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 Δδ
p
(19F) values correlate strongly with Hammett parameters, pK
a of the protonated HBA, and proton-transfer basicity
(pK
BH+).
The relative Lewis acidity of a variety of metal−ligand catalyst complexes is quantified using 31 P NMR spectroscopy. Three 31 P 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.
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.
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