3 Phosphine Oxides and Related Compound

BY B. J. WALKER

1 Introduction

In view of the intense interest in low-coordinate phosphorus compounds it is not surprising that the first examples of three co- ordinate oxides and sulphides have been reported. Interest in phosphine oxide-based olefin synthesis continues and it is clear that the three main phosphorus-based methods are largely complementary.

2 Preparation of Acyclic Phosphine Oxides

Using the now well-established principle of steric protection a primary phosphine oxide (1) and phosphine sulphide ( 2 ) have been prepared from the corresponding phosphine and characterized for the first time Either 1-cyclobutenyldiphenylphosphine oxide ( 3 ) 01

2-alkoxycyclobutyldiphenylphosphine oxide ( 4 ) can be obtained bv hydrolysis of (1-cvclobuteny1)triphenylphosphonium salts depending on the conditions used. The phosphine oxide ( 4 ) undergoes thermo- lysis to give 2-(diphenylphosphinyl)-lY3-butadiene ( 5 ) which can be trapped with dienophiles to give 1-cyclohexenylphosphine oxides. Protected B-(diphenylphosphinoy1)ketones ( 6 ) provide a route to protected 0,y-unsaturated ketones y& olefination of aldehydes and ketones. Synthetic routes to the parent ketophosphonates ( 7 1 ,

possessing a wide variety of substitution patterns, are available from reactions of phosphorus nucleophiles with enones by addition of phosphorus-stabilized carbanions to a-carbonyl cation equivalents and by oxidation of allyldiphenylphosphine oxides. A variety of (2-substituted-aryllphosphine oxides have been prepared by phosphonylation of 2-lithiated substituted benzenes.

have been prepared by reactions of methyl diphenylphosphinite with acid chlorides. On treatment with water, or with diphenylphosphine oxide, the oxides ( 8 ) give the phosphinates ( 9 ) . The acyl- diphosphine dioxides (10) are available from similar reactions of diacid dichlorides.6 Certain homologues ( e 3 . 10, = 4 ) undergo

2

4

A variety of aroyl- and acyl-diphenylphosphine oxides ( 8 )

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80 Organophosphorus Chemistry

spontaneous cyclization to give (11). Reactions of diphenyl- phosphine oxide with a,6-unsaturated acid chi-orides give diphosphine dioxides ( 1 2 ) together with other products! formation of (12) has been extensively investigated, with structural assignments confirmed by X-ray analysis.

The mechanism of

8

BU‘ ( 1 )

( 2 )

x = o x = s

0 + II

O H P P h rrPph3 NaOH ~

R O H , H20

THF, H20 IaoH 0 II

dPPh2

(4)

0

[‘“.IT ( 5 1 1 &H=cHx phzpu: X ( 6 ) X = ‘1

Ph2!& 0

R‘ R (7) x = 0

A new approach to the synthesis of optically active phosphine oxides has been reported.9 phenylvinylphosphine oxide (13), available by crystallization from the diasteromeric mixture, undergoes decarboxylation on heating in wet DMSO containing lithium chloride to give ( 1 4 ) with retention of configuration at phosphorus. The oxide ( 1 4 ) undergoes Michael addition and Diels-Alder reactions at the vinyl group to

( - ) - ( S 1- [(Menthoxycarbonyl )methyl]- -P

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3: Phosphine Oxides and Related Compounds 81

0 II

RCOCl + PhzPOMe - RCOPPh, ( 8 )

R ", / HZ\ O f

PhzPH Ph2P0'\ OPPh, I I 0

'd I I 0

0 II

CLCO(CR,),,COCl + MeOPPhz -

( 1 2 ) (11 1

give derivatives which also decarboxylate. These reactions combined with hydrogenation of (131 , followed by alkylation and decarboxy- lation, provide routes to a wide variety of optically active phosphine oxides (Scheme 1 ) . Full details of the synthesis of optically active phosphine oxides from g-isopropyl methyl- phosphorothioate ( 1 5 1 , which i s readily available as either enantiomer, have appeared." successive nucleophilic substitution with Grignard reagents, provides the appropriate phosphine oxide of 57-75% optical purity

