TY  - JOUR
AU  - Hetényi, Anasztázia
AU  - Martinek, Tamás
AU  - Lázár, László
AU  - Zalán, Zita
AU  - Fülöp, Ferenc
TI  - Substituent-dependent negative hyperconjugation in 2-aryl-1,3-N,N-heterocycles. Fine-tuned anomeric effect?
JF  - JOURNAL OF ORGANIC CHEMISTRY
J2  - J ORG CHEM
VL  - 68
PY  - 2003
IS  - 14
SP  - 5705
EP  - 5712
PG  - 8
SN  - 0022-3263
DO  - 10.1021/jo034417+
UR  - https://m2.mtmt.hu/api/publication/1013527
ID  - 1013527
AB  - 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.
LA  - English
DB  - MTMT
ER  -