When chemist Tehshik Yoon looks out his office window, he sees a source of energy to drive chemical reactions. Plants "learned" to synthesize chemicals with sunlight eons ago; Yoon came to the field a bit more recently.
But this week, in the journal Science, he and three collaborators detail a way to use sunlight and two catalysts to create molecules that are difficult to make with conventional techniques.
In chemistry, heat and ultraviolet (UV) light are commonly used to drive reactions. Although light can power reactions that heat cannot, UV has disadvantages, says Yoon, a chemistry professor at University of Wisconsin-Madison. The UV often used in industry carries so much energy that "it's dangerous to use, unselective, and prone to making unwanted by-products."
Many chemicals exist in two forms that are mirror images of each other, and Yoon is interested in reactions that make only one of those images.
"It's like your hands," Yoon says. "They are similar, but not identical; a left-hand glove does not fit the right hand. It's the same way with molecules in biology; many fail unless they have the correct 'handedness,' or 'chirality.'"
The pharmaceutical industry, in particular, is concerned about controlling chirality in drugs, but making those shapes is a hit-or-miss proposition with UV light, Yoon says.
He says the new technique answers a question posed by a French chemist in 1874, who suggested using light to make products with controlled chirality. "Chemists could never do that efficiently, and so the prejudice was that it was too difficult to do."
When a graduate student asked for a challenging project seven years ago, Yoon asked him to explore powering reactions compounds with metals that are used to capture the sun's energy in solar cells. In a solar cell, these metals release electrons to make electricity.
"We are taking the electrons that these metals spin out and using their energy to promote a chemical reaction," Yoon says.
Plants do the same thing during photosynthesis, he says: absorb light, release high-energy electrons, and use those electrons to bond water and carbon dioxide into sugars. That reaction is the basis of essentially all of agriculture and all food chains.
Once the solar-cell metal supplied electrons, Yoon thought about using a second catalyst to control chirality. He passed the project to Juana Du, another graduate student.
"She must have synthesized 70 different catalysts," says Yoon. "The great thing about grad students here is that they are so bold. At a certain point I was ready give up; this was so hard I did not know where to go next. Juana went to the literature, made things that had no business working, and they worked beautifully."
To control chirality, the second catalyst held the chemicals under transformation in the correct orientation so the electrons could create only the desired chirality. After graduate student Kazimir Skubi found a way to generalize the control mechanism, Yoon says, post-doctoral fellow Danielle Schultz, "made an interesting discovery: if you make a really small tweak to the chiral control catalyst, you get a completely different shape to the product molecules."
Shultz, Du and Skubi are Yoon's co-authors on the Science paper.
Introducing a second catalyst allowed much greater control, Yoon says.
"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 kind 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 or heat. Ultimately, Yoon says, the technique may interest material scientists, who are focusing more on chirality, and especially drug makers.
"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," Yoon says. "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."
Ultimately, Yoon would like to move back to the future and to make chemicals as plants do: powered by sunlight, with all the environmental advantages that implies. It's all in the classic history of photochemistry, Yoon says, pointing to an Italian chemist who advocated powering reactions with sunlight in 1912, and wrote, "… if in a distant future the supply of coal becomes completely exhausted, civilization will not be checked by that, for life and civilization will continue as long as the sun shines!"
- J. Du, K. L. Skubi, D. M. Schultz, T. P. Yoon. A Dual-Catalysis Approach to Enantioselective [2 2] Photocycloadditions Using Visible Light. Science, 2014; 344 (6182): 392 DOI: 10.1126/science.1251511
Cite This Page: