Enantioselective intermolecular [2 + 2] photocycloadditions of vinylazaarenes with triplet-state electron-deficient olefins

Abstract

The development of catalytic asymmetric radical reactions is an attractive but formidable task. The high reactivity of radicals enables the use of readily accessible feedstocks and mild reaction conditions, but it leads to substantial difficulty for chiral catalysts to provide sufficient enantiocontrol. Moreover, a racemic background process is often inevitable, further deteriorating enantioselectivity. In this regard, an effective protocol has been established for enantioselective intermolecular [2 + 2] photocycloadditions to overcome the challenges, which is capitalising on the ground-state preassociations of chiral catalysts with photoactivated substrates. Here, we report the viability of substrate-differentiating synergistic catalysis for this important reaction. In this new platform, energy transfer occurs between DPZ as a photosensitizer and enones or (E)-2-substituted vinylazaarenes for producing triplet-state species, and chiral phosphoric acid interacts with ground-state 2-vinylazaarenes via hydrogen bonding for subsequent enantiofacial cycloaddition. Although all active species are dispersed in the reaction system, valuable enantioenriched mono- and di-azaarene-functionalized cyclobutanes are obtained efficiently and selectively. In addition to constructing all-carbon quaternary stereocentres, flexible modulation of azaaryl groups and other substituents on the cyclobutane ring is also operative.