Phosphine Oxides and Related Compounds

BY B. J. WALKER

- 1 Introduction

In spite of the detailed studies by Warren and his co-workers, i t

has not been possible to conveniently control the stereochemistry of

phosphine oxide-based olefination. The beginnings of such control

may be forthcoming in the report31 that high stereoselectivity can

be introduced in certain cases through modification of the

phosphorus substituents.

2 Preparation of Acyclic Phosphine Oxides

Full details of reactions of Z-(diphenylphosphinyl)-1,3-butadiene

( 2 ) (generated by thermolysis of the butadiene (1)) with various

dienophiles have been rep0rted.l The reactions provide a route to a

variety of functionalized (1-cyclohexeny1)diphenylphosphine oxides

( 3 ) in moderate to good yields. The (1-cyclobuteny1)diphenyl-

phosphine oxide (1) also acts as a dienophile, forming Diels/Alder

adducts (4) with cyclopentadiene (Scheme 1). The phosphine oxide

(7). and the phosphine ( 8 ) and phosphine oxide ( 9 ) are formed

regiospecifically on basic hydrolysis of the cyclic tetraphosphonium

salts (5) and ( 6 ) . respectively.’ The reaction of 2-butyne-1.4-diol

with chlorodiphenylphosphine. previously reported to give

2,3-bis(diphenylphosphinyl)-1,3-butadiene, is now shown to give the

1.4-isomer This is analogous to a similar reaction with

phenylsulphenyl chloride and presumably involves a double

sigmatropic rearrangement (Scheme 2). The diene (10) is

disappointingly unreactive in Diels/Alder reactions. The mechanism

of thermal rearrangement of phosphine (11) to the phosphine oxide

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3: Phosphine Oxides and Related Compounds 71

i i i i

e a g e n t s : I , Heat ; 1 1 ,

H ‘R2

S c h e m e 1

n

Ph2P+ +PPh2 W

( 5 ) X = CH=CH ( 6 ) X = CH2CH2

X = CH=C H /

\ X = CH2CH2

0 0 II II

Ph2PCk$CHzPPh 2

( 7 )

Y Y

PhZPCH,CH,CH2CH,PPh 2 II II

( 8 ) Y = lone pair

( 9 ) Y = O

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72 Organophosphorus Chemistry

foH

R e a g e n e : I , 2 PhZPCL, p y r i d i n e , T H F , 0 OC

Scheme 2

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3: Phosphine Oxides and Related Compounds 73

(13). which involves a formal nucleophilic substitution at

phosphorus, has been investigated. Radical and intermolecular

mechanisms are ruled out by isotope studies and the authors conclude

that reaction probably occurs via (12). A variety o f amino

acid-derived thiocarbamoylphosphine oxides (14) have been prepared

by the reaction of diphenylphosphine oxide with the corresponding

isothiocyanates. The reactions of secondary phosphine oxides with

vinyl halides in the presence of tetra(tripheny1phosphine)palladium

provide a new route to alkenylphosphine oxides.

Chi.ral h.p.1.c. on bonded (R)-N-(3,5-dinitrobenzoyl)phenyl

glycine has been used to separate enantiomers of racemic tertiary

phosphine oxides on a preparative (about 1 g) scale.’

(2)-(9-Methoxypheny1)phenylvinylphosphine oxide (15) has been

prepared from (5)-p-methoxypheny1)methylphenylphosphine oxide

(Scheme 3 ) and used in the synthesis of a number of novel, optically

pure phosphine ligands (e.g. 16, Scheme 4 ) . 8 -

3 Preparation of Cyclic Phosphine Oxides.

A potentially useful new route to the phosphirane oxide (18) has

been reported.’

