NHC features

The chemistry of A-heterocyclic carbenes (NHCs) features strongly in the section on nucleophilic carbenes. In keeping with this interest, the chemistry and reactivity of these species have been reviewed.7 Their reactivity towards small molecules and... 

Shih and coworkers prepared adducts of several organoaluminum compounds with an NHC featuring a pendant JV-t-butylethylamine branch. The resulting complexes were found to undergo insertion reactions with benzaldehyde, with the carbonyl carbon inserting between the C—A1 bond [29]. These trimethylaluminum complexes have also been used in conjunction with [Ni(cod)2] to facilitate the meta- and j ara-alkenylation of pyridine (with 4-octyne) [135]. These complexes were compared to the corresponding triphenylborane adduct and found to be generally more reactive and more prone to react with imsaturated substrates [136]. 

Aside from the ubiquitous NHCs featuring a five-membered heterocycle, significant advances have been made regarding the preparation of NHCs derived from larger heterocycles. The first stable NHC featuring a six-membered aliphatic heterocycle 132a was prepared in 1999 " followed by a number of differently A,A -substituted derivatives. ° Aromatic derivatives 133a are also... 

NHCs featuring an aliphatic seven-membered ring scaffold of type 132b68c,69,i98 obtained by deprotonation of amidinium salts with... 

The characteristic C NMR resonances for the carbene carbon atoms of the NHCs featuring six-membered heterocycles (132a, 133a) fall in the range 5 = 235-245 ppm, similarly to imidazoKn-2-ylidenes 86, whereas the carbene carbon resonances for 132b and 133b appeared more downfield ( = 258-260 ppm). =... 

At this time a large number of NHCs featuring different ring sizes, heteroatoms and substitution patterns are accessible. Still, reports on new NHCs appear regularly in the Kterature. As diverse as the topology of NHCs is their stereochemical and electronic properties. " Different, sometimes surprisingly... 

NHCs featuring an aliphatic seven- or eight-membered membered ring scaffold of type or 147c were also obtained by deprotonation... 

The reactivity of A-heterocyclic carbenes (NHC) with iron has been presented through the various types of complexes that have been described. Their catalytic applications in cross-coupling, allylation, aziridination, hydrosilylation, and as hydrogenase mimics are detailed. A report on cross-coupling reactions catalysed by complexes of iron group metals with NHC features the reaction mechanisms. ... 

Also the novel antifungal antibiotic (-)-PF1163B (211), isolated from Strep-tomyces sp., which features a 13-membered macrocycle incorporating both a lactone and a lactam unit, was synthesized by an RCM route (Scheme 42) [101]. While only poor results were obtained by treatment of diene 210 (containing 8% of an unidentified epimer) with catalyst A, the use of NHC catalyst C led, under the conditions outlined in the scheme, to the corresponding cyclization product in 60% yield along with 10% of a diastereomer resulting from epimer-ization in a previous step. 

As depicted above, there is a large variety of NHCs, and their access is relatively easy. This is a veritable advantage in the use of NHCs for homogeneous catalytic systems. However, it is probably their unique electronic and steric properties that make NHCs an exceptional class of molecules. These unique features are described in the following sections. 

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]. 

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]. 

Catalysts with an unsymmetrical NHC ligand featuring a vinylic side chain have the unique ability to metathesise their own ligand to form a metaUacycle as shown in Scheme 3.7 [119], Ring opening metathesis will then incorporate the monomers, e.g. cyclooctene, into that cycle until a cyclic polymer is cleaved by another intramolecular metathesis step. The catalyst is recovered and can restart this endless route to cyclic polymers [121]. 

Allyl-Pd(ll) complexes featuring a single NHC ligand have been explored for norbomene polymerisation. The first such disclosure appears in a patent of 2006 [39] in which complexes of the form 34 (Fig. 4.12) are activated with AgSbF and LiB(CgFj). The resulting cationic complexes catalyse the polymerisation of norbomene and polar-substituted norbomenes with very high activity. The best result... 

Because of the different kinds of enzyme, several synthetic models have been proposed which can be evaluated with regard to their composition and structure or to particular features of the enzymatic function. One of the most successful, proposed by Crabtree and co-workers some years ago,416 involves the ionic complex [NiL2]Cl2 (144 L = 2-HOC6H4CH = N-NHC(S)NH2). 

Although not a comprehensive treatise on Ir—NHC complexes, this chapter first describes the different types of Ir—NHC described in the literature, with particular attention to their reactivity and special chemical features. Later in the chapter the most relevant catalytic applications of this type of complex are described, mainly in the field of C—H activation. The general atom-numbering scheme for NHCs, which is used throughout the text, is shown in Figure 3.2. 

Five-coordinate complexes of the type shown below have the attrachve feature of containing four alkene ligands which, in principle, are capable of being removed by hydrogenation to reveal an Ir(NHC) cation. Although these, too, are active for transfer hydrogenahon, in related complexes the allyl substituents do not seem to be hydrogenated [42]. 

Plumbene-NHC complex 36 is generated by the reaction of an NHC with a bis(aryl)-lead(II) compound. The NHC-silylene adduct 37 also features a long C-Si bond with significant Si polarity. Pentacoordinated silicon(lV) and... 

Carbenes [74-76], and in particular W-heterocyclic carbenes (NHCs), are today the topics of very intense research [43 8]. Carbenes were originally considered as chemical curiosities before being introduced by Doering in organic chemistry in the 1950s [77], and by Fischer in organometallic chemistry in 1964 [5]. Eater, it was shown that the stability of carbenes could be dramatically enhanced by the presence of heteroatom substituents. After the discovery of the first stable carbene, a (phos-phino)(silyl)carbene, by Bertrand et al. in 1988 [78], a variety of stable acyclic and cyclic carbenes have been prepared. With the exception of bis(amino)cycloprope-nylidenes [79], all these carbenes feature at least one amino or phosphino group directly bonded to the electron-deficient carbenic center. 

Important features favoring the reductive elimination reaction have been discussed based on theoretical and experimental studies [128-130]. The reductive elimination of an azohum salt from a palladium NHC alkyl complex (Fig. 13) proceeds under direct formation of Pd° in an exothermic process with a low activation barrier [128]. The coligands at the palladium atom play an important role. It has been shown that the Caikyi-Pd-CNHC angle becomes more acute during the reductive elimination to allow for an optimal orbital overlap of the groups to be... 

The push-spectator stabilization system enables one to employ various alkyl groups with different types of steric environment, which differentiate amino(alkyl) carbenes dramatically from the NHCs as ligands. Taking advantage of their steric and electronic properties, Bertrand et al. nicely demonstrated the utility of CAACs as ligands in the palladium catalyzed a-arylation of ketones. Depending on the nature of the aryl chloride used, dramatic differences were observed in the catalytic activity of Pd-complexes with CAACs featuring different types of steric environment [36]. 

We reported the use of M-heterocyclic carbene complexes (NHC) for the catalytic activation of methane [55,56]. We found that solutions of N-heterocyclic carbene complexes of palladium(II) in carboxylic acids catalyze the conversion of methane to the corresponding methylesters. The high thermal stability of palladium(II) carbene complexes could be shown for complex 18 (Scheme 22), which we also structurally characterized [120]. An extraordinary feature is the unprecedented resistance of the palladium-NHC-complexes 18-22 under the acidic oxidizing conditions which are necessary for the CH-activation and functionalization. 

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