Palladium catalysis ligand-free

The Mizoroki-Heck reaction in liquid imidazolium salts as the solvent is a special case of an in situ system Under the reaction conditions NHC complexes of palladium are formed as the active catalyst from the solvent and the ligand-free palladium precursor. In general, ionic liquids are novel reaction media for homogeneous catalysis. They allow easy separation of product and catalyst after the reaction. ... 

Leadbeater, N.E. and Marco, M., Ligand-free palladium catalysis of the Suzuki reaction in water using microwave heating, Org. Lett., 2002,4, 2973-2976. 

QCdmJpFJ [QCpmJBr PdCl2 Pd(OAc)2 NaOAc 30 °C. Ligand-free, ultrasound promoted arylation of alkenes and alkynes with aryliodides palladium bis-carbenes and palladium nanoparticles ( 1 nm) are identified after catalysis product extracted with ethyl acetate/petrol ether. [66]... 

Reproduced from Reproduced from Klingensmith LM, Leadbeater NE. Ligand-free palladium catalysis of aryl coupling reactions facilitated by grinding. Tetrahedron Lett 2003 44 765-8. Copyright (2003), with permissbn from Elsevier. 

It was shown that palladacydes 1 [3c, 24] prepared from palladium] I) acetate and tris(o-tolyl)- or trimesitylphosphine are excellent catalysts for the Heck coupHng of triflates and halides including certain aryl chlorides. In some of these cases, a possible involvement of oxidation states +II and +IV in the catalytic cycle has been considered [25]. Similarly, other palladacydes such as 3 [26e,h] or 6 [27] have been used in the Heck reactions (Figure 8.1) [24, 26, 28]. It has been proposed that, at least for NC palladacydes, the reaction proceeds through the classical phosphine-free Pd(0)/Pd(II) catalytic cycle and that the active catalysts are actually slowly formed palladium clusters [29]. Besides classical palladacydes, complexes with pincer-type ligands such as 2 [30] have become very popular in palladium catalysis [31]. 

Recently, Hong [112] developed a selective C-H arylation of xanthene at its C2 position by palladium catalysis. Zhang [113] reported ligand-free conditions under Cu nanoparticles, and Hlavac [114] demonstrated a direct arylation of purines using palladium and copper catalysts in sohd phase. 

Ally lie substitution (the Tsuji-Trost reaction) is among the most synthetically useful processes in palladium catalysis. As the catalytic efficiency of allylic substitution is often moderate (5-10 mol % of Pd catalyst are usually used), and phosphine-free systems are generally inefficient, the recycling of catalyst is the only feasible way to make the process more economical. Various phase-separation techniques have been tried for this reaction. In what concerns the rate of reaction and catalytic efficiency, such ligands as TPPTS are likely to be less effective compared to PhsP.f Thus, the main reason for the use of hydrophilic ligands in allylic substitution is the design of recyclable systems. 

It should be noted that cross-coupling in the presence of phosphine complexes of palladium usually requires high amounts of catalyst, with initial loadings of 25-30 mol% not being uncommon. An entirely new approach to the Suzuki reaction is phosphine-free palladium catalysis. The use of palladium catalysts without the addition of phosphine ligands for cross-coupling with organoboron compounds in aqueous media opened a new chapter in the story of this powerful synthetic method. This approach allows catalyst efficiency to be dramatically increased, and the reaction to be performed under milder conditions. 

Very recently, Stahl et al. reported the first synthesis of a 7-membered NHC ligand [98]. Despite substantial effort, the isolation of the free carbene 21 was not successful. However, palladium complexes of 21 could be formed and structurally characterized. Ligand 21 is C2 symmetric as a result of a torsional twist which is thought to attenuate the antiaromatic character of the 87r-electron carbene heterocycle [101,102]. It will be interesting to see, if the synthesis of conformationally stable analogues and their application in asymmetric catalysis will be feasible. 

The original Sonogashira protocol involves palladium-copper co-catalysis. Attempts have been made over the last few years to overcome some of the limitations in this method, specifically to eliminate the undesired dimerization of terminal alkynes. Various copper-free conditions have been developed in order to reduce the amount of diacetylene formation. The focus seems to have been on changing the ligand. 

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