A new electrochemical process could make triphenylphosphine (TPP) — an important reagent in many organic transformations — more practical for industrial use. Synthesizing TPP results in large volumes of waste in the form of triphenylphosphine oxide (TPPO), which is very energy-intensive to handle. Now, a study led by Christo Sevov, professor of chemistry at The Ohio State University (Columbus, Ohio; www.osu.edu), has demonstrated the efficient conversion of TPPO into TPP. “In industry, people avoid these very simple and reliable reactions with TPP because of the TPPO problem. They have to come up with really circuitous routes to get to the same products that they could access in a single step if they had TPP in greater abundance, or were able to do something with the waste,” explains Sevov. Currently, there is only one large-scale industrial process for recycling TPPO into TPP, and it requires massive energy input, multiple synthetic steps and the use of toxic substances like phosgene.
Beginning with an electrified aluminum container acting as the anode in an electrochemical cell, the aluminum ions are strategically stripped from the anode surface and then utilized as a Lewis-acid activator for the TPPO. “Everything is self-contained, and because you are using the waste of the anode to activate the substrate, you don’t have to add any super-stoichiometric quantities of reagents, and you continuously generate the activator as you perform the electrochemical reaction,” adds Sevov. This continuous generation is key, since this type of reaction normally stalls out after about only 5% completion, shutting down either because the anode is passivated or the selectivity is low.
The team has demonstrated this conversion using various aluminum sources — including soda cans and aluminum mesh fencing. This process could also unlock other organic reactions that are not industrially practical. “We are also looking at hetero-arenes and pyridines. If we can activate these with in-situ-generated Lewis acids, then we can begin doing functionalization reactions on these desirable pharmaceutical platforms,” adds Sevov.