding isomers in each case can be separated by

either gas chromatography (MR3 = siMe3, SiNapMe2/ SnBu3/

SnPh3) and/or by fractional crystallization (MR3 = siPh3,

SnPh3). The ratio of diastereomers shown in Table V were

determined by gas chromatography except for the MR3 = siPh3

case, where the ratio was determined by proton NMR. The

reaction of alkyl lithium reagents with chlorosilanes is an

SN2-Si reaction (8) and should not perturb the

stereochemistry of the «-lithio silane 18. Therefore it

can be concluded that 18 is not configurationally stable in

THF under these conditions, since the ratio of isomeric

products is dependant on the trapping reagent.

These stereochemical assignments for the isomeric

pairs are shown in Table V and are based on a combination

of i3C NMR and x-ray crystal data. As shown in Table VI,

56

there are distinct chemical shift differences for several

of the carbon atoms in each isomeric pair. However,

interpretation of these spectra and correlation with the

absolute configuration of each isomer requires knowledge of

the conformational preferences of these acyclic compounds.

Fortunately there appears to be a strong preference for

conformations similar to that shown above and in Table VI

with a short, non-bonded oxygen-metal interatomic distance

both in the solid state for MR3 = siPh3 and SnPh3 and in

CDCI3 solution for all of the compounds listed.

The crystal structure of 28b (MR3 = SiPh3) was

reported in chapter II. it shows a nonbonded silicon-

oxygen distance of 2.969 A which is considerably shorter

than the sum of their van der Waals radii of 3.62 A (9) and

a silicon-oxygen dihedral angle of only 13.5°. The crystal

structure of 28b (MR3 = SnPh3) has been reported elsewhere

(7) and it shows a similar conformation with a short tin-

oxygen nonbonded distance and a rather small dihedral angle

of 16.9 0.

The i3C NMR spectra of these two compounds indicate

that the same type of oxygen-metal interaction is also

present in CDC13 solution. This statement is based on the

i3C NMR data shown in Table VI and in particular on the

chemical shift differences for the Si-Me group of each

isomeric pair. In both cases this resonance for the B

isomer is found considerably upfield of the corresponding

57

resonance for the A isomer. This is interpreted as being

caused by shielding of the Si-Me carbon atom in the B

isomer by the bulky neopentyl group and indicates that the

conformations shown in Table VI are preferred in solution

as well as in the solid state for these two isomeric pairs

of compounds.

The other isomeric pairs of compounds in Table VI show

the same trends in chemical shift differences for the Si-Me

resonances in the » c NMR spectra. This indicates that the

same type of conformations are preferred for these other

compounds and allows us to assign their absolute

configurations based on the i3c NMR data shown.

TABLE VI

SELECTED i3C NMR DATA FOR DIASTEREOMERS

0 MR Me 0 MR.

Me \ Ph H

B

R3M

Si-Me

A / B

a-C

A / B

i-Ph

A / B

Me 3Si Ph3Si

NpMe2Si Ph3Sn Bu3Sn

-2 . 92/-3.64 -1.49/-3.18 -3.18/-3.20 -3.12/-5.20 -3 . 84/-4.94

9.24/8.85 8.05/7.42 9.37/9.18 9.96/9.63 5.46/5.07

137.98/138.78

137 . 02/138.09

58

In each case the Si-Me resonance of isomer A is found

downfield of the Si-Me resonance of isomer B and again this

is attributed to the shielding effect of the neopentyl

group in the B isomers. A similar effect can be seen for

the ipso phenyl carbons on the compounds for which these

resonances can be distinguished. There is also a clear

trend for the a-carbon resonance of the B isomers to be

upfaeld of the a-carbon resonance of the corresponding A

isomers. The reason for this trend is not clear however.

The compounds 28B (MR3 = SiMe3, SiNapMe 2) correspond

to the methoxytrimethy1si1ane and methoxydimethylnaphthyl

silane adducts to E-l-methyl-l-phenyl-2-neopentylsilene,

9a, as reported in chapter II. These assignments verify

the conclusion that all alkoxysilanes react with silenes in

a stereospecific and syn manner (11).

The compounds 28A and 28B (MR3 = SiMe 3) have been

converted by reactions of known stereochemistry to the

diastereomeric hydrido-, chloro-, and fluoro-silanes of

known relative stereochemistry by other workers (10). The

absolute configurations of these compounds can now be

assigned based on the absolute configuration of the

starting methoxysilanes as reported here. These

assignments are listed in Table VII.

59

TABLE VII

RELATIVE CONFIGURATIONS OF THE DIASTEREOMERIC ADBUCTS TO E- AND Z-l-METHYL-l-PHENYL-2-NEOPENTYLSILENE

addend registry no. rel. config.

H-SiMe3 94597-08-7 94597-09-8

(R,S)(S,R) (R,R)(S,S)

Cl-SiMe3 94597-10-1 94597-11-2

(R,R)(S,S) (R,S)(S,R)

F-SiMe3 94597-12-3 94597-13-4

(R,R)(S,S) (R/ S) (S,R)

Since I was interested in the stereochemistry and

configurational stability of oc-lithio silanes such as 18

under various conditions, and also in the

diastereoselective synthesis of the corresponding <x-metallo

compounds such as those discussed previously in this

chapter, I decided to investigate the production of these

compounds by some other method besides the tert butyl

lithium addition to vinyl silanes sequence which had been

shown to give mixtures of diastereomers.

THF

"ki 2 diastereomers

C1MR.

Me-Si

' 3

60

The production of alkyl lithium compounds from alkyl-

tin compounds via a tin-lithium exchange reaction using

commercially available alkyl lithium reagents is well

documented (12,13,14,15). It has been reported that these

reactions proceed in both polar and nonpolar solvents and

that the exchange is stereospecific and occurs with

retention of configuration at the carbon center (14). It

has also been reported that these exchanges are reversible

(12,13,15).

An investigation into the tin-lithium exchange

reaction between the diastereomeric compounds, 28 (MR3 =

SnPh3, SnBu3), and alkyl lithium reagents was therefore

undertaken. The a-lithium compounds produced by this

metal-metal exchange reaction were trapped by

trimethylchlorosilane to yield mixtures of 28 (MR3 =

SiMe3).

Me-Si + RLi

R = Ph, Bu

R' = Bu, Ph

Me-Si

Me-Si' I Ph

+ RSnR.

Me -vSiCl

OMe SiMe.

