A MNDO and AM1 study of the isomerization of pyrans, thiopyrans, dihydropyridines, and their methoxy derivatives was carried out so as to clarify the nature of the effects causing the observed thermodynamic regioselectivity in nucleophilic addition to pyridinium, pyrylium and thiopyrylium cations. The results show that, in contrast to 2H-thiopyrans, 2H-pyrans and 1,2-dihydropyridines are significantly stabilized by a generalized anomeric effect when the group bonded to the 2 position is OMe. It is pointed out that in the case of reversible additions, a clear distinction should be made between the kinetic and the thermodynamic regioselectivity because, in contrast with current usage, the latter cannot be interpreted in terms of frontier orbital theory but by the presence or absence of a generalized anomeric effect. It is also pointed out that predictions of kinetic regioselectivity based on frontier orbital theory are not supported by literature data. The kinetic regioselectivity seems to be governed by the relative electron density at the carbon under attack, independent of the hard or soft character of the nucleophile.
The Problem of Regioselectivity in Nucleophilic Additions to Pyridinium and Related Cations. Role of Generalized Anomeric Effect / Doddi, G.; Ercolani, G.; Mencarelli, Paolo. - In: JOURNAL OF ORGANIC CHEMISTRY. - ISSN 0022-3263. - STAMPA. - 57:(1992), pp. 4431-4434. [10.1021/jo00042a023]
The Problem of Regioselectivity in Nucleophilic Additions to Pyridinium and Related Cations. Role of Generalized Anomeric Effect
MENCARELLI, Paolo
1992
Abstract
A MNDO and AM1 study of the isomerization of pyrans, thiopyrans, dihydropyridines, and their methoxy derivatives was carried out so as to clarify the nature of the effects causing the observed thermodynamic regioselectivity in nucleophilic addition to pyridinium, pyrylium and thiopyrylium cations. The results show that, in contrast to 2H-thiopyrans, 2H-pyrans and 1,2-dihydropyridines are significantly stabilized by a generalized anomeric effect when the group bonded to the 2 position is OMe. It is pointed out that in the case of reversible additions, a clear distinction should be made between the kinetic and the thermodynamic regioselectivity because, in contrast with current usage, the latter cannot be interpreted in terms of frontier orbital theory but by the presence or absence of a generalized anomeric effect. It is also pointed out that predictions of kinetic regioselectivity based on frontier orbital theory are not supported by literature data. The kinetic regioselectivity seems to be governed by the relative electron density at the carbon under attack, independent of the hard or soft character of the nucleophile.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.