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Chapter 1 the Basic Chemistry of Organopalladium Compounds

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Chapter 1 the Basic Chemistry of Organopalladium Compounds Chapter 1 The Basic Chemistry of Organopalladium Compounds 1.1 Characteristic Features of Pd-Promoted or -Catalyzed Reactions There are several features which make reactions involving Pd catalysts and reagents particularly useful and versatile among many transition metals used for organic synthesis. Most importantly, Pd catalysts offer an abundance of possibilities of carbon–carbon bond formation. Importance of the carbon–carbon bond forma- tion in organic synthesis needs no explanation. No other transition metals can offer such versatile methods of the carbon–carbon bond formations as Pd. Tol- erance of Pd catalysts and reagents to many functional groups such as carbonyl and hydroxy groups is the second important feature. Pd-catalyzed reactions can be carried out without protection of these functional groups. Although reactions involving Pd should be carried out carefully, Pd reagents and catalysts are not very sensitive to oxygen and moisture, and even to acid in many reactions catalyzed by Pd–phosphine complexes. It is enough to apply precautions to avoid oxidation of coordinated phosphines, and this can be done easily. On the other hand, the Ni(0) complex is extremely sensitive to oxygen. Of course, Pd is a noble metal and expensive. Its price frequently fluctuates drastically. A few years ago, Pd was more expensive than Pt and Au but cheaper than Rh. As of October 2003, the comparative prices of the noble metals were: Pd (1), Au (1.8), Rh (2.8), Pt (3.3), Ru (0.2). Recently the price of Pd has dropped dramatically, and Pt is currently the most expensive noble metal. Also, the toxicity of Pd has posed no serious problem so far. The fact that a number of industrial processes, particularly for the production of fine chemicals based on Pd-catalyzed reactions, have been developed and are currently being operated, reflects the advantages of using Pd catalysts commercially. 1.2 Palladium Compounds, Complexes, and Ligands Widely Used in Organic Synthesis In organic synthesis, two kinds of Pd compounds, namely Pd(II) salts and Pd(0) complexes, are used. Pd(II) compounds are mainly used as oxidizing reagents, or catalysts for a few reactions. Pd(0) complexes are always used as catalysts. Pd(II) Palladium Reagents and Catalysts—New Perspectives for the 21st Century J. Tsuji 2004 John Wiley & Sons, Ltd ISBNs: 0-470-85032-9 (HB); 0-470-85033-7 (PB) 2 The Basic Chemistry of Organopalladium Compounds compounds such as PdCl2 and Pd(OAc)2 are stable, and commercially available. They can be used in two ways: as unique stoichiometric oxidizing agents; and as precursors of Pd(0) complexes. PdCl2 is stable, but its solubility in water and organic solvents is low. It is soluble in dilute HCl and becomes soluble in organic solvents by forming a PdCl2(PhCN)2 complex. M2PdCl4 (M = Li, Na, K) are soluble in water, lower alcohols and some organic solvents. Pd(OAc)2 is commercially available. It is stable and soluble in organic solvents. Pd(II) salts can be used as a source of Pd(0). Most conveniently phosphines can be used as reducing agents. For example, when Pd(OAc)2 is treated with PPh3, Pd(0) species and phosphine oxide are formed slowly [1]. A highly active Pd(0) catalyst can be prepared by a rapid reaction of Pd(OAc)2 with P(n-Bu)3 in 1 : 1 ratio in THF or benzene [2]. P(n-Bu)3 is oxidized rapidly to phosphine oxide and a phosphine-free Pd(0) species is formed besides Ac2O. This catalyst is very active, but not stable and must be used immediately; black Pd metal begins to precipitate after 30 min if no substrate is added. The in situ generation of Pd(0) species using n-PBu3 as a reducing agent is very convenient preparative method for Pd(0) catalysts. Pd(OAc)2 + PPh3 + H2O Pd(0) + Ph3PO + 2 AcOH Pd(OAc)2 + P(n-Bu3) Pd(0) O=PBu3 + Ac2O Commercially available Pd(OAc)2, PdCl2(PPh3)2,Pd(PPh3)4,Pd2(dba)3-CHCl3 3 and (η -allyl-PdCl)2 are generally used as precursors of Pd(0) catalysts with or without addition of phosphine ligands. However, it should be mentioned that cat- alytic activities of Pd(0) catalysts generated in situ from these Pd compounds are not always the same, and it is advisable to test all of them in order to achieve efficient catalytic reactions. Pd(PPh3)4 is light-sensitive, unstable in air, yellowish green crystals and a coordinatively saturated Pd(0) complex. Sometimes, Pd(PPh3)4 is less active as a catalyst, because it is overligated and has too many ligands to allow the coordi- nation of some reactants. Recently bulky and electron-rich P(t-Bu)3 