Imidazol-2-ylidene NHC ligands

However, similar NHC architectures employing aromatic side chains have shown more encouraging results. In 2000, Nolan and co-workers reported the synthesis and characterisation of the NHC-Ru complex 20 bearing a sterically more demanding N,N -bis-[2,6-(di-/xo-propyl)phenyl]imidazol-2-ylidene (IPr) ligand [27, 28] (Fig. 3.5). Standard RCM substrate 1 was used to test the catalytic performance of 20. The ring closure was found to be complete after 15 min by using 5 mol% 20 as catalyst at room temperature. Under identical conditions, 15... 

The enantioselective P-borylation of a,P-unsaturated esters with (Bpin) was studied in the presence of various [CuCl(NHC)] or [Cu(MeCN)(NHC)] (NHC = chiral imidazol-2-ylidene or imidazolidin-2-ylidene) complexes. The reaction proceeds by heterolytic cleavage of the B-B bond of the (Bpin), followed by formation of Cu-boryl complexes which insert across the C=C bond of the unsaturated ester. Best yields and ee were observed with complex 144, featuring a non-C2 symmetric NHC ligand (Scheme 2.31) [114]. 

Further improvements in activity of the imidazol-2-ylidene Ru complexes might be attained by the incorporation of a better a-donor substituents with larger steric requirements. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (14 e") of the ruthenium metallacyclobutane intermediate [7]. Both ligand-to-metal a-donation and bulkiness of the NHC force the active orientation of the carbene moiety and thus contribute to the rapid transformation into metallacyclobutane species [7b]. Both can be realized by incorporation of alkyl groups in 3,4-position of imidazol-2-ylidene moiety, lyie Me. Me... 

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. ... 

Given these statements, it is not surprising that NHC complexes of almost all the transition metals have been prepared. In particular, metals incapable of 7i-back-donation such as titanium were only involved in Schrock-carbene complexes until the stable Fischer-type complexes were prepared from TiCU and imidazol-2-ylidenes (IV). The electronic properties of these NHC are also well illustrated in metallocene chemistry (a) 14-electron chromium(II) complexes have been isolated, (b) the displacement of a Cp ligand of chromocene and nickellocene can be achieved by imidazol-2-ylidenes (IV), giving bis(carbene) complexes (Scheme 8.26). 

By far the most popular second donor group on Fc functionalised NHC ligands is a phosphino group, either on the same [145,154,185] or the other [186] Cp ring or with two phosphinoferrocenes flanking the imidazol-2-ylidene unit [148,149] (see Figure 4.52). 

The reaction mechanism is similar to the one employed by Raubenheimer el al. for their chromium(O) thiazol-2-ylidene complex [48], In the case of the ruthenium imidazol-2-ylidene complexes, 4,5-dimethylimidazole stabilised the carbene complex compared with unsubstituted imidazole. Likewise, the carbonyl ligand in trans position was necessary to isolate and crystallise the complex. This can be expected, when an excellent o-donor (NHC) is trans to an excellent tr-acceptor (CO). 

The cyclooctadiene ligand is easily replaced by two molecules of CO. The vCO stretching frequencies of the carbonyl ligands can be used to estimate the electronic properties of the l,3,7,9-tetramethylxanthine-8-ylidene ligand. The electron donor ability is found to be less than for pyrimidine based carbenes [99] or imidazol-2-ylidenes [100]. It is also one of the strongest NHC rr-acceptor ligands known [98]. 

More recently, Nolan and co-workers have reported an example of C-H activation of the same ligand at Rh upon complexation with the free NHC. Treatment of [Rh(COE)2Cl]2 (COE = cyclooctene) with IMes at room temperature led to the total displacement of the COE ligand with simultaneous splitting of the dimer, affording a bis(NHC) coordinated product [Rh(IMes)(IMes) HCl] ((IMes) = cyclometalated l,3-bis(2,4,6-tri-methylphenyl)imidazol-2-ylidene) with a single C-H functionalised orthomethyl group (Fig. 32). 

The dicationic complex [Ru(py-NHC)(terpy)(OH2)] (terpy=2,2 6, 2 -terpyridine) containing the bidentate pyridyl-NHC ligand 3-meth)d-l-(pyridine-2-yl)imidazol-2-ylidene catalyzed the epoxidation of terminal alkenes vrith Phi (OAc)2 in CH2CI2 at room temperature (Table 12.7). In an effort to make the system reusable, a solvent system comprising a 1.2 0.8 mixture of CH2Q2 and the ionic liquid [bmim] [PFg] was employed. Tests with cyclooctene showed that this allowed 10 consecutive epoxidation reactions to be carried out without any drop in catalyst performance [107]. 

N-Heterocyclic carbene ligands (NHCs) are, in molecular chemistry, strongly associated with ruthenium since the work of Grubbs on metathesis. However, these ligands had not been used for the stabilization of NPs. It was, therefore, of interest, after a comprehensive study of phosphine coordination on NPs, to study the interaction between RuNPs and NHCs [35]. The RuNPs were prepared by the decomposition of [Ru(COD)(COT)] in pentane, under dihydrogen atmosphere (3 bar) at room temperature, and in the presence of two different carbenes as stabilizing agents, namely, l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and MA -di(ferf-butyl)imidazol-2-ylidene (I Bu), shown in Fig. 31.6. 

NHC-containing complexes are known to possess an exceptional catalytic activity associated to a better stability than complexes containing phosphine units, allowing performance of many synthetic transformations under harsh conditions [12]. This improvement of the catalyst activity is mainly due to the presence of the electron-rich and sterically demanding NHC ligands. We have therefore synthesized with excellent yields new bis(pyridine) adduct complexes 4b and 4c containing l,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene (SIMes), respectively (Scheme 2). 

A substantial improvement on the above-described class of Ru-vinyUdene complexes has been achieved by Louie and Grubbs through synthesis of a novel generation of complexes coordinating an A-heterocyclic carbene (NHC) ligand (e.g. imidazolylidene). This new set of Ru complexes, e.g. 15 (IMes = l,3-(2,4,6-trimethylphenyl)imidazoT2-ylidene, R = Cy, R = t-Bu) and 16 (iPrlM = l,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene, R = Cy, R = Ph) has been conveniently produced from the bisphosphane-Ru complex 9 (R = Cy) by reaction with free imidazoline carbene or its precursor salts (Scheme 7). 

In 2000, Nolan and Lee reported the successful application of NHC ligands in this particular arylation reaction [101]. By combining palladium acetate and the imidazolium salt IPr-HCl (IPr = l,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene), it was possible to react phenyltrimethoxysUane (as well as vinyltrimethoxysilane) with a variety of aryl bromides and aryl chlorides, including 2-bromopyridine giving the arylated product in high yields (Scheme 1.36) (see Experimental Procedure below). 

In 2005, Saino et al. [38] reported an Fe-catalyzed [2-I-2-I-2] cyclotrimerization using the 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) Al-heterocyclic carbene (NHC) ligand (see Chapters 1 and 2) (Scheme 9.17). The IPr ligand was shown to be more efficient when compared with the IMes ligand. 

In spite of the higher stability of imidazole-2-ylidenes and their metal complexes, tautomerization of imidazoles to the corresponding NHCs are also scarce processes. The relative stability of imidazole N- or C-metal bound isomers was computationally studied by Crabtree and Einsentein, who found a strong dependence on the nature of the metal fragment. Sundberg et al. published in 1974 the first example of N- to C-tautomerization of an imidazole ligand mediated by a Ru(II) complex (Scheme 29), but an acidic media was needed and very low yields were obtained. 

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