Imidazolinylidenes

Despite the planar conformation of nitrogens, several possibilities are available for the introduction of chirality. It is possible to prepare benzim-idazolylidenes, triazolylidenes, imidazolylidenes or unsubstituted-backbone imidazolinylidenes with a stereogenic center on one or two N-substituents (carbenes I and II). The other possibility is to relay the imidazolinylidenes backbone stereogenecity via the N-substituents or to combine stereogenic N-substituents with a chiral backbone (carbenes III or IV). It is at least possible to prepare bis-carbenes of type V with one (or two) stereogenic link between the two carbenes (Fig. 7). 

Lappert was the first to report, in 1983, the synthesis of chiral rhodium(I) and cobalt(I) imidazolinylidene complexes by heating an enantiopure electron rich olefin 3 in the presence of a complex precursor (Scheme 1) [9]. 

Since then, it has been observed that the dimerization of imidazolylidenes and triazolinylidenes does not occur at room temperature. In contrast, the dimerization of imidazolinylidenes is a fast and irreversible process but is prevented by using bulky nitrogen substituents. The benzimidazolylidenes are in equilibrium with the dimer, this equilibrium being affected by steric... 

In 2002, Hoveyda et al. reported the synthesis, structure and reactivity of a chiral bidentate Ru-based catalyst 65, bearing a binaphthyl moiety, for olefin metathesis [33]. Preference for a bidentate chiral imidazolinylidene was based on the hypothesis that such a ligand would induce chirality more efficiently. This catalyst was designed by analogy with similar achiral complexes 66 that... 

In 2004 and 2005, respectively, Bach and Miller independently described the use of chiral thiazolium salts as pre-catalysts for the enantioselective intramolecular Stetter reaction. Bach and co-workers employed an axially chiral A-arylthiazolium salt 109 to obtain chromanone 73 in 75% yield and 50% ee (Scheme 16) [77]. Miller and co-workers found that thiazolium salts embedded in a peptide backbone 65 could impart modest enantioselectivity on the intramolecular Stetter reaction [78]. In 2006, Tomioka reported a C -symmetric imidazolinylidene 112 that is also effective in the aliphatic Stetter reaction, providing three examples in moderate enantioselectivities (Scheme 17) [79]. 

D. Enders and H. Gielen, Synthesis of Chiral Triazolinylidene and Imidazolinylidene Transition Metal Complexes and First Application in Asymmetric Catalysis, J. Organomet. Chem. 2001, 617, 70. 

Imidazolinylidenes contain sp3-carbon atoms in the 4- and 5-position of the heterocycle and thus provide the possibility of a second strategy for the generation of chiral NHCs. By an appropriate choice of substituent (R) at the 4- and 5-position two (homo)chiral centers may be obtained and the chiral information then transmitted to the active space at the metal center of a catalyst by means of the two N-substituents R (Fig. 8). 

Table 1 1,4-addition of diethylzinc to cyclohexanone catalyzed by imidazolinylidene-copper complexes... 

Chiral imidazolinylidenes with N-aryl substituents have been employed by Grubbs and coworkers in the stereoselective ring closing metathesis of olefins [52]. The introduction of the aryl groups as N-substituents was achieved by a palladium catalyzed Buchwald-Hartwig coupling (Scheme 16) [53]. 

Chiral N-arylated imidazolinylidene ligands have been employed in the palladium(II) catalyzed aerobic oxidation of secondary alcohols to the corresponding ketones [55]. The chiral variant of this reaction, which does not generate a new element of chirality, is again based on the kinetic resolution of racemic mixtures. The active catalyst is formed in situ by a combination of two precursors, a dinuclear NHC-palladium(II) complex and an achiral (acetate) or chiral base ((-)-sparteine) (Scheme 18). 

A novel chiral bidentate imidazolinylidene ligand (26) has recently been developed in Helmchen s group [57]. It is related to Grubbs imidazolinyli-... 

In conclusion, while the first chiral imidazolinylidenes tested in asymmetric catalysis only gave a moderate chiral induction, more recent results have clearly indicated that the encoding of chiral information in the backbone of the heterocycle may give rise to highly efficient stereodirecting ligands. The potential versatility of this approach is apparent in a recent contribution... 

Scheme 23 Synthesis of non-symmetrical imidazolinylidenes reported by Hahn et al.

Using the chiral imidazolinylidenes derived from 54 this reaction could be carried out at lower temperatures than those required with the previously employed catalyst [Rh(acac)(C2H4)2]/BINAP (100 °C). This increase in activ-... 

K. Ofele, l,3-Dimethyl-4-imidazolinyliden-(2)-pentacarbonylchrom - Ein neuer Uber-gangsmetall-Carben-Komplex, J. Organomet. Chem. 12, P42-P43 (1968). 

An attempt to produce indenols via RCM reaction of the parent dienes, in the presence ofRuCl2(= CHPh)(imidazolinylidene)(PCy3) at 80 °C, actually led to the corresponding indenones [87]. Whereas the indenols are obtained at room temperature, it was shown that the alkene metathesis ruthenium catalyst is also responsible for the dehydrogenative oxidation of indenols at 80 °C (Scheme 41). 

It may be noted that Grubbs applied this synthetic method to imidazolinylidene Ru complexes. Componnd (14) was found efficient in releasing free SIMes upon heating at 80 °C. Alternatively, tert-bntylate SIMes adduct (15) could be used as a precursor to SIMes, while isolation of the unsaturated analogs proved unsuccessful. NHC-amine and NHC-chloroform adducts (16) and (17) were also... 

Synthesis of chiral imidazolinylidene transition metal complexes and their application in asymmetric catalysis 01JOM(617-618)70. 

In the last decade, very significant developments have been made in the fields of polymerisation (oligomerisation and telomerisation) using NHC complexes with metals. However, we want to pay special attention to those processes only catalysed by NHCs without the need of any metal. In this field, Hedrick et al. disclosed in 2002 a transesterification-ring opening polymerisation (ROP) in which imidazolinylidene carbenes catalyse the synthesis of biodegradable polyesters from cyclic esters (Seheme 2.24). More recently, Liu et al. have shown how a different imizadolium carbene is even able to promote the polymerisation of two different monomers in a controlled fashion. NHC-catalysed processes other than ROP have been described, such as the oxa-Michael addition polymerisation of acrylates recently reported by Matsuoka et al. ... 

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