Mechanism of Pd/Senphos-Catalyzed trans-Hydroboration of 1,3-Enynes: Experimental and Computational Evidence in Support of the Unusual Outer-Sphere Oxidative Addition Pathway.
Mechanism of Pd/Senphos-Catalyzed trans-Hydroboration of 1,3-Enynes: Experimental and Computational Evidence in Support of the Unusual Outer-Sphere Oxidative Addition Pathway.
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DOI：
10.1021/acs.joc.2c02841
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发表时间：
2023-02
期刊：
影响因子：
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通讯作者：
Yuanzhe Zhang;Ziyong Wang;W. Lamine;Senmiao Xu;Bo Li;A. Chrostowska;K. Miqueu;Shih‐Yuan Liu
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文献类型：
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作者：
Yuanzhe Zhang;Ziyong Wang;W. Lamine;Senmiao Xu;Bo Li;A. Chrostowska;K. Miqueu;Shih‐Yuan Liu
采用多种实验技术研究了Pd/ senphos催化1,3-炔反式硼氢化反应的反应机理，包括氘标记实验和双交叉标记实验、模型反应中间体的x射线晶体学表征和反应过程动力学分析。我们的实验数据支持一个不寻常的外球氧化加成机制，其中儿茶酚硼烷作为一种合适的亲电试剂来激活pd0结合的1,3-炔底物，形成Pd-η - 3-π-烯丙基，这已被确定为催化循环的可能静息状态。儿茶酚硼烷的双交叉标记指向硼烷作为氢化物传递梭子的第二个作用。密度泛函理论计算表明，反应的限速过渡态是由儿茶硼烷梭子抽离氢化物，这符合实验确定的速率定律：速率= k[炔]0[硼烷]1[催化剂]1。计算得到的活化自由能ΔG‡= 17.7 kcal/mol和KIE （kH/kD = 1.3）也与实验结果一致。总的来说，这项工作通过实验建立了路易斯酸（如儿茶酚硼烷）作为可行的亲电活化剂参与外球氧化加成反应，并指出这种未充分利用的机制是激活不饱和底物的一般方法。
The reaction mechanism of the Pd/Senphos-catalyzed trans-hydroboration reaction of 1,3-enynes was investigated using various experimental techniques, including deuterium and double crossover labeling experiments, X-ray crystallographic characterization of model reaction intermediates, and reaction progress kinetic analysis. Our experimental data are in support of an unusual outer-sphere oxidative addition mechanism where the catecholborane serves as a suitable electrophile to activate the Pd0-bound 1,3-enyne substrate to form a Pd-η3-π-allyl species, which has been determined to be the likely resting state of the catalytic cycle. Double crossover labeling of the catecholborane points toward a second role played by the borane as a hydride delivery shuttle. Density functional theory calculations reveal that the rate-limiting transition state of the reaction is the hydride abstraction by the catecholborane shuttle, which is consistent with the experimentally determined rate law: rate = k[enyne]0[borane]1[catalyst]1. The computed activation free energy ΔG‡ = 17.7 kcal/mol and KIE (kH/kD = 1.3) are also in line with experimental observations. Overall, this work experimentally establishes Lewis acids such as catecholborane as viable electrophilic activators to engage in an outer-sphere oxidative addition reaction and points toward this underutilized mechanism as a general approach to activate unsaturated substrates.