Rosenberg Murray G, Brinker Udo H
Department of Chemistry , The State University of New York at Binghamton , P.O. Box 6000, Binghamton , New York 13902-6000 , United States.
Institute of Organic Chemistry , University of Vienna , Währinger Strasse 38 , A-1090 Vienna , Austria.
J Org Chem. 2019 Sep 20;84(18):11873-11884. doi: 10.1021/acs.joc.9b01732. Epub 2019 Sep 6.
Ab initio computations of cyclobutylcarbene (-CHCH) were performed using the UMP4(fc)/6-311++G(2df,2p)//UMP2(full)/6-311++G(d,p) theoretical model. The carbene's most striking feature is its :CH-group. It is markedly toward the elongated C1'-C2' bond in the singlet ground-state but not in the triplet state, which is at least 1.1 kcal/mol higher in energy. Nonclassical 3C2E bonding among the C1, C1', and C2' atoms is prominent in the HOMO{-1}. The electron-donating ability of the nonbonding HOMO is thereby enhanced. The intensified nucleophilicity of the singlet carbene is manifested in quantifiable ways. For example, its hard and soft acid and base (HSAB) hardness, HSAB absolute electronegativity, and gas-phase proton affinity rival those of ylide-stabilized -heterocyclic carbenes. It is computed to act as a nucleophile toward alkenes with higher HSAB hardness values. Transition states from singlet cyclobutylcarbene to bicyclo[2.1.0]pentane, cyclopentene, and methylenecyclobutane were computed and confirmed by intrinsic reaction coordinate calculations. Activation energies depend on the singlet's conformation with regard to -CH ring-puckering, :CH-group rotation, and :CH-group bending. The singlet's bent :CH-group favors bicyclo[2.1.0]pentane and cyclopentene formation.
使用UMP4(fc)/6 - 311++G(2df,2p)//UMP2(full)/6 - 311++G(d,p)理论模型对环丁基卡宾(-CHCH)进行了从头算。该卡宾最显著的特征是其:CH基团。在单重态基态中，它明显朝向拉长的C1'-C2'键，而在三重态中则不然，三重态能量至少高1.1千卡/摩尔。C1、C1'和C2'原子之间的非经典3C2E键在最高占据分子轨道{-1}(HOMO{-1})中很突出。由此增强了非键最高占据分子轨道的给电子能力。单重态卡宾增强的亲核性以可量化的方式表现出来。例如，其软硬酸碱(HSAB)硬度、HSAB绝对电负性和气相质子亲和力可与叶立德稳定的-杂环卡宾相媲美。计算表明它对具有较高HSAB硬度值的烯烃起亲核试剂的作用。计算了从单重态环丁基卡宾到双环[2.1.0]戊烷、环戊烯和亚甲基环丁烷的过渡态，并通过内禀反应坐标计算得到了证实。活化能取决于单重态关于-CH环皱曲、:CH基团旋转和:CH基团弯曲的构象。单重态弯曲的:CH基团有利于双环[2.1.0]戊烷和环戊烯的形成。
