Organophosphorus Chemistry Volume 9

A Review of the Literature Published between July 1976 and June 1977

By S. Trippett

The Royal Society of Chemistry

Copyright © 1978 The Chemical Society
All rights reserved.
ISBN: 978-0-85186-086-2

Contents

Chapter 1 Phosphines and Phosphonium Salts By D. W. Allen, 1,
Chapter 2 Quinquecovalent Phosphorus Compounds By S. Trippett, 30,
Chapter 3 Halogenophosphines and Related Compounds By J. A. Miller, 48,
Chapter 4 Phosphine Oxides and Sulphides By J. A. Miller, 66,
Chapter 5 Tervalent Phosphorus Acids By B. J. Walker, 80,
Chapter 6 Quinquevalent Phosphorus Acids By R. S. Edmundson, 101,
Chapter 7 Phosphates and Phosphonates of Biochemical Interest By D. W. Hutchinson, 130,
Chapter 8 Nucleotides and Nucleic Acids By J. B. Hobbs, 151,
Chapter 9 Ylides and Related Compounds By D. J. H. Smith, 182,
Chapter 10 Phosphazenes By R. Keat, 210,
Chapter 11 Physical Methods By J. C. Tebby, 237,
Author Index, 273,

CHAPTER 1

Phosphines and Phosphonium Salts

BY D. W. ALLEN

1 Introduction

Interest in the chemistry of phosphines and phosphonium salts continues at a high level, and, as in previous years, considerable selection has been necessary in the preparation of this Report. A noticeable feature has been the large number of papers concerned with the preparation of chiral phosphines and their use in the homogeneous catalysis of asymmetric synthesis. Of these, only those involving some new aspect of organophosphorus chemistry are included here.

The use and significance of stereochemical reaction cycles in the reactions of chiral phosphines and phosphonium salts have been surveyed, and a major review of the chemistry of polycyclic C — P heterocycles, much of which is concerned with tertiary phosphines and phosphonium salts, has appeared. Procedures for the synthesis of a range of unidentate and polydentate phosphine ligands have been collected together in a single volume. Aspects of the chemistry of methylphosphines have been included in a review of recent developments in the chemistry of simple P–C compounds.

2 Phosphines

Preparation. — From Halogenophosphines and Organometallic Reagents. A series of trimethylsilylcyclopentadienylphosphines, e.g. (1), has been prepared from the appropriate trimethylsilylcyclopentadienyl-lithium and halogenophosphine. Similarly, the reaction of pentamethylcyclopentadienyl-lithium with chlorodimethylphosphine gives the phosphine (2), which is reported to be thermally stable.

Direct metallation of cross-linked polystyrenes, using the n-butyl-lithium–TMEDA reagent, followed by treatment with chlorodiphenylphosphine, affords an improved route to polymeric tertiary phosphine ligands (3) that are suited to the formation of transition-metal catalysts for hydrogenation reactions. The optically active phosphines (4) and (5), of interest for the catalysis of asymmetric hydrogenations, have been prepared from the reactions of chlorodiphenylphosphine with the Grignard reagents derived from (-)-menthyl halides and the optically active 2-halogenomethylpyrrolidines, respectively.

Two unusual fluorinated phosphines, (6) and (7), have been prepared by the reactions of organolithium reagents with appropriate halogenophosphines.

The sterically bulky phosphines (8) have been prepared by the Grignard method from chlorodi(t-butyl)phosphine and chlorodicyclohexylphosphine. In certain iridium(I) complexes, metallation of these phosphines occurs on the terminal olefinic carbon atom. Treatment of α,ω-dialkynyl-lithium reagents with chlorodi-(t-butyl)-phosphine gives the diacetylenic diphosphines (9), which form large ring compounds when they form complexes with transition metals.

Interest in the synthesis of compounds containing the P(CH2CO2R)n grouping continues, and routes involving the reactions of chlorophosphines with sodium enolates of acetate esters and Reformatsky reagents have been reported. A range of N-phosphinylated heterocyclic systems has been prepared by the reactions of chlorophosphines with N-potassio-derivatives of pyrroles and pyrazoles. 13C N.m.r. studies reveal that the product from the reaction of potassiopyrrole with phosphorus trichloride is (10), and not (11) as reported earlier.

