Visible-Light-Triggered Selective Intermolecular [2+2] Cycloaddition of Extended Enones: 2-Oxo-3-enoates and 2,4-Dien-1-ones with Olefins

Photosensitization has recently re-emerged owing to the current interest in visible-light catalysis. One of the photoreactions investigated in this context, namely, photo[2+2]cycloaddition of olefins, is established to show high selectivity and wide generality. Here, we describe the results of our studies on selective intermolecular cycloaddition between extended enones (2,4-dien-1-ones and 2-oxo-3-enoates) and olefins under visible-light sensitization. With Ru(bpy)₃Cl₂ as the triplet energy sensitizer, [2+2] addition of 2,4-dien-1-ones to olefins resulted in the addition to the "ene" part of enones with high efficiency. Generality and functional group tolerance were established by examining a number of enones. 2-Oxo-3-enoates also underwent addition to olefins in the presence of Ru(phen)₃(PF₆)₂. Both additions were more efficient in the presence of the triplet sensitizer than upon direct irradiation. No Paternò-Büchi product was detected. Density functional theory calculations revealed the origin of high selectivity in the two extended enone systems. Together with spectroscopic studies and control experiments, the cycloaddition has been demonstrated to occur from the excited triplet state of these extended enones, which were generated via the energy transfer process.