Preparation of Phosphines by Reduction

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), 73 (84), 7 (85), 75 the chiral 

5 Miscellaneous Methods of Preparing Phosphines. - Transformation of functional groups present in alkyl- and aryl-phosphines has been widely employed in the synthesis of new systems. Further examples of azine formation 

The synthesis of atropisomeric biphenylyldiphosphines has been reviewed. The ferrocenyldiphosphine (115) has been prepared by treatment of l,T-dilithio-ferrocene with an aryl methylphenylphosphinite, and isolated in the form of two diastereoisomers which are separable by recrystallisation from ethanol. Metal-lation of mono- and di-(tetramethylcyclopentadienyl)phosphines, followed by treatment with iron(II) chloride, provides a route to the ferrocenyl systems (116) 

1 Nucleophilic Attack at Carbon. - Rate constants for the forward and reverse formation of the tributylphosphine - carbon disulfide adduct, Bu3P -CS2, have been determined in a range of solvents. The forward reaction constant shows little variation with solvent, whereas the reverse reaction constant 

2-borylphosphinoalkenes with carbonyl compounds and ketenimines. Two reports of the reactions of (trimethylsilyl)phosphines with a,p-unsaturated carbonyl compounds have appeared. These reactions usually lead to transfer of a trimethylsilyl group from phosphorus to oxygen, presumably via the intermediacy of a phosphonioenolate betaine, the initial product of nucleophilic attack by the phosphine at the P-carbon. However, other pathways also arise in the reactions of sterically crowded organotrimethylsilylphosphines, and transfer of a trimethylsilyl group from phosphorus to the a-carbon has been observed.  

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Preparation of Phosphines by Reduction.- A procedure for the reduction of phosphine oxides using trichlorosilane in inert... 

Tertiary phosphine oxides are also produced as significant by-products in several of the reactions of phosphines that have been noted previously, including the Wittig olefination and the conversions of alcohols to haloalkanes with triphenylphosphine as an adjunct reagent. The tertiary phosphine oxides produced in such reactions present a problem in chemical economics, as they themselves possess little chemical utility. The phosphine may be regenerated, but several steps are required, as previously noted with preparations of phosphines by reduction (see Section 3.2). 

Preparation of Phosphines by Reduction.- Trichlorosilane continues to find application for the preparation of chiral phosphines from the corresponding chiral phosphine oxides. This route has been used in the synthesis of the chiral, non-... 

Preparation of Phosphines by Reduction. Trichlorosilane has been employed in the synthesis from the corresponding phosphine oxides of a range of interesting new systems, including the chiral diphosphine (46), the tricyclic phospholane... 

Alkylnickel amido complexes ligated by bipyridine have been prepared that undergo reductive elimination of V-alkyl amines (Equation (54)).207,208 Unlike the phosphine-ligated palladium arylamides, these complexes underwent reductive elimination only after oxidation to nickel(III). Thermally induced reductive elimination of alkylamines from phosphine-ligated nickel complexes appears to occur after consumption of phosphine by arylazides 209... 

Very pure phosphine is formed by the hydrolysis of phosphonium iodide with water, dilute acids or dilute bases 2. 4.107,109-114) or by the reduction of phosphorus trichloride with lithium in diethyl ether 7,us-ii7) Related to the latter is a method for the preparation of phosphine, described in the patent literature, where phosphorus trichloride vapour, diluted with nitrogen, is passed through a column filled with lithium hydride mixed with an inert material, such as sand, NaCl, KCl or similar materials... 

Tertiary phosphines, in the absence of special effects 2 ), have relatively high barriers 8) ca. 30-35 kcal/mol) to pyramidal inversion, and may therefore be prepared in otically stable form. Methods for synthesis of optically active phosphines include cathodic reduction or base-catalyzed hydrolysis 3° 31) of optically active phosphonium salts, reduction of optically active phosphine oxides with silane hydrides 32), and kinetic 3 0 or direct 33) resolution. The ready availability of optically pure phosphine oxides of known absolute configuration by the Grignard method (see Sect. 2.1) led to a study 3 ) of a convenient, general, and stereospecific method for their reduction, thus providing a combined methodology for preparation of phosphines of known chirality and of high enantiomeric purity. 

A variety of processes is available for preparation of phosphine (N.B. very poisonous and flammable) hydrolysis of calcium phosphide7 or aluminum phosphide8 (best if suspended in an inert solvent9,10), white phosphorus,7 a mixture of phosphorus and diphosphorus tetra-iodide,11 or phosphonium iodide 7 thermal disproportionation of phosphorus acid 3,5 or reduction of phosphorus trichloride by lithium tetrahydroaluminate.5... 

Preparation of Phosphines from Metallated Phosphines.- The generation of arylphosphide reagents by the reductive cleavage of carbon-phosphorus bonds using alkali metals has received detailed study for a wide range of functionalised triarylphosphines and related... 

Kyba and eoworkers prepared the similar, but not identical compound, 26, using quite a different approach. In this synthesis, pentaphenylcyclopentaphosphine (22) is converted into benzotriphosphole (23) by reduction with potassium metal in THF, followed by treatment with o "t/20-dichlorobenzene. Lithium aluminum hydride reduction of 23 affords l,2-i>/s(phenylphosphino)benzene, 24. The secondary phosphine may be deprotonated with n-butyllithium and alkylated with 3-chlorobromopropane. The twoarmed bis-phosphine (25) which results may be treated with the dianion of 24 at high dilution to yield macrocycle 26. The overall yield of 26 is about 4%. The synthetic approach is illustrated in Eq. (6.16), below. 

