Mechanistic Investigation on Oxygen-Mediated Photoredox Diels–Alder Reactions with Chromium Catalysts

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• 作者：Yimeng Yang ; Qian Liu ; Liang Zhang ; Haizhu YuZhimin Dang
• 刊名：Organometallics
• 出版年：2017
• 出版时间：February 13, 2017
• 年：2017
• 卷：36
• 期：3
• 页码：687-698
• 全文大小：879K
• ISSN：1520-6041

文摘

The recent dioxygen-mediated, Cr-complex-catalyzed photoredox Diels–Alder reaction between two electron-rich substrates represents the first example of first-row transition-metal photocatalysis. Motivated by the mechanistic ambiguity (such as the inherent interactions of O₂ with Cr complexes and the origin of the high selectivity), we performed systematic density functional theory (DFT) calculations. The calculation results show that O₂ shuttles an electron via an inner-sphere mechanism (in the form of [CrL₃-O₂] complexes, L = Ph₂phen). Meanwhile, the overall transformations involve the excitation and quenching of the CrL₃³⁺ complex, single-electron transfer from the dienophile to the excited state CrL₃³⁺ catalyst (SET-1), asynchronous cycloaddition (instead of synchronous), and the final single electron transfer from the quintet [CrL₃-O₂]²⁺ complex (instead of superoxide) to the radical cationic cycloadduct. The first C–C bond formation in the asynchronous cycloaddition is the rate-determining and selectivity-determining step. On this basis, the origins of the chemo-, regio-, and stereoselectivity have been identified.