An Operationally Simple and Mild Oxidative Homocoupling of Aryl Boronic Esters To Access Conformationaly Constrained Macrocycles

摘要

Constrained macrocyclic scaffolds are recognized as challenging synthetic motifs with few general macrocyclization methods capable of accessing these types of systems. Although palladium catalyzed oxidative homocou-pling of aryl boronic acids and esters to biphenyls has been recognized as a common byproduct in Suzuki-Miyaura cross-couplings for decades, this reactivity has not been leveraged for the synthesis of challenging molecules. Here we report an oxidative boronic ester homocoupling reaction as a mild method for the synthesis of strained and conformationally restricted macrocycles. Higher yields and better efficiencies are observed for intramolecular diboronic ester homocouplings when direcdy compared to the analogous intramolecular Suzuki-Miyaura cross-couplings or reductive Yamamoto homocouplings. Substrates included strained polyphenylene macrocydes, strained cycloalkynes, and a key macrocyclic intermediate toward the synthesis of acerogenin A. Notably, this oxidative homocoupling reaction is performed at room temperature, open to atmosphere, and without the need to rigorously exclude water, thus representing an operationally simple alternative to traditional cross-coupling macrocyclizations. The mechanism of the reaction was investigated indicating that 1-5 nm palladium nanopartides may serve as the active catalyst.