In Situ Spectroscopic Investigation of the Rhenium‐Catalyzed Deoxydehydration of Vicinal Diols

Abstract

The mechanism of the CH3ReO3‐catalyzed deoxydehydration of a vicinal diol to an alkene driven by oxidation of a secondary alcohol was investigated by time‐resolved, in situ IR spectroscopy and was found to occur in three steps: 1) reduction of the catalytically active methyltrioxorhenium(VII) to a rhenium(V) complex (the rate‐limiting step), 2) condensation of the diol and the rhenium(V) complex to a rhenium(V) diolate, and 3) extrusion of the alkene accompanied by oxidation of the Re center and thus regeneration of CH3ReO3. The reaction follows zero‐order kinetics initially but, unexpectedly, accelerates towards the end, which is explained in terms of a deactivating pre‐equilibrium, in which the catalytically active CH3ReO3 condenses reversibly with the diol to form an inactive rhenium(VII) diolate. This conclusion is supported by the direct observation of a catalytically inactive species as well as DFT calculations of the IR spectra of the relevant compounds.

Valorization of Biomass‐derived Small Oxygenates: Kinetics, Mechanisms and Site Requirements of H2‐involved Hydrogenation and Deoxygenation Pathways over Heterogeneous Catalysts

“…The reaction order in the oxygenate (1,4‐anhydroerythritol) was first to fractional at low concentrations but reached zero at 15–20 wt % diol concentration. These more or less intuition‐based interpretations of the reaction orders have to be subjected to re‐examination since the kinetic expression can be quite complex in the case of unproductive binding of surface intermediates (e. g., formation of unreactive Re VII diolate in some pre‐equilibrium which takes place before the catalytic cycle which requires Re V ) …”

“…Note that the oxygenate can bind not only to the reduced Re center but also to the highest valence Re center present on the surface (Figure ); the latter diolate species, typically found to be unreactive, is involved in a pre‐equilibrium that shifts with diol concentration and temperature, causing the kinetics to be affected by these parameters in ways that are linked to both rate constants (e. g., for elimination of alkene) and surface binding constants for the deactivating diolate species. Unusual kinetics sometimes result …”

“…Of the three steps in a catalytic DODH cycle, the rate of the condensation step (i. e., coordination of a dihydroxylated substrate to metal center forming a molecule of water) would be the least affected by deuteration at the carbon atoms. Reduction of the catalytic metal center can be rate‐limiting: for alcohol reductant, a primary KIE might be observed if the reduction step involves C−H bond cleavage of the reductant (including the reactant itself); for H 2 as the reductant, there is likely no KIE for the reduction step. In contrast, a secondary KIE would be expected if oxidative C−C bond cleavage is preferred during the reduction step.…”

“…A complete description of the catalytic DODH cycle requires understanding how the adsorbed species and the active centers on the solid surface evolve during catalysis, which can be studied by an array of spectroscopic tools, e. g., infrared (IR), NMR and X‐ray absorption spectroscopies. As DODH reactions are all performed in liquid phase, in situ IR spectroscopic measurements must be performed with an ATR probe in contact with the hot reaction mixture, allowing one to monitor the progress of a reaction (also by NMR) and temporal evolution of intermediates (if present in significant amounts and having sufficiently strong IR‐active vibrations), or even determination of accurate kinetic parameters . Assignment of surface species on solid DODH catalysts, which features various modes of binding through O‐atoms, can be assisted by isotope labeling of oxygen and the associated isotopic shifts.…”

New Motifs in Deoxydehydration: Beyond the Realms of Rhenium

“…By using ReactIR to follow the DODH of 1,2-tetradecanediol using MTO as the catalyst and 3-octanol as the reductant, Dethlefsen and Fristrup proposed PathwayBas the most probable (see Scheme3). [38] Ar eversible condensation of MTO with Scheme2.Reductiono fM TO by 3-pentanola nd DODH of 1,4-anhydroerythritol.…”