Alkyne–Alkene [2 + 2] cycloaddition based on visible light photocatalysis.

UV-activated alkyne–alkene [2 + 2] cycloaddition has served as an important tool to access cyclobutenes. Although broadly adopted, the limitations with UV light as an energy source prompted us to explore an alternative method. Here we report alkyne–alkene [2 + 2] cycloaddition based on visible light photocatalysis allowing the synthesis of diverse cyclobutenes and 1,3-dienes via inter- and intramolecular reactions. Extensive mechanistic studies suggest that the localized spin densities at sp² carbons of alkenes account for the productive sensitization of alkenes despite their similar triplet levels of alkenes and alkynes. Moreover, the efficient formation of 1,3-dienes via tandem triplet activation of the resulting cyclobutenes is observed when intramolecular enyne cycloaddition is performed, which may serve as a complementary means to the Ru(II)-catalyzed enyne metathesis. In addition, the utility of the [2 + 2] cycloaddition has been demonstrated by several synthetic transformations including synthesis of various extended π-systems. [2 + 2] cycloaddition of alkynes with alkenes would normally require UV light irradiation. Here, the authors report an alkyne–alkene [2 + 2] cycloaddition based on visible light energy transfer photocatalysis, both inter- and intramolecularly, to afford cyclobutenes and 1,3-dienes. [ABSTRACT FROM AUTHOR]