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

The title cluster (Pd3^2+) exhibits a pronounced affinity for Br^- ions to form the very stable Pd3(Br)^+ adduct. Upon a 2-electron reduction, a dissociative process occurs generating Pd30 and eliminating Br- according to an ECE mechanism (electrochemical, chemical, electrochemical). At a lower temperature (i.e. -20 C), both ECE and EEC processes operate. This cluster also activates the C -Br bond, and this work deals with the reactivity of Pd3^2+ with 2,3,4-tri-O-acetyl-5-thioxylopyranosyl bromide (Xyl-Br), both (alpha)- and (beta)-isomers. The observed inorganic product is Pd3(Br)^+ again, and it is formed according to an associative mechanism involving Pd3^2+···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 (5,5ʹ-dimethylpyrroline-N-oxyde). In the second case, only one product is observed, 3,4-di-O-acetyl-5thioxylal, which is issued from the Xyl^- intermediate anion.