2,3-Dihydro-17/-imidazole-2-ylidenes

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... 

Irradiation of matrix-isolated imidazole-2-carboxylic acid gave the 2,3-dihydro-imidazol-2-ylidene-C02 complex (31) characterized by IR spectroscopy and calculated to lie 15.9 kcal mol above the starting material. A series of non-aromatic nucleophilic carbenes (32) were prepared by desulfurization of the corresponding thiones by molten potassium in boiling THF. The most hindered of the series (32 R = Bu) is stable indefinitely under exclusion of air and water and can be distilled without decomposition. The less hindered carbenes slowly dimerize to the corresponding alkenes. Stable aminoxy- and aminothiocarbenes (33 X = O, S) were prepared by deprotonation of iminium salts with lithium amide bases. The carbene carbon resonance appears at 260-297 ppm in the NMR spectrum and an X-ray structure determination of an aminooxycarbene indicated that electron donation from the nitrogen is more important than that from oxygen. These carbenes do not dimerize. 

NHC = Al-heterocyclic carbene PtoH = tris(o-tolyl)phosphine SIAd = l,3-bis(l-adamantyl)-4-5-dihydroimidazol-2-ylidene SIMes = l,3-dimesityl-4,5-dihydroimidazol-2-ylidene SIPr = l,3-bis(2,6-diisopropylphenyl)-4,5-dihydro-imidazol-2-ylidene THF = tetrahydrofuran TM = transition metal tmed = Al,Al,Al, Al -tetramethylethylenediamine ... 

The (SIPr)Pd(cinnamyl)Cl (SlPr= l,3-bis(2,6-diisopropyl phenyl)-4,5-dihydro imidazol-2-ylidene) catalysed anaerobic oxidation of aryl/alkyl, aryl/aryl, and alkyl/ alkyl secondary alcohols to the corresponding ketones in high yields has been achieved using aryl chlorides in toluene with f-BuONa. Domino Pd-catalysed synthesis of a-arylated ketones from secondary aryl alcohols in very good yields was also reported. " ... 

Ni(Gl)Gp(carbene) complexes with the carbenes l,3-bis(2,4,6-trimethylphenyl)-4,5-dihydro imidazol-2-ylidene, l,3-bis(2,6-diisopropylphenyl)-4,5-dihydorimidazol-2-ylidene, and l,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene have also been prepared, structurally characterized, and shown to have catalytic activities in the aryl amination and aryl halide dehalogenation reactions. Structurally similar carbene complexes have also been reported with the novel carbene formed by aryl cyclohexadienyl-ylidene rearrangement, as shown in Equation (11) the solid-state structure of this complex has shown an Ni-Gcarben bond of ca. 189 pm. 

An air-stable catalyst, prepared from dimer 1 and M-hetero-cyclic carbene (iVA -bis(2,6-diisopropylphenyl)4,5-dihydro-imidazol)-2-ylidene), afforded a-arylated ketones from aryl chlorides, bromides, or triflates (eq 87). The reaction conditions are compatible with aryl or alkyl methyl ketones. 

The elimination of an alcohol from a neutral 2-alkoxy-1,2-dihydro-1//-imidazole leads to the formation of NHCs [Eq. (18)]. Upon heating, the elimination of alcohol forms the NHC, which in the case of imidazolin-2-ylidenes dimerizes to... 

Different synthetic routes have been used to prepare these carbenes (Scheme 8.6). The most common procedure is the deprotonation of the conjugate acid. In early experiments, sodium or potassium hydride, in the presence of catalytic amounts of either f-BuOK or the DMSO anion were used. ° Then, Herrmann et al. showed that the deprotonation occurs much more quickly in liquid ammonia as solvent (homogeneous phase), and many carbenes of type IV have been prepared following this procedure. In 1993, Kuhn and Kratz" developed a new and versatile approach to the alkyl-substituted N-heterocyclic carbenes IV. This original synthetic strategy relied on the reduction of imidazol-2(3//)-thiones with potassium in boiling tetrahydrofuran (THF). Lastly, Enders et al." reported in 1995 that the 1,2,4-triazol-5-ylidene (Vila) could be obtained in quantitative yield from the corresponding 5-methoxy-l,3,4-triphenyl-4,5-dihydro-l//-l,2,4-triazole by thermal elimination (80 °C) of methanol in vacuo (0.1 mbar). 

Cycloaddition with activated alkenes Reaction between 3-substi-tuted imidazole 1-oxides 228 and 2,2-bis(trifluoromethyl)ethene-l,l-di-carbonitrile leads to 2-(l,3-dihydro-2H-imidazol-2-ylidene)malononitriles 304 (2006HCA1304). 

I Pr l)3-bis(2,6-diisopropylphenyl)-2,3-dihydro-lfZ-imidazol-2-ylidene IPh 1,3-Diphenyl-2,3-dihydro-lH-imidazole... 

McIPr l,3-diisopropyl-4,5-dimethyl-2,3-dihydro-lfl-imidazol-2-ylidene... 

During the design of steroid mimics as potential ACAC inhibitors, the exocyclic enone functions in 2-[4,4-bis(trifluoromethyl)-2-(4-fluorophenyl)-4,5-dihydro-l//-imidazol-5-ylidene]-l-(4-fluorophenyl)-l-ethanones 815 are selectively reduced with zinc dust to the imidazolines 816 in high yields (Scheme 202) <1996T11153>. 

Fig. 5.7 Structures of 1,3-b is (2,4,6-trim ethyl phenyl) im id azo l-2-yl id en e (IMes) and T,3-bis(2,4,6-tri-methyl phenyl)-4,5-dihydro-1 H-imidazol-2-ylidene (Hj-IMes).

When IPr.Cl is treated with a base (e.g. BuOK, Bu ONa, K2CO3 or CS2CO3), the catbene lrJ-bis(2,6-diisopropylphenyl)-l,3-dihydro-2H-imidazol-2-ylidene [244187-81-3] M 388.6, m 213-217°, is formed and is stable enough to be isolated, stored and is available commercially. The carbene-carbon atom at C2 coordinates with metals and forms C-C bonds, by C-H insertion, with acetylene, MeCN, HCCI3, PhSOCH3, and the stmctures of some of the products have been confirmed by X-ray analysis [Arduengo et al. Helv Chim Acta 82 2348 1999]. 

The amidation of esters and amino alcohols is a less widely investigated area with relatively few literature examples. In 2005, nitrogen-heterocyclic carbenes (NHCs) were reported to be catalytic in this transformation." l,3-Bis(2,4,6-trimethylphenyl)-l,3-dihydro-2i -imidazol-2-ylidene (IMes) was chosen as the most suitable carbene, being readily available, reactive and easy to store. It is required in low catalytic loadings of 5 mol% in THF at 23 °C. These mild conditions are highly desirable so that the reaction is compatible with other functional groups and can potentially be used for enantioselective reactions. Reaction times varied from 1.5-24 hours depending on steric bulk and electrophilicity of the ester. 

---