Reactivity Studies on Organolithium Complexes Bearing Electrophilic Carbenes toward Isonitriles

Isonitriles, as unsaturated substrates, have rarely been reported to react with organolithium compounds containing an electrophilic carbene. The lithium compounds [<i>η</i>¹:<i>η</i>¹:(<i>μ</i>₂-<i>η</i>¹:<i>η</i>¹)-1-(GCH₂)-3-(2,6-^<i>i</i>Pr₂C₆H₃N═CH)-C₈H₄NLi]₂ (G = 2-N-C₅H₁₀NCH₂ (<b>1</b>), CH₃OCH₂ (<b>2</b>), 2-C₄H₇O (<b>3</b>), 2-(CH₂)₄NCH₂ (<b>4</b>)) have been synthesized by applying the corresponding ligands bearing an ene(amido) neighboring electrophilic carbene, which bridges the lithium centers. The reactivities of the lithium compounds toward aromatic isonitriles have been studied, revealing a novel <i>aza</i>-[4 + 1] cyclization that delivers various multifunctionalized 2,4-dihydropyrroloindoline-based lithium complexes <b>5</b>-<b>12</b> <i>via</i> insertion of the isonitriles into the Li-C and Li-N bonds of the corresponding lithium compounds. Furthermore, DFT calculation results support the key mechanistic proposal that the <i>aza</i>-[4 + 1] cyclization processes involve coordination of the isonitrile to the metal center and 1,1-migratory insertion of the isonitrile into the electrophilic carbene to produce the ketenimine intermediate, followed by attack of the nitrogen atom on the central electrophilic carbon atom of the ketenimine. These processes differ sharply from the reported reactivity patterns of lithium complexes toward isonitriles.