Lewis acids are some of the most common catalysts and are used in many important organic transformations including conjugate additions, Diels-Alder and Friedel-Crafts reactions. The quantification of Lewis acid properties can accelerate reaction discovery and catalyst development by assigning specific numbers to catalytically relevant properties. The catalytic activity of metal-ligand complexes for asymmetric catalysis often depends on ligand, counterion or additive effects. The relative Lewis acidity of a wide variety of catalytically relevant metal salts were studied using 31P NMR spectroscopy. Metal salts were assessed for their ability to TEPO (triethylphosphine oxide), resulting in a downfield shift of the 31P NMR signal. In addition to TEPO, more complex bisphosphine oxides were investigated as 31P NMR probes to emulate the bidentate binding motif integral to asymmetric catalysis with Lewis acid complexes. This study quantifies the effects of ligands, counterions, additives and water on the binding properties of Lewis acid complexes, and can provide insight into both electronic and steric effects. For example, the generation of cationic Lewis acid complexes in situ through the addition of silver triflates to ruthenium complexes. This methodology gives us the opportunity to easily quantify the ability of cocatalysts to increase Lewis acidity using a simple NMR experiment. The effect of Brønsted acid-assisted Lewis acid catalysis was also quantified using titanium complexes with various diol-containing ligands and additives. The Brønsted acidity of the diol-containing ligands and Lewis acidity of the cooperative complexes were compared to quantify the effect that Brønsted-acidic ligands and have on solubility and Lewis acidity.

Quantifying Lewis acidity and oxophilicity of Lewis acid metal-ligand complexes for asymmetric catalysis using 31P NMR spectroscopy

255th National Meeting (2018)

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