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
Abstract 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.