Cooperatively assisted N-arylation using organic ionic baseeBrønsted acid combination under controlled microwave heating Rahul Singh, Bharat Kumar Allam, Dushyant Singh Raghuvanshi, Krishna Nand Singh * Department of Chemistry, Centre of Advanced Study, Faculty of Science, Banaras Hindu University, Varanasi 221005, India article info Article history: Received 10 October 2012 Received in revised form 23 November 2012 Accepted 25 November 2012 Available online 29 November 2012 Keywords: Microwave methodology CeN cross coupling Catalyst free Organic ionic base Brønsted acid abstract A new synthetic strategy is developed for the construction of CeN bond through the assistance of Brønsted acid/[DBU][HOAc] without adding any metal catalyst. This is the first efficient S N Ar method- ology utilizing fluoro, chloro, bromo, and iodoarenes as coupling partners, which offers excellent yield up to 94% within a very short time. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The CeN bond formation is an important key reaction having wide applications in the synthesis of organic functional molecules. 1 Usually, CeN bonds are constructed by copper and palladium- catalyzed amination of aryl halides as well as aryl boronic acids. 2 During the recent past, significant advances have been made in the area of amination reactions, which include the development of novel transition metal catalysts and design of new ligands. 3 Although these reaction protocols are endowed with extensive applications, they have limitations, such as usage of toxic transition metals, use of costly ligands, and the successive contamination of the product by these catalysts. Subsequently, there stands enough scope for further exploration and improvement toward amination reactions. In this perspective, transition-metal-free protocol appears particularly attractive. 4 Very recently, the research groups of L aszl oK€ urti, 5a James P. Morken, 5b and Jianbo Wang 5c have revealed some interesting metal-free amination protocols. Ionic liquids (ILs) have emerged as novel catalysts and reaction media, because of their attractive applications and properties. 6 Microwave Assisted Organic Synthesis (MAOS) has attained the status of a new discipline in organic synthesis. 7 According to the current synthetic requirements, relevance of microwave (MW) methodology using ionic liquids is particularly welcome. 8 In CeN bond forming reactions, inorganic bases are usually employed to facilitate the deprotonation/coordination of the nu- cleophile. However, they suffer from demerits, such as low solu- bility, high melting points, and sensitivity toward moisture, thereby compelling the use of more polar solvents and inert atmosphere. To overcome these drawbacks, organic ionic bases have been introduced as new and promising promoters for coupling re- actions. 9 DBU is widely used as an organic base, 10aec and DBU based ionic liquids, such as [DBU][HOAc] are particularly useful as a non- nucleophilic task specific organic ionic base (Fig. 1). 10d The construction of CeN bond can be achieved through S N Ar type substitution pattern. However, the existing reports are appli- cable to highly activated fluorobenzenes and relatively low yields are observed with unactivated fluorobenzenes. 11 Very recently, the research group of Fairlie has described a catalyst free N-arylation strategy with unactivated fluorobenzenes using an excess of K 3 PO 4 (5 equiv) at 150e190  C to achieve the desired transformation. 12 An up to date literature survey reveals that there is no S N Ar substitution protocol, which is applicable to chloro, bromo, and iodoarenes. With an objective to explore an S N Ar type N-arylation Fig. 1. Structure of the DBU[HOAc]. * Corresponding author. Tel.: þ91 0542 2575196; fax: þ91 0542 2368127; e-mail addresses: knsingh@bhu.ac.in, knsinghbhu@yahoo.co.in (K.N. Singh). Contents lists available at SciVerse ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tet.2012.11.078 Tetrahedron 69 (2013) 1038e1042