Radical 4-exo cyclizations via template catalysis.

The mechanism of catalytic 4-exo cyclizations without gem-dialkyl substitution was investigated by a comparison of cyclic voltammetry, EPR, and computational studies with previously published synthetic results. The most active catalyst is a super-unsaturated 13-electron titanocene(III) complex that is formed by supramolecular activation through hydrogen bonding. The template catalyst binds radicals via a two-point binding that is mandatory for the success of the 4-exo cyclization.