Insertion of CO2 Mediated by a (Xantphos)Ni-I-Alkyl Species

The incorporation of CO2 into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of Ni-II to Ni-I by either Zn or Mn, followed by CO2 insertion into Ni-I-alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu-Xantphos)(NiBr2)-Br-II by Zn affords Ni-I species. (tBu-Xantphos)Ni-I-Me and (tBu-Xantphos)Ni-I-Et complexes undergo fast insertion of CO2 at 22 degrees C. The substantially faster rate, relative to that of Ni-II complexes, serves as the long-sought-after experimental support for the proposed mechanisms of Ni-catalyzed carboxylation reactions.