CHAPTER 1
Phosphines and Phosphonium Salts
BY S. TRIPPETT
PART I: Phosphines
1 Preparation
A. From Halogenophosphine and Organometallic Reagent. — For the preparation of tertiary phosphines this continues to be the method of choice when applicable. The lithioacetylide (1) with phosphorus trichloride gave the phosphine (2)1 whose stability at 283° contrasted sharply with the thermal instability of triethynylphosphine. The silicon analogue (3) was prepared in a similar way as well as from bis(trimethylsilyl)acetylene and phosphorus trichloride.
The previously described3 preparation of tris(trifluorovinyl)phosphine from trifluorovinylmagnesium iodide and phosphorus trichloride is now reported to give only polymeric material. Phosphorus tribromide gave the required phosphine.
Among other syntheses of this type, those of the phosphines (4) and (5) and of many fluoroalkylphosphines, e.g. (6), may be mentioned.
β-Ketoalkyltin compounds with halogenophosphines gave the corresponding β-ketoalkylphosphines which are otherwise difficult to prepare, e.g.
B. From Metallated Phosphines. — The synthesis of phosphiran from sodium phosphide and 1,2-dichloroethane in liquid ammonia has been extended to the preparation of both 1- and 2-substituted phosphirans. The 2-ethylphosphiran was a mixture of cis- and trans-isomers.
1-Deuteriophosphiran was obtained from 1,2-dichloroethane and sodium dideuteriophosphide prepared in tris(dimethylamino)phosphine oxide. Alkylphosphines (7) were similarly obtained, e.g. EtPH2 (78%), CH2: CH · CH2 · PH2 (55%).
Convenient syntheses of methyl and dimethylphosphine have been described using dimethylsulphoxide as solvent. Other syntheses using metallated phosphines and alkyl halides include those of the amines (8) and (9) and of the diphosphine (10). Whereas lithium diethyl- and dicyclohexyl-phosphides are stable in refluxing tetrahydrofuran, the corresponding dimethylphosphide rapidly cleaves the solvent to give (11).
Typical of syntheses using vinyl halides were those of the diphosphine (12) and of diphenyl-1-phenylvinylphosphine (13). Perfluoroacyldiphenylphosphines have been obtained from the corresponding perfluoroacid halides or anhydrides.
Aguiar showed that the ready reaction of aryl halides with lithium diphenylphosphide does not involve an aryne. Isslieb has now shown that such intermediates are involved in similar reactions with lithium di-t-butylphosphide (14) and aryl fluorides but not with the diethyl- or dibutyl-phosphides. While this difference was ascribed to the greater nucleophilicity of (14) it may be due to steric hindrance round the phosphorus. The reactions of lithium phosphides with aryl bromides are complicated by metal–halogen exchange. Thus (14) and p-bromotoluene gave only (15) together with the biphosphine (16).
Metallated diphenylphosphine with carbon disulphide in tetrahydrofuran at -50° gave the pale orange-yellow salts (17) which formed stable red solutions in acetone and ethanol and did not react with nitrogen. The corresponding reaction with the tetraphosphine (18) at 60° gave a rearranged salt (19) whose ochre solutions in polar solvents 'greedily' absorbed two molecules of nitrogen to give a species (vN=N 2090 cm-1) assigned a structure of which (20) is one of the contributing forms.
C. By Reduction. — Lithium aluminium hydride and trichlorosilane continue to be the reagents of choice. Among applications of the former are syntheses of the diphosphines (21) and (22) and of dimethylphosphine (70-81%) from tetramethyldiphosphine disulphide.
The triarylphosphines (23) containing functional groups sensitive to lithium aluminium hydride have been obtained by the trichlorosilane reduction of the corresponding oxides. The use of hexachlorodisilane or octachlorotrisilane in refluxing benzene or in chloroform at room temperature has been recommended for the reduction of optically active phosphine oxides. Almost complete inversion of configuration occurs and the mechanism shown has been suggested. The same reagents reduce acyclic phosphine sulphides and cyclic phosphine oxides with retention of configuration.
D. By the Radical Addition of P — H to Olefins. — Primary phosphines with allylamine in the presence of 2,2'-azobis-(2-methylpropionitrile) gave mixtures of the secondary (24) and tertiary (25) 3-aminopropylphosphines.
Similar addition of phenyl phosphine to the terminal dienes (26) gave the diphosphines (27).
Diallyl ether also gave 18% of the monophosphine CH2: CH · CH2 · O · CH2 · CH2 · PHPh. A series of additions of bicyclic secondary phosphines (28) to octa-1,7-diene has been described. The photochemical cyclisation of unsaturated secondary phosphines leads to cyclic tertiary phosphines (29).
E. Miscellaneous. — Tetraphenyldiphosphine on refluxing in aqueous ethanol with formaldehyde and diethylamine gave diethylaminomethyl-diphenylphosphine (30) and the corresponding oxide. A four-centre mechanism is proposed leading to diphenylphosphine and the phosphinite (31).
