Carbenes, imidazoles

Ruthenium and osmium carbene complexes possess metal centers that are formally in the +2 oxidation state, have an electron count of 16 and are penta-coordinated. Ruthenium complexes exhibit a higher catalytic activity when an imidazole carbene ligand is coordinated to the ruthenium metal center (21). 

Imidazoles, carbene complexes with transition metals ... 

Numerous methods for the generation of imidazole carbenes have been reported. For example, starting from an imidazolium halide, the use of systems such as sodium hydride in ammonia or dimethylsulphoxide (DMSO), sodium in ammonia, alkali metals in tetrahydrofuran (THF), metal ferf-butoxides in THF or DMSO, etc. Recently, Seddon and Earle reported a simple procedure for the generation of the imidazolium carbene 2 in 90-95% yield from an imidazolium chloride 1 which does not require solvents, filtrations, or produce noxious waste products (Scheme 4) [40],... 

The catalyst (or catalytic system) is generally composed of a metal and a ligand - most commonly a phosphine, but sometimes an amine or imidazole carbene. For most reactions, the active catalyst is the zerovalent metal i.e. Pd(0), and can be added as such, as a stable complex such as Pd(PPh3)4 - tetrakis(triphenylphosphine) palladium(O) - referred to colloquially as tetrakis . 

Earle MJ, Seddon KR (2001) Preparation of imidazole carbenes and the use thereof for the synthesis of ionic liquids. WO 01/77081... 

WO 01/77081 (2001), to Quill, Preparation of Imidazole Carbenes and the Use Thereof for the Synthesis of Ionic Liquids, Inventors Earle, M.J. Seddon, KR. ... 

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). 

Imidazole with [Cr(CO)jCNCCl3] gives a simple product of substitution of three chlorine atoms followed by deprotonation, [Cr(CO)jCNC(im)3] (95JOM(489)27). The product reacts further with o-propylamine and pyrrolidine to give amino(alkylideneamino)carbenes 36 and 37, respectively. 

It is noteworthy that carbene function may be manifested in the derivatives of tris(imidazol-l-yl)borate 44 (96AGE310). Its reaction first with n-butyllithium, then with iron(II) chloride, and finally with sodium tetraphenylborate gives the iron(III) carbene derivative 45. 

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). 

Free carbenes based on 1,2,4-triazole are not as numerous as those based on imidazole (70ZN(B)1421, 95AGE1021, 97JA6668, 98JA9100). The carbene complex 169 (Ar = Ph, p-Tol) is prepared by the [3 + 2] cycloaddition route from [W(CO)j(C+=NC-HCOOEt)]- and aryldiazonium (930M3241). Oxidative decomplexation causes tautomerization of the 1,2,4-triazole ligand, the products being 170 (Ar= Ph, i-Tol). 

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. 

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