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

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see: https://pubs.acs.org/doi/full/10.1021/acscatal.0c01413
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
Financial support was provided by the European Research Council (ERC-CoG, Contract No. 647550), the Spanish Government (RTI2018-095038–B-I00), “Comunidad de Madrid”, and European Structural Funds (S2018/NMT-4367)