The Diene Effect: The Design, Development, and Mechanistic Investigation of Metal‐Catalyzed Diene‐yne, Diene‐ene, and Diene‐allene [2+2+1] Cycloaddition Reactions

Abstract Metal‐catalyzed diene‐yne, diene‐ene, and diene‐allene [2+2+1] cycloaddition reactions provide new methods for the facile construction of highly functionalized five‐membered rings. These reactions can be conducted with a variety of substrate substitution patterns and functional groups and often in the absence of solvent. The special reactivity of dienes, a key to enabling or enhancing the effectiveness of the [2+2+1] and other reactions, is significantly different from that of alkynes, alkenes, or allenes. For example, the [2+2+1] reaction of a diene‐yne is accelerated compared to that of the corresponding ene‐yne. An even more dramatic “diene effect” is found with diene‐enes and diene‐allenes. While bis‐enes and ene‐allenes are not reported to yield [2+2+1] cycloadducts, the related diene‐enes and diene‐allenes undergo efficient [2+2+1] cycloadditions, providing new routes to cyclopentanones and alkylidenecyclopentanones. Mechanistic studies suggest that the unique reactivity observed with dienes arises from their participation in the putative rate‐determining reductive elimination step by providing an additional energy‐lowering coordination site for the transition metal catalyst.