Organophosphorus Chemistry Volume 28

A Review of the Literature Published Between July 1995 and June 1996

By D.W. Allen, B.J. Walker

The Royal Society of Chemistry

Copyright © 1997 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-0-85404-314-9

Contents

Chapter 1 Phosphines and Pbosphonium Salts By D. W. Allen, 1,
Chapter 2 Pentaco-ordinated and Hexaco-ordinated Compounds By C. D. Hall, 64,
Chapter 3 Tervalent Phosphorus Acid Derivatives By O. Dahl, 80,
Chapter 4 Quinquevalent Phosphorus Acids By R. S. Edmundson, 103,
Chapter 5 Nucleotides and Nucleic Acids By J. A. Grasby and D. M. Williams, 170,
Chapter 6 Ylides and Related Compounds By B. J. Walker, 237,
Chapter 7 Phosphazenes By C. W. Allen, 285,
Chapter 8 Physical Methods By R. N. Slinn and J. C. Tebby, 328,
Author Index, 359,

CHAPTER 1

Phosphines and Phosphonium Salts

BY D.W. ALLEN

1 Phosphines

1.1 Preparation

1.1.1 From Halogenophosphines and Organometallic Reagents. – The application of organolithium reagents has once again dominated this route to phosphines. An improved route to tri-2-furylphosphine (1) is provided by treating furan with butyllithium, followed by cerium trichloride, and then with phosphorus trichloride. Difficulties continue in attempts to prepare the pyridylphosphine (2) by the lithiation of 2,6-dibromopyridine and subsequent treatment with phosphorus trichloride. Terpyridyl systems are the main products, rather than the desired phosphine. A range of functionalised quinolinylmethylphosphines (3) has been prepared. The reaction of 2,2′-dilithiobiphenyl with chlorodiphenylphosphine has been revisited, and the anomalous formation of 5-phenyldibenzophosphole and triphenylphosphine, rather than the diphosphinobiphenyl (4), confirmed. The latter can, however, be prepared by Ullman coupling of o-iodophenyldiphenylphosphine oxide, followed by reduction with trichlorosilane. A study of the energy barrier to axial torsion in the biphenyl (4) reveals that, even if it could be resolved into pure enantiomers, rapid racemisation would occur at temperatures greater than 25°C. Direct metallation at carbon, followed by treatment with chlorodiphenylphosphine, has been used to prepare a range of new diphosphines, e.g., (5), (6), and (7), the latter system having been resolved into chiral forms. Diphosphines bearing other functional groups, some of which are chiral, e.g., (8), (9), and (10), have also been prepared. A route to the benzenetricarbonylchromium-based diphosphine (11) has also been developed. Coordination-directed metallation of hydrazone systems, followed by treatment with chlorodiphenylphosphine, provides a route to the functionalised phosphines (12) and (13). The latter can be easily converted into the chiral α-phosphinoketones (14).

Metallation of 1,3,5-tricyanocyclohexane, followed by treatment with chlorodiphenylphosphine results in the formation of the cis-triphosphine (15), in which all the phosphino centres are available for coordination in a ‘facial’ manner. The organolithium-halogenophosphine route has been applied extensively in the synthesis of ferrocenyl phosphines. New approaches for asymmetric synthesis of chiral ferrocenylphosphines have been reviewed. An improved route to the ferrocenyldiphosphine (16) has been described. Lithiation of the ferrocene system ortho to a chiral group containing an appropriate donor atom has been applied in the synthesis of a considerable number of new systems. Four groups have described the use of chiral oxazolinyl substituents at ferrocene for this purpose, resulting in a range of chiral oxazolinylf errocenylphosphine hybrid ligands, e.g., (17) and (18). Among other chiral systems prepared in a similar manner are the ferrocenophanes (19), the chiral aminoalkylferrocenyl-phosphines, e.g., (20), and the chiral dioxolanyl system (21), a precursor of the aldehydo-phosphine (22), which, with ethylenediamine, yields a tetradentate bis(iminophosphine), having planar chirality. A similar approach has been used in the synthesis of other phosphinoferrocenes having planar chirality. Enantioselective ortholithiation of several aminomethylferrocenes with butyllithium in the presence of a chiral diamine, followed by treatment with chlorodiphenylphosphine, has given a series of ferrocenylphosphines having planar chirality, e.g., (23). The ortholithiation route has also been used in the synthesis of the related ruthenocenyl system (24). Metallation of ferrocenecarboxaldehyde with the lithium derivative of monomethylpiperazine, followed by treatment with chlorodiphenylphosphine, provides a one-pot route to the 1,1′-disubstituted system (25) in moderate yield. A series of phosphaferrocenophanes (26) has been prepared by the reaction of 1,1′-dilithioferrocenes with halogenophosphines. On heating, ring-opening polymerisation occurs to give the polymeric phosphines (27).