Alkylation at sulphur, followed by

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82 Organophosphorus Chemistry

ph\P40 CO (Menthyl) c- Ph P h L p / / 0

CO, ( Men t h y l) <":H3 ( 1 4 ) CH,CH,/

ii i ,i / \ Ph Ph\ //o Ph, 4 0

CH,CH,/p\CH2R CH3CH2/'\ CH,

R

Reagents: i, DMS0,H20,LiC~,180 O C ; ii, H,,Pt;iii, RX;iv, R,CuLi;v, H'; vi,

Scheme 1

0 H 0 I: - I I II

___)

pri o-,"ks H 3 i - 2 C \ p h Pr O.jjP, SR Me Me Me

(15 1 (16)

pri o.jip\Rl

Me

45- 7 0 ' 1 o Opt. pure

Reagents: i, R I ; ii, R'MgX; iii, R'MgX; iv, PhSiH,

Scheme 2

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3: Phosphine Oxides and Related Compounds 83

(Scheme 2 ) . Points of special interest are that the 2-alkyl Q-isopropyl methylphosphorothioate [16, R = (CH2 1 20(CH2) 20CH2CH31 reacts with phenylmagnesium bromide with inversion of configuration rather than the retention normally observed with this class of compound and that the method offers the first route to trialkyl- phosphines of reasonably high optical purity. Chiral phosphine sulphides can be converted to the corresponding selenide(or vice versa) with retention of configuration at phosphorus by methylation with methyl triflate followed by reaction with sodium hydrogen selenide (or sodium hydrogen sulphide 1 . (E)-2-Aminovinyl- diphenylphosphine oxides ( 1 7 ) have been prepared by the base- induced reaction of ethynyldiphenylphosphine oxide with secondary amines . 1 2

The diphosphene monosulphide ( 1 9 ) has been synthesized, by treatment of the corresponding diphosphene with elemental sulphur, and its structure confirmed by X-ray ana1y~is.l~ reaction to give the phosphene (18) was achieved by reaction of (19) with HMPA. Heating or photolysis of ( 1 9 ) gave the thiadiphosphirane ( 2 0 ) (Scheme 3 ) . Another example of stabilization through steric

The reverse

1 Reagents: i, 3 S,; ii, (Me2N)3P; iii, A or h u

Scheme 3

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84 Organophosphorus Chemistry

Ph

( 2 1 1

S II + Mes-P-CC(SiMe3)2 HS ,SiMeg 1

Mes-P=C, >Mes-P

‘S’ SiMe, N C (Si Me3I2

hindrance is the first isolation and X-ray structure of a methylene- oxophosphorane (211, obtained by oxidation of the corresponding methylenephosphine .I4 has been prepared as a mixture with (22) and (24) by reaction of the methylenephosphine (22) with s~1phur.l~ obtained by reaction of (24) with one equivalent of tri-n-butyl- phosphine.

The three-coordinate phosphine sulphide ( 23 1

A pure sample of (23) was

3 Preparation of Cyclic Phosphine Oxides

Although it has not been obtained in the pure form,the phosphorin sulphide (26) appears to be generated in the reaction of the phosphorin (25) with sulphur, as demonstrated by trapping with lY3-dimethylbutadiene and with dimethyl acetylenedicarboxylate (Scheme 4).16 However, the conditions required for the latter reaction suggest that in this case a more stable precursor of (26) is involved. A variety of dihydrophosphorine sulphides (e.g. 27) and oxides have been prepared by the reaction of phospholes with acid chlorides followed by ring-expansion.

