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Issue 8, 2010
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Nucleophilicities and Lewis basicities of imidazoles, benzimidazoles, and benzotriazoles

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Abstract

The kinetics of the reactions of some imidazoles, benzimidazoles and benzotriazoles with benzhydrylium ions (diarylcarbenium ions) have been studied photometrically in DMSO, acetonitrile, and aqueous solution at 20 °C. The resulting second-order rate constants have been used to determine the nucleophile-specific parameters N and s of these azoles according to the linear-free-energy relationship log k (20 °C) = s(N + E). With N = 11.47 (imidazole in acetonitrile), N = 10.50 (benzimidazole in DMSO), and N = 7.69 (benzotriazole in acetonitrile) these azoles are significantly less nucleophilic than previously characterized amines, such as DMAP (N = 14.95 in acetonitrile) and DABCO (N = 18.80 in acetonitrile). For some reactions of the 1-methyl substituted azoles with benzhydrylium ions equilibrium constants have been measured, which render a comparison of the Lewis basicities of these compounds. Substitution of the rate and equilibrium constants of these reactions into the Marcus equation yields the corresponding intrinsic barriers ΔG0. From the ranking of ΔG0 (imidazoles > pyridines > 1-azabicyclooctanes) one can derive that the reorganization energies for the reactions of imidazoles with electrophiles are significantly higher than those for the other amines and that imidazoles are less nucleophilic than pyridines and 1-azabicyclooctanes of comparable basicity.

Graphical abstract: Nucleophilicities and Lewis basicities of imidazoles, benzimidazoles, and benzotriazoles

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Article information


Submitted
18 Jan 2010
Accepted
16 Feb 2010
First published
05 Mar 2010

Org. Biomol. Chem., 2010,8, 1929-1935
Article type
Paper

Nucleophilicities and Lewis basicities of imidazoles, benzimidazoles, and benzotriazoles

M. Baidya, F. Brotzel and H. Mayr, Org. Biomol. Chem., 2010, 8, 1929 DOI: 10.1039/C000965B

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