ALKENPHOS · ALKENPHOS Ligands: Innovative Electronically Responsive Scaffolds for Metal-Catalysed Syntheses

Final Activity Report Summary - ALKENPHOS (ALKENPHOS Ligands: Innovative Electronically-Responsive Scaffolds for Metal-Catalysed Syntheses)

We live in a world where the word ‘carbon’, once only the preserve of chemists, has reached new heights of awareness amongst the general population; carbon footprint, carbon neutral and ‘carbon capture’ are all terms regularly used by the press, with individuals being encouraged to reduce their carbon usage and emissions. It is clearly important to minimise the carbon footprint of chemical processes too, both in terms of feedstocks and energy. In order to achieve these goals, it is essential to start at the beginning of the life cycle of any new synthesis by addressing the underlying chemistry, e.g. the synthesis of elaborate carbon-based molecules used in drugs manufacture in efficient, i.e. of minimal reaction steps and producing negligible waste, clean and environmentally friendly ways. An attractive method for the production of such products is to use metal-catalysed processes that make possible reactions that are not normally achievable in the absence of metal, are selective, implying that we can achieve only one type of reaction with a particular starting material, and are efficient, i.e. a small quantity of metal catalyst is used to generate many of the desired product molecules, with no side reactions or waste, making them important for developing clean, sustainable chemicals’ manufacture. The catalyst is a key since it facilitates and directs the reactions of the starting materials. The catalyst is comprised of a metal atom, namely an acceptor, surrounded by organic components or ligands, i.e. donor molecules, which solubilise the metal and modulate or moderate its reactions. Thus, the judicious choice and design of the ligands that bind to the metal centre is vital. One of the principle objectives of this research project was the preparation of a range of innovative new ligands that possessed a tuneable or responsive ‘pocket’ in which to house the active metal. The design of the pocket was crucial since its size dictated how well the metal was held, since the retention of the metal-ligand ensemble was crucial in achieving good catalysis, and its chemical nature, which dictated the metal reactivity and, thus, its selectivity. Once ligands with varying pockets had been accessed, the project explored how they interacted with metals of relevance to catalysis so as to observe how their behaviour differed from those documented for other established combinations. The novel ligand design incorporated a rigid pocket of a size ideal for accommodating a catalytically-relevant metal atom. In addition, in order not only to ensure a strong metal-ligand interaction, but also to engender the necessary reactivity to the ‘encapsulated’ metal, two different anchor points residing within the pocket were used. Although each of the types of chosen anchor was known to interact favourably with metals, the simultaneous use of these two types was rare. The benefit was that, if two complementary anchors were used, they would each interact in different ways with the metal in between, hence being able to respond and moderate the reactivity of the metal in much the same way as a gyroscope could be used to keep a ship compass level on rolling seas. The project was very successful, with a new class of ligands being developed which combined a very strongly donating site that bound steadily to the metal with an anchor that, while binding to the metal, could vary the extent of interaction. Crucially the synergic effect of the two different anchors enabled unprecedented reactions to be achieved. Significantly, the metal scaffolds that allowed two carbon-based reactants to come together on the metal, joined them and released the new product, thus enabling the active metal to be used again in order to bring two new substrate molecules together were achieved. The successful completion of each of the individual steps of this cycle in exactly the same way every time was the key towards efficient and reusable catalysts. The new ligands were particularly good at ‘releasing’ molecules and allowed totally new types of starting molecules to be brought together and released by the metal. This opened a range of new vistas for the preparation of many different carbon compounds, which were not previously catalytically possible.

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