Carbene complexes with pyridine

Naturally, it is possible to synthesise a similar ligand system without central chirality and in fact without the unnecessary methylene linker unit. A suitable synthesis starts with planar chiral ferrocenyl aldehyde acetal (see Figure 5.30). Hydrolysis and oxidation of the acetal yields the corresponding carboxylic acid that is transformed into the azide and subsequently turned into the respective primary amine functionalised planar chiral ferrocene. A rather complex reaction sequence involving 5-triazine, bromoacetal-dehyde diethylacetal and boron trifluoride etherate eventually yields the desired doubly ferrocenyl substituted imidazolium salt that can be deprotonated with the usual potassium tert-butylate to the free carbene. The ligand was used to form a variety of palladium(II) carbene complexes with pyridine or a phosphane as coligand. 

Oxidative decomposition of cyclopropyl-substituted Fischer-carbene complexes with pyridin-ium iV-oxide stereoselectively leads to cyclopropylcarboxylates. ... 

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

Several reaction sequences have been reported in which Fischer-type carbene complexes are converted in situ into non-heteroatom-substituted carbene complexes, which then cyclopropanate simple olefins [306,307] (Figure 2.22). This can, for instance, be achieved by treating the carbene complexes with dihydropyridines, forming (isolable) pyridinium ylides. These decompose thermally to yield pyridine and highly electrophilic, non-heteroatom-substituted carbene complexes (Figure 2.22) [46]. 

Boratetrahedrane (121) that has the triplet A2 ground state and the carbene Cs structure (122) possess higher energies than that of structure (119e). Similar to borabenzene, the stabilization of boracyclobutadiene (119e) can be achieved through the formation of a complex with pyridine. 

The reaction of these transition metal carbene complexes with some nucleophiles such as isocyanante, thiophenol, hydrazine, orhydroxylamine,have been studied. For example, the carbene part of the complex is converted into vinyl ether by pyridine 100>. 

Oxidation replaces the carbon-metal double bond of metal-carbene complexes with a carbon-oxygen double bond. A variety of oxidizing agents including pyridine iV-oxide (Cotton and Lukehart, 1971), dimethyl sulfoxide (C. P. Casey, R. L. Boggs, and W. R. Brunsvold, unpublished results, 1974), ceric ion (Casey et al., 1972) and oxygen (Fischer and Riedmuller, 1974) have been employed. These oxidations are normally clean, high-yield reactions... 

The activation of silylene complexes is induced both photochemically or by addition of a base, e.g. pyridine. A similar base-induced cleavage is known from the chemistry of carbene complexes however, in this case the carbenes so formed dimerize to give alkenes. Finally, a silylene cleavage can also be achieved thermally. Melting of the compounds 4-7 in high vacuum yields the dimeric complexes 48-51 with loss of HMPA. The dimers, on the other hand, can be transformed into polysilanes and iron carbonyl clusters above 120 °C. In all cases, the resulting polymers have been identified by spectroscopic methods. 

The amidinato(pyridine) complexes also react with the triethylborane adduct of the NHC l,3-diisopropylimidazol-2-ylidene in toluene at reflux temperature to give the corresponding carbene complexes (Scheme 128). ... 

The Cr analogues 6 (Fig. 4.2) of the bis(carbene)pyridine systems were found to be exceptionally active for the oligomerisation of ethylene [10,11]. Activation with MAO led to optimal results, and complexes with Me, Pr and substitution... 

Carbene complexes have also been prepared by transmetallation reactions. Lithiated azoles react with gold chloride compounds and after protonation or alkylation the corresponding dihydro-azol-ylidene compounds, e.g., (381) or (382), are obtained.22 9-2264 Silver salts of benz-imidazol have also been used to obtain carbene derivatives.2265 Mononuclear gold(I) carbene complexes also form when trimeric gold(I) imidazolyl reacts with ethyl chlorocarbonate or ethyl idodate.2266,2267 The treatment of gold halide complexes with 2-lithiated pyridine followed by protonation or alkylation also yields carbene complexes such as (383).2268 Some of these carbene complexes show luminescent properties.2269-2271... 