Unlike the analogous oxonin and azopin, 9-phenyl-9-phospha

- 1 7

18 [61.0]nona-2,4,6-triene ( 2 8 ) is stable to ring-opening. However, apparently the equivalent oxide (29) is not since peroxidation of (28) gives ( 3 1 ) , presumably derived from disrotatory thermal ring-opening of (29) to give (30), followed by cyclization. Some evidence that ( 3 0 ) is an intermediate and is stable at low temperature is provided by 31P and I3C n.m.r. spectra at -2OOC. The phosphirene sulphide (32) has been prepared from the corresponding phosphirene; however, (32) is rather unstable and attempts to prepare the oxide were unsuccessful. 19

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3: Phosphine Oxides and Related Compounds 85

( 2 6 1

Reagents:iJ S,,lOO O C , 24h; ii, S8 , Xylene, 14OoC, 2h; iii, XC=CX,lOO°C, 3h

Scheme 4

O P h / \. PhCH2

The oxide of 1-phosphaadamantane has been prepared by 20 hydrolysis of the corresponding P-phenylphosphonium salt.

4 Structure and Physical Aspects

31P relaxation times are highly dependent on structure and large differences in z1 values of phosphines and the corresponding phosphine oxides have been reported.21 important if 31P n.m.r. spectroscopy is being used to determine the proportion of oxide in a sample of phosphine. 6-Phenyl-1-oxa-6-

This is obviously very

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86 Organophosphorus Chemistry

0 * [ 9 ] - [ Q4;h]

(30 1 I

P P Ph' Ph' NO

(28) (29) > - 20 O C I H

(32 ( 3 1 1

( 3 4 ) X=Na

(35) X = H (36)

phosphaspiro[2.5loctane 6-sulphide ( 3 3 ) has been prepared and its stereochemistry determined by X-ray analysis. 22

5 Reactions at PhosDhorus 23 The chemistry of carboxydiphenylphosphine oxides has been reviewed.

Allylic phosphine sulphides are available from the palladium- catalysed reaction of thiophosphide anions with the appropriate allylic acetate. The main point of interest concerning the reaction

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3: Phosphine Oxides and Related Compounds 87

is that it occurs with retention of configuration at carbon, at least in secondary examples. 24 react with oxiranes to give 1,2-bis(phosphine oxides) ( 3 6 1 , ring-opening to ( 3 4 ) and substitution. 25 dependent on the nature of the solvent; in THF a similar reaction gave only the B-hydroxyalkylphosphine oxide ( 3 5 1 .

In DMF diarylphosphine oxide anions

The reaction is highly

0 II ;:xph H

(39) 0 II

.-,/O\,-. i - i i i ~ ph2pg:H Ph'

Ph

Ph H

Ph

H Ph

Reagents: i, Ph2PLi; i i , HB6; i i i , H20z; iv, Base

Scheme 5

Horner's early work on PO-activated olefination has been reinvestigated. 26 The diastereomeric 1,2-diphenyl-2-@iphenyl- phosphinoy&thanols ( 3 7 ) and ( 3 8 ) were prepared by lithium diphenyl- phosphide ring-opening of (El-stilbene oxide and (Z1-stilbene oxide, respectively, followed by protonation and oxidation [the authors point out that there is insufficient data in Horner's original publication to relate the stereochemistries of ( 3 7 ) and ( 3 8 ) to the

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88 Organophosphorus Chemhtry

same compounds prepared by Horner using the reaction of diphenyl- phosphine oxide anion with stilbene oxides]. Base-treatment of the threo-isomer (38) under a variety of conditions gave (El-stilbene in high yield and with high stereoselectivity. However, similar treatment of the erythro-isomer ( 3 7 ) also gave predominantly (El- stilbene under most conditions phosphine oxide. experiments using m-chlorobenzaldehyde, indicate that the conversion of (37) to (El-stilbene involves fragmentation to the carbanion ( 3 9 ) and benzaldehyde (Scheme 5 ) . This behaviour of the erythro-isomer obviously restricts the use of phosphine oxide-based olefination to the formation of (E)-alkenes in certain cases (see also Section 6).

