Task-oriented use of ionic liquids: efficient acetylation of alcohols and phenols Ignacio López a,⇑ , José Luis Bravo b , Manuel Caraballo b , José Luis Barneto c , Guadalupe Silvero c a Departamento de Química Orgánica e Inorgánica, Escuela Politécnica, Universidad de Extremadura, Cáceres 10003, Spain b Departamento de Química Orgánica e Inorgánica, Facultad de Ciencias, Universidad de Extremadura, Badajoz 06006, Spain c Departamento de Química Orgánica e Inorgánica, Facultad de Veterinaria, Universidad de Extremadura, Cáceres 10003, Spain article info Article history: Received 2 February 2011 Revised 13 April 2011 Accepted 19 April 2011 Available online 27 April 2011 Keywords: Acetylation Alcohols Phenols Ionic liquids abstract The ionic liquid 1-butyl-3-methylimidazolium acetate proves to be an excellent reaction medium for the acetylation of alcohols and phenols, providing better conditions than the acidic 1-butyl-3-methylimida- zolium bisulfate. Reactions were carried at room temperature, with only a 10% excess of acylating agent and with no other solvent or catalyst added. Ó 2011 Elsevier Ltd. All rights reserved. The acetylation reaction of hydroxyl groups in alcohols and phenols constitutes an important synthetic procedure that is widely employed for protection in multi-step synthesis due to its easy reversibility. Protection reactions have to proceed rapidly, readily, quantitatively and keeping costs to a minimum. Typically, the acetylation of hydroxyl groups is performed with an excess of acetic anhydride under basic or acidic catalysis; in some cases acetyl halides have been used. 1,2 In the light of the 12 principles of Green Chemistry, 3 protection/deprotection reactions are intrin- sically not-green, and better avoided whenever possible. Ionic liquids, in particular, have received remarkable attention in the last decade or so as reaction media and/or catalysts in organ- ic synthesis. They have also been employed in the acylation of hy- droxyl groups. Thus, Brönsted acidic 1-H-3-methylimidazolium bisulfate has been used as the catalyst in the acetylation of alcohols and phenols; 4 as well as trimethylammoniumbutanesulfonate bisulfate, 2 and Lewis acidic ionic liquids have been used as well. 5,6 In other cases, the ionic liquid is used as the immobiliser for Lewis acidic rare-earth based catalysts. 7,8 Basic catalytic activity by an io- nic liquid has been reported in the acetylation of alcohols and car- bohydrates. 9,10 The use of enzymes and ionic liquids in the acylation of levoglucosan, 11 and glucosides has also been re- ported. 12 One imidazolium bromide ionic liquid has been em- ployed as the medium for the ultrasonic activation of alcohols with acetic anhydride. 13 Moreover, acylimidazolium derivatives have been used as acylating agents in protection of alcohols. 14–19 In order to take advantage of beneficial ionic liquid features, such as nonvolatility, ‘greenness’ and wide solvent ability, a pur- pose-oriented selection of the melt is essential. 20 For instance, the widely used 1-butyl-3-methylimidazolium hexafluorophos- phate or 1-butyl-3-methylimidazolium tetrafluoroborate em- ployed as solvents for the acetylation of glucose yielded no product. 10 On the other hand, benzoate imidazoliums showed cat- alytic activity in this reaction, yet the acetate product formed com- plicated the recovery of products. 10 Depending on cation/anion combinations, the physico–chemical properties of ILs can be tuned for a particular application, in search of novel functional com- pounds termed as Task-Specific Ionic Liquids (TSILs). 21 In our group, we have been testing the use of ionic liquids as reac- tion media in organic reactions. Thus, new methodologies involving ionic liquids and Lewis acid catalysts, 22 ultrasounds, 23 and mineral catalysts and microwaves have been studied. 24 In further pursuing these investigations, we have now studied the use of two simple io- nic liquids, the acidic 1-butyl-3-methylimidazolium bisulfate [bmim][HSO 4 ], and the mildly basic 1-butyl-3-methylimidazolium acetate, [bmim][OAc]. This choice is based on their distinctive Brön- sted acid–base characteristics. Accordingly, the former could act as an acid catalyst, while the latter might show basic catalysis. Further- more, these two ionic liquids are easy to synthesise and commer- cially available at a lower cost than analogous melts such as dicyanamides, which have proven to be excellent reaction media for acetylation of hydroxyl groups. 9 The acetate melt encompasses some other desirable features, the nontoxic nature of its anion, and the fact that it is a common ion involved in these processes. We report herein that both [bmim][HSO 4 ], and [bmim][OAc] are efficient reaction media for a wide variety of alcohols, including 0040-4039/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2011.04.079 ⇑ Corresponding author. E-mail address: iglomar@unex.es (I. López). Tetrahedron Letters 52 (2011) 3339–3341 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet