(158c) Water-Tolerant Lewis Acids for the Activation of Biomass-Derived Molecules

The depolymerization of the constituents of biomass by way of thermochemical or biological routes results in the production of a wide variety of oxygenated intermediates.  Although the composition of these intermediates is quite diverse, carbonyl functionalities are prevalent (e.g. aldehydes, ketones, carboxylic acids, etc).  In order to transform these functional intermediates into useful chemicals and fuels, robust and active catalysts need to be developed to selectively activate these molecules.  Lewis acids are known to form strong complexes with carbonyl groups.  A particular instance involves the complexation of the lone pair of electrons from the oxygen to the Lewis acid, effectively polarizing the carbonyl carbon atom, thereby making it prone to nucelophilic attack.  Unfortunately, biomass-derived molecules are commonly found in aqueous solutions and, in most cases, Lewis acidity is suppressed in the presence of water, since the hydroxyl group of water also forms strong complexes with Lewis acid sites.  The incorporation of metal centers (e.g. tin, titanium, and zirconium) into the framework of pure silica zeolites results in materials that maintain Lewis acidity in purely aqueous environments.  In this presentation, several interesting Lewis-acid-mediated reactions of biomass-derived molecules, including isomerizations, condensations, and 4+2 cycloadditions will be discussed.