Impact of reducing agent, temperature, and substrate topology on diastereoselectivity of the intermolecular coupling reactions, or how “free” are cobalt-complexed propargyl radicals?

Abstract

Applicability of the term “free radical” to organometallic radicals was studied by using the stereoselectivity of radical C–C bond formation as a diagnostic tool. Based on diastereoselectivity data, it was concluded that the reduction of π-bonded, Co₂(CO)₆-complexed propargyl cations with heterogeneous reducing agents (Zn, Mg) generates “free radicals”, while homogeneous reductants (Cp₂Co, Na–Ph₂CO) produce “sequestered radicals”, presumably associated with reductant-derived oxidized species. The latter are comparable in molecular volume to the requisite radical species, thus restricting the motion and conformational freedoms of converging, transition metal-complexed propargyl radicals. Diastereoselectivity of intermolecular reactions is determined to be much less sensitive toward temperature variation (Δde = 6–22%) than in intramolecular radical cyclizations (Δde = 106%).