Both alkynyl-lithium and alkynyl-Grignard reagents have been used in the synthesis of sterically protected diethynylphosphines and dibutadiynylphosphines, e.g., (28). The reaction of chlorodiethylphosphine with the reagent BrMgC [equivalent to] CMgBr has given the diphosphinoacetylene (29). Cyclohexylmagnesium bromide has been used in the synthesis of the chiral diphosphine (30) from trans-1,2-dichlorophosphinocyclopentane. The Grignard route has also been applied in the synthesis of a series of new chiral phosphines having binding sites of different hardness, e.g.,(31).

In a rare example of the application of other organometallic reagents in phosphine synthesis, the zirconacyclopentanes (32) have been shown to react with dichloro- and monochloro-phosphines to give the heterocyclic phosphines (33) and the diphosphines (34), respectively.

1.1.2 Preparation of Phosphines from Metallated Phosphines. – Both lithium- and sodium-bis(o-methoxyphenyl)phosphides have been isolated as unsolvated solids, the former from the reaction of bis(o-methoxyphenyl)phosphine with butyllithium, and the latter from treatment of tris(o-methoxyphenyl)phosphine with sodium in liquid ammonia. Both reagents decompose in solution in THF over several days. Structural and NMR studies of the dilithiodiphosphides (35) have been reported. The first structural characterisation of a dilithiophosphandiide, in the form of a complex with a ftuorosilane, has been achieved. Organophosphide anions stabilised by coordination to borane have synthetic advantages over the free organophosphides in being only mildly basic. Such reagents have been used to convert chiral ditosylates to chiral diphosphine-borane complexes, from which the free chiral diphosphines, e.g., (36), are easily liberated. Such reagents have also found use in the synthesis of phosphinofullerenes. The triarylphosphine (37) undergoes the expected cleavage of an aryl-phosphorus bond on treatment with lithium metal in THF, and treatment of the resulting diarylphosphide reagent with a chiral ditosylate has given the chiral diphosphine (38). This has been shown to undergo sulfonation at the terminal benzene rings to give a chiral, water-soluble diphosphine, which also shows surface active properties. A related chiral ditosylate-lithium diphenylphosphide route has been employed in the synthesis of the chiral diphosphine (39). Similar routes have been used for the synthesis of the new chiral systems (40), (41), and (42).