has been attracting attention as an important ligand. Interestingly, highly coordinatively unsaturated Pd(t-Bu3P)2 is a stable Pd(0) complex in solid state [3] and commer- cially available. The stability of this unsaturated phosphine complex is certainly due to bulkiness of the ligand. This complex is a very active catalyst in some reactions, particularly for aryl chlorides [4]. = Pd2(dba)3-CHCl3 (dba dibenzylideneacetone) is another commercially avail- able Pd(0) complex in the form of purple needles which contain one molecule of CHCl3 when Pd(dba)2, initially formed in the process of preparation, is recrystal- lized from CHCl3, where Pd(dba)2 corresponds to Pd2(dba)3-dba. In literature, researchers use both Pd2(dba)3 and Pd(dba)2 in their research papers. In this book both complexes are taken directly from original papers as complexes of the same nature. The molecule of dba is not a chelating ligand. One of the dba molecules in Pd2(dba)3-dba does not coordinate to Pd and is displaced by CHCl3 to form Pd2(dba)3-CHCl3, when it is recrystallized from CHCl3.InPd2(dba)3, dba behaves as two monodentate ligands, but not one bidentate ligand, and each Pd Palladium Compounds, Complexes, and Ligands 3 is coordinated with three double bonds of three molecules of dba, forming a 16- electron complex. It is an air-stable complex, prepared by the reaction of PdCl2 with dba and recrystallization from CHCl3 [5]. Pd2(dba)3 itself without phosphine is an active catalyst in some reactions. As a ligand, dba is comparable to, or better than monodentate phosphines. Pd on carbon in the presence of PPh3 may be used for reactive substrates as an active catalyst similar to Pd(PPh3)n. Recently colloidal Pd nanoparticles protected with tetraalkylammonium salts have been attracting attention as active catalysts. They are used for Heck and Suzuki–Miyaura reactions without phosphine ligands [6,7]. Most simply, Pd(OAc)2 is used without a ligand, forming some kind of colloidal or soluble Pd(0) species in situ in reactions of active substrates such as aryl iodides and diazonium salts. Pd on carbon without phosphine is active for some Heck and other reactions, but not always. These Pd(0) catalysts without ligands are believed to behave as homogeneous catalysts [8]. A number of phosphine ligands are used. Phosphines used frequently in Pd- catalyzed reactions are listed in Tables 1.1–1.18. Most of them are commercially available [9]. Among them, PPh3 is by far the most widely used. Any contaminat- ing phosphine oxide is readily removed by recrystallization from ethanol. Bulky tri(o-tolyl)phosphine is an especially effective ligand, and was used by Heck for the first time [10]. The Pd complex of this phosphine is not only active, but also its catalytic life is longer. This is explained by formation of the palladacycle 1, called the Herrmann complex, which is stable to air and moisture and commer- cially available [11]. It is an excellent precursor of underligated single phosphine Pd(0) catalyst. But this catalyst is not active at low temperature, and active above ◦ 110 C. Also a number of palladacycles (Table 1.18) are prepared as precursors of catalysts and show high catalytic activity and turnover numbers. Me Me o-tol o-tol P O O P 2 + 2 Pd(OAc)2 Pd Pd + 2 AcOH P O O 3 o-tol o-tol Me 1 In some catalytic reactions, more electron-donating alkylphosphines such as P(n-Bu)3 and tricyclohexylphosphine, and arylphosphines such as tri(2,4,6-trimeth- oxyphenyl)phosphine (TTMPP) and tri(2,6-dimethoxyphenyl)phosphine (TDMPP), are used successfully. These electron-rich phosphines accelerate the ‘oxidative addition’ step. Furthermore, P(t-Bu)3 was found to be a very important ligand particularly for reactions of aryl chlorides. It is a very bulky and electron-rich phosphine, and has been neglected for a long time and has not been used as a lig- and, because it is believed that the bulkiness inhibits coordination of reactants. Now it is understandable that strongly electron-donating t-Bu3P accelerates the ‘oxida- tive addition’ step of aryl chlorides, because oxidative addition is nucleophilic in nature. Also, the bulkiness assists facile reductive elimination. The following pKa 4 The Basic Chemistry of Organopalladium Compounds values of conjugate acids of phosphines support the high basicity of P(t-Bu)3, which is more basic than P(n-Bu)3: P(t-Bu)3 (11.4), PCy3 (9.7), P(n-Bu)3 (8.4), PPh3 (2.7) Since the first report on the use of P(t-Bu)3 in Pd-catalyzed amination of aryl chlo- rides by Koie and co-workers appeared in 1998 [12], syntheses and uses of a number of bulky and electron-rich phosphines related to P(t-Bu)3 (Tables 1.4–1.6) have been reported [13].
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