Halide Noninnocence and Direct Photoreduction of Ni(II) Enables Coupling of Aryl Chlorides in Dual Catalytic, Carbon–Heteroatom Bond-Forming Reactions

Recent mechanistic studies of dual photoredox/Ni-catalyzed,
light-driven
cross-coupling reactions have found that the photocatalyst (PC) operates
through either reductive quenching or energy transfer cycles. To date,
reports invoking oxidative quenching cycles are comparatively rare
and direct observation of such a quenching event has not been reported.
However, when PCs with highly reducing excited states are used (e.g.,
Ir(ppy)3), photoreduction of Ni(II) to Ni(I) is thermodynamically
feasible. Recently, a unified reaction system using Ir(ppy)3 was developed for forming C–O, C–N, and C–S
bonds under the same conditions, a prospect that is challenging with
PCs that can photooxidize these nucleophiles. Herein, in a detailed
mechanistic study of this system, we observe oxidative quenching of
the PC (Ir(ppy)3 or a phenoxazine) via nanosecond transient
absorption spectroscopy. Speciation studies support that a mixture
of Ni–bipyridine complexes forms under the reaction conditions,
and the rate constant for photoreduction increases when more than
one ligand is bound. Oxidative addition of an aryl iodide was observed
indirectly via oxidation of the resulting iodide by Ir(IV)(ppy)3. Intriguingly, the persistence of the Ir(IV)/Ni(I) ion pair
formed in the oxidative quenching step was found to be necessary to
simulate the observed kinetics. Both bromide and iodide anions were
found to reduce the oxidized form of the PC back to its neutral state.
These mechanistic insights inspired the addition of a chloride salt
additive, which was found to alter Ni speciation, leading to a 36-fold
increase in the initial turnover frequency, enabling the coupling
of aryl chlorides.

近年来，针对双光氧化还原/镍（Ni）催化的光驱动交叉偶联反应的机理研究表明，光催化剂（photocatalyst，PC）的催化循环主要通过还原淬灭或能量转移路径进行。截至目前，提及氧化淬灭循环的相关报道相对较少，且尚未有该类淬灭过程的直接观测研究发表。然而，当使用具备强还原性激发态的光催化剂时（例如三(2-苯基吡啶)合铱（Ir(ppy)3）），将Ni(II)光还原为Ni(I)在热力学上是可行的。近期，有研究开发了一套以三(2-苯基吡啶)合铱（Ir(ppy)3）为催化剂的通用反应体系，可在相同条件下构建C-O、C-N及C-S键——而对于能够氧化这些亲核试剂的光催化剂而言，实现该类转化颇具挑战。在此，通过对该体系的详细机理研究，我们借助纳秒瞬态吸收光谱观测到了该光催化剂（Ir(ppy)3或吩噁嗪）的氧化淬灭过程。物种分布研究表明，反应条件下会生成多种镍联吡啶配合物的混合物，且当配体结合数目多于一个时，光还原反应的速率常数会升高。我们通过Ir(IV)(ppy)3对生成的碘离子的氧化过程，间接观测到了芳基碘化物的氧化加成步骤。值得注意的是，氧化淬灭步骤中生成的Ir(IV)/Ni(I)离子对的存续性，是复刻实验观测到的动力学行为所必需的条件。研究发现，溴离子与碘离子均可将氧化态的光催化剂还原回中性状态。上述机理研究结果启发我们添加氯化物盐添加剂，该添加剂可改变镍物种的分布状态，使初始周转频率提升36倍，从而实现芳基氯的偶联反应。