Although today there exists a number of methods for the synthesis of organic molecules (the main components of certain medicines, foods, additives, paints, fabrics, liquid crystals, etc.), in most cases toxic reagents and solvents have to be used. In these processes, moreover, contaminant waste is produced This is why coming up with a procedure that avoids these drawbacks is a good thing all round. As regards solvents, water is undoubtedly the most suitable, in this respect, due to its abundance, utility, economy, non-toxicity and environmental advantages. And what about if the key to the reaction is using the minimum quantity of catalyst? And if all that involved in the reaction (including the catalyst) is recyclable or can be directly reused?
These precisely, have been the goals of a research group of the Department of Organic Chemistry II at the University of the Basque Country (UPV-EHU) on proposing a new method for the synthesis of diaryl sulphide, objectives which, to a great extent have been met. This is because, for the invention of a method for the synthesis of thioether derivatives, diaryl sulphides are prepared in a very simple way employing a catalyst of copper salt in water. This reaction medium is recyclable. Thus, it is worth studying in depth the research work undertaken by the UPV-EHU and which is patent- protected.
Diaryl sulphides are essential components in certain medicinal drugs and their structure or “skeleton” also appears in pharmacologically efficient molecules. In any case, their methods of synthesis are limited, given that, apart from the fact that they are not viable on applying them in great quantities, they are also quite contaminant. In fact, bonding arene and sulphur (the C(aryl)-S bond) has been no easy task, historically. With the process carried out by doctors M. Carril, R. San Martín and E. Domínguez, water is the only solvent employed in the reaction; it is a cheap solvent, easily handled and does not produce any contamination. Likewise, the initial reagents (arylthiols and halides) are accessible in great quantities. The catalyst sources, i.e. the copper salts are cheap and, as the reaction is produced in an aqueous medium, there is no need for any extreme condition such as an inert atmosphere. Once the reaction is concluded, the results are extracted from the aqueous medium where the catalyst is found and the reagents and base can be added to start the reaction again. As both the solvent and the copper source can be reused, the costs savings are evident. Also, the water, unlike many solvents used in industrial processes, is not toxic, nor is it flammable. Its safety behaviour is such that no safety measure is necessary for its storage or in its handling.
In this respect, it would be desirable for this method for the sustainable development employed in the case of diaryl sulphide to be a reality for other chemical processes.