Iron Tricarbonyl Complexes in Oxidation Chemistry: Regio- and Stereoselective Oxyfunctionalization of Trienes by Singlet Oxygen and Dimethyldioxirane

Abstract

The versatility of the iron tricarbonyl fragment as protecting group in the oxyfunctionalization of trienes has been examined. The photooxygenation of the unsaturated iron complexes 1−3 at low temperatures (−80 °C) afforded after Ph3P reduction the allylic alcohols 4, 4‘, 13, and 22 in good to excellent yields. The persistence of the intermediary allylic hydroperoxides is reflected in the distance between the iron fragment and the reacting free double bond. Only the hydroperoxides derived from the homoallylic (most distant) substrate 1 are persistent enough to undergo the titanium tetraisopropoxide-catalyzed oxygen transfer to β-hydroxy epoxides. Epoxidation of the complexes 1 and 2a with dimethyldioxirane could be optimized in boiling acetone to suppress oxidative destruction of the iron tricarbonyl ligand. The deprotection of the organic ligand with cerium(IV) ammonium nitrate (CAN) enables the convenient preparation of multiply functionalized cyclohexadienes. In contrast to these regio- and stereoselectively controlled transformations, the corresponding unprotected trienes 10 and 19a react with singlet oxygen and dioxirane at all double bonds without any particular preference.