Coupling reactions palladacycles

Phosphites P(OR)3 are much weaker ligands for Pd, and are not capable of supporting Pd° species in solution for the reactions where oxidative addition is rate-limiting therefore they are very rarely used in cross-coupling reactions. Phosphite-derived palladacycles, however, are among the most effective precatalysts (Section 9.6.3.4.8). 

Herrmann WA, Brossmer C, Reisinger CP, Riermaier T, Ofele K, Beller M (1997) Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions. Part 10. Palladacycles efficient new catalysts for the Heck vinylation of aryl halides. Chem Eur J 3 1357-1364 Iyer S, Jayanthi A (2001) Acetylferrocenyloxime palladacycle-catalyzed Heck reactions. Tetrahedron Lett 42 7877-7878 Iyer S, Ramesh C (2000) Aryl-Pd covalently bonded palladacycles, novel amino and oxime catalysts di- x-chlorobis(benzaldehydeoxime-6-C,AT)dipalla-dium(II), di- x-chlorobis(dimethylbenzylamine-6-C,A)dipalladium(II) for the Heck reaction. Tetrahedron Lett 41 8981-8984 Jeffery T (1984) Palladium-catalysed vinylation of organic halides under solid-liquid phase transfer conditions. J Chem Soc Chem Commun 1287-1289 (b) idem,... 

Louie J, Hartwig JF (1996) A route to PdO from Pdll metallacycles in amination and cross-coupling chemistry. Angew Chem Int Ed Engl 35 2359-2361 Mucalo MR, Coouey RP (1989) F.T.I.R. spectra of carbon monoxide adsorbed on platinum sols. J Chem Soc Chem Commun 94—95 Munoz MP, Martin-Matute B, Fernandez-Rivas C, Cardenas DJ, Echavarren AM (2001) Palladacycles as precatalysts in Heck and cross-coupling reactions. Adv Synth Catal 343 338-342... 

Palladacycles [17] have been established as important class of catalysts, an unusual phosphine-free sulfur containing catalyst was introduced by Zim [18] et al. A phosphinite based palladacycle [19] proved to be very efficient in Suzuki coupling reactions. An N,P-Ligand type was synthesized by Kocovsky [20] et al. Tridentate pincer ligands [21] have been proved use-fill in the Heck reaction. Recent developments regarding Heck (22] and Suzuki [23] reactions have been reviewed by Fu and Littke. A new catalyst especially suitable for Heck couplings has been introduced by Beller [24] et al. 

The C-H activation step could, in principle, occur either by oxidative addition of the C-H bond - pathway (a) - or by electrophilic displacement - pathway (b). The oxidative addition pathway would proceed via the formation of a palla-dium(IV) species. Although such intermediates have been postulated in some coupling reactions catalyzed by palladacycles, as yet no conclusive experimental evidence has been presented [14], It is perhaps more likely that C-H activation results from electrophilic displacement of the ortho proton [15]. 

Rosner, T., Le Bars, J., Pfaltz, A., Blackmond, D. G. Kinetic Studies of Heck Coupling Reactions Using Palladacycle Catalysts ... 

Herrmann, W. A., Ofele, K., von Preysing, D., Schneider, S. K. Phospha-palladacycles and N-heterocyclic carbene palladium complexes efficient catalysts for CC-coupling reactions. J. Organomet. Chem. 2003, 687, 229-248. 

Apart from palladacycles, a number of catalyst systems are currently known to show TON up to 100,000 for the Heck and Suzuki reactions of all kinds of aryl bromides. It is important to note that coupling reactions of electron-deficient aryl bromides (e.g. 4-bromoacetophenone), which are often used in academic laboratories, are not suitable as test reactions to judge the productivity of a new catalyst, because simple palladium salts without any ligand give turnover numbers up to 100,000 with these substrates. 

In the cross-coupling reaction, starting from the simple arene (with directing group), palladation by a Pd(II) salt would lead to the formation of the palladacyclic complex (Ar1Pd(II)L) (Scheme 3). After the transmetallation and reductive elimination processes, the biaryl product is obtained together with Pd(0). If the Pd(0) can be further oxidized to Pd(II) catalyst, a catalytic cycle will be formed. By accomplishing this, arenes (C-H) are used to replace the aryl halides (C-X). Similarly, arenes (C-H) can be used to replace the aryl metals (C-M). 

Palladium Catalysts Yu s group has carried out systematic studies on Pd-catalyzed alkylations of aryl C—H bonds. Stille-type cross-coupling reactions have been developed by directed C—H activation (Equation 11.30) [68]. The reaction rate is enhanced by benzoquinone and microwave irradiation. Significantly, carboxylic acid functionality can be used as an efficient directing group for aryl C— H bond activation (Equation 11.31) [69]. The reaction conditions can be applied to the carboxylation of vinyl C— H bonds. The possible intermediacy of a palladacycle has been confirmed by NMR spectra and X-ray crystallography. 

Domino coupling reactions of aryl halides with norbornene and its derivatives provide a simple route to PAHs. In a four component sequence, norbornene (73) is arylated with an excess of iodobenzene to the terphenyl 74, that can be converted to the benz[e]pyrenes 75 and 76 by classical aromatic conversion reactions [131]. The domino sequence is a consequence of the fact that the five-membered intermediate palladacycle 77 a is able to add a second molecule of iodobenzene (77a —> 77b), and the intermediate arylpalladium halide resulting from reductive elimination in 77 b can even add a third molecule of iodobenzene before the final elimination of the Pd(0) complex PdL2 occurs (see Scheme 38). 

Coupling reactions. Various (Heck, Stille, Suzuki, Sonogashira, and Ullmann) coupling reactions are mediated by a stable palladacycle 1. ... 

The area of palladium-catalysed coupling reactions got a new impulse when in 1995 Herrmann and Beller published their work on the discovery of a palladacycle as a catalyst for the Heck reaction (Fig. 10.12) [102, 103]. It had been known earlier that tri-ortho-tolylphosphine is a very good ligand for the palladium-catalysed Heck... 

Palladium species immobilized on various supports have also been applied as catalysts for Suzuki cross-coupling reactions of aryl bromides and chlorides with phenylboronic acids. Polymers, dendrimers, micro- and meso-porous materials, carbon and metal oxides have been used as carriers for Pd particles or complexes for these reactions. Polymers as supports were applied by Lee and Valiyaveettil et al. (using a particular capillary microreactor) [173] and by Bedford et al. (very efficient activation of aryl chlorides by polymer bound palladacycles) [174]. Buch-meiser et al. reported on the use of bispyrimidine-based Pd catalysts which were anchored onto a polymer support for Suzuki couplings of several aryl bromides [171]. Investigations of Corma et al. [130] and Plenio and coworkers [175] focused on the separation and reusability of Pd catalysts supported on soluble polymers. Astruc and Heuze et al. efficiently converted aryl chlorides using diphosphino Pd(II)-complexes on dendrimers [176]. 

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