Electronic and steric effects in thermal denitrosation of N-nitrosoamides.

N-Alkyl-N-nitrosoamides undergo competitive reactions whose rates are dependent upon the interplay of a number of factors. There already exists a significant body of work delineating the effects of pH on the partitioning of the nitrosoamides along their deaminative (-N(2)) and denitrosative (-"NO(+)") pathways. In this paper, the issue of pH dependence is discussed with particular attention to nitrosoamide decompositions in nonaqueous media. The role of the acidity of the medium in the partitioning of the nitrosoamide between deamination and denitrosation and in the choice of deaminative pathways is revisited. In nonaqueous media under near-neutral conditions, the partitioning's pH dependence is evidently accompanied by a sensitivity to structural features in the nitrosoamide. Thus, diminution of steric crowding around the N-nitroso moiety as well as the presence of strongly electron-withdrawing acyl units (i.e., those derived from strong acids, e.g., tosyl and trifyl) increase the relative yield of amides by encouraging the denitrosative pathway. A mechanism for thermal denitrosation of nitrosoamides under near-neutral conditions is proposed in which rapid protonation at the acyl O rather than slow protonation at the amidic N is the first step in the reaction profile. A rate-limiting, bimolecular reaction between the O-conjugate acid and adventitious nucleophiles at the nitrosyl group then occurs followed by rapid tautomerization to amide.

A study of electronic effects on the kinetics of thermal deamination of N-nitrosoamides.

N-4-R-Benzyl-N-nitrosopivalamides (1a-d; R = MeO, Me, H, NO(2)) were allowed to decompose at 18 degrees C in C(6)D(12), CDCl(3), CD(3)CN, and d(6)-DMSO, and the rates of decomposition were followed by (1)H NMR spectroscopy. The half-lives of the nitrosoamides were found to vary in a systematic way with the nature of the R group on the aromatic nucleus. Electron-releasing groups were found to decrease the stability of the starting nitrosoamide, whereas electron-withdrawing ones increased the nitrosoamides' thermal stability. A Hammett-type plot of log(rate constants of deamination) vs sigma(p) was linear (R(2) = 0.986) with a rho-type value of -0.90 indicating development of significant positive charge at the benzylic position in the transition state of the rate-determining step. The thermal stability of the nitrosoamides was also found to be systematically affected by the polarity of the solvent: as the solvent polarity increased, so did the lability of the nitrosoamides. This observation of intra- and intermolecular electronic perturbations of the kinetics of nitrosoamide decomposition appears to be novel. A closer look at the rate-determining step of nitrosoamide thermolysis is made, and a mechanistic framework is proposed that accounts for both steric and electronic modulation of nitrosoamide stability as well as the greater thermal stabilities of the related N-nitrocarboxamides and N-nitrosotosylamides.

Boundary conditions on the partitioning of deaminatively generated benzyl cations.

Nitrogenous entity-separated ion pairs (NESIPs) containing benzyl cations, nitrogen gas, and pivalate anions were generated via thermal deamination of N-benzyl-N-nitrosopivalamide. Some decompositions were performed in methanolic solutions saturated with selected nucleophiles: acetate, azide, or cyanide ions. Trace amounts of benzyl cyanide and tolunitriles were observed; no corresponding products were detected in the acetate and azide cases. Other decompositions were performed in the absence of traditional solvent but in the presence of the nucleophilic salts; again only poor cyanide interception of the cation was observed. The poor showing of the nucleophilic ions, when present, is discussed in the context of the lifetime of the cation, effective nucleophilicity, and cage effects in deamination.

Deaminatively generated carbocations as initiators of styrene polymerization.

[reaction: see text] Addition polymerization is readily accomplished via free radicals and carbanions but conventional initiations via carbocations are limited principally by low initiator reactivity. Thermolyses of N-nitrosamides produce nitrogen-separated ion-pairs (NSIPs) containing exceedingly reactive carbocations. We report here the novel use of this facile mode of carbocation generation in the polymerization of styrene. Polystyrene of viscosity average molecular weight approximately 10(6) was obtained.