Benzimidazolin-2-ylidenes

For rhodium and iridium compounds alkoxo ligands take over the role of the basic anion. Using /z-alkoxo complexes of ( -cod)rhodium(I) and iridium(I)— formed in situ by adding the /r-chloro bridged analogues to a solution of sodium alkoxide in the corresponding alcohol and azolium salts—leads to the desired NHC complexes even at room temperature [Eq. (10)]. Using imidazolium ethoxyl-ates with [(r " -cod)RhCl]2 provides an alternative way to the same complexes. By this method, it is also possible to prepare benzimidazolin-2-ylidene complexes of rhodium(I). Furthermore, an extension to triazolium and tetrazolium salts was shown to be possible. ... 

Imidazolidinium salts can also be transformed into the corresponding diamino ortho-esters by alkaline alkoxylate, and upon alcohol elimination at elevated temperature the imidazolidin-2-ylidenes can be trapped. The reaction of tria-zolium salts with sodium methanolate in methanol yields 5-methoxy-4,5-dihydro-IH-triazole which also eliminates methanol upon heating in vacuo. The resulting triazolin-5-ylidenes can either be isolated or trapped by an appropriate metal precursor [Eq. (19)]. Benzimidazolin-2-ylidenes are similarly accessible by this route. 

Cyclic thiourea derivatives like l,3,4,5-tetramethylimidazole-2(3H)-thione— prepared by condensation of substituted thioureas with a-hydroxyketones—can be converted into the corresponding imidazolin-2-ylidene by desulfurization with sodium or potassium [Eq. (23)]. This method was used to prepare and isolate l,3-bis-neo-pentylbenzimidazolin-2-ylidene with Na/K. With LDA as the base it is also possible to generate free benzimidazolin-2-ylidenes in solution. ... 

Electron-rich olefins are nucleophilic and therefore subject to thermal cleavage by various electrophilic transition metal complexes. As the formation of tetraaminoethylenes, i.e., enetetramines, is possible by different methods, various precursors to imidazolidin-2-ylidene complexes are readily available. " Dimerization of nonstable NHCs such as imidazolidin-2-ylidenes is one of the routes used to obtain these electron-rich olefins [Eq. (29)]. The existence of an equilibrium between free NHC monomers and the olefinic dimer was proven only recently for benzimidazolin-2-ylidenes. In addition to the previously mentioned methods it is possible to deprotonate imidazolidinium salts with Grignard reagents in order to prepare tetraaminoethylenes. " The isolation of stable imidazolidin-2-ylidenes was achieved by deprotonation of the imidazolidinium salt with potassium hydride in THF. ... 

Benzimidazolin-2-ylidene complexes are conveniently prepared from the corresponding enetetramines. In refluxing toluene the olefin is attacked, e.g., by [(f/ -cod)RhCl]2 to form the mono(NHC) or the bis(NHC) complex depending on the reaction conditions. ... 

Ruthenium(n) systems containing imidazol-2-ylidene or imidazolidin-2-ylidene have been used to catalyze the synthesis of 2,3-dimethylfuran starting at (Z)-3-methylpent-2-en-4-yn-l-ol [Eq. (54)]. The activity of the catalyst strongly depends on the nature of the NHC ligand. Benzimidazolin-2-ylidenes give the best results for this transformation. Similar systems have also been used for olefin metathesis reactions. ... 

Although these two methods have found widespread application for the synthesis of free carbenes, they failed for selected saturated imidazolidin-2-ylidenes and especially in the preparation of triazolin-5-ylidenes. In these cases the free carbene species 7 can be obtained from 2-alkoxyimidazolidines 6 [44] or 5-aUcoxytriazoles [36] by thermally induced ot-elimination of an alcohol (Fig. 5). In addition to 2-aUcoxyimidazolidmes, 2-(pentafluorophenyl)imidazolidines [45, 46] have also been used for the generation of NHCs by cx-elimination. The adduct 8 eliminates acetonitrile upon heating [47] to yield the benzimidazolin-2-ylidene 9. In a more exotic procedure, imidazolium salts have been reduced electrochemically to give the free imidazolin-2-ylidenes [48]. 

The reaction of the free carbene with a suitable metal complex is the simplest method for the preparation of an NHC complex (Fig. 8a). In addition, the cleavage of electron-rich enetetramines in the presence of coordinatively unsaturated metal complexes followed by coordination and stabilization of the NHC also constitutes an established method for the preparation of NHC complexes (Fig. 8b) [7, 95], which works particularly well for the preparation of complexes with benzimidazolin-2-ylidene ligands [56, 80, 96]. 

The exclusive coordination of the C5 carbon atom in complex 48 was achieved by a reduction of the bite angle [147]. Blocking of the C4 and C5 positions as seen in benzimidazolin-2-ylidenes led to the complex with the C2-metalated NHC ligand 49 [147]. No transformation of a C5-coordinated NHC ligand into a C2-coordinated one, i.e., conversion of 46 to 47 or vice versa, has been observed so far. 

