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Mechanism and Influence of Acid in Hydrogenation of Ketones by η6-Arene/N-Tosylethylenediamine Ruthenium(II)

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journal contribution
posted on 13.04.2009, 00:00 by Yue Chen, Yanhui Tang, Shubin Liu, Ming Lei, Weihai Fang
Density functional theory calculations have been performed to study the hydrogenation of ketones catalyzed by a η6-arene ruthenium(II) complex in an acidic conditions. Six possible dihydrogen activation (DA) pathways were investigated in this work. The direct DA (path 1) and alcohol-assisted DA (path 2), which will occur in basic/neutral conditions, have higher energy barriers, 19.9 and 18.7 kcal/mol, respectively. If an acid participates in DA in the other three paths (paths 4, 5, and 6), the barrier will substantially decrease to about 3.7, 7.1, and 7.8 kcal/mol, respectively. Compared with paths 1 and 2, the acid-assisted pathways are more favorable. In path 1 and path 2, molecular hydrogen is unable to coordinate with Ru to form a stable η2-H2 ruthenium(II) complex, leading to the increase of the free energy barrier of DA. On the contrary, dihydrogen forms a stable η2-H2 ruthenium(II) complex with Ru in paths 4, 5, and 6. These results indicate that the coordination of dihydrogen with Ru plays an important role in the conversion of H2-hydrogenation in acidic conditions.