Intermolecular Reactions of a Foiled Carbene with Carbonyl Compounds: The Effects of Trishomocyclopropyl Stabilization

<i>endo</i>-Tricyclo­[3.2.1.0^2,4]­oct-8-ylidene is a foiled carbene reaction intermediate. It was generated by thermolyzing Δ³-1,3,4-oxadiazoline precursors dissolved in benzaldehyde and acetophenone. The products appear to stem from direct insertion of the carbene’s divalent C atom into the α-bonds of the carbonyl compounds; however, this is only superficial. The strict stereochemistry observed is due to the topologies of the reaction intermediates of the proposed two-step mechanism. Bimolecular nucleophilic addition generates bent 1,3-zwitterions. The neutral reaction intermediates undergo pinacolic rearrangements to form the observed adducts. Product ratios reflect the migratory aptitudes of the carbonyl compounds’ α-substituents. The carbene reaction was modeled using DFT. The singlet carbene’s bicoordinate C atom bends 31° toward the <i>endo</i>-fused cyclopropane bond, elongating it to <i>r</i> = 1.69 Å. The resulting trishomocyclopropyl HOMO{−1} is a three-center two-electron bond responsible for the electron-deficient carbene’s nucleophilicity. Its calculated properties are consistent with this assertion: (1) singlet–triplet (Δ<i>E</i>_S–T) energy gap of −25 kcal/mol, (2) gas-phase proton affinity (PA) value of 272 kcal/mol, (3) hard and soft acid and base (HSAB) Δ<i>N</i> value of −0.2 in its initial reaction with the carbonyl compounds, and (4) negative frontier orbital interaction values ΔΔ<i>E</i>(PhC­(O)­H) = −4.38 eV and ΔΔ<i>E</i>(PhC­(O)­Me) = −3.97 eV