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 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 (ΔES–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) ΔN value of −0.2 in
its initial reaction with the carbonyl compounds, and (4) negative
frontier orbital interaction values ΔΔE(PhC­(O)­H) = −4.38 eV and ΔΔE(PhC­(O)­Me)
= −3.97 eV.