61

In less polar solvents appreciable amounts of the

hydrolyzed a-lithio silane product, 28 (MR3 = H), were

formed as well. The results of these experiments are shown

in Table VIII.

Good yields of the desired products were only obtained

when THF was the solvent. This could mean either that the

equilibrium for the metal exchange lies further to the

right in this solvent, or that the trapping reaction with

trimethylchlorosilane is too slow in the less polar

solvents to give an accurate picture of the true

equilibrium present at the time the trapping reagent is

added.

28 (M = Sn) -|- R-Li ^ ^ 18 -|- R4Sn

fast Me-,SiCl

V

RSiMe•

slow Me -.SiCl

28 (MR3 = SiMe3)

That is, the trap may react faster with the alkyl lithium

reagent than it does with the a-lithio silane, thereby

shifting the equilibrium to the left.

Overall the triphenylstannyl compounds gave better

62

CO

CU CC

£U CJ z

HH < HH X •H U > X

UJ UJ _ J 2 CO 3 < hh H X

H HH J I

X

II

to C£ 2

00 CM

to

CO

II

to C£ z

00 CN

/V

to <D Z •H CO f-H U

rJ

a:

to Ctf C CO

II

to e: 2

00 CN

H3 <D C o\° o\®

/—N •H o\° Cfc Tf LO LO c ® o\° o\° o\® o\° V) II £ 00 +J • • • • O O -M o O O +-> to e to to vO a> rH to u Q* o

a> rH

3 z o T3 *

O 00 CM

O- w to <D

V) Z vO o LO T3 •H 00 4-> • • • • o O o o O o f-H CO to fH to rH vO to <D 1

> LO 00 LO o

X o 4-> • • • • o o -M o O c 1 CN <N 00 LO

*

CQ rH LO to CN o to to to to to to

• • • • 1 • • • • • • • • 1 1 I 1 1 # # LO 00 o vO

< \o vO vO vO r^ VO

/ — \

•H X •H X •H i—3 TT •H i-3 CN

3 V—t NJ 3 V—/ CQ • flQ *

cd 1 a, 1 G c

•

O* O o o o o O o o o o <SJ o e CO lO LO 00 00 00 o 00 00 o LO 00 <D H 1

CN CN i i

r^ i

to 1

CN r-

<P <3> a> <D <u (D <D C u G G G c C a) <D c

7) <D <D <D <D <D <D 0 c P <D c i—1 > a. Xi N N 3 3 3 cd cd 3 cd u. cd f-H s3 -M G G i-H rH f-H X X X X •H o p V a) a; o O o <D <D o 0 H U </) JO •p +-> 4-> US 4-> & 0) -M cd f-H o © LO LO o CN CN CN CN t^ Z cd a> cn LO LO

£ • • • • • • • • 1 | • • • • • • • • • • • •

d0 •H o o LO LO o 00 00 00 00 to G t+H rH rH RF rH CN CN CN CN LO •H 1 %

LO

T"* GU • • f-H

CO < cd O O o o o o o CN CN CN CN ^r 4-> •H a> a> o o> a> Oi Oi r-. to CO •P • • • • • #> • • • • • • • • • • • • • • • • • • CO

•H o o o o o o o 00 00 00 00 vO s rH i—i rH rH rH rH rH CN CN CN CN vO •H

c CO

4L II X pC X X pC X 3 3 3 3 3 oc a- D- 0- a* C- a* a* CQ CQ CQ CQ CQ

oo* CN

•M • O rH (N Pu o rH (N to rr LO vO r^ 00 rH rH rH X z UJ

63

yields than did the tributylstannyl compounds. When n-

butyl lithium was reacted with the triphenyltin

diastereomers in THF, no exchange of the phenyl ligands was

observed. However, in hydrocarbon solvents the exchange

showed little discrimination between the ligands and all

possible exchange products were observed, including the

tributyltin diastereomers.

n-BuLi, hydrocarbon

28 (MR3 = SnPh3) -a >>28 (MR3 = SnBuPh3)

+• 28 (MR3 = SnBu2Ph)

+ 28 (MR3 = SnBu3)

When phenyl lithium was used in place of n-butyl lithium,

no exchange products could be detected in the reaction

mixture. When the tributyltin diastereomers were reacted

with n-butyl lithium in hydrocarbon solvents no products

arising from the a-lithio silane could be detected and

presumably the butyl groups were exchanging in preference

to the organosilane ligand.

64

In hydrocarbon solvents there was considerable

isomerization of the tetra-alkyl tin starting materials in

those experiments where the a-lithio silane is apparently

produced. The ratios of the mixed-alkyl tin diastereomers,

which were formed as side products, were essentially the

same as those of the starting materials however, as might

be expected since no bonds to either chiral center are

broken in the reactions leading to these products.

However, the ratio of isomers found for the trapping

products (MR3 = SiMe3) was consistently ca. 32:68.

The reaction of alkyl lithium reagents with tetra-

alkyl tin compounds is reported to occur with retention of

configuration at the carbon center (14). Furthermore the

reaction between alkyl lithium reagents and chlorosilanes

is an SN2-Si reaction (8) and should not perturb the ratio

of the a-lithio silanes. This implies that the a-lithio

alkoxysilanes are not configurationally stable in THF or

hydrocarbon solvent under these conditions. The fact that

the same ratio of trapping products is found, regardless of

solvent, is surprising and indicates that the ratio of

a-lithio alkoxysilane diastereomers is the same in both

polar and nonpolar solvents at equilibrium. The

predominant force in determining the conformational and

configurational structures of these compounds is probably

intramolecular coordination of the lithium atom by the

65

alkoxy group. Considering this it seems likely that the

compound 18 is isomerizing via an intermediate such as 29.

MeO Li

I —si*

I Ph

+ MeO-Li

I — Si-

I Ph

29

-LiOMe Ph

* H

\ / Sl=C

/ \

Me N-

However in np case were products detected which indicated

that lithium methoxide had eliminated to produce a silene

intermediate.

In general, better yields of the desired products were

obtained when lower temperatures were employed. Above 0° C

substitution at silicon was competitive with metal-metal

exchange even though methoxide is a very poor leaving group

(8). This is further evidence of the sluggishness of these

reactions.

Silicon is known to stabilize an alpha negative charge

and <x-lithio silanes appear to be thermodynamically more

stable than ordinary alkyl lithiums (16). It is therefore

surprising that these reactions are so sluggish and that,

in fact, the other ligands appear to be exchanged in

preference to the organosilane ligands in hydrocarbon

solvents. The reason for this may be that the <x-stannyl

66

alkoxysilanes are stabilized by a tin-oxygen interaction as

described earlier.

