Bidentate NHCs

Bidentate NHC-Pd complexes have been tested as hydrogenation catalysts of cyclooctene under mild conditions (room temperature, 1 atm, ethanol). The complex 22 (Fig. 2.5), featuring abnormal carbene binding from the O carbon of the imidazole heterocycles, has stronger Pd-C jj, bonds and more nucleophilic metal centre than the bound normal carbene chelate 21. The different ligand properties are reflected in the superior activity of 22 in the hydrogenation of cyclooctene at 1-2 mol% loadings under mild conditions. The exact reasons for the reactivity difference in terms of elementary reaction steps are not clearly understood [19]. 

Fig. 3.26 Catalysts 65-67 bearing chiral NHCs with alkyl side chains 3.1.3.2 Chiral Bidentate NHCs...

Abstract This chapter focuses on carbon monoxide as a reagent in M-NHC catalysed reactions. The most important and popular of these reactions is hydro-formylation. Unfortunately, uncertainty exists as to the identity of the active catalyst and whether the NHC is bound to the catalyst in a number of the reported reactions. Mixed bidentate NHC complexes and cobalt-based complexes provide for better stability of the catalyst. Catalysts used for hydroaminomethylation and carbonyla-tion reactions show promise to rival traditional phosphine-based catalysts. Reports of decarbonylation are scarce, but the potential strength of the M-NHC bond is conducive to the harsh conditions required. This report will highlight, where appropriate, the potential benefits of exchanging traditional phosphorous ligands with iV-heterocyclic carbenes as well as cases where the role of the NHC might need re-evaluation. A review by the author on this topic has recently appeared [1]. 

Fig. 9.3 Rhodium complexes with mixed bidentate NHC ligands...

Chiral silver complexes bearing bidentate NHC ligands (24) have been synthesized. They are used in alkene metathesis and allylic alkylation reactions high diastereos- (g) electivity is observed induced by the chiral backbone on the prochiral biphenyl.27 Ruthenium-based complexes obtained from transmetalation with a Grubbs-Hoveyda complex exhibited high activities and enantioselectivities in ring-opening metathesis/ ... 

Douthwaite et al. have published two types of chiral bidentate NHC ligands [39]. The first of these contains an imidazolylidene and an imine linked by a frans-cyclohexanediamine core according to the synthetic strategy outlined in Scheme 14. The facile modular variation of the peripheral substituents in this class of ligands principally allows for the rapid screening of large ligand libraries. 

Finally, Shi and Qian reported the synthesis of the racemic di-benzimida-zolium 18 obtained in 4 steps from the (+/-) frans-cyclohexanediamine (Fig. 7) [42], The reaction with Pd(OAc)2 gave a dimeric bidentate NHC-Pd complex which was found to be moderately active in Suzuki and Heck reactions. 

For instance, a new bidentate NHC pyrazolate ligand precursor featuring a methylene spacer between the benzimidazol-2-ylidene and the pyrazolate moieties has been used to obtain four new luminescent platinum(ii) complexes along with either phenyl-pyridine or pyridine-triazole luminophores/ ... 

New applications of chiral palladacycle catalysts were carried out by Liu and Shi [67], They synthesized axially chiral cyclometalated bidentate NHC-Pd(II) complexes derived from binaphthyl-2,2 -diamine and used them for the asymmetric addition of indole to N-tosylaryliniines. 

In 2002, Hoveyda and coworkers introduced an alternative concept to install chirality in ruthenium olefin metathesis complexes through a Ci-symmetric bidentate NHC ligand, bearing binaphtholate moieties (Figure 11.33). The NHCs in this type of complexes lacked backbone substitution and it was chelation that prevented free rotation of the ligand [118]. In this case, chiral information installed within the A-substituent was transferred directly to the ruthenium center. Unfortunately, these complexes were found to be less active because of the reduced Lewis acidity at the metal center, mainly due to the exchange of Cl... 

Figure 11.33 Chiral bidentate NHC-bearing ruthenium complexes 160-166.

Tricarbonyl rhenimn (I) bis(carbene) complexes (NHC)2ReBr(CO)3 are easily accessible from [NEtJ [ReBr3(CO)s] precursor and the free carbene source, either a monodentate NHC or a bidentate NHC ligand (75 and 76) (Scheme 14.37)... 

Scheme 2.162 Synthesis of a bidentate NHC and its corresponding rhodium complex.

Synthesis of other bidentate NHCs and their corresponding rhodium complexes. 

Attempts to use chiral bidentate NHC ligands, such as 1-4 (Figure 2.61), in order to solve the problem of reductive elimination that frequently occurs with monodentate NHC ligands during the hydroformylation process, failed [36]. 

Figure 10.1 Chelation introduced through the Cp-NHC unit (a) or through a bidentate NHC-Y hgand (b).

Moreover, Shi and Li s group [34] reported the 1,2-addition of indole to Af-tosylarylimines using axially chiral cyclometallated bidentate NHC-Pd(II) complexes (which were somewhat similar to those reported above) (Scheme 6.27). Moderate yields (71-89%) and enantioselectivities (24-66% ee) were obtained however, the authors found that a small amount (<10%) of an achiral doublealkylation product was detected. 

Figure 3.2 Pd complexes bearing CNC pincer or bidentate NHC ligands.

Complex 33, containing a partially reduced 1,10-phenantroline-based monodentate NHC ligand led to 97% yield and 81% ee in the hydrogenation of methyl acetamidoacrylate (Figure 13.4). A bidentate NHC bearing a PPh2 unit with a chiral pseudo-o-[2.2]paracyclophane linker (34) was found to be a selective catalysts for non-functionalized trisubstituted alkenes, although functionalized alkenes were found to be sensitive to the H2 pressure. Also, the... 

WuJ, Dai W, FamabuJH, etal. Synthesis and catalytic activity of iron complexes with bidentate NHC ligands. Dalton Trans. 2013 42 7404-7413. 

Arnold and co-workers conducted a range of work with tethered (bidentate) NHCs, in which the NHC was anchored to the metal centre covalently as well as via the carbene. This imparted additional stability to the complex, allowing the isolation of species such as lanthanide-NHC complexes 108 and 109 (Figure 2.6). However, it has been shown that the NHC-metal bond is often still quite labile and therefore this might have implications for catalysis. This lability can be used for interesting chemistry such as the functionalisation of silanes and the capture of CO2 and CS2. ... 

The hydroamination of alkenes and alkynes provides a highly atom-economical method for the preparation of substituted amines and imines. Despite substantial efforts and recent progress, the development of a generally applicable functional group-tolerant catalyst for this reaction remains a challenge, and intense research continues in this field. An interesting example has been reported by means of Rh combined with a bidentate NHC ligand [eqn (8.14)]. Complex 32 was found to catalyse the intramolecular hydroamination of aminoalkynes. However, the turnover rates remained modest with values up to 50 h ... 

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