‡Institute for Molecular Science, Okazaki 444-8585, Japan.
§Fukui Institute for Fundamental Chemistry, Kyoto University, Kyoto 606-8103, Japan.
J Am Chem Soc. 2015 Jun 3;137(21):6820-8. doi: 10.1021/jacs.5b01444. Epub 2015 May 22.
The reaction mechanism and origin of regioselectivity of (2 + 2) cycloadditions of benzyne to endohedral metallofullerenes M3N@C80 (M = Sc, Y) were investigated with density functional calculations. The reaction was demonstrated to follow a diradical mechanism rather than a carbene mechanism, in which the formation of the diradical intermediate is the rate-determining step. Through rotation of benzyne moiety on the fullerene surface, the diradical intermediate on 566 site could isomerize to two new diradical intermediates which give rise to two distinct [5,6] and [6,6] benzoadducts, respectively. However, the diradical intermediate on 666 site only produces the [6,6] benzoadduct. The nature of the endohedral cluster not only influences the regioselectivity, but also determines the cycloadduct geometry. For Sc3N@C80, the [5,6] benzoadduct is preferred kinetically and thermodynamically, whereas in the case of Y3N@C80, both [5,6] and [6,6] benzoadducts are favorable. In contrast to closed-cage benzoadducts of Sc3N@C80, Y3N@C80 affords open-cage benzoadducts, making it the first example that the endohedral cluster could alter cycloadducts from the closed cage to open cage. With further analysis, it is revealed that the origin of regioselectivity results from the local strain energy of the fullerene cage.
本征金属富勒烯 M3N@C80（M=Sc，Y）内笼（endohedral）的（2+2）环加成反应的反应机理和区域选择性起源，通过密度泛函计算进行了研究。该反应遵循双自由基机理而不是卡宾机理，其中双自由基中间体的形成是速率决定步骤。通过苯炔部分在富勒烯表面的旋转，566 位的双自由基中间体可以异构化为两个新的双自由基中间体，分别产生两种不同的[5,6]和[6,6]苯加合物。然而，666 位的双自由基中间体仅产生[6,6]苯加合物。内笼簇的性质不仅影响区域选择性，而且决定环加成物的几何形状。对于 Sc3N@C80，[5,6]苯加合物在动力学和热力学上都是优先的，而对于 Y3N@C80，[5,6]和[6,6]苯加合物都是有利的。与 Sc3N@C80 的闭笼苯加合物不同，Y3N@C80 提供开笼苯加合物，这是首例内笼簇可以将环加成物从闭笼改变为开笼的例子。通过进一步分析，揭示了区域选择性起源于富勒烯笼的局部应变能。
