title: 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%).
authors.affiliation: Department of Chemistry and Biochemistry, California State University Northridge, Northridge, California 91330-8262, USA. gagik.melikyan@csun.edu.
authors.forename: Gagik G
authors.initials: GG
authors.lastname: Melikyan
authors.forename: Louis
authors.initials: L
authors.lastname: Carlson
authors.forename: Nare
authors.initials: N
authors.lastname: Sahakyan
authors.forename: Arthur
authors.initials: A
authors.lastname: Floruti
authors.forename: McKinzie
authors.initials: M
authors.lastname: Garrison