A dinickel-catalyzed three-component cycloaddition of vinylidenes.

A dinickel catalyst promotes the [2 + 2 + 1]-cycloaddition of two aldehyde equivalents and a vinylidene. The resulting methylenedioxolane products can be deprotected in one pot under acidic conditions to reveal α-hydroxy ketones. This method provides convenient access to unsymmetrical alkyl-substituted α-hydroxy ketones, which are challenging to synthesize selectively using cross-benzoin reactions. Mechanistic studies are consistent with an initial migratory insertion of the aldehyde into a dinickel bridging vinylidene. Insertion of the second aldehyde followed by C-O reductive elimination furnishes the cycloadduct. Under dilute conditions, an enone side product is generated due to a competing β-hydride elimination from the proposed metallacyclic intermediate. A DFT model consistent with the concentration-dependent formation of the methylenedioxolane and enone is presented.