Revealing regioselectivity in hydrogenation of 1-phenyl-1,2-propanedione on Pt catalysts

Adsorption of 1-phenyl-1,2-propanedione (A), a widely studied molecule in heterogeneously catalyzed enantioselective hydrogenation, and 2,3-hexanedione on a Pt(111) surface was studied using density functional theory. A cluster consisting of two slabs and 31 Pt atoms was used as a model for the catalyst. The results revealed the origin of observed regioselectivity in the hydrogenation of carbonyl groups C1O1 and C2O2 of A and the lack of regioselectivity in the case of 2,3-hexanedione on Pt catalysts. The adsorption modes of A in which the C1O1 carbonyl group next to the phenyl ring is activated toward hydrogenation (ηC1O12 adsorptions) are more stable than the corresponding adsorption modes where the C2O2 group is activated (ηC2O22 adsorption). This indicates that the catalyst surface is covered mainly by reaction intermediates leading to the hydrogenation of the carbonyl group C1O1 and eventually regioselectivity, presuming that the hydrogenation rates of C1O1 and C2O2 are of the same order of magnitude. The adsorption energy of 2,3-hexanedione does not depend on which of the carbonyl groups is adsorbed as η2η2-mode, and thus the hydrogenation on Pt is not regioselective, as has been observed experimentally. Thus the regioselectivity in the hydrogenation of A and 2,3-hexanedione can be explained by the interactions between the substrate and the metal surface.