A Lewis Acidity Scale in Relation to Rate Constants of Lewis Acid Catalyzed Organic Reactions

Abstract

The quantification of Lewis acidity is accomplished by a deuterated quinolizidine probe utilizing 2H NMR spectroscopy. The chemical shifts of the 2H NMR signals for aluminum, boron, titanium, and zinc halides are reported. The rate constants for Lewis acid catalyzed reactions were determined by utilizing UV/Vis spectroscopy for Diels–Alder and Povarov reactions under pseudo‐first‐order conditions. The magnitude of the 2H NMR chemical shifts correlates with the rate constants of the organic transformations for many Lewis acids investigated in the same order, whereas some deviations are identified.

The Correlation of Lewis Acidities of Silyl Triflates with Reaction Rates of Catalyzed Diels–Alder Reactions

“…Inducing the lowest shift differences in every case, tBuPh 2 SiOTf was revealed to be the weakest Lewis acidic silyl triflate, followed by iPr 3 SiOTf as the second weakest. Et 3 SiOTf, tBuMe 2 SiOTf, and Et 2 iPrSiOTf are all more acidic than iPr 3 SiOTf and seem to be fairly equally strong. Interestingly, Me 3 SiOTf exhibits the strongest influence on the ortho shifts and has comparable effects to the former three silyl triflates regarding the meta position.…”

“…We used this approach successfully in our previous work, [3] but it became apparent that a large excess of isoprene results in the formation of polymeric side products when silyl triflates are applied. On the other hand, the formation of polymeric side products was not encountered utilizing equimolar amounts of naphthoquinone, isoprene (5/6 = 1:1), and 5 mol-% of the silyl triflates at 0°C in dichloromethane but still led to reactions following first-order kinetics as monitored by UV/Vis spectroscopy (Scheme 3).…”

“…A rationale for this observation could be the small steric demand of the methyl substituents, which makes the 5·LA adduct easier to access for 6 in the rate-determining cycloaddition. Interestingly, the Brønsted acid HOTf gave the highest catalytic activity, which relates well to the highest-induced NMR shift difference, whereas HBF 4 ·OEt 2 , despite inducing a higher shift difference than the silyl triflates, is only as active as Et 3 Finally, an often-asked key question in silyl triflate catalyzed processes regarding the influence of water leading to proton-catalyzed reactions was addressed. In principle, the hydrolysis side reaction should result in different reaction kinetics.…”

“…Trying to fill this gap, we recently developed quinolizidine probe 1, which was used in 2 H NMR spectroscopy for the quantification of Lewis acidities. [3] Upon complexation of 1 with Lewis acids (LA), the 2 H NMR signal was shifted to a lower field. The chemical shift differences of the Lewis acid adducts of type 2 were used as measures for the Lewis acidities (Scheme 1).…”

“…[3] We then determined the effectiveness of Lewis acids in catalyzed organic reactions in terms of their catalytic activity by measuring the rate constants of the reactions. We could show a good relation of the rate constants with the magnitude of the 2 H NMR chemical shifts, proving that catalytic activity of the Lewis acid relates well with the spectroscopically determined Lewis acidity.…”