Substituent-dependent negative hyperconjugation in 2-aryl-1,3-N,N-heterocycles. Fine-tuned anomeric effect?
The epimerization reactions of conformationally inflexible 2-aryl-1,3-N,N-heterocycles were used as model systems to study the role of the nitrogen lone pair-C2 associated antibonding orbital hyperconjugative interactions in the experimentally observed substituent-dependent generalized anomeric effe...
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| Dokumentumtípus: | Cikk |
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American Chemical Society
2003
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| Sorozat: | JOURNAL OF ORGANIC CHEMISTRY
68 No. 14 |
| doi: | 10.1021/jo034417 |
| mtmt: | 1013527 |
| Online Access: | http://publicatio.bibl.u-szeged.hu/7564 |
| Tartalmi kivonat: | The epimerization reactions of conformationally inflexible 2-aryl-1,3-N,N-heterocycles were used as model systems to study the role of the nitrogen lone pair-C2 associated antibonding orbital hyperconjugative interactions in the experimentally observed substituent-dependent generalized anomeric effect. The measured reaction free enthalpies were found to correlate well with the sum of the hyperconjugative stabilization energies of all the vicinal donor-acceptor orbital overlaps around C2, obtained from ab initio NBO analysis, and both quantities correlated linearly with the Hammett-Brown substituent constant. The individual stereoelectronic interactions (n(N)-sigma*(C2-N), n(N)-sigma*(C2-Ar), n(N-sigma)*(C2-H)) were also observed to exhibit a substituent dependence, despite their distance from the 2-aryl substituent and their nonperiplanar arrangement. The higher the electron-withdrawing effect of the 2-aryl substituent, the larger was the stabilization for n(N)-sigma*(C2-Ar), while the overlaps n(N)-sigma*(C2-N) and n(N)-sigma*(C2-H) changed in the opposite sense. The different polarization of the acceptor sigma* orbitals, caused by the 2-aryl substituent, accounted for the observed propagation of the substituent effect. These results promote a detailed explanation of the useful tautomeric behavior of the 2-aryl-1,3-X,N-heterocycles, and reveal the nature of the connection between the anomeric effect and the Hammett-type linear free energy relationship. |
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| Terjedelem/Fizikai jellemzők: | 5705-5712 |
| ISSN: | 0022-3263 |