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.