Mono- and di-protonation sites in N-ammonio-amidates: a spectroscopic study

Abstract

In a series of N-ammonio-amidates (1)–(6) the site of monoprotonation has been established as the formally negatively charged nitrogen atom by means of i.r., u.v., and ¹H n.m.r. spectroscopy with the aid of O- and N-methyl derivatives. Whereas in more strongly acidic media [65 : 35 (v/v) HSO₃F–CF₃·CO₂H]N-(trimethylammonio)methoxyformamidate (5) is diprotonated, once on nitrogen and once on oxygen, to give the internally hydrogenbonded geometric isomers (16a and b), detectable by ¹H n.m.r. spectroscopy, the ethoxy-analogue (6) is diprotonated on nitrogen : this difference in behaviour is thought to be hyperconjugative in origin. The other N-ammonioamidates are diprotonated on nitrogen. This preference for nitrogen as the location of the second proton, visàvis amides, is ascribed to the strongly electron-withdrawing character of the trimethylammonio-group, which reduces the amide-like lone pair delocalisation in the monocation.