P. Beak, T. S. Woods, D. Mueller
1972
Citations
0
Influential Citations
28
Citations
Journal
Tetrahedron
Abstract
Abstract The effects of 6-chloro, 5,6-benzo, 3,4-benzo, 4-methoxy-6-methyl, 6-methyl-4-one, 5,6-benzo-4-methoxy, and 5,6-benzo-4-one substitution on 2-methoxypyridine-1-methyl-2-pyridone equilibria have been determined by measurement of the heat evolved on equilibration of substituted isomer pairs at 130–150° or by determination of the equilibrium constant at 115, 125 and 140°. In all cases the 2-one isomer is more stable in the liquid phase. Estimates of differences in gas phase enthalpies and in chemical binding energies are made for these isomer pairs. The results provide a quantitative estimate of the stability difference between isomeric 1-methyl 2- and 4-pyridones and show that of the substituents investigated only 6-chloro substitution has a significant enthalpy effect. It is suggested that for substituted compounds in this series extrapolation of enthalpy differences from the parent system will be valid in most cases and that the 1-methyl-2-one isomer will usually be the more stable. Comparison of alkylomeric and protomeric equilibria is considered and it is suggested that, for a single equilibrium, reactions corrected for intermolecular and solvent effects and for kinetic and zero-point energy differences provided the least ambiguous test of approximate quantum mechanical methods. Syntheses of 2-methoxy-1,6-dimethyl-4-pyridone and 2-methoxy-1-methyl-4-quinolone which involve the use of a trimethylsilyl blocking group provide a procedure which should be adaptable to the production of the less available alkylomeric isomers of many isomer pairs.