Stoichiometric and Catalytic Activation of the r- and -2,3,4-Tri-O-Acetyl-5-Thioxylopyranosyl Bromide Inside the Cavity of the Pd 3 (dppm) 3 (CO) 2+ Cluster David Brevet, 1a Yves Mugnier,* ,1a Fre ´de ´ric Lemaı ˆtre, 1a,c Dominique Lucas, 1a Soth Samreth, 1b and Pierre D. Harvey* ,1c Laboratoire de Synthe ` se et d’Electrosynthe ` se Organome ´ talliques, CNRS UMR 5632, Faculte ´ des Sciences Gabriel, UniVersite ´ de Bourgogne, 6 BouleVard Gabriel, 21000 Dijon, France, Laboratoire Fournier, 50 rue de Dijon 21121 Daix, and De ´ partement de Chimie, UniVersite ´ de Sherbrooke, Sherbrooke, Que ´ bec, Canada J1K 2R1 Received December 17, 2002 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 °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 R- and -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. Introduction C-X bond activation is a very important process involved in many organic reactions. 2 An obvious example stressing its importance is certainly the Heck reaction. 3 The title cluster exhibits a pronounced affinity toward anionic species (Figure 1), and we recently reported the heterolytic activation of the alkyl C-X bonds (X ) Br, I) which proceeds efficiently via an electrochemical induction of the Lewis acid cluster Pd 3 2+ , as a CF 3 CO 2 - salt. 4 The inorganic product is the corresponding adduct Pd 3 - (X) + (X ) Br, I), while the organic intermediate “R + ” can be trapped with an appropriate nucleophile such as phenol to generate the corresponding dissymmetric ether. In a parallel study, 5 we investigated in detail the electrochemical behavior of the title cluster in the presence of I - ions, allowing the establishment of the mechanisms by which the Pd 3 (I) + is reduced. The generation of the highly reactive carbocation provides an opportunity to explore applications in organic chemistry, but also the three different oxidation * To whom correspondence should be addressed. E-mail: pharvey@ courrier.usherb.ca (P.D.H.); Yves.Mugnier@u-bourgogne.fr (Y.M.). Phone: (819) 821-2005 (P.D.H.). Fax: (819) 821-8017 (P.D.H.). Phone and fax: (33) 3 80 39 60 91 (Y.M.). (1) (a) Universite ´ de Bourgogne. (b) Laboratoire Fournier. (c) Universite ´ de Sherbrooke. (2) (a) Kulawiec, R. J.; Crabtree, R. H. Coord. Chem. ReV. 1990, 99, 89. (b) Stille, J. K.; Kriesler, S. Y. L. Acc. Chem. Res. 1977, 10, 434. (c) Organometallic Mechanisms and Cata1ysis; Kochi, J. K., Ed.; Academic Press: New York, 1978; p 156. (d) ComprehensiVe Organometallic Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon: Oxford, U.K.; 1995; Vol.12, p 161. (e) Principles and Applications of Organotransition Metal Chemistry; Collman, J. P., Hegedus, L. S., Norton, J. R., Eds.; University Science Books: Mill Valley, CA, 1987. (f) Applied Homogeneous Catalysis with Organometallic Compounds; Cornils, B., Hermann, W. A., Eds.; Wiley: Weinheim, Germany, 1999. (g) Transition Metal Reagents and Catalysts: InnoVations in Organic Synthesis; Tsuji, J., Ed.; Wiley: Chichester, U.K., 2000. (3) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000, 100, 3009 and references therein. (4) Brevet, D.; Lucas, D.; Cattey, H.; Lemaı ˆtre, F.; Mugnier, Y.; Harvey, P. D. J. Am. Chem. Soc. 2001, 123, 4340. (5) Lemaı ˆtre, F.; Brevet, D.; Lucas, D.; Vallat, A.; Mugnier, Y.; Harvey, P. D. Inorg. Chem. 2002, 41, 2368. Inorg. Chem. 2003, 42, 4909-4917 10.1021/ic026279x CCC: $25.00 © 2003 American Chemical Society Inorganic Chemistry, Vol. 42, No. 16, 2003 4909 Published on Web 07/08/2003