NHCs template synthesis

Figure 1.12 Transition metal NHC complexes via thermolysis of electron-rich olefins or isocyanide template synthesis.

In contrast to the methods discussed so far, Hashmi et al, based on Hahn s and Fehlhammer s linear template syntheses of NHC ligands, developed a convergent modular template synthesis of NAC-Au(I) and NHC-Au(I) chlorides [14-16]. ... 

The first starts from common NHC precursor compounds, such as imidazolium salts, in contrast to the other route in which the NHC unit is formed via template synthesis starting from a gold isonitrile complex. 

Scheme 3.6 Templated synthesis of a diprotic benzannulated NHC-ruthenium complex.

Certain transition metal complexes can serve as templates for the synthesis of chelating NHC ligands. For example, 1-phenylphosphole complexes of pal-ladium(II) are attacked in a Diels-Alder reaction by 1-vinylimidazole. If 1,2-dichloroethane is used as the solvent the imidazole is alkylated in situ and then subjected to a spontaneous carbometallation reaction [Eq. (37)]. 

Scheme 10 A tetradentate NHC ligand build up by template-controlled synthesis...

Although arguably the most popular route to NHC is via the azolium salt, there are many other options for their synthesis. An excellent and comprehensive overview of the synthesis of NHC and some of their complexes can be found in Hahn [34]. An interesting route to tri- and tetracarbenes is provided by the template controlled cyclisation of isocyanides, a method pionered by Lappert and coworkers [35,36] (see Fignre 1.2). 

Apart from providing hydrogen bond donors, coordinated protic NHC ligands can be deprotonated at the ring-nitrogen atom followed by reaction with an electrophile. Thus, complexes of types B and C are easily mono- or dialkylated. Such alkylation reactions can be utilized for the template-controlled synthesis of linear or macrocyclic homodonor (E) or heterodonor (F) ligands (Scheme 9.1) [15d]. 

Much simpler than the template-controlled generation of /3-functionalized isocyanides is their direct use in the synthesis of NHC complexes. Hydroxyalkylisocyanides such as 2-hydroxyethyl isocyanide are stable molecules but they are also known to become activated on contact with selected metal ions where they isomerizes to Al,0-heterocycles. FehUiammer et al. [40]... 

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