MeO—> SnR3

Si

I Ph




either oven dried or flame dried. Solvents were distilled

from lithium aluminum hydride, sodium/potassium alloy or

phosphorus pentoxide. Solutions of phenyl lithium and n-

butyl lithium were obtained from Aldrich Chemical Company

and were standardized by the method of Kofran (17). Yields

were determined by GLC analysis, using hexadecane as the

internal standard, on a Perkin-Elmer Sigma-3 FID gas

chromatograph with a 25 m fused silica capillary column

containing methylphenylsilicone stationary phase, SE-54,

equipped with a Hewlett-Packard 3390A recording integrator.

In determining response factors it was assumed that all

isomers of a particular compound have the same response

factor. GC/MS analysis of the reaction mixtures was

performed using a HP 5970A GC/MS and data system.

67

The synthesis and characterization of compounds 28A

and 28B (MR3 = SnPh3 and SnBu3) have been described

elsewhere (7). The synthesis and characterization of the

compounds 28A and 28B (MR3 = SiMe3, SiPh3 and Si<x-NapMe2)

was described in chapter II.

General Procedure for Transmetallation Reactions

(Experiments 1-12)

The a-stannyl alkoxysilane starting material (0.5 -

1.5 mmol) was dissolved in the indicated solvent and cooled

to the desired temperature by using an appropriate cooling

bath. Hexadecane (0.100 - 0.500 mL) was added as an

internal standard. The alkyl lithium reagent (n-butyl

lithium in hexanes, phenyl lithium in cyclohexane/ethyl

ether) was then added to the stirred reaction mixture.

This mixture was kept at the same temperature and stirred

for two hours before an excess (4-5 fold) of

trimethylchlorosilane (freshly distilled) was added. The

reaction mixture was then allowed to warm slowly to room

temperature while being stirred overnight. The next day

the reaction was hydrolyzed with a saturated ammonium

chloride solution and the organic layer washed twice with

water. The combined aqueous fractions were extracted with

petroleum ether and this combined with the organic

fraction. Removal of the solvent left a clear oil which

was analyzed by capillary GLC and GC/MS. The products were

68

identified by comparison with authentic samples as

described earlier. Isomer ratios and yields were

determined by GLC, the latter by the internal standard

method. The results of these experiments are summarized in

Table VIII.

69

TABLE IX

THE CONDITIONS OF ANALYTICAL GLC

Parameter Conditions

Column 25m Fused Silica Capillary with SE-54

Initial Temp. 1000 c

Initial Time 2 min.

Ramp Rate 5° C/min.

Final Temp. 250° C

Inj. Temp. 2500 c

Det. Temp. 250° C

Chart Speed 0.5 cm/min.

Attenuation -2

Threshold -3

TABLE X

RESPONSE FACTORS ON PERKIN-ELMER SIGMA-31

70

Compounds (28) Response Factors

MR3 = SiMe3 .43

MR 3 = H .45

MR3 = SnPh3 .49

MR3 = SnBu 3 .54

^Response factors given in units:

g.(std) area(si) RF= x

g. (si) area(std)

where (si) is the known compound and (std) is the standard, hexadecane,

TABLE XI

CROSS REFERENCE OF EXPERIMENT

NUMBERS AND NOTEBOOK NUMBERS

71


1 II TFB--121

2 II TFB--123

3 H I TFB--3

4 III TFB--5

5 III TFB--9

6 III TFB--11

7 III TFB--21

8 III TFB--13

9 III TFB--15

10 III TFB--17

11 III TFB--19

12 II AHC--66B

72




3. Jones, P.R., Lim, T.F.O., McBee, M.L. and Pierce, R.A., J. Organomet. Chem., 159 (1978) 99-110.

4. Jones, P.R., Lim, T.F.O. and Pierce, R.A., J. Am. Chem. Soc., 102 (1980) 4970-4973.

5. Jones, P.R., Cheng, A.H-B. and Albenesi, T.E., Organometallics, 3 (1984) 78-82.


7. Cheng, A.H-B., Ph.D. Dissertation, N.T.S.U., 1984.

8. Sommer, L.H.; "Stereochemistry, Mechanism and Silicon", McGraw Hill, Inc., New York, 1965.

9. Bondi, A., J. Chem. Phys., 68 (1964) 441-451.





14. Seyferth, D. and Lawrence, G.V., J. Organomet. Chem., 1

(1963) 201-204.

15. Peterson, D.J., J. Am. Chem. Soc., 93 (1971) 4027-4031.

16. Paquette, L.A., Science, 217 (1982) 793-800. 17. Kofran, W.G. and Baclauski, L.B., J. Org. Chem., 41

(1976) 1879.

CHAPTER IV

STEREOCHEMISTRY AND MECHANISM OF THE

ADDITION OF ALCOHOLS TO SILENES

The reaction of silenes with alcohols, to yield

alkoxysilanes, is probably the most widely cited evidence

for the existence of transient silenes (1). Although

alcohols have been shown to be highly efficient and

regiospecific silene traps (1), the mechanism of this

reaction has remained unknown. Apparently non-

stereospecific addition of methanol to certain silenes has

been observed (2), and some discussion of a two step

mechanism has appeared (3).

In chapter II it was reported that the addition of

alkoxysilanes to silenes is a stereospecific syn addition

and a concerted, nonsynchronous mechanism was proposed.

^ / -L-c-Si=C . , /r — > l I. 0 - r c ^ 0 0—rSiR 3 Y iR-

R

73

74

It seems probable that the addition of the polar sigma bond

of alcohols follows the same mechanism as does addition of

the polar sigma bond of alkoxysilanes to silenes. It was

noted in Chapter II that the fleeting existence of a

zwitterionic intermediate such as 1 could not be ruled out.

-Si — C-

\ "

A+. R SlR-

This mechanism would require the silyl group to migrate

much faster than the rate of bond rotation about the Si-C

bond in order to account for the stereospecific nature of

the reaction. The possibility remained that the analogous

reaction of an alcohol with a silene is nonstereospecific.

\ / I I i . i Si=C — S i - = V C — — S i C —

/ ^ i I 0-H ° H

/ R R H

75

In the studies involving methoxytrimethylsilane as a

silene trap, the stereochemistries of the diastereomers

which correspond to the methanol adducts of E- and

Z-l-phenyl-l-methyl-2-neopentyl-2-trimethylsilylsilene, 3,

were determined (4).

