Pd NHC complexes

Since that time various Pd-NHC complexes have... 

Nielsen DJ, Cavell KJ (2006) Pd-NHC complexes as catalysts in telomerization and aryl amination reactions. In Nolan SP (ed) N-Heterocyclic carbenes in synthesis. WUey-VCH, Weinheim, pp 73-102... 

Scheme 6.48 AUyl-modified Pd-NHC complexes as improved catalysts in Buchwald-Hartwig aminations...

Xia and co-workers synthesised a number of Pd-NHC complexes (33, 34, 36) for carbonylative Suzuki reactions (Fig. 9.6) [41], Various aryl iodides were carbonylatively coupled (P = 1 atm) with either phenylboronic acid or sodium tetraphenylborate. All the complexes were highly active, but 33 provided the best results with >76% selectivity for ketone in all the reactions. Xia followed this work with the double carbonylation of various aryl iodides with several secondary amines using the catalysts [CuX(Mes)] (37-X) and [Cu(IPr)X] (38-X) (X = I, Br, Cl) (3 MPa, 100°C, 10 h) (Scheme 9.7) [42],... 

Following this pnblication, the anthors tested a series of Pd-NHC complexes (33-36) for the oxidative carbonylation of amino compounds (Scheme 9.8) [44,45]. These complexes catalysed the oxidative carbonylation of amino compounds selectively to the nreas with good conversion and very high TOFs. Unlike the Cu-NHC catalyst 38-X, the palladium complexes catalysed the oxidative carbonylation of a variety of aromatic amines. For example, 35 converted d-Me-C H -NH, d-Cl-C H -NH, 2,4-Me3-C H3-NH3, 2,6-Me3-C H3-NH3, and 4-Ac-C H3-NH3 to the corresponding nreas with very high TOFs (>6000) in 1 h at 150°C, in 99%, 87%, 85%, 72%, and 60% isolated yields, respectively (Pco,o2 = 3.2/0.8 MPa). 

Axially chiral Pd-NHC complexes reported by Shi and co-workers [26-28] have shown high selectivity in the oxidative kinetic resolution of alcohols without the need of addition of a chiral base. Enantiomeric excesses of up to 99% were obtained (Scheme 10.7). 

Scheme 13.1 Reductive elimination from methyl-Pd-NHC complexes...

Scheme 13.10 Decomposition of Pd-NHC complexes bearing chelating phosphine ligands...

As an example, consider the use of PVPy as a solid poison in the study of poly(noibomene)-supported Pd-NHC complexes in Suzuki reactions of aryl chlorides and phenylboroiuc acid in DMF (23). This polymeric piecatalyst is soluble under some of the reaction conditions employed and thus it presents a different situation from the work using porous, insoluble oxide catalysts (12-13). Like past studies, addition of PVPy resulted in a reduction in reaction yield. However, the reaction solution was observed to become noticeably more viscous, and the cause of the reduced yield - catalyst poisoning vs. transport limitations on reaction kinetics - was not immediately obvious. The authors thus added a non-functionalized poly(styrene), which should only affect the reaction via non-specific physical means (e.g., increase in solution viscosity, etc.), and also observed a decrease in reaction yield. They thus demonstrated a drawback in the use of the potentially swellable PVPy with soluble (23) or swellable (20) catalysts in certain solvents. 

The oxidative addition of 2-chloroimidazolium salts to Pd(0) was also used by Fiirstner and coworkers, providing a series of imidazolylidene complexes of Pd(II). The use of a chiral chloroimidazolium salt provided a enantiopure Pd-NHC complex [149]. A more recent example of C-Cl oxidative add-... 

The oxidative addition of 2-methylimidazolium salts has been studied from the theoretical point of view [ 146,147]. hi fact, the reaction is the reverse of the ubiquitous reductive elimination reaction observed for hydrocarbonyl-Pd- NHC complexes [ 151,152]. The DFT studies predict that the reaction of the 2-methylimidazolium salt with the model complex Pt(PH3)2 is exothermic. The reductive elimination hydrocarbonyl-Pd - NHC complexes has also been studied in detail by DFT calculations [153]. 

Week M, Jones CW (2007) Mizoroki-Heck couphng using immobhized molecular precatalysts leaching active species from Pd Pincers, entrapped Pd Salts, and Pd NHC complexes. 

Scheme 3 Kumada-Tamao-Corriu coupling using well-defined Pd-NHC complex 61...

Figure 9 Structures of well-defined Pd-NHC complexes used in asymmetric Suzuki-Miyaura coupling...

Scheme 25 Preparation of (poly)heterobiaryls using an in situ generated Pd-NHC complex Pd(OAc)/88...

Scheme 29 Reactivity comparison of Pd-NHC complexes 27 and 29 in the Negishi coupling...

S. P. Nolan and co-workers described the use of a Pd/NHC complex for an intramolecular amination/cyclization with an aryl chloride (21). They used this methodology as an approach to the Cryptocarya alkaloids.74... 

The impressive stability of Pd/NHC complexes and their activity in crosscoupling reactions is strongly supported by the recent report of Zhao et al. [49], who developed an environmentally benign protocol for the cross-coupling of aryl bromides with phenylboronic acids in water. The catalytic system is based on a tetradentate NHC ligand. 

Novel approaches such as microwave-assisted heating were successfully apphed in the aryl amination mediated by Pd/NHC complexes. The thermal stabihty of palladium/NHC complexes made them compatible with the high temperatures associated with the microwave protocols [82]. 

In spite of this rosy picture , no system is perfect and relatively recent studies have shown that catalysts of this type could undergo decomposition and therefore hmit success of some catalytic processes. Numerous catalytic reactions can be effectively mediated by Pd/NHC complexes which, because of their simplicity of assembly and use, makes this ligand family a promising platform for further development of new catalysts. In addition to their interesting catalytic activity, their stability under normal storage conditions and activity in simple synthetic protocols could promote them as viable industrial catalysts in one instance (telomerization of butadiene with methanol), it already has. 

Pd-N-heterocychc carbene (Pd-NHC) complex in the form of organic silica was prepared by sol-gel method by using the immobilized IL as a precursor, and its application in Suzuki and Heck reactions was demonstrated [36]. These C-C coupling reactions proceeded efficiently under microwave irradiation with excellent yield, and catalytic activity of the catalyst was not changed in five reaction cycles. Similarly, nano-sized silica particles were used to immobilize Pd-NHC and the resulting material was also utilized for Suzuki and Heck reactions [37]. Because of... 

In situ generation of Pd-NHC complexes in an imidazolium-type ionic liquid matrix and grafting of the catalyst on the surface of silica was reported by Karimi and Enders [41]. Pal-ladium(II) anchored on an ionic Uquid matrix, derived from A-3-(3-trimethoxysilylpropyl)-3-methyl imidazolium chloride, displays high thermal stabihty up to 280 °C. Catalyst 8... 

Fig. 1 Reductive elimination of a C2-methylated imidazolium salt from a Pd"-NHC complex. ...

Rg. 5 Reductive elimination of C2-methylated imidazolium salts from Pd"-NHC complexes containing chelating NHC ligands... 

During their studies on the application of Pd°-NHC complexes in the Heck reaction, Cavell and co-workers reported the formation of 2-arylimidazolium salts (amongst other organic and inorganic products) upon reaction of Pd -his(NHC) complexes with phenyl iodide. The presence of 2-arylimidazolium salts suggests that the Pd°-bis(NHC) complexes are oxidatively adding the phenyl iodide successfully, and that this process is accompanied hy a reductive elimination process to form a 2-arylimidazolium salt (Fig. 8). 

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