Organophosphorus Chemistry Volume 2

A Review of the Literature Published between July 1969 and June 1970

By S. Trippett

The Royal Society of Chemistry

Copyright © 1971 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-016-9

Contents

Chapter 1 Phosphines and Phosphonium Salts By D. J. H. Smith,
Chapter 2 Quinquecovalent Phosphorus Compounds By S. Trippett,
Chapter 3 Halogenophosphines and Related Compounds By J. A. Miller,
Chapter 4 Phosphine Oxides By J. A. Miller,
Chapter 5 Tervalent Phosphorus Acids By B. J. Walker,
Chapter 6 Quinquevalent Phosphorus Acids By N. K. Hamer,
Chapter 7 Phosphates and Phosphonates of Biochemical Interest By D. W. Hutchinson,
Chapter 8 Ylides and Related Compounds By S. Trippett,
Chapter 9 Phosphazenes By R. Keat,
Chapter 10 Photochemistry, Radicals, and Deoxygenation Reactions By R. S. Davidson,
Chapter 11 Physical Methods By J. C. Tebby,

CHAPTER 1

Phosphines and Phosphonium Salts

BY D. J. H. SMITH

PART I: Phosphines

1 Preparation

From Halogenophosphine and Organometallic Reagent. — (4-Bromophenyl)magnesium bromide reacts with chlorodiphenylphosphine below 10 °C to yield diphenyl(4-bromophenyl)phosphine (1). In a similar synthesis, tris(3-fluorophenyl)- and tris(4-fluorophenyl)-phosphines have been prepared from excess of the corresponding Grignard reagent and phosphorus trichloride.

Trimesitylphosphine (2) can be obtained from excess mesitylmagnesium bromide and phosphorus trichloride. However, when the amount of Grignard reagent is limited, the product obtained is tetramesityldiphosphine (3).

A synthesis of phosphines utilising alkyl transfer from boron to phosphorus has been described. No attempt was made to prevent oxidation to phosphine oxides during work-up and hence chlorodiphenylphosphine and tricyclohexylborane yielded the phosphine oxide (4).

Lithioacetylides and diethyl phosphorochloridite give (5), which can be treated further with Grignard reagent to yield dialkyl-(1-alkynyl)phosphines (6).

The reaction of cyclopropyl-lithium with triphenyl phosphite and chlorodiphenylphosphine gave tricyclopropylphosphine and cyclopropyldiphenylphosphine respectively. Tertiary phosphines have been prepared by the treatment of alkyl halides with phosphites, phosphinites, or phosphonites in the presence of sodium, e.g.:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

B. From Metallated Phosphines. — The cis-diphosphine (7) has been obtained from lithium diphenylphosphide and cis-1,2-dichloroethane. Issleib has shown that alkyl-substituted diphosphines can be prepared by exchange reactions with tetraphenyldiphosphine:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The base-catalysed addition of secondary phosphines to vinylphosphines and ethynylphosphines has been described. The reaction is useful for the preparation of poly(tertiary phosphines) with ·CH2CH2 · bridges between phosphorus atoms:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

An acyl-substituted phosphine has been prepared by the reaction of sodium diphenylphosphide with acetyl chloride.

Treatment of lithium diethylphosphide with boron trichloride gave the dimer (8), but with excess silicon halide products of the type (9) were obtained. Similar products may be obtained from lithium diethylphosphide and (methylsilyl)diethylphosphine (10). Methylsilylphosphines have been prepared from potassium silylphosphides and methyl bromide. Alternatively, (methylsilyl)phosphine can be made from silyl bromide and (11).

The reaction of trisodium phosphide with trichlorophenylgermane or trichlorophenylsilane yields heptamers, whereas reaction with dipotassium phenylphosphide gave the tetramers (12).

C. By Reduction. — Lithium aluminium hydride reduction of (+)-benzyl-methylphenylpropylphosphonium bromide proceeds with racemisation, whereas the corresponding arsonium compound gave the arsine with retention of configuration.

A convenient synthesis of methylphosphines involves the reduction of dimethyl methylphosphonite with lithium aluminium hydride. The resulting methylphosphine can be converted into di- or tri-methylphosphine with methyl iodide in methanol, depending upon the conditions used.

