What are Organic Chemists doing to save the World?

By Alessia Petti, Ph.D. candidate in electrosynthesis at the University of Greenwich, guest blog writer

According to the most recent UN report, the last decade (2011-2020) was the warmest on record with carbon dioxide emissions reaching 148 percent of preindustrial levels in 2019.[1] The demand for change is urgent and, as recently emphasized at the COP26 in Glasgow, the time to act is shorting. Given the global nature of the problem, we should all ask ourselves what we are doing/can do to save the Earth. With this post, we intend to answer this question from an organic chemist perspective by summarizing the most significant developments in the field that enable waste reduction and guide us towards a sustainable future.

Figure 1: Earth Warming Up. Credits to: https://www.un.org/en/climatechange/what-is-climate-change

3 ways of fighting Global Warming in Organic Chemistry

  1. Beyond Thermochemical Activation: Enlightened Syntheses

Organic reactions often require thermochemical conditions to take place. Nevertheless, these conditions become incredibly energy-consuming when transferred to large-scale industrial processes. Energy waste is often associated with material waste, especially in the case of conventional redox reactions wherein stoichiometric quantities of redox reagents are necessary. Electrochemistry and photochemistry represent two alternative ways of activating organic molecules in an almost reagent-less fashion. Inexpensive electricity or light, derived from renewable energy sources, allows obtaining extremely selective electron transfers[2,3] while unlocking unprecedented reactivities. Within this context, Britton’s group (Simon Fraser University) in collaboration with Merck reported a photocatalytic method for the C-H fluorination of γ-fluoroleucine, a key intermediate in the preparation of the osteoporosis drug Odanacatib (Figure 2).[4]

Figure 2: Photocatalytic C-H fluorination of γ-fluoroleucine derivative.[5]

While previous fluorination protocols involved a multistep synthesis and the use of hazardous reagents such as hydrofluoric acid, this pathway affords the desired product directly from the unprotected amino acid in high selectivity via UV-irradiation of a decatungstate-catalyst. High productivities were also achieved when translating the reaction in flow (90% yield, 45 g after 2 h of residence time). Although using minimum amounts of metal catalysts represents a substantial improvement, redox reactions can be performed without any catalyst at all. This is the case of direct electrochemical reactions such as the anodic synthesis of ureas[6] realised by Lam’s group (University of Greenwich) (Figure 3). In this work, the carbonyl derivatives are accessed at room temperature by in situ generation of isocyanates followed by the one-pot addition of an amine.

Figure 3: Anodic synthesis of ureas.[6]

Electrolysis is achieved in the presence of cheap carbon graphite electrodes and without using any supporting electrolyte. Additionally, when performing the reaction in flow, no purification is required with the urea products formed in only six minutes (0.3 mmol scale).

2. Cleaner solvents by following Nature’s lead

Solvents have a crucial role in most organic transformations. However, most of those currently used in organic chemistry are petroleum-based and pose serious risks due to their flammability, carcinogenicity, and explosivity. Moreover, their persistence in soils and aquatic ecosystems has a major detrimental impact on the environment.[7] On the other hand, using water as the main solvent represents a cheap, safe, and environmentally benign alternative. Additionally, nature has shown us over a billion years that achieving complex transformations in this medium is perfectly doable. Driven by these considerations, Professor Lipshutz (University of California, Santa Barbara) has developed new synthetic methods using water as a large reaction medium in combination with micellar catalysis. One of his latest works involves a Migita cross-coupling formation of thioethers performed in recyclable water in presence of the surfactant (TPGS-750-M) and low levels of Ni catalyst (Figure 4).[8]

Figure 4: Migita-Like C−S Cross-Couplings in Recyclable Water.[8]

Despite these encouraging results, we are still far from reaching nature’s perfection. For instance, the water along with additives recovered at the end of the process must be treated as “wastewater” and therefore recycled. Aspects involving the toxicity and biodegradability of the surfactant must also be considered.

3. Turning waste into gold

Finally, organic chemistry is often associated with polymer synthesis. Out of the 381 million tonnes of plastic waste per year, only 9% of single-use plastics are recycled.[9,10] This 9% is mostly recycled via mechanical methods which would inevitably convert the polymer into lower-quality materials. Furthermore, many polymers cannot be recycled via mechanical processes meaning that these materials are destined for incineration. In an effort to invert this trend, Professor McNeil (University of Michigan) is exploring the possibility of chemical recycling. In her recent publication[11], a new and mild route for recycling the acrylic-based superabsorbent material used in diapers is disclosed. The transformation into commercially relevant pressure-sensitive adhesives (PSAs) relies first on acid-catalyzed hydrolysis followed by an optional chain-shortening via sonication and lastly, esterification driven by hydrophobicity (Figure 5).

Figure 5: Syntheses of pressure-sensitive adhesives from petroleum feedstocks (industrial approach) compared to waste diaper-based feedstock.[11]

Life cycle assessments conducted on the adhesives synthesized via this approach outperformed the petroleum-derived counterparts on nearly every metric including carbon dioxide emissions and cumulative energy demand.

To conclude, each scientist and innovator has the power to influence our planet’s tomorrow. Hence, start experimenting and be the change that you want to see in the world!

References

[1]        “Climate Reports | United Nations,” can be found under https://www.un.org/en/climatechange/reports, n.d.

[2]        G. E. M. Crisenza, P. Melchiorre, Nat. Commun. 2020, 11, 1–4.

[3]        M. Yan, Y. Kawamata, P. S. Baran, Chem. Rev. 2017, DOI 10.1021/acs.chemrev.7b00397.

[4]        S. D. Halperin, D. Kwon, M. Holmes, E. L. Regalado, L. C. Campeau, D. A. Dirocco, R. Britton, Org. Lett. 2015, 17, 5200–5203.

[5]        G. E. M. Crisenza, A. Faraone, E. Gandolfo, D. Mazzarella, P. Melchiorre, Nat. Chem. 2021, 13, 575–580.

[6]        A. Petti, C. Fagnan, C. G. W. van Melis, N. Tanbouza, A. D. Garcia, A. Mastrodonato, M. C. Leech, I. C. A. Goodall, A. P. Dobbs, T. Ollevier, K. Lam, Org. Process Res. Dev. 2021, DOI 10.1021/acs.oprd.1c00112.

[7]        M. Cortes-Clerget, J. Yu, J. R. A. Kincaid, P. Walde, F. Gallou, B. H. Lipshutz, Chem. Sci. 2021, 12, 4237–4266.

[8]        T.-Y. Yu, H. Pang, Y. Cao, F. Gallou, B. H. Lipshutz, Angew. Chemie Int. Ed. 2021, 60, 3708–3713.

[9]        “100+ Plastic in the Ocean Statistics & Facts (2020-2021),” can be found under https://www.condorferries.co.uk/plastic-in-the-ocean-statistics, n.d.

[10]     “The world’s plastic problem in numbers | World Economic Forum,” can be found under https://www.weforum.org/agenda/2018/08/the-world-of-plastics-in-numbers, n.d.

[11]     P. T. Chazovachii, M. J. Somers, M. T. Robo, D. I. Collias, M. I. James, E. N. G. Marsh, P. M. Zimmerman, J. F. Alfaro, A. J. McNeil, Nat. Commun. 2021, 12, 1–6.