The use of noncovalent interactions to direct transition-metal catalysis is a potentially powerful yet relatively underexplored strategy. Alongside hydrogen bonds, ion-pairing interactions have emerged as a powerful tool for control of enantioselectivity, but an equally important aspect in which careful control is required is regioselectivity. Whilst important advances have demonstrated that multiple hydrogen bonds are able to provide precise molecular recognition in particular situations, ion-pairing interactions have not been explored in the context of addressing regioselectivity challenges.<br/>We will describe the development of an ion pair-directed approach to controlling regioselectivity in the iridium-catalyzed borylation of several classes of aromatic quaternary ammonium salts. To achieve this we took the basic bipyridine ligand and appended a methylene sulfonate tether to the with the hypothesis that ion-pairing with the ammonium group would present the meta C-H bond to the iridium metal in the lowest energy transition state (Scheme, 1). This proved effective for the meta-selective borylation of range of substituted aniline-derived ammonium salts as well as a range of benzylamine-derived ammonium salts. To demonstrate the utility of our products, which contain up to three orthogonal handles for cross-coupling, we show a sequence of three palladium-catalyzed cross-coupling reactions in an iterative manner (Scheme, 2).

Ion pair-directed regiocontrol in transition-metal catalysis: A meta-selective C–H borylation of aromatic quaternary ammonium salts

253rd National Meeting (2017)

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