Five-membered aryne thought to be impossible synthesized in nickel complex

Courtney Roberts doesn’t like being told what is and isn’t possible.

Heteroaromatic rings with five sides and a triple bond are so strained that they were thought to be impossible to make. But where others saw an impossibility, Roberts saw an opportunity. Now, she and her group at the University of Minnesota report that they’ve made 7-aza-2,3-indolynes by using nickel to stabilize the triple bond (Science 2024, DOI: 10.1126/science.adi1606).

It took a few years and a lot of work, but “we gave it the faith and the belief that we were going to get there in the end,” says Roberts. The key to creating the elusive indolyne is back-donation of electrons from the nickel to the aryne, which makes the triple bond longer and more stable than if it wasn’t supported by a metal.

“We had a burden of proof on us,” says PhD student and first author Jenna Humke. The first mention of a 5-membered heteroaryne in the chemical literature was in 1902, but it was impossible to validate at the time. In 2012, researchers at the University of California, Los Angeles published a computational model saying that these highly strained arynes cannot exist on their own (Angew. Chem., Int. Ed., DOI: 10.1002/anie.201108863). But the nickel-supported complex created by Roberts’ team is stable enough to crystallize and characterize spectroscopically.

The new indolyne complex reacts with both nucleophiles and electrophiles, making it a versatile intermediate for adding new functional groups to azaindoles.

“This work highlights how metal-bound arynes can expand not only the types of reactions that arynes can participate in but also the types of arynes that we thought were even possible to access,” says Jessica Kisunzu, who researches aryne chemistry at Colorado College, in an email to C&EN.

Pam Tadross, an organic chemist and technical director for small-molecule manufacturing at Merck & Co., called the paper “an elegantly designed and thoughtfully presented study” that turns a limitation—7-aza-2,3-indolyne’s instability—into a reactivity advantage.

Roberts says that she and her group are excited to take what they’ve learned from 7-aza-2,3-indolynes and apply it to other nitrogen- and oxygen-containing ring systems.