Dihydroimidazol-2-ylidene

In 1991 Arduengo and co-workers42 reported the isolation and characterization of the first stable carbene, l,3-di(l-adamantyl)-2,3-dihydroimidazol-2-ylidene. Since that time many substituted nucleophilic carbenes have been prepared.43 From ab initio studies it can be shown that cyclic electron delocalization is partly responsible for the extraordinary stability of 2,3-dihydroimidazol-2-ylidenes.44... 

Scheme 7 Species involved in the intermolecular hydrogen transfer and dimerization of 2,3-dihydroimidazol-2-ylidene.

Dihydroimidazol-2-ylidenes and their main group element chemistry ... 

Second-generation ruthenium-carbene complex, (tricyclohexylphosphine-[l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene]ruthenium (IV) dichloride),4e was purchased from Strem Chemicals. 

Tricyclohexylphosphine[l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene][benzylidene]ruthenium... 

Apart from imidazol-2-ylidene carbenes, other electron-rich carbenes such as (L27)—generated by deprotonation of a 4,5-dihydroimidazole—have been tested for cross-coupling reactions, but gave much poorer results. The only reported exception is in the cross-coupling of arenediazonium salts with arylboronic acids, in which (L27) (used as hydrochloride) proved to be efficient.369... 

Synthesis and Activity of a New Generation of Ruthenium-Based Olefin Metathesis Catalysts Coordinated with 1,3-Dime-sityl-4,5-dihydroimidazol-2-ylidene Ligands, M. Scholl, S. Ding, C.W. Lee, et al, Org. Lett. 1999, 7, 953-956. 

There are two well-characterized examples of a naked carbon atom bound by a triple bond to a metal center (Fig 14.3.8). The molybdenum carbide anion [CMo N(R)Ar 3]- (R = C(CD3)2(CH3), Ar = C6H3Me2-3,5), an isoelectronic analog of NMo N(R)Ar 3, can be prepared in a multistep procedure via deprotonation of the d° methylidyne complex HCMo N(R)Ar 3. The Mo=C distance of 171.3(9) pm is at the low end of the known range for molybdenum-carbon multiple bonds. In the diamagnetic, air-stable terminal ruthenium carbide complex Ru(=C )C12(LL/)(L = L = PCy3, or L = PCy3 and L = l,3-dimesityl-4,5-dihydroimidazol-2-ylidene), the measured Ru-C distance of 165.0(2) pm is consistent with the existence of a very short Ru=C triple bond. 

Mono- and bicyclic phosphorus heterocycles 199, 200, 202, and 203 were synthesized starting from the bifunctional phosphorylating agent bis(diisopropylamino)ethynyl phosphine 195 via ring-closing enyne metathesis using 4,5-dihydroimidazol-2-ylidene ruthenium benzylidene complex, as a catalyst. Bicyclic phosphorus oxides 199 were obtained in 66-83% yield, whereas phosphorus borane derivative 202 was isolated in 74% yield <2001TL8231>. 

The Pd(OAc)2/SIPr HCl/NaOH system (where SIPr= l,3-bus(2,6-diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene) is efficient for the N-arylation of diverse indoles with aryl bromides (Equation 39) but not effective for the reaction with aryl chlorides <2001JOC7729>. Pd(dba)2 or PdCl2/SIPr-HCl/NaOH systems did not effect the arylation of indole. 

A new acyclic diene metathesis polymerization method has been developed using 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene)benzylidene mthenium(II) dichloride as catalyst. This reaction catalyst was used for preparing oligomers and polymers containing amino acids or polypeptides. 

A small flask was charged with 1,4-butanediol diacrylate (0.45 mmol) dissolved in 2ml of CH2CI2 then treated with l,3-dimesityl-4,5-dihydroimidazol-2-ylidene)ben-zylidene-tricyclopentyl-phosphine ruthenium(II) dichloride (2.7 mg) and cyclooctene (0.45 mmol), the total monomer-to-catalyst ratio being 290 1. The mixture was... 

Decenyl acetate was also prepared using a modified Grubbs catalyst (I) containing the l,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligand. Other synthetic applications of this catalyst are described (2). 

Scholl, M., Ding, S., Lee, C.W., and Grubbs, R.H. 1999. Synthesis and activity of a new generation of ruthenium-based olefin metathesis catalysts coordinated with l,3-dimesityl-4,5-dihydroimidazol-2-ylidene ligands. Org Lett 1, 953-956. 

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