Palladium-NHC catalysts

Using a monodentate ligand does not necessarily mean that only one ligand coordinates to the metal. Since these monoligated metal species are very important for catalytic activity, their synthesis is highly desirable. More details on the development of well-defined and highly active monoligated palladium NHC catalysts will be provided in later parts of this volume [103-109]. 

In 2005, Sames [50] developed a C-H arylation of N-alkyl indoles (indoles with a free NH can also be used) and pyrroles with aryl iodides using a rhodium catalyst. They demonstrated that CsOPiv activates the rhodium catalyst to form Rh(OPiv)2(Ph)L2 (L = P[p-(CF3)CgH4]j). In 2006, Sames [51] also developed a palladium NHC catalyst 53 for the C-H arylation of N-protected indoles, pyrroles, imidazoles, and imidazo[l,2-a]pyridines (Section 17.2.4.5) with aryl iodides. 

A number of nickel(O) catalysts have been synthesized since the initial report of Arduengo et al. containing nickel and palladium NHC catalysts [10]. The work of Matsubara et al. [11] shows the ability to access similar nickel(O) and nickel(II) catalysts from commercially available Ni(acac)2 (Figure 13.2). This synthetic route, which employs sodium hydride to reduce Ni(II) to Ni(0), circumvents the need to use air-sensitive and flammable Ni(COD)2. One other catalyst of note is the 18 e complex synthesized by Spicer and coworkers [12]. The unique structure of the tetradentate NHC ligand serves to stabilize the catalyst as well as facilitate the reduction of several organic structural motifs. 

Further interesting examples can be found in Noel and Buchwald s review, including the adaptation of this method to RTlLs by Ryu and coworkers [83]. In this reaction, iodobenzene was reacted with butyl acrylate with a palladium-NHC catalyst (1 mol%) system in [BMimjNTfj (l-Butyl-3-methylimidazolium). The system was run for 11.5 h giving 115.3 g of tr s-butylcinnamate product. The catalyst could be recycled up to five times. 

Allylic substitutions catalysed by palladium NHC complexes have been studied and the activity and selectivity of the catalysts compared to analogous Pd phosphine complexes. A simple catalytic system involves the generation of a Pd(NHC) catalyst in situ in THF, from Pdj(dba)j, imidazolium salt and Cs COj. This system showed very good activities for the substitution of the allylic acetates by the soft nucleophilic sodium dimethyl malonate (2.5 mol% Pdj(dba)3, 5 mol% IPr HCl, 0.1 equiv. C (CO ), THF, 50°C) (Scheme 2.22). Generation of the malonate nncleophile can also be carried out in situ from the dimethyhnalonate pro-nucleo-phile, in which case excess (2.1 equivalents) of Cs COj was used. The nature of the catalytic species, especially the number of IPr ligands on the metal is not clear. 

Palladium NHC systems for the hydrodehalogenation of aryl chlorides and bromides and polyhalogenated aromatic substrates originate from about the same time as the first reports on Ni chemistry, and show many similarities. Initial efforts showed that the combination of PdCdba) (2 mol%), one equivalent of imidazolium chloride and KOMe produced an effective system for the reduction of 4-chlorotolu-ene, especially upon use of SIMes HCl 2 (96% yield of toluene after 1 h at 100°C) [7]. Interestingly, higher ligand to metal ratios severely inhibited the catalysis with only 7% yield of toluene achieved in the same time in the presence of two equivalents of SIMes HCl 2. Variation of the metal source (Pd(OAc)2, Pd(CjHjCN)jClj), alkoxide (NaOMe, KO Bu, NaOH/ ec-BuOH) or imidazolium salt (IMes HCl 1, IPr HCl 3, lAd HCl, ICy HCl) all failed to give a more active catalyst. 

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). 

The oxidation to methyl ketones without cleavage of the double bond was reported recently for a palladium NHC complex [108]. When the authors used the previously described catalyst 13 in THF with dioxygen for the oxidation of styrene they found that together with the phenylmethylketone a significant amount of y-butyrolactone was formed. Analysis of the mechanism led to the conclusion that THF is oxidized to a hydroperoxide species which is the real oxidant. They therefore tried tert-butylhydroperoxide (TBHP) and found immediate conversion without any induction period. Optimized conditions include 0.75 mol % of the previously described dimeric complex... 

Liao et al. used a carboxylic acid amide functionalised carbene and a phosphane in a mixed NHC/phosphane palladium(II) catalyst [261]. The system shows the usual ligand exchange behaviour meaning that the PPhj ligand can be substituted by PCyj. This made it possible to study the influence of the phosphane ligand on the performance of the catalyst. In the Suzuki-Miyaura reaction between phenylboronic acid and p-chloroacetophenone, the yield changes dramatically. When PCyj is chosen as the phosphane ligand, then quantitative yield is observed (for both the saturated and unsaturated NHC), but in the case of PPhj the yield drops to 8% (unsaturated NHC) or even 4% (saturated NHC). When... 

Figure 3.86 Allylic amination using mixed NHC/phosphane palladium(ll) catalysts.

Figure 4.47 Some rhodium(l) and palladium(ll) catalysts bearing Fc-modified NHC ligands.

Recently, the high activity of palladium/NHC complexes in the Heck reaction was combined with an efficient recyclability process [63]. Bis-carbene pincer complexes of palladium(II) were immobilized on montmorillonite K-10. The catalytic activity of the heterogeneous system is similar to that displayed by their homogeneous counterparts. The stability of the catalyst was tested in the reaction of phenyl iodide and styrene. The product yield decreases from 99 to 79% after ten cycles. 

MCRs Involving Pd-NHC Catalysts Palladium is, along with ruthenium, the most widely used transition metal with NHC ligands in catalytic transformations, and... 

Cycloisomerization of dienes is also possible. Catalysts that have been used include rhodium (Scheme 11.80), ° ruthenium and palladium salts (Scheme 11.81) in alcohol solvents. Palladium NHC complexes are also efficient catalysts.The most likely mechanism involves the formation of a metal hydride, which acts as the catalyst. In some cases, alkene migration may also be observed and, with the right choice of catalyst, may become the exclusive pathway, as in the formation of cyclopentene 11.242. ... 

An efficient procedure of anaerobic MW-assisted oxidation of secondary alcohols (Scheme 18.4) in the presence of N-heterocyclic carbene palladium (NHC)-Pd catalysts (Scheme 18.5) with 2,4-dichlorotoluene or other aryl halides as oxidants was reported recently [16]. 

In 2010, Dorta and coworkers [83] reported a new synthetic strategy to access functionalizable 3-allyl oxindoles bearing a chiral quaternary carbon stereocenter via a direct palladium-catalyzed a-arylation protocol. This elegant methodology, previously accessible only via a two-step procedure involving a Pd-catalyzed intramolecular a-arylation followed by an asymmetric Pd-catalyzed allylic alkylation [84], afforded impressive reactivities, and high chemoselectivities and enantioselectivities were also achieved in the synthesis of oxindoles using a new chiral Pd-NHC catalyst (Scheme 8.45). 

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