syn-Selective aldol reaction of propynal–hexacarbonylcobalt complexes with ketene silyl acetals

Abstract

3-Phenylpropynal–hexacarbonylcobalt complex, prepared from 3-phenylpropynal with octacarbonyl-cobalt, afforded on treatment with the seven-membered ketene trimethylsilyl acetal the syn-aldol product in a highly stereoselective manner. Similar exposure of the above complex to the six- and five-membered ketene trimethylsilyl acetal gave the corresponding syn-aldol products predominantly. The aldol reaction of the cobalt complexes derived from 3-trimethylsilyl- and 3-butyl-propynal with those cyclic ketene silyl acetals proceeded syn-selectively to provide the syn-aldol products. The preferential formation of the syn- over the anti-isomers was also achieved in the reaction of the cobalt complexed 3-trimethylsilylpropynal with acyclic ketene trimethylsilyl acetals, derived from methyl or tert-butyl propionate. In contrast, uncomplexed 3-phenyl-, 3-trimethylsilyl- and 3-butylpropynal furnished the corresponding aldol products nonselectively or anti-isomers predominantly depending on the structure of the starting ketene trimethylsilyl acetal.