Synthesis of unsymmetrical benzils using N-heterocyclic carbene catalysis Yumiko Suzuki * , Abu Bakar Md., Takashi Tanoi, Naoko Nomura, Masayuki Sato School of Pharmaceutical Sciences, University of Shizuoka, 52-1 Yada, Suruga-ku, Shizuoka 422-8526, Japan article info Article history: Received 10 February 2011 Received in revised form 5 April 2011 Accepted 7 April 2011 Available online 20 April 2011 Keywords: N-Heterocyclic carbene Organocatalysis Unsymmetrical benzil Nucleophilic aroylation abstract In this paper, we propose a novel and efficient method for the preparation of various unsymmetrical benzils. We first demonstrate the nucleophilic aroylation of N-phenylbenzimidoyl chlorides with aro- matic aldehydes using N-heterocyclic carbene as the catalyst to afford 1-aryl-2-phenyl-2-(phenylimino) ethanones. These iminoethanones were then converted to 1,2-diaryl-1,2-diketones by acid-promoted hydrolysis. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction In the last decade, N-heterocyclic carbenes (NHCs) have attracted considerable attention as organocatalysts due to their unique cata- lytic properties, facile reactions, and selectivities. 1e5 Many reports have indicated the great potential of NHCs as organocatalysts for a variety of organic reactions. In 1992, our former co-workers, Miyashita and his colaborators reported that the NHC generated from 1,3-dimethylimidazolium iodide (30 mol %) catalyzes nucleophilic substitutions at the imino carbon atom of N-phenylbenzimidoyl chlorides. 6 In this reaction, the chlorine substituent is replaced by the aroyl groups originating from aromatic aldehydes to afford 1,2- diphenyl-2-(phenylimino)ethanones, and moreover, the hydrolysis of the iminoethanones afforded corresponding benzils (1,2-diaryl-1,2- diketones). Unsymmetrical benzils are important building blocks in organic synthesis because they are intermediates for the synthesis of biologically active diarylheterocycles. 7e11 Two common methods are primarily employed for the preparation of unsymmetrical benzils. One route involves an addition reaction of the anion de- rived from cyanohydrins to aromatic aldehydes and the oxidation of the obtained benzoins. 8,12 However, in this method, the use of toxic cyanation reagents is problematic. The other route involves the FriedeleCrafts reaction between benzenes and 2-phenylacetyl chlorides and the oxidation of the products. 9e11,13 In this case, it is difficult to control the regioselectivities in FriedeleCrafts acylation, and moreover, this route is not always applicable to the synthesis due to the directing and activating effects of the substituents. In this study, we present an alternative synthetic route for unsymmetrical benzils (Scheme 1). Our route consists of NHC- catalyzed aroylation of N-phenylbenzimidoyl chlorides with aro- matic aldehydes and the hydrolysis of the aroylated products, i.e., iminoethanones. The substrates of the catalytic reaction, N-phe- nylbenzimidoyl chlorides, are easily obtained by the chlorination of Scheme 1. * Corresponding author. E-mail address: suzuyumi@u-shizuoka-ken.ac.jp (Y. Suzuki). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet 0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.tet.2011.04.020 Tetrahedron 67 (2011) 4710e4715