Si=C

MeOSiMe3 Ph Np MeOH Me

\ / Si=C

/ \ .

MeO SiMe3 Ph SiMe3

Me-Si C-H < Si=C

\

The synthesis of the silenes 3 was therefore undertaken and

is described in this chapter. The stereochemistry and

mechanism of the reaction of methanol with the E- and Z-

isomers of 3 is then reported.

Finally a novel silene isomerization reaction

involving the equivalent of a [1,3] hydride shift to

silicon will be described. This rearrangement is apparently

catalyzed by tert butyl lithium and a preliminary mechanism

will be proposed.

76


The necessary precursor to the silene 3, based on the

methodology used to produce silene 4 (5), is

a-trimethylsilylvinylmethylphenylchlorosilane, 6.

Ph

Me-Si-"*^ 4-

I Cl

Li

Me

\ / Si=C

/ \ Ph H

Me

Ph SiMe,

- 1 ^ f

Cl

•Li

Me

\ / Si=C

/ \ . Ph SiMe3

The silane 6 was obtained in 73% isolated yield by reacting

me thy Ipheny ldi chlorosilane with <x-lithiovinyl-

trimethylsilane. The latter compound was synthesized by

the method of Ong et al. from oc-bromovinyltrimethylsilane

( 6 ) .

Br2/CCl,

Cl3Si""'<:^ hV * Cl^SiCH-CH

Br Br

I I 3„^CH-CH2

quinoline Br

distill Cl3Si

MeMgBr

ether

Br

Me3Si

t-BuLi, ether

Ph(Me)SiCl2 Me

Ph SiMe.

- i i - V

(inverse addn.) Cl

77

In order to determine the stereochemistry of methanol

addition to silene 3, a stereospecific source of a single

isomer of silene 3 was needed. Previous work has

demonstrated the applicability of a thermal retro- Diels-

Alder reaction for the stereospecific generation of silenes

(4,7). The synthesis of the anthracene adduct of silene 3

was therefore undertaken.

Treatment of a benzene solution of the silane 6 and

excess anthracene with tert butyl lithium at room

temperature, followed by hydrolytic work up, gave a 77:23

mixture of 7a and 7b; isolated in 43% yield after column

chromatography (8).

Me

Ph SiMe.

-L-V Si-Me

CI

Si-Ph t-BuLi

O Q SiMe

o o 7a : 7b

(77:23)

Fractional crystallization of this mixture from hexane gave

pure 7a as colorless needles, leaving a solution containing

a 69:31 mixture of 7a:7b. The stereochemistry of the

isomers was not apparent from their NMR spectra and so an

x-ray crystal structure of 7a was obtained (9) and is shown

on the next page.

78

Figure 2. Molecular Structure of (Z)-2-methyl-2-phenyl-3-

trimethylsilyl-3-neopentyl [5,6:7,8] dibenzo-2-silabicyclo

[2.2.2] octane.

79

The anthracene adducts undergo a facile stereospecific

decomposition under vacuum-sealed tube conditions at

temperatures as low as 190° to produce E- or Z-3, the

mildest stereospecific route to a silene yet reported.

When pure 7a was subjected to sealed tube thermolysis at

205° for one hour in the presence of a nine fold excess of

methanol, the amount of anthracene recovered indicated that

a 69% decomposition of 7a had occured. The only other

product detected was the [R,R][S,S] methanol adduct, 5a,

produced in >98% yield based on percent decomposition.

Si—Ph

o o

MeOH

VSTT 2050

69%

Me 0 SiMe-

Ph Me H

(98%)

5a

A control experiment, identical in every respect

except that a 66 fold excess of methanol was used, gave

essentially the same percent decomposition of 7a indicating

that 5a does not arise through a bimolecular reaction of

methanol with 7a. Similar experiments were carried out

using the 69:31 mixture of 7a:7b over a temperature range

of 190° to 250°. The yields of anthracene ranged from 15

to 98%, but in all cases a 69:31 mixture of the

80

diastereomeric adducts 5a and 5b was obtained in near

quantitative yield. A similar experiment run at 160° for

one hour produced only unchanged starting materials. All

of these results are summarized in Table XII.

TABLE.XII

VSTT EXPERIMENTS WITH METHANOL

EX. NO. 7a: 7b MeOH/7 Temp Time Anth 5i 5a: 5b

1 100:0 55 300" . 2h 100% >98% 100:0

2 100:0 16 300° 2h 100% >98% 100:0

3 69:31 8 250 0 2h 98% >98% 69:31

4 69:31 11 210 0 2h 95% >98% 69:31

5 69:31 24 200« lh 60% >98% 69:31

6 69:31 19 190" lh 15% >98% 69:31

7 69:31 19 1600 lh 0% 0% -

8 100:0 66 2050 lh 69% >98% 100:0

9 100:0 9 2050 lh 69% >98% 100:0

1Yields are based on percent decomposition of 7

81

The relative stereochemistries of the diastereomers was

determined by coinjection with authentic samples (4) using

capillary GLC techniques which resolve the diastereomers.

SiMe

SiMe-

MeOH Np-• •H

A Ph—Si-Me

I OMe

SiMe

Np-

Me—Si-Ph

I OMe

7a : 7b - _5a_

1 0 0 : 0 - - - - - - - - 1 0 0

69 : 31 69

5b

0

31

Np = neopentyl

These results show that both E- and

Z-l-methyl-l-phenyl-2-neopentyl-2-trimethylsilylsilene add

methanol stereospecifically syn (8). These results,

combined with the analogous results with alkoxysilanes

reported earlier, suggest a common reaction mechanism for

the addition of these polar sigma bonds to the silicon-

carbon double bond. The initial interaction is undoubtedly

nucleophilic attack at the sp2 hybridized silicon by the

lone pair electrons on oxygen. Wiberg has recently

reported the isolation and characterization of several

82

electron donor-silene complexes. In each case, the electron

donor is coordinated to the silicon atom. As mentioned in

chapter I, Wiberg has also measured the relative rates of

addition for a series of alcohols to

l,l-dimethyl-2,2-bis(trimethylsilyl)silene (11). These

results indicate that the nucleophilicity of the alcohol as

well as their steric bulk are'major factors affecting their

rates of reactivity towards silenes. These results support

the idea of nucleophilic attack by the oxygen lone pair of

electrons as the initial interaction. The highly

stereospecific nature of these reactions can best be

explained by subsequent collapse to a transition state

involving six electrons in a linearly conjugated array of

five orbitals.

