Ion Pair-Directed C-H Activation on Flexible Ammonium Salts: Meta-Selective Borylation of Quaternized Phenethylamines and Phenylpropylamines

Abstract

Ion pairing has unexplored potential as a key catalyst-substrate interaction for controlling regioselectivity and site-selectivity in transition-metal catalysis, particularly in the area of C-H activation. However, there is a significant perceived challenge that has meant that few have investigated this approach to date - that of the low directionality, which could present an unsurmountable challenge if seeking positional selectivity on flexible substrates. Herein, we demonstrate that even flexible substrates with several freely rotatable bonds undergo ion pair-directed C-H borylation with good to excellent levels of regiocontrol for the arene meta-position. Furthermore, we demonstrate that in specially designed competition substrates, ion pair direction prevails over competing hydrogen bond direction. We anticipate that these findings will inspire the greater incorporation of ion-pairing into site-selective catalytic strategies.