Chemical Transformations Scientific Progress
The Chemical Transformations theme of the UK Catalysis Hub has undertaken a wide range of projects broadly concerned with promoting the sustainability and prosperity of the UK manufacturing base by contributing fundamental catalytic science to bulk and fine chemicals synthesis, polymer and pharmaceutical manufacture. All projects have involved strongly multidisciplinary teams utilising expertise from around the UK and internationally which has been facilitated by the Hub funding mechanisms. Some key results and publications from projects, highlighting the breadth of multidisciplinary catalytic science taking place within the theme, are described:
Replacing petrochemically-derived plastics with renewable materials based on biofeedstocks and CO2 is a key challenge for sustainable materials of the future which however, requires new robust and selective catalysts to be developed. Within the Catalysis Hub, a multi-institutional team, together with scientists from Harwell, have made major progress in this area. Using a combination of synthetic, spectroscopic (XAS) and computational (DFT) techniques they have discovered new catalysts and elucidated their mechanisms. These catalysts have the potential to deliver, through sequence control, sophisticated and useful materials which could not be accessed by other methods.
Polymer synthesis: To react the impossible ring: D. Myers, A. Cyriac & C. K. Williams Nature Chemistry 8, 3–4 (2016) doi:10.1038/nchem.2424
A key aspect of this project has been to develop industrially useful methods, which necessitate efficient catalytic processes. A collaborative team of synthetic and process chemists from academia and industry have developed a new method for the catalytic synthesis of amides from nitriles under very mild conditions. Most of the methods reported to date require high temperatures, long reaction times, large excess of reagents or the use of expensive catalysts. It was, therefore, decided to explore a new methodology to obtain amides from nitriles, requiring milder conditions than those previously reported. Consequently, catalytic hydration of nitriles using a simple copper catalyst using hydroxylamine as oxygen source was developed (Chem. Comm., 2016, 52, 1436).
The general applicability of hydrolysis conditions was proven in a variety of substrates affording the corresponding amides in excellent yields, showing the potential of this reaction. Given the widespread nature of primary amides amongst various aspects of fine chemical and pharmaceutical synthesis, this method should find broad utility amongst the academic and industrial synthetic community.
This team has also developed a new method for the conversion of nitroalkanes and nitromethylbenzenes into carboxylic acids under very mild conditions using catalytic amounts of an iodide source and Zn(OAc)2 as Lewis acid (Chem. Comm., 2016, 52, 1013). This transformation is performed in water although, for more complex substrates that are non-compatible or insoluble in water, a combination of toluene and urea can also be employed giving comparable results.
This transformation is compatible with a wide range of substitutes at the phenyl ring. The presence of electron-withdrawing and electron-donating groups were well-tolerated at the ortho-, meta- and para-positions affording the corresponding benzoic acid in excellent yields.
This collaboration brings together expertise in catalysis spanning homogeneous and heterogeneous catalysis, reactor engineering and process engineering. Thus, it addresses many of the fundamental aspects of catalysis. The project has enabled the development of new selective catalytic processes for ring-opening polymerization of commercially important renewable monomers and a detailed study of the properties and performances of various catalysts enabling carbon dioxide/epoxide copolymerization. It has been discovered that some of these catalysts are able to operate switchable polymerization catalysis to generate block sequence selectivity: an important goal in realising sustainable functional materials of the future. This latter phenomenon is very unusual and the collaboration has enabled a detailed study of the both the experimental factors controlling it (kinetics/spectroscopy) and a computational approach to probing the process by DFT calculations.
Calculations reveal both a kinetic and thermodynamic basis for the observed selectivity. So far, the sequence selectivity is a very unusual observation but it has the potential to deliver sophisticated and useful materials which could not be accessed by other methods. Investigations into the properties of such block copolymers have uncovered interesting elastomeric and shape memory performances. Publications arising from the Hub funding in this area include: J. Am. Chem. Soc. 2016, 138, 4120;
This project proposed a new set of materials combining the benefits of single-site homogeneous species with those of heterogeneous materials, by the synthesis of well-defined and, importantly, highly reactive low-coordinate organometallic species protected by the confined molecular environments of platform materials such as MOFs and polymer composites.
A team of molecular, materials and structural scientists has developed, for the first time, robust protocols for the encapsulation of positively charged transition metal organometallic catalysts inside the pores of anionic metal-organic framework (MOF) materials via direct cation exchange. Proof of principle was provided by encapsulating the well-defined Lewis-acidic [CpFe(CO)2L]+ homogeneous catalyst (L = solvent molecule) inside the pores of the [In3(BTC)4]3- anionic MOF, forming [CpFe(CO)2L]+@[In3BTC4]–. Rigorous characterization of this new hybrid material by IR, solid-state and solution NMR spectroscopy, single-crystal and powder X-ray diffraction and ICP-OES conclusively demonstrated that the cationic catalyst is encapsulated intact in one step inside the pores of the anionic host, allowing for the direct transfer of solution-based chemistry to the solid state. To this end, [CpFe(CO)2L]+@[In3BTC4]– was benchmarked as a heterogeneous catalyst in a Diels Alder reaction. This work represents a significant step forward in the area and has been recently published (Chem. Sci., 2016, 2037).
Transition Metal-Free Catalysis
This project aimed to develop main group metal and organic systems able to perform catalytic transformations previously reserved for transition metal homogeneous catalysts. Initial work focused on carbenoid and related systems which led to a major scientific advance in the demonstration of reversible M-C bond formation – i.e. oxidative M-C bond formation and reductive M-C bond breakage for a non-transition metal system. This work represents a breakthrough in redox-based C-X bond formation using Main Group elements (J. Am. Chem. Soc. 2014, 136, 10902).
This chemistry can be taken a step further. N-H bond cleavage at the carbenoid centre has been partnered with subsequent N-X reductive elimination, thereby functionalizing ammonia (as yet only in stoichiometric fashion), and regenerating the metal in a reduced state. As such, not only have two key steps (oxidative addition/reductive elimination) in bond modification chemistry typical of late transition metals have been shown to be viable at a Main Group centre, but such chemistry has also been shown to be possible using the key substrate ammonia as an N1 feedstock. This represents a genuine step-change in Main Group based bond modification, and one which leaves redox-based catalysis by such systems tantalisingly close (J. Am. Chem. Soc., 2016 138, 4555).
An additional avenue of investigation involves a different approach; the utilization of Main Group systems in a constant oxidation state in catalytic processes. This has led to the publication of the first catalytic system based on a gallium hydride active species – a system which catalyses the reduction of CO2 to a methanol derivative. While turnover frequencies are relatively low compared to current state of the art Ru systems (by ca. 1 order of magnitude) they are the best reported to date by a Main Group system (Angew. Chem., 2015, 54, 5098).
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