\ . / S i = C /««

0 — H /*J

R

or

0 / S i — c

It should be noted that the polar sigma orbital of the

nucleophilic reagent and the lone pair orbital are mutually

orthogonal and this transition state is not a pericyclic

one. The relevant orbitals and a scheme of their proposed

interactions are shown on the next page.

When a hexane solution of the silene precursor 6 was

83

IT*

0 ®

<

0 - R

Figure 3. Scheme of Orbital Interactions for the Addition

of Alcohols to Silenes.

84

treated with tert butyl lithium in the absence of any

trapping agents, surprisingly only a very low yield of the

expected 1,3-disilacyclobutanes, 8, could be detected in

the reaction mixture after hydrolysis. Instead, the

apparent product of a [1,3] hydride shift of the silene 3,

2-phenyl-3-trimethylsilyl-5,5-dimethyl-2-silahex-3-ene, 9,

was obtained in 84% yield. Only traces of this product

were detected when anthracene was present in the reaction

mixture indicating that the isomerization reaction is much

slower than the [4 2] cycloaddition reaction between silene

3 and anthracene. The ratio of isomers obtained was ca.

90:10. Also obtained in 7% yield was 2-phenyl-2 -tert

butyl-3-trimethylsilyl-5,5-dimethyl-2-silahex-3-ene, 10.

Me

Ph SiMe.

-Ji-V CI

t-BuLi Ph SiMe.

1.2 eq,-78° Me-Si

hexane H

9 (84%)

Ph I

Me-Si

SiMe-

10 (7%)

Me

I

Ph—Si-

Np -

Np

-SiMe-

Si—Ph

SiMe3 Me

8 (5%)

85

This product may have arisen from the addition of tert

butyl lithium to the silene 3 followed by elimination of

lithium hydride or by the reaction of tert butyl lithium

with 9. It could also have arisen from displacement of

chloride ion from the starting chlorosilane 6 followed by

an addition-elimination sequence with tert butyl lithium.

There is an example in the litererature of such an

addition-elimination sequence involving vinylsilanes and

tert butyl lithium (12). Compound 9 was identified by

GC/MS, elemental analysis and NMR data. It also shows a

strong Si-H stretching band in the IR at 2120 cm-*.

The silene 3 is very sterically hindered, and it might

be expected that dimerization would occur only slowly, if

at all. This has been shown to be the case for other

silenes which contain very bulky ligands (13,14). However,

the concentration of the silene 3 in solution, even in the

absence of any trapping agents, remains very low. When the

reaction is followed by silicon-29 NMR the peaks

corresponding to the starting silane slowly grow smaller

with the concommitent appearance of peaks assigned to the

product 9. However, no peaks which could be assigned to

the silene 3 are observed (15).

The mechanism of this rearrangement has not been

entirely resolved. Perhaps the most obvious mechanism to

draw is that of a concerted [1,3] hydride shift. In fact,

Barton has recently suggested such a mechanism to account

86

in part for the formation of vinylsilane 12, from the

photolysis of a-diazosilane 11 (16). Evidence will

N2

He -.SiCH Me2Si (3%) I

hv H

11 12

be presented which virtually rules out this mechanism for

our system, however.

The silene precursor 7 offered a different approach to

study this silene isomerization reaction. When the

compound 7 (69:31 mixture) was heated in an evacuated

sealed tube to 200° for one hour in the presence of

approximately one equivalent of tert butyl lithium, a 47%

decomposition of 7 occured. The silene isomerization

product 9 was formed in 20% yield along with smaller

amounts of several unidentified products. Another tube

subjected to the same conditions in the absence of tert

butyl lithium produced a trace of anthracene as the only

detectable product. When the anthracene adduct was heated

at 300° for forty-four hours in the absence of any trapping

agents the amount of anthracene recovered indicated that a

48% decomposition of 7 had occurred. A 6% yield of the

isomerization product 9 was produced along with a 14% yield

of silene dimers.

87

We have already shown that in solution at room

temperature, in the presence of tert butyl lithium the

isomerization is much faster than dimerization. This VSTT

experiment shows that in the gas phase at higher

temperatures, in the absence of tert butyl lithium, dimer

formation is faster than isomerization of the silene to

compound 9. However, isomerization to compound 9 occurs in

good yield under VSTT conditions in the presence of tert

butyl lithium.

Another possible mechanism for this unusual

isomerization reaction involves abstraction of an allylic

proton from the silene 3 by tert butyl lithium to form

anion 13.

Ph SiMe.

\ . / Sl=C

Me /

+

SiMe

+

Li H

88

Protonation of this anion by another silene molecule would

regenerate the anion and propagate the reaction. Therefore

only a catalytic amount of tert butyl lithium would be

necessary.

\ SiMe-

/ ,si/ —

Ph

13

Ph SiMe. SiMe3

Si=C > Me-Si + 13

Efforts to trap the proposed silallylic anion, 13, by

quenching the reaction mixture with D20 were unsuccessful.

These results are summarized in Table XIII. However, it is

entirely possible that the concentration of the

intermediate 13 remains very low due to its expected high

reactivity. This would account for the inability to trap

it in any detectable amounts.

While it may be that a [1,3] hydride shift accounts

for the 6% yield of 9 (12% based on anthracene) in the VSTT

experiment run for forty-four hours at 300°, it does not

appear to be a major source of 9 in the other experiments

reported here. The results presented here are consistent

with the anionic, tert butyl lithium catalyzed, mechanism

89

CO H Q-2 U3

£ <

X, -H CL-Cfl-X

CD

3 CQ I

0) M

O X LO •—I CM O

• + u J 5)13 s e x

Cd x o x O <U /-N t- JZ CN

to <u

CO

X «H »-» O u - W U

CD 2

00 rr to rH rH LO +-> 1 o\°

CO o LO o ^r CN rH T3 rH rH rH <D •H > O to LO to o to

r-* CN 00

£ CM i O o i LO o rH vO 00 c&

V)

