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Organophosphorus Chemistry Volume 12
A Review of the Literature published between July 1979 and June 1980
By D. W. Hutchinson, J. A. Miller The Royal Society of Chemistry
Copyright © 1981 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-432-1
CHAPTER 1
Phosphines and Phosphonium Salts
BY D. W. ALLEN
1 Phosphines
Preparation. — From Halogenophosphines and Organometallic Reagents. The Grignard procedure has been used to prepare a range of long-chain (C10 — C19) trialkylphosphines, the p-alkyl-substituted phenylphosphines (1), the substituted vinylphosphines (2) and (3), and a sample of triethylphosphine that is radio-labelled at both phosphorus and the β-carbon.
A mixture of exo- and endo-isomers of the phosphine (4) results from addition of methylmagnesium bromide to a corresponding mixture of isomers of 2-norbornyldichlorophosphine. Surprisingly, the usual mode of addition of the halogenophosphine to the Grignard reagent gives a negligible yield of the phosphines. A convenient Grignard procedure has been described for the synthesis of the 1,2-bisphosphinoethanes (5) from the corresponding bis(dichlorophosphino)ethane, prepared by a modification of the original procedure given in a 1975 patent.
Many examples of the use of organolithium reagents in phosphine synthesis have been reported in the past year. Procedures for the synthesis of the three isomeric tris(trifluoromethylphenyl)phosphines and a range of (perfluoroaryl)-phosphines bearing perfluoroalkyl ether substituents in the aryl rings have been described. A modified preparation of tri-(2-pyridyl)phosphine via the metallation of 2-bromopyridine with butyl-lithium has been developed. Attempts to prepare the corresponding 3-pyridyl- and 4-pyridyl-phosphines from the related bromo-pyridines failed to give clean products. A range of αω-bis(diphenylphosphino)aryl ethers, e.g. (6), has been prepared via the reaction of chlorodiphenylphosphine with the appropriate αω-dilithio-derivatives. Shaw's group have described the synthesis of further chelating diphosphines, e.g. (7), that bear bulky groups at phosphorus and in which the phosphorus atoms are separated by a long alkyl chain.
Allyl(t-butyl)amine undergoes metallation at the terminal carbon of the allyl group, and the reaction of the resulting organolithium reagent with phenyldichlorophosphine gives the heterocycle (8). The reaction of phenyldichlorophosphine with 1,1'-dilithioferrocene yields the phosphino-[1]ferrocenophane (9), which on treatment with organolithium reagents undergoes ring-opening to form the lithium reagent (10); this has been used for the synthesis of mixed phosphino-arsino-ferrocenes.
Interest in the synthesis of chiral phosphines continues, and various methods for the synthesis of such compounds have been reviewed, together with procedures for the synthesis of tertiary phosphines bearing either an o-anisyl or o-di-methylaminophenyl group, involving ortho-metallation of anisole or of NN-dimethylaniline followed by reaction with halogenophosphines. Full details of the preparation of two chiral forms of the phosphinoamine (11), from the reaction of the ortho-lithiated (aminoalkyl)ferrocene with chlorodiphenylphosphine, have now appeared. The use of phosphinites derived from the optically active alkaloid cinchonine in the synthesis of chiral phosphines continues to develop. The diastereoisomeric phosphinites (12) can be separated by crystallization of the related copper(I) cyanide complexes. The reaction of the purified diastereoisomers (freed from copper by treatment with cyanide) with an organo-lithium reagent then gives the chiral phosphines (13) in very high optical purity. In related work, it has been shown that consecutive substitution of chlorine in phenyldichlorophosphine with lithium cinchoninate followed by arylcyano-cuprate reagents leads selectively to the corresponding (R)-phosphinite esters, which are then converted into the chiral phosphines as described above, by treatment with an organolithium reagent. Similar studies of the reactions of organolithium reagents with diastereoisomeric phosphinites derived from menthol have also been reported.
Treatment of acetate esters with lithium bis(trimethylsilyl)amide and diorganohalogenophosphines affords a synthesis of the functionalized phosphines (14). The reaction of phosphorus trichloride with (cyanomethyl)tributylstannane gives a much improved route to the (cyanomethyl)phosphine (15), described as an air-stable, crystalline solid, of low nucleophilic reactivity.
Preparation from Metallated Phosphines. The past year has seen a considerable number of applications of metallo-phosphide reagents in the synthesis of a wide range of new phosphines, many of which are of interest as chiral ligands in transition-metal complexes that are used as catalysts for asymmetric hydrogenation and other reactions.
The reactions with alkyl halides of lithio-phosphide reagents, obtained by cleavage of phenyl groups from alkyldiphenylphosphines by using lithium in THF, have been used to prepare a range of chiral, unidentate phosphines of the type PhPR1R2 (R1 = Me, Et, or Pr; R2 = Et, Pr, or C6H11). The same paper reports the reaction of lithium cyclohexyl(phenyl)phosphide with the dimesylate of 1,4-butanediol, giving the chiral chelating diphosphine (16).
