Complexes metal-carbene, protonated

Ru, Os, and Ir carbene complexes have been prepared from reactions of anionic or low-valent metal complexes with some organic salts or neutral compounds with highly ionic bonds. Oxidative addition of halothiazole and -oxazole species to IrCl(CO)(PMe2Ph)2 affords Ir(III) complexes which on protonation yield cationic carbenes (69), e.g.,... 

The value of -NMR and 13C-NMR spectroscopy in characterizing transition metal carbene complexes was noted in Section III,B,2. The carbene carbon resonance is invariably found at low field (200-400 ppm) in the 13C-NMR spectrum, while protons attached to Ca in 18-electron primary and secondary carbene complexes also resonate at low fields. NMR data for some Ru, Os, and Ir alkylidene complexes and related compounds are given in Table V. 

The strongly basic character of the iminoacyl nitrogen has been demonstrated by protonations with NH4PF6 (4JO) and HBF4 (411) and alkylations with methyl iodide (384) to give metal -carbene complexes. Further examples include the formation of the secondary carbene [Ru C(CgH 3)-NHBu )(CNBu )5](PF6)2 from the reaction of (r], if-Ct H13)Ru... 

It should be noted here that the abstraction of the acidic proton in 2-position of the imidazolium ring by a base is not the only possibility to form a metal carbene complex. Cavell and co-worker have observed the in-situ metal carbene complex formation in an ionic liquid by direct oxidative addition of the imidazolium cation on a metal centre in a low oxidation state (Scheme 5) [53], However, the Pt-carbene complex formed can decompose by reductive elimination. 

A similar mechanism might operate in the activation of an azolium salt by a transition metal compound forming the metal carbene complex. However, since a basic substituent on the metal (acetate, alkoxide, hydride) usually reacts with the H -proton, the proton is removed from the reaction as the conjugate acid and reductive elimination does not occur. 

In general, the termination reactions of these polymerizations are not well understood but, depending upon the metal and the monomer, reductive coupling of the metal carbene fragments to give alkene and reduced metal complexes is one possibility. Another termination reaction appears to be initiated by -Hydride Elimination from the carbene complex. These mechanisms have been observed in well-defined catalyst systems, and are possible in the ill-defined systems also. The fact that most catalysts are sensitive to oxygen and moisture (or other proton sources) means that termination of the polymer chain by added or adventitious sources of water is a common problem, especially for the ill-defined catalysts. 

Rh -mediated hetero-H insertion may proceed by initial coordination of the intermediate metal-car-bene complex with the nonbonding electrons of the heteroatom. Proton transfer would then give the observed product. This can be an efficient process both for forming C—N bonds, as in the cyclization of (152 equation 62), and for C—O bonds, as illustrated by the construction of ether (155 equation 63). ° Si—H insertion of (156) was shown to proceed with retention of absolute configuration, as would be expected for a concerted transition metal carbene mediated process (equation 64). ... 

In alkenyl- and alkynylcarbene complexes the addition of nucleophiles to the carbene carbon competes with the addition to the 3-carbon of the conjugated C-C multiple bond. [17] The regioselectivity of the addition of amines to alkynylcarbene complexes is temperature dependent 1,2-addition is favoured by lower temperatures. [17c] Enolates turned out to be efficient C-nucleophiles for Michael addition reactions to unsaturated metal carbenes. The product distribution may depend on steric factors as shown in Scheme 7 for the addition of different enolates to alkenylcarbene complex 10. The less bulky acetone enolate 11 adds to the carbene carbon protonation of the primary addition product results in demetalation and in the formation of a mixture of isomeric enones 12. In contrast, the more bulky cyclopentanone enolate 13 adds to the less shielded vinylic position. 

However, the formation of the metal-carbene complex was not observed in pure, halide-free [BMIM][Bp4], indicating that the formation of carbene depends on the nucleophilicity of the ionic liquid s anion. To avoid the formation of metal-carbene complexes by deprotonation of the imidazolium cation under basic conditions the use of 2-methyl-substituted imidazolium is frequently suggested. However, it should be mentioned here that strong bases can also abstract a proton to form the vinyl imidazolidene species which may also act as a strong ligand to electrophilic metal centers. 

The photochemistry of metal carbyne complexes is in many ways similar to the photochemistry of metal carbene complexes, but the reactions have not been developed or become as synthetically useful as the photochemistry of metal carbene complexes. Among reported reactions are couplings with ancillary CO ligands to form ketenyl complexes, protonation of the carbyne carbon, insertions into C-H bonds, addition of the carbyne carbon to an alkyne to produce a cyclopropenyl complex, and electron-transfer reactions. ... 

Protons attached to the a-carbon atom in metal-carbene complexes undergo rapid base-catalyzed hydrogen-deuterium exchange with hydroxylic solvents (Kreiter, 1968). For example, the half-life for exchange of the a protons in VII in acetone-D20 with no added base is 23 min at 40° (Casey, 1970). This rapid exchange implies the existence of an intermediate carbanion. These... 

Metal-carbene complexes are stable in dilute aqueous acid, but strongly basic aminocarbene complexes react with HCl or HBr in ether at -40° to give products derived from protonation of the carbon-metal bond (Fischer et ai, 1973b). Alkoxy-substituted carbene complexes react with mixtures of HCl... 

Several methodologies for the preparation of metal-carbene complexes have been developed (Scheme 2.153). In the best case, imidazolium salts are submitted directly to a solution of a metal complex. The active catalyst is formed under catalytic conditions. Mandatory removal of the acidic proton and subsequent formation of the carbene is carried out in situ and can be promoted by a basic ligand already present in the metal salt. This task can be fulfilled, for example, by acetate in the palladium precursor, which is basic enough to remove the proton and to form the carbene-metal complex. Also, counter-ions of imidazolium salts can participate in the coordination reaction of the newly formed carbene complex by the displacement of less chelating ligands from the metal center [2]. 

Fischer-type metal-carbene complexes reaction of a carbanion on a metal carbonyl the anions obtained can be protonated or alkylated on the oxygen atom. 

---