0> • vO

T3 to PU rH £ to LO o o CN 00 vO vO to O II • • • • • • • • •

u to LO to to i—4

<D \ rH rH rH rH rH rH rH rH 1—4 <D \

o £

2 vO

<4H o i—1 LO LO TH rH 00 O Tf LO

+-> II • • • • • • • • •

e LO r* 00 00 a> o 00

<D <U 1-H rH rH rH rH rH rH rH rH u

<U £

0) cu w

o CN

cd CN a; t in a* o LO O LO a II • • • • • • • • • a II

Tf LO rf LO LO rr LO rH <D CN CN CM CM CN CN CN CM CN + v,^ 2 £

u o w

o o C O o o o o o o o CN CM CM CN <N CN I CN CN 1 CN CN CM X X X X a Q a X Q a a

<u tn M •H o Qi

* CO

0) rH *c PC PC x £ £ JC £ £ ,£ o

CO 00 00 00 O LO CN o LO

<D -o i-H i-H rH rH LO LO CN to rH

£ >s

•H

• •H > J o CM O o o o rH o O •H 3 • • • • • • • • •

3 00 1—4 rH CM rH CN rH rH rH

O- 1 . 0) +-M

• •

X o LO vO 00 a> o rH CN

tu z rH rH rH r-H rH rH CM CN CN

90

as the major source of 9 in these other reactions. If this

mechanism is correct it is the first reported instance of a

silallylic anion.




either flame dried or oven dried. Solvents were distilled

from lithium aluminum hydride, sodium/potassium alloy or

phoshorus pentoxide immediately prior to use. Solutions of

tert butyl lithium were obtained from Aldrich Chemical Co.

and were standardized by the method of Kofran (17). Unless

otherwise noted, yields were determined by the internal

standard method using a Perkin-Elmer Sigma-3 FID gas

chromatograph with a 25 m or 3 m fused silica capillary

column containing SE-54 stationary phase and equipped with

a Hewlett-Packard 3390A recording integrator. In

determining response factors it was assumed that all

isomers of a particular compound have the same response

factor.

GC/MS analysis of some of the reaction mixtures was

performed using an HP 5970A GC/MS and data system. NMR

spectra were determined on a JEOL FX-90Q 90 MHz or VXR-300

300MHz spectrometer with CDCl3 or D20 a-S the lock solvents.

Chemical shifts are reported in parts per million downfield

91

from tetramethylsilane. Elemental analyses were performed

by Galbraith Laboratories, Knoxville, TN.

1. (x-bromovinyltrichlorosilane.

To a solution of 80.8 grams (0.500 mmoles) of

trichlorovinylsilane in 400 mL of dry carbon tetrachloride

in a 1 L round bottom pyrex flask irradiated with a sunlamp

was added dropwise 79.9 grams (0.500 mmoles) of bromine in

100 mL of additional carbon tetrachloride over about two

hours. The color of the bromine was consumed by the

vinylsilane almost immediately and occasional cooling with

an ice/water bath was necessary. The carbon tetrachloride

was removed under vacuum and the residue transferred under

an inert atmosphere to a 200'mL round bottom flask and 84.0

grams (0.650 moles) of freshly distilled quinoline was

added very slowly with cooling. The impure product was

then distilled from the heterogeneous reaction mixture at

approximately 50 torr, boiling range 65-75°C. A fractional

distillation at atmospheric pressure yielded 90.0 grams

(75%) of the title compound, BP 147-149°C, (lit. 70-71°C/53

torr (6)).

2. «-bromvinyltrimethylsilane.

To 216 ml of a 2.9 M solution (627 mmoles) of methyl

magnesium bromide (Alfa) in 500 mL of dry ethyl ether was

added dropwise so as to maintain a gentle reflux, 45.6

grams of a-bromovinyltrichlorosilane in 100 mL of

92

additional ether. The mixture was then refluxed for an

additional five hours. After removal of the solvent, the

yellow residue was vacuum distilled at approximately 25

torr to yield 27.2 grams (70%) of the title compound, BP

27-28°C (lit. BP 56.7°C/67 torr(6)).

3^ (x-trimethylsilylvinylmethylphenylchlorosilane. (6 ) .

To a stirred solution of 21.8 grams (122 mmoles) of

a-bromovinyltrimethylsilane in 200 mL of dry ethyl ether at

-78°C was added dropwise 79.0 mL of a 1.7 M solution of

tert butyl lithium in pentane (134 mmoles). After stirring

at -78° for two hours the mixture was slowly added, via an

outlet at the bottom of the flask, to 35.0 grams (183

mmoles) of methylphenyldichlorosilane in 200 mL of

additional ether which had been previoulsy cooled to -78°.

The entire addition took about 40 minutes. After stirring

for an additional hour at -78° the mixture was allowed to

warm to room temperature while stirring overnight.

The salt was allowed to settle and the liquid portion

decanted under an inert atmosphere to a dry flask. The

salt was washed with hexane and this washing combined with

the other liquid. The solvent was removed under vacuum and

the residue carefully vacuum distilled through a column

packed with glass beads and wrapped with heating tape, at

0.4 torr to yield 22.67 grams (73%) of the title compound,

BP 82-84°C.

93

4. 2 - m e t h y l - 2 - p h e n y l - 3 - n e o p e n t v l - 3 - t r i m e t h y l s i l y l [ 5 , 6 : 7 , 8 ]

dibenzo-2-silabicyclo [2.2.2] octane. (7a,b).

To a mixture of 1.28 grams (5.00 mmoles) of

(X-trimethylsilylvinylmethylphenylchlorosilane, 6, and 1.78

grams (10.0 mmoles) of anthracene in 125 mL of dry benzene

was added 4.41 mL (7.50 mmoles) of a 1.7 M solution of tert

butyl lithium in pentane. After stirring for 16 hours the

brown reaction mixture was hydrolyzed with 25 mL of a

saturated ammonium chloride solution. The organic layer was

separated and washed twice with distilled water and

combined with a petroleum ether extract of the aqueous

layers. After drying the combined organic fractions over

magnesium sulfate, the solvents were removed under vacuum

to yield a light colored solid. The solid was extracted

three times with cool petroleum ether. This left 1.14

grams (6.4 mmoles) of recovered anthracene. The solvent

was removed from the extracts to yield a sticky, yellow,

solid material. This was taken up in a small amount of hot

benzene and chromatographed on silica gel and eluted with

hexane to give 0.967 grams (43%) of the title compound as a

77:23 mixture of diastereomers (7a:7b). This ratio was

determined by *H NMR and also by GLC, using a 3-meter

capillary column. This white solid was dissolved in hot

hexane and placed in a freezer for two hours. The

resultant solid was filtered and redissolved in hot hexane.

