20830-11-9Relevant academic research and scientific papers
Nucleophilic Addition to Olefins. 9. Kinetics of the Reaction of Benzylidenemalononitrili with Malononitrile Anion
Bernasconi, Claude F.,Zitomer, Janie L.,Fox, John P.,Howard, Keith A.
, p. 482 - 486 (2007/10/02)
The rates of the reversible addition of - to benzylidenemalononitrile, to form -, have been measured in water and in 50percent Me2SO - 50percent water.In water at 20 deg C, k1 = 2.30*1E5 M-1 s-1, k-1 = 5.95 s-1, and K1 = 3.86*1E4 M-1.In 50percent Me2SO - 50percent water at 20 deg C, k1 = 9.50*1E5 M-1 s-1, k-1 = 6.52 s-1, and K1 = 1.45*1E5 M-1.The pKa of the adduct is about 5 pK units lower than that of malononitrile.K1 for the addition of - is much higher than K1 for piperidine addition, despite the slightly lower proton basicity of -.This reflects the common observation that the carbon basicity of carbanions is higher than that of amines.The intrinsic rate contante for nucleophilic attack by - appears to be somewhat smaller than for amine attack, just as the intrinsic rate constant for protonation of - is lower than for the protonation of amines.This is not unexpected, since structural factors should affect the coordination of bases with Lewis acids in a similar way as coordination with the proton.The change from water to 50percent Me2SO - 50percent water has the effect of increasing k1 by a slighthly larger factor (4.13) than K1 (3.75).This implies that the intrinsic rate constant is higher in the less aqueous solvent, which is consistent with the notion that solvent reorganization contributes to the intrinsic barrier of the reaction.
Nucleophilic Addition to Activated Olefins. 3. Reactions of Piperidine and Morpholine with Benzylidenemalononitrile in 50percent Dimethyl silfoxide-50percent Water. Intrinsic Barriers in Nucleophilic Addition
Bernasconi, Claude F.,Fox, John P.,Fornarini, Simonetta
, p. 2810 - 2816 (2007/10/02)
The reactions of piperidine and morpholine with benzylidenemalononitrile give rise to two observable kinetic processes.The first is measurable by the stopped-flow temperature-jump technique and is associated with the formation of a zwitterionic addition complex (T+/- in Scheme I) and subsequent deprotonation to form the anionic complex T-.The second process, which was monitored in the stopped-flow apparatus, refers to the cleavage of the addition complex T- into benzaldehyde and malononitrile (Scheme II).In the first process nucleophilic attack is rate limiting for the piperidine reaction but proton transfer is partially rate limiting in the morpholine reaction at low amine concentrations.This result implies that base catalysis in nucleophilic vinylic substitutions by amines might be a consequence of rate-limiting proton transfer in some cases.For a given equilibrium constant for the nucleophilic addition to an activated olefin the rate constants are higher for benzylidenemalononitrile (cyano activation) than for olefins activated by alkoxycarbonyl groups or the nitro group, implying a lower intrinsic barrier to nucleophilic attack in the former.The similarity of these results with finding on proton transfers involving carbon acids activated by the same substituents is discussed and related to a common origin.Analysis of the data referring to the cleavage of T- shows that protonation of T- on carbon, to form T0, is rapid, while cleavage of T0 into the iminium ion, PhCH=N+R2, and malononitrile anion is rate limiting.
