Journal of Organic Chemistry p. 2878 - 2885 (1989)
Update date:2022-08-24
Topics:
Bernasconi, Claude F.
Killion, Robert B.
Piperidine and morpholine add to substituted benzylidenemalononitriles (Z-C6H4CH=C(CN)2) to form a zwitterionic adduct, Z-C6H4CH(R2NH+)C(CN)2-(T+/-), which is in rapid acid-base equilibrium with the anionic adduct, Z-C6H4CH(R2N)C(CN)2-(T-).Rate constans for amine addition (k1) were determined by direct rate measurements while equilibrium constans for addition (K1) as well as pKa+/- values of the zwitterions were obtained spectrophotometrically.The bulk of the measurements was carried out in 50percent Me2SO-50percent water with piperidine, while a smaller number of experiments were performed with morpholine, and with both amines in water and in 70percent Me2SO-30percent water.The reactions show the typical bahavior of a carbanion-forming process in which the carbanion derives a good part of its stabilization from polar effects while resonance effects play a more modest role.This behavior includes a high intrinsic rate constant (k0 = k when K = 1), a small transition-state imbalance, and a relatively small solvent effect on the intrinsic rate constant.The observation of an imbalance suggests that the deprotonation of malononitrile derivatives by carboxylate ions should also have an imbalanced transition state.The fact that none has been observed is attributed to a solvation effect of the carboxylic acid, which enhances the Broensted βB value, as recently suggested by Murray and Jencks.The 4-Me2N substituent leads to strong resonance stabilization of the olefin as indicated by a low K1 value.Contrary to expectation of a lowered intrinsic constant, this resonance stabilization has little effect on k0.This suggests theoperation of a compensating factor which increases k0 and which can be understood as an attenuation of the reduction in k0 caused by late development of resonance at the carbanionic center of the adduct.
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