Palladium-catalyzed Suzuki–Miyaura reaction using saturated N-heterocarbene ligands _ Ismail O ¨ zdemir a, *, Bekir C¸ etinkaya b , Serpil Demir a , and Nevin Gu¨rbu¨z a a Department of Chemistry, Ino ¨nu ¨ University, 44069 Malatya, Tu ¨rkiye b Department of Chemistry, Ege University, 35100 Bornova- _ Izmir, Tu ¨rkiye Received 9 February 2004; accepted 12 May 2004 The incorporation of saturated N-heterocyclic carbenes into palladium pre-catalysts give high catalyst activity in the Suzuki coupling of deactivated aryl chloride substrates. R Cl + B(OH) 2 R Pd(OAc) 2 (1.5mol%) K 2 CO 3 (2 equiv) 60 0 C, dioxane LHX (1.5mol%) The complexes were generated in the presence of Pd(OAc) 2 by in situ deprotonation of bis(imidazolinium) bromides LHX (3) which were characterized by conventional spectroscopic methods and elemental analyses. KEY WORDS: carbene; Suzuki; palladium; imidazolidin-2-ylidene; aryl chlorides; phenylboronic acid. 1. Introduction The palladium-catalyzed cross-coupling reaction of aryl halides with arylboronic acids, the so-called Suzuki–Miyaura reaction, is one of the most versatile and powerful methods for C–C bond formation and has attracted much current interest [1,2]. The low reactivity of aryl chlorides in cross-coupling reactions is generally ascribed to their reluctance to oxidatively add to Pd(0) [3]. Current interest focuses on the use of aryl chlorides since they are cheaper and more readily accessible than bromides and iodides [4]. The reaction is normally promoted by a palladium catalyst precursor, a ligand that binds to the palladium center to stabilize the catalyst during the reaction process, and a base that captures the boronic acid moiety. The choice of the right ligand is a pivotal factor in determining the rate of the reaction. Triaryl phosphines are traditionally employed as ligands for the reaction. Recently electron- rich, bulky phosphines [5] and phosphine oxides [6] have been reported to be effective ligands. However, the major drawback of these is that the phosphine ligands are comparatively difficult to make or rather expensive. Furthermore, tertiary phosphines require air-free handling to prevent their oxidation and are susceptible to P–C bond cleavage at elevated tem- peratures [7]. On the other hand, palladium complexes of N-heterocyclic carbene ligands(NHC’s) [8], in particular have proved to be excellent catalysts not only for the Suzuki and Heck reaction, but also for Stille and Sonagashira reactions [9]. Also ruthenium N-heterocyclic carbene complexes have been found effective catalysts for the formation of furans, cyclopropanation [10,11] alkene metathesis [12,13] and cycloisomerisation [14]. The NHC complexes are cost efficient to prepare, insensitive to air and moisture and are thermally stable in both the solid state and in solution; the carbenes are non-dissociative ligands. However, the development of new ligands or the application of existing ligands in these reactions, particularly those involving aryl chlorides as substrates, is still of considerable importance. Recently, we have developed improved procedures Heck and Suzuki reactions of aryl chlorides making use of novel ligands 1,3- bis(dialkyl)imidazolium salts [15], 1-alkylimidazoline, a-bis(imine) [16]. Although the nature of the NHC ligand on complexes has a tremendous influence on the rate of catalyzed reactions, the use of saturated NHC ligands in coupling reactions is a neglected area. In order to find more efficient palladium catalysts we have prepared a series of new bis(imidazolinium) bromides LHX, 3 (scheme 1), containing a saturated imidazole ring and we report here in situ Pd-carbene based catalytic system for the Suzuki coupling reaction. 2. Experimental All reactions were performed using Schlenk-type flask under argon and standard high vacuum-line techniques. Solvents were analytical grade and distilled under Ar from sodium benzophenone (Et 2 O, dioxane). 1 H NMR *To whom Correspondence should be addressed. E-mail: iozdemir@inonu.edu.tr Catalysis Letters Vol. 97, Nos. 1–2, August 2004 (Ó 2004) 37 1011-372X/04/0800–0037/0 Ó 2004 Plenum Publishing Corporation