title: 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(E)N(+)'' + 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.
authors.affiliation: Chemistry Unit, Grove Building, School of Medicine, Swansea University, Swansea, Wales, UK SA2 8PP. t.w.bentley@swansea.ac.uk
authors.forename: T William
authors.initials: TW
authors.lastname: Bentley
mesh_terms: D000588:Amines / Q000737:chemistry*; D000596:Amino Acids / Q000737:chemistry*; D045563:Indolequinones / Q000737:chemistry*; D015394:Molecular Structure; D010455:Peptides / Q000737:chemistry*; D014867:Water / Q000737:chemistry
chemical_list: D000588:Amines; D000596:Amino Acids; D045563:Indolequinones; D010455:Peptides; D014867:Water; C068040:quinone methide