Dienamine-activated cycloalkenones participate in various higher-order cycloadditions with tropone and heptafulvenes. The mechanisms and selectivities of these first organocatalytic stereoselective [6+4] and [8+2] cycloadditions have been investigated by combined experimental and computational studies.<br/>The [6+4] reaction of 2-cyclopentenone and tropone is much faster than the potentially competing [4+2] cycloaddition. Furthermore, the rate- and stereodetermining transition state features a strong hydrogen-bond that was shown to be crucial for obtaining high yield and excellent stereoselectivity.<br/>The reactions of 2-cyclohexenone and different heptafulvenes give either [4+2] or [8+2] cycloadducts. The reason for this change in periselectivity was evaluated by comparison of free energy reaction profiles. These studies additionally allowed for prediction of novel reaction partners that could be verified experimentally. In all cases, stepwise processes were established as the lowest energy reaction paths compared to alternative concerted mechanisms. Isolation of a reaction intermediate provided compelling evidence for the computed stepwise mechanisms.

Organocatalytic stereoselective higher-order cycloadditions - mechanisms and selectivities

ACS Spring 2022

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