Inaccessibility of beta-hydride elimination from -OH functional groups in Wacker-type oxidation

Area of Science:

• Computational Chemistry
• Catalysis
• Organic Chemistry

Background:

• The Wacker process is a vital industrial method for converting alkenes to ketones.
• The mechanism of alcohol dehydrogenation, a related transformation, is crucial for understanding catalytic cycles.
• Current understanding often relies on beta-hydride elimination (BHE) as the key product formation step.

Purpose of the Study:

• To investigate the energetic feasibility of different reaction mechanisms for alcohol dehydrogenation relevant to the Wacker process.
• To compare the activation barriers of proposed pathways using quantum mechanics.
• To assess the catalytic role of water in these transformations.

Main Methods:

• Density functional theory (DFT) calculations using B3LYP and MPW1K functionals.
• Analysis of transition states and activation enthalpies for proposed reaction pathways.
• Inclusion of water as a catalytic species in the computational models.

Main Results:

• The commonly accepted beta-hydride elimination (BHE) pathway has a high activation enthalpy (36.2 kcal/mol).
• An alternative five-bodied reductive elimination (RE) pathway exhibits a significantly lower activation enthalpy (18.8 kcal/mol).
• Water catalysis lowers activation barriers for both pathways but does not alter their relative energetic favorability, favoring RE.

Conclusions:

• The reductive elimination (RE) pathway is energetically more favorable than BHE for alcohol dehydrogenation in this context.
• The assumption of BHE mechanisms may need reevaluation, especially when the beta atom is not an alkyl group.
• Water plays a catalytic role, but the fundamental mechanistic preference remains.