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 C1double bond; length as m-dashO1 and C2double bond; length as m-dashO2 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 C1double bond; length as m-dashO1 carbonyl group next to the phenyl ring is activated toward hydrogenation (image adsorptions) are more stable than the corresponding adsorption modes where the C2double bond; length as m-dashO2 group is activated (image adsorption). This indicates that the catalyst surface is covered mainly by reaction intermediates leading to the hydrogenation of the carbonyl group C1double bond; length as m-dashO1 and eventually regioselectivity, presuming that the hydrogenation rates of C1double bond; length as m-dashO1 and C2double bond; length as m-dashO2 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 image-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.