Electrophilicity parameters of 5-benzylidene-2,2-dimethyl[1,3]dioxane-4,6-diones (benzylidene Meldrum's acids) |
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Authors: | Kaumanns Oliver Mayr Herbert |
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Institution: | Department Chemie und Biochemie, Ludwig-Maximilians-Universit?t München, Butenandtstr. 5-13 (Haus F), 81377 München, Germany. |
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Abstract: | 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. |
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