IN-SITU STUDIES OF CARBONYL COUPLING - COMPARISONS OF LIQUID-SOLID AND GAS-SOLID REACTIONS WITH REDUCED TITANIUM REAGENTS
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The kinetics and mechanism of the liquid-solid, titanium-induced acetone coupling reaction were investigated in order to compare liquid-solid and gas-solid carbonyl coupling. The reaction utilizing both TiCl4- and TiO2-derived reagents reduced with LiAlH4 was monitored using in situ total internal reflectance infrared spectroscopy to probe reactive intermediates and observe the reaction progress. These studies demonstrated that carbon-carbon bond formation via carbonyl coupling occurs below 300 K in both the liquid-solid and gas-solid versions of this reaction. In situ Fourier transform infrared spectroscopy and ex situ gas chromatography/mass spectrometry verified the previously proposed reaction sequence in which pinacols are initially formed, followed by deoxygenation to form the olefin. In addition, these spectroscopic studies confirmed the commonality of the nature of the active titanium reagent, independent of the titanium source. The broad mechanistic and energetic similarities between the liquid-solid and gas-solid versions of this reaction, combined with the low temperatures at which carbon-carbon bond formation can occur, suggest that the principal barrier to operation of carbonyl coupling as a catalytic cycle lies in concurrent catalyst reduction. 1995, American Chemical Society. All rights reserved.
