Synthesis of Tebbes Reagent

N-Heterocyclic carbene complexes can be generated from the stable, free carbenes or by reaction of carbene precursors. Three examples of the s)mthesis of N-heterocyclic carbene complexes from carbene precursors are shown in Equations 13.14-13.16. The reaction of an imidazolium salt with a complex containing a basic ligand forms N-heterocydic carbene complexes (Equation 13.14), as does transfer of the carbene from a silver-carbene complex (Equation 13.15), - or reaction of an imidazolium-2-carboxylate, with accompanying decarboxylation (Equation 13.16).  

Dihydroimidazolium carbene precursors or saturated N-heterocyclic carbene precursors are generated by a related sequence that includes reaction of a diamine, often generated by reduction of the glyoxaldimine, with triethylorthoformate, in the presence of acid. This reaction generates the dihydroimidazolium salts, wliich can be deprotonated to form the free carbene or used directly in the synthesis of carbene complexes. Finally, the acid-catalyzed reaction of the diamine with formaldehyde, followed by iodine and base, generates the free carbene. The silver complexes are typically generated by reaction of the protonated carbenes with Ag O.  

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