The development of a new eco-friendly process is often associated with advanced reaction technologies. An aspect that sometimes imposes a careful balance between the environmentally benign character and the economic/ safety feasibility of the process itself. The use of supercritical CO2 (scCO2) is an example: scCO2 is among the most attractive green alternatives to replace conventional solvents, though its handling requires high-pressure operations (usually at P > 130 bar) which are energy-consuming and potentially dangerous.12 This approach becomes a green solution only when real chemical benefits (higher selectivities, rates, yields) are achievable in the supercritical fluid.
Also, in the case of DMC, reaction conditions apparently are not green: the methylating ability of DMC can be exploited at a temperature >160°C (above the boiling point of DMC itself: 90°C), which implies an autogenic pressure (> 3 bar) for batchwise processes. Such conditions are not prohibitive, however, especially according to the industrial practice, where pressures up to 20-30 bar and temperatures up to 250°C are not a concern.
Moreover, from the environmental standpoint, advantages must be seen on the global balance: the DMC-mediated alkylation reactions are much safer than any other alkylation method known that uses conventional reagents. Not only the features of the reaction itself, but the peculiarity of the reagent(s), the base (truly catalytic), and the absence of wastes are also key aspects.
Discontinuous (batch) processes are carried out in pressure vessels (autoclaves) where DMC is maintained as liquid by autogenous pressure. Instead, CF reactions at atmospheric pressure require that both DMC and the reagent(s) in the vapor phase come into contact with a catalytic bed: a constraint that has spurred the development of new applications and alternative reaction engineering, namely, GL-PTC and the continuously fed stirred-tank reactor (CSTR).
Accordingly, under different conditions, DMC is used as a methylating reagent for a variety of substrates: phenols, thiols, thiophenols, aromatic amines, arylace-tonitriles, arylacetoesters, aroxyacetonitriles, aroxyacetoesters, alkylarylsulfones, benzylarylsulfones, and lactones, either under CF or in batch conditions.
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