For a variety of reasons efforts to induce asymmetry at phosphorus have had little success. In the latest attempt the reduction of phosphine oxides with lithium aluminium hydride in the presence of optically active diols or diamines has been studied. Reductions of the 2-phospholene oxide ( 4 0 ) were complicated by competing reduction of the alkene group, which appears to be much faster for the phosphine oxide than for the corresponding phosphine. Experiments where this complication could not occur, e.g. - reduction of benzylmethylphenylphosphine oxide, gave phosphines with small but significant enantiomeric excesses.

together with some benzyldiphenyl- The formation of this last compound,and trapping

27

The previously reported oxygen-insertion reaction of phosphole oxide dimers (41) with m-chloroperbenzoic acid to give ( 4 2 ) (analogous to the Baeyer-Villiger reaction of ketones) has been extended to a variety of other systems.28 regio- and stereoselective, appears to be general for strained phosphorus e.g. ( 4 3 ) , and providesa useful synthetic route to a variety of l,?-oxaphospha ring systems.

The reaction is highly

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3: Phosphine Oxides and Related Compounds 89

Ph, 40

MCPBA - P P

o4 ‘Ph O4 ‘Ph

( 4 1 1 (42)

bh ( 4 3 1

6 Reactions of the Side-chain

1-Phenylphosphole oxide and 1,2,5-triphenylphosphole oxide each show different modes of reaction with cyclopentadiene to give the adducts (44) and (461, respectively. 29 Acetone-sensitized irradiation of the former adduct, which has no bridgehead phosphorus, gives the cage product (45), a reaction which is of interest in connection with energy storage. Irradiation of the adduct (46) in either acetone or methanol gives high yields of the dihydroindene (47). In the latter case methyl phenylphosphinate is also isolated through solvent trapping of C6H5P0. of l-phenyl-cis-3a,7a-dihydrophosphindole and its oxide ( 4 8 ) have been investigated with a view to developing syntheses of other phosphorus heterocycles. 30 The oxide ( 4 8 1 undergoes gas-phase thermolysis to give the isomeric 2,3-dihydrophosphindole 1-oxide ( 4 9 ) in low yield. Attempts to reduce (48) with chlorosilane led to rearrangement to ( 5 0 ) and (51) (Scheme 6). Reaction of the phosphorus-stabilized diazomethyl carbanions ( 5 2 ) with the pyrylium salt ( 5 3 ) leads to the formation of the previously unknown 4-diazomethyl-4FJ-pyrans (541, which can be converted to oxepins by treatment with catalytic amounts of p-ally1 palladium chloride. 31 The mechanism of acetolysis of diphenylphosphinyl(phenyl1diazo- methane (551 has been investigated using l80 labelling techniques.

The chemistry

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90 Organophosphorus Chemktry

b 0’’ Ph

h v MeOH - (44 1

0 Ph \pH

0 II + PhPHOMe & h V , MeOH ;r

Ph Ph

( 4 5 )

Ph

( 5 0 1 ( 5 1

Reagents: i , 380-410 O C ; ii, HSiCL,, T E A ; i i i , HzO, OH‘

Scheme 6

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3: Phosphine Oxides and Related Compounds 91

THF, Et20 + LiCR II -70 O C

!, + ,; BUt 0 But N2

B F4- ( 5 2 ) ( 5 4 )

( 5 3 ) R = Ph,P(O), Ph(MeO)P(O), (MeO)2 P ( 0 )

Ph 1

( 5 5 1

Ph ,P(O)C = N,

Reagents: i , BuLi; i i , R 2 CHO; i i i , R 2 COX; iv, NaBH,; v , NaH

Scheme 7

0 I1 y ph2px”

R%? o**..L .. . ( 5 9 ) +

0 ’3

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92 Organophosphorus Chemistrv

