Quantification of the Ambident Electrophilicities of Halogen-Substituted Quinones

Kinetics and mechanisms of the reactions
of p-quinone,
2,5-dichloro-p-quinone, 2,3,4,5-tetrachloro-p-quinone (chloranil), 2,3,4,5-tetrafluoro-p-quinone (fluoranil), and 3,4,5,6-tetrachloro-o-quinone
with π-nucleophiles (siloxyalkenes, enamines) and amines have
been investigated. Products arising from nucleophilic attack at all
conceivable sites, that is, at C and O of the carbonyl groups (pathways a, b) as well as at halogenated and nonhalogenated
conjugate positions (pathways c, d), were observed. The partial rate constants for the C-attack pathways
(a, c, d), which
are derived from the photometrically determined second-order rate
constants and the product ratios followed the linear free energy relationship
log k (20 °C) = sN(E + N) (Mayr, H.; J. Am. Chem. Soc. 2001, 123, 9500−9512). It was, therefore, possible to calculate the electrophilicity
parameters E of the different positions of the quinones
from log k (20 °C) and the N and sN parameters of the nucleophilic
reaction partners, which have previously been derived from their reactions
with benzhydrylium ions. Almost all rate constants for the C-attack
pathways (a, c, d) were considerably larger than those calculated for the corresponding
SET processes, indicating the operation of polar mechanisms. SET mechanisms
may only account for the formation of the products formed via O-attack.
With the E parameters determined in this work, it
is now possible to predict rate constants for the reactions of these
quinones with a large variety of nucleophiles and, thus, envisage
unprecedented reactions of quinones.