Yang Yimeng, Wen Yongqiang, Dang Zhimin, Yu Haizhu
Department of Polymer Science and Engineering, University of Science and Technology Beijing , Beijing 100083, PR China.
Department of Chemistry and Center for Atomic Engineering of Advanced Materials, Anhui University , Hefei 230601, PR China.
J Phys Chem A. 2017 Jun 15;121(23):4552-4559. doi: 10.1021/acs.jpca.7b02995. Epub 2017 Jun 6.
The recent thioxanthone-sensitizer-catalyzed intermolecular [2 + 2] cycloaddition induced by visible-light irradiation set the stage for the future development of feasible photocycloadditions. Nonetheless, the mechanism of this reaction still remains under debate, especially on the activation mode of the thioxanthone photosensitizer (energy transfer, bielectron exchange, and hydrogen transfer are all possible mechanisms). To settle this issue, systematic density functional theory calculations have been carried out. The results indicate that the energy-transfer pathway is more favorable than the bielectron-exchange and the hydrogen-transfer pathways. Meanwhile, the overall transformations involve the complexation and excitation of photosensitizer, the first C-C bond formation, the dissociation of the sensitizer, the triplet-to-singlet electronic state crossing, and the second C-C bond formation. The first C-C bond formation is the rate- and selectivity-determining step, and synergistic energy and electron transfer from photosensitizer to substrate moieties takes place along this process. On this basis, the effect of olefin substrates (ethyl vinyl ketone vs vinyl acetate) on the stereoselectivity was finally analyzed.
近期，由可见光照射引发的噻吨酮敏化剂催化分子间[2 + 2]环加成反应为可行的光环加成反应的未来发展奠定了基础。尽管如此，该反应的机理仍存在争议，尤其是噻吨酮光敏剂的活化模式（能量转移、双电子交换和氢转移都是可能的机制）。为了解决这个问题，已经进行了系统的密度泛函理论计算。结果表明，能量转移途径比双电子交换和氢转移途径更有利。同时，整个转化过程包括光敏剂的络合和激发、第一个C-C键的形成、敏化剂的解离、三重态到单重态的电子态跃迁以及第二个C-C键的形成。第一个C-C键的形成是速率和选择性的决定步骤，在此过程中发生了从光敏剂到底物部分的协同能量和电子转移。在此基础上，最终分析了烯烃底物（乙烯基乙基酮与醋酸乙烯酯）对立体选择性的影响。
