Disrupting Aryl-CH···O Interactions on Pt(111) Through the Coadsorption of Trifluoroacetic Acid and 2,2,2-Trifluoroacetophenone (TFAP): Inhibition of Competing Processes in Heterogeneous Asymmetric Catalysis

A study of the coadsorption of trifluoroacetic acid (TFA) and 2,2,2-trifluoroacetophenone (TFAP) on Pt(111) was carried out using scanning tunneling microscopy (STM) measurements. The investigation is based on literature reports that the conversion rate and the enantiomeric excess for the hydrogenation of TFAP on cinchona modified Pt/Al 2 O 3 are very sensitive to the presence of small amounts of TFA in the solvent. As previously reported, STM and density functional theory (DFT) studies show that TFAP forms aryl-CH···O bonded dimers on Pt(111) at room temperature. In the present study, STM measurements show that TFA disrupts aryl-CH···O bonding through insertion into TFAP dimers to form isolated trimolecular and bimolecular assemblies. Structural models are proposed for the TFAP–TFA aggregates. The implications of these structures for the rate and enantioselectivity of TFAP hydrogenation are discussed in terms of H-bonding activation of the carbonyl function. It is proposed that the acid additive operates on the enanantioselective hydrogenation of TFAP through the inhibition of the racemic reaction at sites remote from the chiral modifier. This action is assumed to occur in parallel with effects, already proposed in the literature, due to the formation of modifier–acid complexes and the protonation of the modifier.