Catalyst-Substrate Helical Character Matching Determines the Enantioselectivity in the Ishihara-Type Iodoarenes Catalyzed Asymmetric Kita-Dearomative Spirolactonization

• Published in: Journal of the American Chemical Society Vol. 145; no. 13; pp. 7301 - 7312
• Main Authors: Zheng, Hanliang, Cai, Liu, Pan, Ming, Uyanik, Muhammet, Ishihara, Kazuaki, Xue, Xiao-Song
• Format: Journal Article
• Language: English
• Published: WASHINGTON American Chemical Society 05.04.2023; Amer Chemical Soc
• Subjects: Chemistry; Chemistry, Multidisciplinary; enantioselectivity; halogens; hydrogen bonding; naphthols; organocatalysts; oxidation; Physical Sciences; reaction mechanisms; Science & Technology; stereochemistry; HYPERVALENT; DFT; OXIDATIVE DEAROMATIZATION; MECHANISTIC INSIGHTS; ORGANIC-REACTIONS; IODINE CATALYSTS; NONCOVALENT INTERACTIONS; SPIROCYCLIZATION; REARRANGEMENTS; SELECTIVITY
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.2c13295

Catalyst design has traditionally focused on rigid structural elements to prevent conformational flexibility. Ishihara’s elegant design of conformationally flexible C 2-symmetric iodoarenes, a new class of privileged organocatalysts, for the catalytic asymmetric dearomatization (CADA) of naphthols is a notable exception. Despite the widespread use of the Ishihara catalysts for CADAs, the reaction mechanism remains the subject of debate, and the mode of asymmetric induction has not been well established. Here, we report an in-depth computational investigation of three possible mechanisms in the literature. Our results, however, reveal that this reaction is best rationalized by a fourth mechanism called “proton-transfer-coupled-dearomatization (PTCD)”, which is predicted to be strongly favored over other competing pathways. The PTCD mechanism is consistent with a control experiment and further validated by applying it to rationalize the enantioselectivities. Oxidation of the flexible I­(I) catalyst to catalytic active I­(III) species induces a defined C 2-symmetric helical chiral environment with a delicate balance between flexibility and rigidity. A match/mismatch effect between the active catalyst and the substrate’s helical shape in the dearomatization transition states was observed. The helical shape match allows the active catalyst to adapt its conformation to maximize attractive noncovalent interactions, including I­(III)···O halogen bond, N–H···O hydrogen bond, and π···π stacking, to stabilize the favored transition state. A stereochemical model capable of rationalizing the effect of catalyst structural variation on the enantioselectivities is developed. The present study enriches our understanding of how flexible catalysts achieve high stereoinduction and may serve as an inspiration for the future exploration of conformational flexibility for new catalyst designs.