Coordination of iron(III) cations to beta-keto esters as studied by electrospray mass spectrometry: implications for iron-catalyzed Michael addition reactions.

Solutions of Fe(III) salts and beta-keto esters have been investigated by means of electrospray ionization mass spectrometry. The complexes formed in such solutions have been considered previously as active intermediates in Fe(III)-catalyzed Michael additions. By using different Fe(III) salts with a set of beta-keto esters, cation and anion mass spectra were analyzed and the effects of ester concentration, the role of the counterion, and the structure of the ester employed are discussed. Depending on the basicity of the ester, an increase of its concentration may lead to a decrease in the concentration of iron complexes observed in the mass spectra. Counterions with strong binding affinities to iron are found to capture the metal as ferrates, thereby removing the metal from the catalytic cycle.