Evidence for Single-Electron Pathways in the Reaction between Palladium(II) Dialkyl Complexes and Alkyl Bromides under Thermal and Photoinduced Conditions

Palladium(II) dialkyl complexes have previously been studied for their formation of alkanes through reductive elimination. More recently, these complexes, especially L2Pd-(CH2TMS)(2) derived from Pd(COD)(CH2TMS)(2), have found general use as palladium(0) precursors for stoichiometric formation of oxidative addition complexes through a two-electron reductive elimination/oxidative addition sequence. Herein, we report evidence for an alternative pathway, proceeding through single-electron elementary steps, when DPEPhosPd(CH2TMS)(2) is treated with an alpha-bromo-alpha,alpha-difluoroacetamide. This new pathway does not take place through a palladium(0) intermediate, neither does it afford the expected oxidative addition complexes. Instead, stoichiometric amounts of carbon-centered alkyl radicals are formed, which can be trapped in high yields either by TEMPO or by an arene, leading to alpha-aryl-alpha,alpha-difluoroacetamides. The same overall transformation takes place under both thermal conditions (70 degrees C) and irradiation with a household light bulb (at 30 degrees C). It is also demonstrated that DPEPhosPdMe(2), made in situ from Pd(TMEDA)Me-2, displays a similar initial reactivity. Finally, electronically and structurally different alkyl bromides were evaluated as reaction partners.