Enantioselective decomposition of chiral alkyl bromides on Cu(643)R&S:: Effects of moving the chiral center

The enantioselective surface chemistry of two chiral alkyl halides, S-1-bromo-2-methylbutane and R-2-bromobutane, have been compared on the naturally chiral Cu(6 4 3)(R+S) surfaces. Temperature programmed reaction spectroscopy was used to quantify the yields of the various decomposition products during heating. A fraction of the adsorbed alkyl bromides desorb intact while the remainder decomposes by debromination to form either S-2-methyl-1-butyl or R-2-butyl groups on the surfaces. The S-2-methyl-1-butyl group then reacts by P-hydride elimination to form 2-methyl-1-butene or by hydrogenation to form 2-methylbutane. The R-2-butyl group reacts by Phydride elimination to form butene or by hydrogenation to form butane. This surface chemistry on Cu(643)(R+S) is not enantioselective at low coverages but is enantio selective at high coverages. In R-2-bromobutane the chiral carbon atom coincides with the debrommation reaction center while the P-hydride elimination centers are achiral. In S-1-bromo-2-methylbutane the chiral carbon atom coincides with the beta-hydride elimination reaction center while the center for debromination is achiral. Results show that the enantioselectivities are influenced by the surface structure to a greater extent than they are by the adsorbate structure. (c) 2006 Elsevier B.V. All rights reserved.