Regio- and diastereoselective intermolecular [2+2] cycloadditions photocatalysed by quantum dots.

Light-driven [2+2] cycloaddition is the most direct strategy to build tetrasubstituted cyclobutanes, core components of many lead compounds for drug development. Significant advances in the chemoselectivity and enantioselectivity of [2+2] photocycloadditions have been made, but exceptional and tunable diastereoselectivity and regioselectivity (head-to-head versus head-to-tail adducts) is required for the synthesis of bioactive molecules. Here we show that colloidal quantum dots serve as visible-light chromophores, photocatalysts and reusable scaffolds for homo- and hetero-intermolecular [2+2] photocycloadditions of 4-vinylbenzoic acid derivatives, including aryl-conjugated alkenes, with up to 98% switchable regioselectivity and 98% diastereoselectivity for the previously minor syn-cyclobutane products. Transient absorption spectroscopy confirms that our system demonstrates catalysis triggered by triplet-triplet energy transfer from the quantum dot. The precisely controlled triplet energy levels of the quantum dot photocatalysts facilitate efficient and selective heterocoupling, a major challenge in direct cyclobutane synthesis.