Organophosphorus Chemistry Volume 6

A Review of the Literature Published between July 1973 and June 1974

By S. Trippett

The Royal Society of Chemistry

Copyright © 1975 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-056-5

Contents

Chapter 1 Phosphines and Phosphonium Salts By D. J. H. Smith, 1,
Chapter 2 Quinquecovalent Phosphorus Compounds By S. Trippett, 27,
Chapter 3 Halogenophosphines and Related Compounds By J. A. Miller, 42,
Chapter 4 Phosphine Oxides, Sulphides, and Selenides By J. A. Miller, 62,
Chapter 5 Tervalent Phosphorus Acids By B. J. Walker, 74,
Chapter 6 Quinquevalent Phosphorus Acids By N. K. Hamer, 97,
Chapter 7 Phosphates and Phosphonates of Biochemical Interest By D. W. Hutchinson, 124,
Chapter 8 Nucleotides and Nucleic Acids By J. B. Hobbs, 141,
Chapter 9 Ylides and Related Compounds By S. Trippett, 160,
Chapter 10 Phosphazenes By R. Keat, 182,
Chapter 11 Photochemical, Radical, and Deoxygenation Reactions By R. S. Davidson, 204,
Chapter 12 Physical Methods By J. C. Tebby, 221,
Author Index, 259,

CHAPTER 1

Phosphines and Phosphonium Salts

BY D. J. H. SMITH

1 Phosphines

Preparation. — From Halogenophosphine and Organometallic Reagents. An improved synthesis of trimethylphosphine from phosphorus trichloride and methyl-lithium at -78 °C has been described. Another ‘improved high yield’ synthesis of the same phosphine uses the reaction of triphenyl phosphate with methylmagnesium iodide. Other trialkylphosphines have also been prepared by this latter method.

Triarylphosphines with formyl or acetyl groups substituted into the aromatic rings can be prepared by a Grignard reaction using ethylene keta derivatives. The resulting phosphines (1) are treated with toluene-p-sulphonic acid. The acetyl derivatives may also be prepared by oxidation of the ethyl derivatives (2) followed by reduction with trichlorosilane.

A series of phosphines (3) containing alkenyl groups has been prepared by reaction of the chlorophosphine with the appropriate Grignard reagent. Tertiary arylethynylphosphines, e.g. (4), can be easily prepared by heating copper arylacetylides with the corresponding chlorophosphine in a polar aprotic solvent containing a lithium salt.

Organosilylphosphines, e.g. (5), are obtained directly from the reaction of chlorophosphines and trimethylchlorosilanes in the presence of magnesium.

From Metallated Phosphines. The preparations of a number of flexible aliphatic ligands, e.g. (6), containing the dimethylphosphino-group have been described, in which the sodium dimethylphosphide used was prepared from tetramethyldiphosphine.

The chiral phosphines (7) and (8) have been obtained by the reaction of sodium diphenylphosphide with menthyl chloride and neomenthyl chloride, respectively.

(Mercaptoalkyl)phenylphosphines, e.g. (9) and (10), may be prepared by treatment of chloro-thiols or episulphides with sodium phenylphosphide. The S — H is more acidic than the P — H in these compounds.

Michael addition of sodium phosphides to alkenes containing nitro- or sulphonyl-groups gave the expected (2-nitroalkyl)- and (2-sulphonylalkyl)-phosphines.

The reaction of potassium phenyl(trimethylsilyl)phosphine with bromine or iodine in benzene gave pentaphenylpentaphospholan and the phosphine (11). This phosphine can also be obtained from dipotassium phenylphosphide, prepared by ring cleavage of pentaphenylpentaphospholan with potassium, and trimethylchlorosilane.

The phosphane (12) is ring-expanded by metallation and subsequent reaction with dichloromethane.

Dialkylphosphinobis(dimethylamino)methanes (13) are prepared from formamidinium salts or the ethers (14) by addition of lithium phosphides.

Bis(diphenylphosphino)amine (15) was the unexpected product from the reaction of 1,2,4,5-tetrabromobenzene and sodium diphenylphosphide in liquid ammonia.

Alkylalkoxydiphosphines (16) are obtained by the addition of dialkoxyphosphines to dialkylchlorophosphines or, less satisfactorily, from the condensation of dialkoxychlorophosphines and dialkylphosphines in the presence of triethylamine.