Continuing the investigation of phosphine oxide-based olefination, Warren's group has unambiguously established the stereochemistry of the erythro- and threo-isomers of the inter- mediates in these R 2 = C6H5) is readily obtained from the reaction of ethyldiphenyl- phosphine oxide carbanion with benzaldehyde, followed by chromatographic separation of the minor amount (12%) of threo-isomer. The threo-isomer is most easily prepared by reduction of the ketone (57, R1 = Me, R 2 = C6H5) (Scheme 7 ) . R1 = Me, R 2 = C6H5) was confirmed by X-ray analysis and a study of a number of different pairs of erythro- and threo-isomers demonstrated that n.m.r. spectroscopy can be used for their stereo- chemical assignment. Base-catalysed decomposition of threo-isomers (58) generally gives (E)-alkenes highly stereoselectively, while erythro-isomers (56) usually give (E:?) mixtures. However, the results are interpreted in terms of stereospecific syn-elimination from (56) and (58); the l o s s of stereospecificity in the case of (56) resulting from dissociation to aldehyde and phosphine oxide carbanion (see also Section 5). Although it is always the major isomer obtained from reactions of phosphine oxide carbanions with aldehydes, the proportion of erythro-(56) is maximized in these reactions by the use of THF, low temperatures ( - looo) , and the presence of complexing agents.34 formation of (56) in these cases is pictured as (59) and this is supported by the effect of substituents R1 and R2 on the stereo- chemistry of the reaction; only when R1 or R 2 are secondary alkyl groups is there a significant fall in the proportion of erythro- isomer. Alternatively the proportion of threo-isomer 1 5 8 ) obtained from reduction of the corresponding 6-ketophosphine oxide (571.i~ maximized by use of sodium borohydride in ethanol as the reducing agent. 35 prepared by this method. Stereoselective, phosphine oxide-based olefin synthesis has also been applied to the production of unsaturated alcohols. 36 The appropriate 6-hydroxyalkylphosphine oxides (61) and ( 6 2 ) can be prepared by the routes shown (Scheme 8 ) . However, both of these methods lead to a predominance of threo- isomer and hence (E)-alkene. Intermediates (63) containing major amounts of erythro-isomer [and hence offering a route to ( g ) - alkenes] are available from the reaction of (60) with lithium butyl and aldehydes. Protected B-(diphenylphosphinyl) ketones (64) have

The erythro-isomer (56, R1 = Me,

The stereochemistry of (56,

The transition state for the

(E)-Isosaffrole, (E)-anethole, and feniculin have been

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3: Phosphine Oxides and Related Compounds 93

0 0 I I

Ph P 0 'CHR'

(60 1 H

11 1 i , i i I iii, i v Ph2PCH, R * obC(CH2),0H

i v i ,v i i (61 1

0 0 J Ph P II R' I P h 2 ! r o H

II ocoR2 viii ~ 'C(CH,),OH iii, i v I

R o//c\ R 2 HO 'R H

(62 1

ph2pY-

-i. 0 R 2 L 0 "

0 II F' ph2proHL H R & OH

It Ph2PCH2d 2 x i # ix =

HO

(63)

-co 0

L o Reagents: i, B u L i ; ii, (CH,), I ; i i i , NaBH,, i v , Separate; v, NaH, DMF; v i , u;

2 v i i , R COCI, Py; v i i t , LDA; ix, R2CH0,-78 O C

S c h e m e 8

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94 Organophosphorus Chemistry

0 R2

Ph2P R’ (64 1 R3 i , i i ~ ph2p---------- HO R

( 6 5 )

R 4 5 Reagents: i , BuLi; ii, R R CO; iii, NaH

Scheme 9

Me

I C0,Me

(66)

0 II

( 6 8 ) R’ = Ph

Reagents:i, L D A ; i i , RCHO; i i i, A ; iv, H,, Pt02 , AcOH; v , HO-

Scheme 10

37 been used to synthesize 8,~-unsaturated ketones (Scheme 9 ) .

Although these reactions offer good yields of alkene the stereo- selectivity in forming ( 6 5 1 , and hence the alkenes, is generally low.

