Fischer Carbene Complexes: A Glance at Two Decades of Research on Higher‐Order Cycloaddition Reactions

The structure of Fischer carbene complexes (FCCs) is electron deficient. If bearing an α,β‐unsaturated system, it can generate a wide variety of compounds by undergoing many different transformations, including higher‐order cycloadditions. The latter are described as pericyclic reactions in which more than six electrons participate. These reactions have been employed in various areas of organic synthesis, resulting in highly selective compounds with a broad range of scaffolds. The first studies on higher‐order cycloadditions involving FCCs frequently yielded competing byproducts. Many groups have attempted to increase selectivity by exploring distinct reaction conditions, reagents and co‐catalysts (e. g., metal‐mediated cycloadditions). The present review is the first to focus exclusively on using higher‐order cycloadditions involving FCCs to synthesize carbocycles and heterocycles. Based on two decades of reports, an analysis is made of the main aspects of the mechanisms proposed for higher‐order cycloadditions and the structural diversity obtained by the substituent effect. This review describes the reports over the past 20 years on higher‐order cycloadditions (HOC) involving Fischer carbene complexes (FCCs) to obtain diverse carbocycles and heterocycles. The FCCs analyzed contain group VI metals, covering alkenyl, alkynyl and alkyl carbene complexes, which reacted with alkynes, allenes, enolates, fulvenes, heptafulvenes and other polyenic species through concerted or stepway process, some of them co‐catalyzed by other metals. An analysis is made of the main aspects of the mechanisms proposed for HOC.