Nucleophilic Addition to Olefins. 10. Kinetics of Cleavage of the Piperidine and Morpholine Adducts of α-Cyano-4-nitrostilbene and α-Cyano-2,4-dinitrostilbene

Article abstract of DOI:10.1021/ja00323a033

Rates of cleavage of the anionic piperidine and morpholine adducts (T^-) of α-cyano-4-nitrostilbene (1-NO2) and α-cyano-2,4-dinitrostilbene (1-(NO2)2) into PhCH=⁺NR2 and 2-X-4-nitrophenylacetonitrile anion were determined.For the adducts of 1-(NO2)2 there is a change from rate-limiting carbon protonation (to form T⁰) at low amine concentrations to rate-limiting cleavage of T⁰ into products at high concentrations.For the adducts of 1-NO2 cleavage is rate limiting throughout.Compared to the protonation of the anion of (2,4-dinitrophenyl)acetonitrile (2-(NO2)2), protonation of T^- derived from 1-(NO2)2 is slightly enhanced when the acid is water, strongly reduced when the acid is morpholinium or piperidinium ion (R2NH2⁺), and strongly enhanced with H3O⁺.The slightly enhanced rate of the water reaction is attributed to an enhanced pK_a⁰ of the adduct, the strongly depressed rate for the R2NH2⁺ reactions to a steric effect.The enhanced rate with H3O¹⁺ is ascribed either to an intramolecular pathway via the nitrogen-protonated adduct (T^+/-) or to a stabilization, by the adjacent amine moiety, of the transition state for protonation by H3O⁺.Problems with either interpretation exist, though, and are discussed.Even after taking into account the different leaving group basicities, the cleavage of T⁰ derived from 1-NO2 is much slower than that of the previously studied T⁰ derived from benzylidenemalononitrile, indicating a higher intrinsic barrier for the departure of the more delocalized (4-nitrophenyl)acetonitrile anion compared to ^-CH(CN)2.This is consistent with similar patterns observed with other carbanion-forming reactions such as deprotonations of C-H acids and nucleophilic additions to olefins.If one allows for a steric enhancement of the cleavage of T⁰ derived from 1-(NO2)2, it appears that the intrinsic barrier for departure of 2-(NO2)2^- is also higher than that for the somewhat less delocalized (4-nitrophenyl)acetonitrile anion.