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Enantioconvergent Cu-Catalyzed Intramolecular C–C Coupling at Boron-Bound C(sp3) Atoms of α‑Aminoalkylboronates Using a C1‑Symmetrical 2,2′-Bipyridyl Ligand Attached to a Helically Chiral Macromolecular Scaffold

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journal contribution
posted on 16.10.2020, 14:16 by Yukako Yoshinaga, Takeshi Yamamoto, Michinori Suginome
Enantioconvergent intramolecular coupling of α-(2-bromobenzoylamino)­benzylboronic esters was achieved using a copper catalyst having helically chiral macromolecular bipyridyl ligand, PQXbpy. Racemic α-(2-bromobenzoylamino)­benzylboronic esters were converted into (R)-configured 3-arylisoindolinones with high enantiopurity using right-handed helical PQXbpy as a chiral ligand in a toluene/CHCl3 mixed solvent. When enantiopure (R)- and (S)-configured boronates were separately reacted under the same reaction conditions, both afforded (R)-configured products through formal stereoinvertive and stereoretentive processes, respectively. From these results, a mechanism involving deracemization of organocopper intermediates in the presence of PQXbpy is assumed. PQXbpy switched its helical sense to left-handed when a toluene/1,1,2-trichloroethane mixed solvent was used, resulting in the formation of the corresponding (S)-products from the racemic starting material.