Mechanistic and Kinetic Insights into Intermolecular [2+2] Photocycloadditions

Polypyridyl metal complexes of ruthenium(II) have emerged as efficient single electron transfer (SET) reagents for photocatalytic reactions, allowing a variety of organic transformations to be undertaken and facilitating access to organic products that can be otherwise difficult to obtain. However, despite ongoing advances in design, less than ideal reactivity can be obtained if the excited state properties of the photocatalyst are not investigated under photocatalytically relevant conditions. Herein, we have used a combination of in situ 1H NMR photoirradiation, steady state and time-resolved photophysical measurements, including transient absorption spectroscopy, to reveal kinetic and mechanistic details for the intermolecular [2+2] photocycloaddition of trans-anethole using [Ru(bpm)3]2+ (bpm = 2,2′-bipyrimidine) as the catalyst. Using this approach, we show that formation of the cyclobutane homodimer is mediated by both closed cycle and catalytic chain reaction mechanisms, with the former appearing to be predominant. Interestingly, the reaction leads to appreciable generation of photoreduced [Ru(bpm)2(bpm•–)]+ in situ which appears to be surprisingly long-lived. We suggest the reduced photocatalyst is not turned over by molecular 3O2, and instead, the role of singlet oxygen as a critical redox modulator is proposed and supported by other spectroscopic evidence.