Open AccessTricyclo[2.1.0.02,5]pent-3-ylidene: Stereoelectronic Control of Bridge-Flapping within a Nonclassical Nucleophilic Carbene
Author(s) -
Murray G. Rosenberg,
Udo H. Brinker
Publication year - 2020
Publication title -
journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.2
H-Index - 228
eISSN - 1520-6904
pISSN - 0022-3263
DOI - 10.1021/acs.joc.0c02414
Subject(s) - carbene , chemistry , singlet state , nucleophile , delocalized electron , allene , crystallography , stereochemistry , computational chemistry , photochemistry , physics , excited state , atomic physics , catalysis , organic chemistry , biochemistry
Tricyclo[2.1.0.0 2,5 ]pent-3-ylidene is a carbene foreseen to rearrange to pyramidane ( c -C 4 H 4 )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 E 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 C s -symmetric singlet (ω flap = ±44 deg) and C 2 v -symmetric triplet (ω flap = 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 2 v -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 2 v -symmetric bridge-flapping TS has the proper geometry to initiate enyne fragmentation.