Extended sulfonated bipyridine ligands targeting the para-selective borylation of arenes

Abstract

Iridium-catalysed borylation of arenes is one of the most widely used metal-catalysed C–H activation processes to elaborate aromatic rings. Exercising catalyst control over regioselectivity of the C–H oxidative addition step remains an area of active research. In this paper we describe the synthesis of a selection of sulfonated bipyridine ligands in which there is an arene spacer between the sulfonate group and the bipyridine backbone. In comparison to our previous work which achieved meta-selective borylation and in which the sulfonated bipyridine bore no spacer, we hoped that these extended ligands may allow a larger macrocyclic transition state for C–H activation thus favouring the para-position. The synthesised ligands have been evaluated on a series of arenes bearing amides and quaternary ammonium salts at varying distances from the aromatic ring.