Electronic and steric effects in thermal denitrosation of N-nitrosoamides

N-Alkyl-N-nitrosoamides undergo competitive reactions whose rates are depen dent upon the interplay of a number of factors. There already exists a sign ificant body of work delineating the effects of pH on the partitioning of t he nitrosoamides along their deaminative (-N-2) and denitrosative (-"NO+") pathways. In this paper, the issue of pH dependence is discussed with parti cular attention to nitrosoamide decompositions in nonaqueous media. The rol e of the acidity of the medium in the partitioning of the nitrosoamide betw een deamination and denitrosation and in the choice of deaminative pathways is revisited. In nonaqueous media under near-neutral conditions, the parti tioning's pH dependence is evidently accompanied by a sensitivity to struct ural features in the nitrosoamide. Thus, diminution of steric crowding arou nd the N-nitroso moiety as well as the presence of strongly electron-withdr awing acyl units (i.e., those derived from strong acids, e.g., tosyl and tr ifyl) increase the relative yield of amides by encouraging the denitrosativ e pathway. A mechanism for thermal denitrosation of nitrosoamides under nea r-neutral conditions is proposed in which rapid protonation at the acyl O r ather than slow protonation at the amidic N is the first step in the reacti on profile. A rate-limiting, bimolecular reaction between the C-conjugate a cid and adventitious nucleophiles at the nitrosyl group then occurs followe d by rapid tautomerization to amide.