Regiodivergent Hydroboration/Cyclization of Unactivated 1,6-Enynes: Synthesis of Vinyl and Alkyl Boronates by Taming the Co-NHC Catalyst

A sustainable and regiodivergent hydroboration/cyclization strategy for unactivated 1,6-enynes has been developed using earth-abundant cobalt catalysts supported by N-heterocyclic carbene (NHC) ligands. This method enables the selective synthesis of either vinyl- or alkylboronates, valuable intermediates in pharmaceutical and materials chemistry, under mild conditions with broad functional group tolerance. Product selectivity is precisely controlled through the strategic modulation of the ligand environment around the cobalt center, allowing access to two mechanistically distinct pathways. A well-defined tridentate NHC–ligated Co(I)–methyl complex, [(MesCNC)Co–CH3], facilitates the formation of vinylboronates via a cobalt–boryl intermediate. In contrast, employing a bis-NHC-cobalt(II) precursor, [(MesCC)CoBr2], in the presence of NaHBEt3, directs the reaction through a cobalt–hydride intermediate to furnish alkylboronates. Detailed mechanistic studies confirm that the ligand framework dictates the nature of the active intermediates, thereby offering a rational approach to the regiodivergent hydroboration/cyclization of unactivated enynes. This approach exemplifies a resource-efficient and mild route to valuable organoboron compounds using nonprecious metal catalysis, aligning with green chemistry principles for sustainable synthesis.