Neutral vs. cationic rhodium (I) complexes of bulky N-phosphino sulfinamide ligands: Coordination modes and its influence in the asymmetric hydrogenation of Z-MAC

Here we report the synthesis of a new N-di-tert-butylphosphino-tert-butylsulfinamide (PNSO) ligand and its corresponding p-tolylsulfinamide analog. The coordination of these compounds to rhodium to form a neutral and apolar complex is described, followed by the subsequent protonation of said complexes to quantitively form the more orthodox, cationic rhodium species containing a tetrafluoroboric counterion. The crystallographic structure of the tert-butylsulfinamide-derived cationic species was obtained and is elucidated. It outlines coordination from the sulfinamide group to the rhodium atom and shows no preference between O- and S-coordination as both complexes can be seen in one unit cell of the crystal. The efficacies of the neutral species and the salt species were tested in the asymmetric hydrogenation of methyl (Z)-α-acetamido cinnamate (Z-MAC). The p-tolylsulfinamide-derived complexes gave no hydrogenation while the tert-butylsulfinamide-derived ones produced hydrogenation with complete conversion but low enantioselectivities. The stereochemical outcome of the reaction was analyzed by means of the quadrant method.