Monodentate NHCs

Fig. 2.12 Silver, gold and platinum complexes with monodentate NHC ligands as catalysts for the diboration of alkenes and alkynes...

In 2008, Grisi et al. reported three ruthenium complexes 65-67 bearing chiral, symmetrical monodentate NHC ligands with two iV-(S)-phenylethyl side chains [74] (Fig. 3.26). Three different types of backbones were incorporated into the AT-heterocyclic moiety of the ligands. When achiral triene 57 was treated with catalysts 65-67 under identical reaction conditions, a dramatic difference was observed. As expected, the absence of backbone chirality in complex 65 makes it completely inefficient for inducing enantioselectivity in the formation of 58. Similarly, the mismatched chiral backbone framework of complex 66 was not able to promote asymmetric RCM of 57. In contrast, appreciable albeit low selectivity (33% ee) was observed when the backbone possessed anti stereochemistry. 

Simple monodentate NHCs are somewhat susceptible to dissociation when coordinated to early transition metals [6], so in most cases multidentate chelating hgands are employed in which the carbene is tethered to a strongly coordinating anchoring group. This is not universally the case however, and simple monodentate NHC complexes of Zr 1 (Fig. 4.1) have been studied [7]. The complexes were activated with MAO and tested for ethylene polymerisation, leading to moderate activities between 7 and 75 kg mol bar h for linear polyethylene. 

It is interesting to note, that apart from the impressive series of transition metal complexes with monodentate NHCs, tetracarbene palladium(II) and platinum(II) as well as hexacarbene iron(III) complexes with bi- and tridentate chelating carbene ligands were prepared. 

Note Chelating carbenes are significantly more stable towards reductive elimination than monodentate NHC. 

Soon after the first isolation of a stable carbene, homogenous catalysis became a major field of application for this new ligand class [72,73]. For some time, simple monodentate NHC like the archetypical IMes all but dominated the scene. This is hardly surprising as NHC were a new ligand class whose properties were investigated in virtually every catalytic reaction. When publications with these readily available carbenes began to proliferate, protocols for functionalised NHC were developed and these hybrid ligands were then used in catalysis. 

Note Selectivity issues in phosphane rhodium catalyst hydroformylation reactions are often discussed in terms of the ligand bite angle [109-111 ]. This feature is entirely absent in the monodentate NHC ligands discussed here. 

Another example showing the challenge and difficulty of enantiocontrol of chiral monodentate NHC ligands was reported by Fernandez s group (Scheme 3.22) [42]. The authors prepared three neutral monodentate NHC-Rh... 

Recently, the same group extended the application of monodentate NHC ligand 77 to the conversion of acyclic precursors into 2-substituted and 2,3-disubstituted indolines via C(sp )-H activation coupling (Scheme 3.43) [69]. In substrates 78 with symmetric NCHR2 groups, very high asymmetric induction (89-98% ee) and reactivities (84-96% yields) were obtained. 

Later, the group of Tomioka developed a copper-NHC-catalyzed allylic arylation of cinammyl bromides with aryl magnesium bromides in 2009 [84]. An air-tolerant monodentate chiral NHC-CuCl catalyst 96 showed excellent enan-tioselectivities (up to 98% ee) and y-selectivity (up to 97 3) in this transformation (Scheme 3.57) at low temperature. Subsequently, the authors performed steric and electronic tuning of the monodentate NHCs and developed a copper-catalyzed asymmetric allyllic arylation of aliphatic allyUic bromides [85]. 

In 2010, Hoveyda and coworkers disclosed a Cu-catalyzed method for enantio-selective boronate conjugate additions to trisubstituted alkenes of acyclic a.P Unsaturated carboxylic esters, ketones, and thioesters, resulting in the formation of p-substituted quaternary carbon stereogenic centers (Scheme 3.65) [94]. By using 5 mol% of a chiral monodentate NHC 103 copper complex at low temperature, the products were obtained in good yields (up to 98%) and enantiose-lectivities (up to 96% ee). Moreover, transformations involving unsaturated thioesters gave the best enantiocontrol. 

In the same context of enantioselective hydroborations, Lee and Hoveyda reported in 2009 the fimctionalization of nonactivated cyclic alkenes with the use of copper-NHC complexes (Scheme 3.66) [95]. In addition, asymmetric hydrobo-ration of two cyclic olefins proceeded with good enantioselectivities (72 and 89% ee s) in the presence of a catalytic amount of monodentate NHC 104 and CuCL... 

Simple monodentate NHC adducts of molybdenum (0) and tungsten (0) carbonyl complexes have been the subjects of numerous studies [28,75,90-103]. 

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)... 

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]. 

Jarvo et al. have reported the use of isolated NHCP palladium allyl complexes of the type [(R-2)Pd(i -C3H5)]X (X = C1, OAc, BF ) as catalysts for nucleophihc allylation reactions. They investigated a series of electron-rich monodentate and bidentate ligands (bisphosphines, functionahzed NHCs, monodentate NHCs) for their ability to promote nucleophilic attack of the corresponding palladium allyl complexes on electrophiles in, for example, the conjugate addition of allylboronic esters to a variety of ,p-unsaturated Af-acylpyrroles [15a] as well as the allylation of aldehydes by allylstannanes (Scheme 10.4) [15b]. 

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