The Dimroth rearrangement. Part XIII. The small effect of p-substitution on rearrangement rates for 1,2-dihydro-2-imino-1-methyl-5-phenylpyrimidines

Rates have been measured for the Dimroth rearrangements of 1,2-dihydro-2-imino-1-methyl-5-(p-substituted phenyl)pyrimidines. Although the mesomeric effects of the p-substituents are severely attenuated by the presence of a considerable interplanar angle between the benzene and pyrimidine rings, rearrangement rates decrease in the order NO2 > F > Cl > Br > Me > OMe > NH2 > NMe2, following qualitatively the σρ values for the groups. 1,2-Dihydro-2-imino-1,4,6-trimethyl-5-phenylpyrimidine and its p-nitro- and p-amino-derivatives, for which u.v. spectra and pKa values indicate even less through-conjugation, behave similarly.Syntheses are described for the three trimethylated imines and their products of rearrangement; for 1,2-dihydro-2-imino-1,6-dimethylpyrimidine, which rearranges more rapidly than its 1,4-dimethyl-isomer; and for 1,2-dihydro-2-imino-5-methoxy-1-methylpyrimidine, required for comparison of its rearrangement with that of the p-methoxyphenyl analogue and with those of quinazoline analogues. Structures were confirmed by 1H n.m.r. spectra.