Well‐Dispersed Trifluoromethanesulfonic Acid‐Treated Metal Oxide Nanoparticles Immobilized on Nitrogen‐Doped Carbon as Catalysts for Friedel–Crafts Acylation

Abstract

Although strong acid-treated metal oxides are useful heterogeneous superacid catalysts for various organic transformations, they usually have a limited density of acidic sites due to their low surface areas. Herein, heterogeneous trifluoromethanesulfonic acid immobilized nitrogen-doped carbon-incarcerated titanium nanoparticle (NP) catalysts have been developed that are composed of well-dispersed, small Ti NPs (ca 7 nm) that are otherwise difficult to achieve using acid-treated metal oxides. The catalysts showed high activity for Friedel-Crafts acylation with low titanium loading (2 mol%, < 1 mg of metal for 1 mmol of substrate). A range of microscopic, spectroscopic and physicochemical studies revealed that the nitrogen-doped carbon immobilized the trifluoromethanesulfonic acid and that the addition of metals further changed the nature of the acidic species and enhanced catalytic activity.

Development of Trifluoromethanesulfonic Acid-Immobilized Nitrogen-Doped Carbon-Incarcerated Niobia Nanoparticle Catalysts for Friedel–Crafts Acylation

“…An ammonia solution (28% in water) was purchased from Tokyo Chemical Industry Co., Ltd. Trifluoromethanesulfonic acid was provided by Central Glass Co., Ltd. Friedel–Crafts acylation reactions were conducted with a Carousel. Anisole and acetic anhydride were purchased from Sigma-Aldrich Co. Ltd. Propionic anhydride, n -butyric anhydride, n -hexanoic anhydride, benzoic anhydride, thioanisole, mesitylene, 1,2-dimethoxybenzene, and thiophene were purchased from Tokyo Chemical Industry Co., Ltd. Nitromethane was purchased from Wako Pure Chemical Co. Poly­(4-vinylpyridine) and NCI-Ti-TfOH were prepared by following previously reported methods …”

“…Anisole and acetic anhydride were purchased from Sigma-Aldrich Co. Ltd. Propionic anhydride, n-butyric anhydride, n-hexanoic anhydride, benzoic anhydride, thioanisole, mesitylene, 1,2-dimethoxybenzene, and thiophene were purchased from Tokyo Chemical Industry Co., Ltd. Nitromethane was purchased from Wako Pure Chemical Co. Poly(4-vinylpyridine) and NCI-Ti-TfOH were prepared by following previously reported methods. 7 Preparation of NCI-Nb. To a stirring solution of poly(4vinylpyridine) (667 mg) in ethanol (30 mL) was added dropwise a solution of the metal salt (0.7 mmol) in ethanol (4 mL) under air.…”

“…To address these problems, we recently developed welldispersed trifluoromethanesulfonic acid-immobilized nitrogendoped carbon-incarcerated titanium nanoparticle catalysts (NCI-Ti-TfOH). 7 robustness, and low cost. 8 However, these catalysts were mostly limited to redox reactions.…”

“…To address these problems, we recently developed well-dispersed trifluoromethanesulfonic acid-immobilized nitrogen-doped carbon-incarcerated titanium nanoparticle catalysts (NCI-Ti-TfOH) . Nitrogen-doped carbon-supported metal nanoparticles (NPs) have recently attracted attention in organic chemistry because of their unique catalytic activity, robustness, and low cost .…”

“…H NMR (CDCl 3 , 500 MHz): δ 7.46 (d, 1H, J = 7.9 Hz), 7.41 (s, 1H), 6.78 (d, 1H, J = 7.9 Hz), 3.83 (s, 3H), 3.82 (s, 3H), 2.45 (s, 3H). 13 C{ 1 H} NMR (CDCl 3 , 125 MHz): δ 25.9, 55 7,. 55.8, H NMR (CDCl 3 , 500 MHz): δ 7.83 (d, 2H, J = 8.5 Hz), 7.23 (d, 2H, J = 8.5 Hz), 2.53 (s, 3H), 2.49 (s, 3H).…”