N-N '-bis(2,2-dimethylpropyl)benzimidazolin-2-ylidene: A stable nucleophilic carbene derived from benzimidazole

N,N'-Dialkylated benzimidazol-2-thiones 6 can be prepared in a three-step p rocedure from o-phenylenediamine. Thione 6a was reduced with sodium/potassi um under desulfurization leading to carbene 4. Compound 4 is the first stab le free carbene derived from benzimidazole. It exhibits the topology of an unsaturated carbene including a short C4-C5 bond. Its C-13 NMR spectrum [de lta(C2) = 231.47] and selected structural parameters [angle N1-C2-N3 103.5( 1)degrees and 104.2(1)degrees], however, correspond to those of saturated c arbenes of the imidazolidin-2-ylidene type 3. The observation that the unsa turated carbene 4 behaves like a saturated imidazolidin-2-ylidene is corrob orated by the tendency of the sterically less bulky N,N'-dialkylated thione 6b to yield the olefin 8 after reduction with Na/K. Carbene 3 can be coord inated to a W(CO)(5) fragment yielding the complex 9, which contains a nonp lanar carbene ring.