Catalysis of Organic..

Wang et al. 27Schiff’s base formation occurs by condensation of the free amine base withaldehyde A in EtOAc/MeOH. The free amine base solution of glycine methyl esterin methanol is generated from the corresponding hydrochloride and triethylamine.Table 4 shows the reaction concentration profiles at 20-25°C. The Schiff’s baseformation is second order with respect to both the aldehyde and glycine ester. Theequilibrium constant ( ratio k(forward)/ k(reverse)) is calculated to be 67.Table 4. Reaction kinetics of Schiff base formationTime Aldehyde A Glycine methyl. Schiff’s base B(min) (molar) ester·HCL(molar) (molar) .0.65 0.365 0.294 0.0312.42 0.303 0.232 0.0935.29 0.242 0.171 0.1547.52 0.211 0.14 0.18510.8 0.18 0.109 0.21616.18 0.149 0.078 0.24727.21 0.118 0.047 0.27844.28 0.10 0.029 0.29672.2 0.09 0.019 0.306 .k(forward) = 0.006695 L/mol.sec, k(reverse) = 0.0001 L/mol.secImpurity Fate and Byproduct FormationThe Schiff’s base hydrogenation is the second step of a telescoped reductiveamination and is carried out in the presence of the aldehyde. When the Schiff’s baseis initially prepared, the magnitude of the equilibrium concentration of aldehyde A istwo orders lower than the Schiff’s base B. In the reaction network, catalytichydrogenation of A and B occur simultaneously. Based on the adsorption strengthand catalytic activity comparison between A and B shown in Table 4, k2 x KB is tentimes higher than k3 x KA. Therefore, the ratio of alcohol to product formation, Eq(5), is about 10 -3 . This result indicates that the alcohol formation is not significant inthe reaction network. Since the Schiff’s base is the dominant adsorbed species on thecatalyst during hydrogenation, the product C molecules do not compete strongly withthe Schiff’s base for the palladium surface adsorption. No E from the reduction ofproduct C is expected until the Schiff’s base transformation is almost complete.The Schiff’s base catalytic hydrogenation rate can be expressed asdC/dt = k [H2] L [Schiff base*]= k2 KB [B] [H2] L / ( 1 +KB [B] )where[B] is the Schiff ‘s base concentration in the reaction solution[H2] L is the hydrogen concentration in the reaction mixture