Substituent effects on the reactivity of selected amine-boranes ; Amine-borane reduction of nitroso compounds

Abstract

Studies of the hydrolysis of amine-boranes and of the reaction of amine-boranes with hypochlorite have been extended to a series of substituted benzylamine-boranes. The hydrolysis of selected substituted benzylamine-boranes takes place by acid-independent and acid-dependent pathways consistent with mechanisms previously proposed in the hydrolysis of substituted aniline- and quinoline-boranes. Steric and electronic effects associated with the difference in hydrolysis rates of various amine-boranes studied here and elsewhere are discussed. Primary and secondary substituted benzylamine-boranes were found to react with NaOCl in a 1:4 molar ratio in processes involving hydride oxidation followed by N-chlorination of amine, whereas the tertiary amine-borane, N,N-dimethylbenzylamine-borane, was found to react with NaOCl in a 1:1 molar ratio representing B-chlorination. Isoquinoline- and quinoline-borane display a 1:3 stoichiometry with NaOCl, but appear to react via the oxidation pathway without subsequent chlorination of amine. Substrate and solvent isotope effects, studies of the pH dependence of reaction, as well as activation parameters obtained in the reaction of various amine-boranes with NaOCl, show the entropy terms to be dominant in the barrier to activation and suggest a four-centered activated complex involving hypochlorous acid as the kinetically important form of the oxidant. Denitrosation of N-methyl-N-nitrosoaniline (NMNA) was found to occur in dilute hydrochloric acid with trimethylamine-borane (TMAB) in the presence of thiourea. The kinetics of the reaction are similar to those found by previous workers in the acid-catalyzed denitrosation of NMNA by thiourea in the presence of nitrite trapping agents. A mechanism is proposed which involves attack of thiourea on protonated NMNA leading to nitrosated thiourea and N-methylaniline followed by rapid regeneration of thiourea by TMAB. Rate measurements of the reaction between TMAB and acidified sodium nitrite under a variety of conditions show that the reaction most likely involves reaction of protonated nitrous acid. The bimolecular rate constant for attack by H$\sb2$NO$\sb2\sp+$ on TMAB in perchloric acid was extracted by a kinetic analysis of the variation of the rate constant with acidity and indicated the rate of reaction is close to that calculated for the diffusion limit. A first order dependence on (TMAB), detection of N$\sb2$O as a reaction product, and the observed reaction stoichiometry lead to a proposed mechanism involving the rate-limiting formation of a nitrosated borane intermediate through a four-membered transition state.