Aromatization as the driving force for single electron transfer towards C–C cross-coupling reactions Monojit Roy, Dhananjay Dey, Abhishek Kundu, Subhankar pal, Debashis Adhikari IISER Mohali, India Well-established previous methods along this endeavor heavily relied on palladium-, rhodium- and ruthenium- based catalysts. However, significant demand for the absence of a transition-metal impurity in pharmaceutical molecules motivated chemists to seek transition-metal-free alternatives. Toward this end, electron transfer catalysis has emerged as a viable alternative to the traditional heavy metal mediated two-electron chemistry. For such C–H functionalization reactions, one of the elegant methods is to promote the reaction using a radical, where such a radical steers a substitution reaction on an arene or heteroarene to fabricate a biaryl product, often referred to as base-promoted homolytic aromatic substitution (BHAS). The generation of such a radical in the reaction medium is often triggered by a single electron transfer (SET) event. Multiple small molecules, either in their neutral or monoanionic form, have been observed to be sufficiently reducing in nature, so they promote SET and break a C–X bond (X = halide, diazonium, iodonium, etc.). Intuitively, different molecules have varying reducing ability towards aryl radical generation, which is dictated by the electronic nature of the reducing species. Aromaticity is a dictating force for a plethora of reactions, and a molecule may elicit a specific pattern of reactivity, if it is propelled by a thermodynamic driving force to gain aromaticity. Herein we present a small molecule, dihydrophenazine (DPh), that has a clear driving force to gain aromaticity upon undergoing 2e−/2H+ redox processes. DPh molecule is capable of promoting SET under photochemical conditions in its deprotonated form to break a bond in aryldiazonium salts and generate an aryl radical. Such an aryl radical has been utilized to build a synthetic protocol for C–C cross-coupling chemistry including a large array of arenes and heteroarenes. The governing principle behind such electron transfer is the aromaticity gain which thermodynamically drives such a process.
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