Hahn Konstanze R, Seitsonen Ari P, Baiker Alfons
Department of Physics, University of Cagliari, Cittadella Universitaria, I-09048 Monserrato, Italy.
Phys Chem Chem Phys. 2015 Nov 7;17(41):27615-29. doi: 10.1039/c5cp04792g.
The cinchona alkaloids cinchonidine and cinchonine belong to the most efficient chiral modifiers for the noble metal-catalyzed enantioselective hydrogenation of C=O and C=C bonds. Under reaction conditions these modifiers are coadsorbed on the noble metal surface with hydrogen. Using density functional theory, we studied the effect of coadsorbed hydrogen on the adsorption mode of cinchonidine and cinchonine on a Pt(111) surface at different hydrogen coverages. The theoretical study indicates that the presence of coadsorbed hydrogen affects both the adsorption geometry as well as the stability of the adsorbed cinchona alkaloids. At all hydrogen coverages the cinchona alkaloids are found to be adsorbed via anchoring of the quinoline moiety. In the absence of hydrogen as well as at low hydrogen coverage the quinoline moiety adsorbs nearly parallel to the surface, whereas at higher hydrogen coverage it becomes tilted. Higher hydrogen coverage as well as partial hydrogenation of the quinoline part of the cinchona alkaloid and hydrogen transfer to the C[double bond, length as m-dash]C double bond at 10, 11 position of the quinuclidine moiety destabilize the adsorbed cinchona alkaloid, whereas hydrogen transfer to the nitrogen atom of the quinoline and the quinuclidine moiety stabilizes the adsorbed molecule. The stability as well as the adsorption geometry of the cinchona alkaloids are affected by the coadsorbed hydrogen and are proposed to influence the efficiency of the enantiodifferentiating ability of the chirally modified platinum surface.
金鸡纳生物碱辛可尼定和辛可宁是用于贵金属催化的C=O和C=C键对映选择性氢化反应的最有效的手性修饰剂。在反应条件下，这些修饰剂与氢气共吸附在贵金属表面。我们使用密度泛函理论研究了在不同氢覆盖度下共吸附的氢对辛可尼定和辛可宁在Pt(111)表面吸附模式的影响。理论研究表明，共吸附氢的存在既影响吸附几何结构，也影响吸附的金鸡纳生物碱的稳定性。在所有氢覆盖度下，发现金鸡纳生物碱通过喹啉部分的锚定作用进行吸附。在没有氢以及低氢覆盖度时，喹啉部分几乎平行于表面吸附，而在较高氢覆盖度时它会倾斜。较高的氢覆盖度以及金鸡纳生物碱喹啉部分的部分氢化和氢转移到奎宁环部分10、11位的C=C双键会使吸附的金鸡纳生物碱不稳定，而氢转移到喹啉和奎宁环部分的氮原子则会使吸附的分子稳定。金鸡纳生物碱的稳定性以及吸附几何结构受共吸附氢的影响，并被认为会影响手性修饰铂表面的对映体区分能力的效率。
