| Fisher carbene complexes have found many applications in synthetic organic chemistry. There is tremendous amount of interest in developing new methods for their synthesis, and in exploring new chemistry of these versatile compounds.; We developed a new synthesis of pyridyl and quinolinyl carbene complexes that were previously difficult to prepare. These heteroaromatic carbene complexes undergo annulation reactions with internal and terminal alkynes to afford various heterocyclic compounds such as pyrrolizines, quinolizones, cyclopentaquinoline-ones, and cyclopentapyridine-ones, etc.; We also developed a general and efficient synthesis of heterocylo[b]-fused carbazoles using a novel addition and intramolecular Diels-Alder reactions of Fisher carbene complexes and isonitriles. The reactions go through a metal-bound conjugated ketenimine intermediate. We can rapidly construct various structurally complex and potentially biologically important carbazoles such as indolo[2,3-b]carbazoles, indolo[3,2-b]carbazoles, pyrrolo[2,3-b]carbazoles, and furo[2,3-b]carbazoles. There exists two possible reaction pathways for the conjugated ketenimine intermediate, namely the 6π electrocyclization and the intramolecular Diels-Alder reaction. Calculations were performed using density functional theory at the B3LYP level with a 6-31G* basis set to locate the transition structure energy and geometry for the two pathways. Calculation results clearly show that intramolecular Diels-Alder reaction is the favored pathway for the conjugated ketenimine system; Lastly, we synthesized the first β-keto Fischer carbene complexes that bears alpha protons. The alpha protons, being doubly activated by both the carbonyl group and the metal carbene moiety, are very acidic, and thus pose a great synthetic challenge. These β-keto carbene complexes, upon treatment with bases, undergo elimination reactions to afford enol ethers. |