Reduction of the bisphosphonium salts (13a) with sodium hydride results in the cleavage of the bridge, irrespective of the substituents on phosphorus. It is suggested that the reaction proceeds with initial attack of hydride ion at phosphorus to give a phosphorane which subsequently decomposes. However, when lithium aluminium hydride is used the loss of the bridge is competitive with loss of the benzyl group.

The same phosphonium salts can also be reduced very efficiently with cyanide ion. Ethylenebis(triphenylphosphonium) bromide was reduced with 2 moles of potassium cyanide in DMSO to triphenylphosphine and succinonitrile. One mole of cyanide gave the β-cyanoethyl salt (13), indicating that an elimination-addition sequence is the probable reaction pathway.

In a study of the mechanism of reduction of phosphine oxides with trichlorosilane Mislow has shown that the stereochemical course of the reduction of benzylmethylphenylphosphine oxide depends upon the base used. Weak bases (pKb > 7) give predominant retention, whereas strong bases (pKb<5) give predominant inversion. Complex formation does not appear to be important, but reduction with inversion proceeds via a product of the base decomposition of the chlorosilane, whether a derived perchloropolysilane or a trichlorosilyl anion as shown. This work naturally led to the use of hexachlorodisilane for the reduction of acyclic phosphine oxides with inversion of configuration. In contrast, the reduction of phosphetan oxides (14) with hexachlorodisilane proceeds with retention. These reductions are faster than their acyclic analogues and it is suggested that the reaction proceeds with nucleophilic attack at phosphorus.

However, the desulphurisation of acyclic phosphine sulphides proceeds with retention, presumably via attack of the trichlorosilyl anion on the sulphur atom of the intermediate trichlorosilylmercaptophosphonium ion (15).

Decyldichlorophosphine (16) can be reduced catalytically with hydrogen over palladium in the presence of triethylamine.

D. By the Radical Addition of P–H to Olefins. — Dimethyl- and bis(tri-fluoromethyl)-phosphines yield tertiary phosphines, e.g. (17) and (18), with olefins and trifluoroethylene on u.v. irradiation. Photolysis of ethereal solutions of alkylphosphines with divinyl ether leads to perhydro-1,4-oxaphosphorins (19).

Phosphine and divinyl ether in the presence of AIBN gave (19; R = H), which could be converted to (19; R = C8H17) by photolysis in oct-1-ene. The bridged phosphine (20) can be converted to the highly condensed system (21) by photolysis through Pyrex, in contrast to the oxide (see Chapter 10, Section 1). Tricyclic phosphines have also been made by irradiation of cyclododeca-1,5,9-triene with a 60Co source in the presence of phosphine. Treatment of the resulting product with AIBN in hexane gave a mixture of phosphines.

2 Reactions

A. Nucleophilic Attack on Carbon. — (i) Activated Olefins. The reaction of diethylphosphine with α-chloroacrylonitrile at room temperature and some β-substituted acrylonitriles in the presence of triethylamine led to diethylphosphinoacrylonitrile (22). In the absence of triethylamine at -15 °C, α-chloroacrylonitrile gave the phosphine (23).

Chlorodiethylphosphine and acrylonitrile gave a 1:1-adduct which, it is claimed, might have the structure of an epiphosphonium salt (24). Tris(hydroxymethyl)phosphine and acrylonitrile gave the phosphine (25).

Addition reactions of fumaric acids and esters with acrylic compounds are catalysed by tricyclohexylphosphine.

The vinyl phosphonium salt (26) has been isolated from the reaction of triphenylphosphine with trans-β-bromovinylphenylsulphone.

For the reaction of diphenylcyclopropenones with triphenylphosphine see Chapter 8, Section 1A.

(ii) Activated Acetylenes. Phosphines and diacylacetylenes have been shown to give 1,2-alkylidenediphosphoranes (27) which are thermally less stable and more reactive than the corresponding phosphoranes stabilised by ester groups. In the same paper an alternative synthesis of the diphosphorane (28) starting from a secondary phosphine was described. This synthesis could not be used to prepare the acyl diphosphoranes (27) since the phosphine adds preferentially to the carbonyl group rather than to the acetylenic bond.

