Study of exchange processes in 4,4,6-trimethyl-2-phenylamino-4H-1, 3-thiazine derivatives

It is shown that, in contrast to model structures with fixed amine and imine forms, the form and position of some of the signals in the PMR spectra of 4,4,6-trimethyl-2-phenylamino-4H-1, 3-thiazine derivatives depend on the temperature; this is associated with rotational isomerism about the nitrogen-heteroring bond, which is realized at a higher rate in the model derivative with an amine structure and is slowed down in the tautomeric derivatives owing to the formation of hydrogen bonds with solvent molecules (in deuteroacetone) or in dimeric associates (in deuterochloroform). According to data from the low-temperature spectra, the percentages of the syn and anti forms in acetone solutions are comparable. In chloroform solution, the equilibrium between the two rotational forms is shifted markedly to favor the anti isomer as the temperature is lowered because of the formation of cyclic dimeric structures.