3 Phosphine Oxides and Related Compounds

BY B. J. WALKER

1 Preparation of Phosphine Oxides Diastereomerical ly pure menthyl phosphinates (1) are reduced stereospecifically by lithium 4,4'-di-tert-butylbiphenylide with retention of configuration at phosphorus. The phosphine oxide anion produced in this reaction can be trapped by benzyl bromide to provide a new synthesis of optically active phosphine oxides with optical purities as high as 958.1 The method was also applied to the synthesis of ( S , S ) and (R,R)-1,2- ethanediylbis(methylpheny1phosphine oxide) (2 ) . The vinylphosphonium salt (3) has now been shown to react with ethoxide ion to give exclusively the phosphine oxide ( 4 ) * and not the ylide (5) as previously reported.3 Depending on the nature of the reagents, the reaction of nitrones with vinylphosphines can provide the cycloadducts (6 ) and (7) or lead to initial oxidation of the phosphine by the nitrone to give the corresponding phosphine oxide.4 In the latter cases cycloadducts of the phosphine oxide with nitrone were also isolated in low yield.

1 -Phenyl-1 -benzophosphepine oxide (9) has been prepared by flash vacuum pyrolysis of 2a,2b-dihydro-3-phenyl-3H-cyclobut[b]phosphindole 3-oxide (8) (Scheme l).5 Treatment of (9) with trichlorosilane provides the first example of a 1-benzophosphepine (10) which, unlike (9). is thermally unstable in solution and gradually decomposes to naphthalene. Full details have appeared of the synthesis of the cis-isomer ( 1 2 ) of the tetracyclic phosphine oxide previously prepared as the t r ans - i somer (13).6 The isomeric mixture obtained depends on the conditions used for hydrolysis of the intermediate salt ( 1 1 ) to give (12) and ( 1 3 ) . I n addition to investigating a number of reactions of (12) and (13) which retain the tetracyclic structure the authors report detailed 13C, IH, and 3 1 P n.m.r. studies which not only allow determination of stereochemistry but, due to the rigidity of the ring system, provide useful data relating coupling constants to geometry.

The macrocycles ( 1 4 ) 7 and ( 1 5 ) g containing the phosphine oxide function have been prepared. Oxidation of ( 1 5 ) produces only the d , l - fo rm (16) of the disulphoxide. This differs from the corresponding oxidation of the acyclic analogue (17) which gives all four possible stereoisomers of (18). I t is suggested that the selective formation of ( 1 6 ) is due to the

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3 . Phosphine Oxides and Related Compounds 71

R2 I p- +

I R

(8 ) (9) (1 0 )

Reagents: i, FVP, 550 OC, 6 x mmHg; ii, SiHCI3, PhH, 55 OC Scheme 1

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72

.But

CH H3C I I

X\\ ,N-N P I \

GH CH3 N-N X ' //

P /I Ph' 'N-N N-N Ph

' 1 CH3

-78 "C

Organophosphorus Chemistry

qL@ s o s

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3: Phosphine Oxides and Related Cbmpounds 73

macrocycle (1 5 1 being conformationally strained in solution and this is supported by variable temperature 13C n.m.r. studies and X-ray diffraction data .

Th ioxophosph ines ( 1 9 ) have been generated by the reaction of dibromotriphenylphosphorane ( 2 0 ) with bis(triphenylmethylmercapt0)- phosphines (21) as evidenced by trapping with 1.3-dienes to give cyclic thiophosphinate analogues, e.g. ( 2 2 ) i n low yield.9 Oxidation of diphosphiranes ( 2 3 ) with ozone at low temperature has been used to generate the dioxide (24 ) which decomposes at room temperature to give the phosphaalkene oxide ( 2 5 ) and products apparently derived from ArP=O.lo

2 St ruc tu re and Physical Aspects Structural studies on a variety of phosphine oxide binary and ternary co- crystallization compounds have been reported. The molecular structure of the highly stable HMPA-primary amine adduct ( 2 6 ) has been determined by X-ray crystallography and molecular orbital calculations have been used to make structural comparisons with adducrs of other phosphine oxides.] 1

