Pyridine-ylidenes

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. 

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). ... 

CN 4-(8-chloro-5,6-dihydro-l lJ/-benzo[5,6]cyclohepta[l,2-fo]pyridin-l l-ylidene)-l-piperidinecarboxylic acid ethyl ester... 

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... 

Like the parent Cp2Be, the beryllocenes 10, 11, and 12 are sublimable, air- and oxygen-sensitive solids. They do not react with pyridine, CO, or l,3,4,5-tetramethylimidazol-2-ylidene, but do react with CNC6H3Me2-2,6 to give half-sandwich iminoacyl products via insertion into the Be-C cr-bond, described in Section 2.02.2.3.2. 

Following on the triazole mohf a series of new iridium(I) l,2,4-triazole-3-ylidene NHC complexes [lr(cod)(NHC)L]BF4 (56a-c, L=PPh3, pyridine Scheme 4.23) were synthesized and tested and good results found for TH on C=0, C=N and C=C double bonds in 2-propanol with K2CO3 [52]. 

Condensation of 3-amino-2-(methylamino)pyridine 647 with diethyl 2-oxomalonate in boiling ethanol afforded 2-carbethoxy-4-methylpyrido[2,3- ]pyrazin-3(4//)-one 213 <1996JHC1737>. On the other hand, condensation of 647 with diethyl oxaloacetate gave ethyl [2(l//)-oxopyrido[2,3- ]pyrazine-3(4//)-ylidene]carboxylate 648 in addition to the formation of pyridodiazepine 649 as a by-product <1996JHC1737>. However, the condensation of 647 with diethyl 2-oxoadipate gave the 2-ethoxycarbonylpropyl analogue 650 (Scheme 30) <1994FA259>. 

Azine, This term has the following meanings a)Pyridine(Ref 2) b)Sym di-ylidene derivs of hydrazones of ketones or aldehydes, such as acetone azine. (CHj CiN-N CfCHj) (Ref 3) c)The group (Ns)a is called free azine" or nitrine. According to Walden Audrieth (Ref 1), the halides of azine are extremely expl substs which undergo spontaneous de-compn at RT... 

The pyridine-like nitrogen of the 2H-pyrrol-2-ylidene unit tends to withdraw electrons from the conjugated system and deactivates it in reactions with electrophiles. The add-catalyzed condensations described above for pyrroles and dipyrromethanes therefore do not occur with dipyrromethenes. Vilsmeier formylation, for example, is only successful with pyrroles and dipyrromethanes but not with dipyrromethenes. 

Azacyclohexatriene-2-ylidene (3), the 2-isomer of pyridine, has been generated by one-electron oxidation of the corresponding radical cation in neutralization-reionization mass spectrometry.17 It was determined by ab initio H-F calculations that the charge polarization of the radical fonned by H-abstraction from pyrazine can be... 

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... 

Dimethyl-4,6-dioxo-1,3-dioxane-5-ylidene)-2,6-diphenyl-4/7-tellurin4 1.44 g (10 mmol) of Meldrum s acid are dissolved in 25 ml of pyridine, 5.0 g (10 mmol) of powdered 2,6-diphenyl-4-ethoxytellurinium fluorosulfate are added, and the dark red solution is immediately concentrated under vacuum. The residue is chromatographed on 100/200 mesh Florisil with dichloromethane as the mobile phase yield 4.0 g (80%) m.p. 199-201°. 

Phenyl-7-methoxybenzotellurinium perchlorate was oxidized to the 7h,7e -dioxide of the 4-(4//-l-benzotellurin-4-ylidene)-4//-l-bcnzotellurin by triphenylphosphane oxide at 220° or by air in pyridine in the presence of triphenylphosphane1. 

Burstein C, Lehmann CW, Glorius F (2005) Imidazo[l,5-a]pyridine-3-ylidenes-pyridine derived N-heterocyclic carbene ligands. Tetrahedron 61 6207-6217... 

---