No abstract

A convenient and highly efficient method for the Lewis acid-catalyzed trans-selective hydrosilylation of alkenes has been developed. The mechanism of this novel protocol operates via direct addition of silylium type species across C=C bond followed by trapping of the resultant carbenium ion with boron-bound hydride. A number of diversely substituted silanes possessing both aryl and alkyl groups at silicon atom were efficiently prepared using this hydrosilylation methodology. The possibility to employ aryl-containing hydrosilanes in this reaction opens broad capabilities for the synthesis of alcohols via a trans-selective hydrosilylation/Tamao-Fleming oxidation sequence, complementary to the existing cis-selective hydroboration/oxidation protocol.

The primary alcohols 1a-e and ethers 4a-d were effectively reduced to the corresponding hydrocarbons 2 by HSiEt(3) in the presence of catalytic amounts of B(C(6)F(5))(3). To the best of our knowledge, this is the first example of catalytic use of Lewis acid in the reduction of alcohols and ethers with hydrosilanes. The secondary alkyl ethers 4j,k enabled cleavage and/or reduction under similar reaction conditions to produce either the silyl ethers 3m-n or the corresponding alcohol 5a upon subsequent deprotection with TBAF. It was found that the secondary alcohols 1g-i and tertiary alcohol 1j, as well as the tertiary alkyl ether 4l, did not react with HSiEt(3)/(B(C(6)F(5))(3) reducing reagent at all. The following relative reactivity order of substrates was found: primary >> secondary > tertiary. A plausible mechanism for this nontraditional Lewis acid catalyzed reaction is proposed.