Visible Light-promoted Alkyne – Alkene [2+2] Cycloaddition and Enyne Photometathesis

Abstract

Cyclobutenes are good synthetic intermediates for their diversity and found in complex natural products and pharmaceuticals. Therefore, the development of synthetic method for cyclobutenes has attracted a lot of attention from the synthetic society. Because alkyne – alkene thermal [2+2] cycloaddition is a forbidden pathway, [2+2] cycloaddition of alkyne with alkene via UV activation has become an critical mean for the synthesis of cyclobutenes. While widely adopted, we were interested in exploration the complementary methods because the reaction with UV light has limited for an energy source. A variety of complementary methods for the formation of cyclobutenes using Lewis acids and transition metal catalysts have been studied. However, there is a limitation that certain functional groups are required on the substrates for activation. In a mean time, energy transfer (EnT) mechanism has been increasingly attracted attention as a substitute for visible-light photocatalysis in transformations. Here we developed [2+2] cycloaddition of alkyne with alkene through triplet energy transfer under visible light enabling the synthesis of various cyclobutenes via intermolecular reaction. Broad mechanism studies propose that the productive triplet excitation of alkenes in spite of the similar triplet energies of alkynes and alkenes can be explained by the localized spin densities on olefinic carbons. In addition, the intramolecular cycloaddition of enyne achieved the efficient synthesis for conjugated dienes through triplet tandem excitation of the formed cyclobutene products, that may provide an alternative approach to the transition metal-catalyzed ring closing enyne metathesis. 1,3-Dienes are useful intermediates in synthetic chemistry in that they can be easiely found in various transformations. Moreover, We have demonstrated the utility of our synthetic method by achieving several synthetic applications such as the synthesis for a variety of extended π-systems.