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

Abstract

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.

Isolable Silicon‐Based Polycations with Lewis Superacidity

“…Figure S26). This is in agreement with the lower FIA compared to the strained silacyclobutane and silafluorene derivatives 2 2+ and 3 2+ (Δ 31 P=28.6, 27.7 ppm), which exceed the reported Δ 31 P for B(C 6 F 5 ) 3 ⋅OPEt 3 in CH 2 Cl 2 (26 ppm) [24] . X‐Ray structure analyses confirmed the formation of octahedral [ 2 (OPEt 3 )] 2+ and [ 3 (OPEt 3 ] 2+ (Figures S101 and S102).…”

“…This is in agreement with the lower FIA compared to the strained silacyclobutane and silafluorene derivatives 2 2+ and 3 2+ (D 31 P = 28.6, 27.7 ppm), which exceed the reported D 31 P for B-(C 6 F 5 ) 3 •OPEt 3 in CH 2 Cl 2 (26 ppm). [24] X-Ray structure analyses confirmed the formation of octahedral [2(OPEt 3 )] 2+ and [3(OPEt 3 ] 2+ ( Figures S101 and S102). With 4[OTf] 3 and 5[OTf] 3 , mixtures of bis-adducts and other unidentified species formed (D 31 P ranging from 35.5 to 48.9 ppm), which hampered a meaningful scaling with the GB method (see Supporting Information).…”

Isolierbare Silicium‐basierte Polykationen mit Lewis‐Superacidität

“…Abbildung S26). Dies stimmt gut mit der niedrigeren FIA im Vergleich zu den gespannten Silafluoren‐ und Silacyclobutanderivaten 2 2+ und 3 2+ (Δ 31 P=28.6, 27.7 ppm) überein, deren Δ 31 P den Wert für B(C 6 F 5 ) 3 ⋅OPEt 3 in CH 2 Cl 2 (26 ppm) übersteigen [24] . Kristallstrukturanalysen belegen die Bildung von oktaedrischem [ 2 (OPEt 3 )] 2+ und [ 3 (OPEt 3 ] 2+ (Abbildung S101 und S102).…”

“…Dies stimmt gut mit der niedrigeren FIA im Vergleich zu den gespannten Silafluoren-und Silacyclobutanderivaten 2 2+ und 3 2+ (D 31 P = 28.6, 27.7 ppm) überein, deren D 31 P den Wert für B(C 6 F 5 ) 3 •OPEt 3 in CH 2 Cl 2 (26 ppm) übersteigen. [24] )] 3+ übersteigt nicht nur D 31 P von OPEt 3 -Silylium-Addukten (43.9 AE 2.7 ppm), [9,22,25] sondern auch den D 31 P-Wert des Boreniumadduktes [(catecholat)B(OPEt 3 )] + (60.7 ppm). [9,22] Wegen des großen Einflusses von sterischen und HSAB-Effekten auf die GB-Methode sind Vergleiche zwischen verschiedenen Verbindungsklassen mit Vorsicht zu…”

On the Use of Aqueous Metal-Aqua pKa Values as a Descriptor of Lewis Acidity

“…On the other hand, we were surprised to find that the Gutmann-Beckett method, in which Lewis acidity is interrogated by 31 P nuclear magnetic resonance (NMR) spectroscopy with a suitable phosphorus-containing probe molecule, typically a phosphine oxide, has not been employed for systematic quantification of the Lewis acidity of redox-inactive metal ions. 22,23 As this method has been found to be effective for alkaline earth metal complexes and some transition metal compounds, 24,25 we anticipated that studies of the wider family of redox-inactive metal ions by this method might enable direct comparisons with the common aqueous pKa scale as well as provide insights for new studies of Lewis acid-modulated chemistry.…”