Solid-Phase Synthesis of 2-Arylbenzothiazole Using Silica Sulfuric Acid under Microwave Irradiation

Abstract

The condensation of several aromatic/heteroaromatic aldehydes with 2-aminothiophenol catalyzed by silica sulfuric acid under microwave irradiation afforded 2-arylbenzothiazoles in high yields and short reaction times under solvent-free conditions. The major advantages of the present method are good yields, ecofriendly, reusable catalyst, mild and solvent-free reaction conditions.

An efficient synthesis of 2-arylbenzothiazoles using silica sulfuric acid, oxalic acid and aluminum chloride hydrate as heterogeneous and homogeneous catalyst systems

“…As previously suggested [37][38][39][40][41][42][43][44][45][46][47][48][49][50] , the reaction may proceed through the formation of imine I from aldehydes and 2-aminothiophenol. Subsequent cyclization to 2,3-dihydrobenzothiazole II followed by oxidation in air would afford 2-arylbenzothiazoles (Scheme 2).…”

“…It has been shown that application of microwave irradiation reduce the reaction time and increases the product yield 49 . The use of domestic microwave ovens involves disadvantages such as: without control of temperature, power and pressure inside the reaction vessel.…”

“…Other synthetic routes employ microwave-assisted reaction of 2-aminothiophenol with β-chlorocinnamaldehydes 32 , palladium-catalyzed direct coupling of benzothiazoles with arylbromides 33 , palladium-catalyzed condensation of arylhalides with 2-aminothiophenol followed by dehydrative cyclization 34 , reaction of copper(I) thiobenzoate with 2-iodoanilines 35 , and the aquatic reaction of the corresponding thioamidinium salts and 2-aminothiophenol 36 . Recently, some simple one-step methods of syntheses of 2-arylbenzothiazoles have been found to be effective using perchloric acid-doped polyaniline 37 , ceric ammonium nitrate (CAN) 38 , tungstophosphoric acid impregnated zirconium phosphate 39 , zirconium(IV) oxide chloride and anhydrous copper(II) sulfate 40 49 . However, many of these methods have one or more disadvantages such as high reaction temperature, expensive metallic compounds, requirement for excess of reagenets, harsh reaction conditions, air sensitive catalysts, multistep processes, prolonged reaction times, use of toxic and costly catalysts, requirement for hazardous and carcinogenic organic solvents.…”

Aromatization of 1,3,5-Trisubstituted of 4,5-Dihydro-1H-Pyrazoles by In-Situ Generation of I+from Hydrogen Peroxide/Acids/Iodide Potassium or Sodium Systems

“…[45][46][47][48][49][50][51][52][53][54][55] The salient catalytic activities attributed to silica sulfuric acid have encouraged us, in the second part of the present paper, to use various solvents like CH 3 CN, MeOH, H 2 O, EtOH, EtOAc, CH 2 Cl 2 and CCl 4 used for aromatization of 1,3,5-triphenyl-2-pyrazolines (1a) as model reaction, EtOH:H 2 O (10:1, v/v) was the solvent of choice as best results in terms of the yield was obtained using SSA (0.02 g), H 2 O 2 (1 mL) and KI (0.016 g) for 1 mmol of 1,3,5-triphenyl-2-pyrazolines when the reaction was run at 80 o C for 15 min in 90% yield (Table 3). Similarly, to illustrate the scope and usefulness of this methodology, a number of reactions were conducted (Scheme Literature data.…”

Cerium‐catalyzed tandem synthesis of 2‐substituted benzothiazoles in PEG

“…[7] One of the most common methods for the synthesis of benzothiazoles involves the direct condensation of the 2-aminothiophenol with aldehydes, carboxylic acids or its derivatives in the presence of various promoting agents, such as polyphosphoric acid (PPA), [8] polyphosphate ester, [9] a mixture of methanesulfonic acid and phosphorous pentoxide, [10] FeCl 3 , [11] Sc(OTf) 3 , [12] TCCA, [13] ionic liquid under microwave irradiation, [14] molecular iodine, [15] acetic acid under microwave irradiation [16] and silica sulfuric acid under microwave irradiation. [17] Some other general methods include palladium-catalyzed intramolecular cyclization of o-bromophenylthioureas, [18] Suzuki biaryl coupling of 2-bromobenzothiazole with arylboronic acids, [19] coupling of benzothiazoles with aryl bromides, [20] microwave-mediated reaction of 2-aminothiophenol with β-chlorocinnamaldehydes, [21] the reaction between thiophenols and aromatic nitriles, [22] and the intramolecular cyclization of thiobenzamides via aryl radical cations as reactive intermediates. [23] Very recently, we reported a method of preparing 2-substituted benzothiazoles from the reaction of 2,2 -disulfanediyldianiline with aldehydes promoted by sodium dithionite in water.…”