Mono- and dioxygenation of rhodium and iridium olefin fragments : solution versus solid state reactivity

Abstract

Partial oxygenation of olefins is a very important method for the conversion of these cheap starting materials into useful products. Often, such oxygenation reactions are metal-catalyzed. Not much is known, however, about the mechanisms involved. We have investigated stoichiometric reactions of N4-ligand rhodium(I) and iridium(I) olefin complexes with hydrogen peroxide, dioxygen and air, to obtain more insight into the mechanistic aspects of metal-catalyzed olefin oxygenation. Chapter 2 describes the synthesis of the starting metal(I) olefin complexes. Formation of a mono-ethene or a bis-ethene complex is found to depend on the metal center as well as the ligand. Chapter 3 deals with the mono-oxygenation of N3-ligand and N4-ligand rhodium(I) olefin complexes by hydrogen peroxide in solution. This results in the formation of 2-rhodaoxetanes. Upon protonation, 2-rhodaoxetanes react with nitriles to C-O coupled six-membered metallacyclic imino-esters. Their subsequent rearrangement to C-N coupled metallacyclic amides is easier for a 4-methyl-2-rhodaoxetane than for an unsubstituted 2-rhodaoxetane and is promoted by electron withdrawing groups in the starting nitriles. The same trend is followed for (-hydrogen elimination from the metallacyclic amides. Chapters 4 and 5 describe the dioxygenation of N4-ligand rhodium(I) and iridium(I) ethene complexes by dioxygen in the solid state and the reactivity of the resulting unsubstituted 3-metalla-1,2-dioxolanes. Remarkably, reaction of dioxygen with N4-ligand metal(I) ethene complexes in the solid state is much more selective than in solution. Two stereo-isomeric N4-ligand 3-metalla-1,2-dioxolanes are obtained in varying ratios dependent on the ligand, the metal center and the counterion used. The obtained N4-ligand metalladioxolanes rearrange to formylmethyl hydroxy complexes upon exposure to photons or protons. Obtained rhodium(III) formylmethyl hydroxy fragments are protonated to formylmethyl aqua species and insert CO2 reversibly into their M-OH bond, to give formylmethyl hydrogen carbonate species. Chapter 6 compares possible mechanisms for the dioxygenation of metal(I) olefin fragments in the solid state.