Dual Photoredox/Cobalt‐Catalyzed Regio‐ and Stereoselective Reductive Coupling of Allenes and Formaldehyde to Z ‐Homoallylic Alcohols

Comprehensive Summary Herein, we report a cooperative photoredox/cobalt catalytic system for the direct synthesis of trisubstituted Z ‐homoallylic alcohols via the reductive coupling of allenes with formaldehyde (formalin solution or paraformaldehyde). This method delivers products in high yield with excellent regioselectivity and stereoselectivity. The reaction is mild, water compatible, scalable, and exhibits broad functional group tolerance. The reaction proceeds under exceptionally mild conditions at room temperature, features low catalyst loadings (as low as 0.1 mol% for the photocatalyst and 2 mol% for the cobalt catalyst), and exhibits excellent functional group tolerance. A wide range of aryl, heteroaryl, and alkyl‐substituted allenes undergo smooth conversion, delivering the desired Z ‐olefin products in moderate to good yields with complete γ‐regioselectivity (>19 : 1 rr) and high to excellent Z ‐stereoselectivity (up to >19 : 1 Z / E ). The synthetic utility of this method is demonstrated through a gram‐scale synthesis (72% yield, 2.2 g) and diverse subsequent transformations of the homoallylic alcohol product, including hydrogenation, epoxidation, Mitsunobu reaction, and Appel bromination, all of which proceed without erosion of the Z ‐alkene geometry. Mechanistic studies support an oxidative cyclometalation pathway.