Under H-2, the radical cyclization of appropriate dienes can be catalyzed by cobaloximes. H center dot can be abstracted from an intermediate (presumably a cobalt: hydride) by trityl radicals (Ar3C center dot) or by TEMPO. The rate-determining step in these reactions is the uptake of H-2, which is second order in cobalt and first order in hydrogen; the third-order rate constant is 106(3) M-2.s(-1).