Light‐ and Manganese‐Initiated Borylation of Aryl Diazonium Salts:Mechanistic Insight on the Ultrafast Time‐Scale Revealed by Time‐Resolved Spectroscopic Analysis

Manganese‐mediated borylation of aryl/heteroaryl diazonium salts emerges as a general and versatile synthetic methodology for the synthesis of the corresponding boronate esters. The reaction proved an ideal testing ground for delineating the Mn species responsible for the photochemical reaction processes, i.e. involving either Mn radical or Mn cationic species, which is dependent on the presence of a suitably strong oxidant. Our findings are important for a plethora of processes employing Mn‐containing carbonyl species as initiators and/or catalysts, which have much potential in synthetic applications.