An investigation of Miyaura borylation and one-pot Suzuki-Miyaura cross-coupling reaction under air in sustainable Deep Eutectic Solvents

Within the framework of green chemistry, the solvent's choice is strategic in the design of a new chemical process: an alternative solution to common organic solvents is represented by deep eutectic solvents (DESs). The straightforward zero-waste synthesis, the possibility of using inexpensive and environmentally benign components, and their unique physicochemical properties like low volatility and high thermal stability make these solvents extremely appropriate for many applications. In this work, DESs have been employed as green solvents for both Miyaura borylation and Suzuki-Miyaura cross-coupling reactions. To the best of our knowledge, no examples of such comprehensive protocols in DES are reported in the literature so far. Herein, an optimized protocol for the Pd-catalyzed Miyaura borylation and the resulting one-pot Suzuki-Miyaura cross-coupling reaction of (hetero)aromatic iodides, bromides, chlorides, and triflates in Deep Eutectic Solvents made of choline chloride/glycerol (1:2) and choline chloride/glucose (2:1) DES is presented. The fine-tuning of each reaction component led to robust conditions for the borylation of both electron-poor and electron-rich (hetero)aryl substrates with moderate-to-high yields. This protocol allowed us to reduce the Pd-loading down to 1%, without the need for an inert atmosphere. The so-obtained boronic esters were isolated and characterized or directly employed for Suzuki-Miyaura cross-coupling in a one-pot fashion to introduce different bromides. The optimized protocol was successfully applied for the simple preparation of conjugated organic compounds with potential application in optoelectronics. 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.