Schoeller Wolfgang W, Frey Guido D, Bertrand Guy
Fakultät für Chemie der Universität, Universität Bielefeld, Bielefeld, Germany.
Chemistry. 2008;14(15):4711-8. doi: 10.1002/chem.200701926.
Cyclopropenylidenes are Hückel aromatic pi systems in which one of the ring atoms is a carbene center. Quantum chemical calculations at the density functional level of theory, supplemented by coupled-cluster calculations, indicate that there is a sizeable energy separation between the lowest-energy singlet and triplet states of these species. Amino groups considerably increase the energy difference between these two states, whereas electron-withdrawing substituents decrease it. The 1,1-dimerization products of cyclopropenylidenes, namely, triafulvalenes, have been investigated. The calculations show that, without steric hindrance and considerable electronic stabilization, cyclopropenylidenes are kinetically unstable and dimerize. Different substituents (alkyl, silyl, terphenyl, amino, and phosphoraneiminato) were probed to tune the frontier orbital energies of cyclopropenylidenes. Accordingly, it is predicted that by a suitable choice of substituents at the olefinic positions, cyclopropenylidenes can be more nucleophilic than their five-membered ring congeners, namely, imidazol-2-ylidenes.
环丙烯叉是休克尔芳香π体系，其中一个环原子是卡宾中心。在密度泛函理论水平上进行的量子化学计算，并辅以耦合簇计算，结果表明这些物种的最低能量单重态和三重态之间存在相当大的能量间隔。氨基显著增加了这两个态之间的能量差，而吸电子取代基则使其减小。对环丙烯叉的1,1-二聚产物，即三亚富瓦烯进行了研究。计算表明，在没有空间位阻和可观的电子稳定作用的情况下，环丙烯叉在动力学上是不稳定的，会发生二聚反应。研究了不同的取代基（烷基、硅基、三联苯、氨基和磷亚胺基）来调节环丙烯叉的前沿轨道能量。因此，预计通过在烯烃位置适当选择取代基，环丙烯叉可以比其五元环同系物，即咪唑-2-亚基更具亲核性。
