Changing the kinetic order of enantiomer formation and distinguishing between iminium ion and imine as the reactive species in the asymmetric transfer hydrogenation of substituted imines using a cyclopentadienyl iridium (III) complex

The iridium (III) complex of pentamethylcyclopentadiene and (S,S) or (R,R)-1,2-diphenyl-N′-tosylethane-1,2-diamine is an effective catalyst for the asymmetric transfer hydrogenation of imines under acidic conditions. However, the enantiomeric excess (ee) of the product amines from the reduction of 1-methyl-3,4-dihydroisoquinolines in either acetonitrile or dichloromethane, decreases exponentially. The dominant cause of the enantioselectivity is the difference in kinetic order of the formation of the two enantiomers with the S-enantiomer being formed in a first-order process whereas that for the R-enantiomer follows zero-order kinetics when (R,R)-TsDPEN is employed, due to different rate-limiting steps for the two processes. A series of 1-fluorinated methyl-3,4-dihydroisoquinolines were synthesised to change the rate-limiting dissociation of the (R) amine product from Ir (III) so that both enantiomers are formed with the same kinetic order. This results in almost complete removal of the enantioselectivity of the reduction. It has been suggested that reduction of imines using transition metal complexes occurs through the neutral imine rather than the more reactive iminium-ion. α-Substituted imines with electron-withdrawing groups make protonation more difficult but enhance the electrophilicity of the imine carbon facilitating nucleophilic attack. The pKa of the iminium ions of 1-fluorinated methyl-3,4-dihydroisoquinolines were determined. Using the relative rates of the cyclopentadienyl iridium (III) complex catalysed reduction of these 1-fluorinated methyl-3,4-dihydroisoquinoline in acetonitrile and, under the acidic conditions of a 5:2 ratio of formic acid:triethylamine, showed that the iminium ion is the reactive species.

Determination of Kinetic Order of Reaction for its Duration in the Study of Solvation Factor Effect on Cyclohexene Hydrocarbomethoxylation Catalyzed by Palladium-Phosphine Systems

The purpose of the study was to investigate homogeneous catalytic reaction of cyclohexene hydrocarbomethoxylation leading to the methyl cyclohexanecarboxylate formation. Pd(PPh3)2Cl2 and Pd(OAc)2 which were promoted by free PPh3 and p-toluenesulfonic acid were used as catalytic precursors. In the temperature range 358--393 K, we studied the methanol concentration effect on the value of kinetic order of the reaction with respect to cyclohexene for the duration of the reaction. Findings of research show that the kinetic order increased from 1 to 10 with temperature increase and methanol concentration rise. We compared the kinetic order values for the duration of the reaction and the concentration order with respect to cyclohexene, which is equal to 1, set for the initial rate region. Based on this comparison, it was suggested that the formation of inactive palladium complexes is progressed by the action of excess methanol at elevated temperatures. We found that the regularities of methanol effect on the cyclohexene hydrocarbomethoxylation rate for the duration of the reaction were agreed with the regularities discovered for the initial rate region. With consideration of the data about enthalpy change in the ligand exchange reactions between the palladium complexes with participation of CH3OH, СО and PPh3, we draw the conclusion about the dominant contribution of specific solvation to the catalyst deactivation in the conditions of high methanol concentrations