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

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• 作者：Ingrid Malene Apeland ; Murray G. Rosenberg ; Vladimir B. Arion ; Hanspeter K盲hlig ; Udo H. Brinker
• 刊名：Journal of Organic Chemistry
• 出版年：2015
• 出版时间：December 4, 2015
• 年：2015
• 卷：80
• 期：23
• 页码：11877-11887
• 全文大小：668K
• ISSN：1520-6904

文摘

endo-Tricyclo[3.2.1.0^2,4]oct-8-ylidene is a foiled carbene reaction intermediate. It was generated by thermolyzing 螖³-1,3,4-oxadiazoline precursors dissolved in benzaldehyde and acetophenone. The products appear to stem from direct insertion of the carbene鈥檚 divalent C atom into the 伪-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鈥?伪-substituents. The carbene reaction was modeled using DFT. The singlet carbene鈥檚 bicoordinate C atom bends 31掳 toward the endo-fused cyclopropane bond, elongating it to r = 1.69 脜. The resulting trishomocyclopropyl HOMO{鈭?} is a three-center two-electron bond responsible for the electron-deficient carbene鈥檚 nucleophilicity. Its calculated properties are consistent with this assertion: (1) singlet鈥搕riplet (螖Eb>S鈥揟b>) energy gap of 鈭?5 kcal/mol, (2) gas-phase proton affinity (PA) value of 272 kcal/mol, (3) hard and soft acid and base (HSAB) 螖N value of 鈭?.2 in its initial reaction with the carbonyl compounds, and (4) negative frontier orbital interaction values 螖螖E(PhC(O)H) = 鈭?.38 eV and 螖螖E(PhC(O)Me) = 鈭?.97 eV.