title: Stoichiometric and catalytic activation of the alpha- and beta-2,3,4-tri-O-acetyl-5-thioxylopyranosyl bromide inside the cavity of the Pd3(dppm)3(CO)2+ cluster.
abstract: The title cluster (Pd(3)(2+)) exhibits a pronounced affinity for Br(-) ions to form the very stable Pd(3)(Br)(+) adduct. Upon a 2-electron reduction, a dissociative process occurs generating Pd(3)(0) and eliminating Br(-) according to an ECE mechanism (electrochemical, chemical, electrochemical). At a lower temperature (i.e. -20 degrees C), both ECE and EEC processes operate. This cluster also activates the C-Br bond, and this work deals with the reactivity of Pd(3)(2+) with 2,3,4-tri-O-acetyl-5-thioxylopyranosyl bromide (Xyl-Br), both alpha- and beta-isomers. The observed inorganic product is Pd(3)(Br)(+) again, and it is formed according to an associative mechanism involving Pd(3)(2+).Xyl-Br host-guest assemblies. In an attempt to render the C-Br bond activation catalytic, these species are investigated under reduction conditions at two potentials (-0.9 and -1.25 V vs SCE). In the former case, the major product is Xyl-H, issued from a radical intermediate Xyl(*) abstracting an H atom from the solvent. Evidence for Xyl(*) is provided by the trapping with TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) and DMPO (5,5'-dimethylpyrroline-N-oxyde). In the second case, only one product is observed, 3,4-di-O-acetyl-5-thioxylal, which is issued from the Xyl(-)() intermediate anion.
authors.affiliation: Laboratoire de Synthèse et d'Electrosynthèse Organométalliques, CNRS UMR 5632, Faculté des Sciences Gabriel, Université de Bourgogne, 6 Boulevard Gabriel, 21000 Dijon, France.
authors.forename: David
authors.initials: D
authors.lastname: Brevet
authors.forename: Yves
authors.initials: Y
authors.lastname: Mugnier
authors.forename: Frédéric
authors.initials: F
authors.lastname: Lemaître
authors.forename: Dominique
authors.initials: D
authors.lastname: Lucas
authors.forename: Soth
authors.initials: S
authors.lastname: Samreth
authors.forename: Pierre D
authors.initials: PD
authors.lastname: Harvey