Multifunctional Pyridylsiloxane Ligands in Palladium (II) Oxidation Catalysis: Control of Catalyst Structure and Mechanism through Selective Oligomerization

The palladium-catalyzed selective aerobic oxidation of alcohols to carbonyl compounds represents a significant advance in the utilization of molecular oxygen as an environmentally-friendly reagent in fine chemicals synthesis.  According to a recent mechanistic study of the Pd(OAc)₂/pyridine catalytic system, pyridine stabilizes the Pd⁰ intermediate against Pd-Pd bond formation, followed by reoxidization with O₂.  However, pyridine also attenuates the catalytic activity—another example of the frequently-encountered tradeoff between high activity and high turnover number.  In an attempt to modify this tradeoff towards improved catalytic activity and resistance to deactivation, we prepared a series of novel 3-pyridylsiloxane ligands of well-defined molecular structure.  The metal-ligand binding and catalytic properties were measured as a function of the multiplicity of pyridine groups, the pyridine-pyridine separation, and the inclusion of secondary functionality in the siloxane structure.

The functional monomer (3-pyridyl)dimethylsilane was synthesized by metal-halogen exchange with isopropylmagnesium chloride in THF, followed by nucleophilic substitution with dimethylchlorosilane.  Using this monomer as a reactive terminus, a series of (3-pyridyl)siloxanols was prepared by catalytic oxidation over Pd(OH)₂/C and heterofunctional condensation with (CH₃)₂SiHCl.  The finished pyridylsiloxane oligomers were prepared by a convergent synthesis route, coupling n equivalents of the pyridylsiloxanol oligomer with a central chlorosilane unit containing n reactive sites.  In this way, the multifunctional siloxane compounds could be obtained in good yield, and purified by flash chromatography (isolated yields 40-75%).

The oxidation of benzyl alcohol was studied under 1 atm of O₂ at various pyridyl:Pd ratios.  With (3-pyridyl)pentamethyldisiloxane as the ligand, the initial rate increased dramatically as the py:Pd ratio was decreased from 4 to 1.5.  This qualitative behavior was maintained for bis(pyridyl)siloxanes containing from zero to two Si-O spacer units between the pyridylsilyl end groups.  Interestingly, when three spacer units were included, the initial rate became almost independent of py:Pd ratio under the range of conditions studied.  This transition is attributed to the formation of a strainless trans bidentate complex, which is not possible for the shorter oligomers.  These experimental results may prove useful in the design of efficient silica-supported Pd complexes with improved activity and resistance to Pd-Pd bond formation.