Arynes from triaryloxonium ions

• A collaborative study including two Oxford Chemistry research groups has demonstrated an ultra-mild and functional group tolerant method to synthesise versatile arynes.
• The team showed that triaryl oxonium ions can be transformed into arynes – useful reactive intermediates for the functionalization of aromatic rings – by treatment with solid potassium phosphate at room temperature.
• Their study, published in Nature Synthesis, is conducted under milder conditions than other similar methods, and should stimulate further investigations into the usefulness of aryl oxonium ions.

Arynes – aromatic molecules containing a triple carbon–carbon bond – have a particularly rich history as a broadly applicable reactive intermediate for organic synthesis. They are often generated using strong bases or fluoride ions, which can sometimes limit their applicability when other functional groups on the molecule don’t tolerate these conditions.

Now, a team involving the Burton and Smith groups from Oxford Chemistry, as well as the Paton group at Colorado State University, have shown that triaryl oxonium ions can be transformed into arynes through treatment with solid potassium phosphate at room temperature. In their study, published today in Nature Synthesis, they demonstrated synthetically that a wide range of arynes containing useful functional groups can be generated, and subsequently trapped by cycloaddition reactions (whereby the carbon–carbon triple bond reacts with an arynophile, such as furan) in high yield.

The team used quantum chemical calculations, in conjunction with a kinetic isotope study, to further investigate the mechanism behind benzyne formation from the oxonium ion. Their calculations unveiled a step-wise E1cB type elimination mechanism via a zwitterionic intermediate, in contrast to the concerted manner of benzyne formation from iodonium ions.

Their synthetic and computational investigation has shown that the oxonium ion is both a powerful electron-withdrawing group, and a particularly effective leaving group. They anticipate that triaryloxonium salts such as those investigated may be useful for generating other functionalized arynes.

Final year Oxford DPhil student Maddie Hindson, who contributed to the study, said:

It has been really exciting to be part of the team working on triaryl oxonium ions - we have a lot to learn about their chemistry and it will be interesting to see where their reactivity takes us next.

You can read more about the study in Nature Synthesis.