The
molecular complex chloro(2,2′:6′,2″-terpyridine-4′-carboxylic
acid)palladium(II) chloride was synthesized and was attached to the
surface of amorphous silicon dioxide to generate a molecular/heterogeneous
catalyst motif. This catalytic system exhibited excellent selectivity
(>99%) for hydrodeoxygenation of oxygenated aromatics under mild
reaction
conditions. A kinetic analysis showed that this molecular/heterogeneous
catalyst was an order of magnitude more active than analogous homogeneous
catalysts. Characterization techniques such as XRD and solid-state
NMR, in conjunction with ICP-MS, indicate that the molecular catalyst
is present on the surface of SiO2 and the formation of
unwanted metallic Pd nanoparticles can be avoided. Computational modeling
shows the catalysts can adhere to the oxide surface through a hydrogen
bonding interaction, via a Coulombic attraction between the charged
molecule and the oxide surface, or through covalent bonding. Postreaction
analysis of the surface-modified oxide catalysts confirmed prolonged
molecular integrity of the catalyst and sustained binding of the catalyst
to the oxide surface when nonpolar solvents were employed for reactions.
These surface-attached molecular catalysts thus were recycled through
multiple catalytic reactions.
Ten ruthenium pincer complexes were evaluated as catalysts for the hydrodeoxygenation (HDO) reaction on a lignin monomer surrogate, vanillyl alcohol. Four of these complexes are reported herein with the synthesis and full characterization data for all and single-crystal X-ray diffraction data for three complexes bearing OH/O − , NMe 2 , and Me substituents on the pincer. A systematic study of these CNC pincer complexes revealed that the π-donor substituent on the pyridine ring plays a key role in enhancing the yield of the desired deoxygenated product. While OMe, OH, and NMe 2 are all effective as π-donor substituents on the central pyridine ring in the pincer, the highest conversion to products and the best selectivity was observed with OH substituents and added sodium carbonate as a base. Base serves to deprotonate the OH group and form 1 Oas observed spectroscopically. Furthermore, efforts to use other catalysts have revealed that free or labile sites are needed on the ruthenium center and an electronically rich and nonbulky CNC pincer is optimal. At low catalyst loadings (0.01 mol %), the OH-substituted catalyst 1 OH in the presence of base serves as a homogeneous catalyst and is able to achieve quantitative and selective conversion of vanillyl alcohol to desired the HDO product, creosol, with up to 10000 turnovers. With this knowledge in hand, we can design the next generation of homogeneous catalysts with increased reactivity toward all of the oxygenated sites on lignin-derived monomers.
A method has been developed for the silanolysis of alcohols using an abundant and non-corrosive base K2CO3 as a catalyst. Reactions between a variety of alcohols and hydrosilanes generate silyl ethers under mild conditions. The use of hydrosilanes leads to the formation of H2 as the only byproduct thus avoiding the formation of stoichiometric strong acids. The mild conditions lead to a wide scope of possible alcohol substrates and good functional group tolerance. Selective alcohol silanolysis is also observed in the presence of reactive C-H bonds, lending this method for extensive use in protection group chemistry.
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