Green Chemistry Advancements: Chemistry in Water with Hydroxypropyl Methylcellulose

Organic solvents have been commonly used in chemical reactions for many years. However, these solvents negatively affect the environment and pose risks to workers. They contribute significantly to chemical waste and are responsible for over 80% of chemical waste generation. In response, a global push for sustainability and green chemistry has emerged as a beacon of hope for minimizing the environmental impact of chemical processes. The ACS Green Chemistry Institute Pharmaceutical Roundtable has advocated for 12 green chemistry principles that have revolutionized how chemists approach chemical transformations. One of the main objectives of green chemistry is reducing chemical waste.

The research conducted by Professor Sachin Handa and his team at the University of Louisville (now at the University of Missouri) in collaboration with Dr. Wilfried Braje at AbbVie Deutschland GmbH & Co. KG has been published in the peer-reviewed journal ChemSusChem. This research represents a significant step forward in the field of green chemistry. The team has developed catalytic processes that completely eliminate the need for organic solvents, aligning with the principles of green chemistry and contributing to a more sustainable future.

The research team has introduced a new concept called aqueous catalysis in hydrophobic pockets, which can be a huge step forward in sustainable chemistry. The idea is to use an aqueous medium to facilitate and co-catalyze chemical reactions without the need for organic solvents in the catalytic process. The researchers used plant-based material hydroxypropyl methylcellulose (HPMC), a compound that has hydrophobic pockets created by alkyl ether side chains. These pockets can initiate the formation of metal nanoparticles (NPs) as the active catalyst from corresponding metal salts, which can help solve the solvent problem. The team conducted various experiments to validate their hypothesis that stable metal NPs could form on HPMC or within its hydrophobic pockets. Advanced imaging techniques, such as scanning transmission electron microscopy-based high-angle annular dark-field imaging (STEM-HAADF) and high-resolution transmission electron microscopy (HRTEM), showed that ultrasmall copper (Cu) NPs from the Cu salt (CuI), with an average size of 3.8 nm, were uniformly dispersed on the surface or within the pockets of HPMC. These NPs were stabilized within the hydrophobic environment of HPMC. Thus, HPMC facilitates the instantaneous formation of active NPs in their hydrophobic pockets and also solubilize the substates for efficient catalysis. Overall, this approach can be a significant step towards developing sustainable chemistry by reducing the reliance on organic solvents in catalytic processes.

According to the authors, these stabilized Cu NPs, residing within the hydrophobic pockets of HPMC, play a pivotal role in catalyzing chemical reactions. Specifically, they enable a highly efficient tandem click reaction, forming triazoles from in-situ generated benzylic azides and alkynes via 1,3-dipolar cycloaddition. This type of “click chemistry” is widely recognized for its significance in accessing biologically important heterocycles.

This study has shown that the use of organic solvents from the catalytic process to the pure product isolation can be eliminated. The researchers were able to achieve impressive yields of up to 94% simply by filtering the reaction mixture, with no need for organic solvents at any stage. This reduction in chemical waste and environmental impact aligns with the goals of green chemistry.  The study also explored the wide applicability of this new technology by testing various functional groups and complex molecular structures, all of which were tolerated. Furthermore, the scalability of the process was demonstrated on a gram-scale, highlighting its potential for practical applications in industrial settings.

The authors highlighted the benefits of the recyclability and stability of the Cu NPs. They demonstrated that the catalyst and reaction medium could be reused at least once without significant activity loss, making the process more environmentally friendly. Moreover, the stability of Cu NPs, even when exposed to air, indicates the system’s durability.

Professor Sachin Handa and Dr. Wilfried Braje have significantly contributed to the field of green chemistry by eliminating the need for organic solvents. Their development of chemistry in water with HPMC as a versatile catalyst and reaction medium supports the principles of green chemistry. It offers a practical and sustainable solution to the challenge of organic solvents in chemical transformations. The Cu NPs exhibit a broad substrate scope, scalability, recyclability, and stability, further highlighting this technology’s potential for widespread adoption in industrial settings. The technology also offers a safer solution to handle explosive azides.