Cobalt-mediated radical reactions of 1,4-enynes are of significant synthetic value providing access to organic molecules with complex topology, in particular those with vicinal quaternary carbon centers. The vinyl group in 1,4-enynes allows for varying the nature of substituents and degree of substitution along the carbon framework, as well as for projecting a radical center within an allylic triad. In this project, stereo-, regio-, and chemoselectivities of radical dimerization reactions in dicobalthexacarbonyl-1,4-enyne complexes were studied for substrates containing substituents with opposing electronic effects (γ-EDG, γ-EWG) and topologically diverse carbon frameworks (α-, β-, γ-, α,γ-, β,γ-, γ,γ-, α,β-, α,γ,γ-, β,γ,γ-). Generation of highly delocalized α-vinyl propargyl cations was conducted under acidic conditions (HBF4), followed by reduction with zinc or cobaltocene. γ-Phenyl- and γ-methyl substituted 1,4-enynes reacted with nearly complete migration of the reaction center – alpha-to-gamma –, affording γ,γ-dimers as main products (92-97%). Stereoselectivity of the carbon-carbon bond formation is found to be dependent upon the proximity to a bulky Co2(CO)6-metal core, i.e. while α,α-dimers exhibited major preference for a d,l-configuration (de 50-100%), respective γ,γ- dimers were formed with a lower degree of stereoselectivity (de 10-20%). Radical reactions with β-Me derivatives featured significant acceleration (135 times!), undergoing spontaneous dimerization even in the absence of the reducing agents. Cyclic 1,4-enynes selectively dimerized at the gamma position, since alpha tertiary propargyl radicals become persistent in the presence of a bulky cobalt cluster. Bis-cobalt complexes were oxidized with ceric ammonium nitrate to polysubstituted 3E,7E-diene-1,9-decadiynes in four-step synthetic sequences. Experimental data on dimerization of cyclic 1,4-enynes with double bonds positioned inside and outside of the five- and six-membered carbocycles, α-ethoxycarbonyl captodative radicals, and functionally diverse heterocycles such as α,β-unsaturated γ-lactones and 3,4-dihydro-2H-pyrans will also be presented.

Allylic triad as a dichotomous reaction site in cobalt-mediated radical transformations of 1,4-enynes

255th National Meeting (2018)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.