KETONE COUPLING ON REDUCED TIO2 (001) SURFACES - EVIDENCE OF PINACOL FORMATION

Reductive coupling of acetone and acetophenone was investigated in tem perature-programmed desorption (TPD) studies on both reduced (Ar+-bomb arded) and oxidized TiO2 (001) surfaces. The principal reaction produc t of either ketone on the reduced surface was a symmetric olefin with twice the carbon number of the reactant. 2,3-Diphenyl-2-butene compris ed over 65% of the volatile carbon-containing species desorbed from th e reduced surface following acetophenone adsorption. The main side rea ctions which yielded products of the same carbon number as the reactan ts included deoxygenation to form olefins and deoxygenation plus hydro genation to yield saturated species. The yield of reduction products w as greatly diminished on the oxidized TiO2 (001) surface; the yield of 2,3-dimethyl-2-butene, the reductive coupling product of acetone, dec reased 10-fold with respect to the yield from the reduced surface. Thi s decrease in activity for reductive coupling is similar in scale to t hat observed for benzaldehyde coupling on the same surfaces, supportin g the conclusion that both ketone and aldehyde coupling reactions occu r at ensembles of Ti cations able to undergo a four-electron oxidation . Phenyl groups adjacent to the carbonyl carbon have the greatest effe ct on the carbonyl coupling reaction, giving significantly higher yiel ds of the coupled olefin product. The observation of a small amount of the pinacol, 2,3-diphenyl-2,3-butanediol, during acetophenone TPD is the first direct evidence that the carbonyl coupling reaction on reduc ed TiO2 surfaces proceeds through a pinacolate intermediate, as it doe s for the McMurry reaction carried out in liquid-solid slurries.