Electrophilicity parameters of 5-benzylidene-2,2-dimethyl[1,3]dioxane-4,6-diones (benzylidene Meldrum's acids).
Kinetics of the reactions of four benzylidene Meldrum's acids 1 with acceptor-substituted carbanions 2 were studied photometrically in DMSO at 20 degrees C. The reactions follow second-order kinetics, and the second-order rate constants were found to follow the correlation log k2 (20 degrees C) = s(N + E) (eq 1), which was used to calculate the electrophilicity parameters E for compounds 1. Hammett correlations are given, which allow one to assign electrophilicity parameters for various beta,beta-acceptor substituted styrenes and thus to predict a large number of absolute rate constants for a manifold of Michael additions. The reactions of primary and secondary amines are approximately 2 orders of magnitude faster than predicted by the correlation (1), supporting transition states which are stabilized by hydrogen bridges from NH to the carbonyl groups of the benzylidene Meldrum's acids.
Kaumanns
Oliver
O
Department Chemie und Biochemie, Ludwig-Maximilians-Universität München, Butenandtstr. 5-13 (Haus F), 81377 München, Germany.
Mayr
Herbert
H