Unimolecular Reactivity of [Cu(R)(CF3 )3 ]- Complexes (R=Organyl): Stepwise versus Concerted Mechanism in Copper-Mediated Trifluoromethylation.

Abstract

The cuprate complexes [Cu(R)(CF3 )3 ]- (R=organyl) offer an efficient synthetic access to valuable trifluoromethylation products RCF3 . Here, electrospray-ionization mass spectrometry is used to analyze the formation of these intermediates in solution and probe their fragmentation pathways in the gas phase. Furthermore, the potential energy surfaces of these systems are explored by quantum chemical calculations. Upon collisional activation, the [Cu(R)(CF3 )3 ]- complexes (R=Me, Et, Bu, s Bu, allyl) afford the product ions [Cu(CF3 )3 ]⋅- and [Cu(CF3 )2 ]- . The former obviously results from an R⋅ loss, whereas the latter originates either from the stepwise release of R⋅ and CF3 ⋅ radicals or a concerted reductive elimination of RCF3 . The gas-phase fragmentation experiments as well as the quantum chemical calculations indicate that the preference for the stepwise reaction toward [Cu(CF3 )2 ]- increases with the stability of the formed organyl radical R⋅. This finding suggests that the recombination of R⋅ and CF3 ⋅ radicals may possibly contribute to the formation of RCF3 from [Cu(R)(CF3 )3 ]- in synthetic applications. In contrast, the [Cu(R)(CF3 )3 ]- complexes (R=aryl) only yield [Cu(CF3 )2 ]- when subjected to collision-induced dissociation. These species exclusively undergo a concerted reductive elimination because the competing stepwise pathway is disfavored by the low stability of aryl radicals.