Shining Light on Cyclobutane Synthesis: Ir Catalyzed [2+2] Cycloadditions of Vinylogous Esters Through Energy Transfer

Cyclobutane rings can be found in many different natural and artificial small molecules but the high ring strain associated the 4-membered ring makes their preparation difficult. Most commonly, cyclobutanes are formed through the [2+2] cycloaddition of two alkenes, which is forbidden under thermal conditions, but is photochemically allowed. UV light catalyzed reactions have several drawbacks including the production of unwanted side products and potential decomposition of desired products. This work reports the use of an iridium transition metal catalyst to promote [2+2] cycloadditions of vinylogous ester compounds under lower energy visible light. These reactions offer modest yields and excellent regio- and diastereoselectivity. Absorption and phosphorescence data are reported for targeted substrates, reagents, catalysts, and additives to help elucidate the mechanism of these cycloadditions, which is believed to proceed via triplet energy transfer from the metal catalyst to a conjugate organic substrate