Challenge and progress: palladium-catalyzed sp3 C–H activation

Palladium-catalysis has been broadly applied to sp2 C–H activation. Recently, palladium-catalyzed sp3 C–H activation has also been considered as an important strategy to construct synthetically useful C–C/C–X bonds. Allylic sp3 C–H bonds can be successfully activated by Pd(II) species to produce η3-coordinated palladium species for further transformations, while activation of general sp3 C–H bonds mainly proceeds through directed pathways with the assistance of proper directing groups or initiated by oxidative addition. Various catalytic mechanisms were extensively investigated through either Pd(II)/Pd(0), Pd(II)/Pd(IV) or Pd(0)/Pd(II) catalytic cycles. The challenges faced in this area have also been addressed in this perspective article.