Lithiophosphide reagents have also found application in the synthesis of a range of chiral phosphines based on carbohydrate systems, e.g., (43), the key step being nucleophilic ring-opening of epoxide derivatives with lithium diphenylphosphide. A lithiophosphide-tosylate route has been used in the synthesis of the carbohydrate-based diphosphine (44). Conjugate addition of lithium diphenylphosphide to α,β-unsaturated carboxylic esters is the key step in the synthesis of β-phosphinocarboxylic acids, e.g., (45). The ferrocenylborylphosphine system (46), in which there are ‘through space’ intramolecular boron-phosphorus interactions, has been prepared by the reaction of lithium diphenylphosphide with dibromodiborylferrocene precursors. Treatment of the cyclopropenium salt (47) with lithium diphenylphosphide results in the formation of the cyclopropenylphosphine (48), which does not undergo thermal or photochemical rearrangement or ring-opening. A range of new amphiphilic phosphines, e.g., (49), containing polyether chains, has been prepared via the use of phosphide reagents obtained by lithium-induced cleavage of a phenyl group from either octyldiphenylphosphine or isopentyldiphenylphosphine, and their subsequent reactions with chloroalkyl ethers. The phosphide reagent derived from the secondary phosphine (50, R=H) has been alkylated to give the tertiary phosphines (50; R=PhCH2 or MeOCH2). Several groups have reported lithiophosphide routes to tripodal polydentate mixed donor phosphine ligands, e.g., (51), (52), (53), and (54). Lithiophosphide reagents have also been employed in the synthesis of a range of pyridylphosphines, e.g., (55), (56), and (57). The chiral phosphinoaryloxazolines (58) have been obtained by nucleophilic displacement of fluorine from the related o-fluoroaryl systems, using lithium diarylphosphide reagents. Dilithiodiphosphide reagents are key intermediates in the synthesis of the heterocyclic systems (59) and (60), and a range of macrocyclic diphosphines, e.g., (61). The difunctional reagent, dilithiumphenylphosphide, has also been used in heterocyclic synthesis. With 1,3-dichloropropane or 1,2-dichloroethane, the simple phosphetane (62) and phosphirane (63), respectively, result, both of which can be isolated by vacuum distillation. The phosphetane rapidly polymerises in the neat state, but is stable in solution in benzene, in which it has a remarkably low field 31P NMR shift (13.9 ppm). In contrast, the phosphirane exhibits an even more remarkable high-field shift (–236 ppm). The selenium-phosphorus system (64), isolated as the related phosphine oxide, has been obtained from the reaction of dilithiumphenylphosphide with bis(o-bromomethylphenyl)selenide. This phosphide reagent has also been employed in the synthesis of a range of six-membered P2B4 heterocycles. Dilithio-organophosphide reagents have also been used in the synthesis of the phosphines (65), albeit in lowish yield. It has been shown that whereas methyl ftuoroformate reacts with an equimolar quantity of lithiumbis(trimethylsilyl)phosphide to give an inseparable mixture of tris(methoxycarbonyl)phosphine (66) and tris(trimethylsilyl)phosphine, colourless crystals of a lithium bis(methoxycarbonyl)phosphide-dimethoxyethane solvate are obtained in high yield from the reaction of the ftuoroformate ester with lithium phosphide in a 2:3 molar ratio. Protonation of the bis(methoxycarbonyl)phosphide yields the secondary phosphine (67), which, unlike other diacylphosphines, does not show the presence of an enol tautomer even in non-polar solvents. Lithiumbis(diorganophosphino)phosphide reagents have been used in the synthesis of phosphino-phosphinidene-phosphoranes, R2PP=P(R)Bu12, involving two-, three-, and four-coordinate, directly bonded phosphorus atoms. A wide range of alkali metal phosphide reagents has been utilised in the synthesis of silylphosphine systems, including an improved preparation of tris(trimethylsilyl)heptaphosphine, and a variety of heterocyclic Si-P compounds. The phospholenes (68) have been obtained from the reactions of the 1-lithiophospholenide reagent with group 14 halides.

Applications of sodio- and potassio-organophosphide reagents also continue to appear. A mixed sodio-potassio-diphenylphosphide has been used in the synthesis of the chiral systems (69). Interest has also continued in exploring SRN1 processes involving metallophosphide reagents in liquid ammonia, some of which are synthetically useful. Potassiophosphide reagents have been applied in the synthesis of the quinolylphosphine (70), the ferrocenyl ligand (71), and the water-soluble triphosphine (72). The heterocyclic systems (73) have been isolated from the reactions of 1,4- and 1,5-dihaloalkanes with the phosphide reagent generated from red phosphorus and potassium hydroxide in aqueous dioxan, in the presence of phase-transfer catalysts. Dipotassiodiphosphide reagents have been used in the synthesis of heterocyclic P-Sn and P-B systems. Alkylation of 1,2-bis(phosphino)benzene (in the form of a copper(I) complex) can be achieved in the presence of potassium t-butoxide and alkyl halides, enabling the synthesis of, e.g., the heterocyclic systems (74) and (75), and the bis(secondary)phosphine (76).

