Acquiring and Exploiting Persistency of Propargyl Radicals: Novel Paradigms

Abstract

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 Å3), two different thresholds for persistency in secondary propargyl radicals were established306.5 and 576.0 Å3dependent upon the molecular architecture and the nature of the substituents populating the radical centers. Three alternative molecular platforms were designed to exploit a newly acquired dichotomy in allylic radicals (α-persistent-γ-transient) and trichotomy in pentadienyl radicals (α-persistent-γ-transient-ε-transient), providing access to molecular assemblies with contiguous 4°–4°, 4°–3°, and 3°–3° carbon atoms.