Metal-carbene complexes NHCs

Given the success of the Grubbs-type NHC-Ru catalysts in metathesis polymerisation (Chapter 3), it is somewhat surprising that more research has not been done on mid-transition metal carbene complexes for coordination-insertion polymerisation. At this stage however, there are only a few reported attempts with the metals Co, Fe and Ir. 

This article presents the principles known so far for the synthesis of metal complexes containing stable carbenes, including the preparation of the relevant carbene precursors. The use of some of these compounds in transition-metal-catalyzed reactions is discussed mainly for ruthenium-catalyzed olefin metathesis and palladium-Znickel-catalyzed coupling reactions of aryl halides, but other reactions will be touched upon as well. Chapters about the properties of metal- carbene complexes, their applications in materials science and medicinal chemistry, and their role in bioinorganic chemistry round the survey off. The focus of this review is on ZV-heterocyclic carbenes, in the following abbreviated as NHC and NHCs, respectively. 

There are essentially three different types of transition metal carbene complexes featuring three different types of carbene ligands. They have all been named after their first discoverers Fischer carbenes [27-29], Schrock carbenes [30,31] and WanzUck-Arduengo carbenes (see Figure 1.1). The latter, also known as N-heterocycUc carbenes (NHC), should actually be named after three people Ofele [2] and Wanzlick [3], who independently synthesised their first transition metal complexes in 1968, and Arduengo [1] who reported the first free and stable NHC in 1991. Fischer carbene complexes have an electrophilic carbene carbon atom [32] that can be attacked by a Lewis base. The Schrock carbene complex has a reversed reactivity. The Schrock carbene complex is usually employed in olefin metathesis (Grubbs catalyst) or as an alternative to phosphorus ylides in the Wittig reaction [33]. 

Deprotonation of an azolium salt with a strong base renders the free carbene which can then be reacted with a suitable transition metal complex to yield the transition metal carbene complex. In many cases, the NHC is not isolated, but prepared in situ prior to adding the metal complex. 

Linker-free oxazoline functionalised NHC can be synthesised using a modular approach. In this modular approach, a standard aUcyl or aryl substituted imidazole (module 1) is reacted with a 2-halo substituted oxazoline (module 2) to form the corresponding oxazoline functionalised imidazolium salt (see Figure 3.25). In a second step, the oxazoline functionalised NHC can then be reacted with a suitable metal precursor to form the corresponding transition metal carbene complex [93,98,102] (see Figure 3.26). 

This approach makes the synthesis of neutral zwitterionic transition metal carbene complexes possible, with valuable advantages over cationic complexes [230]. The concept was previously applied to N, P and S ligands [231,232] and has a carbene precursor in Roesler s backbone boron substituted NHC ligands [233] the latter has not been coordinated to transition metals yet. 

Note Kuhn s thione method to prepare free NHC can be amended to produce 2-chloro-imidazolium salts that can in turn be used to synthesise transition metal carbene complexes by oxidative addition. 

The additional adamantyl substituent on the phenol moiety was introduced by an acid catalysed reaction with 1-adamantol prior to the reduction step. The significance of this ligand is that it stabilises a Pd-alkyl group cis to the NHC ligand in a palladium(II) carbene complex. These transition metal carbene complexes with a cis alkyl ligand are still rare... 

Moore et al. [274] introduced a sulfonato functionality into a NHC wingtip group to make the resulting transition metal carbene complexes water soluble. Introduction of the functional group was achieved in a modification of the epoxide method previously used by Arnold et al. [33] and Glas et al. [12] for the synthesis of alkoxide functionalised carbenes (see Section 4.1). Reaction of an N-substituted imidazole with... 

Disadvantage Usually forms weaker transition metal carbene complexes compared with unsaturated NHC with chiral wingtip groups. 

There are no examples of isolated and characterised stable benzothiazol-2-ylidenes found in the literature. That is not surprising since benzo-annulation decreases the stability of the free NHC [38] and thus facilitates dimerisation. As the nonannulated thiazole system is already prone to dimerisation, benzothiazol-2-ylidene can be expected to require very considerable steric shielding to make the isolation of the monomeric NHC possible. Thus, benzothiazoUum salts are mainly used to synthesise the respective transition metal carbene complexes rather than the free ligands. 

The chemistry of transition metal carbene complexes with NHC derived from purines or xanthines has its roots in the synthesis of xanthinium betaines [75], the xanthine analogues of imidazolium salts. The synthesis is sttaightforward and involves the methylation of the xanthine with methyl tosylate (see Figure 6.32). 

In the early 1960s, Wanzlick published the first investigations concerning the chemistry of NHCs [19]. In 1968, the synthesis of first stable crystalline transition metal-carbene complexes based on imidazolium salts with mercury(II) and chromium(O), respectively, were published [20, 21]. In the following years, NHCs... 

Simple addition of a NHC to yield an early transition metal carbene complex in a high oxidation state was applied to synthesize [(IMes)VCl3(0)] 15 (Figure 6.2). The NHC ligand conferred a high stability to the complex, and while other tri-chloro-oxo-vanadium(V) adducts e.g. [VCl3(0)(MeCl ]) were easily hydrolyzed in air, compound 15 was air-stable over months even in a dichloromethane solution. A major contribution to this stability may arise from the highly unusual Cl-Ccarbene interactions determined from the crystal structure (vide infra). 

Another type of metal-carbene complexes has recently attracted much attention. These have N-heterocyclic carbenes (NHC) as ligands. 

AT-heterocyclic carbenes show a pure donor nature. Comparing them to other monodentate ligands such as phosphines and amines on several metal-carbonyl complexes showed the significantly increased donor capacity relative to phosphines, even to trialkylphosphines, while the 7r-acceptor capability of the NHCs is in the order of those of nitriles and pyridine [29]. This was used to synthesize the metathesis catalysts discussed in the next section. Experimental evidence comes from the fact that it has been shown for several metals that an exchange of phosphines versus NHCs proceeds rapidly and without the need of an excess quantity of the NHC. X-ray structures of the NHC complexes show exceptionally long metal-carbon bonds indicating a different type of bond compared to the Schrock-type carbene double bond. As a result, the reactivity of these NHC complexes is also unique. They are relatively resistant towards an attack by nucleophiles and electrophiles at the divalent carbon atom. 

The metal-carbene bond distances in this family of complexes (2.082 (2) A for Ag, 1.9124 (16) A for Cu, and 2.035 (12) A for Au) are within the range of reported values for typical group 11 metal NHC complexes (23). The metal carbene units are almost linear, with a C-M-C bond angle of 178.56 (13)°, 177.70 (9)°, and 177.7 (6)° for Ag, Cu, and Au, respectively. The imidazole units for 2 -Ag, 2 -Cu, 2Me Au exhibit structural parameters typically observed for coordinated NHC ligands. There are no inter- or intramolecular metal-metal interactions in these complexes. 

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