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

• Published in: Journal of organic chemistry 2021-01, Vol.86 (1), p.878-891
• Main Authors: Rosenberg, Murray G, Brinker, Udo H
• Format: Article
• Language: English
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/acs.joc.0c02414

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 (ΔE S–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 C 2v -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 C 2v -symmetric transition state (TS) (E a = 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 C 2v -symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.