A greener strategy for functionalizing carbon-carbon double and triple bonds has been developed by American scientists.1 The researchers borrowed an electrochemical technique from the field of energy storage and applied it to organic synthesis, creating a new method that does not require chemical reductants and is more efficient and selective than the conventional approach.
Hydrogen atom transfer (HAT) – the concerted movement of a proton and an electron between two substrates – is a useful process with great potential in chemical synthesis. “Particularly for the functionalization of unsaturated carbon-carbon bonds, which is one of the most ubiquitous chemical functionalities that organic chemistry has to offer,” explains Samer Gnaim from the Scripps Research Institute. “But due to technical difficulties, such as safety issues, low atom economy and expense, this reaction is still considered a niche tool in the chemical industry. We have introduced a fundamentally new concept that can solve these problems, establishing the HAT as an accessible instrument for industrial and academic applications.
Gnaim explains that in a common HAT process, an active catalytic transition metal hydride species is generated by exposing an appropriate metal complex to an excess of a reductant such as silane. In many cases, a peroxide or other oxidant is also needed, he adds. “Our new electrocatalytic approach, inspired by the field of hydrogen storage, can overcome this limitation.” The process involves an electrochemically generated cobalt hydride species and requires no additives or complicated experimental protocols.
This technique has in fact been known for several decades and is gaining importance as a promising and efficient means of producing hydrogen for energy applications. “The electrochemical approach is appealing because it uses electricity, which is a clean, renewable source of energy,” comments Smaranda Marinescu, an organometallic chemist at the University of Southern California in the United States. She believes that the use of this strategy in organic synthesis has been limited by the knowledge of how to intercept cobalt hydride species with organic substrates. “This method not only improves the sustainability, efficiency and scalability of reactions, but it also enables the synthesis of chemicals that cannot be obtained using conventional techniques.”
Gnaim points out that electrocatalytic HAT (e-HAT) is versatile and can be applied to different types of reactions involving alkenes and alkynes, such as isomerization, selective reduction, and hydrofunctionalization. The team selected alkene isomerization as the transformation model (using tin as the cathode and cobalt bromide as the metal source), but depending on the reaction, different types of electrodes, proton sources, and compounds of cobalt are available. The researchers note that the system can be set up in minutes using a simple undivided cell and a commercial potentiostat. They also confirmed the scalability of e-HAT in batch and stream configurations.
“By combining various techniques, including analytical electrochemistry, kinetic analysis, and computational studies, we were able to probe the elementary steps of this process,” says Gnaim. “At first, a low-valence cobalt species is formed by direct cathodic reduction.” This species is then protonated to generate the cobalt hydride catalyst in place, so that the reaction can ultimately take place through a carbon-centered radical, which is in equilibrium with an alkyl-cobalt intermediate.
Gnaim mentions that the e-HAT approach has also led to the discovery of a new class of transformation called “e-selective alkyne semi-reduction”. “This type of reaction was not known in the field of HAT before,” he says. “The chemistry that can be achieved with our method proceeds with distinct chemoselectivity and sometimes even allows for reactivities that cannot be recapitulated using purely chemical procedures.”
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