title: Oxyanion steering and CH-π interactions as key elements in an N-heterocyclic carbene-catalyzed [4 + 2] cycloaddition.
abstract: 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.
authors: Department of Chemistry, Roy and Diana Vagelos Laboratories, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA.
authors: Scott E
authors: SE
authors: Allen
authors: Jessada
authors: J
authors: Mahatthananchai
authors: Jeffrey W
authors: JW
authors: Bode
authors: Marisa C
authors: MC
authors: Kozlowski
mesh_terms: D000838:Anions / Q000737:chemistry; D002384:Catalysis; D003500:Cyclization; D006571:Heterocyclic Compounds / Q000737:chemistry*; D007783:Lactones / Q000138:chemical synthesis / Q000737:chemistry*; D008697:Methane / Q000031:analogs & derivatives* / Q000737:chemistry; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D000838:Anions; D006571:Heterocyclic Compounds; D007783:Lactones; C030011:carbene; D008697:Methane; D010100:Oxygen