Normal carbenes

Once formed by this process, the carbene may undergo any of the normal carbene reactions (see p. 250). When the net result is substitution, this mechanism has been called the SnIcB (for conjugate base) mechanism. Though the slow step is an SnI step, the reaction is second order first order in substrate and first order in base. 

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]. 

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. 

From the three components, the 2-phosphino substituted imidazolium salt cannot serve as a normal carbene ligand, but both the amino functionalised imidazoUum salt (given the role as the base) and the nonfunctionalised imidazolium salt can act as carbene ligands and would enhance the catalytic activity of the system, if they did [149]. Furthermore, the amino functionalised imidazolium salt would stabilise the Pd(0) species due to its hemila-bile behaviour. One should use these ionic liquid systans with caution. 

Various terminologies have been used for specific subclasses of non-classical carbenes normal carbenes include all carbenes that can be represented by a neutral canonical resonance form and abnormal carbenes those for which a valence bond representation requires the introduction of formal charges on some nuclei (for example, C4-bound imidazolylidenes. Scheme 5.1)." Carbenes with no heteroatom in the position a to the carbene carbon atom are denoted remote carbenes. Hence, remote carbenes can be either normal or abnormal (E or I respectively. Figure 5.1). 

Figure 5.1 Metal complexes comprising the classical imidazol-2-ylidene ligand (A) and representative non-classical carbene ligands (B-N), including normal carbenes (B-E), abnormal carbenes (F-I), remote carbenes (E, G, I), cyclic alkyl(amino)carbenes (J), acyclic carbenes (K, L, M) and amino(ylide)-carbenes (N). Substituted nitrogen centres may be replaced by oxygen or sulfur. The M=C bond representation— while strongly over-emphasizing the differences in the nature of the metal-carbon bond in these non-classical carbene complexes— was used to accentuate normal and abnormal bonding.

Oxidative addition provided an alternative method towards the synthesis of non-classical carbene complexes (Scheme 5.3), and also avoided protection protocols in order to specifically inhibit normal carbene formation. This methodology was developed for the metalation of non-classical imidazolyli-dene, pyrazolylidene, and pyridylidene ligand precursors as well as for CAACs and allowed for installing various metals including molybdenum and the metals of the group 10. ... 

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. 

In a similar study, the catalytic activity of the classical carbene complex 31 and its abnormal thiazolylidene analogues 32 and 33, obtained by oxidative addition, were tested in the Suzuki coupling of activated aryl bromides (Figure 5.9). At 70 °C, the catalytic activity decreased in the order 31 >32 >33. The abnormal thiazolylidene complexes were thus less active than their normal counterpart. It is worth noting that the steric impact of the ligand in the normal carbene... 

The statements abnormal and mesoionic carbenes (aNHCs and MICs) are synonyms and are referred to free carbenes for which it is not possible to draw a structure resonance formula without formal charges (Fig. 6). Differ-endy, remote carbenes (rNHCs) are carbenes without any heteroatom in the a and positions of the carbene center. These three types of ligands are characterized by a different structure with respect to the one usuaUy owned by NHCs. In the case of normal carbenes (nNHCs), the carbene donor is stabilized by the presence in the a and a positions of two heteroatoms. To emphasize this difference, aNHCs, MICs, and rNHCs are also caUed... 

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