Life cycle assessment of multistep benzoxazole synthesis: from batch to waste-minimised continuous flow systems

In this contribution we have focused on the progress of synthesis methods for the preparation of 2-aryl benzoxazoles as highly interesting materials with increasing relevance in the pharmaceutical industry as well as in optical applications. The traditional production methods of 2-aryl benzoxazoles clearly have some drawbacks related to the use of strong acids and/or toxic reagents leading to a large production of waste. Importantly, comprehensive analysis of the associated risk in terms of safety, environmental impact and disposal cost is lacking. In this regard, the life cycle assessment (LCA) methodology is herein applied to ultimately evaluate the environmental profile of the available routes to access 2-aryl benzoxazoles. Seven batch synthesis approaches and two continuous-flow (CF) approaches (small and large scale) are closely compared. The superiority of the CF technology is ultimately proven among the analysed environmental impact categories. The main finding is that the oxygen-flow chemistry intensification fortified the sustainability of the green chemistry principles (towards the catalyst/solvent) themselves by ensuring the regeneration of OMS catalysts and reduction of manganese leaching to the minimum by the CPME solvent, which also provided high solvent recyclability. In this way, it adds circularity in the sense of its 10R framework (e.g. R standing for recycle, repair, rethink, and refuse). As a result, for example, our flow approach reduces carbon emissions by 85% in comparison with our batch approach, the latter exhibiting lower environmental impact than the six batch approaches from the literature. In addition, our flow chemistry process has lower energy consumption and solvent load, whose share is up to 88% of the environmental impact.