Borane-Substituted Imidazol-2-ylidenes: Syntheses, Structures, and Reactivity⋆

Reactions of the Lewis acids BH3 and BEt3 with trimethylimidazole (1) lead to the borane adducts 2a and 2b. Deprotonation of 2a with n-butyllithium results in the formation of the novel N-borane-substituted imidazol-2-ylidene anion 3a– whereas deprotonated 2b rearranges unexpectedly to the anionic compound 3b–. This can be transformed into the carbene–borane adduct 4 by methylation. The reaction of 3a– with [Mn(CO)5Br] yields the carbene complex 5. Surprisingly, 3a– attacks Fe(CO)5 at a carbon atom which leads to the iron acyl complex anion 6–. The compositions of the products follow from spectroscopic and analytical data and from X-ray structure analyses for Li(thp)+3a–, Li(thf)2+3b– and Li(thp)3+6–.