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

Article abstract of DOI:10.1246/bcsj.60.1887

Metal ion catalyzed reduction of substituted nitrosobenzenes by 1-benzyl-3,5-bis(1-pyrrolidinylcarbonyl)-1,4-dihydropyridine (BPDH) in acetonitrile has been studied for seven bivalent metal ions.The reduction of N-methylacridinium salt (MA) has also been examined.In the former cases, it was found that the metal ions catalyze the reduction by forming a 1 : 1 complex with BPDH according to a Michaelis-Menten type saturation kinetics which allowed to derive the association constants, K_M, for the complexation and the second-order rate constants, k₂, for the reduction.A linear relationship was found between log k₂ and the ionization potentials of metal ions with a positive slope.A linear Hammett relationship was also observed between log k₂ and the Hammett ? constants with a positive p value for three metal ions.These results suggest that a bivalent metal ion is sandwiched between the BPDH and the substrate and acts as a Lewis acid to stabilize the incipient N-oxide anion of the substrate which is formed by hydride transfer in the transition state.In the cases of MA, all metal ions inhibited the reduction.Repulsion between the positive charges of the metal ion-BPDH complex and the substrate salt appears to be prevailing.