N,N'-Bis(2,2-dimethylpropyl)benzimidazolin-2-ylidene: a stable nucleophilic carbene derived from benzimidazole.
N,N'-Dialkylated benzimidazol-2-thiones I (R = CH2CMe3, Me) can be prepd. in a three-step procedure from o-phenylenediamine. Thione I (R = CH2CMe3) was reduced with sodium/potassium under desulfurization leading to carbene II. Compd. II is the first stable free carbene derived from benzimidazole. It exhibits the topol. of an unsatd. carbene, including a short C4-C5 bond. Its 13C NMR spectrum [d(C2) = 231.47] and selected structural parameters [angle N1-C2-N3 103.5(1) Deg and 104.2(1) Deg], however, correspond to those of satd. carbenes of the imidazolidin-2-ylidene type. The observation that II behaves like a satd. imidazolidin-2-ylidene is corroborated by the tendency of the sterically less bulky I (R = Me) to yield olefin III after redn. with Na/K. Carbene II can be coordinated to a W(CO)5 fragment yielding a complex which contains a nonplanar carbene ring.
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