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Abstract
The use of noncovalent interactions to direct transition-metal catalysis is a potentially powerful yet relatively underexplored strategy, with most investigations thus far focusing on using hydrogen bonds as the controlling element. We have developed an ion pair-directed approach to controlling regioselectivity in the iridium-catalyzed borylation of two classes of aromatic quaternary ammonium salts, leading to versatile meta-borylated products. By examining a range of substituted substrates, this provides complex, functionalized aromatic scaffolds amenable to rapid diversification and more broadly demonstrates the viability of ion-pairing for control of regiochemistry in transition-metal catalysis.
Highlights
The use of noncovalent interactions to direct transitionmetal catalysis is a potentially powerful yet relatively underexplored strategy, for addressing issues of regioselectivity in synthetic chemistry
Quaternary ammonium salts are readily accessed, and many methods exist for their elaboration in sp[2], sp3, and heteroatom cross coupling,[13] reduction,[14] conversion to boronate esters,13d,15 partners for C−H activation,[16] displacement with 18F,17 and [1,2] and [2,3] rearrangement chemistry[18] among others
We investigated whether aniline-derived quaternary ammonium salts may exhibit similar trends (Scheme 2)
Summary
The use of noncovalent interactions to direct transitionmetal catalysis is a potentially powerful yet relatively underexplored strategy, for addressing issues of regioselectivity in synthetic chemistry. Monosubstituted or 1,2disubstituted arenes generally give inseparable mixtures of isomers (Chart 1, eq 3), unless a particular substituent can direct either ortho6c,8 or meta,[9] or a bulky ligand favor para.[10,11] In particular, accessing the meta position is desirable due to the relative paucity of methods to access this position, despite recent interest.[5,9,12] To address this, we sought to investigate cationic arene substrates in iridium-catalyzed borylation, anticipating that the charged center would be able to ion pair with an anionic bifunctional ligand (Chart 1, eq 4).
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