Natural Kaolinitic Clay: a Remarkable Catalyst for Highly Regioselective
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Indian Journal of Chemistry Vol. 45B, April 2006, pp. 972-975 Natural kaolinitic clay: A remarkable catalyst for highly regioselective chlorination of arenes with Cl2 or SO2Cl2 B Jayachandran, Prodeep Phukan,# Thomas Daniel & A Sudalai* National Chemical Laboratory, Pune 411 008, India E-mail: [email protected] Received 30 September 2004; accepted (revised) 7 November 2005 Natural kaolinitic clay containing transition metals s as Fe and Ti in its lattice has been found to exhibit unusual regioselectivity in the liquid-phase chlorination of arenes with either Cl2 or SO2Cl2 as the chlorinating agent para-Chlorinated products are predominant for most of the substrates with an exceptional case of ortho-selectivity for the chlorination of aniline. Keywords: Chlorination, chlorine, sulfuryl chloride, aniline, ortho-chloroaniline IPC: Int.Cl.7 C 07 C Introduction of chlorine into aromatic rings by acids such as zeolites for the chlorination of arene4. electrophilic substitution is an important synthetic Although para-selectivity was achieved using zeolite4 transformation because chlorinated hydrocarbons are because of it’s shape selective nature, there is not such recognized as versatile starting materials and additives report for ortho-selective chlorination. Sheldon in th e p rod uctio n o f h igh qu ality in secticides, reported a highly regioselective ortho-chlorination of 1 fungicides, herbicides, dyes, pharmaceuticals etc. phenol with SO2Cl2-amine systems in homogeneous Moreover, they can serve as precursors for numerous phase5. In recent years, acid catalysis of organic functionalities, such as phenols, aromatic ethers and transformations by clay aluminosilicates6 is an area of thioethers, amines, aryl hydrazines, benzonitriles, considerable potential and interest due to the ease of benzaldehyd es, fluoro aromatics and arom atic handling and w ork-up, absence o f toxicity and hydrocarbons2. In general, chlorination of aromatic ring corrosion and low cost of clays. Due to their Br¥onsted is achieved either by using chlorine or SO 2Cl2 as and Lewis acidities, clays function as efficient catalysts chlorine source in presence of Lewis acid catalysts for a variety of important transformations7. An such as Al3+, Fe3+, Sn4+ or Zn2+ chlorides, acetic acid interesting feature of clay catalyst is that it gives etc.3 But use of homogenous catalyst has the drawback unexpected ortho-selective products in a few cases, such as poor regioselectivity, difficulty in separation which is very specific for a particular combination of of the catalyst from the reaction mixture, generation substrates8. This peculiar property is sometimes very of toxic waste etc. Secondly, use of mineral acids and much useful for the synthesis of some important ortho- metallic halides create work-up and effluent problem substituted compounds. Herein is reported the use of in the industrial scale production of these compounds. natural kaolinitic clay for the chlorination of arenes Use of solid acid is an easy, safe and economical with substantial degree of regioselectivity with Cl2 or process, which does not create any environmental SO2Cl2 as the chlorinating agent (Scheme I). problem. Several efforts have been made for the The k ao lin itic clay w as pro cu red fro m th e replacement of these homogeneous catalysts with solid Padappakara mine of Quilon District, Kerala, India and # Present address: Department of chemistry, Gauhati University, Guwahati 781 014, Assam JAYACHANDRAN et al.: CATALYST FOR REGIOSELECTIVE CHLORINATION 973 R R R Kaolinitic clay, Cl or SO Cl Cl 2 2 2 + o CCl4,80 C Cl R = H, CH3, Cl, OMe, OH, NH2 Scheme I it was subsequently purified9 by separating the coarser In a typical reaction procedure, a mixture of aniline mineral impurities from clay particles. This is achieved and clay catalyst (10 weight % based on aniline) in by suspending the crude lumps in water (500 g of the CCl4 was heated to reflux. To this mixture, chlorine clay lump in 5 L water), stirring vigorously and gas was bubbled slowly for 2 h. The catalyst was allowing the coarser mineral to separate. Particles of filtered off, the products analyzed by GLC capillary size below 45 µm are collected and washed free of column (ortho-product 79.5% and para-product solubles. Clay particles thus separated from coarser 20.5%) and purified by flash chromatography to mineral impurities are dried and calcined at 550°C for afford 2-chloroaniline (77%) and 4-chloroaniline 6 h. The calcined clay sample (1 part by wt.) is boiled (20 %). The products were characterized by comparing