A new cobalt catalyst designed by Paul Chirik and coworkers at Princeton University gives chemists two mechanisms for the price of one. The team’s alkene hydrogenation reaction runs differently depending on whether they activate the catalyst with heat or light (ACS Catal 2020, DOI: 10.1021/acscatal.0c05136). When the team heats the reaction, the mechanism involves a radical-based H atom abstraction. However, when they hit the reaction with visible light, it goes through a coordination-insertion mechanism. This opens up new possibilities in switchable catalyst design, where scientists could use heat to hydrogenate one position or light to reduce another, without changing catalysts. Using a variety of alkenes and a set amount of catalyst, the team ran reactions for 18 h activated by either heat or blue light. Yields with the light reaction were typically near 99%, while heat produced low-to-moderate yields. Chirik and coworkers also found that the light reaction can activate both cyclic and terminal alkenes, while the heat reaction leaves the cyclic alkene untouched (shown in scheme). This means that the two approaches can produce different reaction products from the same starting materials in some cases.
“The way it works turned out to be not the way it’s designed to,” Chirik says. Generally, radical mechanisms tend to go with photochemical reactions, but here the opposite is true, he says. Through a series of radical-trapping and deuterium-labeled reactions, the team determined that at 100 °C, the catalyst transfers an H atom to the substrate, leaving a radical at the Co center. This quickly grabs the alkene, which reacts to produce the reduced alkane. But under the blue light of a kessil lamp, a CO ligand jumps ship, transforming the 18-electron, bench-stable catalyst into a 16-electron, reactive catalyst. This opens a spot for the alkene to bind to the catalyst, resulting in the hydrogenated alkane.
That this catalyst shows selectivity based on the mechanistic pathway helps push first-row transition metal catalysis research forward, says Christopher Teskey, an organometallic chemist at Aachen University in Germany. The paper also shows the potential of easy-to-handle catalysts that can be turned on with light. “A lot of people don’t have glove boxes, but almost every synthetic group has a set of blue LEDs these days,” Teskey says.