Publication date: 3rd July 2020
Tetrasubstituted cyclobutyl structures are precursors to, or core components of, many important bioactive molecules, including prospective drugs. Light-driven [2+2] cycloaddition is the most direct strategy for construction of these structures. [2+2] photocycloadditions that proceed through the triplet excited state are advantageous because (i) their scope is not limited by the electrochemical potentials of the substrate, (ii) triplets have long enough lifetimes to mediate intermolecular cycloadditions, and (iii) triplets can be accessed using visible (as opposed to UV) light through excitation of a triplet sensitizer, such as a transition metal complex or organic chromophore, followed by triplet-triplet energy transfer (TT EnT). Synthetic applications of [2 + 2] photocycloadditions however also demand high selectivity, not only for specific coupling products, but also for particular stereo- and regioisomers of those products. Achieving selectivities for (i) a particular regioisomer of the coupled product, (ii) a particular diastereomer of the coupled product, and (iii) homo- vs. hetero-coupling within a mixture of reactive olefins still remains a challenge. Here, we discuss the use of colloidal CdSe quantum dots (QDs) as visible light absorbers, triplet exciton donors, and scaffolds to drive homo- (photodimerization) and hetero- (cross coupling) intermolecular [2+2] photocycloadditions of 4-vinylbenzoic acid derivatives, with (i) perfect and switchable regioselectivity and (ii) 97-98% diastereoselectivity for the previously minor syn-head-to-head (HH) or syn-head-to-tail (HT) configurations of the adducts. The diasteromeric ratios (d.r.) we achieve are a factor of 5 - 10 higher than those reported with all other triplet sensitizers. Furthermore, the size-tunable triplet energy of the QD enables regioselective hetero-intermolecular couplings through selective sensitization of only one of the reagent olefins.
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