Polystyrene-supported enantiopure 1,2-diamines: Development of a most practical catalyst for the asymmetric transfer hydrogenation of ketones

Chlorosulfonylated polystyrene, a commodity resin, reacts with enantiopure 1,2-diamines to afford, in a single step, high loading catalytic resins involving monosulfonylated 1,2-diamino moieties. These functional polymers form stable (p-cymene)ruthenium chloride [RuCl(p-cymene)] complexes that efficiently catalyze (down to S/C=150) the asymmetric transfer hydrogenation (ATH) of alkyl aryl ketones with formic acid-triethylamine under essentially solvent-free (down to 0.25 mL mmol^-1) reaction conditions. Among these resins, the immobilized version of TsDPEN stands out as a most practical catalyst for ATH: Uniformly high enantioselectivities are achieved with its use at low catalyst loading, and the resin can be recycled with virtually no limits.