Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors
Metathesis Activity Encoded in the Metallacyclobutane Carbon-13 NMR Chemical Shift Tensors
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DOI：
10.1021/acscentsci.7b00174
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发表时间：
2017-07-26
影响因子：
18.2
通讯作者：
Eisenstein, Odile
中科院分区：
文献类型：
--
作者：
Gordon, Christopher P.;Yamamoto, Keishi;Eisenstein, Odile
金属环丁烷是一类重要的[gjrapic]有机金属中间体，因其在烯烃复分解中的作用。它们既可以是平面的，也可以是具有特殊化学和物理性质的皱环。甲基化活性金属环丁烷具有较短的M- c - α / α ‘和M…c - β距离，较长的c - α / α ’- c - β键长度，以及各向异性的C-13化学位移，α -碳和β -碳的过渡金属化合物分别出现在约100和0 ppm时。对于d(0)体系，不表现出复分解活性的金属环丁烷在典型的40 ppm和30 ppm分别具有α -碳和β -碳的C-13化学位移，对于具有较高d(n)电子计数（n = 2和6）的金属循环的a-碳，其向上位移约为-30 ppm。固体核磁共振测量化学位移张量，结合轨道（自然化学位移，NCS）分析其主成分(δ (11) >= δ (22) >= δ(33))和双组分计算表明，甲基化活性金属环丁烷的特定化学位移来自位于具有局部pi*(M-C-a/a')特征的金属环丁烷平面上的低空轨道。因此，在复分解活性金属环丁烷中，α -碳保留了一些残余的烷基烯特征，而它们的β -碳被屏蔽了，特别是在垂直于环的方向上。总的来说，化学位移张量直接提供了关于金属环丁烷成为烯烃转化中间体能力的预测值的信息。
Metallacyclobutanes are an important class of[GJRAPHIC]organometallic intermediates, due to their role in olefin metathesis. They can have either planar or puckered rings associated with characteristic chemical and physical properties. Metathesis active metallacyclobutanes have short M-C-alpha/alpha' and M...C-beta distances, long C-alpha/alpha'-C-beta bond length, and isotropic C-13 chemical shifts for both early d(0) and late d(4) transition metal compounds for the alpha- and beta-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have C-13 chemical shifts of the alpha- and beta-carbons at typically 40 and 30 ppm, respectively, for d(0) systems, with upfield shifts to ca. -30 ppm for the a- carbon of metallacycles with higher d(n) electron counts (n = 2 and 6). Measurements of the chemical shift tensor by solid-state NMR combined with an orbital (natural chemical shift, NCS) analysis of its principal components (delta(11) >= delta(22) >= delta(33)) with two-component calculations show that the specific chemical shift of metathesis active metallacyclobutanes originates from a low-lying empty orbital lying in the plane of the metallacyclobutane with local pi*(M-C-a/a') character. Thus, in the metathesis active metallacyclobutanes, the alpha- carbons retain some residual alkylidene character, while their beta-carbon is shielded, especially in the direction perpendicular to the ring. Overall, the chemical shift tensors directly provide information on the predictive value about the ability of metallacyclobutanes to be olefin metathesis intermediates.