with base gives (18) and the olefinic by-product (19). The

stereochemistry of (18) was deduced from the (Z)-stereochemistry o f

(19) (which is presumably derived from allowed conrotatory

ring-opening of (18)) and from the product of a Pummerer reaction of

(18). Oxidation of P-tertbutyl- and P-phenyl 9-phosphabicyclo-

[6.1.0]nona-2,4,6-trienes (20) with peroxide offers a route to

phosphonin oxides (22) .lo However, these last compounds undergo

intramolecular cyclization at 2S0C to give trans-3a. 7a-dihydro-

phosphindoles (23). Oxidation of (20. R=Ph) with oxygen provides

instead anti-9-phenyl-9-phosphabicyclo[4.2.l]nona-2,4,7-triene-9-

oxide ( 2 4 ) . the structure of which is supported by an - X-ray analysis

Reaction of the a-bromoalkylphosphine oxide (17)

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74 Organophosphorus Chemistry

0 0 I , I I ~ I I I I

P h I t y - M c + Me-S-Ph i P h m P -CCH2CH2-S-Ph I

I 8 II

AT NMc Ar NMe II

OMe

Ar = b’ J I l l

Reagents : I , L D A ; 1 1 , C u C l ; I I I , x y l e n e I r e f l u x

Scheme 3

0 It

(15) -b (PhDy-CCH2CHZ) ,PPh I I

AT

Ph

4 AT

R e a g e n t s : I , P h P H Z , K O H , R T , 5 h ; I I I H S I C L ~ , R 3 N , M e C N

Scheme C

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3: Phosphine Oxides urid Related Compounds 75

of a phosphine derivative. Attempts to stabilize either the

phosphonin structure (22) o r the phosphirane structure (21) by

steric effects have also been reported.”

2,4,6-tri(tertbutyl)phenyl substituted triene (25) with tertbutyl

hydroperoxide gave the corresponding phosphonin oxide (26) which

slowly decomposed to give cyclooctatetraene; possibly by

retrocycloaddition through elimination of ArP=O. Apart from

reporting only the second example o f a phosphirane oxide, this paper

also contains useful and interesting 31P and l 8 O n.m.r. chemical

shift data.

Oxidation of the

Reactions of aryl and alkyl dialkylphosphinites with dimethyl

acetylenedicarboxylate followed by treatment with hydrogen bromide

give various amounts of phosphole oxides (27).12 These last

compounds are difficult t o isolate from the reaction mixture. partly

because they readily add water to give the phospholenes (28) (Scheme

5). Various stereoisomers of hydroxyphospholane oxides ( 2 9 ) have

been obtained from the reaction of 1,4-diketones with phenyl-

phosphine. l 3 The a n t i - 7 - p h o s p h a n o r b o r n e n e phosphinite (30) is

converted stereospecifically to the corresponding syn-7-

phosphanorbornene oxide (32) by an Arbusov reaction with the

appropriate alkyl halide.14

corresponding chloride (31) are both converted, by hydrolysis at

room temperature, to the secondary phosphine oxide ( 3 3 ) . Phosphorin

sulphides (35). generated by thermolysis of the 1-phospha-2-

thiabicyclic compounds (34). can be trapped with dienophiles to give

phosphine sulphides (3. 36 1. l5

-

The phosphinite (30) and the

The hexaoxide ( 3 8 ) has been obtained from the remarkably

air-stable hexa(tertbuty1)octaphosphine (37) by oxidation with

peroxides o r peracids at room temperature.16

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76 Organophosphorus Chemist?

(201 R = P h , But

Ph

O4 ‘R

(24) (23)

( 2 5 ) X = \one pair ( 2 6 ) X = 0

R e a g e n t s : I , M c O Z C C E CCOZMe ; ii , H Br ; I I I , H 2 0

Scheme 5

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3: Phosphine Oxides und Relared Compound.$

( 2 9 1

X .Op

Me*Me

04'\ N Me

(301 X = OMe ( 3 1 ) X = C I

77

Ph \ p//o

R I - X = OMe

( 3 2 1

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78 Organophosphorus Chemistry

3 . Structure and Physical Aspects

An ab initio study of the tautomeric equilibrium (39) has been

reported.17

expected by comparison with experimental observations; this is

rationalised in terms of solvent and substituent effects. Ab initio

calculations have been carried out on FP=O (in the gas phase) and

the report includes an investigation. by mass spectrometry and

infrared, of the same molecule isolated in an argon matrix.18

Generally the results are somewhat different from those

The conformation of 2-phosphorus-substituted-1.3-dithiane

derivatives ( 4 0 ) has been studied using 1 3 C n.m. r . spectroscopy.”

