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Catalytic C–F Bond Functionalization for the Fixation of Environmentally Persistent Fluorocarbons

Periodic Reporting for period 4 - FluoroFix (Catalytic C–F Bond Functionalization for the Fixation of Environmentally Persistent Fluorocarbons)

Reporting period: 2020-10-01 to 2021-09-30

The rapid expansion of the fluorochemicals market has led to a notable advancement in our quality of life. Fluorinated organic molecules play a pivotal role in chemical manufacture. Among their many uses, they find applications as refrigerants, in polymeric materials and as solvents and surfactants. It has been estimated that approximately 20–25% of pharmaceuticals and 30–40% of agrochemicals contain at least one fluorine atom. For example, Lipitor contains a single fluorine atom and led the market for pharmaceutical sales from 1996–2012 generating €93 billion in revenue over 14.5 years.

The carbon–fluorine bond is the strongest known single carbon–element bond, and it is unsurprising that synthetic fluorocarbons persist in the environment. Hydrofluorocarbons (HFCs) are known to contribute to climate change. For example, HFC-23 has a global warming potential approximately 10,000 times greater than CO2. From the 1st of January 2015, as part of climate change action, the European Union introduced new legislation to control the use of fluorinated gases, including HFCs. This regulation seeks to cut, by containment, reduction and recovery, the emission of fluorinated gases by two-thirds of current levels by 2030. Hydrofluoroolefins (HFOs) are proposed as greener alternatives to HFCs and have been billed as next generation refrigerants. While the global warming potentials of HFOs are lower than HFCs, the long-term effect of these fluorinated gases, and their decomposition products, on the environment is not yet clear. In contrast to mankind, Nature uses fluorine in organic chemistry sparingly. The vast majority of naturally occurring fluorine is in the form of inorganic fluoride, present in mineral forms such as fluorite (CaF2) and cryolite (Na3AlF6).

If new methods could be developed that transform, low-value, environmentally persistent HFCs and HFOs into high-value products, such as pharmaceuticals or agrochemicals, it could re-align the use of these molecules within the fluorochemicals market. Volatile fluorine-containing gases could be used not as end products but as chemical intermediates that never leave the plant. Existing HFCs could be recycled into useful products following recovery at the end of their equipment’s lifetime. If this method also resulted in the formation of inorganic fluoride from fluorinated gases it would represent an environmentally responsible way to return fluorine to the environment and an important step to closing the fluorine cycle.

The decisive objective of this project is to develop new methods to transform environmentally persistent hydrofluorocarbons (HFCs) and hydrofluoroolefins (HFOs), into reactive chemical building blocks that can be used in chemical manufacture.
During the grant period (1st April 2016 to 30th September 2021) new synthetic methods were developed to transform unreactive fluorocarbons into reactive chemical building blocks. The focus of these studies was the design, development, and application of new main group reagents to effect reactions that break the strong carbon–fluorine bonds in fluorocarbons including HFOs and HFCs. The main group reagents developed are based on some of the most inexpensive and abundant metal and elements in the Earth’s crust. These include simple molecules based on boron, magnesium, aluminium, silicon, and zinc.

The reaction of these molecules with fluorocarbons – namely fluoroarenes, fluoroolefins (HFOs) and hydrofluorocarbons (HFCs) – has been investigated with the goal of developing processes which selectively transform a C–F bond into a C–B, C–Mg, C–Al, or C–Si bond. Two approaches have been developed, those which rely on the use of a transition metal catalyst to lower the activation energy of the reaction and help break the strong carbon–fluorine bond, and those which do not. A comprehensive understanding of the mode of action of these new reactions has been developed. This includes a detailed understanding of how the main group reagent breaks strong carbon–fluorine bonds. In the cases where a catalyst is present, we have also developed an intimate knowledge of how the transition metal and main group fragments combine to generate reactive species.

In addition to developing methods that transform unreactive fluorocarbons into new fluorinated main group reagents, we have been developing the applications of these reactive fluorinated building blocks. This includes developing new reactions that upgrade organometallic intermediates into useful organic molecules by either carbon–carbon or carbon–heteroatom bond formation.
In this project, we developed methods to upgrade fluorocarbons through reactions with main group nucleophiles. These transformations involve the direct breaking of the strong carbon–fluorine bonds in fluorocarbons. We have reviewed aspects of our achievements in this field (Synlett 2019, 30, 2233-2246).

Our work includes a large scope of fluorocarbons including fluoroarenes, fluoroalkanes and fluoroalkenes. We developed both non-catalytic and catalytic methods to transform C–F bonds into C–B, C–Mg, C–Al, and C–Si bonds. As part of these studies, we investigated the interaction between transition metal (Pd) and main group fragments (Al, Mg, Zn) and uncovered some remarkable heterometallic complexes containing multiple metals in proximity. Complete details of the progress beyond the state-of-the-art is detailed in the Major Achievements section of this report.

In the last year of this project, we also initiated a new direction of research based on the idea of moving fluorine atoms, or fluorine containing groups between two organic molecules. So far, we have described two different catalyst systems for fluoride metathesis (Org. Lett. 2020, 22, 9351-9355) along with HF transfer (ChemRxiv 2021, DOI: 10.26434/chemrxiv-2021-72q5b). These concepts and preliminary findings were used to construct a successful ERC CoG application.
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