Enhanced reactivity in radical cyclizations of hydrazones using the silicon-tethered 1-bromovinyl group

Abstract

A silicon-tethered 1-bromovinyl group was shown to function as a radical precursor for tin-mediated vinyl additions to chiral α- or β-hydroxyhydrazone. In contrast to related thiyl-mediated methods, these vinyl bromides were not limited to the 5- exo cyclization mode. A series of Si-tethered 5- exo and 6- exo cyclizations formed the corresponding five- and six-membered exo-methylene-substituted oxasilacycles. Treatment with fluoride cleaved the Si–C and Si–O bonds to afford the corresponding allylic hydrazines. Diastereoselectivities ranged from 2:1 to 25:1 ( anti: syn) for the 5- exo cyclizations, depending on the size of the exocyclic substituent, but 6- exo cyclization was not diastereoselective. A variant involving Tamao oxidation of the exo-methylene oxasilacyclopentane intermediate afforded a methyl ketone, a net process corresponding to addition of a radical acyl anion equivalent.