The Dimroth rearrangement. Part VII. Measurement of equilibria in reversible examples from the pyrimidine series

Abstract

Both 1-ethyl-1,2-dihydro-2-methyliminopyrimidine and its 2-ethylimino-1-methyl isomer separately undergo Dimroth rearrangement in alkaline solution to yield the same mixture of both compounds. The equilibria figures revealed by n.m.r. spectra for several such pairs of 1-alkyl-2-alkylimino-1,2-dihydropyrimidines indicate that their ratio is controlled by two factors, namely, the relative steric bulk of the alkyl groups and their respective inductive effects. Thus, the isomer accommodating the bulkier and/or more electron-withdrawing (or less electron-releasing) group on extracyclic nitrogen will predominate at equilibrium. The preparations, ultraviolet spectra, and pK_a values of the imines are recorded.