Being used in the catalytic production of more than half of all the commercial chemicals, oxidation is one of the most significant industrial reactions, second only to polymerisation. Not surprisingly, the U.S. Department of Energy identified selective oxidation of organic chemicals to be the most important research area to impact the future chemical industry. However, it remains "one of the reactions with the greatest potential for improvement", primarily because of the low selectivity encountered in the vast majority of oxidation reactions and the widespread use of stoichiometric, toxic and hazardous oxidants, such as CrO3, H2S2O8, PhIO, HNO3. Using catalysts and environmentally benign oxidants is undoubtedly the most realistic way to address these issues. In this regard, developing iron-catalysed aerobic oxidation is most appealing, due to the unrivalled advantage of abundance, low cost and benign nature of Fe and O2. However, although iron-containing metalloenzymes are capable of selectively oxidizing various substrates with O2 under mild conditions, few man-made iron catalysts are known that can catalyse efficient, selective aerobic oxidation.
We recently uncovered a novel class of well-defined iron complexes bearing pyridine bisimidazoline (PyBisulidine) ligands, which allow for highly chemoselective oxygenation of ethers and olefins. Building on this success, this project seeks to develop the next generation of more active iron catalysts for selective oxygenation of more challenging substrates. In particular, we will concentrate on two reactions, depolymerisation of lignin and cleavage of aliphatic C=C double bonds, under aerobic conditions. These reactions, which are vastly different in nature and can thus demonstrate the wide scope of the iron catalysts, are of both fundamental and commercial significance.
Lignin is the only natural polymers made of aromatic units and could be used to produce a wide range of platform aromatic compounds. However, this requires the depolymerisation of the lignin ether linkage in the first place. Considering the huge scale of any possible processes toward this end, the catalyst to be used should ideally be based on a cheap metal such as iron.
Oxidative cleavage of alkenes into carbonyls is a widely used transformation. However, ozone is most often used in industrial operations, and this comes with the well-known safety issues of explosivity, the cost of special equipment, and the large amount of waste generated. Thus, there has been a strong incentive to develop catalytic methods to replace ozone. Iron-based catalysts are particularly interesting, considering not only the environmental benefit of iron but also the ability of iron oxygenases to oxidize olefins to carbonyl compounds with exquisite selectivity.
For both of these reactions, few iron catalysts are known that can make use of O2 as oxidant. The Fe-PyBisulidine complexes are expected to bring about a step change in addressing these challenges.
The new catalysts will find applications in other reactions as well. The ether linkage is one of the most ubiquitous bonds found in nature and manmade chemicals, ranging from pharmaceuticals and agrochemicals through household products to lignin and coal. Thus, using the new catalysts and O2, compounds containing an ether bond may be transferred into highly value-added products, and polluting plastics, agrochemicals and detergents may be degraded in air. To further demonstrate the value of the iron catalyst, collaboration with a SME to produce, via acylation of arenes, compounds of direct business interest will be implemented.
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