PREPARATION OF 2-CHLOROBUTA-1,3-DIENES BY DICHLOROCYCLOPROPANATION OFALLYLSILANES FOLLOWED BY DESILYLATION

Allylsilanes react with dichlorocarbene to form dichlorocyclopropanes which, in turn, afford 2-chlorobuta-1,3-dienes when treated with cesiu m fluoride in DMF under reflux. The Z stereochemistry of the products differs from that of the starting allylsilanes. Lengthy reaction times were necessary to induce desilylation because of the lack of steric c ongestion in the intermediate silylmethyl-sub stituted cyclopropanes. 5-Methyl-7-trimethylsilylhepta-1,5-diene was cyclopropanated regiosele ctively at the inner ene function in spite of unfavourable steric cong estion, possibly as a result of the electron-donating effect of the al lylic silyl group induced by the sigma-pi conjugation.