PROPARGYL CHLORIDES AS SOURCES FOR COBALT STABILIZED GAMMA-CARBONYL CATIONS

Hexacarbonyldicobalt complexes of gamma-chloroalkynones and gamma-chlo roalkynoates (5) were found to undergo silver-mediated Nicholas conden sation reactions with silyl enol ethers or silyl ketene acetals (10) t o give 1,6-dicarbonyl complexes 11 in fair to good yields. Substrates 5 with gamma-alkyl substitution formed diastereomeric products with en ol silanes 10a and 10b. Reactions with propiophenone trimethylsilyl en ol ether (10b) gave good levels of syn diastereoselection, whereas rea ctions with cyclohexanone trimethylsilyl enol ether (10a) were only sl ightly diastereoselective, favoring the anti diastereomer. When conduc ted in acetonitrile, the silver-mediated reaction of 5e gave a radical dimerization product 13, isolated as the syn diastereomer.