Imidazole-2-thione, carbene complex

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. 

Although deprotonation of NHC salts have proven to be an efficient synthetic route, the basic reaction conditions can prove incompatible with certain complex synthesis in a one-pot procedure. Thus, other synthetic strategies to generate free carbene have been developed. Khun reported the synthesis of imidazol-2-ylidenes bearing a small alkyl group on the NHC backbone by potassium reduction of the corresponding thiones (10) (equation 5). This method has also been applied in the synthesis of benzimidazole derivatives, by reduction of the corresponding sulfones with Na/K alloy at room temperature. Synthesis of unsymmetrically substituted... 

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