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
79
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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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