Bis(diphenylphosphino)methane (29) with one equivalent of dimethyl acetylenedicarboxylate gave 5H-diphosph(v)ole (30). In contrast to 1,2-alkylidenediphosphoranes one obtains a P=C-P=C conjugation. Variable temperature n.m.r. indicated the presence of two conformers resulting from restricted rotation about one ester group.

Spectroscopic evidence indicated that the buff-coloured, unstable solid from the reaction of cis-1,2-bis(diphenylphosphino)ethylene and dimethyl acetylenedicarboxylate was the 1,4-diphosph(v)orin (31), but the bis-ylide (32) obtained from l,2-bis(diphenylphosphino)ethane and the same acetylene hydrolyses only slowly in water.

The structure of the yellow adduct obtained from 1,2,5-triphenylphosphole and dimethyl acetylenedicarboxylate has been shown to be the phosphorane (33), and not that previously reported, which rearranges to the cyclic phosphine (34) in refluxing chloroform.

Dideuteriated olefins (35) can be prepared from triphenylphosphine and activated acetylenes in the presence of deuterium oxide.

Sodium diphenylphosphide and 1-bromobut-2-yne in liquid ammonia gave (but-2-ynyl)diphenylphosphine (36). However, the reaction with 3-bromoprop-1-yne yielded propyne and diphenylphosphinoamine with no product from attack on carbon being observed. The corresponding chlorides react by nucleophilic attack on carbon.

(iii) Carbonyls, etc. Triphenylphosphine and NN’-dibenzoyl-o-benzoquinonedi-imide in benzene gave the benzimidazole (37) which is thought to have arisen as shown.

Diethylphosphine reacted with carbon disulphide in the presence of base to yield the diethylphosphoniobisdithioformate (39) whereas the reaction with diphenylphosphine stopped at the phosphinodithioformate (38) stage.

(iii) Miscellaneous. α-Halogenobenzyl phenyl ketones and triphenylphosphine afford the ketophosphonium halide (40) and/or the enolphosphonium salt (41) depending upon the reaction conditions. If α-mesyloxybenzylphenyl ketone (42) is used, only the ketophosphonium salt is obtained. The formation of the ketophosphonium salt is best explained by a direct displacement of halide ion by phosphorus, while a mechanism involving attack on halogen followed by recombination of the resulting ion pair is favoured for formation of the enolphosphonium salt. It has been shown that these reactions are not base-catalysed as previously reported, but that the presence of base simply prevents the acid-catalysed debromination reaction.

B. Nucleophilic Attack on Halogen. — The scope of the reaction by which alcohols can be converted into halides with tertiary phosphine and perhalogenocarbon has been extended. The reaction shows a remarkable tendency to give inversion products even when solvolysis of the corresponding esters is assisted and gives retention products, e.g. (43).

Tributylphosphine and carbon tetrachloride gave a polymeric waxy solid (44) which could be hydrolysed to tributylphosphine oxide.

A relatively stable compound (45) was the product of the reaction of tri(chloromethyl)phosphine and chlorine in carbon tetrachloride.

Highly chlorinated ketones are dechlorinated by trivalent phosphorus compounds to α,β-unsaturated products (46).

In a related reaction, dehalogenation of 2,2,3-tribromopropionitrile has been achieved using triphenylphosphine.

1,2-Dibenzoylethane was the major product from the reaction of triphenylphosphine with the epoxyketone (47). This interesting compound presumably arises from a reaction sequence as shown. The other products from the reaction can be visualised as being produced via an intermediate phosphonium alkoxide formed by initial attack at carbon. Triethyl phosphite and (47), however, gave the phosphonate (48).

Borowitz has shown that α-halogenoketones can be dehalogenated with diphenylphosphine. The reaction is not acid-catalysed as is the reaction with triphenylphosphine. A reaction mechanism involving a six-centred transition state has been proposed. Evidence for this includes a Hammett ρ value of -0.74 and the fact that sterically hindered ketones do not react any slower than unhindered ones.

C. Nucleophilic Attack on Other Atoms. — A Hammett plot of the rates of reaction of triphenylphosphine with ozonides of substituted styrenes gave ρ = +0.72. There was no significant isotope effect in this reaction, which suggests the formation of an unstable phosphorane (49) in the rate-determining step.

