Direct arylation reaction: a powerful tool for the synthesis of photoactive compounds, both in conventional and sustainable media like Deep-Eutectic Solvents

The formation of new C(sp2)-C(sp2) bonds has always attracted the interest of synthetic organic chemists for the possibility of assembling (hetero)aryl-containing building blocks aimed to the preparation of new conjugated photoactive molecules. Classical cross-coupling reactions, like Stille-Migita, Negishi and Suzuki-Miyaura, have allowed an infinite number of transformations, but all of them require, at least, an additional step for the preformation of organometallic intermediates, which inevitably corresponds to an increase in terms of waste of materials, solvent, and energy. From this point of view, the introduction of direct arylation (DA) reactions has been a revolution in this field, since new carbon-carbon bonds can be formed starting from a (hetero)aromatic bromide and a (hetero)arene with an aromatic C-H bond which could be activated under proper reaction conditions, without employing preformed organometallic compounds. In the last years, we have been able to synthesize several new photoactive compounds thanks to the employing of direct arylation reactions, with great savings in terms of number of both synthetic and purification steps, reagents, and waste. These new molecules have been successfully applied as photosensitizers for Dye-Sensitized Solar Cells (DSSCs) and/or organic emitters for Luminescent Solar Concentrators (LSCs). Recently, we tried to improve the sustainability of the direct arylation developing a general synthetic protocol for the DA reaction of 3,4-ethylenedioxythiophene (EDOT) and other substituted thiophenes with (hetero)aromatic bromides in Deep Eutectic Solvents (DESs) under air and in non-anhydrous conditions, with moderate-to-high yields. This DA protocol was demonstrated to tolerate many functional groups, like aldehydes, ketones, nitriles, esters, carboxylic acids, and nitro-groups. The sustainability of the protocol was established through calculations of green metrics, such as Eco-scale and E-factor, and compared with the literature, when possible. Remarkably, such a procedure can be successfully applied for the simple preparation of conjugated organic compounds with potential applications in optoelectronics and photovoltaics.