Air-stable carbenes

Air-stable carbenes the reactions of diaminocarbenes with water and oxygen... 

In 2003, Sigman et al. reported the use of a chiral carbene ligand in conjunction with the chiral base (-)-sparteine in the palladium(II) catalyzed oxidative kinetic resolution of secondary alcohols [26]. The dimeric palladium complexes 51a-b used in this reaction were obtained in two steps from N,N -diaryl chiral imidazolinium salts derived from (S, S) or (R,R) diphenylethane diamine (Scheme 28). The carbenes were generated by deprotonation of the salts with t-BuOK in THF and reacted in situ with dimeric palladium al-lyl chloride. The intermediate NHC - Pd(allyl)Cl complexes 52 are air-stable and were isolated in 92-95% yield after silica gel chromatography. Two diaster corners in a ratio of approximately 2 1 are present in solution (CDCI3). 

The ruthenium complexes were prepared in 50-80% yield by treatment of the imidazolium salts with potassium hexafluoro-t-butoxide, and then by (PCy3)2Cl2Ru = CHPh. A single phosphine is displaced by the carbene affording the desired complexes as air-stable solids that were purified by silica gel... 

This group showed that isolable silver(I) diaminocarbene complexes can be use in situ instead of free carbenes, to generate the copper carbene complex. The silver salts that precipitates during the formation of the copper complex have not any negative effect on the conversion. This method is advantageous since most of the silver complexes are isolable, air-stable and easily obtained by treatment of the corresponding imidazohnium salt by 0.5 equiv of silver oxide (Scheme 53). The solid structure of 78 was analyzed by X-ray diffraction. 

There have been many reports of the use of iridium-catalyzed transfer hydrogenation of carbonyl compounds, and this section focuses on more recent examples where the control of enantioselectivity is not considered. In particular, recent interest has been in the use of iridium A -heterocyclic carbene complexes as active catalysts for transfer hydrogenation. However, alternative iridium complexes are effective catalysts [1, 2] and the air-stable complex 1 has been shown to be exceptionally active for the transfer hydrogenation of ketones [3]. For example, acetophenone 2 was converted into the corresponding alcohol 3 using only 0.001 mol% of this... 

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. 

The isocyanide and carbene complexes are all colorless or white, air-stable, crystalline solids. The molecular structures of the cis isomers have been established by X-ray crystallography.2 The isocyanide complexes have a strong 1R absorption [(N-C) stretching] in the range 2150-2280 cm"1 for example, cis-PtCl2(PhNC)(PEt3) has at 2190 cm"1 (Nujol mull). The carbene com-... 

The synthesis of unnatural chiral amino acids is a considerable challenge to the organic chemist. One approach is via chiral organochro-mium(0) carbenes (56). The starting material is easily prepared from chromium hexacarbonyl and organolithium reagents (Fig. 5), and the resulting carbenes are air-stable yellow to red solids. 

Unusual bridging (//-cyclopropyIidene)diiron complexes having a tetrahedral carbene carbon have been studied as model intermediates in carbon-carbon bond formation in the Fischer-Tropsch synthesis248. The cyclopropylidene complexes cis- and trans-[Cp(Co)Fe]2(/(-Co)(//-C,H4) were readily prepared by cyclopropanation in ether, of the corresponding cis- and mww-vinylidene complexes [CpCoFe](//-CO)(//-CH2) with diazomethane in the presence of CuCl (equation 181). Both isomers are air stable in the solid state. Solutions of the complexes are air stable for several hours, provided they are kept in the dark. The pure //-cyclopropylidene isomers slowly interconvert in solution, like their parent /z-vinylidene and other alkylidene complexes. The final equilibrium ratio cis .trans = 4.8 1 is reached after two weeks. 

In a logical continuation of this work, carbene addition to an iron-iron double bond has also been exploited for the simple synthesis of the first /. -methylene complex in the nitrosyl series. The readily available /x-nitrosyliron complex [(Tj5-C5H5)Fe(/u.-NO)]2 (26) exhibits the same structural features as the rhodium dimer 21 (157) and reacts with diazomethane in the temperature range -80-25°C to give the expected /z-methylene derivative 27 (Scheme 14) as a black, air-stable compound in... 

Both the fully unsaturated and the hydrogenated alkyl indolizines were rather unstable in air. Attempted carbene additions to the relatively stable 2-phenyldihydro compounds proved unsuccessful. 

Some carbenes of these types are stable enough for isolation, for example, 346 is air stable <1997JA12742>. 

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... 

---