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.钌(II)多吡啶基金属配合物已成为光催化反应中高效的单电子转移（single electron transfer, SET）试剂，可驱动多种有机转化反应，助力获取常规路径下难以合成的有机产物。尽管在催化剂设计方面已有持续进展，但倘若未在光催化相关反应条件下探究光催化剂的激发态性质，往往难以获得理想的反应活性。本文中，我们结合原位1H核磁共振光辐照（in situ 1H NMR photoirradiation）、稳态与时间分辨光物理表征（steady state and time-resolved photophysical measurements，含瞬态吸收光谱（transient absorption spectroscopy）），揭示了以[Ru(bpm)₃]²⁺（bpm=2,2′-联嘧啶）为催化剂时，反式茴香脑的分子间[2+2]光环加成反应（intermolecular [2+2] photocycloaddition）的动力学与机理细节。研究表明，环丁烷同二聚体（cyclobutane homodimer）的生成同时由闭环循环机制（closed cycle）与催化链反应机制（catalytic chain reaction）介导，且前者占据主导地位。值得注意的是，该反应会原位生成大量光还原态物种[Ru(bpm)₂(bpm•–)]⁺（photoreduced [Ru(bpm)2(bpm•–)]+），其寿命出乎意料地长。我们提出，该还原态光催化剂无法被分子态三线态氧（molecular ³O₂）完成循环再生；与此同时，我们通过其他光谱学证据支持了单线态氧（singlet oxygen）作为关键氧化还原调节剂（redox modulator）的作用。