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3: Phosphine Oxides and Related Compounds 95

Ph,P II 0

i - i v I ?5iPh2But

P O - O Reagents: i, LDA,THF, -78 O C ; i i ,

(69) 6u' Ph,SiO

+ - iii, KOBut, THF, 0 O C ; iv, B u Z N F,THF, 0 - 2 5 O C

Scheme 11

0 i II Ph,PCH, (CH=CH),-,CH2PPhZ + 2

( Me

a-Aminocarbanionequivalents are provided by the phosphine oxides ( 6 6 1 . 38 2-oxazolidones ( 6 7 ) which on heating eliminate diphenylphosphine oxide to give the corresponding 2-oxazolone derivative. Hydro- genation and hydrolysis provide stereospecific routes to ( 5 ) - conhydrine, (+I-ephedrine,and (+)-!-methylephedrine ( 6 8 ) (Scheme 1 0 ) .

Base-induced reaction with aldehyde gives the

The highly mutagenic pentaene (701, recently isolated from human faeces, has been synthesized by reaction of the carbanion of

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96 Organophosphorus Chemistry

hepta-2,4-dienyldiphenylphosphine oxide with the aldehyde ( 6 9 )

of the homologous polyenes ( 7 1 ) using reactions of diphosphine dioxides with aldehydes were generally unsuccessful, 40 although higher members (71, n_ = 2 , 3 ) were obtained in reasonable yields. A variety of spiro-ketals, including the sex pheromone ( 7 3 ) of the olive fly Dacus oleae and the common wasp pheromone ( 7 5 1 , have been synthesized by olefinations with the phosphine oxides ( 7 2 ) and (74) followed by acid-catalysed cyclization (Scheme 12) .4L phosphine oxide-stabilized carbanion ( 7 6 ) has been used to prepare the vinylcyclopropane ( 7 7 ) en route to the fungal prohormone

(Scheme 11).39 Attempts to prepare lower members (e.g. 71, n = 0 ) - -

The

methyl trisporate (Scheme 1 3 ) . 42

ii i - (CH, 1 OTHP

0

Reagents: i, LDA, THF, -78 O C ; ii, THPO(CH,),CHO; iii, H30+;

Scheme 1 2

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3: Phosphine Oxides and Related Compounds 97

Me + . . . L i

Reagents: i, Ph2P(0)CHCH2?/s> ; ii, H 3 0 ; I I I , KH, THF

Scheme 13

7 Phosphine Oxide Complexes and Extractants

A 31P n.m.r. study indicates that dimethylphenylphosphine oxide forms an inclusion complex with cyclodextrin in water. 43

A high proportion of the many reports of complexes of phosphine oxides, sulphides,and selenides with electron-pair acceptors are concerned with purely inorganic or physical aspects. 31P n.m.r. spectroscopic studies have been carried out on tin complexes ( 7 8 ) with phosphine oxides44 and zinc complexes with oxides, sulphides, and selenides .45 Copper complexes of the types (79) and and a variety of cobalt and nickel complexes of secondary phosphine oxides and ~ u l p h i d e s ~ ~ have been synthesized.

Sn X 4 L,

(78) L = Bu,PO

X = Br, CI

[ CICU(R,PO)~I ' (80 1

Rhodium complexes with mixed phosphine-phosphine oxide ligands have been used as catalysts for the hydroformylation of alkenes. The controversy over the interaction of manganese phosphine complexes with oxygen continues. 49 suggests that both reversible oxygen uptake and irreversible oxidation to phosphine oxide complexes are possible.

48

A sophisticated infrared study

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98 Organophosphorus Chembtry

Txi-n-octylphosphine oxide continues to be the most popular

51 extractant for cations. Among many reports of extractions of metals by phosphine oxides are tervalent berkelium, 50 actinium, uranium,52 and rare earths.53 variety of organic acids between aqueous buffer solutions and n-decane containing 0-0.16 M trioctylphosphine oxide has been studied.

The distribution of water and a

54

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