Issue 19, 2011

Nucleophilicity parameters for amines, amino acids and peptides in water. Variations in selectivities for quinone methides

Abstract

Second order rate constants for reactions of 4,4′-dimethylaminobenzhydrylium cations with amines and other nucleophiles in water define a scale of nucleophilicity (N+′′ = log k + 2.63). The N+′′ scale can be extended by linking directly to an established N+ scale based on reactions of methyl vinyl pyridinium cations with amine nucleophiles. Logarithms of rate constants for other benzhydrylium ions and quinone methides (QMs) are correlated by the equation: log k = sEN+′′ + constant, having a nucleophilicity parameter (N+′′ defined as in the Ritchie N+ equation with N+′′ = 4.75 for hydroxide ion), and an electrophile's response (selectivity) parameter (sE, as in the Swain-Scott equation). Correlations for other benzhydrylium cations require only one slope and one intercept per cation, and fit data for up to 54 amines, amino acids and peptide nucleophiles; the slope sE increases as the reactivity of the cation decreases. Contrary to recent reports, sE is significantly less than unity for reactions of o- and p-benzoquinone methides. As the reactivities of QMs decrease, sE increases and the response of sE to changes in reactivity is larger for QMs than for cations.

Graphical abstract: Nucleophilicity parameters for amines, amino acids and peptides in water. Variations in selectivities for quinone methides

Article information

Article type
Paper
Submitted
06 May 2011
Accepted
21 Jun 2011
First published
02 Aug 2011

Org. Biomol. Chem., 2011,9, 6685-6690

Nucleophilicity parameters for amines, amino acids and peptides in water. Variations in selectivities for quinone methides

T. W. Bentley, Org. Biomol. Chem., 2011, 9, 6685 DOI: 10.1039/C1OB05715D

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