Oxazaborolidinones: Steric Coverage Effect of Lewis Acidic Boron Center in Suzuki-Miyaura Coupling Reactions.

It was demonstrated that α-hydroxycarboxamide is an excellent boron-protecting group. The reaction between α-hydroxycarboxamide and organoboronic acids produced stable oxazaborolidinones (OxBs), in which the sp 2 ${{_{{\rm sp}{^{2}}}}}$ -hybridized boron atom was sterically protected by α-hydroxycarboxamide. The alkyl groups of the α-hydroxycarboxamide moiety can dynamically cover the p-orbital of the sp 2 ${{_{{\rm sp}{^{2}}}}}$ -hybridized boron center, creating a small space around the boron atom, allowing for smooth transmetalation by a Pd catalyst and easy deprotection by water. This protecting phenomenon is effective for readily purification, Suzuki-Miyaura coupling reactions with unstable boronic acids and iterative cross-couplings.