The effect of salt formation on structure and charge distribution in imines. Part 4. Energy barriers to isomerisation about the C–N bond in 2,6-dimethyl-4-aryliminopyrans and their salts: solvent and substituent effects, and evidence for isomerisation mechanisms
- 1 January 1981
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in Journal of the Chemical Society, Perkin Transactions 2
- No. 11,p. 1501-1507
- https://doi.org/10.1039/p29810001501
Abstract
Free energies of activation, ΔG‡, have been calculated for isomerisation about the CN bond in a series of 2,6-dimethyl-4-aryliminopyrans (1), and their hydro-[(2) and (3)] and metho-[(4) and (5)] salts. Independent values were measured from coalescence of two different sets of 1H n.m.r. signals in each compound. The effects of temperature, solvent, and aryl substituent X on ΔG‡ indicate that the metho-salts isomerise by CN bond rotation, the imines by inversion or a mechanism intermediate between inversion and rotation, and the hydro-salts by the previously unreported deprotonation–imine isomerisation–reprotonation mechanism. Hydro-salts (3) having a –M substituent in the aryl para-position give anomalous results for ΔG‡ at the lower coalescence temperature.Keywords
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