Perrhenate-Catalyzed Deoxydehydration of a Vicinal Diol: A Comparative Density Functional Theory Study

Area of Science:

• Computational Chemistry
• Catalysis
• Organic Synthesis

Background:

• Oxo-rhenium compounds, particularly perrhenate salts, are effective catalysts for deoxydehydration (DODH) reactions in the presence of reductants.
• Understanding the precise mechanisms of these DODH reactions is crucial for optimizing catalytic efficiency and designing new synthetic routes.

Purpose of the Study:

• To provide the first computational details of reported deoxydehydration (DODH) mechanisms using density functional theory (DFT).
• To investigate the conversion of vicinal diols to alkenes catalyzed by perrhenate (ReO₄⁻) in the presence of a reductant (PPh₃).
• To compare the energetic feasibility of two distinct reaction pathways (Pathway A and Pathway B) for the DODH of phenyl-1,2-ethanediol to styrene.

Main Methods:

• Density Functional Theory (DFT) calculations employing the M06/6-311+G(d,p)/LANL2DZ level of theory.
• Evaluation of Gibbs free energy changes (ΔG°_g and ΔG°_sol) for intermediates and transition states in both gas and solvent phases (chlorobenzene, methanol).
• Calculation of activation energy barriers for key transition states, including nucleophilic attack, ligand dissociation, condensation, and product extrusion.

Main Results:

• Both Pathway A (condensation then reduction) and Pathway B (reduction then condensation) are thermodynamically feasible for the perrhenate-catalyzed DODH reaction.
• Pathway B further branches into routes B1 and B2, involving different intermediate steps and reduction stages (Re(V)-diolate or Re(III)-diolate).
• Pathway A exhibits the lowest overall activation energy barrier compared to Pathway B, identifying it as the kinetically favored mechanism.

Conclusions:

• The DFT calculations elucidate the detailed mechanistic steps and energetic profiles for perrhenate-catalyzed DODH reactions.
• Pathway A is identified as the most efficient route for the conversion of phenyl-1,2-ethanediol to styrene due to its lower activation barrier.
• These findings provide valuable insights into the catalytic cycle of rhenium-based DODH catalysts, aiding in catalyst design and reaction optimization.