Challenge and progress: palladium-catalyzed sp(3) C-H activation

Palladium-catalysis has been broadly applied to sp(2) C-H activation. Recently, palladium-catalyzed sp(3) C-H activation has also been considered as an important strategy to construct synthetically useful C-C/C-X bonds. Allylic sp(3) C-H bonds can be successfully activated by Pd(II) species to produce eta(3)-coordinated palladium species for further transformations, while activation of general sp(3) 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.