Researchers at the University of Regensburg have succeeded in hydroformylating olefins at room temperature and atmospheric pressure using iron catalysis and light. This method uses inexpensive 1,3,5-trioxane instead of toxic carbon monoxide and could replace the use of expensive rhodium in the long term.
The hydroformylation of olefins, also known as oxosynthesis, is one of the most important industrial processes in chemistry. Developed in 1938 by Otto Roehlen (Ruhrchemie), more than ten million tons of so-called oxoproducts are now produced each year using this process. The main catalysts used are rhodium catalysts, with which olefins can be converted very efficiently into the desired products, albeit at high temperatures (150-200°C) and pressures (150 to 300 bar hydrogen/carbon monoxide). Rhodium is one of the rarest metals on earth: 1 gram costs around 140 euros. By contrast, ubiquitous iron costs just 120 euros per tonne, making it around a million times cheaper and also far less toxic.
Researchers at the University of Regensburg have now succeeded in hydroformylating electron-poor olefins at room temperature and atmospheric pressure by combining iron catalysis and light. Inexpensive 1,3,5-trioxane, a trimer of formaldehyde, is used instead of toxic carbon monoxide. The developed process could also be extended to related hydroacylations and hydrocarboxlyations. “There is undoubtedly still a long way to go before this approach can compete with rhodium-catalyzed processes,” explains Prof. Dr. Oliver Reiser, Institute of Organic Chemistry at UR, “but the replacement of valuable metals such as rhodium or iridium with readily available alternatives is essential for the development of resource-saving processes on the way to sustainable chemistry.”
Photocatalysis, i.e. the use of light for chemical synthesis, is one of the active areas of research in organic chemistry. In order for organic molecules to be effectively stimulated by light, mediating catalysts are used. Precious metals such as ruthenium or iridium - less common than gold - are also predominantly used for this purpose. It has long been assumed that metals such as iron or copper, which are abundant on earth, are less suitable as photocatalysts, as their excited states initiated by light are very short-lived (nanoseconds or even picoseconds) and therefore do not enable reactions with molecules.
Prof. Reiser's research group, supported by the Collaborative Research Center 325 “Assembly Controlled Photocatalysis” funded by the German Research Foundation (DFG), was initially able to use copper and more recently also iron for photocatalysis. The “trick” is that substrates first form a complex with the catalyst molecules, which is split by light in an ultra-fast reaction. The resulting reactive fragments can then trigger chemical reactions. The hydroformylation that has now been developed also follows this principle: light-induced cleavage of an iron-chlorine bond generates chlorine radicals, which subsequently activate the trioxane for the hydroformylation of olefins by hydrogen atom cleavage.
Original Publication: V. Klöpfer, A. Chinchole, O. Reiser, Tetrahedron Chem 2024, 10, 100073: Dual iron- and organophotocatalyzed hydroformylation, hydroacylation and hydrocarboxylation of Michael-acceptors utilizing 1,3,5-trioxanes as C1-Synthone; https://doi.org/10.1016/j.tchem.2024.100073 (external link, opens in a new window)
Contacts
Prof. Dr. Oliver Reiser
Institute of Organic Chemistry
University of Regensburg
E-Mail: [email protected]
Tel.: +49 (0)941-9434631