Closing the circle: Rh^I-catalyzed [2+2+1] cycloadditions are achieved under mild reaction conditions and in good to excellent yields with catalyst loading as low as 0.1 mol %. The diene–allenes react selectively with the proximal double bonds of the allene and diene units, thus allowing for a variety of substitutions. DCE=1,2-dichloroethane, TFE=2,2,2-trifluoroethanol, Ts=tosyl.

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.