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

endo-Tricyclo[3.2.1.02,4]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 endo-fused cyclopropane bond, elongating it to r = 1.69 A. The resulting trishomocyclopropyl HOMO{−1} is a three-center two-electron bond responsible for the electron-deficient c...