Direct Substitution of the Hydroxy Group in Alcohols with Silyl Nucleophiles Catalyzed by Indium Trichloride

摘要

Substitution of the hydroxy group in alcohols by nucleophiles intrinsically requires an equimolar (or greater) amount of acid because of the poor leaving ability of the OH group. To avoid the use of excessive amounts of acid, alcohols are usually transformed into the corresponding halides or related compounds that have good leaving groups before the treatment with the nucleophiles. In this context, direct substitution of alcohols in a catalytic manner under nearly neutral conditions would be a fascinating and ideal procedure for synthetic organic chemistry. We have previously reported the direct dehydroxylation of alcohols under catalytic conditions by using a chlorosilane/catalytic InCl_3 system. We have now turned our attention to CC bond formation by a direct substitution system with allylic nucleophiles. In 1982, Cella reported allylation/dehydroxylation of alcohols by allylsilane in the presence of an excess amount of a Lewis acid. This system, however, is only applicable to a narrow range of alcohols and gives a significant amount of side products. As a special case, hemiacetal is effectively alkylated by allylsilane to give the corresponding alkenes in high yields but this reaction requires more than an equimolar amount of BF3. Rubin and Gevorgyan recently reported allylation of certain alcohols in the presence of a boron catalyst although dehydration prevents the desired alkylating reaction in some cases. Herein we report the direct substitution of the hydroxy group in alcohols by allyl-, propargyl-, and alkynylsilanes catalyzed by indium chloride. The system allows the desired alkylation of a wide range of applicable substrates under neutral conditions.