Professor David Allen is Emeritus professor of chemistry at the Sheffield Hallam University, UK. His main research interests are in phosphonium salts and related compounds. Current interests include the preparation of phosphonioalkyl derivatives of biologically active molecules, the phosphonioalkyl group facilitating the passage of the biologically active agent through cell membranes, and studies of the formation of biologically active surface-functionalised gold nanoparticles.

Organophosphorus Chemistry Volume 36

A Review of the Literature Published between July 2004 and December 2005

By D.W. Allen, J.C. Tebby

The Royal Society of Chemistry

Copyright © 2007 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-354-5

Contents

Preface John Tebby and David Allen, 7,
Phosphines and related tervalent phosphorus systems D. W. Allen, 15,
Phosphine chalcogenides, phosphonium salts and P-ylides G. Keglevich, 73,
Tervalent phosphorus acid derivatives A. T. Hewson, 121,
Quinquevalent phosphorus acids A. Skowronska and K. Owsianik, 135,
Pentacoordinated and hexacoordinated compounds J. C. Tebby, 184,
Nucleic acids and nucleotides: mononucleotides Marie Migaud, 197,
Nucleotides and nucleic acids; oligo- and polynucleotides David Loakes, 232,
Phosphazenes J. C. Van de Grampel, 313,

CHAPTER 1

Phosphines and related tervalent phosphorus systems

D. W. Allen

DOI: 10.1039/b603257p

1 Introduction

As this chapter covers eighteen months of the literature relating to the above area, it has been necessary to be somewhat selective in the choice of publications cited. Nevertheless, it is hoped that most significant developments have been noted. The period under review has seen the publication of a considerable number of review articles, and most of these are cited in the relevant sections. Papers from the 16th International Conference on Phosphorus Chemistry, held in the UK in July 2004, have been published in a special issue of the Journal of Organometallic Chemistry, (2005, 690, issue 10). A special issue of Tetrahedron: Asymmetry, (2004, 15, issue 14), devoted to the synthesis and catalytic applications of chiral phosphine ligands, also contains much of interest and this area has continued to be a main stimulus for new work in phosphine chemistry. A theoretical study of the influence of phosphorus-containing substituents on organic molecules also has general interest in phosphine chemistry.

2. Phosphines

2.1 Preparation

2.1.1 From halogenophosphines and organometallic reagents. This route continues to be widely applied, with most work involving the use of organolithium reagents, Grignard reagents now finding few applications. Grignard routes have, however, found use for the synthesis of fluoroarylphosphines, e.g., (1), one of a range of electron-poor fluoroarylphosphines investigated as ligands for rhodium-promoted hydroformylation, in which they behave poorly in comparison with arylphosphite ligands, and also the pincer-diphosphine (2). The low-temperature reactions of perfluorovinyl lithium (generated from CF3CH2F and two equivalents of n-butyl lithium) with diorganochlorophosphines have given a range of perfluorovinylphosphines (3), which have proven to be air- and moisture-stable over a period of months. The reactions of these phosphines with a variety of reagents have also been studied with respect to reactions both at phosphorus and at the perfluorovinyl substituent. Considerable interest has been shown in the synthesis of sterically-crowded triarylphosphines. Two groups have reported the use of organolithium reagents for the preparation of a series of ‘bowl-shaped’ phosphines, e.g., (4), these having improved ligand properties in some metal-catalysed reactions compared with conventional triarylphosphines and P(t-Bu). The reaction of methyl lithium with (2,6-dimesitylphenyl)dichlorophosphine has given the phosphine (5). Arylcopper reagents derived from an aryl Grignard precursor by treatment with copper(I) chloride have been used to arylate the crowded diarylchlorophosphine (6) to give the phosphines (7).

However, overall yields were low (7–15%) as a result of a reductive side reaction of the chlorophosphine to form the dibenzophosphole (8). Also reported in the same paper were the phosphines (9), again prepared via the use of arylcopper reagents. Among other bulky arylphosphines prepared via organolithium or Grignard routes are the biphenylyl system (10), the phenanthrylphosphine (11), and the binaphthophosphepins (12). Low temperature organolithium routes have been used to prepare series of crown ether-functionalised arylphosphines (13) and poly (dimethylsiloxane)-derived phosphines (14) and related phosphinite esters.

