A greener strategy for functionalizing double and triple carbon-carbon bonds has been developed by American researchers.1 The researchers borrowed an electrochemical technology from the energy storage field and applied it to organic synthesis, which created a new method that does not require chemical reducing agents and is more efficient and selective than the classical method.
Hydrogen atom transfer (HAT) – the coordinated movement of a proton and an electron between two substrates – is a useful process with great potential in chemical synthesis. “Especially for the functionalization of unsaturated carbon-carbon bonds, which is one of the most common chemical functions that organic chemistry has to offer,” says Samer Gnaim from the Scripps Research Institute. “However, due to technical difficulties, such as safety issues, poor nuclear economy and costs, this reaction is still considered a niche tool in the chemical industry. We have introduced a fundamentally new concept that can solve these challenges and establish HAT as an easily accessible instrument for both industrial and academic applications. ‘
Gnaim explains that in a conventional HAT process, an active catalytic transition metal hydride species is generated by subjecting a suitable metal complex to an excess of reducing agent such as silane. In many cases, a peroxide or other oxidant is also required, he adds. “Our new electrocatalytic approach, inspired by the hydrogen storage field, can overcome this limitation.” The process involves an electrochemically generated cobalt hydride species and requires no additives or complicated experimental protocols.
In fact, this technology has been known for decades and is becoming increasingly important as a promising and efficient way to produce hydrogen for energy applications. “The electrochemical approach is attractive because it uses electricity, which is a clean and renewable energy source,” 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 capture the cobalt hydride species with organic substrates. “This method not only improves the durability, efficiency and scalability of the reactions, but it also enables the synthesis of chemicals that cannot be obtained with conventional techniques.”
Gnaim points out that electrocatalytic HAT (e-HAT) is versatile and can be applied to various types of reactions involving alkenes and alkynes, such as isomerization, selective reduction and hydrofunctionalization. The team chose alkenisomerization as a model transformation (with tin as the cathode and cobalt bromide as the metal source), but depending on the reaction, different types of electrodes, proton sources and cobalt compounds are available. The researchers note that the system can be set up in a few minutes using a simple undivided cell and a commercial potentiostat. They also confirmed the scalability of e-HAT in both batch and flow configurations.
“By combining different techniques, including analytical electrochemistry, kinetic analysis and computational studies, we were able to investigate the elementary steps in this process,” says Gnaim. “In the beginning, a low-value 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 finally take place through a carbon-centered radical, which is in equilibrium with an alkyl-cobalt intermediate.
Gnaim mentions that the e-HAT method 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 HAT field before,” he says. “The chemistry that can be achieved with our method progresses with distinct chemoselectivity and sometimes even enables reactivities that cannot be recapitulated with purely chemical procedures.”
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