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Concerted base‐promoted elimination in the decomposition of N ‐halo amino acids
Author(s) -
Armesto X. L.,
Canle M.,
García M. V.,
Losada M.,
Santaballa J. A.
Publication year - 1996
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/(sici)1099-1395(199608)9:8<552::aid-poc820>3.0.co;2-r
Subject(s) - chemistry , halo , base (topology) , decomposition , amino acid , computational chemistry , organic chemistry , stereochemistry , combinatorial chemistry , biochemistry , mathematical analysis , mathematics , physics , quantum mechanics , galaxy
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 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 xh D H D N instead of the stepwise A xh D H ‡ + D N proposed earlier.