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Kinetic and mechanistic study of the reactions of aryl chloroformates with quinuclidines
Author(s) -
Castro Enrique A.,
Aliaga Margarita,
Campodonico Paola R.,
Leis J. Ramón,
GarcíaRío Luis,
Santos José G.
Publication year - 2006
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/poc.1116
Subject(s) - chemistry , alicyclic compound , reactivity (psychology) , amine gas treating , aryl , aqueous solution , tetrahedral carbonyl addition compound , medicinal chemistry , ionic strength , computational chemistry , organic chemistry , nucleophile , alkyl , catalysis , medicine , alternative medicine , pathology
The reactions of quinuclidines with phenyl, 4‐methoxyphenyl, 4‐chlorophenyl, and 4‐nitrophenyl chloroformates (PClF, MOPClF, ClPClF, and NPClF, respectively) in aqueous solution (25 °C, ionic strength 0.2 M KCl) are followed spectrophotometrically and evaluated kinetically. Under amine excess, pseudo‐first‐order rate coefficients ( k obsd ) are found. Plots of k obsd versus [quinuclidine] are linear, with the pH‐independent slope ( k N ). The Brønsted‐type plots (log k N vs . p K a of quinuclidinium ions) are linear with slopes (β) of 0.32, 0.34, 0.31, and 0.23 for the reactions of PClF, MOPClF, ClPClF, and NPClF, respectively. The magnitude of the slopes suggests that these mechanisms are stepwise, with the formation of a zwitterionic tetrahedral intermediate (T ± ) being the rate‐determining step. The sensitivity of log k N to the basicity of the nonleaving group (β nlg ) is −0.16. By comparing the reactions under investigation between each other and with similar aminolyses, the following conclusions can be drawn: (i) the mechanisms for the quinuclidinolysis of the four chloroformates studied are stepwise. (ii) The reactivity increases in the sequence MOPClF < PClF < ClPClF < NPClF. (iii) The change of the leaving group from 2,4‐dinitrophenoxide to chloro changes the mechanism from concerted to stepwise. (iv) Quinuclidines are more reactive toward aryl chloroformates than isobasic secondary alicyclic amines. Copyright © 2006 John Wiley & Sons, Ltd.