Treatment of tris-(hydroxymethyl)phosphine with phenacyl bromides followed by internal acetal formation and base-catalysed elimination of formaldehyde gave the interesting bicyclic phosphines (32). Oxidation with hydrogen peroxide in methanol gave the acyclic oxides.
F. Optically Active Phosphines. — t-Butylmethylphenylphosphine has been resolved via the asymmetric platinum(n) complex (33) obtained from the binuclear compound (34) and (+ )-deoxyephedrine. Fractional crystallisation of (33) gave two diastereoisomers. Treatment of one of these with methanolic potassium cyanide liberated the optically active phosphine which was characterised as the oxide and as the optically active complex (35) having [α]D = -11°. The extension of this method to the resolution of other tervalent phosphorus compounds, e.g. phosphites, was proposed.
A method for determining the optical purity of phosphines has been described, which involves quaternisation of the phosphine with the optically active bromide (36) and analysis of the 1H n.m.r. spectrum of the resulting salt taking advantage of the chemical shift non-equivalence of the diastereotopic protons in the product mixture.
Inversion of configuration at the phosphorus of the phosphetans (37) has been studied by n.m.r. techniques. The methyl phosphetans did not invert at 162° for 4 days while the t-butyl and phenyl phosphetans inverted remarkably rapidly in view of the increased strain expected in the four-membered ring in the transition state.
2. Reactions
A. Nucleophilic Attack on Carbon. — (i) Activated Olefins. Tricyclohexylphosphine catalysed the addition of acrylonitrile and ethyl acrylate to aldehydes to give the unsaturated alcohols (38), presumably via the betaines (39; R = C6H11). In contrast, the corresponding betaines from triphenylphosphine transfer a proton to give the ylides (40) before reacting with the aldehyde in a normal Wittig olefin synthesis.
Triphenylphosphine and N-substituted maleimides in acetic acid gave the stable ylides (41). The reaction is analogous to that previously described with maleic anhydride. With either cis- or trans-β-haloacrylic acids, esters, or nitriles, tributyl- and triphenyl-phosphines in benzene at room temperature gave the trans-vinylphosphonium salts (42), probably by an addition-elimination mechanism. No reaction occurred with the α- or β-methyl-β-haloacrylates. β-Bromoacrylic acid and triphenylphosphine also gave the bis-salt (43) which was formed exclusively at higher temperatures. The salt (42; R = Ph, X = CO2H) was not an intermediate in this reaction which may involve dehydrobromination of the β-bromoacrylic acid and addition of triphenylphosphonium bromide to the resulting propiolic acid. The last reaction is now reported to give a high yield of the bis-salt (43).
1,2-Dichloroperfluorocycloalkenes (44) and perfluorocycloalkenes with tertiary phosphines in wet acetic acid gave the stable ylides (45) when n = 1 or 2 but not when n = 3, the major product in this case being the phosphine oxide together with tars and the 1-chlorocyclohexene (46).
Triphenylphosphine and an excess of perfluorocyclobutene formed a 1:1-adduct, which with water gave the ylide (45, R = Ph, n = 1), and for which, on the basis of 31P and 19F n.m.r. data, the unlikely looking structures (47) or (48) were suggested.
Diphenylphosphine with 1,2-dichlorotetrafluorocyclobutene in dimethylformamide gave the mono- (49, 46%) and di-phosphines (50, 20%) whereas in the absence of solvent only trifluorodiphenylphosphorane and diphenylphosphinyl fluoride had been identified. The same phosphine with 1,2-dichlorohexafluorocyclopentene in dimethylformamide gave only the monophosphine (51; R = Ph, 78%) while dicyclohexylphosphine also gave 8% of the diphosphine (52; R = C6H11). The diphosphine (50) had previously been obtained (11%) from diphenylphosphine and perfluorocyclobutene in the absence of solvent.
Preparation of the phosphanone (53) has been improved by catalysis with sodium alkoxides at 120 — 130°.
The addition of dimethylphosphine to vinylsilanes is catalysed by lithium dimethylphosphide, although with diphenyldivinylsilane vigorous polymerisation resulted.
Potassium diphenylphosphide added to 1,1-diphenylethylene gave a low yield of the phosphine (54). Carbonation of the intermediate anion from the addition to stilbene resulted in the isolation of 6% of the acid (55).
(ii) Activated Acetylenes. The initial adducts (56) from the addition of triphenylphosphine to the acetylenic carboxylic esters (57) have been trapped in the presence of sulphur dioxide and water as the betaines (58), also obtained, when R = Ph, CO2Me, by the addition of bisulphite anion to the vinylphosphonium salts (59).
The yellow 1:2 adduct formed from triphenylphosphine and dimethyl acetylenedicarboxylate in refluxing ether has now been shown to be the stable ylide (60) formed by rearrangement of an intermediate phosphorane. Compound (60) gave a colourless perchlorate and reduction with zinc and acetic acid gave the oxide (61). It seems probable that the yellow 1:2-adduct formed from 1,2,5-triphenylphosphole and the same acetylene was the result of a similar rearrangement and has the structure (62; X = CO2Me). For the reactions of diphenyl-1-phenylvinylphosphine with this acetylene, with epoxides, and with activated olefins see Chapter 8, section 1A.