The results indicate that the axial preference is Ph2P(S)>Ph2P(0)

and are explained by a combination of the anomeric effect and the

relative bulk and electronegativities of sulphur and oxygen. Other

applications of n.m.r. include investigations of the isomerisation

of 2-thioxo-(41) and 2-seleno-(42)-2-phosphabicyclo[4.4.Oldecan-

5-ones in the presence of acid and base by 31P n.m.r.” and the

nuclear shielding of selenium in phosphole selenides by 77Se

n. m. r . 21

Although previously reported as the arsin oxide structure (43).

- X-ray structural and spectroscopic investigations now show that the

compound actually exists as an arsorane-type st~ucture in the solid

state and in dilute solution.22

various phosphine oxides has been investigated using calculated and

experimental dipole moments. 23

The conformational equilibria of

5 Reactions at Phosphorus

Attempts to use silanes to reduce the dimer (44) of Z-phenyliso-

phosphindole oxide to the corresponding phosphine (which is a

potential precursor of the isophosphindole system) lead instead to

carbon-carbon bond cleavage and formation of the diphosphine

monoxide (45).24 An n.m.r. study of the interactions of

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3: Phosphine Oxides and Related Compounds

R \

( 3 7 ) X = lone pair

138) X - 0

H,P=X H , P - X - H

(391 X = O , CH2 NH

X II

pTPPh* y-S

( L O ) Y = M e 2 C , S , CH, X = O , S

Ph x phm

H S I C ~ ~

PY 25 O C

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80 Organophosphorus Chemistry

dibenz[c,fl[l,51azaphosphocine 12-oxide derivatives (46) and (47)

with thionyl chloride or dimethyltertbutylsiloxytriflate indicates

that transannular species (e.g. 48, R=Me) are formed.Z5 Photo-

induced cleavage of acylphosphine oxides (49) has been shown by

chemically induced dynamic electron polarisation to involve a Type I

reaction. 2 6

-

6 Reactions at the Side-Chain

The reaction of ketones with the lithio derivatives of allyl-(50) or

(l-buten-3-~1)-(51) diphenylphosphine oxide provides a convenient

synthesis of 1.3-dienes with high (E)-stereoselectivity. 27

approach based on Whitham's method of interconverting alkene isomers

via their epoxides provides the first viable synthesis of

(E,E)-1,5-cyclooctadiene ( 5 4 ) (Scheme 6) The diastereomeric

phosphine oxides (52) and (53) were separated and then identified by

formation of their (-1-menthoxyacetic esters. Fortunately only the

oxide (52) gives a volatile alkene on base treatment, hence

separation of (52) and (53) is not required for the synthesis of

( 5 4 ) . The reactions of (a-1ithioalkyl)diphenylarsine oxides ( 5 5 )

with electrophiles have been investigated and provide routes to a

variety of organoarsenic compounds. 29 Reduction of arsine oxides

and treatment with halogens leads t o As-C bond cleavage and hence a

synthesis of a variety of halogen compounds (Scheme 7). The

reaction of (55) with certain carbonyl compounds is highly

stereoselective. for example benzaldehyde gives almost exclusively

the erythro-adducts (56). 30

(57) on the other hand generally show low stereoselectivity on

reaction with carbonyl compounds. However, Kauffmann has now

shown31 that the introduction of an ortho-substituent in the phenyl

groups of (57) greatly increases diastereoselectivity in these

reactions, for example, reaction of (58) with benzaldehyde

An -

(a-Lithioalkyl )diphenylphosphine oxides

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3: Phosphine Oxides and Related Compounds 81

0 II

Cl I

( 4 6 ) R = Me (47) R = Ph

Is Ar, P C O A r

( d 8 1

Scheme 6

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82 Organophosphorus Chemistn

