Homogeneously catalyzed cleavage of C-O bonds and related reactions
Final Report Abstract
The original purpose of the project was the investigation of hydrogenolysis of alcohols catalyzed by homogeneous palladium complexes using formic acid as reducing agent, and the development of further applications, based on our preliminary work using a bulky bidentate P,P-ligand. Since it was not possible to identify a more efficient ligand and reaction conditions, we shifted our efforts towards the development of reductive rearrangement of glycol derivatives using the original Pd-based catalyst. This transformation constitutes a useful extension of the simple hydrogenolysis of benzylic alcohols. It proceeds via initial formation of benzylium cation, which undergoes a [1,2]-H-shift reaction. The resulting aldehyde is reduced upon palladium catalysis. The project included a detailed study of the structural requirements on the substrate, as well as mechanistic investigations. Secondly, we shifted our focus to search for alternative, ideally non-precious metal catalysts for the C–O bond cleavage of alcohols. This was possible using titanocene dichloride as precatalyst, in the presence of manganese as reducing agent and silane as hydride donor. We showed that the used silane Me(EtO)2SiH also acts as regenerating agent for the active catalytic species (Ti(III)). This methodology was applied to a wide range of primary and secondary benzylic alcohols bearing a variety of functional and protecting groups. Sequential deoxygenation–dehalogenation and deoxygenation–hydrogenation reactions were possible, and the procedure was applied to the exhaustive reduction of lignin model substrates. A mechanistic study confirmed a one-electron reduction of the substrate.
Publications
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Tandem Acid/Pd‐Catalyzed Reductive Rearrangement of Glycol Derivatives. Chemistry – A European Journal, 26(16), 3641-3646.
Schmidt, Tanno A.; Ciszek, Benjamin; Kathe, Prasad & Fleischer, Ivana
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Titanium-catalysed deoxygenation of benzylic alcohols and lignin model compounds. American Chemical Society (ACS).
Caciuleanu, Alexandru; Vöhringer, Felix & Fleischer, Ivana
