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  - Nyulászi, László
AU  - Veszprémi, Tamás
TI  - Nature of bonding in cyclic conjugated ylides
JF  - JOURNAL OF PHYSICAL CHEMISTRY US
J2  - J PHYS CHEM US
VL  - 100
PY  - 1996
IS  - 16
SP  - 6456
EP  - 6462
PG  - 7
SN  - 0022-3654
DO  - 10.1021/jp951740a
UR  - https://m2.mtmt.hu/api/publication/2610099
ID  - 2610099
AB  - The conjugative ability of the lambda(5)-P=C bond has been compared to its lambda(3)-P=C counterpart at the MP2/6-31G*//MP2/6-31G* level of theory, using isodesmic reaction energies. Investigating heterobutadienes, it has been observed that compounds containing a lambda(3)-P=C bond show similar delocalization energy as those with C=C units. As for lambda(5)-phosphabutadienes, however, stabilization is achieved only in the case of C substitution. This behavior has been rationalized by perturbation theory arguments with the conclusion that while for the lambda(3)-P=C and C=C bonds a two-way interaction, for the lambda(5)-P=C system a one-way interaction is operational, Comparing cyclic systems containing lambda(3)- and lambda(5)-P=C bonds, similar structures (bond lengths) and stabilizations (as concluded from bond separation and homodesmic reaction energies) were obtained for six-membered rings including benzene and other rings containing one and three (symmetrically arranged) phosphorus atoms. The four-membered rings (1 lambda(3),3 lambda(3)- and 1 lambda(5),3 lambda(5)-diphosphacyclobutadienes), however, show completely different behavior, since the lambda(5)-P derivative does not show destabilization upon ring formation and has equal bond lengths, while lambda(3)-P rings are clearly antiaromatic. Rationalization has been given in terms of the differences in the one-way and two-way conjugative interactions.
LA  - English
DB  - MTMT
ER  -