Abnormal NHC complexes

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. 

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. 

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. 

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

Interestingly, Crabtree and coworkers found that abnormal binding of NHCs is also possible in Ag - NHC complexes when the C-2 position of the initial imidazolium salt is blocked by a phenyl group. Transmetallation to [IrCl(cod)]2 affords stable abnormal Ir-NHC complexes when bulky substituents are introduced in the C-4 position, also protecting the Ir(I) complex from decomposition through protonolysis (Scheme 37) [125]. 

Treatment of the yttrium(III) adduct 60 with potassium naphthalenide in dme-diethyl ether mixture results in deprotonation of the C4 carbon and migration to afford the abnormal carbene complex 63 (Fig. 13).72 The C2 binding carbon migrates from the yttrium(III) centre to the incorporated potassium(I) cation. The C4 carbanion forms a short bond with the yttrium(III) centre in the solid state (2.447(2) A) and exhibits a large jYc coupling constant of 62 Hz in solution. Complex 63 may be quenched with a variety of electrophiles. For example, reaction with Me3SiCl silylates the NHC backbone to afford 64. 

The abnormal carbene complex 27 (bonded through C3) is formed from the reaction between M3(CO)i2 (M = Ru, Os) and the bulky NHC ImAd2 (l,3-di(adamantyl)imidazol-2-ylidene) the reaction with the ruthenium precursor occurs readily in thf at room temperature, whereas the osmium reaction requires heating at 70 °C. Thermolysis of 27 affords 28.30... 

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. 

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. 

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. 

As a versatile alternative to the generation of free carbenes, Hashmi and co-workers developed a methodology for the synthesis of abnormal NHCs where the carbene is constructed directly at the metal center. Thus, a [3 + 2] dipolar cycloaddition of azomethine ylides and gold isonitriles afforded the abnormal saturated imidazolinylidene gold eomplexes 9 [eqn (3.3)]. This methodology circumvents the regioseleetivity issue of C4 vs. C5 eoordination because the carbenic carbon is pre-coordinated to the metal center in the isonitrile precursor. Moreover, this procedure provided aeeess to a new subclass of abnormal carbenes and yielded the first abnormal earbene complexes featuring a saturated heteroeyele, unlike all previous abnormal imidazolylidene complexes, derived from unsaturated heteroeyeles. 

Other interesting examples arise from the tautomerization of normal NHCs to form abnormal NHCs, a process that is generally assumed to be driven by the release of steric strain. For example, Dagorne and co-workers observed the tautomerization of complex 10 to form abnormal carbene complex 11, which contains only one bulky t-Bu group adjacent to the carbenic carbon (Scheme 3.2). Complex 11 constitutes a rare example of an abnormal NHC main group complex (see Chapter 5 for further examples). While the mechanism of this tautomerization remains elusive, the authors noted that the process is fast in a coordinating solvent such as THF (minutes), and slower in toluene [ca. 20 hours), which hints at a dissociation of the free carbene and Cnhc A1 bond cleavage. 

The propensity of bulky carbenes to tautomerize was used by Hevia and co-workers as an alternative methodology for the synthesis of abnormal NHC-gallium complexes. When starting from the IMes complex, isomerization was slightly endothermic, whereas bulkier carbenes such as IPr and ItBu induced formation of the abnormal carbene complexes, which were thermodynamically favored by 1.5 and 6.6 kcal mol", respectively. This observation further supports steric arguments as the major driving force for tautomerization and formation of the abnormal carbene isomer. 

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