HOMOGENEOUS HYDROGENATION OF CONJUGATED DIENES CATALYZED BY NAPHTHALENETRICARBONYLCHROMIUM

Conjugated dienes were selectively 1,4-hydrogenated using (Nap)Cr(CO)3 (Nap = naphthalene) at atmospheric pressure of hydrogen and room temperature in polar coordinating solvents. Kinetic studies show that the rate of hydrogenation has a first-order dependence at low catalyst and substrate concentrations and low partial pressures of hydrogen. The rate is either zero order or fractional order at higher concentrations of substrate and at higher partial pressures of hydrogen. The kinetic isotope effect was studied using D2 gas, and k(H)/k(D) was found to be 1.15. Addition of free naphthalene to the reaction mixture decreases the rate of hydrogenation. It was found that naphthalene in (Nap)Cr(CO)3 was substituted by coordinating solvents, as evident from in situ IR and UV-Vis spectral studies.