钌/镍双金属光氧化还原催化C—P键交叉偶联反应机理的理论研究

Theoretical mechanistic study of Ru^(Ⅱ)/Ni^(0)-metallaphotoredox catalyzed C—P cross-coupling reaction

光介导的钌/镍双过渡金属光氧化还原催化策略可以实现室温条件下温和地构建C—P键,但由于金属镍的催化循环历程和外加碱的详细机理仍不够明确,本文利用密度泛函理论计算研究了二苯基氧化膦和芳基碘化物C—P键交叉偶联反应的机理.计算结果表明,结合光催化剂还原淬灭(Ru^(Ⅱ)-^(*)Ru^(Ⅱ)-Ru^^(Ⅰ)-Ru^(Ⅱ))和镍催化循环(Ni0-Ni^(Ⅰ)-Ni^(Ⅲ)-Ni^^(Ⅰ)-Ni^(0))的自由基机制是优势路径,其中无机碱Cs_(2)CO_(3)发挥重要作用,它辅助底物二苯基氧化膦参与光催化剂淬灭,并通过分步质子耦合电子转移过程产生重要中间体磷自由基.该研究结果有助于加深研究者对光介导的氧化还原构建C—P键偶联反应机理的认知,我们希望可以为实验化学家进一步设计过渡金属镍和光催化剂协同催化构建C—X键提供思路.

Photoredox-mediated ruthenium(Ⅱ)/nickel(0) dual catalysis has successfully triggered challenging carbon-phosphorus cross-coupling reactions. However, detailed mechanisms, such as the catalytic cycles for dual catalysts and the role of base additive, remain controversy in these reactions. In this study, the C—P bond-forming reaction mechanism of diarylphosphine oxide and aryl iodide has been investigated at the SMD(MeOH)/(U)M06/[6-311++G(d, p)/SDD(Ni, I, Ru, Cs)]//(U)M06/[6-31G(d)/LanL2DZ(Ni, I, Ru, Cs)] level. The density functional theory calculations suggest that the radical mechanism merging reductive quenching(Ru^(Ⅱ)-^(*)Ru^(Ⅱ)-Ru^^(Ⅰ)-Ru^(Ⅱ))and nickel catalytic cycles(Ni0-Ni^(Ⅰ)-Ni^(Ⅲ)-Ni^^(Ⅰ)-Ni^(0)) is favorable. The photoredox catalytic cycle starts with a thermodynamically and kinetically favorable single electron transfer(SET) process, where photoexcited*RuⅡis reductively quenched by activated dimer of Cs_(2)CO_(3)-diphenylphosphine oxide to generate the ground-state RuIand phosphorus-centered radical cation complex. The inorganic base Cs_(2)CO_(3)plays an important role in the reaction process. It promotes the reductive quenching of excited-state photocatalyst*RuⅡand activates diphenylphosphine oxide via a stepwise proton-coupled electron transfer(PCET) process to generate a key phosphorus-centered radical. The phosphorus-centered radical with high activity and short life could quickly attacks Ni~0 catalyst to obtain the active NiIintermediate. A subsequent two-electron oxidative addition of aryl iodide to NiIand reductive elimination from NiⅢrealizes the C—P bond cross-coupling. Finally, the second exergonic SET process between nickel(Ⅰ) iodide and RuIcomplex can regenerate Ni~0 and RuⅡto restart the dual catalytic cycle. The oxidative addition is the rate-determining step with an energy barrier of 42.3 kJ·mol-1. Such conclusions deepen our understanding of the C—P cross-coupling reaction mechanism and would provide inspiration for experimental chemists to further design photoredox/nickel dual catalysis in the construction of C-heteroatom bonds.