Transition State for β-Elimination of Hydrogen from Alkoxy Groups on Metal Surfaces

Experimental investigations of β-hydrogen elimination from alkoxy and alkyl groups bound to a Cu(111) surface have been coupled with computational studies of gas-phase analogues to provide insight into the transition state for catalytic hydrogenation and dehydrogenation on metal surfaces. Previous studies have shown that fluorination increases the activation barrier (Δ E act ) to β-hydrogen elimination in ethoxy groups (RCH 2 O (ad) → RCH O (ad) + H (ad), where R = CH 3, CFH 2, CHF 2, CF 3 ) and propyl groups (RCH 2 CH 2,(ad) → RCH CH 2,(ad) + H (ad), where R = CH 3, CF 3 ) on the Cu(111) surface. The increase in barrier height with increasing fluorination was attributed to the inductive influence of fluorine, which energetically destabilizes a hydride-like transition state of the form [RC δ+ ···H δ- ] ‡ . In this paper, deuterium kinetic isotope effects (DKIE) show that fluorination does not alter the mechanism for β-hydrogen elimination from ethoxy groups. Furthermore, the DKIE measurements confirm that the effects of fluorine on the kinetics of β-hydrogen elimination do not result from the change in mass when hydrogen is substituted by fluorine. A systematic study of fluorine substitution of surface-bound isopropoxy groups reveals combined steric and electronic effects. An excellent correlation is found between the Δ E act for β-hydrogen elimination in adsorbed alkoxy groups and the calculated reaction energetics (Δ H rxn ) for gas-phase dehydrogenation of fluorinated alcohols in trans antiperiplanar conformations (e.g., RCH 2 OH (g) → RCH O (g) + H 2,(g), where the hydroxyl hydrogen is antiperiplanar to a carbon and the oxygen is antiperiplanar to a fluorine). Hammett plots for β-hydrogen elimination give a reaction parameter of ρ = −26. These correlations both suggest that the transition state for β-hydrogen elimination develops a greater partial positive charge on the carbinol carbon than is found in the adsorbed reactant. Furthermore, the transition state is energetically late in the reaction coordinate for β-hydrogen elimination.