Cobalt-Mediated Radical Cyclizations: Stereoselective Synthesis of Cyclopentanes, Cyclohexanes, and Tetralins

Abstract

A stereoselective method for assembling five- and six-membered, mono- and binuclear carbocyclic compounds by intramolecular cyclization of Co2(CO)6-complexed propargyl radicals was developed. The formation of requisite bis-propargyl cations A is enabled by a stabilizing metal cluster, while converging transient radicals are stereodirected by a bulky metal core and substituents at the acetylenic termini. With the flexible three- or four-carbon tethers, formation of 1,2-dialkynylcyclopentanes and 1,2-dialkynylcyclohexanes occurred with a high stereoselectivity (91–100% trans-), noticeably enhanced by a γ-TMS group that exclusively directs the cobalt-alkyne cores into diaxial positions (1,3-steric induction). The main cyclization parameters were fully preserved with an aromatic ring as a rigid tether, thus providing rapid access to trans-tetralins with terminal and internal acetylenic moieties. The observed stereoselectivity in cyclization reactionswith both flexible and rigid tetherswas interpreted in terms of the selective stereomutation of the alpha stereocenters occurring in intermediate cations or radicals. Organic ligands were separated from metal cores under oxidative conditions (Ce4+), providing access to vicinally substituted bis-alkynyl cyclopentanes, cyclohexanes, and tetralins of trans-configurations, prone to secondary transformations, and of immediate relevance to the angularly and linearly fused estrogen mimics.