From Metallated Phosphines. The reactions of organophosphide anions with alkyl tosylates have been used to prepare the chiral diphosphines (12) and (13), and also a range of phosphines bearing chiral substituents derived from various natural products.

The reaction of lithium diphenylphosphide with a bis-benzylic halide has been employed in the synthesis of the diphosphine (14), which is of interest as a trans-spanning ligand. Displacement of halide ion from a vinylic carbon atom occurs in the reaction of cis– and trans-β-chlorovinyldiphenylarsines with lithium diphenylphosphide, which proceeds stereospecifically with the formation of the corresponding cis– and trans-phosphine-arsines (15). Surprisingly, the reaction of lithium diphenylphosphide with a thirty-fold excess of cis-1,2-dichloroethene yields only the cis-diphosphine (16).

Further instances of the probable attack of phosphide anions on halogen have appeared. Lithium bis(trimethylsilyl)phosphide reacts with 1,2-dibromoethane to form the diphosphine (17), together with ethylene. Similarly, the reaction of lithium diphenylphosphide with 1,2-di-bromo- or -di-iodo-adamantane affords the anti-Bredt olefin adamantene (isolated as the dimer), in addition to 1- and 2-diphenyl-phosphino-adamantanes (Scheme 1).

Dimetallodiphosphide reagents of type (18) react with difunctional halogen derivatives to form five-, six-, or seven-membered heterocycles of types (19) and (20). The reagent (18; R=Ph, M=Li, n=2) is conveniently prepared by the cleavage of 1,2-bis(diphenylphosphino)ethane, using lithium.

By Addition of P — H to Unsaturated Compounds. There has been a marked reduction in the number of papers concerned with this route in the past year, but nevertheless a number of interesting studies have been reported. Thus, for example, the primary phosphine (21) undergoes free-radical-induced intramolecular cyclization to form the bicyclic phosphine 1-phosphabicyclo[3,3,1]nonane (22).

The addition of secondary phosphines to vinylaminophosphines, e.g. (23), occurs under both free-radical and base-catalysed conditions to form, e.g., (24) or (25). Similar addition of primary phosphines to (23) occurs to form either polymers, e.g. (26), or diphosphacyclohexanes, e.g. (27), depending on the mode of initiation.

Phosphine and primary phosphines add to vinyl acetate to form (2-acetoxyethyl)-phosphines, e.g. (28), which can be hydrolysed to form (2hydroxyethyl)phosphines, e.g. (29). The addition of phenylacetylene to phenylphosphine that is co-ordinated to a dicarbonylcyclopentadienylmanganese unit occurs stereospecifically, with the formation of (co-ordinated) phenylbis(trans-β-styryl)phosphine (30).

By Reduction. Both conformational isomers of the phosphepin (31) have been obtained by reduction of the related conformationally isomeric oxides, using either trichlorosilane or a mixture of trichlorosilane and triethylamine, the reactions proceeding with retention of configuration at phosphorus. The chiral chelating diphosphine (32) has been prepared similarly by reduction of the corresponding oxide, using the trichlorosilane-triethylamine reagent. A procedure for the reduction of triarylphosphine oxides by heating with hydrogen under pressure in the presence of a sulphur (or selenium)–silicon tetrachloride catalyst has been described.

Miscellaneous. A number of routes to phosphines bearing reactive groupings have been reported. The reactions of trimethylsilylphosphines with chloroacetonitrile afford a convenient route to cyanomethylphosphines (33), and the aminomethylphosphine (34), which is accessible by the aminomethylation of di-isopropylphosphine, reacts with acid chlorides to form the acylphosphines (35). The rather unusual (hydroxymethyl)phosphine (36) is formed in the reaction of guanosine with tetrakis(hydroxymethyl)phosphonium chloride, and procedures for the preparation of the phenolic phosphines (37) by déméthylation of the corresponding methyl ethers have been developed.

Cyclization procedures involving the reactions of primary phosphines with carbonyl compounds have been described for the synthesis of the heterocyclic phosphines (38) and (39).

The preparation and reactions of ‘organotin phosphines’ continue to attract interest, and have been reviewed. Cyclodehydrogenation of the secondary phosphine (40) with azobenzene in the presence of AIBN gives the 1,2-stannaphospholans (41). Radical-initiated addition of di-n-butylstannane to the dialkynylphosphines (42) gives a mixture of (43) and (44). The latter is of value for the preparation of diphosphacyclohexadiene systems by reaction with aryldichlorophosphines.