The isotropic 31 P chemical shift for hydrogen-bonded co-crystals formed between triphenylphosphine oxide and aryl sulphonamides has been correlated with the number of NH---OP hydrogen bonds formed and this data can now be used to derive information on the hydrogen bonding patterns in related co-crystals.12 The ternary co-crystallization compound formed by mixing N,N-dimethyl-o-phenylenediamine, tr iphenylphosphine oxide and aqueous fluoroboric acid has been shown by X-ray crystallography to have an unusual structure (27) where the oxygen atom of the phosphine oxide acts as a double acceptor through hydrogen bonding.13

A range of diastereomeric 2-diphenylphosphinoyl-l,3-dioxanes have been synthesized ei ther by Arbuzov react ions of i s o p r o p y I diphenylphosphini te with the appropriate (1 .3-dioxa-2-y1)tr imethyl - ammonium iodide (Scheme 2) or by transacetylization between 1,3-diols and diphenyl(diethoxymethy1)phosphine oxide (Scheme 3).14 The anomeric effect i n these compounds was studied by n.m.r. spectroscopy and X-ray cry s t a 11 o g r a ph y . The conform at i on a I be h a v i o u r of 2 -(dip h e n y 1 p h o sp h i n o y 1 ) - 1,3-dithiane (28) has been studied between 223K and 258K by IH n.m.r.15 The linear plots of In K versus 1/T obtained allowed assessment of both the enthalpic and entropic contributions of the S -C-P(0 ) anomeric effect. Comparison with the corresponding data obtained from the cyclohexyl analogue ( 2 9 ) suggests that the enthalpic anomeric effect i n ( 2 8 ) is approximately 3.4 kcal/mol.

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74 Organophosphorus Chemistry

Scheme 2

H !? PhS03H + (EtO)&HPPh2

f

Scheme 3

i-iii - (30) (31)

Reagents: i, 2 x Bu'Li, THF, 0 "C; ii, Ar2C0, 0-50 "C; iii, H30' Scheme 4

R'

PPh2 I

TBSO"' &TBS

H 0' 0 I t

(33) R' = Me2P, R2 = H;

:: R' = Me2P, R2 = OH

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3: Phosphine Oxides and Reluted ('orrtpouncis 75

A detailed study of 77Se and 3 ' P nuclear spin relaxation in tri( rertbuty1)phosphine selenide has been reported16 and the kinetics and mechanism of formation of tetracoordinate P(V) sulphides from the reaction of tricoordinate phosphorus compounds with diary1 trisulphides have been investigated.17

3 Reactions at Phosphorus The isolable oxaphosphetanes (31 ) have been prepared by treatment of the phosphine oxide ( 3 0 ) with two equivalents of butyllithium followed by reaction with substituted benzophenones (Scheme 4).18 I t has been reported that treatment of triphenylphosphine oxide with organolithium or Grignard reagents leads to ligand exchange even at -9SOC.19 Although no such examples are given i n the report, a similar reaction occurring with mixed arylalkylphosphine oxides would obviously pose problems in phosphine oxide carbanion chemistry.

4 Reactions at the Side-Chain A number of phosphine oxide derivatives of the A-ring of vitamin D3 and related compounds have been synthesized20 and used in the synthesis of vitamin D and its analogues. These include the phosphine oxide (32), which has been used in the synthesis of 25-phosphorus analogues (33) of vitamin D3.21 The 1,25-dihydroxy vitamin D3 metabolite (36) and the l a - f l u o r o analogue (37) have been synthesized from vitamin D3 by conversion to the phosphine oxide ( 3 4 ) followed by olefination with the ketone ( 3 5 ) , itself obtained by degradation of vitamin D3.22 Attempts to prepare 9-fluoro vitamin D3 using the standard olefination reaction of the appropriate carbonyl compound (39) with the A-ring phosphine oxide ( 3 8 ) gave instead the 9-hydroxy derivative (40).23 9-Fluoro vitamin D3, prepared by the corresponding Wittig reaction using the ylide analogue of (38) , did not undergo hydrolysis to the 9-hydroxy derivative. Phosphine oxide-based olefinations with (41) have been used in the synthesis of la, 25-dihydroxy- 19-nor-vitamin D3 24 and its side-chain homologated analogue (42).*5 The phosphine oxides ( 4 3 ) and (44 ) . which are enantiomeric synthons for the preparation of dihydrotachysterols, have been synthesized from the appropriate di hydrocarvones.2 6