After crystals began to form the vial was stoppered

94

overnight. The next day clear, needle-like crystals were

collected and determined to be 100% 7a by NMR. The

mother liquor contained a 69:31 mixture of 7a to 7b. All

attempts to further isolate 7b were unsuccessful.

General Procedure for Vacuum Sealed Tube Thermolysis (VSTT)

Experiments.

All of the thermolyses were carried out in sealed 6.3

mm (OD) x 120 mm Pyrex tubes (thick wall, 1.2 mm). The

reaction mixture of the silene precursor, the trapping

reagent and eicosane as the internal standard dissolved in

dry cyclohexane was transferred to the thermolysis tube

through a capillary tube. After degassing under vacuum the

thermolysis tube was placed in a vertical pyrolysis oven

that was preheated to the temperature indicated. After the

indicated time the tube was removed, allowed to cool and

opened. If any solids compounds were present, sufficient

benzene was added until a homogeneous solution was

obtained. Analysis of the product mixtures and

determination of yields was accomplished by GLC, as

described earlier.

5-13. VSTT of 7 with Methanol

In a typical experiment the silene precursor, 7a,

(0.0112 grams), methanol (0.0066 grams), eicosane (0.0100

grams) and .300 mL of cyclohexane were heated at 205° for

one hour. Anthracene and the methanol adduct of the silene

95

2 — 3, 5a./ were the only products detected. The yields and

the results of other similar experiments are listed in

Table XII.

14-22. 2-phenyl-3-trimethylsilyl-5,5-dimethyl-2-silahex-

3-ene. (9). General Procedure.

To a. solution of 1.28 grams (5.0 mmoles) of

a-trimethylsilylvinylmethylphenylchlorosilane, 6, and

0.1792 grams of eicosane in 100 mL of dry hexane was added

slowly with stirring 3.0 mL (5.1 mmoles) of tert butyl

lithium in pentane. After about twenty minutes a.n intense

yellow color began to develop. The solution then became

cloudy but remained bright yellow after 16 hours. The

color disappeared when the reaction was hydrolyzed with a

saturated ammonium chloride solution. The clear organic

layer was washed twice with distilled water and the solvent

removed under vacuum before analysis by GLC and GC/MS. See

Table XIII for the yields and for other experiments run

under different conditions. The title compound and

2-phenyl-2-tertbutyl-3-trimethylsilyl-5,5-dimethyl-2-sila-

hex-3-ene, 10, were isolated by preparative gas

chromatography for characterization purposes. One isomer

of the dimer, 8, was isolated by crystallization from the

reaction mixture over about 2 months.

23. VSTT of 7 with No Trap.


96

precursors, 7a and 7b, (0.0083 grams, 69:31 mixture)

eicosane (0.0173 grams) and .300 mL of cyclohexane. After

forty-four hours at 300° the thermolysate was analyzed by

GLC and GC/MS. GLC indicated a 6% yield of compound 9 and

a 34% yield of the silene dimers along with several other

unidentified compounds in very low yield and a 48% yield of

anthracene.

24. VSTT of 7 with Tert-Butyl Lithium


precursors, 7a and 7b, (0.0089 grams, 0.020 mmoles, 69:31

mixture) and tert butyl lithium 0.011 mL (0.020 mmoles) in

pentane along with .300 ml of cyclohexane. A control tube

which contained the same reagents except for the tert butyl

lithium was run at the same time. After one hour at 200°

the thermolysates were analyzed. A 47% yield of anthracene

was obtained in the first tube along with a 40% yield of

compound 9 (based on percent decomposition of 7) and

several unidentified products in smaller amounts. The

control tube showed a 4% yield of anthracene as the only

detectable product.

97

TABLE XIV

THE CONDITIONS OF THE ANALYTICAL GLC

Parameter Conditions A Conditions B

Column 25 m 3 m

Initial Temp. 60<» c 60° C

Initial Time 2 min. 2 min.

Ramp Rate 5° C/min. 5° C/min.

Final Temp. 250° C 2500 c

Inj. Temp. 250° C 2500 c

Det. Temp. 2500 C 2500 c

Chart Speed 0 .5 cm/min. 0.5 cm/min.

Attenuation -2 -2

Threshold -3 -3

98

TABLE XV

RESPONSE FACTORS ON PERKIN ELMER SIGMA-3i

Compound GLC Conditions Response Factor

5a,b A or B .64

6 A or B .54

8 B .56

9 A or B .60

10 A or B .66

Anthracene A or B .79

iResponse factors given in units:

g.(std) area(si) RF = x

g. (si) area(std)

where (si) is the known compound and (std) is the standard, eicosane.

99

TABLE XVI

CROSS REFERENCE OF EXPERIMENT NUMBERS AND NOTEBOOK NUMBERS


1 3TFB-51 2 3TFB-53 3 4TFB-49 4 3TFB-81 5 3TFB-87a 6 3TFB-87b 7 3TFB-91a 8 3TFB-91b 9 3TFB-91C 10 3TFB-91d 11 3TFB-91e 12 3TFB-93a 13 3TFB-93b 14 3TFB-59 15 3TFB-69 16 4TFB-53 17 4TFB-55 18 4TFB-57 19 4TFB-59 20 4TFB-61 21 4TFB-63 22 4TFB-65 23 4TFB-43 24 4TFB-45

100

Characterization of New Compounds

6. «-trimethvlsilylvinylmethylphenylchlorosilane

NMR: iH-NMR : -0.35 (s, 9H) (CH3)Si; 0.27 (s, 3H) CH3Si;

6.06 (s, 2H) vinyl protons; 6.78-6.85 (m, 3H),

7.09-7.20 (m, 2H) aryl protons. i3C-NMR : -0.58 (q,

(CH3)3Si); 1.37 (q, CH3Si); 127.53 (d), 129.74 (d),

133.38 (d), 134.94 (s), aryl carbons; 144.18 (t,

=CH2); 149.51 (s, Si2C=).

MS: m/e (relative intensity), 254 (1) P, 239 (73) P-15,

155 (74), 145 (95), 121 (95), 73 (100).

Anal: Calcd. for C12H19Si2Cl : C, 56.54; H, 7.51. Found .

C, 56.35; H, 7.76.

