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 - TY - JOUR AU - Neuvonen, K AU - Fülöp, Ferenc AU - Neuvonen, H AU - Koch, A AU - Kleinpeter, E AU - Pihlaja, K TI - Substituent influences on the stability of the ring and chain tautomers in 1,3-O,N-heterocyclic systems: Characterization by C-13 NMR chemical shifts, PM3 charge densities, and isodesmic reactions JF - JOURNAL OF ORGANIC CHEMISTRY J2 - J ORG CHEM VL - 66 PY - 2001 IS - 12 SP - 4132 EP - 4140 PG - 9 SN - 0022-3263 DO - 10.1021/jo001114w UR - https://m2.mtmt.hu/api/publication/1013896 ID - 1013896 AB - Substituent effects on the stabilities of the ring and chain forms in a tautomeric equilibrium of five series of 2-phenyloxazolidines or -perhydro-1,3-oxazines possessing nine different substitutions at the phenyl moiety have been studied with the aid of C-13 NMR spectroscopy and PM3 charge density and energy calculations. Reaction energies of the isodesmic reactions, obtained from the calculated energies of formation, show that electron-donating substituents stabilize both the chain and ring tautomers but the effect is stronger on the stability of the chain form than on that of the ring form. The C-13 chemical shift changes induced by the phenyl substituents (SCS) were analyzed by several different single and dual substituent parameter approaches. The best correlations were obtained by equation SCS = rho (F)sigma (F) + rho (R)sigma (R). In all cases the rho (F) values and in most cases also the rho (R) values were negative at both the C=N and C-2 carbons, indicating a reverse behavior of the electron density. This concept could be verified by the charge density calculations. The C-13 chemical shifts of the C=N and C-2 carbons show a normal dependence on the charge density (q(tot)), but the charge density shows a reverse dependence on substitution. Correlation analysis of the C-13 chemical shifts, solvent effect (CDCl3 vs DMSO-d(6)) on the NMR behavior as well as the effect of substituents on the electron densities and on the stabilities of the ring and chain tautomers show that the substituent dependence of the relative stability of the ring and chain tautomers in equilibrium is governed by several different electronic effects. At least intramolecular hydrogen bonding between the imine nitrogen and the hydroxyl group as well as polarization of the C=N bond seem to contribute in the chain form. Stereoelectronic and electrostatic effects are possible to explain the increase in stability of the ring form by electron-donating substituents. LA - English DB - MTMT ER -