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Catalytic Vinylidene Transfer and Insertion Reactions
Metal-stabilized carbenes, most commonly formed through the decomposition of diazoacetates, are extensively employed in organic synthesis. However, several classes of carbenes, such as vinylidenes, are challenging to utilize in transition metal catalysis due to the instability of the required diazo precursors. To overcome this challenge, most transition metal-catalyzed vinylidene transfer and insertion methods rely on alkynes as vinylidene precursors. Only catalysts that form stable M=C multiple bonds and weak M(π-C≡C) interactions can promote this alkyne isomerization, and the resultant metal(vinylidene) species is often less reactive compared to free vinylidenes. The discovery of 1,1-dihaloalkenes as precursors to transition metal vinylidene complexes has significantly expanded the scope of vinylidene transfer and insertion reactions. Dinuclear catalysts were found to promote the reductive cyclization of 1,1-dichloroalkenes containing pendant alkenes to form methylenecycloalkenes, and mechanistic studies are consistent with the formation of a Ni2(vinylidene) species. Furthermore, these catalysts promote reductive three-component cycloaddition reactions with 1,1-dichloroalkenes and aldehydes to generate methylenedioxolanes, which upon treatment with aqueous acid provides access in one step to new, unsymmetrical aliphathic α-hydroxy ketones that would be difficult to access with existing methods. Under dilute conditions, an enone byproduct is formed and a DFT model is presented that accounts for concentration-based reaction selectivity.