Insights into the Mechanism of an Allylic Arylation Reaction via Photoredox-Coupled Hydrogen Atom Transfer
Insights into the Mechanism of an Allylic Arylation Reaction via Photoredox-Coupled Hydrogen Atom Transfer
复制标题
DOI：
10.1021/acs.joc.1c02235
复制
发表时间：
2021-12-09
影响因子：
3.6
通讯作者：
Swierk, John R.
中科院分区：
文献类型：
--
作者：
Spielvogel, Ethan H.;Stevenson, Bernard G.;Swierk, John R.
尽管作为一种合成方法广泛使用，但光氧化还原偶联氢原子转移（HAT）反应的精确机制和动力学仍然知之甚少。这是由于缺乏详细的动力学信息以及副反应和产物的鉴定造成的。在本报告中，报道了使用 Ir(ppy)(3) 光催化剂和硫醇 HAT 催化剂偶联环己烯和 1,4-二氰基苯 (DCB) 的典型串联光氧化还原/HAT 反应的机理研究。通过电化学、光化学和光谱测量的结合，确定了关键的非生产途径和副产物，并提取了主要化学步骤的速率常数。发现反应量子产率在反应过程中迅速下降。一种未报道的氰醇副产物被鉴定出来，并被认为在反应中作为质子受体发挥着关键作用。瞬态吸收光谱 (TAS) 和量子化学计算表明，反应机制涉及亲核 DCB 自由基阴离子与环己烯的自由基加成，并发生协同 HAT 作为再生烯烃的最后一步。使用 TAS 得出的速率常数进行反应动力学建模，表明反应效率受到激发的 Ir(ppy)(3) 和氰醇光产物之间的寄生吸收和无效猝灭的限制。
Despite widespread use as a synthetic method, the precise mechanism and kinetics of photoredox coupled hydrogen atom transfer (HAT) reactions remain poorly understood. This results from a lack of detailed kinetic information as well as the identification of side reactions and products. In this report, a mechanistic study of a prototypical tandem photoredox/HAT reaction coupling cyclohexene and 1,4-dicyanobenzene (DCB) using an Ir(ppy)(3) photocatalyst and thiol HAT catalyst is reported. Through a combination of electrochemical, photochemical, and spectroscopic measurements, key unproductive pathways and side products are identified and rate constants for the main chemical steps are extracted. The reaction quantum yield was found to decline rapidly over the course of the reaction. An unreported cyanohydrin side product was identified and thought to play a key role as a proton acceptor in the reaction. Transient absorption spectroscopy (TAS) and quantum chemical calculations suggested a reaction mechanism that involves radical addition of the nucleophilic DCB radical anion to cyclohexene, with cooperative HAT occurring as the final step to regenerate the alkene. Kinetic modeling of the reaction, using rate constants derived from TAS, demonstrates that the efficiency of the reaction is limited by parasitic absorption and unproductive quenching between excited Ir(ppy)(3) and the cyanohydrin photoproduct.