Hydrogenation carbene ligands

We quickly established that these complexes are not particularly stable. They tend to split off the carbene ligand with a simultaneous hydrogen shift thereby liberating aldehydes, a fact that Japanese investigators also discovered (13). Very recently, we learned how to prepare these hydroxy-carbene complexes analytically pure (14) Previously, these complexes, without isolation, had been successfully converted to the substantially more stable methoxycarbene compounds by treatment with diazomethane (12). 

We can also observe reactions that lead to a more stable arrangement of the whole system very probably via primary addition and subsequent rearrangement (4 in Fig. 3). In addition, it can be established that because of the electron withdrawal of the M(CO)5 moiety, hydrogen atoms in cv-alkyl positions to the carbene carbon develop such an acidic character that their acidity corresponds to that of nitromethane (5 in Fig. 3). Finally, by cleavage of the carbene ligand from the metal complex, pathways in synthetic organic chemistry are opened (6 in Fig. 3). 

The second example shows that such a reaction need not always lead to an uncharged system. If, instead of thiocarbene complexes, amino-carbene complexes are treated with hydrogen halides, products of saltlike character are isolable (66). One finds the halogen at the metal and the hydrogen at the removed carbene ligand, and one obtains iminium halo-genopentacarbonylmetalates ... 

Our carbene complexes should open the way to new organic chemistry if the carbene ligand can be successfully cleaved from the metal under not too severe reaction conditions. This expectation can be realized, for instance, by utilizing hydrogen halides in methylene chloride at — 78°C (71) ... 

Since the shift of a hydrogen atom is not possible with the methoxy-(phenyl) carbene ligand, it can only dimerize in reactions with bases and in thermal liberations (75). 

In Eq. (24), Hughes proposed that H2 displaces H20 and transfers a proton to form HF and a carbene ligand that is then hydrogenated to give a fluorohydrocarbon product [Eq. (25)] (93). 

As mentioned above, the electrophilic metal carbene complexes are stabilised by the presence of heteroatoms or phenyl rings at the divalent carbon atom, while hydrogen or alkyl groups stabilise the nucleophilic complexes. Therefore, there is a distinction between carbenoids and alkylidenes when designing carbene ligands corresponding to the former or the latter class. 

The use of the Rh complex 29 bearing a tridentate carbene ligand has also been described for the transfer hydrogenation of ketones and imines [74], The catalyst was found to be highly active, needing only 0.001 mol% (up to 68 000 TON) to completely reduce to substrates. 

It is interesting to note that C-H activation on ruthenium NHC complexes is not limited to intramolecular protons located in the N-sidechain of the carbene, but occurs inter-molecularly as well. Leimer et al. reacted [MesIRuH PCyj] with toluene-dg at ambient temperature and observed a rapid H/D exchange reaction involving the four hydride hydrogen atoms on ruthenium, the methyl protons of the mesityl substituents of the carbene ligand and the deuterium atoms on the meta positions of toluene-dg. The ortho-, para- and methyl-deuterium atoms of the solvent did not participate [145]. 

In a first model reaction, Danopoulos et al. [472] reacted a free pincer carbene ligand with [RuCPPhjljCl ] and obtained the corresponding octahedral pincer carbene adduct (see Figure 3.156). The complex lacks the yhdene functionality necessary for activity of the complex in olefin metathesis. Instead, the compound was successfully employed in the transfer hydrogenation of cyclohexanone, acetophenone and benzylidene anihne. Reaction temperatures were mostly low to moderate (25-55 °C) and catalyst loadings in the range of 0.015 to 0.1% with TONs of only 150 to 8800. 

Neumann (777) showed that irradiation of the carbene complexes (CO)sW CPh2, (CO)5 V C(OCH3)Ph, and (CO)5Cr CO(CH2)2CH2 under CO atmosphere led to CO incorporation into the complexes, a result which implies photoinduced CO dissociation. They also irradiated these complexes under H2 atmosphere and observed slow, if any, hydrogenation of the carbene ligands in the tungsten complexes but more rapid hydrogenation for the Cr complex [Eq. (137)] (777). 

Methyllithium (and likewise BuLi and allyllithium) also adds to the carbene ligand of (CO)5W[C(OMe)Ph]. However, the reaction of the resulting anionic adduct with Si02/pentane at —40°C yields pentacarbonyl(n -olefin)-W complexes, probably via the intermediary formation of the methyl(phenyl)carbene complex and following rearrangement via 1,2-hydrogen shift ... 

Recent research in the application of supercritical (sc) fluids and ionic liquids (IL) as solvents in homogeneous catalysis (see Sections 7.3 and 7.4), opened the way to the development of biphasic water/scCOz [171, 172] and water/IL [173] systems for the hydrogenation of various substrates, e.g., alkenes, aldehydes, etc. with water-soluble catalysts. The catalytically highly active, versatile and robust transition metal - N-heterocyclic carbene complexes [174] have also been applied for hydrogenation reactions [175], Given that water-soluble complexes with N-heterocy-clic carbene ligands are known [176], catalytic applications in aqueous systems are also foreseen. 

More interestingly, acyloxycarbene- chromium complexes, which are generated in situ by acylation of the corresponding tetraalkylammonium metal acylates, transfer their carbene ligand to silyl enol ethers under very mild conditions and with excellent simple diastereoselectivity to provide 2-(/m-butyldimethylsilyloxy)cyclopropyl acetates. When R2 is hydrogen the cu-iso-mers predominate, whereas formation of the trans-product is favored when R2 is phenyl18. 

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