Unraveling the Selectivity Patterns in Phosphine-Catalyzed Annulations of Azomethine Imines and Allenoates.

The mechanism and selectivity of phosphine-catalyzed [3 + 2] and [3 + 3] annulations of azomethine imines and allenoates have been computationally studied. Exploration of the potential energy surface reveals that the cyclization step is a key step controlling the selectivity of the process. This contrasts with previous studies on related transformations where the initial nucleophilic addition involving the activated allenoate was found to exclusively control the regioselectivity of the transformation. Among the possible reaction pathways, the energetically low-lying reaction channel involves an intramolecular Michael addition leading to the experimentally observed [3 + 2] product. The factors controlling the observed regioselectivity have been quantitatively rationalized by means of state-of-the-art computational methods, namely, the activation strain model of reactivity in combination with the energy decomposition analysis.