Abnormal carbenes

By convention, the carbenes displayed in Fig. 1.7 are normal NHCs as they are coordinated to the metal centre throngh the atom. By contrast, abnormal (also named non-classical or unnsnal) are those bound through the atom. Abnormal is also a term used for NHCs having a valence representation requiring additional charges. Remote is a term nsed to describe a carbene which does not have any heteroatom on the a-position to the carbenic carbon (Fig. 1.8) [57]. 

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 abnormal metallation is also favored when the conjugated pyridine-imid-azolylidene precursor (with a smaller bite angle) is used. For these precursors, an abnormal binding is produced even with a small wingtip group, as shown in Scheme 3.12. Under the same conditions, the pyridine-benzimidazolium analogue afforded the C2 carbene complex (Scheme 3.12) [27]. 

In addition to complexes of type 44 with a C2 coordinated NHC ligand, complexes of type 45 with abnormal C4 or C5 bound carbene ligands have recently been described (Fig. 16) [143, 144]. The carbene carbon atom in these complexes is stabilized by only one nitrogen atom. A similar situation has been observed for the cyclic (alkyl)(amino)carbene (type 16, Fig. 6) [38, 39]. 

The formation of complexes with abnormal carbene ligands is controlled by steric, electronic, and kinetic effects as well as by the counter ion present in the azolium salt [145-148]. In selected cases the base used for the deprotonation of the azolium salt [149, 150] also plays a significant role. Crabtree et al. demonstrated in a detailed study that A-pyridyl functionalized imidazolium salts react with... 

Fig. 16 Complex with normal and abnormal bound carbene ligands...

In principle, this is a simple process we determine the causes of instability in the target molecule and eliminate them by appropriate transformations (substitution, complex-ation, etc.). Indeed many abnormal molecules with varied structures have now been made.32 For each problem, several solutions are possible. There is not enough space to illustrate every conceivable approach here, so we will merely consider four representative molecules norcaradiene (p. 226), cyclobutadiene, trimethylenemethane and carbene. 

For imidazolium salts 1, an alternative pathway with deprotonation and carbene generation at the C4/C5-position was observed previously the carbenes thus generated are called abnormal carbenes [79-81]. Likewise, suitably substituted imidazo[l,5-a]pyridinium salts can be deprotonated to mesoionic carbenes 10b and the corresponding silver, iridium and rhodium complexes were formed. 

Blocking the C2 position with alkyl groups may afford C - H oxidative additions of the imidazolium salts yielding abnormal carbenes. This strategy was followed in the reaction a Pt(0) complex with C2-methylated imidazolium salts, which provided the oxidative addition of the C4,5 - H bond, as shown in Scheme 42 [158]. This behavior provides evidence that the substitution of imidazolium-based ionic liquids at the C2 may not be enough to prevent their involvement in reactions for which they are solvents. 

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... 

Note The chelate carbene complex with normal carbene coordination (C ) has a smaller bite angle and is therefore more strained than the chelate carbene complex with abnormal carbene coordination (C )... 

Note The chelate carbene complexes with normal carbene coordination (O) do exist. They have even bulkier wingtip groups (tert-butyl instead of isopropyl) and a more crowded equatorial plane since the abnormally coordinated (C ) carbene chelate ligands are faced with sterically uniquely undemanding hydride ligands. 

Four years after Grundemann et al. s original observation [49], Peris and coworkers [40] proved that steric strain is not the decisive factor in the abnormal coordination of the carbene in these particular iridium(ni) hydride complexes. We may well suspect that differences in... 

The corresponding reactions with iridium(I) precursors again behave differently. When the phosphino functionalised imidazolium salt is reacted with [IrlcodlCl], the phosphane adduct with a pendant imidazolium moiety is formed. A similar reaction with the more reactive [Ir(cod)( Li-H)(p,-Cl)2]2 yields a five coordinate iridium(I) complex that might be described as having square pyramidal geometry with the bromide in apical position and the carbene in abnormal coordination mode [47-49] (see Figure 3.97). 

Within the same triad, iron has the smaller ionic radius compared with ruthenium [20], although electronically and structurally the two elements should form the same complexes. They essentially do, but when a second pincer carbene ligand is coordinated, it coordinates with one NHC moiety in abnormal coordination mode [476] as opposed to ruthenium, where both pincer carbene ligands are coordinated normally [468], In the absence of coordinating anions (BPh instead of bromide) octahedral cationic complexes are formed instead of square pyramidal neutral ones (see Figure 3.158). The additional n-donor ligand compensates the positive charge electronically. 

Note Steric constraints can result in the formation of abnormal carbene complexes [47,48]. 

Carbene complexes have been synthesized by various CH activation routes. For example, amines and ethers can undergo double geminal CH activation to yield a Fischer-type (heteroatom substituted) carbene and an imidazolium salt can yield iV-heterocyclic carbenes directly in the same way. In the latter case, abnormal binding via C-4 can occur as well as the usual C-2 binding. 

Direct SN2 displacement of CHC13 by PhO- has been dismissed, cf. Ref. 73a. Moreover, Coulombic repulsion between C13C and PhCU would hamper that interaction. Finally, C-H insertion of PhO by carbene-like CO, derived from the base-mediated hydrolysis of CC12, cannot be ruled out a priori, cf. Gattermann-Koch reaction (CO) and Kolbe-Schmitt reaction (C02). However, the presence of abnormal, ring-expanded aryl chlorides supports the involvement of CC12... 

These types are particularly involved whose one of the leaving groups is part of the chain. Thus, in the abnormal Wolff-Kishner reduction 4,5-dihydro-lH-pyrazoles are formed (e.g. 311), whose pyrolysis leads to [2,3,(4)I,(2)3(4)]-eliminations of nitrogen with-cyclopropane formation. In this way 311 forms 312 (75%)163). The three assistant figures are necessary in order to fix which chain-bonds migrate and where the second leaving group is located. Even the eliminations of carbenes (formally)... 

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