NDLI: Structure-Reactivity Effects on Primary Deuterium Isotope Effects on Protonation of Ring-Substituted α-Methoxystyrenes

Content Provider	PubMed Central
Author	Tsang, Wing-yin Richard, John P.
Abstract	Primary product isotope effects (PIEs) on L+- and carboxylic acid-catalyzed protonation of ring-substituted α-methoxystyrenes (X-1) to form oxocarbenium ions X-2+ in 50/50 (v/v) HOH/DOD were calculated from the yields of the α-CH3 and α-CH2D labeled ketone products, determined by 1H NMR. A plot of PIE against reaction driving force shows a maximum PIE of 8.7 for protonation of 4-MeO-1 by Cl2CHCOOH (ΔGo = 1.0 kcal/mol). The PIE decreases to 8.1 for protonation of 4-MeO-1 by L3O+ (ΔGo = −2.8 kcal/mol) and to 5.1 for protonation of 3,5-di-NO2-1 by MeOCH2COOH (ΔGo = 13.1 kcal/mol). The PIE maximum is around ΔGo = 0. Arrhenius-type plots of PIEs on protonation of 4-MeO-1 and 3,5-di-NO2-1 by L3O+ and on protonation of X-1 by MeOCH2COOH in 50/50 (v/v) HOH/DOD give similar slopes and intercepts. These were used to calculate values of [(E a)H − (E a)D] = −1.2 kcal/mol and (A H/A D) = 1.0 for the difference in activation energy for reactions of A–H and A–D and for the limiting PIE at infinite temperature, respectively. These parameters are consistent with reaction of the hydron over an energy barrier. There is no evidence for quantum mechanical tunneling of the hydron through the barrier. These PIEs suggest that the transferred hydron at the transition state lies roughly equidistant between the acid donor and base acceptor, and contrast with the recently published Brønsted parameters [Richard, J. P.; Williams, K. B. J. Am. Chem. Soc. 2007, 129, 6952–6961], which are consistent with a product like transition state. An explanation for these seemingly contradictory results is discussed.
Related Links	http://dx.doi.org/10.1021/ja905080e
Ending Page	13962
Page Count	11
Starting Page	13952
File Format	PDF
ISSN	00027863
e-ISSN	15205126
Journal	Journal of the American Chemical Society
Issue Number	39
Volume Number	131
Language	English
Publisher Date	2009-10-07
Access Restriction	Open
Subject Keyword	Colloid and Surface Chemistry Biochemistry Chemistry(all) Catalysis Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Chemistry Colloid and Surface Chemistry Biochemistry Catalysis

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