Complex 61 is also accessible from the 2-nitrophenyl isocyanide complex 68 by reduction of the nitro group with Sn/HCl. Incomplete reduction of the nitro group in 68 with Raney-Nickel/hydrazine yields, after intramolecular cyclization, the complex 70 with the NH,NOH-stabilized benzimidazolin-2-ylidene ligand. Complex 69 with the 2-hydoxylamin-substituted phenyl isocyanide ligand presumably occurs as an intermediate in this reaction. The alkylation of both the NH,NH- and the NH, NOH-stabilized NHC ligands in 67 and 70, respectively, proceeds readily (Fig. 23) [184, 185]. 

Fig. 23 Template-controlled synthesis of benzimidazolin-2-ylidene ligands...

NHCs are often compared to phosphines and theoretical studies by Frenking et al. [211-213] showed that the substitution of the phosphines in 89 for two NHCs leads to compounds with a carbon(O) atom that is formally stabilized by two NHC C donor-acceptor bonds. The first carbodicarbene 90 was prepared by Bertrand et al. who selected benzimidazolin-2-ylidenes as carbene donors and the deprotonation of the conjugated acid of an allene as the synthetic strategy (Fig. 29)... 

Hahn, F.E., Jahnke, M.C. and Pape, T. (2007) Synthesis of pincer-type bis(benzimidazolin-2-ylidene) palladium complexes and their application in C-C coupling reactions. Organometallics, 26,150-4. 

In 1999, the first annulated carbene derived from benzimidazole, l,3-bis(2,2-dimethylpropyl)benzimidazolin-2-ylidene, was prepared by the reduction of A/,N -dialk ylated benzimidazol-2-thione with Na/K in toluene [44]. Frequently, benzimidazolium ions containing less bulky Af-alkyl groups interconvert rapidly into enetetramine derivatives. Even when the formation of these compounds can be avoided by sterically demanding substituents, the final generation of the carbene may fail. 

The mechanism for the formation of complexes 63—66 has not been established unambiguously yet. As mentioned before, related complexes bearing bidentate imidazolin-2-ylidene/donor or benzimidazolin-2-ylidene/donor ligands (Scheme 9.18) have been described as formed by tautomerization of the azole to the NR,NH-substituted NHC that then together with the tethered donor coordinates to the metal center. This rather simple type of reaction mechanism is not very likely to be operative. 

The monoanionic CCC pincer ligand 7 was introduced by Hollis [15] (Figure 9.3). Metallation of the ligand precursor was achieved by deprotonation with tetraamido zirconium(IV) complexes. Modifications were done by Chianese [16] who used stronger donating benzimidazolin-2-ylidene moieties in 8, and by... 

The monoanionic CCC ligand type was also used by Chianese and coworkers to prepare the Ir complexes 47a-f with benzimidazolin-2-ylidene moieties to prevent the formation of abnormal carbenes. They treated the imidazolium salt 8 2HBr with 30 equiv of triethylamine as base and 0.5 equiv of [(p-Cl)Ir(COD)]2 in acetonitrile at 80 °C to obtain the Ir(III) complexes 47 (Scheme 9.8). However, a clean reaction was obtained only in the case of 47 bearing Al-mesityl (a) or A/ -2,6-di-isopropylphenyl (b) substituents at the NHC moiety. With A/ -3,5-dimethylphenyl (c), A/ -3,5-di-tert-butylphenyl (d), N-tert-hutyl (e), and Al-l-adamantyl (f) substituents, they had to use a slight excess of CsF as base to obtain the respective Ir(III) complexes 47 [16]. For complex 47b, the group had to raise the temperature in an autoclave up to 150 °C [16b]. 

Palladium complexes of monodentate and chelating bidentate benzimidazolin-2-ylidene ligands have been prepared in moderate to good yield, via m situ deprotonation, with Pd(OAc)2 in To effect formation of... 

Palladium(ii) complexes of imidazoline-2-ylidenes with pendant 1- and 1,3-di-methylferrocenyl substituents, 78 and 79, were prepared by the reaction of the relevant benzimidazolium salt with Pd(OAc)2 and a crystal structure of 79 obtained. " In common with other related carbene complexes, the plane of the benzimidazole rings are twisted with respect to the palladium coordination plane the dihedral angle is 85.17(23)°. The closely related l-ferrocenyl-3-methyl-benzimidazolin-2-ylidene complex (in which there is no methyl linker between the N and the ferrocenyl group) has also been prepared by a similar route. " l-Ferrocenylmethyl-3-benzylimidazolidinium and 1-ferrocenyl-methyl-3-(2,4,6-trimethylbenzyl)imidazolidinium iodide salts were prepared and complexed to Pd by an in situ deprotonation route. " Cyclic voltammetry studies on the salts and the Pd complex show a number of redox processes, indicating that the ferrocenyl substituents are electronically isolated from the remaining molecular framework. 

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