0

0 ll

II Ph As 0 H

I I , l l I 'PR' Ph2AsC(Li)HR' 4 / C - - - H

Ho \Ph ( 5 5 )

R e a g e n t s : I , R 2 X ; 1 1 , PhCHO; I I I , H20 ; I V , L I A I H ~ ; v , x 2

Scheme 7 0

0 II

A r 2 P C ( L i ) R

(57) Ar: P h OMe

(58 ) Ar = @, R=Pr"

PhCHO - 2 R' R'

UMe t OMe (60) (611

0

I I R ~ L I Ph2! SR

ph2pYsR - R'

0

Ph2P SR II w -

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3: Phosphine Oxides and Related Compounds 83

gives the ervthro-adduct (59) exclusively. This has obvious

implications for the use of phosphine oxides in olefin synthesis as

well as for the mechanism of the Wittig reaction. Of further

interest is the increase in selectivity towards other aldehydes

caused by changing lithium to chromium or titanium as the counter

ion in (57).

The carbanions ( 6 0 ) . derived from a-methoxyallylphosphine

oxides. react with Si, S and P electrophiles to give the products

(61) of y-attack highly regioselectively in almost all cases. 32

Full details have appeared of the generation of the

a-thioalkylphosphine oxide carbanions (63) via nucleophilic

addition to a-thiovinylphosphine oxides ( 6 2 ) . 33

particularly the regiochemistry, of y-thioalkylphosphine oxide

carbanions ( 6 4 ) has also been reported. The enolate ( 6 6 ) formed on

reaction of (E)-but-2-enyldiphenylphosphine oxide carbanion ( 6 5 )

with 2-methylcyclopent-2-enone can be trapped by vinyl sulphones to

give ( 6 7 ) highly stereoselectively and in excellent yield. 34

phosphine oxides ( 6 8 ) and (70) (which are formed in a similar way to

( 6 6 ) from the reaction of cyclopenten-3-one with the carbanions of

(E)- and (Z)-(but-2-eny1)diphenylphosphine oxides, respectively) can

be cyclized stereospecifically to the bicycloheptanes ( 6 9 ) and (71)

or, in the case of (70). to the bicyclooctanol (72) (Scheme

- The reactivity,

-

The

7 Phosphine Oxide Complexes and Extractants

The structure of the stable complex (73) of triphenylphosphine oxide

with 5-methyl-6-phenyl-l,2,3-oxathiazin-4(3H)-one 2.2-dioxide has

been investigated by X-ray and n.m. 1:. spectroscopic methods.36

Trimethyl- and triphenylphosphine oxides have been used as probe

molecules in studies of adsorption sites on surfaces by 31P n.m.1.

spectroscopy. 37

Diphenylphosphine oxide platinum(I1) complexes (75) are

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84 Organophosphorus Chemistry

( 7 1 1 0 0 ( 7 0 1

\IL @IPh2 \ v

'H H Me

(72)

R e a g e n t s : I K O B u t , THF , R T ; I I L D A ,THF - 78 O C

0

pp"2 Ph2P :2 PMePh, II

NoOH \ / \ /

Ph2 H2 0

/Pt\,/Pt\ PPh2 P t U 2 - /\

Ph2McP cH2\ p/ NH 3

I/

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3: Phosphinr Orities arid Relutetl Conipounds 85

reported to be formed on reaction of the phosphine complex (74) with

sodium hydroxide in Uses of phosphine oxides and

sulphides in the solvent extraction of metals include the use of

tri(isobuty1)phosphine and triarylphosphine sulphides in the

extraction of Pd( I I I 3 ’ and Hg( 11) ,40 respectively, from hydrochloric

acid.

1. 2.

3 . 4. 5 . 6. 7.

8. 9. 10.

11. 12.

13.

14. 15. 16.

17. 18.

19.

20.

21. 22.

23.

24. 25. 26.

27. 28. 29. 30. 31. 32.

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86 Orgunophosphorus Chemistrv

33.

34. 35. 36.

37.

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