7a. (Z)-2-methyl-2-phenyl-3-neopentyl-3-trimethylsilyl

[5,6:7,8] dibenzo-2-silabicyclo [2.2.2] octane

NMR: iH-NMR : -0.18 (s, 3H); 0.02 (s, 9H); 1.19 (s, 9H);

1.56 (m, 2H); 4.19 (s, 1H); 4.93 (s, 1H); 7.00-7.65

(m, 13H). i3C-NMR : -0.14 (q, SiCH3); 2.6-3.0

(broad, Si(CH3)3); 23.87 (s, C-Si2); 33.13 (q,

(CH3)3C); 34.84 (s, C(CH3)3); 40.41 (d, bridgehead

C); 47.69 (t, CH2); 52.06 (d, bridgehead C); 124-143

(complex, aryl carbons).

MS: m/e (relative intensity), 454 (2) P, 439 (2) P-15,

355 (12), 277 (11), 205 (25), 197 (36), 193 (26),

135 (100), 73 (53).

Anal: Calcd. for C 3 0H 3 8Si 2 : C, 79.23; H, 8.42. Found :

C, 78.94; H, 8.55.

101

7b.

NMR: 13C-NMR : 0.29 (q, CH3Si); 2.10 (q, (CH3)3Si); 24.61

(s, C-Si2); 32.69 (q, (CH3)3C); 34.36 (s, C(CH3)3);

41.31 (d, bridgehead C); 49.89 (t, CH2); 52.42 (d,

bridgehead C); 124-143 (complex, aryl carbons).

8. 1,3-dimethyl-l,3-diphenyl-2,4-dineopentyl-2,4-di-

trimethylsilyl-1,3-disilacyclobutane

Due to low yield, only one isomer could be isolated.

It is the cis-neopentyl, trans-methyl isomer. At

least three other isomers could be detected by

GC/MS.

NMR: iH-NMR : 0.20 (s, 18H); 0.74 (s, 18H); 0.77 (s, 3H);

0.96 (s, 3H); 1.98 (broadened doublet, J=16 Hz, 2H);

2.27 (d, J=16 Hz, 2H); 7.3-7.8 (complex, 20H). i3C-

NMR : 4.50 (q, SiCH3); 5.14 (q, SiCH3); 6.51 (broad,

Si(CH3)3); 13.20 (s, C-Si2); 31.94 (q, (CH3)3C);

31.80 (s, C(CH3)3); 46.24 (t, CH2); 127.14 (d),

129.01 (d), 129.10 (d), 135.33 (d), 136.76 (d),

138.7 (s), 141.2 (s), aryl carbons.

Anal: Calcd. for C 3 2H 5 4Si 4 : C, 69.48; H, 10.20. Found :

C, 68.77; H, 10.12.

9_. 2-phenyl-3-trimethylsilyl-5,5-dimethyl-2-silahex-3-ene

NMR: iH-NMR : (major isomer) 0.00 (s, 9H); 0.50 (d, 3H)

1.02 (s, 9H); 4.96 (q, 1H); 6.9 (s, 1H); 7.2-7.5 (m,

5H). (minor isomer) 0.11 (s); 0.37 (d); 1.10 (s);

4.65 (q); 6.92 (s); 7.2-7.5 (m). 13C-NMR : (major

102

isomer) -3.44 (q, SiCH3); 0.14 (q, Si(CH3)3); 30.44

(q, (CH3)3C); 36.88 ( s, C(CH3)3); 127B.27 (d),

128.44 (d), aryl carbons; 132.15 (s, Si2C=); 134.10

(d, aryl C); 136.76 (s, ipso C); 169.93 (d, =CH).

MS: m/e (relative intensity), 261 (2) P-15, 219 (49) P-

C(CH3)3, 145 (43), 135 (100), 121 (38), 73 (95).

Anal. Calcd. for C 1 6H 2 8Si 2 : C, 69.48; H 10.20. Found :

C, 69.61; H, 10.22.

10. 2-phenyl-2-tertbutyl-3-trimethylsilyl-5,5-dimethyl-

2-silahex-3-ene

Samples of 10 decompose in a few days, even when

sealed under argon. A satisfactory elemental

analysis could not be obtained.

NMR: iH-NMR : -0.31 (broad, 9H); 0.09 (s, 3H); 0.39 (q,

3H); 0.68 (s, 9H); 0.88 (s, 9H); 6.86-7.15 (m, 5H);.

13C-NMR : -4.35 (q, SiCH3); 4.17 (q, Si(CH3)3);

18.73 (s); 27.84 (q); 30.63 (q); 37.14 (s); 127.01

(d); 128.14 (d); 131.95 (s, Si2C=); 134.42 (d);

138.58 (s); 170.19 (d, =CH).

MS: m/e (relative intensity), 275 (22) P-C(CH3)3, 201

(36), 135 (100), 73 (41).

103


1. Raabe, G. and Michl, J., Chemical Reviews, 85 (1985)

419-509.

2. Brook, A.G., Safa, K.D., Lickiss, P.D. and Baines, K.M., J. Am. Chem. Soc., 107 (1985) 4338-4339.




6. Chan, T.H., Mychajlowskij, W., Ong, B.S. and Harpp, D.N., J. Org. Chem., 43 (1978) 1526-1532.


8. Jones, P.R. and Bates, T.F., J. Am. Chem. Soc., 109 (1987) 913-914.

9. This structure was solved courtesy of Dr. Alan H. Cowley and co-workers at UT-Austin. These results will be published at a later date.

10. Wiberg, N., Wagner, G., Reber, G., Riede, J. and Muller, G., Organometallics, 6 (1987) 35-41.



13. Brook, A.G., Ryburg, S.C., Abdesaken, F., Gutekunst, B., Gutekunst, G., Kalbury, R.K.M.R., Poon, Y.C., Chang, Y-M. and Wong-Ng, W., J. Am. Chem. Soc., 104 (1982) 5667-5672.

14. Wiberg, N. and Wagner, G., Angew. Chem., Int. Ed. Engl., 22 (1983) 1005.

15. silicon-29 spectra of several silenes have been reported. For a compilation of data see: Brook, A.G. and Baines, K.M., Advances in Organometallic Chemistry, Stone, F.D.A. and West, R., eds.; Academic Press, New York, Vol. 25 (1985) pp 1-38.

104

16. Yeh, M-H., Linder, L., Hoffman, D.K. and Barton, T.J., J. Am. Chem. Soc., 108 (1986) 7849-7851.

17. Kofran, W.G. and Baclauski, L.B., J. Org. Chem., 41 (1976) 1879.

37?/67531/metadc332323/...2-silahex-3-ene. these silenes were also trapped as their [4+2]...

Documents