Notable examples of general synthetic procedures in Volume 47 include the synthesis of aromatic aldehydes (from dichloro-methyl methyl ether), aliphatic aldehydes (from alkyl halides and trimethylamine oxide and by oxidation of alcohols using dimethyl sulfoxide, dicyclohexylcarbodiimide, and pyridinum trifluoro-acetate the latter method is particularly useful since the conditions are so mild), carbethoxycycloalkanones (from sodium hydride, diethyl carbonate, and the cycloalkanone), m-dialkylbenzenes (from the />-isomer by isomerization with hydrogen fluoride and boron trifluoride), and the deamination of amines (by conversion to the nitrosoamide and thermolysis to the ester). Other general methods are represented by the synthesis of 1 J-difluoroolefins (from sodium chlorodifluoroacetate, triphenyl phosphine, and an aldehyde or ketone), the nitration of aromatic rings (with ni-tronium tetrafluoroborate), the reductive methylation of aromatic nitro compounds (with formaldehyde and hydrogen), the synthesis of dialkyl ketones (from carboxylic acids and iron powder), and the preparation of 1-substituted cyclopropanols (from the condensation of a 1,3-dichloro-2-propanol derivative and ethyl-... 

The enantioselective 1,4-addition addition of organometaUic reagents to a,p-unsaturated carbonyl compounds, the so-called Michael reaction, provides a powerful method for the synthesis of optically active compounds by carbon-carbon bond formation [129]. Therefore, symmetrical and unsymmetrical MiniPHOS phosphines were used for in situ preparation of copper-catalysts, and employed in an optimization study on Cu(I)-catalyzed Michael reactions of di-ethylzinc to a, -unsaturated ketones (Scheme 31) [29,30]. In most cases, complete conversion and good enantioselectivity were obtained and no 1,2-addition product was detected, showing complete regioselectivity. Of interest, the enantioselectivity observed using Cu(I) directly in place of Cu(II) allowed enhanced enantioselectivity, implying that the chiral environment of the Cu(I) complex produced by in situ reduction of Cu(II) may be less selective than the one with preformed Cu(I). 

C. By Reduction.—The cyclic secondary phosphines phospholan and phosphorinan have been prepared by reduction of the corresponding chlorophosphines with lithium aluminium hydride. ... 

Two contrasting conclusions have been reported in the reactions of lithium aluminium hydride in THF with phosphine oxides and phosphine sulphides respectively. The secondary oxide, phenyl-a-phenylethylphos-phine oxide (42), has been found to be racemized very rapidly by lithium aluminium hydride, and this observation casts some doubt on earlier reports of the preparation of optically active secondary oxides by reduction of menthyl phosphinates with this reagent. A similar study of the treatment of (/ )-(+ )-methyl-n-propylphenylphosphine sulphide (43) with lithium aluminium hydride has revealed no racemization. These results have been rationalized on the basis of the preferred site of attack of hydride on the complexed intermediate (44), which, in the case of phosphine oxides (X = O), is at phosphorus, and in the case of the sulphides (X = S), is at sulphur. Such behaviour is comparable to that observed during the reduction of phosphine oxides and sulphides with hexachlorodisilane. ... 

Various type (967) dinuclear complexes with phosphido bridges have been prepared, usually by reduction of [(R3P)2NiX2] with, for example, sodium.2372-237 The terminal phosphines can be exchanged by other phosphines or by CO, yielding (968) and (969) in the latter case.2373,2374 Diphenylphosphide Ni1 complexes have also been prepared electrochemically.2376... 

Tetrakisphosphine complexes of Ni° are usually prepared either from labile Ni° precursors (such as [Ni(cod)2] or [Ni(CO)2(cod)]) or by reduction of an appropriate Ni11 complex in the presence of phosphine. Despite the large number of homoleptic Ni-phosphine complexes NiL4, however, relatively few X-ray crystal structures are known. Some examples are collected in Table 23. Typical Ni°—P distances are in the range 2.14-2.21 A. [Ni(PMePh2)4] was obtained serendipit-ously upon treatment of [NiBr(NPMe3)]4 with LiCCPh. 

Phosphite complexes of platinum(0) have received substantially less attention than have phosphine complexes.44 [Pt P(OC6H4OMe-2)3 3] can be prepared by reduction of the [PtCl2 P-(OC6H4OMe-2)3 2] complex in the presence of the phosphite or by the reaction of the phosphite with Lris(//2-norbornene)platinum(II) 44 Alkene complexes of bis(phosphite)platinum(II) can be prepared in a similar manner to the analogous phosphine complexes. 

Nickel and palladium complexes also catalyze the formation of the carbon-phosphorus bonds in phosphorus(V) and phosphorus(III) compounds. Indeed, this chemistry has become a common way to prepare phosphine ligands by the catalytic formation of phosphine oxides and subsequent reduction, by the formation of phosphine boranes and subsequent decomplexation, or by the formation of phosphines directly. The catalytic formation of both aryl and vinyl carbon phosphorus bonds has been accomplished. 

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