Dialkyl t-butyl phosphates (50) can be prepared in low yield from the reaction of triphenylphosphine with the corresponding dialkyl t-butyl peroxyphosphates.

Differences in the reactions of tri(o-tolyl)phosphine and the meta– and para-isomers have been reported. The latter compounds gave phosphine oxide upon reaction with thionyl chloride, whereas tri(o-tolyl)phosphine gave phosphine oxide and sulphide. Tri(o-tolyl)phosphine produced the compound (51) ‘stabilised by specific attractions between o-methyl groups and ligands’ upon reaction with liquid sulphur dioxide. The other isomers were unreactive.

A complex mixture is obtained from benzotrifuroxan (52) and triphenylphosphine, containing five compounds whose structures were elucidated by X-ray crystallography.

Triphenylphosphine reacted with thiodehydrogliotoxin (53) to give the disulphide (54) with retention of configuration at the asymmetric carbon atoms. However, the disulphide gave the monosulphide (55) more slowly and with inversion of configuration of the asymmetric carbon atoms as judged by circular dichroism. Desulphurisation of trisulphides obviously occurs preferentially at the sulphur-bonded sulphur atom.

A 1,3-dipole (56) is thought to be the correct structure for the product from triphenylphosphine and dimethyl azodicarboxylate.

A pyrazole (57) was isolated from the reaction of (56) with dimethyl acetylenedicarboxylate.

Isocyanates and isothiocyanates react in a similar way. Other reactions of the 1,3-dipole are described in Chapter 2, Section 7.

D. Miscellaneous. — It has been shown that allylmethylphenylphosphine does not undergo an allylic rearrangement. Racemisation, which is slower than racemisation of methylphenylpropylphosphine, must occur via pyramidal inversion. The rate of racemisation of t-butylmethyl-phenylphosphine is similar to that of the two phosphines above, indicating that steric effects are not significant. Electron-withdrawing substituents in the para-position of the phenyl ring increase the rate of racemisation, indicating that the (p-p)π conjugation affects the barrier to rotation. Similar electronic effects have been observed in a study of the rates of racemisation of diphosphines.

Hydroxylamine has inadvertently been used as an oxidising agent for tertiary phosphines in the preparation of the oximes (58).

Hydrogenation of unsaturated phosphines (59) was found to be possible over a palladium catalyst if the nickel(II) complex was used.

The addition of phosphorus trichloride to dilithiophenylphosphine gave hexaphenyldecaphosphine (60).

Various lithiated tetra-, tri-, and di-phosphines are produced from the addition of phenyl-lithium to ‘phenylphosphorus’ (PhP)n.

Tertiary phosphines and picric acid give, at room temperature, deeply coloured picrates believed to be covalent in character. At lower temperatures yellow charge-transfer complexes are obtained.

An equimolar mixture of triphenylphosphine and NN’-bisbenzene-sulphonyl-p-benzoquinone di-imide (61) has been used as a dehydrating agent in the preparation of anhydrides, amides, and esters.

Peptide synthesis employing triphenylphosphine and 2,2′-dipyridyl disulphide in an oxidation–reduction condensation has been described. High reactivity with high optical purity is observed, which is rationalised by the intermediate formation of an acyloxyphosphonium salt (62) with predominant pentacovalent character, which reacts rapidly with the amino-component.

The chromatography of phosphines using various adsorbents has been reported.

For the reaction of triphenylphosphine with TCNE and related compounds see Chapter 10, Section 2.

PART II: Phosphonium Salts

1 Preparation

The quaternisation of triarylphosphines has been achieved using benzyne intermediates. The reaction of o-lithiofluoroaromatics with the phosphine at -75 °C leads to a mixture of betaines which can be protonated with fluorene. The same mixture is obtained from the lithiofluoroaromatics (63) and (64).

Tetra-arylphosphonium salts have been formed under Ullmann conditions by the reaction of triarylphosphines with iodobenzene in the presence of copper powder and cuprous iodide in DMF as solvent.

(Continues…)Excerpted from Organophosphorus Chemistry Volume 2 by S. Trippett. Copyright © 1971 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
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