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COBALT-CATALYZED ENANTIOSELECTIVE RING OPENING OF UNSTRAINED HETEROCYCLES VIA VINYLIDENE ADDITION AND BETA-HETEROATOM ELIMINATION
Ring opening of heterocyclic compounds through C-X bond cleavage is a useful strategy that provides rapid access to highly functionalized acyclic building blocks. In recent years, much work has focused on using transition metal catalysts to activate the C-X bonds of heterocycles and initiate ring opening. Metal-catalyzed ring opening of unstrained heterocycles is less prevalent than catalytic activation of strained heterocycles, which is advantageously driven by relief of ring strain. Methods for catalytic ring activation of unstrained heterocycles exist but are limited. Herein, we report the use of chiral cobalt complexes as catalysts for enantioselective ring opening of dihydrofuran and nitrogen-protected pyrrolines by utilizing dichloroalkenes as vinylidene precursors, Zn as a reductant, and ZnCl2 as an additive. Based on preliminary mechanistic studies, we believe this method proceeds through [2 + 2] cycloaddition between the ligated cobalt vinylidene species and the heterocycle, followed by β-heteroatom elimination, cobalt to zinc transmellation, and protonation to give rise to synthetically useful chiral allylic alcohol and amine products.