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Two independent methods reported in 2019 use iridium-catalyzed C–H borylation and a bulky tetrabutylammonium counterion to favor boron installation at an aromatic ring’s para position. The counterion does not attract the catalyst to that remote site; it blocks one competing meta position. The approach depends on the substrate—including a substituent at the ring’s 2-position—and is not a universal para-directing method.
What para-selective C–H borylation does
Aromatic C–H borylation replaces a hydrogen attached to an aromatic ring with a boron-containing group, creating an aryl boron compound. The para position is opposite a reference substituent on the ring; reaching it can be difficult when other positions react more readily.
Aryl boron compounds are useful intermediates for further synthesis. One important next step is Suzuki cross-coupling, a common way to form carbon–carbon bonds, including in drug-discovery research. The Phipps group’s 2019 paper describes one of the reported para-selective approaches.
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The strategy uses an aromatic sulfonate salt paired with a bulky tetrabutylammonium cation. The cation’s size sterically shields a nearby meta position, making that competing site less accessible. It does not work by drawing the catalyst toward the para position.
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Blocking one meta site is not enough by itself: a substituent at the ring’s 2-position is needed to disfavor the other meta site left unshielded. Together, these effects can make para borylation more favorable for suitable substrates. This is a non-covalent way to influence selectivity rather than a conventional large, covalently attached directing group that must later be removed.
Which substrates and ligand choices were reported?
The 2019 report describes compounds that can be temporarily converted to sulfonate salts, including anilines, phenols, benzylamines, and benzyl alcohols. The teams used off-the-shelf reagents, but performance depended on the substrate and ligand; the described scope should not be read as applying to every aromatic compound.
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| 2019 approach | Ligand described in the report | What is established |
|---|---|---|
| Robert J. Phipps’s team | Standard bipyridine | Reported para-selective C–H borylation using ion-pairing with a bulky counterion; detailed yields and selectivity ratios are not stated in the cited report. |
| Robert E. Maleczka Jr. and Milton Smith’s team | Methoxy-substituted bipyridine | The modified ligand improved para selectivity in that team’s system; detailed yields and selectivity ratios are not stated in the cited report. |
The two groups reported their work independently in 2019. The Phipps paper is M. T. Mihai, B. D. Williams, and R. J. Phipps, “Para-Selective C-H Borylation of Common Arene Building Blocks Enabled by Ion-Pairing with a Bulky Countercation,” Journal of the American Chemical Society 141 (2019), 15477–15482, doi:10.1021/jacs.9b07267. The second paper, by J. R. Montero Bastidas and colleagues, appeared in the same journal and volume at page 15483, doi:10.1021/jacs.9b08464.
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The conceptual advance is a way to influence remote-site selectivity through ion pairing and steric blocking, without relying on a large substrate-bound directing group. Ángeles Fernández-Ibáñez summarized the broader lesson as: “we should start looking to non-covalent interactions to control reactivity and selectivity in organic transformations”.
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The reported approach is conditional, not a general recipe for placing boron para to any substituent. The cited news account describes qualitative substrate and ligand effects, but does not provide verifiable substrate-by-substrate yields or selectivity ratios; those figures should not be inferred from the qualitative description. A later Phipps group publication is titled “Extended Sulfonated Bipyridine Ligands Targeting the Para-Selective Borylation of Arenes,” Tetrahedron 117 (2022), 132831; the group’s publication listing records it.
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