Decomposition of N′‐Benzoyl‐N‐nitrosoureas in Aqueous Media

Abstract

AbstractThe decomposition of N′‐benzoyl‐N‐methyl‐N‐nitrosourea (BMNU) in aqueous media over the 0−14 pH range has been studied. In basic and neutral media (6 < pH < 14) the reaction proceeds through abstraction of the acidic proton of BMNU (pKa = 7.8) and subsequent decomposition of the conjugate base of the thus formed nitrosourea, via an intermediate benzoyl isocyanate. Support for this mechanism is provided by the presence of N,N′‐dibenzoylurea in the final reaction mixtures, as the result of the trapping of benzoyl isocyanate with benzamide generated from hydrolysis of the former. The hydrolysis of BMNU takes place through three competitive pathways: spontaneous decomposition of the conjugate base of BMNU, and buffer‐catalyzed and hydroxide ion catalyzed water addition to the carbonyl group of the deprotonated nitrosourea. N′‐Benzoyl‐N,N′‐dimethyl‐N‐nitrosourea (BDMNU), a benzoyl nitrosourea lacking the acidic proton of BMNU, is hydrolyzed in basic media by attack of hydroxide ion on the carbonyl group of the urea. In acid media (0 < pH < 6), BMNU gives only deamination products, differing from the reported behavior of other N‐nitroso compounds and of the isoster nitrosoguanidine, in which denitrosation is almost quantitative. The reaction is acid‐catalyzed in the 0−2.5 pH range and pH‐independent in the 3−5 pH range. The presence of general acid catalysis (α = 0.60), the absence of nucleophilic catalysis, and the thermodynamic activation parameters for the reaction support the mechanism proposed in the literature for the deamination of N‐nitrosoureas in acidic media. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)