Résumé
The catalytic aerobic coupling of organometallic lithium and Grignard reagents using MnCl2 is a useful method to synthesize homo or heterocoupled products, including biaryls, dialkynes or arylalkynes. For the case of aryllithiums, we herein provide insights into the mechanistic scenario using a combined study involving [18O] labelling experiments, mass spectrometry and DFT calculations. A highly reactive manganate complex [MnPh3]- could be detected in ESI-MS and reacted in the gas phase with molecular oxygen to provide biphenyl and a Mn-oxygen complex, whereas in the absence of oxygen only very limited amounts of biphenyl were produced. An alternative C-O bond forming reaction was also unveiled from this Ion-Molecule Reaction. Based on these results, a theoretical study was conducted to shine some light on the gas phase and solution processes. Taking into account the specificities of the condensed phase, DFT calculations have revealed a number of potential mechanistic scenarios for this oxidative C-C coupling reaction.