The FeII-FeIV and FeIII-FeV manifolds in an expanded world of Gif chemistry

Abstract

The Gif systems for the selective functionalization of saturated hydrocarbons based on the reactions of Superoxide with FeII and of hydrogen peroxide with FeIII are described. Both systems are relatively efficient, but not nearly so efficient as the electrochemical system developed in collaboration with Prof. G. Balavoine and Dr. Aurore Gref (Universite de Paris-Sud-Orsay, France). All of the systems afford mainly ketones. This is an unusual selectivity, which implies a non-radical mechanism. It has been proven for the FeIII-H2O2 system that the activation of the FeIII is independent of the formation of ketone, which involves a hydroperoxide (derived from oxygen) as an intermediate. This intermediate controls the formation of ketone and of secondary alcohol. The addition of a number of trapping reagents such as BrCCl3 diverts the reaction from oxygenation to bromide formation. Although BrCCl3 is indeed a good trap for carbon radicals, the pattern of selectivity across a range of saturated hydrocarbons is completely different for Gif chemistry when compared with normal radical bromination. The chemistry is explained in terms of an FeV oxenoid species that inserts itself into secondary C-H bonds (a compromise between bond strength and steric hindrance). This gives an FeV intermediateA with an iron-carbon bond, which is probably rapidly reduced to the FeIII state by hydrogen peroxide. Then oxygen is inserted into the FeIII-C bond. Hydrolysis affords the isolateable intermediate hydroperoxide (intermediateB). A system based ontert-butyl hydroperoxide (TBHP) is described. This is similar to the above Gif systems, but the kinetic isotope effect is very different and the selectivity for adamantane substitution is different. However, FeIII is activated by TBHP to an FeV oxenoid which, after reaction with a hydrocarbon, reacts with oxygen to give a hydroperoxide. So the pattern of intermediatesA andB is maintained with TBHP. Radical chemistry is involved in some of the reactions that involve ionic coupling to saturated hydrocarbons. The importance of the FeII-FeIV manifold in providing a mechanism that permits the selective functionalization of saturated hydrocarbons by ionic trapping with chloride, azide, and other anions is made manifest. Comparison is made with the FeIII-FeV manifold where ionic trapping is never seen. Traditional Fenton chemistry (hydroxyl radical formation) is not operative here, but the trapping does involve the formation of carbon radicals. These react very efficiently with anions bonded to FeIII.