Bis(pyridyl)siloxane–Pd(II) complex catalyzed oxidation of alcohol to aldehyde: Effect of ligand tethering on catalytic activity and deactivation behavior

Ligands containing two pyridyl groups tethered to a flexible methylsiloxane spacer are effective ligands in the Pd(OAc) 2-catalyzed aerobic oxidation of benzyl alcohol to benzaldehyde. The stabilization effect was the strongest for spacers just long enough to favor bidentate coordination of Pd to form a monomeric ring complex. [Display omitted] ▶ Bis(pyridyl)siloxane ligands are effective in stabilizing Pd(II) complexes. ▶ Ability to stabilize Pd(II) complexes depends strongly on length of ligand. ▶ Bis(pyridyl)siloxane ligands that form monomeric rings preferentially are more effective than others. A series of oligomeric methylsiloxane compounds functionalized with pyridyl groups was synthesized and used as ligands in the aerobic Pd(OAc) 2-catalyzed oxidation of benzyl alcohol to benzaldehyde at 353 K. The effect of tethering two pyridine ligands at the terminal positions of linear siloxanes of varying length was systematically investigated, as was the effect of the attachment point of the pyridine ring ( meta or para). It was found that meta-substituted pyridylsiloxanes were generally more effective in protecting the catalyst against Pd agglomeration. For this purpose, the optimal meta-pyridylsiloxane ligands had 4–6 silicon atoms or 10 silicon atoms for para-pyridylsiloxane ligands. The metal–ligand binding properties were used to interpret the catalytic behavior, and the ability of the catalyst stability correlated with ability of the bis-pyridyl ligand to form a mononuclear cyclic complex with Pd.