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The development of manganese-based alkene epoxidation, cis-dihydroxylation and alkane C-H oxidation catalysts

Periodic Reporting for period 1 - MANGANOXI (The development of manganese-based alkene epoxidation, cis-dihydroxylation and alkane C-H oxidation catalysts)

Reporting period: 2015-09-01 to 2017-08-31

Hydrocarbons are widely available from fossil fuel sources but have little utility in the synthesis of more complex molecular structures in either academic or industrial settings due to their limited chemical reactivity. As a result they are converted into higher value fine chemicals through an array of industrial processes that are undertaken on a huge scale worldwide. Amongst the many transformations that can be used, oxidative processes are highly desirable due to their ability to rapidly introduce significant reactivity and structural complexity in simple organic structures, but are underutilised, frequently due to their high environmental impact. In this project we sought to develop new catalytic methodology for the environmentally benign oxidation of hydrocarbon substrates using hydrogen peroxide as the oxidant. We were particularly interested in developing systems capable of generating epoxides from alkenes, substrates which are widely used in the bulk chemical, fine chemical, pharmaceutical and agrochemical industries, which therefore have significant importance in wider society.
The overall objectives of the project were to (i) investigate whether the addition of Lewis acid additives to a catalytic system that is known to be active in oxidative catalysis, which we have previously worked on, would result in an increase in reactivity; (ii) to develop a range of new ligand architectures to coordinate metals in which we included triazole moieties generated through ‘click’ reactions; the rationale for including the ‘click’ triazoles being that that they provided a similar structural motif to the commonly used pyridine ligand in other oxidative systems, allowed for modular syntheses to be developed and ultimately might provide a means of attaching the catalyst to solid supports; (iii) in the final objective we envisioned the use of solid supports to develop routes to heterogenised versions of the catalytic systems developed in (ii) so that they could be applied in flow reactors.
In the first Work Package (WP1) very extensive efforts were devoted to the optimisation of a catalytic system for the epoxidation of styrene through the addition of small quantities of a range of Lewis acids. After very careful optimisation of reaction conditions it was found that fresh distillation of the reaction solvent was essential, as was the catalytic loading of the Lewis acid, the quantity of catalyst used and reaction time, and we were able to completely convert styrene to styrene oxide in the remarkably fast three minutes. These conditions were then applied to a number of other substituted styrenes and it was found that whilst the conditions developed were effective for electron deficient styrenes, electron rich styrenes were too reactive and over oxidised/polymerised products resulted. The system was also found to oxidise a number of other relatively inert organic materials, presumably through C-H oxidation, including anisole. Preliminary mechanistic investigations using a classical radical clock substrate indicate that the system proceeds via a non-radical mechanism. This work has recently been submitted for publication. A planned secondment to The Karlsruhe Institut Für Technologie (under the supervision of Professor D. Bräse) was then undertaken to transfer knowledge of the immobilisation of catalytic systems on metal organic frameworks (MOFs). Attention then turned to the design and synthesis of new ligand scaffolds containing ‘click’ triazoles for the synthesis of manganese complexes for application in oxidative catalysis as set out in WP2. A number of new ligands were prepared and screened in a range of oxidative transformations, but none showed the meaningful efficacy required for them to be taken forward to WP3. Due to the extensive developmental time required to establish the optimal condition in WP1 there was insufficient remaining project time to develop alternative ligand frameworks and efforts to develop an analogue of the optimised catalytic system that could be attached to a solid support were unsuccessful. Nonetheless, an extensive review covering the use of heterogenised manganese complexes and their application in the oxidation of hydrocarbon substrates has been written and submitted for publication, which will provide a valuable resource for the research community in this and wider fields.
The catalytic system developed for the epoxidation of styrene and electron deficient substituted styrenes represents one of the most active systems of its type reported and has the potential to impact on the bulk chemical and fine chemical, pharmaceutical and agrochemical industries, which are central to Europe's ongoing and future economic success, and are directly relevant to a number of theme priorities in Horizon 2020.