Nucleophilicity of the bis(4-nitrophenyl)methyl anion
Stefan T. A. Bergera, Tadeusz Lemekb, and Herbert Mayra*
aDepartment Chemie und Biochemie, Ludwig-Maximilians-Universität München,
Butenandtstr. 5–13 (Haus F), 81377 München, Germany
bDepartment of Chemistry, Agricultural University of Cracow, 21 Mickiewicz Ave., 31-120 Cracow, Poland
E-mail:
[email protected]
Dedicated to Professor Nouria Al-Awadi in recognition of her achievements for science in Kuwait
Abstract
Kinetics of the reactions of the bis(4-nitrophenyl)methyl anion 1 with quinone methides and other Michael acceptors have been measured in dimethyl sulfoxide solution at 20 °C. The second-order rate constants k2 have been used to determine the nucleophilicity parameters N = 19.9 and s = 0.67 of 1 according to the linear free energy relationship log k(20 °C) = s(N + E).
Keywords: Kinetics, linear free energy relationship, nucleophilicity, intrinsic barriers
Introduction
In previous work, we have demonstrated that the rates of the reactions of carbocations and Michael acceptors with nucleophiles can be calculated by Equation 1,
log k2(20 °C) = s(N + E) (1)
where E represents the reactivity of the electrophile, N is a nucleophilicity parameter and s is a nucleophile-specific slope parameter.1-4
By using colored benzhydrylium ions and structurally related quinone methides as reference electrophiles, a comprehensive nucleophilicity scale has been developed, which includes a large variety of s-, n- and p-nucleophiles. This scale allows us to compare the reactivities of different types of nucleophiles on the basis of the nucleophilicity parameters N and s.
The fast characterization of a large number of nucleophiles became possible, because spectrophotometry provides an efficient tool to determine rate constants for the reactions of
