Catalysis of the Michael reaction by iron(III): calculations, mechanistic insights and experimental consequences

The FeIII catalysed Michael reaction of β-oxo esters 1 with methyl vinyl ketone (2a) and methyl acrylate (2b) as acceptors is investigated. The proposed mechanism, based on experimental results, is supported for the first time by ab initio calculations. Thereby different FeIII complexes 4, derived from the catalyst and 1, with either neutral (H2O) or anionic (a second diketonate 1) coligands are considered and their stabilities calculated. Ligand exchange in 4 by acceptors 2 gives intermediates 5 and subsequent CC bond formation affords complexes 6. Both 5 and 6 are found to be minima on the potential hypersurface and a pathway from 5 to 6 via transition states 7 could be identified. For the reaction of 4a (L=H2O) with 2a, b no reactivity difference was found. The anionic coligand, however, causes a significant change in the reaction profile: due to the higher reaction barrier for the CC coupling for 2b (5d to 6d) compared to 2a (5f to 6f) a fast subsequent reaction and thus, catalytic turnover is inhibited.