Role of the sulfonamide moiety of Ru(II) half-sandwich complexes in the asymmetric transfer hydrogenation of 3,4-dihydroisoquinolines

The role of the sulfonamide moiety of Noyori-Ikariya [Ru(II)Cl(eta (6)-p-cymene)(S,S)-(N-arylsulfonyl-DPEN)] (where DPEN = 1,2-diphenylethylene-1,2-diamine) half-sandwich complexes in the asymmetric transfer hydrogenation (ATH) of imines (1-methyl-3,4-dihydroisoquinoline and 6,7-dimethoxy-1-methyl-3,4-dihydroisoquinoline) was investigated. Nine complexes were synthesized and characterized, most of which have not been previously reported and a majority of the corresponding ligands (N-arylsulfonyl-DPEN) have not been described in imine ATH. The study demonstrates that the structure of the sulfonamide fragment strongly affects the catalytic activity. By monitoring the reaction kinetics, it was found that the reactivity of certain complexes was moderately enhanced and the enantioselectivity was affected as well, albeit to a lesser extent. No simple structure-activity pattern was found, suggesting that extensive screening experiments are necessary in order to obtain the optimal catalyst for a particular substrate. The study complements other previously reported works on structure-activity relationships concerning Ru(II)-catalyzed ATH by adding a new dimension of investigation.