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The regioselective hydrogenation of cinnamaldehyde in basic aqueous media catalyzed by [RuH2(mtppms)x] [x = 3,4; mtppms = meta-sulfonatophenyl-diphenylphosphine] is analyzed by means of theoretical calculations. The water solvent is modeled by the inclusion of a (H2O)3 cluster in addition to a continuum model. Two Ru complexes are evaluated as active species for the reduction process: the major identified species [RuH2P4] and the related phosphine dissociated complex [RuH2P3]. Several reaction mechanisms are computationally evaluated, and their analysis suggests that the reaction takes place in several steps. The first hydrogenation process takes place by means of a hydrogen transfer from the metal catalyst, whereas the second hydrogenation process is performed by a water solvent molecule. This mechanism can account for the selectivity of the CO versus CC double bond reduction experimentally observed under basic reaction conditions and can be directly related to the mechanism found in acidic media, where the opposite regioselectivity (CC vs CO) is observed.
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Rossin et al. (2006) studied this question.
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