Wittig reactions potassium hexamethyldisilazide

Z)-OL, -Unsaturated esters,l Wittig-Homer reactions generally show a preference for formation of (E)-alkenes. Thus (E)-a,p-unsaturated esters are obtained preferentially on reaction of aldehydes with trimethyl phosphonoacetate under usual conditions (potassium f-butoxide). Use of a highly dissociated base can favor (Z)-selectivity. The most effective base for this purpose is potassium hexamethyldisilazide, KN[Si(CH3)3]2, in combination with 18-crown-6, although even potassium carbonate with the crown ether is fairly effective. The (Z)-selectivity can be further enhanced by use of 1 as the phos-phonoester. Under these conditions, (Z)-unsaturated esters can be prepared from aliphatic and aromatic aldehydes with Z/E ratios as high as 50 1. The method is also useful for transformation of unsaturated aldehydes to (E,Z)-dienoates and (E,E,Z)-trienoates. 

E)-Styrenes. The phosphoranium salts are prepared by alkylation of the phosphine immediately after its generation from the corresponding phosphine oxide. The stereoselectivity of the Wittig reaction with ArCHO is very sensitive to the base used. Potassium hexamethyldisilazide is the preferred base for obtaining ( )-styrenes. 

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