Boronic acids accelerate hydrolyses of the Schiff’s bases derived from salicylaldehyde and primary amines. The reversible complex between arylboronic acids and imines has a rate of hydrolysis that follows a typical Michaelis-Menten enzyme-substrate kinetics, when the imine is saturated with the benzene boronic acid (BBA). Herein we present a structure-activity relationships (SAR) for different substituted BBAs and imine substrates undergoing hydrolysis. We hypothesized that electron withdrawing groups would enable BBAs to be more easily ionized, thus becoming better hydrogen bond donors and therefore, achieving higher affinities for the imine substrates. SAR experiments were performed with different fluoro-substituted benzeneboronic acids which highlighted that all fluoro-BBAs have higher catalytic efficiencies of imine hydrolysis than the unsubstituted benzene boronic acids. We report novel catalysts for the hydrolysis of salicylidene-3-hydroxyaniline imine, including 2,3,5-trifluoroBBA and 2,3,4,5-tetrafluoroBBA, characterized by kcat/km values of at least 1000-fold higher than the unsubstituted BBA. The pH dependencies for BBA catalyzed hydrolysis of salicylaldehyde-3-hydroxyaniline shows that rate constants are maximal at lower pH values and decrease with increasing pH. The plots of Log10 (Km), Log10 (kcat), Log10 (kcat/Km) vs. Hammett sigma substituent constants indicate that kcat, Km and kcat/Km dependent on electron withdrawing substituent groups present in the benzeneboronic acids.  In addition, we showed that the hydrogen bonding network provided by the hydroxyl or amino groups in the imine substrates could assist the BBA catalysts in binding tighter to the substrate, resulting in higher catalytic efficiencies during hydrolysis of selected imine substrates.<br/>

Hydrolysis of salicylaldehyde imines by arylboronic acid catalysts

2019 Fall National Meeting

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