Mechanism of Carboxylic Acid-Catalyzed Reduction of Substituted Nitrosobenzenes by 1-Benzyl-3,5-bis(1-pyrrolidinylcarbonyl)-1,4-dihydropyridine in Acetonitrile

Mechanism of carboxylic acid-catalyzed reduction of substituted nitrosobenzenes by 1-benzyl-3,5-bis(1-pyrrolidinylcarbonyl)-1,4-dihydropyridine has been studied in acetonitrile at 25 °C. The major product was hydroxylamine when the dihydropyridine was used in excess over nitrosobenzene, while that was azoxybenzene in the reverse case. The observed second-order rate constant k2R are dependent on acid concentration in a first-order manner with the acid-catalyzed third-order rate constant kHR. The Hammett relationship for kHR for substituted nitrosobenzenes gave a ρ=−0.93. The Brönsted plot of log kHR vs. pKa of acids gave a straight line with a slope of −0.34. The deuterium isotope effect for the catalysis by dichloroacetic acid was found to be kHR⁄kDR=2.80. These results indicate that a general acid catalysis by carboxylic acids plays an important role in the reduction of nitrosobenzene by dihydropyridine in acetonitrile.