Phosphine-free Pd complexes

Littke and Fu reported an interesting and important suggestion that commercially available Pd[P(f-Bu)3]2 is a resting state of the system in the HR, and not a catalytically active species. Change of the ratio of Pd to phosphine from 1 1 to 1 2 leads to a marked decrease in the rate of reactions, and the Pd monophosphine adduct PdP(f-Bu)3 is the active catalyst. The combination of Pd[P(t-Bu)3]2 and a phosphine-free Pd complex generates in situ the active 1 1 adduct [7]. 

It seems that palladium complexes with high oxidative states and basic ligands can be involved in oxidative addition as weU as in o-bond metathesis. Cheng has demonstrated that phosphine-free Pd complexes together with alkenyl or aryl iodides are very efficient catalysts for 1,2-diboration of allenes. This reaction is completely regioselective and highly... 

Scheme 18 1,2-Diboration of allenes with B2pin2 by phosphine-free Pd complexes.

The reaction is catalyzed by palladium complexes either pre-formed, as [Pd(TPPMS)3] [13], or prepared in situ from (usually) [Pd(OAc)2] and various phosphines [21,23-27], TPPTS being one of the most frequently used [14]. Other precursors, e.g. [ PdCl(T -C3H5) 2] and so-caUed ligandless (phosphine-free) Pd-catalysts can also be effective. In fact, in several cases a phosphine inhibition was observed [23]. The solvent can be only slightly aqueous (5 % water in CH3CN, [14]) or neat water [26]. In the latter case a biphasic reaction mixture (e.g. with toluene) facilitates catalyst separation albeit on the expense of the reaction rate. A short selection of the reactions studied in aqueous solvents is shown on Scheme 6.9. 

The acylpalladium halide complex 84 is an intermediate of catalytic decarbony-lation of aroyl halides 83 [53]. The decarbonylation of 84 generates the aryl-palladium intermediate 85 at higher temperature which undergoes facile alkene insertion. Therefore, similar to aryl halides, acyl halides can be used for the alkene insertion. The reaction is carried out with a phosphine-free Pd catalyst in the presence of tertiary amines [54]. Higher yields were obtained by using a mixture of K2CO3 and benzyltrioctylammonium chloride [55]. 

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]. 

The palladium-catalyzed coupling of aiyl and vinyl halides to organotin compounds, known as Stille coupling, is one of the most important catalytic methods of carbon-carbon bond formation. The reaction is generally conducted in polar organic solvents, such as dimethylformamide, with tertiary phosphine complexes of palladium, although phosphine-free complexes or simple Pd-salts are also frequently used as catalysts [8]. 

C4)4N]Br [C CpmJBr Pd(OAc)2 Pd-benzthiazole carbene complex NaHC03 Na2C03 NaOAc 130 °C. Phosphine-free arylation of a-substituted acrylates, allylic alcohols and other substrates using arylhalides. Nanoparticles are identified and [(C4)4N]OAc appears to play a crucial role in their formation product extracted with cyclohexane. [68- 74]... 

C.C mlfPFe] Pd-bis-imidazole complexes Et3N NaOAc Na2C03 120-160 °C. Phosphine-free arylation of butyl acrylate and other olefins with aryliodides and bromides catalyst recycled five times without loss of activity product extracted with Et20. [78]... 

The cis-trans isomerization of PtCl2(Bu P)2 and similar Pd complexes, where the isomerization is immeasurably slow in the absence of an excess of phosphine, is very fast when free phosphine is present. The isomerization doubtless proceeds by pseudorotation of the 5-coordinate state. In this case an ionic mechanism is unlikely, since polar solvents actually slow the reaction. Similar palladium complexes establish cis/trans equilibrium mixtures rapidly. Halide ligand substitution reactions usually follow an associative mechanism with tbp intermediates. Photochemical isomerizations, on the other hand, appear to proceed through tetrahedral intermediates. 

Following Zhang and He s work, Ackermann and Barfuesser developed a protocol using a Pd-complex derived from air-stable heteroatom-substituted secondary phosphine oxides (HASPO) for the selective C3-arylation of a variety of functionalized NH-free indoles with bromoarenes (09SL808). Optimized conditions gave good-to-high yields and allowed the use of sterically hindered substrates (Scheme 32). 

In conclusion, for C-C bond forming reactions on aryl bromides and iodides there is no need for the use of complexes, palladacycles or pincers as simple ligand-free Pd(OAc)2 will perform as well. For C—C bond forming reactions on aryl chlorides some palladacycles may be the catalyst of choice. For problematic cases, the palladium complexes based on bulky electron-rich phosphines or catalysts based on carbene Hgands are likely to give the best results. 

The application of ultrasound in Mizoroki-Heck reactions is not well described. The first application of ultrasound irradiation in Mizoroki-Heck reactions performed in ionic liquids catalysed by palladium-carbene complexes and palladium nanoparticles was published in 2001 [ 190]. The Pd/C-catalysed reaction of iodobenzene and methyl methacrylate in NMP at room temperature resulted in an increased reaction rate under sonochem-ical conditions [191]. The catalyst showed high activity under phosphine-free ambient conditions and could be reused. The coupling products could be obtained in up to 70% yield. 

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