Department of Chemistry, School of Science, Tianjin University, Tianjin 300072, China.
College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108, China.
Org Biomol Chem. 2021 May 5;19(17):3850-3858. doi: 10.1039/d1ob00446h.
Density functional theory calculations have revealed the mechanism and origins of the reactivity and regioselectivity of the CpIr(iii)/CpRh(iii)-catalyzed allylic C-H amidation of alkenes and dioxazolones. Generally, the catalytic cycle consists of alkene coordination, C(sp3)-H activation, dioxazolone oxidative addition, reductive elimination and proto-demetallation to give the final amidation product. The C-H activation is found to be the rate-determining step, and it controls the reactivity of the reaction. For the CpIr(iii)-catalyzed system, the C-H activation undergoes an Ir(iii)-assisted proton transfer process with a low energy barrier, elucidating its high reactivity. In contrast, the C-H activation step is more like a direct deprotonation in the CpRh(iii)-catalyzed system, which is responsible for its higher barrier and lower reactivity. The branched-selectivity arises from the electronic effect of the alkyl group on the charge distribution over the allylic moiety. Herein, iridium(v) polarizes the allylic group greater than that of the rhodium(v) system, which accounts for its good regioselectivity. The mechanistic insights will be useful for the further development of transition metal-catalyzed selective C-H amination reactions.
密度泛函理论计算揭示了 CpIr(iii)/CpRh(iii)催化烯丙基 C-H 酰胺化反应的反应性和区域选择性的机制和起源。通常，催化循环包括烯烃配位、C(sp3)-H 活化、二恶唑酮氧化加成、还原消除和原脱金属化，以得到最终的酰胺化产物。C-H 活化被发现是速率决定步骤，它控制着反应的活性。对于 CpIr(iii)催化体系，C-H 活化经历了一个 Ir(iii)辅助的质子转移过程，具有较低的能量势垒，解释了其高反应性。相比之下，在 CpRh(iii)催化体系中，C-H 活化更像是直接去质子化，这导致了其更高的势垒和更低的反应性。支链选择性源于烷基对烯丙基部分电荷分布的电子效应。在此，铱(v)比铑(v)体系更能极化烯丙基部分，这解释了其良好的区域选择性。这些机理见解将有助于进一步发展过渡金属催化的选择性 C-H 胺化反应。
