Transition Structures of the Ene Reactions of Cyclopropene
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- 作者：Qiaolin Deng ; Bert E. Thomas IV ; K. N. Houk ; and Paul Dowd
- 刊名：Journal of the American Chemical Society
- 出版年：1997
- 出版时间：July 23, 1997
- 年：1997
- 卷：119
- 期：29
- 页码：6902 - 6908
- 全文大小：179K
- 年卷期：v.119,no.29(July 23, 1997)
- ISSN：1520-5126
文摘
The transition structures for the ene reactions of cyclopropenewith ethylene, propene, and cyclopropenehave been located with ab initio molecular orbital calculations and the6-31G* basis set and by DFT calculationswith the Becke3LYP functional and the 6-31G* basis set. Several ofthe transition structures have also been locatedwith CASSCF calculations. Energies of all stationary points werealso evaluated with second-order M
ller-Plessettheory using the RHF/6-31G* optimized geometry. The geometries ofeach transition structure and the energeticsof each reaction are discussed and compared to the ene reaction ofpropene with ethylene. Calculations show thatthe cyclopropene ene reactions have much lower activation barriers thanthe propene-ethylene ene reaction, inagreement with experimental results. The transition structureshave varying degrees of asynchronicity. Thestabilitiesof the possible radical intermediates for each reaction are reflectedin the geometries of the transition structures.The relief of strain in a cyclopropene, when acting as theenophile, accounts for the energetic differences in thesereactions. The endo transition structure for thedimerization is lower in energy than the exo transitionstructure by2.7 kcal/mol at the Becke3LYP/6-31G* + ZPE level of theory.Secondary orbital overlap of a CH bond of theenophile with the 
-system at the central carbon of the ene isproposed to account for the preference for theendotransition structure. Barely stable diradical intermediates havebeen found for both endo and exocyclopropenedimerization reactions, but it is likely that they are artifacts of thecurrent level of theory.
ller-Plessettheory using the RHF/6-31G* optimized geometry. The geometries ofeach transition structure and the energeticsof each reaction are discussed and compared to the ene reaction ofpropene with ethylene. Calculations show thatthe cyclopropene ene reactions have much lower activation barriers thanthe propene-ethylene ene reaction, inagreement with experimental results. The transition structureshave varying degrees of asynchronicity. Thestabilitiesof the possible radical intermediates for each reaction are reflectedin the geometries of the transition structures.The relief of strain in a cyclopropene, when acting as theenophile, accounts for the energetic differences in thesereactions. The endo transition structure for thedimerization is lower in energy than the exo transitionstructure by2.7 kcal/mol at the Becke3LYP/6-31G* + ZPE level of theory.Secondary orbital overlap of a CH bond of theenophile with the -system at the central carbon of the ene isproposed to account for the preference for theendotransition structure. Barely stable diradical intermediates havebeen found for both endo and exocyclopropenedimerization reactions, but it is likely that they are artifacts of thecurrent level of theory.