Acquiring and Exploiting Persistency of Propargyl Radicals: Novel Paradigms

Three complementary methods for altering an intrinsic nature of propargyl radicals, from transient to persistent, were developed by fine-tuning the bulkiness and degree of substitution around the secondary and tertiary propargylic carbons, as well as by sterically enhancing a π-bonded Co2(CO)6 metal core. The latter was employed, as a mechanistic tool, for precluding an acetylene–allene rearrangement, stabilizing propargyl cations, and creating a steric hindrance that could provide for said transition from transient to persistent propargyl radicals. A window of opportunities was identified wherein the steric bulkiness in propargyl radicals remains below the “persistency threshold”, providing good to excellent stereoselectivities in radical dimerization reactions (d,l 62–100%). Along with the persistency threshold for tertiary propargyl radicals (278.2 A3), two different thresholds for persistency in secondary propargyl radicals were established—306.5 and 576.0 A3—dependent upon the molecular architecture ...