Competitive asymmetric transfer hydrogenation of 3,4-dihydroisoquinolines employing Noyori-Ikariya catalytic complexes

Competitive asymmetric transfer hydrogenation (ATH) of three differently methoxy-substituted 1-methyl-3,4-dihydroisoquinolines (1-Me-DHIQs) was carried out to examine the differences in their reactivity with six ruthenium complexes of the Noyori-Ikariya type having the general formula [Ru(II)Cl(eta(6)-arene)(N-arylsulfonyl-DPEN)] (DPEN = 1,2-diphenylethylene-1,2-diamine). The reaction kinetics of two or three substrates at once was followed in situ by H-1 NMR spectroscopy. A method originally developed for heterogeneous catalysis was used to evaluate the experimental data, providing selectivities of the catalysts to the particular substrates and affinity of these substrates to the active site. The higher reaction rate was usually connected with both higher selectivity and affinity. However, in several cases, the opposite behavior was observed, pointing to a higher selectivity towards the less reactive substrate, which can inhibit the reaction due to its higher affinity. No competitive behavior was manifested in terms of enantioselectivity. As the structure of the Noyori-Ikariya catalytic complexes is highly variable and previous structure-activity studies have often been inconclusive, the presented method may aid in the disentanglement of the complex relationships important for rational catalyst design.