Para-Selective Borylation of Monosubstituted Arenes Using a Transient Mediator

Herein, we conceptualized a transient mediator approach that has the capability of para-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 para selectivity is related to the incredible electrophilicity and steric hindrance of phenoxathiine or thianthrene radical cation. The versatility of this approach was demonstrated via para-borylation of various monosubstituted simple aromatics combining the sulfonium salt formation with further photocatalyzed transformation.