Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, Texas 77842, United States.
J Org Chem. 2021 May 7;86(9):6305-6313. doi: 10.1021/acs.joc.1c00099. Epub 2021 Apr 23.
C kinetic isotope effects (KIEs) for the photoredox-promoted [2 + 2] cycloaddition of enones were determined in homocoupling and heterocoupling examples. The only significant KIEs were observed at the β carbon, indicating that C-C bond formation is irreversible. However, these KIEs were much lower than computational predictions, suggesting that product selectivity is determined in part by a step prior to C-C bond formation. The results are explained as arising from a competition between C-C bond formation and electron exchange between substrate alkenes. This idea is supported by a relatively small substituent effect on substrate selectivity. The possible rates for electron transfer and bond-forming steps are analyzed, and the competition appears plausible, particularly if the mechanism involves a complex between reduced and neutral enone molecules.
在偕二酮的光氧化还原促进的[2+2]环加成的同偶联和异偶联实例中，确定了 C 动力学同位素效应（KIE）。仅在β碳上观察到明显的 KIE，表明 C-C 键的形成是不可逆的。然而，这些 KIE 远低于计算预测，表明产物选择性部分取决于 C-C 键形成之前的步骤。结果解释为 C-C 键形成和底物烯烃之间的电子交换之间的竞争所致。这一想法得到了对底物选择性的相对较小取代基效应的支持。分析了电子转移和键形成步骤的可能速率，并且该竞争似乎是合理的，特别是如果该机制涉及还原和中性偕二酮分子的复合物。
