Oxyanion Steering and CH−π Interactions as Key Elements in an N-Heterocyclic Carbene-Catalyzed [4 + 2] Cycloaddition

The N-heterocyclic carbene catalyzed [4 + 2] cycloaddition has been shown to give γ,δ-unsaturated δ-lactones in excellent enantio- and diastereoselectivity. However, preliminary computational studies of the geometry of the intermediate enolate rendered ambiguous both the origins of selectivity and the reaction pathway. Here, we show that a concerted, but highly asynchronous, Diels–Alder reaction occurs rather than the stepwise Michael-type or Claisen-type pathways. In addition, two crucial interactions are identified that enable high selectivity: an oxyanion-steering mechanism and a CH−π interaction. The calculations accurately predict the enantioselectivity of a number of N-heterocyclic carbene catalysts in the hetero-Diels–Alder reaction.