Once again, there has been strong interest in the synthesis and characterisation of organophosphido derivatives of other main group elements, notably aluminium, gallium, indium, and germanium, tin, and lead. In addition, organophosphido derivatives of lanthanide, actinide, and d-block transition elements, have been described.

Interest also continues in the chemistry of phosphines metallated at an adjacent carbon. The use of phosphinomethanide reagents in the synthesis of novel heteroorganic compounds has been reviewed, and further examples described. The borane adduct of methyldiphenylphosphine can be metallated at the methyl group using sec-butyllithium, to generate the borane-protected reagent (77), the key intermediate in the synthesis of the tripod ligands (78). Arylphosphines bearing organosilyl substituents, e.g., (79), have been obtained via the intermediacy of C-metallated systems. A further study of the products of iodine-oxidation of the bis(diphenylphosphino )methanide ion has been reported.

1.1.3 Preparation of Phosphines by Addition of P-H to Unsaturated Compounds. – This route has not received much attention over the past year. A stereoselective synthesis of tris(Z-styryl)phosphine is offered by the addition of phosphine to phenylacetylene in a superbasic system (HMPA-H2O-KOH). In a similar vein, the reaction of phosphine with styrene and a-methylstyrene in a superbasic medium (DMSO-KOH) provides a route to the primary phosphines, (2-phenylethyl)phosphine and (2-methyl-2-phenylethyl)phosphine, respectively. Transition metal phosphine complexes have been shown to catalyse the α-hydroxylation, β-cyanoethylation, and β-alkoxycarbonylethylation of phosphine. Addition of primary phosphines to acrylic esters has been used for the synthesis of the phosphines (80). A similar addition of diphenylphosphine to acrylic esters and amides has given a series of hydrophilic phosphines (81). The bis(phosphorinanyl)ethane (82) is formed in the photochemical addition of 1,2-bis(phosphino)ethane to 1,4-pentadiene.

1.1.4 Preparation of Phosphines by Reduction. – The reduction of phosphine oxides in the final stage of phosphine synthesis remains a common strategy, with silane reagents often being used. Trichlorosilane has found application in the synthesis of the axially chiral systems (83), (84), (85), the chiral diphosphine (86), and the diphosphinobiferrocenyls (87). This reagent has also found use in the synthesis of the phospholene (88), whereas phenylsilane was the reagent of choice for the isomeric system (89). Following protection of (88) and (89) with borane, attempts have been made to achieve ring-expansion on treatment with chlorocarbenes. Phenylsilane has also found use in the synthesis of the poly(arylene-ether-triarylphosphine) system (90). A mild and practical synthesis of secondary P-ethynylphosphines (91) is provided by reduction of the easily available oxide precursors with phenylsilane for the P-alkyl systems, and a mixture of phenylsilane and phenyltrichlorosilane for the P-phenyl system. Various hydrosilane reagents have also been used in the synthesis of the heterocyclic systems (92). Full details have now appeared of the synthesis of the secondary phosphine (93), isolated as one resolved diastereoisomeric form. The chiral bis(primary)phosphine (94) has been obtained by the reduction of a diastereoisomerically pure trans-1,2-cyclo-pentanobisphosphonite ester using lithium aluminium hydride. Aluminium hydride has been employed in the synthesis of a range of arylphosphine-functionalised silasesquioxide materials, precursors for the synthesis of organometallic-functionalised silica gel.

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