Abnormal NHCs

Theoretical calculahons suggested that the normal binding is thermodynamically favored when the anion-free complexes are considered, but ion-pairing has 

Formation of the reaction products, in the case of the methylene linker, was rationalized by means of density functional theory (DFT) calculations with the inclusion of a solvent effects polarized continuous model (PCM). The calculations 

---

Fig. 2.5 Palladium-based hydrogenation catalysts with bidentate normal and abnormal NHC ligands...

Fig. 19 Synthesis of complexes with abnormal NHC ligands by oxidative addition...

Analogues of imidazolylidene bearing two adjacent boron atoms in the heterocycle (3) have been synthesized and their coordination chemistry has been studied.16 They were shown to be better er-donors than their classical counterparts. The imidazo[l,5-ajpyridine has been shown to be a modular base for new stable NHCs (4).17 The same platform also allowed the isolation of abnormal NHCs (5) with a mesoionic structure and a strong a-donor character. 

Figure 7.10 Schematic representation of the tautomers of N-heterocyclic carbenes. (a) normal NHC (imidazol-2-ylidene) NHC(n). (b) abnormal NHC (imidazol-4-ylidene) NHC(a). (c) Imidazol NHC(I).

A last example to be mentioned here is the case of peculiar abnormal NHC (143) in which the carbene centre is no longer located between the nitrogen atoms but between nitrogen and carbon atoms. Compound (143) has been established as an efficient organocatalyst for ring-opening polymerization of cyclic esters.This report represents the first use of an abnormal NHC as catalyst in an organic transformation. 

During a study examining the electronic effect of abnormal NHC coordination on the properties of Pd complexes, Alhrecht and co-workers observed the apparent occurrence of an NHC-NHC reductive elimination reaction to form a bis(imidazolium) salt. Reaction of a Pd°-bis(NHC) complex with molecular chlorine in acetonitrile led to the formation of the bis(imidazolium) salt (containing a Pd°-based anion) (Fig. 11), which could be isolated in moderate yields. The product could arise as a result of the initial oxidative addition of the chlorine to the Pd°-bis(NHC) complex to form a Pd" intermediate, which then decomposes via an NHC-NHC reductive elimination reaction to form the bis(imidazolium) salt product. 

The first discussed example was reported by Bertrand and coworkers. By using different iminium salts they reported the coordination of an array of different abnormal NHC ligands, so-called CAAC ligands, to an AuQ fragment. Here, depending on the bulkiness of the free carbene used, the formation of mono- or biscarbene gold complexes is observed [12]. The synthesis of the most known representative of this class of Au(I) complexes in gold catalysis is shown in Scheme 9.3. The unique thermal stabiUty of this NHC-Au(I) complex is discussed in more detail in Section 9.3. 

In addition to these findings, the most remarkable example of the improvement of the stability of a gold catalyst was reported by Bertrand and coworkers. In their studies of intermolecular hydroamination reactions of allenes and alkynes, which were either conducted with ammonia or hydrazine, they showed that their gold catalysts based on abnormal NHC ligands have amazing thermal stability [38]. Due to the fact that the amines used are capable of coordinating to the open side of the catalytic species and therefore can block the catalyst, these reactions require high reaction temperatures. Therefore, the reactions are carried out at temperatures of 160-200 °C (see Scheme 9.19) [38]. 

Treatment of ItBu or IPr with M-butyllithium led to the generation of a lithium salt of the NHC with the negative charge at the C5 position. Addition of tris (pentafluorophenyl)borane in an attempt to generate a frustrated Lewis pair [25,26] led instead to the formation of a crystallographically characterized abnormal NHC-borate adduct with Uthium coordinated to 2 equivalents of THF at the C2 position of the NHC, as shown in Figure 15.6 [27]. 

Tamm and coworkers [98] overcame the tendency for this FLP to form the abnormal NHC-borane adduct through the C4 carbon by using a saturated analog of the NHC. The same hydrogen activation and THF ring-opening reactions were observed for the imidazolidin-2-ylidene as shown in Scheme 15.6. 

