Deuterium isotope effects and the bunnett w factor in elimination reactions of 4-alkoxyimino-5,6-dihydro-6-alkoxyaminopyrimidin-2(1H-)one in strong acid media

Abstract

Rate coefficients for the elimination of hydroxylamines from 4-alkoxyimino-5,6-dihydro-6-alkoxyaminopyrimidin-2(1H)-ones (1a) show maxima at Ho values of –1.8 (for O-benzylhydroxylamine) and –0.8 (for hydroxylamine) in a variety of mineral acids. The hitherto unknown values of Ho for trifluoromethanesulphonic acid are reported and this acid also gives a rate maximum at –1.8 for the elimination of O-benzylhydroxylamine from (1a; R2= Bz). Bunnett w values for these eliminations fall in the range 6.0–16.0 (dependent upon the acid) implying that water is involved as a proton transfer agent. In contrast, eliminations in formic acid show low and variable w values and formate ion is involved as the proton transfer agent which explains the lack of a rate maximum in this acid. Deuterium isotope effects reveal a highly stereoselective elimination and support the proposal of an E1cB (irreversible) mechanism. The inhibition of elimination by the 5-fluoro-substituent [as in (1b)] is probably due to destabilisation of the intermediate carbanion by the lone pair electrons on fluorine.