“Release and Catch” Effect of Perfluoroalkylsulfonylimide‐Functionalized Imidazole/Pyridine on Brønsted Acids in Organic Systems

Abstract

A “release and catch” method was developed by utilising the scavenging effect of a fluorous zwitterion on a homogeneous triflic acid (TfOH) catalyst in Michael addition and Rupe rearrangement. Both TfOH and the zwitterion were recycled with >90 % recovery using toluene. The zwitterions were designed by functionalising imidazole/pyridine with the perfluoroalkylsulfonylimide group. The “caught” TfOH was delivered to ethyl acetate and re‐used. The smooth delivery was primarily because of the fluorous tail of the zwitterion, the hydrophobicity of which probably weakened the ability of the zwitterion to form H bonds, so that retro‐ion‐exchange occurred towards the formation of the acid and zwitterion. The method was universal for other strong Brønsted acids such as H2SO4 and p‐MeC6H4SO3H. The method combined the significant advantages of homogeneous catalysis and heterogeneous isolation. Based on the H0 acidity function and the 31P NMR chemical shift of Et3P=O adducts, it is reasonable to deduce that the decrease of the acid strength of the formed composites of Brønsted acids and the zwitterion drove the scavenging effect.

A New Amphiphilic Brønsted Acid as Catalyst for the Friedel–Crafts Reactions of Indoles in Water

“…The stronger acidity, the stronger hydrogen bond, and the lower electron density at the phosphorus, and the larger 31 P chemical shift. The chemical shift difference (Δ δ 31P ) of 1a /Et 3 P=O adducts to the reference (C 6 H 5 ) 2 ClP=O was 46.44, very close to 46.40 of H 2 SO 4 /Et 3 P=O adducts . These results are also supported by the initial conductances of 1a and H 2 SO 4 with the same concentrations, which are both 0.136 ms/cm, meaning the close ionization ability of 1a and H 2 SO 4 .…”

“…The results are summarized in Table . H 0 was obtained by UV/Vis spectrophotometry method, which is a semiquantitative measure of acidity used for concentrated solutions of strong acid . As shown, the H 0 value of 1a was –0.52, definitely comparable with tested –0.33 of concentrated H 2 SO 4 .…”

“…As shown, the H 0 value of 1a was –0.52, definitely comparable with tested –0.33 of concentrated H 2 SO 4 . The acidity was further determined by the 31 P NMR shift of the Et 3 P=O probe according to the classical Gutmann method , . The probe formed Et 3 P=O ··· H H‐bond with acids.…”

“…The acid strength, based on the H 0 values, has the following order: CF 3 SO 3 OH (–3.52) > H 2 SO 4 (–0.33) ≈ 1a (–0.52) > o ‐C 12 H 25 C 6 H 4 SO 3 H (ca. 0.44–0.58) . It was thus reasoned that the hydrophobic segments ‐C 12 H 25 of o ‐C 12 H 25 C 6 H 4 SO 3 H formed micelles, locating the organic substrates and facilitating the reaction.…”

“…Even relatively short ‐C 4 F 9 chains exhibit significant hydrophobicity. Our previous research demonstrated that ‐C 4 F 9 hindered access of water molecules to adjacent SO 2 NHSO 2 acid sites, and hydrogen was impeded to bind with the negative conjugate base SO 2 N – SO 2 . Similarly, in Wang's reports, pyrrolidine perfluorobutyl sulfonamide was used in aqueous catalysis .…”