Carbenes, imidazol-2-ylidene

Imidazoline-2-thiones are reduced by potassium metal in THF forming stable carbenes (imidazol-2-ylidenes) 756 that react very readily with electrophiles (see also Section 3.4.1.8.5). 

A conformational analysis of N,N -dinaphthyl heterocyclic carbenes (imidazol-2-ylidenes and imidazolin-2-ylidenes 578a-h ) has been carried out at the B3LYP/6-31G(d) level (11SC1087). The potential hypersurface is complex (Figure 44) since the 16 minima are connected by 32 transition states. The calculated values agree with the experimental results of Dorta et al. (09EJIC1861). 

Imidazol-2-ylidene and its N,N -disubstituted derivatives as stable carbenes 99ACR913. 

One method to transform imidazolium salts (00AGE3773) into carbene ligands, imidazol-2-ylidenes, is by deprotonation with sodium hydride or other suitable hydride in a mixture of THE and liquid ammonia (73JCS(D)514, 96CEJ1627, 98IC6412). 

Co-condensation reaction of the vapors of l,3-di-rcrt-butylimidazol-2-ylidene and nickel, palladium, or platinum gives the coordinatively unsaturated 14-electron sandwiches [L M] (M=Ni, Pd, Pt) of the carbene type (990M3228). Palladium(O) carbene complexes can also be prepared by the direct interaction of l,3-R2-imidazol-2-ylidenes (R=/-Pr, r-Bu, Cy, Mes) (L) with the palladium(O) compound [Pd(P(o-Tol)3)2] (OOJOM(595)186), and the product at the first stage is [(L)PdP(o-Tol)3l, and then in excess free carbene [PdL ]. 

Diethylzinc l,3-bis(l-adamantyl)imidazol-2-ylidene adduct is known (93JOM(462)13). Bis(l,3-diphenylimidazol-2-ylidene) mercury(II) carbene complexes follow from the corresponding imidazolium perchlorate and mer-cury(II) chloride (68AGE682). 

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. 

Silicon analogues of imidazole-2-ylidenes are stable carbenes that form adducts where the metal-silicon bond is relatively weaker than that between metal and carbon atoms. 

The versatile starting material lCp RuCI 4 (1) reacts rapidly with sterically demanding phosphines (PCy and P Pr ) as well as with the nucleophilic carbene ligands (L) to give deep blue, coordinatively unsaturated Cp Ru(L)CI complexes 2-8 (L= l,.Tbis(2,4,6-lrimethylphenyl) (IMes. 2) 1,3-R2-imidazol-2-ylidene = cyclohcxyl (ICy, 3) 4-methylphenyl (ITol, 4) 4-chlorophenyl (IPCl, 5) adamanlyl (lAd, 6) 4..5-dichloro-1,3-bis(2.4,6-trimethylphenyl) (IMesCI, 7) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidenc (IPr. 8) in high yields according to Eq. (4). 

Further improvements in activity of the ruthenium carbene complexes were achieved by incorporation of methyl groups in 3,4-position of imidazol-2-ylidene moiety. Introduction of sulfur in the trara-position to the N-heterocyclic carbene leads to increased stability of the resulting ruthenium complexes. The synthesis and the first applications of these new rathenium complexes are described herein. 

Further improvements in activity of the imidazol-2-ylidene Ru complexes might be attained by the incorporation of a better a-donor substituents with larger steric requirements. The ligands that most efficiently promote catalytic activity are those that stabilize the high-oxidation state (14 e") of the ruthenium metallacyclobutane intermediate [7]. Both ligand-to-metal a-donation and bulkiness of the NHC force the active orientation of the carbene moiety and thus contribute to the rapid transformation into metallacyclobutane species [7b]. Both can be realized by incorporation of alkyl groups in 3,4-position of imidazol-2-ylidene moiety, lyie Me. Me... 

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... 

B(3,5-(CF3)2C6H3)4-.512 Palladium complexes with a hemilabile terdentate carbene ligand, 1,3-bis(pyl)imidazol-2-ylidene, were active toward the catalytic polymerization of CO/norbornylene.513 Palladium complexes of cz s-bidentate C4-bridged diphosphines cis- and trans- 1,2-bis [(diphenylphosphino)methyl]cyclohexane, e fl o,e fl o-2,3-bis[(diphenylphosphino)methyl] norbornane,... 

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