Low temperature lithiation of 2,5-dimethylazaferrocene followed by treatment with chlorodiphenylphosphine yields a mixture of phosphines (15) and (16), separable by chromatography, and a similar procedure has given a series of phosphinocyclo-pentadienylrhenium ligands, e.g., (17). Direct lithiation of five-membered ring heteroaromatic compounds has continued to be applied in the synthesis of related heteroarylphosphines. New phosphines of this type include series of 3,4-ethylene-dioxy-substituted-2-thienylphosphines (18) and 2-indolylphosphines, e.g, (19). An attempt to prepare the 2-selenophenylphosphine (20) by this route resulted in the isolation of only the phosphine oxide. Among other related heteroarylmonophosphines prepared are the phosphinotriazoles (21), and a family of modular chiral phosphinothienyl-oxazolines, e.g., (22). Structurally similar phosphinophenyl-oxazolines (23) and -pyrazoles (24) have also been prepared by organolithium (and other) routes. Halogen-lithium exchange has continued to be applied in the synthesis of the new pyridylphosphines (25). Direct lithiation at the methyl group of 2-methylpyridines has been used again to prepare further examples of chiral phosphinomethylphosphines, e.g., (26).

Organolithium-halogenophosphine procedures have also been used in the synthesis of a variety of monophosphines bearing other donor groups. Among these are amidoarylphosphines, e.g., (27) and (28) (and its aminoalkyl reduction product),the phosphinoacetals (29) and (30), the ketiminophosphine (31), and the peri-disubstituted naphthalene (32), in which peri-N -> P interactions may be present.

The organometallic reagent-halogenophosphine route has also been applied widely in the synthesis of new diphosphines. Among new systems having aromatic backbones are the diphenylether (33), and the terphenyl (34). Rigid phosphinoarylene-alkenes, e.g., (35), have been designed as ‘Precipiton’ reagents for use in the Staudinger reaction, the phosphine oxide side-products undergoing photoisomerisation from a soluble Z-form to an insoluble E-form, thereby aiding product separation. The colourless bisphosphinodithienylethylene (36) undergoes photoisomerisation on irradiation with 313 nm light to form (37), purple in colour, which reverts to (36) on irradiation with light of > 434 nm, thereby providing a basis for photoswitchable devices.

A wide range of new C1-symmetric diphosphine ligands, (38) and (39), having a heteroaryl-aryl bridge, is afforded by direct double lithiation of heteroaryl-arenes with butyl lithium in the presence of TMEDA, followed by treatment with chlorodiphenylphosphine. A similar strategy has given a series of atropisomeric chiral diphosphines (40), based on a thienyl-camphor system.

Double deprotonation of 1,2-dibromo-4,5-difluorobenzene and 1-bromo-2-chloro-4.5-difluorobenzene using LDA at low temperature is the key step in the synthesis of the para- bis(phosphino)phenylenes (41). Related work starting with 1,4-dibromo-2.5-difluorobenzene led to the isolation of the 1,2,4,5-tetraphosphinobenzenes (42). A direct lithiation approach has also been used to prepare further examples of diphosphine ligands based on metallocene units such as ferrocene and arenechromium tricarbonyls. Among new ferrocene systems reported are the diphosphinoferrocenophane (43), ferrocenyl bis(phosphines) bearing sulfinyl, sulfonyl or sulfenyl groups, planar chiral diphosphinoferrocene-oxazoline ligands, and the chiral JOSIPHOS-type diphosphines (44)

A modular approach has been developed to a family of chiral diphosphines (45) based on the benzene chromium tricarbonyl unit, and offering a wide range of substituents at phosphorus. This work has also been extended to include the synthesis of the chiral [indane-Cr(CO)3]-based diphosphines, (46). Treatment of 1,2-bis(dichlorophosphino)ethane with 4-(methylthio)phenyl lithium has given the diphosphine (47, R = Me), which can be demethylated with sodium in liquid ammonia to form the related tetrathiolated diphosphine (47, R = H). Alkylation of bis(dichlorophosphino)methane with a benzylic Grignard reagent has given (48)

