(Cyclopentadienone)iron-Catalyzed Transfer Dehydrogenation of Symmetrical and Unsymmetrical Diols to Lactones.

Air-stable iron carbonyl compounds bearing cyclo-pentadienone ligands with varying substitution were explored ,as catalysts in dehydrogenative diol lactonization reactions using acetone as both the solvent and hydrogen acceptor. Two catalysts with trimethylsilyl groups in the 2- and 5-positions, [2,S-(SiMe3)(2)-3,4-(CH2)(4)(eta(4)-C4C=O))Fe(CO)(3) (1) and [2,5-(SiMe3)(2)-3,4(CH2)(3)(eta(4)-C4C=O)]Fe(CO)(3) (2), were found to be the most active, with 2 being the most selective in the lactonization of diols containing both primary and secondary alcohols. Lactones containing five-, six-, and seven-membered rings .were successfully synthesized, and no over-oxidations to carboxylic acids were detected. The lactonization of unsymmetrical diols containing two primary alcohols occurred with catalyst 1, but selectivity was low based on alcohol electronics and modest based on alcohol sterics. Evidence for a transfer dehydrogenation mechanism was found, and insight into the origin of selectivity in the lactonization of 1 degrees/2 degrees diols was obtained. Additionally, spectroscopic evidence for a trimethylamine-ligated iron species formed in solution during the reaction was discovered.