A,j9-Unsaturated carbonyls

Palladium(II) compounds have unique characteristics suitable for efficient catalysed cyclopropanation of electron-deficient alkenes using diazoalkanes. Neither copper nor rhodium(II) catalysts have shown comparable reactivity with diazoalkanes, although these catalysts are superior to palladium(II) catalysts for cyclopropanation with diazocarbonyl compounds. A few examples of palladium(II) catalysed cyclopropanation of a,j9-unsaturated carbonyl compounds with diazoalkanes are shown in equations 20-24 °. 

The tendency for a,j9-unsaturated carbonyl compounds to undergo nucleophilic addition is thus due not simply to the electron-withdrawing ability of the carbonyl group, but to the existence of the conjugated system that permits formation of the resonance-stabilized anion 1. The importance in synthesis of a,j3-un-saturated aldehydes, ketones, acids, esters, and nitriles is due to the fact that they provide such a conjugated system. 

Taking into account the competitive hydrolysis of the silyl enol ether, this reaction is remarkable. The method was shown to be general and was extended to a variety of aldehydes and several a,j9-unsaturated carbonyl compounds giving uniformly 1,4-addition with aldehydes and a mixture of 1,4- and 1,2-adducts in the case of ketones [187]. Later, this aqueous version of the Mukaiya-ma reaction was shown to give near quantitative yields in the presence of a water-tolerant Lewis acid such as ytterbium triflate [188]. Keeping with the same concept,copper(II) triflate [189],indium(III) trichloride [190],tris(pentafluoro-phenyl)boron [191] and scandium(III) triflate in the presence of a surfactant [192] have proved to be active catalysts. 

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