(611b) 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.