The Stereoselectivity of the Dehydrogenation of Alkyl Groups on Pt(111) Single-Crystal Surfaces

The thermal chemistry of erythro- and threo-2-butyl-3-d(1) species adsorbed on Pt(111) single-crystal surfaces was studied under ultrahigh vacuum (UHV) by temperature-programmed desorption (TPD). The alkyl intermediates were prepared via thermal activation of the appropriate erythro- and threo-2-iodobutane-3-d(1) precursors, which were made in house via a three-step synthesis starting with cis- and trans-2,3-epoxybutanes, respectively. Several products were detected in the TPD experiments, including butenes and butanes with different degrees of isotope substitutions and hydrogen from deep dehydrogenation reactions. At least two distinct temperature regimes were identified at higher coverages for the beta-hydride elimination steps that produce the olefins, the focus of this work, but one single high-temperature regime could be isolated by working with low coverages of the iodoalkanes. On clean Pt(111), an inverse kinetic isotope effect was observed for the conversion of 2-butyls to 2-butenes, and a slight preference for trans- (rather than cis-) 2-butene production. In addition, a reversal in relative rates for 2-butene-d(0) versus 2-butene-d(1) was detected in the high-temperature side of the TPD peaks obtained with the two alkyl epimers, an observation that we explain by a high degree of reversibility of the alkyl-to-alkene conversion. Coadsorption of hydrogen or deuterium on the surface reverses the kinetic isotope effect, but not the selectivity for trans olefin production. However, these effects, in particular, the preference for trans olefin formation, are small, indicating an early transition state for the beta-hydride elimination step and a thermodynamic (rather than kinetic) control of selectivity in olefin isomerization reactions.