In continuing studies of stereoselectivity i n reactions of phosphine oxides, Warren has shown that the introduction of a diphenylphosphinoyl group to create a chiral centre next to the hydroxyl group in allylic alcohols allows epoxidation with high diastereoselectivity, especially in the eryrhro- isomers, e.g. (45) (Scheme 5).27 This high diastereoselectivity was retained when a third chiral centre was introduced. The epoxides produced could be ring-opened with thiolate anions i n a highly diastereoselective manner and

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76 Organophosphorus Chemistry

PPh2 I

R3SiO"' & (34)

Oe PPh2 I

TBSO"' &

'H\3 ,CH3 (CH2)3C,

OH Base

____t

0 (35)

(36) X = F (37) X = OH

(CH2)3CH

i, BuLi, THF

ii, TBAF c

PPh2 I

0 (39)

R3SiO**' OSiR3

HO"

HO"

OTBDMS (43)

OTBDMS (44)

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3: Phosphine Oxides and Related Compounds 77

the hydroxy phosphine oxides produced undergo the expected stereospecific Horner-Wittig elimination to provide compounds, e.g. (46) , with all stereo centres defined. The reaction of the bis(phosphine su1phide)-stabilized carbanion (47 ) with aldehydes depends on the counter cation involved.2 8

Reactions with the lithium salt of ( 4 7 ) lead to the vinylbis(phosphine sulphides) ( 4 8 ) whereas the potassium salt of (47) gives the expected olefination product (49) (Scheme 6). I t is worth noting that the lithium salt of the corresponding phosphine oxide does not react under the same conditions whereas the potassium salt gives the product analogous to (49) , although in lower yield than that obtained from the sulphide.

A new route to 2 4 5 2 ) and 3 4 5 1)-(2-aminovinyI)indoles in excellent yield is provided by the reaction of 2- and 3-acylindoles with the carbanions of 1 -aminoalkyldiphenylphosphine oxides (50) (Scheme 7 ) . 2 9

The previously established method of diene synthesis using tandem Wittig reactions of the phospholanium salt (53) has been applied to the synthesis of the sex pheromone ( 5 4 ) from the pedal gland of the bontebok (Darnaliscus dorcas dorcas) and to various 1,4-diketones (Scheme 8).30

Diphenyl coumarin-3-phosphinyl oxides ( 5 5 ) and the corresponding phosphonates ( 5 6 ) have been synthesized i n one-step via a Knoevenagel reaction of acetoxysalicylaldehydes with diphenylphosphinyl- and triethyl phosphonoacetate, respectively.31 2,5-Dimethoxyphenyldiphenylphosphine oxide ( 5 7 ) undergoes lithiation, predominantly at the 6-position of the dimethoxyphenyl ring, on treatment with tertiarybutyllithium in THF under conditions of thermodynamic control at low temperature.32 The carbanion ( 5 8 ) formed can be trapped with a variety of electrophiles. The corresponding phosphine sulphide, although less reactive, is lithiated exclusively at the 4-position.

The (E,E)-isomer of 1,2,5-triphenylphospholane oxide (60) has been identified as the product obtained by catalytic hydrogenation of the phosphole (59).33 Compound ( 6 0 ) can be isomerized exclusively to the thermodynamically more stable (E,Z)-isomer (6 1) by catalytic amounts of strong base. Treatment with one mole equivalent of base leads to a mixture of isomers. The individual isomers of the (E,Z)-isomers (61) were obtained by chiral supercritical fluid chromatography. The reaction of phosphole sulphide derivatives ( 6 2 ) with ethyl diazoacetate has been investigated with a view to providing a new route to phosphinines (64).34 Reaction at high temperature provided the corresponding homophosphole (63 ) which could be converted to the phosphinine ( 6 4 ) by heating with triphenyl phosphite. The stereochemistry of ( 6 3 ) , which was determined by X-ray crystallography, led to suggestions for the mechanism of the rearrangement of (63) to (64 ) .

