Substituent-dependent negative hyperconjugation in 2-aryl-1,3-N,N-heterocycles. Fine-tuned anomeric effect?

Hetenyi, A N [Hetényi, Anasztázia (NMR spektroszkópi...), szerző] Gyógyszerkémiai Intézet (SZTE / GYTK); Martinek, T A [Martinek, Tamás (Gyógyszerkémia), szerző] Gyógyszerkémiai Intézet (SZTE / GYTK); Lazar, L [Lázár, László (Szerves kémia, gy...), szerző] Gyógyszerkémiai Intézet (SZTE / GYTK); Zalan, Z [Zalán, Zita (Szerves kémia, gy...), szerző] Gyógyszerkémiai Intézet (SZTE / GYTK); Fulop, F [Fülöp, Ferenc (Kémia), szerző] Gyógyszerkémiai Intézet (SZTE / GYTK)

Angol nyelvű Tudományos Szakcikk (Folyóiratcikk)
Megjelent: JOURNAL OF ORGANIC CHEMISTRY 0022-3263 1520-6904 68 (14) pp. 5705-5712 2003
  • SJR Scopus - Organic Chemistry: D1
Azonosítók
Szakterületek:
    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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    2020-08-09 02:24