Divergent concerted and stepwise cycloadditions via enantioselective cross-conjugated iminium-ion catalysis

Cycloadditions are described as either concerted or stepwise processes, where a catalyst can influence both the mechanism and the peri-, regio- and stereoselectivity. We report experimental and DFT-computational investigations that reveal the versatile reactivities of cross-conjugated unsaturated iminium-ion intermediates. These undergo cycloadditions with cyclic and linear dienes via both stepwise and concerted pathways, respectively, providing two distinct classes of enantioenriched cycloadducts. Activation of 2-formyl-1,3-cyclohexadiene with a chiral 1,2-diaminocatalyst generates an acyclic cross-conjugated iminium-ion intermediate, which by utilizing 4π-electrons, undergoes enantioselective inverse-electron-demand [4 + 2] cycloadditions with cyclic dienes via a stepwise mechanism, as well as indene and a styrene, affording cycloadducts in high yields and up to 96% ee. In contrast, only 2π-electrons of the cross-conjugated iminium-ion intermediate are involved in the [4 + 2] cycloaddition with linear dienes, proceeding via an ambimodal asynchronous concerted pathway, yielding decalin derivatives in up to 96% ee. The generality of this catalytic enantioselective concept is expanded to also include acyclic cross-conjugated iminium-ion intermediates, which react with both cyclic and linear dienes following the same two distinct reaction pathways. In contrast, the analogous iminium-ion, generated from 1-formylcyclohexene, does not react under the same conditions.