1.3- Diisopropyl-imidazol-2-ylidene

Hetero-[2- -2- -2] cycloaddition of CO2 to diynes 2.192a,b occurs at atmospheric pressure in the presence of bis(l,5-cyclooctadiene)nickel catalyst and N-heterocyclic carbene ligand IPr [IPr = l,3-bis(2,6-diisopropyl)imidazol-2-ylidene] 2.194. Pyrones 2.193a,b are formed in good yield (93% and 75%, respectively) (Scheme 2.67) [117]. 

McIPr l,3-diisopropyl-4,5-dimethyl-2,3-dihydro-lfl-imidazol-2-ylidene... 

Kuhn and Krantz in 1993 reported [35] that l,3-diisopropyl-4,5-dimet-hylimidazole-2-thione reacted with potassium metal to give the imidazol-2-ylidene (Scheme 5). 

A CO2 adduct of one imidazol-2-ylidene is known (l,3-diisopropyl-4,5-dimethylimidazolium-2-carboxylate). It is surprisingly stable thermally - it was purified by sublimation at 90 °C, [49] but thermolysis of 16, Scheme 6, still appears an attractive proposition. 

The (SIPr)Pd(cinnamyl)Cl (SlPr= l,3-bis(2,6-diisopropyl phenyl)-4,5-dihydro imidazol-2-ylidene) catalysed anaerobic oxidation of aryl/alkyl, aryl/aryl, and alkyl/ alkyl secondary alcohols to the corresponding ketones in high yields has been achieved using aryl chlorides in toluene with f-BuONa. Domino Pd-catalysed synthesis of a-arylated ketones from secondary aryl alcohols in very good yields was also reported. " ... 

In some cases the metal-NHC bond can be sensitive to water, with hydrolysis causing ring-opening of the NHC to give formamide. This was found to occur when [Cu(IPr)(OAc)2] (IPr = l,3-bis(2,6-diisopropyl-phenyl)imidazole-2-ylidene) was dissolved in wet chloroform, producing the corresponding formamide within hours (Fig. 21). ... 

A substantial improvement on the above-described class of Ru-vinyUdene complexes has been achieved by Louie and Grubbs through synthesis of a novel generation of complexes coordinating an A-heterocyclic carbene (NHC) ligand (e.g. imidazolylidene). This new set of Ru complexes, e.g. 15 (IMes = l,3-(2,4,6-trimethylphenyl)imidazoT2-ylidene, R = Cy, R = t-Bu) and 16 (iPrlM = l,3-diisopropyl-4,5-dimethyl-imidazol-2-ylidene, R = Cy, R = Ph) has been conveniently produced from the bisphosphane-Ru complex 9 (R = Cy) by reaction with free imidazoline carbene or its precursor salts (Scheme 7). 

In a related study, Hedrick, Waymouth and coworkers reported that NHCs were also able to act as catalysts for the ROP of trimethylene carbonate (target DP 50) [31]. The appHcation of l,3-bis(2,6diisopropylphenyl)-imidazol-2-ylidene resulted in >99% monomer conversion within 30 min, producing polymers with predictable molecular weights and a narrow PDl (1.06). However, the less-hindered and more basic l,3-diisopropyl-4,5-dimethylimidazol-2-ylidene produced polymers with much broader polydispersities (>2), albeit within a few seconds. 

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