Alkyldiphenylphosphine oxides

Taking a simple general example to explain the usefulness of this reaction, treatment with base (usually butyllithium) of the alkyldiphenylphosphine oxide gives the anion which then reacts with an aldehyde to form the corresponding-erythro threo diastereoisomeric mixture of alcohols (cf. p. 517) in unequal proportions only one enantiomer of each diastereoisomeric pair is formulated [(8)... 

An alternative route to the erythro.threo diastereoisomeric mixture of alcohols results from the acylation of the alkyldiphenylphosphine oxide with an ester or lactone to yield the /J-ketophosphine oxide, followed by reduction with sodium borohydride. This reduction shows f/ireo-selectivity, so that, following separation of the diastereoisomers, a preparatively useful route to ( )-alkenes is achieved. 

The nucleophilic addition of the a-lithiated alkyldiphenylphosphine oxide B to the carbonyl group of an aldehyde at the beginning of a Wittig-Horner reaction results in the phos-phorylated lithium alkoxide D. If the alkene synthesis is carried out in a single step, the Li of the intermediate D is, without workup, reversibly replaced by K by adding potassium-ferf-butoxide. In this way, the phosphorylated potassium alkoxide F is made available. Only in F... 

Preparation of alkyldiphenylphosphine oxides. General procedure from phospho-nium salts. Triphenyl phosphine is heated under reflux with an excess of alkyl halide. The precipitated phosphonium salt is filtered off, washed well with ether, and then heated with 30 per cent w/w aqueous sodium hydroxide (c. 4 ml/g) until all the benzene has distilled out. The mixture is cooled and extracted with dichloromethane, and the extracts are dried (magnesium sulphate) and evaporated to dryness. In this way ethyldiphenylphosphine oxide is obtained from triphenyl phosphine (65.6 g, 0.25 mol) and iodoethane (42.9 g, 0.275 mol) in dry toluene (250 ml) to give first the phosphonium salt (102.4 g, 97.9%) after 3.5 hours, from which the phosphine oxide is obtained as needles (53.2 g, 92.5%), m.p. 123-124 °C (from ethyl acetate) p.m.r. 5 (CDC13, TMS) 1.9-13 (m, 10H, Ph2PO), 2.3 (m, 2H, CH2) and 1.2 (dt, 3H, JHm, = 7 Hz, JMeP = 17 Hz, Me). 

If further evidence of the difficulty of reducing the P=0 bond were needed, the recent findings of Yamashita et al ° provide interesting confirmation. Many a-hydroxyalkyldiphenylphosphine oxides (13) were converted into the corresponding alkyldiphenylphosphine oxides (14) in 33-73% yield, with PCb-KI (equation 21). No competing reduction of the P==0 bond was apparently observed. Majew-ski has reported the disproportionation of dialkylphosphine oxides into dialkyIphosphines and phos-phinic acids (equation 22). The reaction proceeds in yields of over 90% and the phosphines are easily isolated by distillation. 

The order of ease of displacement of groups from the phosphonium salts is allyl, benzyl > phenyl > methyl > 2-phenylethyl > ethyl, higher alkyls. The usefulness of this method for the preparation of various alkyldiphenylphosphine oxides from the available triphenylphosphine is particularly noteworthy. In contrast, the preparative value of this method is diminished in cases where the carbanions correspond ing to the leaving groups on the phosphorus have approximately the same stability. In some cases the reaction is used to prepare the carbon compound (equations 9 to 14). ... 

The isomeric isoxazole-substituted phosphine oxides (61) and (63) have been synthesised and used in Horner-Wittig reactions to prepare isoxazoles of types (62) and (64) regiospecifically.32 An alternative approach to (62) and (64) involves reaction of the isoxazole aldehydes (e.g. 65) with alkyldiphenylphosphine oxide anions (Scheme 11). The routes described have been applied to the synthesis of isoxazoles containing leukotriene-like carbon chains. 

The Horner modification of the Wittig reaction, in which a diphenylphos-phinoyl group is the anion-stabilizing function, can be used to prepare olefins of either E- or Z-geometry. Reaction of a lithiated alkyldiphenylphosphine oxide with an aldehyde gives predominantly the erythro- cohol (29) (precursor of the Z-olefin), whereas successive acylation and reduction gives predominantly the r/ireo-alcohol (30) (Scheme 11). ... 

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