Cyclohepta-amylose-catalysed hydrolysis of 2-oxo-4,4,5,5-tetramethylimidazolidin-1-oxyl and of related carbamoyl and ester nitroxides. Nitroxide as spin label and activating group

Abstract

Base hydrolysis and cyclohepta-amylose-catalysed hydrolysis of 2-oxo-4,4,5,5-tetramethylimidazolidin-1-oxyl (1), its thiocarbonyl analogue (6), the carbamoyl nitroxides (7a–c), and the ester nitroxide (8) were studied by e.s.r. spectroscopy. On base hydrolysis in the absence of cyclohepta-amylose, the intermediate radical anions RN(–O˙)CO₂^–(R = Bu^t and NH₂CMe₂CMe₂–) were detected before decarboxylation. Hydrolysis of the amide group of the carbamoyl nitroxides is faster than that of 4,4,5,5- tetramethylimidazolidin-2-one (9) and therefore there is activation by the nitroxide group. For rate-determining nucleophilic addition to the carbonyl group, values of k_e.s.r. correlate inversely with a_{N– 1} values, and this is explained in terms of the predominant activating inductive effect of the nitroxide group. When the ester (8) reacted, the carbonyl nitroxide group was hydrolysed before the ester group. Complexing of the compounds (1), (6), (7a–c), and (8) with cyclohepta-amylose caused anisotropic effects in the e.s.r. spectra, reduced a_{N– 1} values, and increased rates of hydrolysis. These effects were greatest for carbamoyl nitroxide (1). In the cyclohepta-amylose-catalysed reaction, cycloamylose alkoxide ion acts as nucleophile but no intermediates are detected since both bonds of the carbamoyl and ester nitroxide systems are rapidly hydrolysed.