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

Abstract

The Lewis acidity of various silyl triflates was quantified by utilizing [D5]pyridine as a 2H NMR spectroscopy probe. The chemical shifts of the 2H NMR signals for pyridine–silyl adducts are reported. The rate constants of silyl triflate catalyzed Diels–Alder reactions were determined by using UV/Vis spectroscopy. A correlation of the magnitude of the 2H NMR chemical shifts with the rate constants for most silyl triflates investigated was observed, but Me3SiOTf exhibits a surprisingly large deviation. Control experiments indicate that proton catalysis of the Diels–Alder reaction can be excluded.

Kinetic and Binding Studies Reveal Cooperativity and Off‐Cycle Competition for H‐Bonding Catalysis with Silsesquioxane Silanols

“…Comparing measurements for POSS‐triols 1 a and 1 b , the phenyl‐substitution ( 1 a , Δ δ =11.1 ppm) displays enhanced H‐bonding abilities relative to the alkyl‐substitution ( 1 b , Δ δ =7.4 ppm). [D 5 ]Pyridine was also compared using 2 H NMR spectroscopy and determined to be a less responsive probe (Δ δ 2 H<0.1 ppm, shift for the para position) . Silylated POSS‐silanols 2 and 3 were synthesized and 31 P NMR shifts are notably lower compared to diol 6 a and silanol 4 respectively.…”

Stereoselective Synthesis of β‐Sulfinylamino Isocyanides and 2‐Imidazolines

“…Also, 2‐furyl‐ and 2‐thienyl‐substituted imines ( R )‐ 9h and ( R )‐ 9i gave corresponding heteroaromatic products (2 S )‐ 14h and (2 S )‐ 14i stereoselectively in satisfactory yields (Table 2, entries 7 and 8). However, reaction of 3‐pyridyl‐substituted imine ( R )‐ 9j with 13 did not give the desired product, probably owing to competition for coordination of the Lewis acid between the sulfinyl group and the pyridine nitrogen atom, which thereby led to side reactions 19. The use of other α‐acidic isocyanides such as p ‐nitrobenzyl isocyanide and methyl α‐phenyl isocyanoacetate did not lead to the desired products.…”

Borenium ionic liquids as catalysts for Diels–Alder reaction: tuneable Lewis superacids for catalytic applications

“…DielsAlder cycloaddition was selected, being a popular model reaction to test the catalytic performance of Lewis acids. 7 Diels-Alder reaction is a powerful tool in organic chemistry, allowing for a one-step synthesis of six-membered carbocycles from a diene and a dienophile (Scheme 2). Lewis acids catalyse Diels-Alder reactions by coordination to the dienophile, influencing both the reaction rate and the selectivity to endo:exo or ortho/para isomers.…”

“…It also showcases that Diels-Alder cycloaddition is very sensitive to Lewis acid strength, making it an excellent model system. 7 However, by altering the anion and/or the ligand on the boron centre -Lewis acidity and thus catalytic activity can be finetuned. The endo:exo selectivities remained constant (94:6 -96:4), irrespective of the ionic liquid used.…”