Pyridoxal Catalysis in the α,β-Elimination of S-(p-Substituted phenyl)cysteines

The pyridoxal-catalyzed α,β-elimination of S-(p-substituted phenyl)cysteines was studied at 50°C and μ=0.10 (KNO3) in 79% (v/v) aqueous acetonitrile in the presence of organic bases of different strength. The rate increased with an increase in basicity. The kinetic solvent isotope effect (kH2O⁄kD2O) of 1.15 is consistent with the reaction mechanism in which the α-proton abstraction occurs by the direct attack of a base on the proton. The elimination rate was insensitive to a change in the electronic nature of a leaving thiophenol, indicating that the α-proton abstraction can be referred to the rate-determining step. The rate showed a considerable dependence on the water content, as reflected on the activation parameters for reactions in 79% (v/v) aqueous acetonitrile as well as in water. The larger destabilization due to water-solvation at the transition state in the aqueous reaction system seems to cause a lower reactivity relative to the organic solvent system. The catalysis of benzene analogs demonstrated that the hydroxyl group adjacent to the aldehyde in pyridoxal plays a major role in the nonenzymatic α,β-elimination reactions.