Who Wins: Pesci, Peters, or Deacon? Intrinsic Reactivity Orders for Organocuprate Formation via Ligand Decomposition

There are three metal-mediated ligand decomposition reactions that give rise to organometallics: (i) decarboxylation of a metal carboxylate, the Pesci reaction; (ii) desulfination of a metal sulfinate, the Peters reaction; (iii) desulfonation of a metal sulfonate, the Deacon reaction. Despite growing interest in their use in applications in organic synthesis, little is known about the relative ease of thermal extrusion of CO2 versus SO2 versus SO3 in metal complexes. Here the intrinsic reactivity orders for organocuprate formation via ligand decomposition have been studied in the gas phase for the first time. A combination of low-energy collision-induced dissociation experiments in an ion trap mass spectrometer and DFT calculations was used. Simple ligand competition experiments in the heterocopper complexes [MeX-OCuOYMe](-) - (where X = CO or SO and Y = SO or SO2), formed via electrospray ionization, show that desulfination occurs more easily than decarboxylation, which in turn is more facile than desulfonation. This is consistent with DFT calculations at the M06/SDD/cc-pVTZ//M06/cc-pVDZ level of theory, which show that the barriers associated with the transition states follow the order SO2 < CO2 < SO3.