The reactions of lithium diphenylphosphide with alkyl halides have been used in the synthesis of αω-bis(diphenylphosphino)alkanes, e.g. (17), and with a polymer-bound tosylate to give the polystyrene-bonded (long-chain alkyl)diphenylphosphine (18). Treatment of the readily accessible spiro-hydrocarbon (19) with lithium diphenylphosphide gives the isomeric phosphines (20). In related work, (lithiomethyl)diphenylphosphine has been used in the synthesis of the (phosphinoalkyl)cyclopentadienide ligand (21), from which a series of (phosphinoalkyl)metallocenes and related macrocyclic complexes may be prepared.
Sodium diphenylphosphide reacts with the mesylate of (S)-s-butyl alcohol to give (R)-s-butyldiphenylphosphine, and with the tosylate (22) it gives, unexpectedly, the phosphine (23), in which the carbon skeleton has been rearranged. This reagent also causes the cleavage of the aryl-oxygen bond in coumaran, giving a route to the phenolic phosphine (24).
Potassium diphenylphosphide has been used in the synthesis of the chiral (β-aminoalkyl)phosphine (25), derived from ephedrine.
The reaction of lithium dimethylphosphide (conveniently obtained by cleavage of dimethylphenylphosphine by lithium, in THF) with o-dichlorobenzene gives a route to the otherwise difficultly accessible diphosphine (26; X = PMe2). Similar reactions with a range of o-bromophenylalkyl ethers, amines, arsines, and stibines yield the chelating ligands (26; X=OMe, SMe, NMe2, AsMe2, or SbMe2). In related work, the reaction of sodium methylphenylphosphide (obtained from the cleavage of methyldiphenylphosphine with sodium in liquid ammonia) with o-dichlorobenzene yields a mixture of the diastereoisomeric forms of the diphosphine (27), which have been resolved via a new large-scale procedure involving palladium complexes of optically active amines.
The reactions of lithio-phosphides derived from bis-secondary phosphines with alkyl halides have been employed in the synthesis of a range of macrocyclic phosphines, e.g. (28) and (29). A similar procedure, using a related lithioarsenide, has given the phosphino-arsino-macrocycle (30).
The reactions of metallo-phosphides with tosylates have been widely employed in the synthesis of chiral, chelating diphosphines. In some cases, naturally occurring chiral substances have been used as the starting material, thereby eliminating the need for optical resolution at some stage. Thus (31) is derived from tartaric acid, and (32) and (33) are derived from carbohydrates. Amongst other new systems prepared are (34), (35), and (36). Polymer-bound chiral diphosphines have also been prepared.
Chiral unidentate phosphines have also been prepared by the tosylate route from mannitol, xylose, and glucose.
Metallo-phosphide reagents have also been used in a new route to (β-hydroxy-ethyl)phosphines, in the synthesis of a range of multidentate phosphines, including the hitherto unknown methylidynetrisphosphine (Me2P)3CH, and in the preparation of a number of new PP-diphosphines bearing bulky alkyl groups. Primary phosphines co-ordinated to transition metals also undergo metallation, and the resulting co-ordinated metallo-phosphide may be alkylated or treated with halogenophosphines to form co-ordinated PH-functional di-, tri-, and tetra-phosphines.
The reactions of lithium bis(trimethylsilyl)phosphide with dichlorophosphines give a series of silylated triphosphines; on heating, these are converted into cyclic polyphosphorus systems. Baudler's group has continued to apply dimetallated phosphide reagents in the synthesis of both homocyclic and heterocyclic phosphines. Among new systems reported in the past year are the three-membered-ring P2Si and As2P heterocycles. The synthesis and properties of three-membered-ring phosphines have been reviewed.
Preparation by Addition of P — H to Unsaturated Compounds. This method continues to be used extensively for the preparation of multidentate phosphine ligands, which may also involve other donor atoms. A key step in the synthesis of the tetraphosphine (37) is the radical-catalysed addition of 1,3-bis(phenylphosphino)propane to allyl alcohol. Photochemically induced addition of diphenylphosphine to unsaturated silanes has been employed in the synthesis of the phosphines (38) and (39). The latter, on treatment with cyclopentadienyllithium, followed by butyl-lithium, is converted into a ligand similar to (21), bearing both phosphine and cyclopentadienide-anion donor sites. Aminopolyphosphine ligands, e.g. (40), prepared by the photochemical addition of 2-cyanoethylphosphine to two equivalents of vinyldiphenylphosphine followed by reduction of the cyano-group, have been anchored to a controlled-pore glass support, and subsequently converted into a transition-metal catalyst system. Meek's group has also described the synthesis of a variety of new multidentate ligands, e.g. (41), by a combination of addition of P — H to unsaturated systems with other well-established synthetic routes in phosphorus chemistry. Addition reactions of diphenylphosphine to vinyldiphenylphosphine that is co-ordinated to a transition metal, and of co-ordinated diphenylphosphine to vinyldiphenylphosphine, have also been reported. Full details have now appeared of the addition of secondary phosphines to alkynyldiphenylphosphines that are coordinated to platinum or palladium.
