Suzuki Cross-Coupling Reaction Catalyzed by the Palladium Complex Pd[ N -MorphC(S)NP(O)(O i Pr) 2 - O , S ] 2
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Suzuki Cross-Coupling Reaction Catalyzed by the Palladium Complex Pd[N-MorphC(S)NP(O)(OiPr)2-O,S]2 Damir A. Safin Æ Maria G. Babashkina Æ Axel Klein
Received: 4 September 2008 / Accepted: 11 November 2008 / Published online: 3 December 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Suzuki cross-coupling reaction between phenyl bromide and phenylboronic acid, catalyzed by the palladium complex Pd[N-MorphC(S)NP(O)(OiPr)2-1,5-O,S)]2 in acetonitrile, toluene, THF or DMF has been investigated. Bases employed for the reaction were Na2CO3, K2CO3 or Cs2CO3. Varying largely the experimental conditions we found that excellent yields of the product were obtained using toluene and K2CO3 at 100 °C at the catalyst amount of 0.02 mmol. Keywords Cross-coupling Phenylboronic acid Phenyl bromide Suzuki-Miyaura reaction N-Phosphorylthiourea Palladium
1 Introduction The Suzuki-Miyaura cross-coupling reaction catalyzed by palladium containing compounds has become very popular and one of the most efficient methods for the synthesis of bi-or polyaryl compounds by means of carbon-carbon bond formation [1–12]. The Suzuki reaction usually proceeds in the presence of a base and various bases as carbonates have been tried so far. The choice of solvent is also crucial to the reaction. Various bases and solvents have been tried; however, both the choice of base and solvent is still empirical. D. A. Safin (&) M. G. Babashkina A. M. Butlerov Chemistry Institute, Kazan State University, Kremlevskaya St. 18, 420008 Kazan, Russian Federation e-mail: [email protected] D. A. Safin M. G. Babashkina A. Klein Institut fu¨r Anorganische Chemie, Universita¨t zu Ko¨ln, Greinstrasse 6, 50939 Cologne, Germany
Another important condition for palladium catalysts is the nature of the ligands. The key role of the ligand is to stabilize the palladium catalyst. Tertiary phosphine ligands were traditionally used together with catalyst precursors in Suzuki cross-coupling reactions. However, many of these ligands are air sensitive and therefore require special conditions. Moreover, this rather erratic approach does not allow insight into reaction mechanisms, since the reactive species remain uncertain. During the last decade, the application of various new ligands to improve efficiency and selectivity of this cross-coupling reaction has become very popular. Therefore, such compounds as N-heterocyclic carbines [13], Pd(OAc)2 system in combination with various additional ligands [14], thiourea [15, 16], and many others were widely used. Herein we present a new class of palladium catalyst system based on the N-phosphorylated thiourea Pd[NMorphC(S)NP(O)(OiPr)2-1,5-O,S)]2 (PdL2), containing a tertiary nitrogen atom at the thiocarbonyl group (Fig. 1). The palladium complex PdL2 was synthesized as previously described [17]. According to the NMR, IR and X-ray data it was established that the palladium atom is coordinated by two deprotonated ligands through the sulfur atoms of the thiocarbonyl groups and oxygen atoms of the phosphoryl groups both
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