Using sunlight and two catalysts, chemists at the University of Wisconsin, Madison, Wis., have created molecules that are typically difficult to make with conventional methods. The reaction precisely controls chirality and could be useful in drug making and materials science, where correct shape is essential.
Inspired by photosynthesis, the first step involved exploring metals used to capture the sun’s energy in solar cells.
“We are taking the electrons that these metals spin out and using their energy to promote a chemical reaction,” says Tehshik Yoon, a chemistry professor at the university who led the research.
Next, Yoon’s team investigated using a second catalyst to control chirality. The second catalyst holds the chemicals under transformation in the right position to enable the electrons to create the desired chirality. After finding a way to generalize the control mechanism, the team discovered that tweaking the chiral control catalyst resulted in product molecules with a completely different shape.
“One reason this field has failed is that a single catalyst had to both absorb light and control the chirality,” says Yoon. “If you tweak the single catalyst, you change its effects. By separating the two roles, you can make all kinds of changes to chirality without messing up the photochemical catalyst. To get this to work, two stars have to be aligned.”
The experiments to date have made square structures with four carbons that would be difficult to make with UV light or heat. The second catalyst allows much greater control; the team reports enantiomeric excesses of 92%. A recent Science article includes more details.
“Drug companies need compounds with well-defined chirality, and they want structures that nobody has made, and we have structures that are really strained, exotic, with controlled chirality,” notes Yoon. “These are part of an unexplored space in molecular diversity. Now that we have a platform for using these catalysts in tandem, we are looking more broadly to see what else we can do.”
The team now is pursuing a few potential directions. “One is to determine what we might be able to do with these enantioenriched cyclobutanes. They are unique structures, so we wonder if they might have unique physical properties or biological effects, and we are pursuing collaborations along those lines. So certainly, continuing to understand and optimize the conditions for enantioselective synthesis is a continuing goal,” Yoon explains.
“We’ve already begun to explore other enantioselective reactions, and we have some very exciting results already,” he continues. “Now that we know that chiral Lewis acids and photocatalysts are mutually tolerant of one another, we can start to apply everything that synthetic chemists have learned about chiral Lewis acid catalysis over the past several decades to photochemical activation and really quickly discover some new chemistry.”
Yoon says one key challenge is to better understand the structure of the Lewis acid. “We have some evidence that the active catalyst might not be a simple 1:1 complex of ligand to metal … to start the process of rationally improving selectivities, we will need to have some sense of what the important structural parameters are.”
Yoon admits that a dual-catalyst system is susceptible to deactivation by any poison that affects either of the catalysts. “I think it’s fair to expect that we will need to be mindful of managing cross-talk between the activities of the two catalytic cycles. That being said, the ruthenium photocatalyst is surprisingly robust. It is octahedrally saturated and seems to be quite stable to a number of chemical insults. I think this is what makes our dual-catalyst approach successful in the first place,” he explains.
Currently, there are no plans to scale up the process. “This is simply a proof-of-concept study,” notes Yoon. “A few pharmaceutical companies have approached us to see whether these reactions might be useful in their investigations, and as part of this, they have helped us demonstrate that we can run photochemical reactions on multi-gram scales quite readily,” he adds.
The dual-catalyst approach has the potential to be a valuable platform for developing a wide range of broadly useful stereo-controlled reactions, such as controlling photo-induced radical processes. In fact, Yoon already has been studying this and hopes to have results within a few months.