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

✍ Scribed by X. L. Armesto; M. Canle; M. V. García; M. Losada; J. A. Santaballa

• Publisher: John Wiley and Sons
• Year: 1996
• Tongue: English
• Weight: 590 KB
• Volume: 9
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(199608)9:8<552::aid-poc820>3.0.co;2-r

✦ Synopsis

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-trifluoroethanoI 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 a-carbon. With bulky alkyl substituents on the a-carbon, and in particular in the case of Nalkylamino acids, the catalytic effect increases as the base strength decreases. To characterize the transition state, Br~nstedS p and fiC 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,D,D, instead of the stepwise A,,D,* + D, proposed earlier.