1.3- Dimethyl imidazol-2-ylidene

A one-step construction of y,y-difunctionalized y-butyrolactones from benzoins or benzaldehydes via a tandem reaction promoted by l,3-dimethyl-imidazol-2-ylidene in the presence of methyl acrylate was reported by Zhai et al. [63]. So far, the exact mechanism of the reaction, as well as the role of the catalyst, has not been clarified. 

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). 

Some 30 years later, Wanzlick s work was reinvestigated by Arduengo et al., who were able to prepare and to isolate the imidazol-2-ylidene IVc (R = adamantyl) in near quantitative yield (Scheme 8.4). The deprotonation of the 1,3-di-l-adamantylimidazolium chloride (4c) was carried out with sodium or potassium hydride, in the presence of catalytic amounts of either f-BuOK or the dimethyl sulfoxide (DMSO) anion. Carbene IVc is thermally stable in the solid state (colorless crystals, mp 240-241 °C). 

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

By contrast, 1,2,4-triazole carbene 354 displays electrophilic character. Thus, it reacts with alcohols and amines producing triazoline derivatives 355 in quantitative yields. Oxygen or sulfur gives triazolinone and triazolinethione derivatives 356 (similar reaction with tellurium is known for imidazol-2-ylidenes). Reactions of 354 with dimethyl maleate or dimethyl fumarate lead to compounds 357, probably via ring opening of a cyclopropane intermediate with subsequent 1,2-hydrogen shift. 

The complex [Y(L) N(SiMe3)2 2] (L = l-f t7-butyl-3-(2-fi t7-butylaminoethyl)imidazol-2-ylidene) showed a Cc(carbene) resonance at 186 ppm with a V -C(carbene) coupling of 54.7 Hz, larger than any Vyc value teported for an s-bound yttrium alkyl or an yttrium-NHC adduct <2003AGE5981>. In comparison, the J c value fot Y[N(SiHMe2)2]3(N,N -dimethyl-NHC) was 49.6 Hz <19970M682>. 

Two new N/O-functionalized NHC palladiiun complexes, fr s-[(l-ben2yl-3-(3,3-dimethyl-2-oxobutyl)2PdBr2] and cis-[(l-benzyl-2-oxobutyl)imidazole-2-ylidene)2PdBr2] (23 and 24, respectively) were reported by Ghosh and coworkers [35]. These complexes were effective catalysts for the Sonogashira cross-coupling of a range of aryl iodides with substituted acetylenes in air. 

The mononuclear complex 13 was also employed for carbene transfer reactions from ethyl diazoacetate (EDA) in a range of reactions that led to the alkenation of aldehydes, cyclopropanation, and carbene insertion into N—H and O—H bonds [32]. The complex proved particularly adept at the last process, especially aromatic amines and aliphatic alcohols. Addition of the PIN ligand (l-isopropyl-3-(5,7-dimethyl-l,8-naphthyrid-2-yl)imidazol-2-ylidene) to [Ru2(CO)4(OAc)2], foUowed by treatment with Na[BAr 4] gave the dinuclear complex [Ru2(PIN)2(CO)4][[BAr 4] 2, which showed some improved reactivity compared to 13, particularly in the transfer of CH(C02Et) to aldehydes [109]. 

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