Eduardas Skucas and David W. C. MacMillan
J. Am. Chem. Soc.
The stereoselective preparation of α-vinyl carbonyl compounds is a challenging task for synthetic chemists mainly due to their tendency to racemize under reaction conditions. MacMillan and Skucas report a multicatalysis protocol for the enantioselective α-vinylation of aldehydes under very mild conditions. Using vinyl iodonium triflate species as starting materials, an imidazolidinone organocatalyst to induce stereoselectivity, and a Cu(I) salt, they have devised an efficient and useful synthetic tool for the enantioselective preparation of β,γ-unsaturated aldehydes.
They propose that the Cu(I) catalyst undergoes an oxidative addition to the vinyl iodonium triflate substrate to form a highly electrophilic Cu(III) vinyl complex. At the same time, the imidazolidinone organocatalyst reacts with the aldehyde substrate to form the corresponding enamine. Complexation of the enamine with the Cu(III) species and subsequent reductive ellimination liberates the Cu(I) metal completing one of the catalytic cycles. Hydrolysis of the resulting imminium species yields the desired α-vinyl aldehyde and the imidazolidinone organocatalyst completing the second catalstic cycle.
The authors also investigated the scope of the reaction for both the aldehyde and vinyl coupling partners demonstrating that the protocol can tolerate sterically demanding β-branched aldehydes, protected heteroatoms, electron-poor styrenes as well as trisubstituted carbocycles. The stereochemical induction has been demonstrated to be completely under control of the organocatalyst as preexisting stereocentres do not influence the stereochemical outcome.
Utilising routine reactions, α-vinyl aldehydes can be transformed into a variety of compounds and used as versatile precursors for the synthesis of larger molecules.