The Exploration of a Strategy for Regioselective Arene Amination Utilising Non-Covalent Interactions
The formation of arene C-N bonds for the preparation of anilines is one of the most used reactions in industry and academia. Numerous methods which utilise reactive N-centred radicals for the synthesis of anilines from arenes have recently been developed. However, regiochemical control is a major challenge associated with these methods, with mixtures of regioisomers commonly obtained in most protocols. Non-covalent interactions have been used in a limited number of examples to control regioselectivity in radical reactions. We therefore decided to investigate whether non covalent interactions between an arene substrate bearing a suitable directing group and an incoming N-centred radical could be a viable strategy to selectively target the arene ortho position. Initial investigations focussed on using hydrogen bonding interactions between substrate and radical to direct radical addition to the arene ortho position. Whilst promising reactivity was seen in many cases, the regiochemical outcome was poor. Later studies focussed on the use of anionic substrates and investigating whether ion pairing interactions with a cationic N-centred radical could direct ortho-selective radical addition. It was found that reacting anionic phenylsulfamate substrates with reagents which generate aminium radical cations resulted in an ortho-selective radical amination, allowing access to ortho-phenylenediamine products. This work was subsequently expanded upon to include arenesulfonates and arenecarboxylates as substrates for ion pair-directed ortho-selective amination. In these cases, the reaction proceeds via rearrangement of readily accessible O- (arenesulfonyl)hydroxylamines and O-benzoylhydroxylamines, respectively, and constitutes a remarkably facile way to access the ortho-aminated products. This work provides a blueprint for further development of regioselective amination reactions and more generally showcases the potential for non-covalent interactions to control regioselectivity in radical chemistry.