Synthesis of BenzannulatedN-Heterocyclic Carbene Ligands by a Template Synthesis from 2-Nitrophenyl Isocyanide

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Abstract
The reaction of 2‐nitrophenyl isocyanide 2 with [M(CO)5(thf)] (M=Cr, Mo, W) yields the isocyanide complexes [M(CO)5(2)] (3: M=Cr; 4: M=Mo; 5: M=W). Complexes 3–5 react with elemental tin under reduction of the nitro function of the isocyanide ligand to give the complexes with the unstable 2‐aminophenyl isocyanide ligand. The coordinated 2‐aminophenyl isocyanide ligand in all three complexes reacts spontaneously under intramolecular nucleophilic attack of the primary amine at the isocyanide carbon atom to yield the complexes with the NH,NH‐benzimidazol‐2‐ylidene ligand (6: M=Cr; 7: M=Mo; 8: M=W). An incomplete reduction of the nitro group in 3–5 is observed when hydrazine hydrate is used instead of tin. Here the formation of complexes with a coordinated 2‐hydroxylamine‐functionalized phenyl isocyanide [(CO)5M‐CN‐C6H4‐‐2‐N(H)‐OH] is postulated and this unstable ligand again undergoes intramolecular cyclization to give the NH,NOH‐stabilized benzimidazol‐2‐ylidene complexes 9–11. The tungsten derivative 11 can be allylated stepwise by a deprotonation/alkylation sequence first at the OH and then at the NH position to yield the monoallylated and diallylated species 12 and 13. The molecular structures of 3–5 and 12–13 were established by X‐ray crystallography.