NHC ligands with a pendant group that enforces chelation have also been coordinated to copper centers. The reaction of Cu20 with pyridine fV-functionalized carbene ligand led to the formation of several compounds.91 In the case of mesityl derivatives, a dinuclear complex with a weak metal-metal interaction was isolated 60,91 whereas for the bulkier 2,6-diisopropylphenyl group, a monomeric complex was formed and characterized 61 (Figure 25).91... 

Thiols like pyridine-2-thiol yield (carbene)gold thiolates, and onium salts give cationic (carbene)gold ylide complexes, isolated and characterized as the perchlorates.170 The reaction of dithiocatechol with (cyclohexylisocya-nide)gold chloride affords a carbene complex [(CyNC)AuC(NHCy)2]+Cl-, which co-crystallizes with a neutral tetranuclear complex [(CyNCAu)2Au2(S2C6H4)2].224... 

Few examples of the preparation of six-membered heteroaromatic compounds using Fischer-type carbene complexes have been reported [224,251,381]. One intriguing pyridine synthesis, reported by de Meijere, is sketched in Figure 2.35. In this sequence a (2-aminovinyl)carbene complex first rearranges to yield a complexed 1 -azadiene, which undergoes intermolecular Diels-Alder reaction with phenylacetylene. Elimination of ethanol from the initially formed adduct leads to the final pyridine. 

The abnormal metallation is also favored when the conjugated pyridine-imid-azolylidene precursor (with a smaller bite angle) is used. For these precursors, an abnormal binding is produced even with a small wingtip group, as shown in Scheme 3.12. Under the same conditions, the pyridine-benzimidazolium analogue afforded the C2 carbene complex (Scheme 3.12) [27]. 

Extensive studies on diastereoselectivity in the reactions of 1,3-dipoles such as nitrile oxides and nitrones have been carried out over the last 10 years. In contrast, very little work was done on the reactions of nitrile imines with chiral alkenes until the end of the 1990s and very few enantiomerically pure nitrile imines were generated. The greatest degree of selectivity so far has been achieved in cycloadditions to the Fischer chromium carbene complexes (201) to give, initially, the pyrazohne complexes 202 and 203 (111,112). These products proved to be rather unstable and were oxidized in situ with pyridine N-oxide to give predominantly the (4R,5S) product 204 in moderate yield (35-73%). 

The Au-Au distances are 3.1882(1) A and they are considered to be responsible for the emission band that appears at 460 nm at room temperature. When the anion in the carbene complex is BF4, the structure is similar although the Au-Au distances are substantially longer (3.4615(2) A). The different distance leads to a different emission band that is blue-shifted. This indicates a greater orbital interaction in the former, consistent with its shorter Au-Au distance. Nevertheless, the behavior in solution is similar for both. Thus, at room temperature in solution they lose their emissive properties but they recover them in frozen solutions at 77 K. Interestingly, the emission differs in color, depending on the solvent, ranging from orange (acetone) to blue (pyridine), which would seem to result from the self-association... 

In connection with these catalytic cyclopropanation reactions, it should be mentioned that the isolable ruthenium-carbene complex 162, which is obtained from 19, [RuCMp-cymene)]2 and 2,6-bis(4-isopropyl-l,3-oxazolin-2-yl)pyridine, reacts with styrene at elevated temperature in a carbene transfer reaction83 (equation 41). Since complex 162 is also catalytically active for (alkoxycarbonyl)carbene transfer to olefins, this reaction represents one of the few connecting links between catalytic and stoichiometric carbene transfer reactions of metal-carbene complexes. 

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

Pyrrole synthesis.3 Imino carbene complexes such as 1 react with alkynes in hexane at 70°, possibly by a [2 + 2]cycloaddition, to form pyrroles in 80-98% yield. Usually, only a single pyrrole is obtained. This heteroannelation is unusual because carbon monoxide is not incorporated to give a six-membered product. However, O-alkyl imidate carbene complexes such as 3 react with 1-alkynes to form 3-hydroxy-pyridines as the major product. 

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