Ion-Pair-Directed Borylation of Aromatic Phosphonium Salts.

Abstract

Control of positional selectivity in C-H activation reactions remains a challenge for synthetic chemists. Non-covalent catalysis has the potential to be a powerful strategy to address this challenge. As part of our ongoing investigations into the use of ion-pairing interactions in site-selective catalysis, we demonstrate that several classes of aromatic phosphonium salts undergo iridium catalyzed C-H borylation with high selectivity for the arene meta position. This is achieved using a bifunctional bipyridine ligand bearing a pendant sulfonate group which had previously been successful for borylation of aromatic ammonium salts. In this case, the phosphonium salts give higher meta-selectivity than the corresponding ammonium salts. We propose that the high selectivity occurs due to an attractive electrostatic interaction between substrate and ligand in the transition state for borylation.