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Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Two independent studies reported in 2019 showed how to install boron at the para position of certain aromatic rings using iridium-catalyzed C–H borylation. Their key idea is not to pull the catalyst toward the para site: a bulky tetrabutylammonium counterion shields a competing meta site, while a substituent at the ring’s 2-position helps suppress the other. The approach can make remote functionalization possible for suitable substrates, but it is not a universal para-directing method.
What para-selective C–H borylation does
Aromatic C–H borylation replaces a carbon–hydrogen bond on an aromatic ring with a carbon–boron bond. In the reported methods, iridium catalyzes that transformation, with the reaction favoring the para position—the ring position opposite an existing substituent.
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Aryl boron compounds are useful intermediates for further synthesis. One important follow-on reaction is Suzuki cross-coupling, which forms carbon–carbon bonds and is widely used in synthetic chemistry, including drug-discovery work. Robert Maleczka described this versatility as a reason that installing boron on aromatic molecules is valuable.
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The strategy uses aromatic sulfonate salts paired with tetrabutylammonium. The bulky cation creates steric crowding near one of the ring’s competing meta positions, making reaction there less favorable. In the explanation reported by Chemistry World, the counterion does not act as a conventional long-range director that attracts the catalyst to para; it mainly blocks one competing site.
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Blocking one meta position is not enough by itself. A substituent at the ring’s 2-position helps disfavor the other meta position, leaving the para site comparatively favored. The selectivity therefore comes from the combination of substrate structure and steric shielding, rather than from the counterion alone.
Which substrates and ligands were reported
The 2019 report describes aromatic compounds that can be converted to sulfonate salts, including anilines, phenols, benzylamines and benzyl alcohols. These methods use off-the-shelf reagents rather than an elaborate covalently attached directing group, but the substrate still needs to suit the strategy, including the relevant 2-substitution.
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The two teams used different ligands, and the reported performance depended on both ligand and substrate:
| 2019 approach | Ligand detail reported | What is established about performance |
|---|---|---|
| Robert J. Phipps’s group | Standard bipyridine ligand | Qualitative para-selective approach; substrate-by-substrate yields and selectivity ratios are not stated in the cited report. |
| Robert E. Maleczka Jr. and Milton Smith’s team | Methoxy-substituted bipyridine improved para selectivity in that system | Qualitative, substrate-dependent performance; comparative yields and selectivity ratios are not stated in the cited report. |
The Phipps paper, by M. T. Mihai, B. D. Williams and R. J. Phipps, appeared in Journal of the American Chemical Society 141 (2019), pages 15477–15482, under the title “Para-Selective C-H Borylation of Common Arene Building Blocks Enabled by Ion-Pairing with a Bulky Countercation”. The second team’s paper is cited in the news report as J. R. Montero Bastidas and colleagues, Journal of the American Chemical Society 141 (2019), page 15483, DOI 10.1021/jacs.9b08464.
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Why the result matters—and where it stops
Many aromatic substituents favor reaction at neighboring ortho positions, while conventional remote functionalization can require a large directing group that must later be removed. Counterion-based steric shielding offers another way to influence which C–H bond reacts, using a non-covalent interaction rather than relying only on a substrate-bound directing group. As Ángeles Fernández-Ibáñez put it, chemists should look to non-covalent interactions to control reactivity and selectivity in organic transformations.
This is a method for particular synthetic substrates, not a general rule that boron can be placed at para on any arene. The described selectivity depends on a suitable sulfonate-forming substrate, a 2-substituent, and the ligand–substrate combination. Detailed numerical scope, yields and selectivity ratios are not established by the reported qualitative account, so they should not be inferred from it.
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Later work on para-selective borylation
The Phipps group’s publication record also lists a 2022 paper by J. L. Douthwaite and R. J. Phipps, “Extended Sulfonated Bipyridine Ligands Targeting the Para-Selective Borylation of Arenes,” in Tetrahedron 117 (2022), article 132831. Its listing shows continued work on ligand design, but does not by itself establish additional scope or performance figures for the 2019 methods.
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