2.1.2 Preparation of phosphines from metallated phosphines. Metallophosphide reagents have been used for the synthesis of a wide range of new phosphines. Lithio-organophosphides remain the reagents of choice, although the sodium-, potassium-, and caesium-analogues also continue to be used. The structure of lithium diphenylphosphide in solution is very much dependent on the solvent, according to Li pulsed gradient spin-echo diffusion methods. In THF, it exists as a mononucleated solvated species, whereas in diethyl ether, a dinuclear structure is found. The reactions of lithium diorganophosphides with ate-type copper carbenoids derived from lithio (triorganosilyl)dichloromethanes provide a route to new bulky, highly basic but air-stable mono- and di-phosphines, e.g., (49). Lithium diphenylphosphide has been shown to cause a regio- and stereo-specific ring-opening of silyl- and disilyl-epoxides, with the loss of a silyl substituent, giving vinylphosphines, e.g., (50), usually isolated as the corresponding oxides or methylphosphonium salts. Fluorobenzenechromiumtricarbonyl complexes undergo nucleophilic displacement of fluoride ion on treatment with borane-protected P-chiral lithiophosphides to give a series of new P-chiral phosphinoboranes, (51), in which the stereochemistry at phosphorus is retained during the SNAr process. A new route to sterically-protected 1,4-diphosphafulvenes (52) is provided by treatment of secondary aryl (ethynyl)phosphines with a ca. 0.25 molar amount of butyllithium, the reaction involving intermediate phosphaallenes and related anions. The key step in the synthesis of the new 2-phosphabicyclooctane ligand (53) is the reaction of the dilithium derivative of (3,5-di-t-butyl-4-methoxyphenyl)phosphine with a chiral cyclic sulfate ester, the phosphine being stored as either a borane derivative or as a salt with HBF4.

An interesting range of new phosphines bearing other donor groups has also been obtained via the use of lithioorganophosphide reagents. Included in this group is a series of chiral 5-diphenylphosphino-1,2,3,4-tetrahydroacridines, e.g., (54), derived from terpenes and steroids, chiral phosphinocarboxylic acid derivatives, e.g., (55), derived from myrtenal, and various phosphino-pyridines, e.g., (56), -oxazolines, e,g., (57) and -alkylamines, e.g., (58). Also reported are the electron-rich, bulky, methoxybenzylphosphine (59), and the phosphinoisoindole (60).

Diisopropyl- and dicyclohexyl-carbodiimides insert into the lithium-phosphorus bond of lithium diphenylphosphide to form structurally-associated lithium salts which, on protonation, give the guanidinophosphines (61). Treatment of the dilithiophosphide ButPLi2 with cyclohexylisocyanide resulted in the consumption of six molar proportions of the latter with the eventual formation of the salt (62). Lithioorganophosphide reagents have also been employed in the synthesis of a range of di- and tri-phosphines. Among new diphosphine ligands are the diphosphinoether (63), the DIOP analogues (64), the diphosphinoquinoxaline (65), the anthracene-bridged P,N,N’ donor system (66), the bisphosphinocyclopropane (67) and a range of related phosphinothioethers.

Also reported are the diphosphinoindane (68), and the 1,3-diphosphine (69). Monolithiation of the primary arylphosphine (70), followed by treatment with dichlorophosphinoferrocene has given the air-stable bulky 1,3-dihydrotriphosphine (71) as a mixture of three diastereoisomers.

Various benzylic diphosphines have also been reported, including the PCP-pincer ligands (72) and (73), the C-symmetric 1,4-diphosphines (74), the biphenyl (75) and the terphenyl (76).

The dilithiophosphide-cyclic sulfate route has found further application in the synthesis of the bisphosphetane (77) and a related bis(phospholane). Among other new chiral bis(phospholanes) also prepared in this way are a series of alkylene-bridged systems, e.g., (78), the thienyl system (79), and a series of P-chirogenic bis(phospholanes), e.g, (80).

