Reaction with C2-functionalized Carbene Adducts

Another alternative to generate and handle the free carbene is to mask the C2-position of the imidazol-2-ylidene with various thermally labile groups, such that heating these adducts in the presence of transition metal precursors gives 

Grubbs and co-workers found that heating this methanol adduct in a toluene solution of [(PCy3)2(Cl)2Ru=CHPh] displaced one equivalent of tricyclohex-ylphosphine generating the Enders-NHC analogue of the famous Grubbs II catalyst This methodology was extended to saturated carbenes, which can be protected as methanol, tcrt-butanol or chloroform adducts.  

Even fluorinated aromatics such as can react with carbenes and 

Crabtree and co-workers have recently reported the use of imidazole-2-carboxylates as suitable precursors for the formation of a variety of transition metal NHC complexes. These air and moisture stable adducts can be prepared from iV-substituted imidazoles by reaction with dimethylcarbonate giving CO2 adducts.  

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