Differences in the activity of neutral and ionized β-cyclodextrin on the nitrosation of amines by phenylpropyl nitrites

Abstract

The influence of β-cyclodextrin (β-CD) on the base-catalyzed hydrolysis reaction of 1-phenyl-1-propyl, 2-phenyl-1-propyl and 3-phenyl-1-propyl nitrites and on the nitrosation of pyrrolidine, piperidine and N-methylcyclohexylamine by the aforementioned alkyl nitrites (RONO) is studied in aqueous buffers of the amines and in an alkaline medium with [OH−] = 0.20 M. The hydrolysis reaction is catalyzed by the presence of β-CD owing to the formation of reactive 1:1 inclusion complexes between the alkyl nitrite and the ionized β-CD; the addition of potential inhibitors, such as dodecyltrimethylammonium bromide monomers, accelerates the reaction even more. The effect is quite significant in the case of 1-phenyl-1-propyl nitrite and is viewed as a case of allostery. In the presence of neutral β-CD, the nitrosation by 1-phenyl-1-propyl nitrite, either of pyrrolidine or piperidine, is inhibited by β-CD addition; however, the nitrosation reaction of piperidine by 2-phenyl-1-propyl nitrite is catalyzed (passing through a maximum) by β-CD, whereas the nitrosation of pyrrolidine promoted by 3-phenyl-1-propyl nitrite exhibits practically no change upon β-CD addition. In alkaline media (containing ionized β-CD) the nitrosation of pyrrolidine by 1-phenyl-1-propyl nitrite is inhibited by the presence of β-CD; in contrast, the nitrosation of both piperidine and N-methylcyclohexylamine is catalyzed in all cases, but the degree of catalysis depends not only on the alkyl nitrite structure, but also on the type and concentration of the amine. Kinetic results are quantitatively interpreted on the basis of the proposed reaction mechanism in each case, and the kinetic rate constants of the different steps are determined. Comparison of the results obtained in aqueous alkaline media and in aqueous buffers of the amines themselves allows us to establish important characteristics of the transition state of the reaction.