Selective heterogeneously catalyzed hydrogenations

Selective hydrogenation of ethyl-benzoylacetate to 3-hydroxy-3-phenyl-propionate catalyzed by Pd/C in EtOH as a solvent in the presence of KOH: The role of the enolate ion on the reaction mechanism

“…It occurs with formation of a H-C-OH moiety which may undergo further hydrogenation by the C-OH bond splitting. The selectivity of different metals towards this functional group has attracted much interest [3][4][5][6][7][8][9][10][11][12][13][14][15][16].…”

“…Among the several metals active in ketones hydrogenation Pt, Pd and Rh are the most effective. The selectivity of the catalyst is related to the type of the metal and to the support [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Pd catalysts, under mild conditions, present the advantage that ring hydrogenations are completely avoided [4][5][6][7][12][13][14][15].…”

“…The selectivity of the catalyst is related to the type of the metal and to the support [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17]. Pd catalysts, under mild conditions, present the advantage that ring hydrogenations are completely avoided [4][5][6][7][12][13][14][15]. In addition, employing alkaline solution of ethanol in the presence of Pd/C catalysts the C-OH hydrogenolysis is practically suppressed [7,12,13] with selectivity comparable or higher than that of the Cu-based catalysts, which are highly selective but much less active [16,17].…”

“…In addition, employing alkaline solution of ethanol in the presence of Pd/C catalysts the C-OH hydrogenolysis is practically suppressed [7,12,13] with selectivity comparable or higher than that of the Cu-based catalysts, which are highly selective but much less active [16,17]. The role of the base in the selective hydrogenation of ketones has not yet well understood, as well as that of the enolate anion, which is formed by interaction with KOH [4,5]. Kinetic studies on the hydrogenation of substituted acetophenones suggest that the enol form is not involved in the hydrogenation step, thus suggesting that keto-enol equilibrium is not a limiting step of the kinetics [8][9][10].…”

“…In these papers, surface complex between the ketones and the modifiers are claimed and the chiral nature of the modifier is responsible for the enantioselective hydrogenation [18][19][20][21][22][23][24][25][26]. However, the role of the enolate ion on the reaction mechanism is not investigated and only in the old paper is mentioned [4,5].…”