By Reduction. Phenylsilane reduces cyclic and acyclic phosphine oxides to the corresponding phosphines with complete retention of configuration and in high yields. Phosphine oxides and phosphonium salts containing a t-butyl group can be reduced satisfactorily with lithium aluminium hydride, also with retention of configuration. The reduction of triphenylphosphine oxide with chlorodisilanes has been discussed.

The synthesis of polyphosphines containing combinations of primary, secondary, and tertiary phosphorus atoms by the base-catalysed addition of P — H across the carbon-carbon double bond of vinyl phosphonates, followed by reduction with lithium aluminium hydride, has again been described. The preparation of 1,2-bis(phosphino)ethane from the bis-phosphonate (17) by reduction with lithium aluminium hydride has been reported in detail.

Methylated poly(tertiary)phosphines, e.g. (18), can be made by the base-catalysed addition of P — H to vinylphosphine sulphides. The protecting sulphur atom(s) are removed by treatment with lithium aluminium hydride. The desulphurization of diphosphine disulphides with tributylphosphine has been used to prepare tetramethyldiphosphine (19).

The effects of temperature and cathode material and the use of aluminium electrodes on the electrolysis of phosphonium salts have been studied.

Miscellaneous. Patent specifications have appeared for the convenient resolution of tertiary phosphines by complexation with the asymmetric palladium(II) complex (20).

Carbonyl bis(diphenylphosphide) (21), which is stable at room temperature, has been isolated28 from the reaction of phosgene with diphenyl(trimethylsilyl)phosphine at -110 °C.

Bis(trifluoromethyl)(trimethylsilyl)phosphine (22) has been prepared by an exchange reaction using bis(trifluoromethyl)phosphine. Fluoroalkylphosphines (23) may also be obtained by treatment of (fluoroalkyl)iodophosphines with trifluoromethyl iodide in the presence of antimony powder.

A convenient preparation of phosphine from the addition of aqueous sulphuric acid to aluminium phosphide has been described in detail.

Reactions. — Nucleophilic Attack on Carbon. Activated olefins and acetylenes. The full paper describing addition of P — H bonds to vinyl isocyanides has been published. The reaction of diphenylphosphine with vinyl isocyanide in the presence of base proceeds normally, whereas the corresponding reaction with phenylphosphine gave the 1,3-azaphosphole (24).

The reactions of primary phosphines and the corresponding alkyl phosphides with αβ-unsaturated ketones (25) have been discussed in some detail.

Tetrafluoroethylene with an excess of dimethyl phosphine in the gas phase gives (26) by a reaction which is thought to be initiated by the bimolecular abstraction of a hydrogen atom from dimethylphosphine by tetrafluoroethylene. Tetrafluoroethylene also reacts with tetramethyldiphosphine by a radical process to give 1,2-bis(dimethylphosphino)tetrafluoroethane (27).

Tertiary phosphines have been shown to be very effective catalysts for Michael reactions. They appear to participate by nucleophilic addition to the activated olefin. The cyclic phosphine (28) has been prepared by a double Michael addition of phenylphosphine to 1-propenylcyclohexenyl ketone. Similarly, the dihydrophosphepin (29) can be obtained from cycloaddition of phenylphosphine and hexa-1,5-diyne.

Triphenylphosphine has been reported to react with TCNE at room temperature to give (30).

Carbonyls. Several papers have appeared this year from Issleib’s group describing the synthesis of heterocyclic phosphorus compounds by acidcatalysed condensations of phosphines with carbonyl compounds. (Mercaptoalkyl)phenylphosphines (31) react with aldehydes or ketones to form 1,3-thiaphospholans or 1,3-thiaphosphorinans. The intermediate compound (32) can be isolated from a similar reaction with phenylisothiocyanate and is converted into a thiaphospholan by intramolecular loss of hydrogen sulphide.

In similar reactions 1,3-azaphosphorinans and 1,3-azaphosphepans have been obtained from the condensation of aminophosphines (33) and carbonyl compounds. 1,2-Azaphospholans (34) are produced by oxidation of the aminophosphines with bromine.

The addition of the (carboxymethyl)phosphine (35) to Schiif bases or semi-carbazones gives 1,3-azaphospholan-5-ones (36).

Bis(hydroxymethy1)phosphines catalyse the polymerization of phenylisocyanate. However, high yields of (37) are also obtained.

