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. Tuning the selectivity for [2+2] photocycloadditions remains challenging. Now, triplet–triplet energy transfer from CdSe quantum dots enables the homo- and heterocouplings of 4-vinylbenzoic acid derivatives via [2+2] photocycloaddition. Preorganization of substrates on the quantum dots reverses intrinsic stereoelectronic preferences to yield cyclobutane products with unprecedented diastereo- and regioselectivity.