Catalytic Enantioselective Allylboration and Related Reactions of Isatins Promoted by Chiral BINOLs: Scope and Mechanistic Studies

• Published in: Journal of organic chemistry 2023-02, Vol.88 (3), p.1469-1492
• Main Authors: Braire, Julien, Macé, Aurélie, Zaier, Rania, Cordier, Marie, Vidal, Joëlle, Lalli, Claudia, Martel, Arnaud, Carreaux, François
• Format: Article
• Language: English
• Subjects: Chemical Sciences
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/acs.joc.2c02476

An improvement in the catalytic enantioselective allylboration of isatins with 2-allyl-1,3,2-dioxaborolane in the presence of chiral BINOL derivatives is reported, offering an efficient one-step access to enantioenriched N-unprotected 3-allyl-3-hydroxy-2-oxindoles. This catalytic process is also effective for the crotylboration reaction with enantiomeric ratios (er) up to 97:3, as well as for the asymmetric synthesis of homopropargylic alcohols via an allenyl addition to indoline-2,3-diones. Origins of the high enantioselectivity in chiral BINOL-catalyzed allylboration of isatins were examined by DFT calculations. A hypothetical scenario suggested a crucial internal hydrogen bonding between the amide group (CO···H–O) and the ethylene hydroxyl of the transient chiral mixed boronate ester, generating a rigid and stabilized system that favors the addition of the allylboron species to the Re face of the ketone function. The key role of the alcohol additive (t-BuOH or t-AmOH) in the enantioselective allylboration reaction of isatins has also been shown on the basis of a kinetics study and computational calculations by favoring the transesterification of the 2-allyl-1,3,2-dioxaborolane with BINOL via proton transfer processes.