Multivalent, Sulfur-Rich PyBox Asterisk Ligands in Asymmetric Metal Catalysis



Multivalent PyBox asterisk ligand 1 and monovalent ligand 2 are reported. The innovative feature in 1 is a sulfur-rich persulfurated benzene core as a template joining six PyBox units in close proximity. An efficient persulfuration of C6Cl6 with p-methoxybenzenethiol (95 %, >99 % per C–S bond) and deprotection led to hexaphenol 4. Six O-arylations of 4 with Cl-PyBox 3 (94 %, >99 % per C–O bond) provided 1. The electronic delocalization of 1 and 2 was analyzed by UV/Vis spectroscopy and cyclic voltammetry. PyBox ligands 1 and 2 were successfully used for Rh-catalyzed hydrosilylation of acetophenone, in spite of competitive bindings from six proximal sulfur atoms. The concept of multivalency and the presence of donating and oxidizable sulfur atoms in asymmetric metal catalysis were tested. The reactivities and the enantiocatalytic behavior of 1 and 2 with varying rhodium content were determined. Lability of the Rh complexes under certain conditions, was demonstrated by some crossover experiments with other PyBox ligands. Conformational and competitive metal binding studies of 1 with the aid of molecular modeling (GenMol™) helped to explain the results. This work suggests new directions for simple multivalent ligands in metal-mediated catalysis.