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Propargyl Radical Chemistry: Renaissance Instigated by Metal Coordination

Accounts of Chemical Research2015Vol. 48(4), pp. 1065–1079

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Abstract

Over the last two decades, radical chemistry of propargyl systems was developed into a potent synthetic field providing access to classes of organic compounds that are otherwise hardly accessible. The levels of diastereoselection thus achieved (up to 100%) are unprecedented for free propargyl radicals, as well as for organic radicals π-bonded to transition metals. These advances were enabled by the coordination of the triple bond to a Co2(CO)6 core that precluded an acetylene-allene rearrangement, stabilized requisite propargyl cations, created conformational constraints at the carbon-carbon bond formation site, configurationally altered the acetylenic moiety allowing for 1,3-steric induction upon the newly formed stereocenters, increased bulkiness of propargyl triads thus controlling the spatial orientation of converging radicals, and allowed for α-to-γ projection of the reaction site and alteration of the transiency of radical intermediates. In the course of these studies, a number of popular "beliefs" were proven to be untrue. First, cobalt-complexed propargyl cations, which have long been considered to be thermally labile species, were engaged in synthetically meaningful transformation at temperatures as high as 147 °C. Second, in radical dimerization reactions, higher reaction temperatures did not adversely impact the yields and levels of d,l-diastereoselectivity. Third, π-bonded organometallic radicals, deemed unruly, were effectively controlled with complementary mechanistic tools, thus achieving the highest levels of stereoselectivity (up to 100%) in inter- and intramolecular reactions. Fourth, meso stereoisomers, being thermally labile and kinetically disfavored, were discovered to be major products in intramolecular cyclizations induced by cobaltocene. Fifth, propargyl cations were synthesized in the absence of strong acids, thus increasing the functional tolerance and achieving a long sought after compatibility with acid-sensitive functionalities. A concept of sequestered propargyl radicals was introduced to explain disparity in diastereoselectivity data: heterogeneous reducing agents allegedly produce "free" radicals, while homogeneous reductants generate "sequestered" radicals associated with reductant-derived oxidized species. Among mechanistic tools, a 1,3-steric induction was found to be most efficient for controlling the stereoselectivity of radical reactions (up to 100% d,l). In intramolecular reactions, a d,l-to-meso reversal of stereoselectivity was discovered with zinc being replaced with cobaltocene as a reductant. Among efficient tools for controlling the stereoselectivity in intramolecular cyclizations is a rigidity of the carbon tether that provides for an exclusive formation of d,l-diastereomers. Two novel reactions that belong to a new field of unorthodox organometallic radical chemistry were discovered: the spontaneous conversion of cobalt-complexed propargyl cations to radicals and the THF-mediated process wherein a THF molecule assumes a new role of an initiator in radical reactions. A multistep mechanism involves a THF-induced alteration of propargyl cations that facilitates a redox process between metal clusters. Novel stereoselective methods provide access to topologically and functionally diverse 3,4-diaryl and 3,4-dialkyl-1,5-alkadiynes, 3,4-disubstituted 1,5-cycloalkadiynes (C8-C12), 3,4-dialkoxy-1,5-(cyclo)alkadiynes, and 3,7-diene-1,9-alkadiynes, which can be used in targeted syntheses of organic assemblies of relevance to medicinal chemistry, materials science, and natural product syntheses. Novel mechanistic tools and methodologies for controlling stereoselectivity in radical reactions can be expanded toward new types of π-bonded unsaturated units (dienes, arenes, diynes, and enynes) and transition metals other than cobalt (Fe, Cr, Mo, W, and Mn).

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Abstract

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