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 = s_EN₊′′ + 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 (s_E, 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 s_E increases as the reactivity of the cation decreases. Contrary to recent reports, s_E is significantly less than unity for reactions of o- and p-benzoquinone methides. As the reactivities of QMs decrease, s_E increases and the response of s_E to changes in reactivity is larger for QMs than for cations.