Concerted base-promoted elimination in the decomposition of N-halo amino acids

Article abstract of DOI:10.1002/(SICI)1099-1395(199608)9:8<552::AID-POC820>3.0.CO;2-R

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 substitueras 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, Brtonsted's β and β_lg 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 A_xhD_HD_N instead of the stepwise A_xhD_H^? + D_N proposed earlier.