Photosensitized activation of diazonium derivatives for C-B bond formation

摘要

Aryl diazonium salts are ubiquitous building blocks in chemistry, as they are useful radical precursors in organic synthesis as well as for the functionalization of solid materials. They can be reduced electrochemically or through a photo-induced electron transfer re-action. Here, we provide a detailed picture of the ground-and excited-state reactivity of a series of nine rare and earth-abundant photosensitizers with 13 aryl diazonium salts, which also included three macrocyclic calix4arene tetradiazonium salts. Nanosecond transient absorption spectroscopy confirmed the occurrence of excited-state electron transfer and was used to quantify cage -escape yields (i.e., the efficiency with which the formed radicals separate and escape the solvent cage). Cage-escape yields were large; they increased when the driving force for photo-induced elec-tron transfer increased and also tracked with the C-N2+ bond cleav-age propensity, among others. A photo-induced borylation reaction was then investigated with all the photosensitizers and proceeded with yields between 9 and 74.