Pair Of Catalysts Builds Chiral Rings Using Visible Light

By combining two catalysts, researchers have used photochemistry to build enantiomerically enriched rings in high yields. The advance could add to the tool kit for building motifs present in agrochemicals and pharmaceuticals.

For more than a century, chemists have talked about generating chirality with photochemistry because it can access products not available by other routes. But photochemistry’s march toward enantioselectivity has lagged behind those for transition-metal catalysis or organocatalysis. The reason is that once a molecule absorbs a photon of light, it reacts before it can be reined in by a stereochemistry-controlling catalyst, explains Tehshik P. Yoon, who led the new work. The few existing enantioselective options require specialized light sources or carefully designed catalysts.

Yoon and his coworkers at the University of Wisconsin, Madison, reported in Science a different approach. Their method makes chiral cyclobutanes from the visible-light-promoted [2 + 2] photocycloaddition of α,β-unsaturated ketones (2014, DOI: 10.1126/science.1251511). It requires two catalysts: Ru(bpy)₃, a transition-metal complex that absorbs visible light, and a chiral Lewis acid made from the lanthanide element europium.

“We’re using wavelengths of light that pass through organic molecules,” so they don’t enter the excited state that leads to willy-nilly reactivity, Yoon says. It’s Ru(bpy)₃ that absorbs this light, which comes from a compact fluorescent bulb, and then “spits out an electron” to trigger cyclobutane ring formation, Yoon explains. The reaction occurs under chiral control because the chiral Lewis acid coordinates to the ketone substrate.

The work cleverly mimics photosynthesis in that it decouples the harvesting of light energy from bond-breaking and bond-forming steps, explains University of Neuchâtel, Switzerland, chemist Reinhard Neier in an accompanying commentary (DOI: 10.1126/science.1252965).

“I think the concepts are general” and will yield more than just cyclobutanes, Yoon says.