A number of unusual chelating diphosphines have been prepared. The bicycloalkenyl compound (45) reacts with diphenylphosphine to form the diphosphine (46). Diphenyl(o-vinylphenyl)phosphine is dimerized on heating with rhodium(III) chloride in 2-methoxyethanol to form the diphosphine (47), isolated as a rhodium complex from which it may be freed by treatment with sodium cyanide solution. The chiral diphosphines (48) and (49) have been synthesized and used, as rhodium complexes, to promote asymmetric hydrogenation reactions.

The reaction of t-butyldichlorophosphine with magnesium in THF gives a mixture of the cyclopolyphosphines (50) and (51), from which the hitherto unknown trimer is easily separated. The cyclopentaphosphine (52) is conveniently prepared by the reaction of methylphosphine with dibenzylmercury.

The unstable phospha-alkenes CFa=PH, CHa=PCl, and CH2=PH have been identified by microwave spectroscopy as products of the pyrolysis of CF3PH2, CH3PCl2, and (CH3)2PH, respectively. These species also undergo further pyrolysis to produce the phosphyl HC[equivalent to]P. The related compound CH3C[equivalent to]P has also been detected in the pyrolysis products of ethyldichlorophosphine.

The synthesis of the enantiomeric forms of the phosphine (53) has been reported, resolution being achieved via the carboxylic acid group.

Reactions. — Nucleophilic Attack at Carbon. A number of studies of the kinetics of quaternization of phosphines have been reported, all of which lend support to an earlier suggestion that the transition state for such reactions is reactantlike. From the rates of quaternization of a series of heteroaryldiphenylphosphines (54) with α-bromoacetophenone, it was concluded that the π-excessive heterocyclic substituents are not significantly involved in pπ-dπ, conjugative stabilization of the developing phosphonium centre in the transition state of the reaction. Similarly, there is little evidence of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] conjugative effects in the transition state for quaternization of a series of trisdialkylaminophosphines (55) with iodomethane. A comparison of the rates of quaternization of triphenylphosphine and triphenylarsine with iodomethane and various 4-substituted benzyl halides has led to the conclusion that the transition states for the quaternization of the phosphine and the arsine are at quite different positions along the reaction co-ordinates, there being a much smaller degree of bond-making in the transition state for the quaternization of the phosphine.

N-Methylpyridinium salts are easily demethylated by triphenylphosphine in DMF, the reaction being accelerated by electron-withdrawing substituents in the pyridine ring. Similarly, triphenylphosphine has been used to debenzylate benzylarsonium salts in the synthesis of asymmetric tertiary arsines.

The competitive elimination (E2) and substitution (SN2) reactions of cyclohexyl tosylate with triphenylphosphine have been examined. Triphenylphosphine is considered to be representative of neutral weak bases which have good nucleophilic affinity for carbon, but it is a poor reagent for elimination when compared with anionic weak bases that are also good carbon nucleophiles. The reaction of triphenylphosphine with cyclohexyl bromide occurs with almost complete substitution.

Tertiary phosphines react with fluorosulphonyl isocyanate and with isothiocyanates to form the zwitterionic adducts (56) and (57).

Activated olefins are reduced rapidly and stoicheiometrically by some alkylphosphines in anhydrous methanol. The reaction is thought to proceed via ylide formation, as shown in Scheme 2.

The reaction of triethylphosphine with dimethyl acetylenedicarboxylate in the presence of p-chlorobenzaldehyde is reported to lead to the olefin (58) and the bicyclic lactone (59). In the presence of water, the initial dipolar adduct (60) is hydrolysed, with the formation of dimethyl fumarate and the phosphine oxide.

Triphenylphosphine reacts with the methyl 2-bromoalkanoates (61) to form either the betaine (62) or the ylide (63), depending on conditions and the nature of the solvent. In the presence of aldehydes, the betaines (62) undergo Wittig reactions via the ylides (63) without the addition of base.

Treatment of the α-bromovinylphosphonate esters (64) with tri-n-butylphosphine gives the trans-betaines (65). Trimethylphosphine reacts with dichloroacetylene to give the bis-ylide (66).

Nucleophilic Attack at Halogen. The reactions occurring in the triphenylphosphine–carbon tetrachloride system continue to attract attention. The salt (67), which is the first isolable product from the reactions of the above system, undergoes ready dechlorination on treatment with trisdimethylaminophosphine (TDAP) to form the ylide (68) and the dichlorophosphorane (69). This reaction offers a convenient route to the ylide (68), and enables the course of other reactions occurring in the triphenylphosphine–carbon tetrachloride system to be clarified. Thus it was not clear as to whether the phosphorane (70) is formed directly from the reaction of the ylide (68) with triphenylphosphine or with the dichlorophosphorane (71) (also present in the reaction mixture) and triphenylphosphine in an autocatalytic process, via the salt (72), that leads to the regeneration of (71). It has now been discovered that in fact the ylide (68) and triphenylphosphine do not react, and thus the latter route, involving the bisphosphonium salt (72), is implicated. This hypothesis is supported by the isolation of (72) from the reaction of the ylide (68) with the dichlorophosphorane (71) in the absence of triphenylphosphine. Not surprisingly, the salt (72) is rapidly decomposed by triphenylphosphine, the phosphorane (70) being formed.

The reaction of the phosphorane (70) with TDAP offers a convenient route to hexaphenylcarbodiphosphorane (73), which it has hitherto been difficult to prepare. Trimethylphosphine and dimethylphenylphosphine react with carbon tetrachloride in dichloromethane solution to form (74) and (75); the dichlorophosphoranes (74) are insoluble in the solvent, and evaporation of the filtrate affords the pure phosphoranes (75).

The reaction of tertiary alkyl- or aryl-phosphines with hexachloroethane results in the formation of dichlorophosphoranes (76) and tetrachloroethene. In contrast, the silylphosphines (77) react with equimolar amounts of hexachloroethane to give a halogenophosphine, chlorotrimethylsilane, and tetrachloroethene. The reaction of the silylphosphine and hexachloroethane in a 2:1 mole ratio also provides a route to tetraorganodiphosphines.

A full account has now appeared of the reactions of tri-t-butylphosphine with germanium and tin tetrahalides, preliminary details of which were noted in last year’s Report.

Details of new applications of phosphine–carbon tetrachloride and phosphine–halogen reagents in synthesis continue to appear. The use of an insoluble cross-linked polymer-supported phosphine–carbon tetrachloride reagent in peptide synthesis was noted in the previous Report; superior to this is the use of linear soluble polymer-supported phosphine–carbon tetrachloride reagents as condensing agents, enabling peptide synthesis in homogeneous solution. At the end of the reaction period the polymer is precipitated quantitatively and removed by filtration. High yields of dipeptides have been achieved by using this technique. The succinimides (78) mainly undergo chlorination to form (79) on treatment with triphenylphosphine and carbon tetrachloride, although the lactams (80) are also formed, arising from Wittig-type reactions with the ylide (68) that is present in the reaction mixture. Some abnormal reactions of saturated (5α- and 5β-)19-hydroxy-steroids with phosphine–halogen reagents have been reported.

The rates of dehalogenation of α-bromo- and α-iodo-m-cyanobenzyl phenyl sulphones (81) in aqueous DMF by series of alkyldiphenyl- and substituted triarylphosphines have been studied. The reaction of optically active benzyl(methyl)phenylphosphine with (81) proceeds with inversion of configuration at phosphorus, consistent with a mechanism involving attack of phosphine on halogen followed by hydrolysis of an intermediate halogenophosphonium salt.

Rate data for the triphenylphosphine-promoted dehalogenation of meso-1,2-dibromo-1,2-diarylethanes to form trans-stilbene in DMF are consistent with a concerted anti-elimination mechanism involving the attack of phosphine on bromine. Attack of triphenylphosphine on halogen also occurs in its reaction with the N-chloropyrrolidine-2,5-diones (82), leading eventually to the betaines (83). The reaction of triphenylphosphine with trichloro(phenyl)methane gives the cis-olefin and dichlorotriphenylphosphorane.

(Continues…)Excerpted from Organophosphorus Chemistry Volume 9 by S. Trippett. Copyright © 1978 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.