Tricyclo[2.1.0.02,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic Carbene

Tricyclo­[2.1.0.02,5]­pent-3-ylidene is a carbene foreseen to rearrange to pyramidane (c-C4H4)­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 (ΔES–T = −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 Cs-symmetric singlet (ωflap = ±44 deg) and C2v-symmetric triplet (ωflap = 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 C2v-symmetric transition state (TS) (Ea = 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 proximal conformation is preferred when R is electron-donating and a distal one is favored when R is electron-withdrawing. Finally, carbene rearrangements via 1,2-C atom shift or enyne fragmentation were computed. The C2v-symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.

三环[2.1.0.0²,⁵]戊-3-亚基（tricyclo[2.1.0.0²,⁵]pent-3-ylidene）是一种卡宾（carbene），理论上可重排为金字塔烷（pyramidane (c-C4H4)C）——一种含有反置碳原子的高张力分子。采用(U)MPWB1K/cc-pVTZ//(U)MPWB1K/6-311G(d)理论模型对该卡宾进行建模分析，结果显示其具有较大的单重态-三重态能隙（ΔE_S–T = -45 kcal/mol）、较高的气相质子亲和能（PA = 258 kcal/mol），且对缺电子烯烃表现出偏好性，这些性质均符合亲核性卡宾的特征。Cs对称单重态（ωflap = ±44°）与C2v对称三重态（ωflap = 0°）之间的结构差异，源于前者存在非经典电子离域，而后者并无该现象。针对该卡宾单重态主桥（包含活性二价C3原子）的简并桥折叠过程，计算结果表明其反应速率较慢，这是由于C2v对称过渡态（TS）的活化能垒较高（Ea = 17 kcal/mol）。构象平衡的位置受立体电子控制。该卡宾的1-取代衍生物（R ≠ H）对σ诱导效应较为敏感：当R为给电子基团时，近端构象更稳定；当R为吸电子基团时，则更倾向于远端构象。此外，研究还计算了通过1,2-碳原子迁移或炔烯断裂发生的卡宾重排反应，其中C2v对称桥折叠过渡态具备引发炔烯断裂的适宜几何构型。