Later, Tamm and coworkers investigated the combination of l,3-bis(di-tert-butylphenyl)imidazolin-2-ylidene with a new the fluorinated borane, tris[3,5-bis (trifluoromethyl)phenyl]borane (B(w-XyFg)3), which offers thermally induced FLP reactivity with, for example, CO2, C—H bonds, and H2. Again, however, this FLP tends to form abnormal NHC-borane adducts [100]. The number of FLP combinations involving carbenes is constantly growing and there are a number that have not been included here. Tamm and coworkers have recently published a detailed review specifically of NHCs in FLP chemistry [101]. 

The first NHC-AlMes adduct was prepared by Robinson and coworkers in 1996 [133]. Interestingly, a similar adduct, prepared with IfBu in 2010, was found to isomerize in THF or toluene solution to form the abnormal NHC-AlMes adduct coordinated through the C4 position. Decomposition into unknown products was subsequently observed in dichloromethane. Addition of excess trimethylaluminum resulted in the generation of an imidazolium salt with a trinuclear aluminate anion (Scheme 15.12) [134]. 

Scheme 15.12 Reaction of IfBu-AIMea to generate either the "abnormal" NHC-AIMes adduct (right) or an imidazolium aluminate salt (left) [134],...

The formation of the products 15 and 16 obviously involved complex, multistep reactions, and proposing a meaningfijl mechanistic sequence would require additional work. Nevertheless, the outlined results demonstrate several points, including the deprotonation of N-alkylimidazole at C5 and subsequent formation of abnormal NHCs, the formation of alkoxycarbenes via intramolecular attack of a C2-deprotonated N-alkylimidazole ligand to a CO coHgand, the CO activation by the attack of two C2-deprotonated N-alkyhmidazole ligands, and the dramatic effect of the employed strong base on the nature of the products. 

Particularly interesting new work includes the development of remote and abnormal NHCs (see Chapter 5 for further details). Also, NHCs have not only been used as ligands in transition metal chemistry but also as nucleophilic organocatalysts. In an extension of this concept, Lavallo and Grubbs presented recently the first organometallic transformation catalyzed by NHCs. This illustrate nicely that almost 20 years after Arduengo s first report on an N-heterocyclic carbene, new NHCs as well as new applications for these interesting and versatile molecules are still and will be for some time the subject of intensive research. 

The increased donor ability of abnormally bound imidazolylidenes increased the nucleophilicity of the metal centre. Abnormal NHC-palladium complexes were thus shown reactive towards Lewis acids. When the abnormal NHC complex 15 was treated with AgBp4, the adduct 18 was formed while normal carbene complexes underwent the expected halide abstraction to form 17 (Scheme 5.5). Crystallographic analysis revealed short Ag- -Pd distances of 2.8701 A, suggesting a strong metal-metal interaction. Theoretical calculations indicated that the palladium centre acted as a Lewis base in this adduet, despite its formal dipositive charge. No such adduct formation was observed with analogous normal NHC-palladium complexes. 

The reduction of compound 57 or its samarium analogue 58 by potassium naphtalenide alforded the bimetallic dimers 59 and 60 with K-bound abnormal NHCs, which also bound to yttrium or samarium via the regioselectively deprotonated backbone (Scheme 6.7). Reaction of 58 with potassium-intercalated graphite (KCg) in dimethoxyethane (DME) yielded the methoxy-bridged dimer 61 as a product of DME cleavage. Quenching the yttrium dimer... 

NHC)Ru(p-cymene)] complexes (NHC = imidazol-2-ylidene, imidazolin-4-ylidene or pyrazolin-2-ylidene) were also used as catalysts for the p-alkylation of secondary alcohols in the presence of KOH, in refluxing toluene. Notably, abnormal NHC complexes were better performing than normal [(IMe)Ru-Cl2(p-cymene)] 75. For further details the reader is referred to Chapter 5. 

Zuo W, Braunstein P. N-Heterocyclic dicarbene iridium(III) pincer complexes featuring mixed NHC/abnormal NHC ligands and their application in the transfer dehydrogenation of cyclooctane. Organometallics. 2012 31 2606-2615. 

The organometallic chemistry of abnormal carbenes has developed continuously since the previous edition of this book, with a major focus on complexes containing either abnormal imidazolylidenes or abnormal 1,2,3-triazolylidenes. The latter class of complexes has been reviewed recently." Herein, we will cover highlights and representative recent advances in this area. Given the common features of the different subclasses of abnormal carbenes, they have been grouped together into sections focusing sequentially on the synthesis, stability and reactivity, and catalytic application of such abnormal NHC complexes. 

---