Synthetic journey towards transition metal-free arylations

Abstract

In general, aryl–aryl transformations were investigated via transition metal-catalysed reactions. However, transition metal-mediated organic synthesis provided higher yields but has some flaws such as high cost, and the need for careful exclusion of air and moisture during the reactions. Possibly, there may be contamination of the product from traces of heavy metals present. To overcome these kinds of disadvantages, continuous efforts have been focused on the recent development of aryl–aryl formation through transition metal-free (TMF) reactions. However, most of these cross-couplings reaction were limited to hetero and electron-rich arenes which require strong inorganic bases. Our critical review includes a synthetic pathway to prepared X-aryl (X = C, N, O and S) motif by a transition metal-free scenario. Formerly, TMF reactions were carried out by various sources like strong bases, diazodium salt, aryliodonium salt, organo-catalysts and versatile energy sources like microwave heating, visible light and photochemical reactor. In this review, we have focused on the transition metal-free arylations.