Mechanistic Insights for Iodane Mediated Aromatic Halogenation Reactions

Abstract

Stable iodanes such as PhI(OAc)2 (Ac=acetyl) may be readily activated by electrophiles and salts for useful functional group transfer reactions under mild conditions, but the underlying reaction mechanism remains controversial. In this work, a general mechanism of iodane-mediated aromatic halogenation is revealed by extensive DFT calculations using PhI(OAc)2 and anisole PhOMe as model iodane and aromatic substrate, respectively. It is shown that facile OAc/X (X=Cl, Br) ligand exchange with electrophiles can be induced by electrophilic attack at acetyl C=O groups of PhI(OAc)2 to form more reactive iodanes, especially the non-symmetric iodanes PhI(X)OAc that can be activated by electrophiles to reach reactive iodonium PhIX+ for further electrophilic halonium X+ transfer to aromatic substrates. Direct anionic OAc−/X− exchange of iodane are 4∼7 kcal/mol endergonic, suggesting the crucial role of electrophilic iodane activation. Hetero- and homolytic I−X bond cleavages of neutral iodanes are more than 28 kcal/mol endergonic and thus unlikely under ambient conditions without electrophilic activation.