Nucleophilic Addition to Activated Olefins. 3. Reactions of Piperidine and Morpholine with Benzylidenemalononitrile in 50percent Dimethyl silfoxide-50percent Water. Intrinsic Barriers in Nucleophilic Addition

Article abstract of DOI:10.1021/ja00528a049

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.