The structure and reactivity of quinoline overlayers and the adsorption geometry of lepidine on Pt{111}: model molecules for chiral modifiers in enantioselective hydrogenation

NEXAFS, variable temperature STM and TPR have been used to study model adsorbates of relevance to heterogeneous chiral hydrogenation on platinum surfaces. In both the presence and absence of co-adsorbed hydrogen, quinoline molecules are uniformly though randomly distributed on Pt{1 1 1} at 300 K. At 350 K, irreversible surface reactions denude the Pt terraces of modified adsorption sites. Under conditions of hydrogen starvation, extensive polymerisation of quinoline occurs. In the presence of hydrogen, polymerisation is accompanied by hydrogenolysis. In both cases, the {1 1 1} terraces are depleted of quinoid-modified adsorption sites. Quinoline lies approximately flat at 300 K ( alpha similar to 15degrees), bonded to the surface predominantly via the aromatic pi system. This adsorption geometry remains largely unaffected by heating to 360 K, although the data suggest that molecular tilt increases somewhat with surface coverage (alpha similar to 20degrees). Lepidine exhibits a greater tilt angle than quinoline on Pt{1 1 1} (alpha similar to 32degrees), the likely result of steric hindrance due to the substituent at the C4' position-a characteristic that is present in all known efficient chiral modifiers for the reaction of interest. These findings are discussed with respect to the mechanism of chiral induction and the origin of enantioselectivity collapse in the asymmetric hydrogenation of alpha-ketoesters on Pt catalysts. (C) 2001 Elsevier Science B.V. All rights reserved.