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. 2021 Jan 1;86(1):878-891. doi: 10.1021/acs.joc.0c02414. Epub 2020 Dec 23.
Tricyclo[2.1.0.0]pent-3-ylidene is a carbene foreseen to rearrange to pyramidane (-CH)C, a highly strained molecule featuring an inverted C atom. Modeling of the carbene, using the (U)MPWB1K/cc-pVTZ//(U)MPWB1K/6-311G(d) theoretical model, indicated a large singlet-triplet energy gap (Δ = -45 kcal/mol), a high gas-phase proton affinity (PA = 258 kcal/mol), and a preference for electron-poor alkenes. These properties are consistent with those of nucleophilic carbenes. Structural differences between the -symmetric singlet (ω = ±44 deg) and -symmetric triplet (ω = 0 deg) stem from nonclassical electron delocalization in the former and the lack thereof in the latter. Degenerate bridge-flapping of the singlet's main bridge, which comprises the reactive divalent C3 atom, is computed to be slow due to a high activation barrier of the -symmetric transition state (TS) ( = 17 kcal/mol). The position of the conformeric equilibrium is subject to stereoelectronic control. 1-Substituted derivatives of the carbene (R ≠ H) are sensitive to σ inductive effects. A conformation is preferred when R is electron-donating and a one is favored when R is electron-withdrawing. Finally, carbene rearrangements via 1,2-C atom shift or enyne fragmentation were computed. The -symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.
三环[2.1.0.0]戊-3-亚基是一种预计会重排为棱晶烷(-CH)C的卡宾，棱晶烷是一种具有高度张力且含有一个反转碳原子的分子。使用(U)MPWB1K/cc-pVTZ//(U)MPWB1K/6-311G(d)理论模型对该卡宾进行建模，结果表明其单重态-三重态能隙较大(Δ = -45千卡/摩尔)，气相质子亲和力较高(PA = 258千卡/摩尔)，并且偏好缺电子烯烃。这些性质与亲核卡宾的性质一致。-对称单重态(ω = ±44度)和-对称三重态(ω = 0度)之间的结构差异源于前者存在非经典电子离域而后者不存在。单重态主桥（包含反应性二价C3原子）的简并桥摆动经计算由于-对称过渡态(TS)的高活化能垒( = 17千卡/摩尔)而较慢。构象平衡的位置受立体电子控制。卡宾的1-取代衍生物(R ≠ H)对σ诱导效应敏感。当R为供电子基团时，优选一种构象；当R为吸电子基团时，另一种构象更受青睐。最后，计算了通过1,2-碳原子迁移或烯炔断裂进行的卡宾重排。-对称桥摆动TS具有引发烯炔断裂的合适几何结构。