The reaction of lithiophosphides with P-chlorodiazaphospholene has given novel P–P bonded compounds, e.g., (81), in which the P–P bond is substantially longer than in conventional diphosphines, thought to be a result of its increased polarity arising from the diazaphospholenium cation-phospholide anion combination. Treatment of the bisphosphinophosphide (82) with dicyclopentadienyltitanium dichloride results in the formation of the hexaphosphine system (83).

There has also been a limited use of sodio- and potassio-organophosphide reagents in phosphine synthesis. Among new chiral monophosphines prepared using sodiophosphides are the ferrocene (84), the heterocyclic phosphine (85), the α-methylbenzylphosphine (86), and the water-soluble phosphine (87).

This approach has also been used to prepare the chiral diphosphines (88) and (89), and a series of new polyphosphines, e.g, (90). Potassium organophosphides have been the reagents of choice for the synthesis of the phosphinoarylimidazoline (91), the chiral phosphinoarylpyridines, (92), phosphine-functionalised N-het-erocyclic carbene-precursor ligands, e.g., (93), the chiral diphosphine (94), and the calix[4]arene tetraphosphine (95).

Further studies of the reactions of red or white phosphorus with KOH-dioxan ‘superbase’ systems have appeared, subsequent reactions of the generated phosphides with allyl halides having given a mixture of isomeric allyl- and prop-1-enyl-phosphine oxides. The reactions of alkali metal tris(t-butyl)silylanides with white phosphorus have given a range of metallo(silyl)polyphosphides which includes the pπ-bonded organophosphide (96). The reactions of metallic sodium or potassium with organodichlorophosphines and phosphorus trichloride have given various alkali metal tetraorganylcyclopentaphosphides and tetraorganyltetraphosphine-1,4-diides. A series of disecondary phosphines PhPH(CH2)nPHPh (n = 1-6) has been obtained from the caesium hydroxide-promoted reactions of alkylene dibromides with phenylphosphine in DMF.

The synthesis of organophosphido derivatives of metals other than those of main group 1 has continued to attract interest. Organophosphide complexes of calcium, strontium and barium have been prepared from the potassiophosphide (97). The reagents LiAl(PH2)4 and LiAl(PHMe)4 have been used to prepare the silylphosphines [Me4Si2(PHR)2] (R = H or Me), from which a range of Si/P/Li complex cage phosphides has been derived by treatment with butyllithium. New organophosphido complexes of aluminium, gallium, and indium have been described and the structural chemistry of organophosphido complexes of these metals has been reviewed. Unusual phosphido complexes of tin(II) and germanium(II)have also been prepared and characterised. Studies of the reactivity of lanthanum organophosphide complexes have continued to attract attention. Studies of the protonation of an alkyllanthanum organophosphide have shown that protonation occurs at the P–La bond rather than at the C–La bond. The scope and mechanism of the reactions of lanthanum organophosphides in the synthesis of phosphine-terminated polyethylenes have also received detailed study. Strategies for the synthesis of chiral, bidentate bis(perfluoroorganyl)phosphines involving the stabilisation of the P(CF3)2- ion as a complex with tungsten pentacarbonyl (or alternative silver or mercury acceptors), and also the use of the P(CN)2- ion as an intermediate for the introduction of bis(pentafluorophenyl)phosphino units, have been reviewed. Organosilylphosphide complexes of tungsten pentacarbonyl have also been studied. The reactivity of organophosphide complexes of iron, palladium, and gold has received attention. Synthetic and structural studies of zinc- and cadmium-complexes of the di(benzothiazol-2-yl)phosphide ion, and organophosphido-zincate complexes, have also attracted interest.

Synthetic applications of phosphines metallated at atoms other than phosphorus have continued to appear. Straightforward applications of lithiomethylphosphines have given the hybrid donor ‘scorpionate’ ligand (98), the anionic phosphine [Ph2PCH2SnB11H11]-, based on a stanna-closo-dodecaborate system, and a series of new 2-dimethylphosphinoethanols, (99).

(Continues…)Excerpted from Organophosphorus Chemistry Volume 36 by D.W. Allen, J.C. Tebby. Copyright © 2007 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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