Water-Soluble Organometallic Catalysts from Carbohydrates. 2. A Strategy for the Preparation of Catalysts with Pendant Quaternary Ammonium Groups Using d -Salicin

A series of water-soluble chelating bis-phosphinite ligands have been prepared from D-salicin (2-(hydroxymethyl)phenyl beta-D-glucopyranoside). The 4- and 6-hydroxyl groups of salicin were protected as a cyclic ketal. Mitsunobu reaction with phthalimide at the benzylic position was used to install the aminomethyl side-chain in the C(1)-aromatic substituent. Formation of the bis-2,3-O-diarylphosphinite was accomplished by reaction of the resulting diol with chlorodiarylphosphine. Quaternization with Meerwein's salt (Me(3)O(+) BF(4)(-)) followed by reaction with Rh(+)(COD)(2) BF(4)(-) gave precatalysts with limited aqueous solubility. Deprotection of the ketal group with acidic resin in methanol gave water-soluble cationic Rh complexes that are competent to carry out highly efficient hydrogenation of acetamidoacrylic acid derivatives in organic, aqueous, or biphasic media. However, enantioselectivities of these reactions in neat aqueous or biphasic media are generally lower than those observed in organic medium.