with 2M HCl (4 parts by wt) for 45 min. The leached the IR and 1H NMR data with those reported in the samples are then washed free of chloride ions and dried literature11. at 110°C for 12 h. The clay has been thoroughly Table I lists the results for the various aromatic characterized by FT IR, XRD, UV-Vis, EPR, SEM and sub strates, w hich un derw ent regioselectiv e chemical analysis by atomic absorption spectroscopy. chlorinations with chlorine as well as SO2Cl2 catalyzed The XRD pattern of the clay exhibits 3 major peaks by the clay. As can b e seen from Ta ble I, the located at 2q values of 26.8°. The FT IR spectra of conversions are quantitative in all substrates except the clay showed the presence of two different benzene with strong electrophile Cl2 whereas it is only -1 bands: one at 3710 cm assigned to SiOH and the moderate in the case of weak electrophile, SO2Cl2. A other band centered at 3590-3490 cm-1 assigned to n ov el feature o f th is catalyst is the un usual hydroxyl groups in the clay lattice. The presence of regioselectivity exhibited in case of aniline (ortho- Lewis and Br¥onsted-acid sites in the clay has been 79.5 %; para-20.5% with Cl2 and ortho-90%; para- confirmed by its FT IR study of adsorbed pyridine. The 10% with SO2Cl2). For all other substrates, para- IR bands at 1630, 1495 and 1445 cm-1 is indicative of selectivity is the dominant feature. It is also remarkable its Lewis acid sites while the IR bands at 1630, 1549 that for all the cases studied, poly-chlorination did not and 1495 cm-1 observed for pyridine adsorbed clay occur with the clay catalyst. In the case of toluene, it samples indicate the Br¥onsted acid sites. The SEM of is to be noted that no side-chain chlorination took the clay shows average particle size of 2-5 µm in the place. The observed ortho preference with aniline may form of clusters. The UV-Vis spectrum of the clay be due to H-bonding of anilinium cation with the shows absorption maximum, l max at 320-240 mm hydration sphere of these cations. The catalyst was indicating the presence of Ti (anatase form) as mostly recovered and reused 5 times in case of toluene with its oxide. A broad EPR spectrum of the clay recorded virtually no loss of activity and selectivity. at 293K (the g value of 2.13) is indicative of Fe3+ state In conclusion, an efficient, simple, and economic in the clay. Finally, the chemical component of the clay process for highly regioselective chlorination of arenes was determined by wet chemical analysis10 (in %): has been demonstrated. Chlorine was found to be more SiO2 = 67.45, Al2O3 = 22.20, TiO2 = 3.45, Fe2O3 = 6.1 effective as compared to SO 2Cl2 for this process. and K = 0.8. Altho ugh para-selectivity was the p redominant 974 INDIAN J. CHEM., SEC B, APRIL 2006 Table I—Kaolinitic clay-catalyzed chlorination of arenes a Compd Substrates With Cl2 With SO2Cl2 1 Conv.b p/o ratiob Isolated Yield (%) Conv.b p/o (%) (GC) para ortho (%) ratiob a 70 c 67 2 d b 100e 3.00 72 23 24 1.20 Cl c 100 2.76 71 25 25 3.00 OMe d 100 9.00 85 9 100 9.00 OH e 100 2.77 70 23 30 1.5 NH2 f 100 0.26 20 77 55 0.11 a) Reaction conditions: Substrate (5 mmol); clay: 10% ass; SO2Cl2 (5 mmol); Solvent CCl4; 360K, 2hr; b) Determined by GLC; HP-5880 silicon gum capillary column; c) 41% chlorobenzene + 56% 1,4-dichlorobenzene +3%, 1,2-dichlorobenzene; d) only 2% chlorobenzene is formed; e) in the absence of catalyst, the conversions were <2%. outcome for most of the substrates, exceptional otho- purified by flash chromatography using pet. ether – selectivity was achieved in case of chlorination of EtOAc (15-25%) to afford 2-chloroaniline (1.9 g,77 %) aniline. and 4-chloroaniline (0.5 g, 20%). Experimental Section Procedure for chlorination with SO2Cl2 A mixture of aniline (0.465 g, 5 mmoles), clay Procedure for chlorination with Cl 2 catalyst (0.046 g, 10 weight %) and SO2Cl2 (0.68 g, 5 A mixture of aniline (1.86 g, 20 mmoles) and clay mmoles) were taken in CCl4 (10 mL) and refluxed for catalyst (186 mg, 10 weight %) was taken in CCl4 (30 2 h. The catalyst was filtered off and the products were mL) and heated to reflux. Chlorine gas was bubbled analyzed by GLC. slowly through this mixture for 2 h. After the reaction was complete (TLC), the catalyst was filtered off and Acknowledgement the products analyzed by GLC. After evaporating the The authors (BJ and PP) thank CSIR (New Delhi) solvent under reduced pressure, the crude product was for the award of research fellowship. JAYACHANDRAN et al.: CATALYST FOR REGIOSELECTIVE CHLORINATION 975 References (b) Smith K & Butters M, Synthesis, 1985, 1157. 1 Frieter E R, Kirk-Othermer Encyclopedia of Chemical (c) Kumar S B & Singh A P, J Catal, 150, 1994, 430.