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

Abstract

The Fe III catalysed Michael reaction of b-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 Fe III complexes 4, derived from the catalyst and 1, with either neutral (H 2 O) 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 Ã/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 0/H 2 O) 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 Ã/C coupling for 2b (5d to 6d) compared to 2a (5f to 6f) a fast subsequent reaction and thus, catalytic turnover is inhibited. #

Michael additions catalyzed by transition metals and lanthanides species. A review. Part 1. Transition metals

“…6H 2 O. 65 The reaction is first-order in iron, which is compatible with a one-center template reaction. The authors suggested simultaneous coordination of iron with the enolate and the carbonyl group of the electrophile.…”

Coordination of Iron(III) Cations to β‐Keto Esters as Studied by Electrospray Mass Spectrometry: Implications for Iron‐Catalyzed Michael Addition Reactions

“…[2][3][4][5][6][7] Furthermore, kinetic investigations and a suitable choice of substrates have provided evidence that corroborates the proposed mechanism of a one-center template reaction, in which the iron is assumed to act as a Lewis acidic center to facilitate both deprotonation of the coordinated dicarbonyl compound and coupling by means of template effects (Scheme 1). [8] While these mechanistic proposals are in accordance with experimental observations, direct experimental evidence for the active species within the catalytic cycle and the intermediates is lacking so far. However, theoretical investigations support the proposed mechanism and shed light on the effect of the counterion on the reaction rate.…”

“…In the first, the catalytically active species possesses no chloride but only water and ester ligands. [8] Therefore, it would be necessary that the chloride ligands present at iron be removed prior to the formation of the active catalyst. In the case of Fe(ClO 4 ) 3 , heterolysis is more facile due to the lower charge density and coordination ability of the counterion.…”

“…However, theoretical investigations support the proposed mechanism and shed light on the effect of the counterion on the reaction rate. [8] Nevertheless, three crucial points remain to be solved: 1) Which complexes are present in a solution of Fe III salts and b-keto esters? 2) What exactly is the role of the counterion?…”

“…Furthermore, comparison of 1 a, 1 b, and 1 e permits the investigation of possible steric effects of the ester group. Based on the synthetic procedures developed in the condensed phase, [8] the counterion of FeX 3 was selected to study the observed acceleration of the Fe-mediated Michael addition when the counterion is changed from chloride to perchlorate.…”