The addition of trimethylsilyl keten to diphenylphosphine results in the formation of the acyl phosphine (38) which is stable at room temperature but rearranges on heating.

The reaction of germyl- or silyl-phosphines with biacetyl leads to cyclic products as well as acyclic derivatives derived from 1,1- and 1,2-addition (see Scheme 1). Condensation of hydrometal-phosphines (39) with biacetyl gives mono-insertion products which cyclize in the presence of H2PtCl6 into germaor sila-dioxolan derivatives.

Treatment of epoxides with lithium diphenylphosphide followed by oxidation gives β-hydroxydiphenylphosphine oxides which can be fragmented to olefins stereospecifically, thus constituting an olefin inversion (Scheme 2).

Nucleophilic Attack at Halogen. Tertiary phosphine–carbontetrahalide adducts continue to be exploited for halogenation or dehydration reactions. Among those described this year is the addition of triphenylphosphine–carbon tetrachloride to cholesterol or i-cholesterol to give a complex mixture of products which suggests that both reactions are proceeding via an ion-pair (39a).

Aziridine can be obtained in good yields by the simultaneous action of triphenylphosphine, carbon tetrachloride, and triethylamine on N-substituted β-amino-alcohols (40).

A simple one-step preparation of cyclotriphosphazenes (41) and cyclotetraphosphazenes (42), which uses condensation of bis(diphenylphosphine)amine in the presence of carbon tetrachloride and triethylamine, has been described.

Substituted ureas can be converted into chloroformamidines (43) by treatment with tertiary phosphine–carbon tetrachloride.

French workers prefer the use of tris(dimethylamino)phosphine–carbon tetrachloride for reactions of this type. These reagents are used to substitute one hydroxy-group in 1,3-diols. Heating the salt (44) gives the chloride directly, or the phosphine oxide may be displaced by added nucleophiles. Addition of sodium methoxide gives the oxetans (45). The same reagents can be used to activate selectively the primary hydroxy-group of hexoses and hence allow it to be displaced by added nucleophiles.

In a similar reaction it has been shown that benzoic acid may be converted into its anhydride by addition of tris(dimethy1amino)halogenophosphonium salts (46).

Triphenylphosphine or tris(dimethylamino)phosphine in aqueous solvents reduces benzyl αα’-dichlorobenzyl sulphoxide to a mixture of diastereomeric benzyl α-chlorobenzyl sulphoxides. Full details of a kinetic study of the reduction of α-halogenobenzyl phenyl sulphoxides have been published.

The mechanism of the formation of betaines from the reaction of triphenylphosphine with pyrrolidine dione derivatives (47) has been discussed.

Difluorocarbene can be generated by treatment of the phosphonium salt (48) with sodium methoxide or more conveniently by the reaction of tertiary phosphines with dihalogenodifluoromethane and potassium fluoride (Scheme 3).

Nucleophilic Attack at Other Atoms. A Lossen rearrangement occurs when aromatic hydroxamic acids are allowed to react with the triphenylphosphinediethyl azodicarboxylate complex in the presence of ethanol, to give the hydroxamates (49).

Attempted 1,3-dipolar additions of acetylenic phosphines to sodium azide gave only iminophosphoranes (50). No cyclic compounds were isolated. The formation of iminophosphoranes from reaction of diazocyclopentadienes (51) and triphenylphosphine continues to be studied.

Solvent effects on the oxidation of triphenylphosphine by perbenzoic acid have been reported. The second-order rate constants are directly proportional to the dielectric constant of the solvent. Oxidation of methylphenyl-propylphosphine with 3-chloroperbenzoic acid or ozone proceeds with retention of configuration. The reaction of alkyl- or aryl-phosphines with dialkyl peroxides or polyperoxides in aqueous solvents leads to the formation of alcohols or glycols, respectively.

Desulphurization of β-keto-sulphides by tris(dimethylamino)phosphine is thought to proceed via a phosphonium salt intermediate, e.g. (52), which can collapse to give a variety of products depending upon the substrate used and the reaction conditions.

Sulphimides (53) are reduced by the corresponding sulphides by triphenylphosphine in DMF. The kinetics of the reaction indicate that the initial reaction is nucleophilic attack by phosphorus at the sulphinyl sulphur atom. In the presence of alcohols a complex mixture of products is obtained which, the authors claim, indicates the initial formation of a 1,3-dipole. The related reactions of sulphoxides and sulphimides with triphenylphosphine in the presence of p-tosyl isocyanate have also been studied.

