NMR Quantification of Hydrogen-Bond-Accepting Ability for Organic Molecules.

The hydrogen-bond-accepting abilities for more than 100 organic molecules are quantified using 19F and 31P NMR spectroscopy with pentafluorobenzoic acid (PFBA) and phenylphosphinic acid (PPA) as commercially available, inexpensive probes. Analysis of pyridines and anilines with a variety of electronic modifications demonstrates that changes in NMR shifts can predict the secondary effects that contribute to H-bond-accepting ability, establishing the ability of PFBA and PPA binding to predict electronic trends. The H-bond-accepting abilities of various metal-chelating ligands and organocatalysts are also quantified. The measured Δδ(31P) and Δδp(19F) values correlate strongly with Hammett parameters, pKa of the protonated HBA, and proton-transfer basicity (pKBH+).

NMR quantification of H-bond donating ability for bioactive functional groups and isosteres.

The H-bond donating ability for 127 compounds including drug fragments and isosteres have been quantified using a simple and rapid method with 31P NMR spectroscopy. Functional groups important to medicinal chemistry were evaluated including carboxylic acids, alcohols, phenols, thioic acids and nitrogen group H-bond donors. 31P NMR shifts for binding to a phosphine oxide probe have a higher correlation with equilibrium constants for H-bonding (log KHA) than acidity (pKa), indicating that these binding experiments are representative of H-bonding ability and not proton transfer. Additionally, 31P NMR binding data for carboxylic acid isosteres correlates with physicochemical properties such as lipophilicity, membrane permeability and plasma protein binding. This method has been used to evaluate the H-bond donating ability of small molecule drug compounds such as NSAIDs and antimicrobials.

NMR Quantification of the Effects of Ligands and Counterions on Lewis Acid Catalysis.

The relative Lewis acidity of a variety of metal-ligand catalyst complexes is quantified using 31P NMR spectroscopy. Three 31P NMR probes, including two new bidentate binding probes, are compared on the basis of different binding modes (i.e., monodentate vs bidentate) and the relative scale of their downfield shift upon binding to Lewis acid complexes. Bidentate coordination of catalyst complexes including metal catalysts, ligands, and counterions were assessed due to their importance to asymmetric catalysis. The effect of ligands, counterions, and additives on Lewis acidity is quantified and correlated to reaction yield at an early time point as an approximation for catalytic activity/efficiency and chelation mode in two organic transformations. Binding studies were performed under catalytically relevant conditions, giving further applicability to synthesis. Insight into activation modes are revealed through this analysis.

Lanthanum(III)-Catalyzed Three-Component Reaction of Coumarin-3-carboxylates for the Synthesis of Indolylmalonamides and Analysis of Their Photophysical Properties.

New methodology has been developed for the Lewis acid catalyzed synthesis of malonamides. First, the scandium(III)-catalyzed addition of diverse nucleophiles (e.g., indoles, N,N-dimethyl-m-anisidine, 2-ethylpyrrole, and 2-methylallylsilane) to coumarin-3-carboxylates has been developed to afford chromanone-3-carboxylates in high yields as a single diastereomer. Upon investigating a subsequent lanthanum(III)-catalyzed amidation reaction, a new multicomponent reaction was designed by bringing together coumarin-3-carboxylates with indoles and amines to afford indolylmalonamides, which were identified to exhibit fluorescent properties. The photophysical properties for selected compounds have been analyzed, including quantum yield, molar absorptivity, and Stokes shift. Synthetic studies of several reaction byproducts involved in the network of reaction equilibria for the three-component reaction provide mechanistic insight for the development of this methodology.