Imidazole stable carbenes

Imidazol-2-ylidene and its N,N -disubstituted derivatives as stable carbenes 99ACR913. 

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

As mentioned in the Introduction, one of the synthetic approaches to stable nucleophilic carbenes involves C-deprotonation of imidazol-ium and related cations with alkali metal salts of strong bases such as NaH and KO Bu. Accordingly, the interactions with alkali metal cations with stable nucleophilic carbenes could prove to be important for understanding the solution behavior of the latter. Until recently, however, there were no examples of complexes of stable carbenes with... 

Is the stability of 8Ad due to unfavorable kinetics, i.e., the bulky adamantyl groups blocking reaction, or to unfavorable thermochemistry, i.e., loss of aromaticity of the imidazole ring as a result of reaction, or both The distinction is potentially important as understanding could assist in designing stable carbenes. To decide, compare the kinetics and thermodynamics of the insertion of 8Ad into the central CH bond in propane with reactions of 8Me, which should also be aromatic but lacks shielding groups , and 9, which is neither aromatic nor crowded. 

Especially the report of the successful isolation of a stable carbene by Ar-duengo [6,7] (Scheme 1) and the publication of the solid-state structure of a stable imidazole-2-ylidene in 1991 led to a renaissance of these nucleophilic carbenes, which certainly also had an influence on the 2005 Nobel price in chemistry ... 

The realization of the extraordinary properties of these new ligands stimulated research in this area and many imidazole-2-ylidenes have been synthesized in the last 10 years [8]. The 1,3-diadamantyl derivative 1 of the imidazole-2-ylidenes is stable at room temperature and the 1,3-dimesityl-4,5-dichloroimidazole-2-ylidene [9] is reported to be even air-stable. A variety of stable carbenes has been synthesized in between and it could be shown that steric bulk is not a requirement for the stability (the 1,3-dimethylimidazoline-2-yhdene can be distilled without decomposition [10]), although it certainly influences the long-term stability by preventing dimerization. 

Imidazoline-2-thiones are reduced by potassium metal in THF forming stable carbenes (imidazol-2-ylidenes) 756 that react very readily with electrophiles (see also Section 3.4.1.8.5). 

Jones and co-workers investigated reactions of the stable carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene with a series of halide sources... 

The first paper on the synthesis of a thermally stable carbene, which could be kept in a bottle, appeared in 1991. It was entitled A Stable Crystalline Carbene and the main author was Anthony Arduengo (Fig. 8.1) from the Experimental Station of DuPont at Wilmington in the US [14]. He had set out to find a simple preparative route to industrially important imidazole-2-thiones and for this purpose treated imidazolium salts with strong bases and then added sulfur to form the required C=S bond [15, 16]. The ease with which these reactions succeeded, even in dry air, led Arduengo to believe that the respective carbenes, formed by deprotonation of the imidazolium ions, were less unstable than expected. Since the dimerization, as observed in the case of 6, was thought to... 

The synthesis and characterization of Et2Zn adducts with nucleophilic carbenes see Carbene Complexes) have been described. They are directly available and in good yields from a smooth reaction between Et2Zn and stable carbenes see Carbene Complexes) such as l,3-dimesityhmidazol-2-yhdene or l,3-di(l-adamantyl)imidazol-2-ylidene (equation 13). The solid-state structure of [Et2Zn l,3-di(l-adamantyl)imidazol-2-ylidene ] shows a zinc atom in a rather unusual trigonal planar arrangement, certainly due to a steric... 

Further, ferrocenyl(alkyl)imidazole compound (30) was made by a similar method. Reaction of compound 29 with 1 in CH3CN affords imidazole (30) in excellent yield. After methylation with methyl iodide followed by deprotonation, 30 was transformed into a planar-chiral stable carbene (31) (eq 25). 

Our final method is in some ways the most straightforward in terms of mechanism simple removal of a proton from a stable cation. This is the method used to make very stable carbenes, and it works because both the cation used as the starting material and the carbene product are stabilized by one or more adjacent lone pairs. Here is an example. Imidazoles are nucleophilic, and can be alkylated to give relatively stable imidazolium cations, which we can represent with the charge delocalized between the two nitrogen atoms, although there is another possible representation with the charge on carbon. 

Most carbenes are very short lived, although persistent carbenes are known. A persistent carbene (also known as a stable carbene or a Arduengo carbene) is a type of carbene demonstrating particular stability. The best-known examples are diaminocarbenes with the general formula (R2N)2C , where the R represents various functional groups. The groups can be bridged so that the carbon with unfilled orbitals is part of a heterocycle, such as imidazole or triazole. Persistent... 

Imidazole is characterized mainly by the T) (N) coordination mode, where N is the nitrogen atom of the pyridine type. The rare coordination modes are T) - (jt-) realized in the ruthenium complexes, I-ti (C,N)- in organoruthenium and organoosmium chemistry. Imidazolium salts and stable 1,3-disubsti-tuted imidazol-2-ylidenes give a vast group of mono-, bis-, and tris-carbene complexes characterized by stability and prominent catalytic activity. Benzimidazole follows the same trends. Biimidazoles and bibenzimidazoles are ligands as the neutral molecules, mono- and dianions. A variety of the coordination situations is, therefore, broad, but there are practically no deviations from the expected classical trends for the mono-, di-, and polynuclear A -complexes. 

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