“We grabbed a well-known catalytic intermediate iridium and presented a completely new application,” says Paolo Melchiorre from the Institute for Chemical Research of Catalonia (ICIQ) in Spain. His team has shown that a conventional allylic substitution catalyst completely switches reactivity under blue light to enable enantioselective cross-coupling.1

Photochemical reactions expand the toolbox of organic chemists and provide access to reactivities that cannot be achieved with conventional methods. These processes often require a catalyst and antenna – a photoactive molecule that traps light, usually intense ultraviolet (UV), and creates reactive species like radicals.

An image showing a long test tube illuminated by blue light

The ICIQ team has now found that an ordinary chiral iridium complex combines both functions – and does not require an intense UV light source. “We activated our photoactive iridium complex with blue, visible light, which is widely available and inexpensive,” explains Giacomo Crisenza from ICIQ, who also worked on the project. When exposed to light, the iridium catalyst becomes a one-electron oxidizer. “This opens the door to unprecedented carbon-carbon cross-coupling, creating products that are inaccessible under thermal conditions,” he continues.

Although the discovery appears accidental, it is based on rational design and experience. “A few years ago, our group observed that certain intermediates in organocatalysis reacted to light activation, which led to unexpected products,” says Melchiorre. “After developing similar solutions for enamines, iminium ions, and other species, we always check that our catalyst works without an external antenna to absorb light,” he adds.

Melchiorre points out that other chemists have described similar behavior in certain organometallic compounds.2 “However, these complexes were designed specifically for this purpose,” he explains.

“Even if they were not developed for photocatalytic processes, the light excitation is the old one” [organometallic] Catalysts can uncover new mechanistic pathways that are inaccessible under conventional thermal conditions, ”says Anabel Lanterna, expert on photocatalysis at the University of Nottingham, UK. “Developing robust catalysts is challenging, time-consuming and expensive,” she says. “The reuse of known organometallic complexes could accelerate the development of photoredox catalysis.”

A scheme that shows that the iridium complex undergoes either an allylic substitution or a cross-coupling reaction, depending on the conditions

Lanterna also emphasizes the value of using visible light. ‘[It] eliminates the generation of wasteful by-products [and] also reduces process costs. ‘ Deep ultraviolet sources require specialized and expensive quartz reactors, she explains. In fact, the ICIQ researchers examined the absorption properties of both the reagents and the catalyst and selected the wavelength that maximized the efficiency and selectivity towards the active iridium species.

These cross-coupling reactions could find application in the development of fine chemicals and pharmaceuticals. Also, photochemistry would make these reactions greener. ‘[We] could drastically reduce [the] Carbon footprint and make chemicals and medicines more affordable, ‘says Lanterna.

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