Several tervalent phosphorus compounds readily remove selenium from triphenylmethyl isoselenocyanate (54) at room temperature forming the isocyanide quantitatively.

Miscellaneous. The barriers to pyramidal inversion of a series of acyl phosphines (55) [RCOP(CHMe2)2] have been measured. Electron-withdrawing substituents in R facilitate the inversion whereas electron-donating groups hinder it because of the increase or decrease of interaction of the phosphorus lone-pair with the carbonyl group. The Hammett ρ constant for the inversion of phosphines has been determined using substituent constants derived from the inversion of 1-aryl-2,2-dimethylaziridines. The pyramidal inversion barriers for phosphines and arsines have been reviewed.

Tertiary phosphines substituted at the a-carbon by electronegative groups, e.g. (56), react with boron trihalides to give products derived from carbon–phosphorus bond cleavage. Phosphines containing only hydrocarbon groups do not react.

Triphenylphosphine reacts with styrene in the presence of palladium(II) acetate to give trans-stilbene.

Full details of the demethylation of the pentacyclic diether (57) with lithium diphenylphosphide have been published. Selective cleavage of the methoxy-group is achieved even when a four-fold excess of phosphide is present.

Calculations show that the hypothetical reaction of phosphine with acetylene to give (58; X = Ph) should be possible in the ground state whereas the reaction of ethylene with phosphine to give (58; X = PH3) requires a photochemically excited state.

Various structural parameters and the conformation of biphosphine have been determined from the microwave spectra of biphosphine and deuteriated derivatives. Rotational isomerism in tetramethyldiphosphine has been detected using photoelectron spectroscopy. The dipole moments of a series of methyl-substituted triaryl phosphines have been measured.

2 Phosphonium Salts

Preparation. — The reaction of triphenylphosphine with 1-bromoalkyl ketones has been described in which the initially formed labile enolic salts (59) are converted irreversibly into phosphonium salts via ion-pairs (Scheme 4). When R is larger than ethyl the ion-pair is not formed and the enol salts decompose in the presence of atmospheric moisture to give alkyl aryl ketones. No enol phosphonium salts are isolated from the reaction of bromo-diketones with triphenylphosphine in ether. The phosphonium salts (60) are precipitated directly.

β-Bromo-β-nitrostyrenes undergo deoxygenation with triphenylphosphine in aprotic solvents to give high yields of cyanomethylphosphonium salts (61). When this reaction is carried out in methanol the salts (62) are the products. Different products, thought to be derived from an azirine intermediate, are also obtained if electron-withdrawing substituents are present in the styrene aromatic nucleus.

Cyanoethylphosphonium salts can be prepared by the addition of phosphines to acrylonitrile in the presence of a dialkylanilinium salt.

The diuretic phosphonium salts (63) are obtained by cleavage of oxaphospholes by dry hydrogen chloride in benzene. Treatment of trioctylphosphine with benzyl chlorides gives a series of salts which are anti-spasmodic and anti-ulcerogenic.

Reaction of 7-chloronorbomadiene with triphenylphosphine in the presence of silver tetrafluoroborate gives the phosphonium salt (64) and a tricyclic salt (65) which is unstable at room temperature.

A mixture of isomers of 7-chloro-7-methoxynorbomene gives an isomeric mixture of phosphonium salts (66) and (67) on reaction with triphenyl phosphine in liquid sulphur dioxide at -60 °C. Both salts can be converted into the non-classical dication (68). The salt (67) isomerizes to (66) at temperatures above -14 °C.

The spiro-phosphonium salts (69) can be prepared by the reaction of the dilithio-derivatives (70) with triphenyl phosphate, followed by addition of sodium tetrafluoroborate.

Cyclopropyltriphenylphosphonium bromide is conveniently prepared from 3-bromopropyltriphenylphosphonium bromide (71) by treatment with equimolar sodium ethoxide in absolute ethanol.

Several α-(trisubstituted-stannyl)phenacyltriphenylphosphonium salts (72) have been isolated from the reaction of acylphosphoranes with chloro-tin compounds.

(Continues…)Excerpted from Organophosphorus Chemistry Volume 6 by S. Trippett. Copyright © 1975 The Chemical Society. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.