Isomerization pathways of (acylcycloheptatriene)iron tricarbonyl complexes

Article abstract of DOI:10.1021/om00037a058

The (7-exo-acylcycloheptatriene)Fe(CO)₃ complexes (7-acyl) prepared by acylation of the mild organornetallic nucleophile (η³-C₇H₇)Fe(CO)₃^- isomerize readily to their 5-acyl and 6-acyl isomers. The 5-acyl isomer can be prepared in high yield by deprotonation of 7-acyl or 6-acyl followed by a proton quench. Refluxing 5- or 7-acyl in methanol gives the 6-acyl isomer. Rate studies have been carried out on the sequential isomerization of 7-acyl to 5-acyl to 6-acyl. Data were obtained by ¹H NMR monitoring in methanol-d₄ at 40°C. The results indicate that both steps of the isomerization involve an intermolecular proton-exchange mechanism. Solvent deprotonation of any of the three acyl isomers gives a common conjugate base that can be reprotonated at three sites. Kinetic protonation gives 5-acyl, and thermodynamic protonation gives 6-acyl. Mass spectral analysis shows small but significant amounts of multiple deuterium incorporation, suggesting that the common anion intermediate can also be protonated at the metal to give a transient iron hydride.