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78 Organophosphorus Chemistry

(75 : 25/syn : anti) (45) (1 00 : O/syn : anti ) Scheme 5

S

( Ph2P)2C=CHR

$ 9 (48) (Ph2P)2CH2

(47) > fi Ph2pMH +

H R (49)

Reagents: i, Bu"Li, PhH; ii, RCHO; iii, BU'OK, THF

Scheme 6

CHNR32 ~ - c o ~ Ph2PCH2NR32 !

R2 R2 (50) (51 1

Reagents: i, Bu"Li, THF, -78 "C; ii, BU'OK, THF, -78 "C Scheme 7

(52)

- i, ii f 0 - P h 2 P m

R-L PhNp\ Ph Clod- (53)

4 v i V R 0 SMe (54) R = n-C5HI1

Scheme 8 Reagents: i, BU'OK; ii, RCHO; iii, LDA; iv, MeSSMe; v, HCHO; vi, HgCI2, 50 "C

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3: Phosphine Oxides and Related ('ompounds 79

0 I I

0 I I CH2Ci2 NaOH RippR32 R32PCH2C02Et

0

R2

Ph Ph Ph Ph

MeLi

Ph Ph Ph Ph

* ? + RX Bu3SnH p b l P h 2 AlBN

0 s I

O Y O BU'

hv -78 "C

But

,Me

? y p \ - M e Ar

S y J

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80 0 rga n o p hosp h orus Chemistry

0 - N 0 -N Ph2!+R2 + Ph2!+ R2

R’ Ph2P

R’ anti -(70) syn -(70)

1 ii 1 ii OH NH:, OH NH2

Ph2!+ R2 Ph2!,p), R2

R’ R’ anti -( 7 1 ) S Y ~ -(71)

iii, iv ! iii, iv 1 ( E )-(72) (Z 1472) + -

Reagents: i, R2CEN-0, ultrasound; ii, NiC12.6H20, NaBH,; iii, NaH, DMF; iv, HCI

Scheme 9

ReCI( N2) (P h2PCH2CH2PPh2) (76)

( W 4

(77) (78)

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3: Phosphine Oxides and Related Compounds 81

Boron trifluoride catalyses the 1,3-phosphotropic rearrangement of the a-phosphorylated imine (65) to give (66).35 This reaction takes place at room temperature, whereas the uncatalysed reaction requires heating to 150-2OOoC. High yields of the addition products (67) have been obtained from the reaction of carbon-centred radicals with diphenylvinylphosphine oxide.36 Radical addition to the chiral phosphine oxides (68) using Barton‘s method provides diastereomeric ratios of up to 9:l in the case of ( 6 9 ) .

Allylic diphenylphosphine oxides undergo 1,3-dipolar cycloadditions with nitrile oxides to give A*-isoxazolines ( 7 0 ) with a n t i - p r e f e r r e d stereoselectivities of up to 5:1.37 Separate reduction of syn-and ant i - (70) to the hydroxy amines (71), followed by Wittig-Horner elimination provides stereoselective syntheses of the homoallylic amines (72) (Scheme 9). A study of the effect of substituents on phosphorus on t h e diastereoselectivity in the cycloaddition of nitrones to vinylphosphine oxides (73) and sulphides (74 ) has been reported.38 In certain cases diastereoselectivities of >90% were achieved.

Interestingly, while 2-hydroxyalkylphosphines undergo Rabbit gastric lipase-catalysed acylation, the corresponding phosphine oxides and sulphides either react more slowly or not at all.39

5 Phosphine Oxide Complexes The synthesis and X-ray crystal structure of the macrocyclic bisphosphine oxide manganese complex (75) have been reported.40 A stable P-bonded phosphinidene oxide complex (77) of rhenium ( I ) has been prepared from (76) by nitrogen replacement with C-tertiarybutylphosphaalkyne followed by hydrolysis.41 X-Ray crystallography was used to determine the structure of (77) . What is reported to be the first complex (78) with a PO ligand has been prepared.42 The molecular structure of (78) , determined by X-ray crystallography, shows an exceptionally short PP distance.

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