Enantioselective chemisorption on model chirally modified surfaces: 2-Butanol on r-(1-naphthyl)ethylamine/Pd(111)

The enantioselective adsorption of propylene oxide and 2-butanol is explored on α-(1-naphthyl)-ethylamine (NEA) covered Pd(111) using temperature-programmed desorption (TPD), reflection-absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). The saturation coverage of NEA is ∼0.1 monolayers (ML), and it thermally decomposes to desorb hydrogen and HCN. 2-Butanol adsorbs enantioselectively on NEA-modified Pd(111), while propylene oxide does not, emphasizing the importance of hydrogen-bonding interactions. An enantioselectivity ratio, ER, of ∼2 (corresponding to an ee of ∼33%) is found at an NEA coverage of ∼0.055 ML, where 2-butanol adsorbs on the Pd(111) substrate. A second regime is found in which 2-butanol adsorbs on an NEA-covered surface with a maximum ER ∼ 1.8 (corresponding to an ee of ∼29%). This interaction appears to cause the NH₂ group to reorient to facilitate hydrogen bonding interactions between 2-butanol and the amine group, and the heat of adsorption of ∼35 kJ/mol is typical of - OH..."NH₂ hydrogen bond strengths.