Nitrosation of dialkylhydroxylamines, and computational and NMR investigations of the interconversion of conformational isomers of N nitroso dimethylhydroxylamine
Résumé
The effect of acidity upon the rate of nitrosation of N benzyl,O-methylhydroxylamine (3) in 1:1 H2O:MeOH has been investigated. The first-order rate constant (kobs) for loss of HNO2 decreases as [H3O+] increases. This is compatible with two parallel reaction channels. One involves reaction of the free hydroxylamine with HNO2 and the other involves reaction of the free hydroxylamine with NO+. In contrast, there is only a slight increase in kobs with increasing [H3O+] for nitrosation of N,O dimethylhydroxylamine in water at 25 oC to give N nitroso dimethylhydroxylamine, 5. This also fits a two-channel mechanism. Now, one involves nitrosation of the free base by NO+ but the other involves catalysis by chloride. The interconversion of cis and trans conformers of 5 has also been investigated by temperature-dependent NMR spectroscopy in CDCl3, methanol-d4, toluene-d8, and DMSO-d6. Enthalpies and entropies of reaction and of activation have been determined, and compared with computational values (B3LYP/6-31G*). The cis form is slightly more stable at normal temperatures and no solvent effects upon the thermodynamics or kinetics of the conformational equilibrium were predicted or detected. Key molecular parameters have been calculated for the cis and trans forms, and for the TS for their interconversion. These results indicate electronic delocalisation in the ground states of 5 but not in the TS for their interconversion.
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