Enantioselective Aminocatalytic [2 + 2] Cycloaddition through Visible Light Excitation

An asymmetric aminocatalytic activation strategy to obtain enantioenriched cyclobutanes through [2 + 2] photocycloaddition under visible light irradiation is presented. This metal-free process does not require the use of any external photocatalyst, as it is catalyzed by a simple diamine which, upon condensation with an enone substrate, forms an iminium ion intermediate that absorbs in the visible light region. The direct excitation of such an intermediate leads to a charge transfer (CT) excited state that unlocks a stereocontrolled intermolecular photocycloaddition with good enantiomeric and diastereoisomeric ratios and high yields. Finally, DFT calculations and experiments were performed to support the charge transfer behavior and the mechanistic proposal.