Para-Selective Borylation of Monosubstituted Arenes Using a Transient Mediator

Herein, we conceptualized a transient mediator approach that has the capability of <i>para</i>-selective C–H functionalization of monosubstituted aromatics. This approach is enabled by in situ generation of a versatile sulfonium salt via highly electrophilic phenoxathiine or thianthrene radical cation intermediate which can be readily generated from its sulfoxide with trifluoromethanesulfonic anhydride. Preliminary mechanistic study unveiled that the remarkable <i>para</i> selectivity is related to the incredible electrophilicity and steric hindrance of phenoxathiine or thianthrene radical cation. The versatility of this approach was demonstrated via <i>para</i>-borylation of various monosubstituted simple aromatics combining the sulfonium salt formation with further photocatalyzed transformation.