Fe. Hahn et al., N-N '-bis(2,2-dimethylpropyl)benzimidazolin-2-ylidene: A stable nucleophilic carbene derived from benzimidazole, CHEM-EUR J, 5(6), 1999, pp. 1931-1935
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.