An additional experiment supports the claim that MeCN is a suitable solvent owing to more than just its solvation properties: when 5 vol% of MeCN are added to toluene, 57% of SA, with 92% selectivity are obtained, which is significantly higher than the one obtained in toluene alone in similar conditions (4%, Table for DMF, respectively) 40,41 and a lower kinetic lability than MeCN (kinetic lability for solvent (S) exchange in [Co(S)6] 2+ : ~10 5 s -1 for MeCN and 3.9 x 10 -1 s -1 for DMF, respectively). Although the exact active species can only be hypothesized, it seems reasonable to propose that the activation of the lactone requires the prior decoordination of the LB from the cobalt (di)cation, which would be less favoured in DMF than in MeCN.