Concerted base-promoted elimination in the decomposition ofN-halo amino acids

Abstract

N-Chloroamino acids are unstable in aqueous solution and decompose through different pathways depending on the reaction conditions, yielding precursors of carcinogenic and/or mutagenic compounds. One of these pathways is a 1,2-elimination process, which has scarcely received any attention and for which no systematic analysis is available. The process is first order relative to the N-chloroamino acid and to that of hydroxide ion. The use of 2,2,2-trifluoroethanol and 1,1,1,3,3,3-hexafluoropropan-2-ol buffer solutions established that the process is general-base catalysed. The reaction rate is affected by the presence of a methyl group on the nitrogen atom and the nature of the leaving group. The results show an important steric effect due to the alkyl substituents on the α-carbon. With bulky alkyl substituents on the α-carbon, and in particular in the case of N-alkylamino acids, the catalytic effect increases as the base strength decreases. To characterize the transition state, Brønsted's β and βlg were used. A More O'Ferrall-Jencks diagram shows the transition state structure changing from carbanion-like to nitrenium-like, a large perpendicular effect being evident. The reaction proceeds through a concerted mechanism AxhDHDN instead of the stepwise AxhDH‡ + DN proposed earlier.