Examples of base-catalysed addition reactions have also been reported. Addition of the bis-primary phosphine (42) to diphenylvinylarsine yields the new ligand (43). Base-catalysed addition of neomenthylphenylphosphine [obtainable from the naturally occurring, inexpensive (–)-menthol] to diphenylvinylphosphine gives the diphosphine (44), which can be separated (by fractional crystallization) into pure diastereoisomers that differ only in the configuration of the chiral phosphorus atom; this is the first example of a self-resolving chiral ditertiary phosphine. The related compounds (45) and (46) have also been obtained as mixtures of diastereoisomers, by the addition of neomenthylphenylphosphine to phenyldivinylphosphine and dimethylvinylphosphine sulphide, respectively.
Preparation by Reduction. Reduction of phosphine oxides with trichlorosilane has been used in the synthesis of unsaturated heterocyclic phosphines, e.g. (47), and of the chiral phosphines (48) and (49). The latter is one of the most easily prepared optically active chelating diphosphines, and it gives one of the highest optical yields reported for asymmetric hydrogenation reactions that are catalysed by phosphine–rhodium(I) complexes. Phosphine oxides with at least one phosphorus–aryl bond are reduced to the phosphine by equimolar amounts of magnesium and dicyclopentadienyltitanium dichloride in boiling THF. The titanium reagent is an inoffensive solid which can be handled in air, and is commercially available. Although easier to use than the silane reagents, its action would appear to be less general. Reduction of phosphonates using lithium aluminium hydride has been employed in the synthesis of the methylene-diphosphines (50). A wide range of phosphine dihalides has been reduced to the tertiary phosphines, using hydrogen under pressure, in pyridine as solvent.
Mathey has reported further applications of the nickelocene-allyl iodide reagent for the reduction of phosphine sulphides. It has now been shown that this reagent selectively reduces P=S in the presence of P — O bonds or other sensitive, reducible, functional groups, and studies with chiral compounds show that the reductions proceed with full retention of configuration at phosphorus. The new chiral diphosphine (51) has been obtained by reduction, by sodium, of the corresponding disulphide, which is conveniently accessible by Diels–Alder addition of trans-vinylenebisdiphenylphosphine sulphide to the chiral, naturally occurring diene (–)-α-phellandrene.
Miscellaneous Methods. A synthetic route to the phosphorus analogue (52) of the alkaloid carnegine has been developed with the aid of a computer program. A number of applications of (hydroxymethyl)phosphines in the synthesis of heterocyclic systems, e.g. (53) and (54), have been reported. Complete silylation of tris(hydroxymethyl)phosphine gives the (trialkylsiloxymethyl)-phosphines (55), and tris- and bis-(hydroxymethyl)phosphines react with optically active secondary amines to give optically active (aminomethyl)phosphines, e.g. (56). (Aminomethyl)phosphines have also been prepared from the reactions of secondary phosphines with diaminomethanes. The synthesis of the chiral (β-aminoethyl)phosphines (57) from optically active amino-acids has been reported.
Japanese workers have continued to develop the range of chiral N-acylpyrrolidino-diphosphines (58). Coupling reactions with acid chlorides and related compounds have been devised which permit the facile conversion of the chelating diphosphine (59) into a wide variety of water-soluble diphosphines, e.g. (60), which in the form of rhodium(I) complexes are of value as homogeneous catalysts for reactions conducted in an aqueous medium. A range of new chiral aminophosphines, e.g. (61), has been prepared.
In recent years, efforts have been made to anchor phosphine–transition metal catalysts to polymeric supports for use as 'heterogeneous–homogeneous' catalyst systems. However, nearly all such supported catalysts have a low phosphorus content, and the phosphine function is unevenly distributed over the polymer chain. This problem has now been overcome by the polymerization of the (vinylaryl)phosphines (62) to give the phosphinated polymers (63).
The reaction of α-bromo-ketones with di-t-butylphosphine, followed by treatment with base, has given the new β-keto-phosphines (64), which have been shown to co-ordinate to metals in the form of the bidentate enolate ion. Similarly, the reaction of diphenylphosphine with ethyl chloroacetate in the presence of ethylene oxide gives the phosphine (65). Silylphosphines have been employed in a new route to such functionalized phosphines, the reaction of tris(trimethylsilyl)phosphine with chloroacetic acid esters giving the phosphinoesters (66). Treatment of the trimethylsilyl ester (66; R = SiMe3) with methanol gives phosphinotriacetic acid (66; R = H) in good yield and high purity. Other phosphinopolycarboxylic acids have also been prepared by this method.
The synthesis and reactivities of dicyanoalkylidenephosphines, e.g. (67), which are analogues of acyl-phosphines, have been studied. Diorgano(styryl)tin chlorides react with sily 1-phosphines to give the stannyl-phosphines (68). The reaction of secondary phosphines with p-dimethylaminobenzaldehyde gives the diphosphines (69).
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Excerpted from Organophosphorus Chemistry Volume 12 by D. W. Hutchinson, J. A. Miller. Copyright © 1981 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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