Dibutylphosphine and dibutylethynylphosphine oxide at 80° gave a low yield of the trans-oxide (63).
The tetramer of dimethyl acetylenedicarboxylate with triphenylphosphine gave the red ylide (63a), probably identical with the compound previously obtained from (impure) dimer.
(iii) Carbonyls, etc. Secondary phosphines added to keten and to bis(trifluoromethyl)-keten to give the acylphosphines (64). For the reaction of tris(dimethylamino)phosphine with dimethylketen see Chapter 2, section 6.
Fluorosulphonyl isocyanate and triphenylphosphine in ether at room temperature gave a dipolar adduct (65). N-Isothiocyanatodi-isopropylamine with trimethyl and triethylphosphine formed similar 1:1-adducts (66). With secondary phosphines the products were di-isopropylamine thiocyanate and the diphosphine. A six-membered cyclic transition state is suggested.
Diphenylphosphine and cyanic acid gave the amide (67) which with toluene-p-sulphonyl isocyanate did not form a urea but instead a low yield of a compound assigned the structure (68).
Addition of diphenylphosphine to the Schiff's bases (69) gave the phosphines (70).
(iv) Miscellaneous. The mode of quaternisation of tertiary phosphines with triphenylmethyl chloride has been found to depend on the size of the phosphine. Small phosphines, e.g. Et3P, PhPMe2, gave the expected triphenylmethylphosphonium salts (71) but more bulky phosphines, e.g. Ph3P, Ph2PMe, PhButPMe, gave instead the 4-(diphenylmethyl)-phenylphosphonium salts (72).
The quaternisation of triphenylphosphine with α-bromoketones is base catalysed. Thus α-bromopropiophenone in acetonitrile at room temperature gave none of the salt (73) in the absence of base while in the presence of a catalytic amount of triethylamine 56% of (73) formed in 2 hr. Other effective catalysts included aqueous potassium cyanide and hydroxide. Phenacyl bromide and triphenylphosphine in refluxing benzene–methanol gave acetophenone (87%) via nucleophilic attack on halogen, while in the presence of triethylamine at room temperature in the same solvent mixture 92% of the phenacylphosphonium salt was produced. These remarkable effects were thought to involve addition of the base to the carbonyl group followed by attack of the phosphine, the transition state being 'stabilised by mesomeric electron release from the negatively charged oxygen atom.'
The hindered isobutyrophenones (74; X = Cl, Br, O·SO2·Me) with triphenylphosphine in aprotic solvents gave methacrylophenone in an elimination reaction. Subsequent addition of triphenylphosphonium salt then gave the 3-ketophosphonium salts (75). In protic solvents the bromo-compound gave isobutyrophenone. Similar elimination–additions had previously been observed with α-halogenocyclohexanones.
B. Nucleophilic Attack on Halogen. — The conversion of alcohols into chlorides on treatment with a tertiary phosphine and carbon tetrachloride has been shown to involve inversion of configuration at the carbon and undoubtedly proceeds via the alkoxyphosphonium chloride (76). Thiols are similarly converted into chlorides with inversion and carbon tetrabromide may be used for the preparation of alkyl bromides. Trioctyl, triphenyl, and tris(dimethylamino)phosphine have been used, the last allowing particularly easy isolation of product. The intermediate alkoxyphosphonium salt in the reaction of tris(dimethylamino)phosphine with pentan-1-ol and carbon tetrachloride has been trapped as the hexafluorophosphate.
Benzotrichloride has been used in similar reactions, the ease of reaction increasing with the nucleophilic character of the phosphine, and the sequence is suggested as a method for reducing suitable trichloromethyl to dichloromethyl compounds.
With the trichloromethylcyclohexadienone (77) competitive reactions using tris(dibutylamino)phosphine showed that (77) was almost as reactive as benzotrichloride, and homoallylic stabilisation of the anion (78) was suggested.
The trichloromethyl anion formed from carbon tetrachloride and tris(dimethylamino)phosphine has been trapped by addition to carbonyl compounds to give the alcohols (79).
A similar sequence using esters or amides of trichloroacetic acid gave the glycidic esters (80) or amides, while tributyltin trichloroacetate (81) and triphenylphosphine in the presence of benzaldehyde gave, after treatment with aqueous sodium hydrogen carbonate, the dichloro-acid (82). The suggested intermediates here were dichloroketen and the β-lactone (83).
Debromination of α,α'-dibromodibenzylsulphone (84) with triphenylphosphine was stereospecific involving inversion at both centres, the meso-form giving cis-stilbene and the ([+ or -])-form leading to trans-stilbene. The mechanism is illustrated for the former case.
The dechlorination of dichlorodiphenylmethane with tributylphosphine to give tetraphenylethylene (50%) was unaffected by the presence of butanol and was therefore held not to involve the formation of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]. Debromination of the dibromide (85) formed a convenient preparation of diphenylketen, the dibromophosphorane being insoluble in the reaction mixture.
