Gordon Christopher P, Yamamoto Keishi, Liao Wei-Chih, Allouche Florian, Andersen Richard A, Copéret Christophe, Raynaud Christophe, Eisenstein Odile
Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1-5, 8093, Zürich, Switzerland.
Department of Chemistry, University of California, Berkeley, California 94720, United States.
ACS Cent Sci. 2017 Jul 26;3(7):759-768. doi: 10.1021/acscentsci.7b00174. Epub 2017 Jun 14.
Metallacyclobutanes are an important class of 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 and M···C distances, long C-C bond length, and isotropic C chemical shifts for both early d and late d transition metal compounds for the α- and β-carbons appearing at ca. 100 and 0 ppm, respectively. Metallacyclobutanes that do not show metathesis activity have C chemical shifts of the α- and β-carbons at typically 40 and 30 ppm, respectively, for d systems, with upfield shifts to ca. -30 ppm for the α-carbon of metallacycles with higher d electron counts ( = 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 (δ ≥ δ ≥ δ) 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 π*(M-C) character. Thus, in the metathesis active metallacyclobutanes, the α-carbons retain some residual alkylidene character, while their β-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.
金属环丁烷是一类重要的有机金属中间体，因其在烯烃复分解反应中的作用。它们可以具有与特征化学和物理性质相关的平面或褶皱环。复分解活性金属环丁烷具有较短的M-C和M···C距离、较长的C-C键长，并且对于早期d和晚期d过渡金属化合物，α-和β-碳的各向同性C化学位移分别出现在约100和0 ppm处。不显示复分解活性的金属环丁烷对于d体系，α-和β-碳的C化学位移通常分别为40和30 ppm，对于具有较高d电子数（ = 2和6）的金属环的α-碳，其化学位移向高场移动至约-30 ppm。通过固态NMR测量化学位移张量，并结合其主成分（δ ≥ δ ≥ δ）的轨道（自然化学位移，NCS）分析进行双组分计算表明，复分解活性金属环丁烷的特定化学位移源于位于金属环丁烷平面内具有局部π*(M-C)特征的低能空轨道。因此，在复分解活性金属环丁烷中，α-碳保留了一些残余的亚烷基特征，而它们的β-碳受到屏蔽，特别是在垂直于环的方向上。总体而言，化学位移张量直接提供了关于金属环丁烷作为烯烃复分解中间体能力的预测价值的信息。
