EQUILIBRATION OF N-(2-CYANOETHYL)PYRIDINIUM CATIONS WITH SUBSTITUTED PYRIDINES AND ACRYLONITRILE - A CHANGE IN RATE-DETERMINING STEP IN AN E1CB REACTION

The rates of equilibration of N-(2-cyanoethyl)pyridinium cations (1) with the corresponding pyridines and acrylonitrile have been measured in aqueous solutions of ionic strength 0.1 at 25 °C. Second-order rate constants (kOH) have been obtained for the hydroxide ion catalyzed elimination reactions of 16 ring-substituted 1 having pyridine leaving groups of pKBH in the range 1.5-9.7. BrØnsted plots of log kOH vs pKBH are “concave down” with two distinct linear regions having β1g = -0.30 (for pKBH < 5.8) and β1g = -0.93 (for pKBH > 5.8). This observation is consistent with a change in rate-determining step within an E1cb reaction mechanism from rate-determining deprotonation of 1 (i.e., (E1cb)irrev) for pKBH < 5.8 to rate-determining leaving-group expulsion from the carbanionic intermediate (i.e., (E1cb)rev) for pKBH > 5.8. This interpretation is supported by 1H NMR spectral observations in basic D2O, which show no incorporation of deuterium into the acrylonitrile product for pKBH < 5.8 but do show D for H exchange of the methylene protons that are α to the cyano group at a rate that is faster than elimination for pKBH > 5.8. Rates of nucleophilic attack of pyridines and pyridinone anions (pkBH > 6) upon acrylonitrile have also been measured. These display a linear BrØnsted plot of βnuc = 0.20. Combination of β1g and βnuc gives βeq = 0.13 for the Michael-type addition of pyridinium cations to acrylonitrile to produce 1. Although the rates of the addition of pyridines of pKBH < 6 are too slow for convenient measurement in the current study, the combination of the measured rate and equilibrium Bronsted parameters allows the demonstration of the change in rate-determining step in these addition reactions from rate-determining nucleophilic attack (carbanion formation) with βnuc = 0.20 for pyridines of pKBH > 5.8 to rate-determining protonation of the carbanionic intermediate with βnuc = 0.83 for pyridine nucleophiles of pKBH < 5.8. General-base catalysis of the elimination reactions is observable in the (E1cb)irrev region but is extremely weak under the current experimental conditions. © 1990, American Chemical Society. All rights reserved.