title: Intermolecular Reactions of a Foiled Carbene with Carbonyl Compounds: The Effects of Trishomocyclopropyl Stabilization.
abstract: endo-Tricyclo[3.2.1.0(2,4)]oct-8-ylidene is a foiled carbene reaction intermediate. It was generated by thermolyzing Δ(3)-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 (ΔE(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) 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.
authors.affiliation: Institute of Organic Chemistry, University of Vienna , Währinger Strasse 38, A-1090 Vienna, Austria.
authors.forename: Ingrid Malene
authors.initials: IM
authors.lastname: Apeland
authors.affiliation: Department of Chemistry, The State University of New York at Binghamton , P.O. Box 6000, Binghamton, New York 13902-6000, United States.
authors.forename: Murray G
authors.initials: MG
authors.lastname: Rosenberg
authors.affiliation: Institute of Inorganic Chemistry, University of Vienna , Währinger Strasse 42, A-1090 Vienna, Austria.
authors.forename: Vladimir B
authors.initials: VB
authors.lastname: Arion
authors.affiliation: Institute of Organic Chemistry, University of Vienna , Währinger Strasse 38, A-1090 Vienna, Austria.
authors.forename: Hanspeter
authors.initials: H
authors.lastname: Kählig
authors.affiliation: Institute of Organic Chemistry, University of Vienna , Währinger Strasse 38, A-1090 Vienna, Austria.|Department of Chemistry, The State University of New York at Binghamton , P.O. Box 6000, Binghamton, New York 13902-6000, United States.
authors.forename: Udo H
authors.initials: UH
authors.lastname: Brinker