Acquiring and Exploiting Persistency of Propargyl Radicals: Novel Paradigms

作者：Melikyan Gagik G; Davis Ryan; Cappuccino Samantha

来源：Organometallics, 2016, 35(17): 2854-2867.

DOI：10.1021/acs.organomet.6b00435

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 pi-bonded Co-2(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 angstrom(3)), two different thresholds for persistency in secondary propargyl radicals were established-306.5 and 576.0 angstrom(3)-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 (alpha-persistent-gamma-transient) and trichotomy in pentadienyl radicals (alpha-persistent-gamma-transient-epsilon- transient), providing access to molecular assemblies with contiguous 4 degrees-4 degrees, 4 degrees-3 degrees, and 3 degrees-3 degrees carbon atoms.

• 出版日期2016-9-12