Modified Silico-Tungstate: An Easy and Efficient Catalyst for the Acetylation of Amines Under Solvent-Free Condition R. K. Sharma • Deepti Rawat Received: 17 April 2010 / Accepted: 3 June 2010 / Published online: 23 June 2010 Ó Springer Science+Business Media, LLC 2010 Abstract In this present work, organo-functionalization of silica gel was carried out using 3-aminopropyltriethox- ysilane (APTES) as a reactive surface modifier. Tungstate ion (WO 4 2- ) was immobilized onto the modified silica gel and its catalytic activity was investigated for acetylation of amines under solvent-free conditions. The method is highly chemoselective—amine group was selectively acetylated in presence of the hydroxyl group. The above catalyst gave acetylated products in excellent yield in shorter duration of time. The catalyst was characterized by various techniques such as BET surface area measurements, FTIR, CHNS, 13 C-CPMAS-NMR, ED-XRF and diffuse reflectance UV– vis techniques. The catalyst was reused five times without significant loss of catalytic activity. Keywords Acetylation Á Amines Á Modified silica Á Tungstate Á Solvent-free 1 Introduction The acetylation of alcohol and amines is a fundamental process of organic chemistry and widely used transforma- tion in organic synthesis [1, 2]. This method is used as one of the means for protection of alcohols and amines in multistep synthesis [3]. Typically, acylation is performed under catalysis with acetyl chloride or acetic anhydride in the presence of catalysts such as Et 3 N and pyridine in presence of suitable organic solvent [4]. A variety of acidic and basic catalysts such as p-toluenesulphonic acid [5], 4-(dimethylamino)pyridine (DMAP)/4-(1-pyrrolidino)pyr- idine (PPY) [6], tributylphosphine [Bu 3 ]P [7], ZnCl 2 [8], CoCl 2 [9], NbCl 5 [10], Cu(ClO 4 ) 2 [11], RuCl 3 [12], I 2 [13], Cp 2 ZrCl 2 [14], TaCl 5 [15], InCl 3 [16], ZrOCl 2 Á8H 2 O[17] including metal triflates such as Bi(OTf) 3 [18], Cu(OTf) 2 [19, 20] Gd(OTf) 3 [21], Al(OTf) 3 [22], Ce(OTf) 3 [23], Sc(OTf) 3 [24, 25] have been reported to be effective cat- alysts for acylation. These homogeneous catalysts are corrosive, toxic, expensive, susceptible to air and moisture. The existing protocol involves long reaction times, stringent conditions, use of halogenated solvents, and side reactions with acid sensitive groups. The use of large amounts of acylating agents and activators should be avoided to pro- mote green chemistry and atom efficiency. From both eco- nomic and environmental point of view, there is a growing demand for new, environmentally benign and economic heterogeneous catalytic methods which will allow facile recovery of the catalyst from the reaction media. Solid acid catalysts such as heteropoly acids [26], aluminum-supported MoO 3 [27], cation-exchanged montmorillonite K-10 clay [28], sulfamic acid [29], NaHSO 4 –SiO 2 [30], phosphomo- lybdic acid [31], polymer-supported gadolinium triflate [32], polystyrene-bound tetrafluorophenylbis (triflyl)-methane [33], BiFeO 3 [34], heterogeneous cobalt (II) salen [35] have been used as heterogeneous catalyst for acetylation reaction. Therefore, there is always a need to develop reusable cata- lyst for this type of widely used reaction to achieve high atom economy. In view of this, modified silico-tungstate catalyst has been synthesized and well characterized by CHNS, FTIR BET measurements, 13 C-CP MAS NMR, thermal analysis, UV–Visible diffuse reflectance and EDXRF techniques. The performance of the catalyst was evaluated in the acetylation of amines to corresponding amides under sol- vent-free condition. R. K. Sharma (&) Á D. Rawat Green Chemistry Network Centre, Department of Chemistry, University of Delhi, Delhi 110007, India e-mail: rksharmagreenchem@hotmail.com 123 J Inorg Organomet Polym (2010) 20:698–705 DOI 10.1007/s10904-010-9385-2