The [2+2] photocycloaddition is the most commonly used photochemical reaction in organic synthesis. Recent progress in the field which employed Lewis acid to assist the sensitization of the triplet excited state of chalcone derivatives realized stereo-selective photocatalytic [2+2] cycloaddition. Here, we demonstrated that CdSe QDs can efficiently sensitize this type of reactions, without using the Lewis acid as the co-catalyst to lower the triplet state energy of the substrates. Compared to widely used organometallic triplet sensitizers such as Ru(bpy)<sub>3</sub>, CdSe QDs have three advantages: 1) their absorption cross-section is an order of magnitude bigger than those organometallic complexes; 2) due to their very small singlet-triplet energy gap, the QD photocatalyst can maximize the use of visible photons and reduce the risk of side-reactions caused by UV light; 3) QDs are robust against photodegradation and they can be easily recycled from the reaction mixture. The reactivity of the QD catalyst can be enhanced by optimizing the efficiency of triplet energy transfer via surface modification that assists the adsorption of chalcone substrates. The formation of the QD-chalcone intermediate also provides the possibility to control the stereochemistry of [2+2] cycloaddition.

Photocatalytic [2+2] cycloaddition sensitized by triplet energy transfer from CdSe quantum dots

256th National Meeting (2018)

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