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N-heterocyclic carbene adducts of cyclopalladated ferrocenylpyridine containing chloride or iodide anions: synthesis, crystal structures and application in the coupling of terminal alkynes with arylboronic acids Hong-Mei Li • Chen Xu • Lu-Meng Duan • Xin-Hua Lou • Zhi-Qiang Wang • Zhen Li • Yao-Ting Fan Received: 30 November 2012 / Accepted: 7 January 2013 / Published online: 24 January 2013 Ó Springer Science+Business Media Dordrecht 2013

Abstract Two air-stable N-heterocyclic carbene (NHC) adducts of cyclopalladated ferrocenylpyridine containing chloride or iodide anions 2–3 have been easily prepared from the bridge-splitting reaction of the palladacyclic dimer 1 and anion exchange reaction, respectively. They were characterized by elemental analysis, IR, 1H, and 13C NMR. Additionally, the structures of these complexes were determined by single-crystal X-ray analysis. These adducts have been successfully applied to the coupling of terminal alkynes with arylboronic acids under atmospheric conditions.

Introduction The Sonogashira reaction has become an extremely powerful method in organic synthesis for construction of aryl alkynes [1–3]. Due to the value of the products, the development of new catalyst systems and modifications has received considerable attention [4]. The reaction generally does not work well for electron-deficient alkynes in many methods [2–5]. To overcome this limitation, palladium- or coppercatalyzed Sonogashira-type couplings of terminal alkynes with arylboronic acids have been investigated [6–11]. In 2003, Zou and co-workers firstly reported the above coupling reaction using a palladium/silver system [6]. Subsequently, PPh3 adduct of palladacycle and nano-palladium/ PPh3 catalytic systems were reported by the groups of Wu H.-M. Li
C. Xu (&)
X.-H. Lou
Z.-Q. Wang College of Chemistry and Chemical Engineering, Luoyang Normal University, Luoyang 471022, Henan, China e-mail: [email protected] L.-M. Duan
Z. Li
Y.-T. Fan Department of Chemistry, Zhengzhou University, Zhengzhou 450052, Henan, China

[10] and Sun [11], respectively. In these palladium-catalyzed coupling reactions, the phosphine ligands are generally used. As an alternative, N-heterocyclic carbenes and their transition metal complexes have been the focus of intense research in organometallic chemistry and homogeneous catalysis [12–14]. To the best of our knowledge, there was no report concerning the NHC palladium complex-catalyzed coupling of terminal alkynes with arylboronic acids. Moreover, palladacycles are one of the most developed and studied classes of catalyst precursors because of their structural versatility and easy synthetic accessibility [15, 16]. In recent years, part of our research effort has focused on the synthesis and application of adducts of palladacycles [17–20]. These adducts combine the stability induced by the presence of a palladacycle framework with the high activity commonly associated with phosphine or carbene ligands and are generally far more active than the corresponding dimeric palladacycles. Base