Dendrimeric Organotelluride Catalysts for the Activation of Hydrogen Peroxide. Improved Catalytic Activity through Statistical and Stereoelectronic Effects Khalid Ahsan, Michael D. Drake, Donald E. Higgs, Amy L. Wojciechowski, Brian N. Tse, Margaret A. Bateman, Youngjae You, and Michael R. Detty* Department of Chemistry, University at Buffalo, The State University of New York, Buffalo, New York 14260 Received March 28, 2003 Dendrimeric polyorganotellurides are prepared in high yield using propyloxy spacers to connect the organotelluride groups to the core molecules. The polyorganotellurides catalyze the oxidation of thiophenol with hydrogen peroxide to give diphenyl disulfide in homogeneous solutions (5% CH 2 Cl 2 /MeOH or 46% CH 2 Cl 2 /MeOH). The polyorganotellurides with two, three, four, and six catalytic groups show roughly statistical increases for the number of catalytic groups relative to the corresponding monotellurides. Catalysts containing [4-(dimethylamino)- phenyl]telluro groups and n-hexyltelluro groups are oxidized more rapidly by hydrogen peroxide and also show greater catalytic activity than the corresponding catalysts containing phenyltelluro groups. A combination of statistical effects and stereoelectronic effects give a 26-fold increase in catalytic activity from 1-phenoxy-3-(phenyltelluro)propane (23a; ν 0 ) 12 μM min -1 ) to dendrimer 22c with six n-hexyltelluro groups (ν 0 ) 312 μM min -1 ) for the oxidation of 1.0 × 10 -3 M PhSH with 3.75 × 10 -3 MH 2 O 2 in the presence of 1.0 × 10 -5 M catalyst. The rate of appearance of PhSSPh, with a molar extinction coefficient, ǫ, of 1.24 × 10 -3 L mol -1 cm -1 at 305 nm, was monitored at 305 nm. While H 2 O 2 is a powerful oxidant thermodynamically, many of the reactions of H 2 O 2 are limited by the kinetics of reaction, as illustrated by the oxidation of halides to the corresponding halogen/hypohalous acid 1 and the oxidation of thiols to disulfides. 2 Nature has developed a variety of peroxidase enzymes to accelerate these reactions of H 2 O 2 and other peroxy compounds, and chemists have designed synthetic catalysts to mimic the peroxidase enzymes. 3 Among these latter catalysts, diorganotellurides have been excellent catalysts for the activation of H 2 O 2 in these particular reactions. 2,4 The diorganotellurides undergo two-electron redox processes at the Te atom during the catalytic cycle, as shown in Scheme 1. 2,4,5 Peroxide oxidation of the dior- ganotelluride gives the corresponding oxide (or its hydrate), which then acts as an oxidant (kinetically superior to H 2 O 2 ) for a variety of substrates (Sub-H). The diorganotelluride is regenerated in the process to resume the catalytic cycle. The rate-limiting step in the catalytic process is the rate of oxidation of the diorgano- telluride. 4a,5b For the diorganotellurides, catalytic activity with H 2 O 2 will be a balance between the rate of oxidation of the Te atom with H 2 O 2 and the rate of reductive elimination to form product and to regenerate catalyst. Traditionally, the molar activity of catalysts has been optimized through structure-activity relationships de- rived from substituent changes. However, stereoelec- tronic effects can only go so far with respect to increas- ing rates of oxidation of the Te atom. We have shown enhanced catalytic activity in dendrimeric 6 diorgano- telluride catalysts 7 in which statistical increases in catalytic activity in two-phase systems were noted by (1) Mohammed, A.; Liebhafsky, H. A. J. Am. Chem. Soc. 1934, 56, 1680. (2) (a) Detty, M. R.; Gibson, S. L. Organometallics 1992, 11, 2147. (b) Engman, L.; Stern, D.; Pelcman, M.; Andersson, C. M. J. Org. Chem. 1994, 59, 1973. (c) Vessman, K.; Eksto ¨rm, M.; Berglund, M.; Andersson, C. M.; Engman, L. J. Org. Chem. 1995, 60, 4461. (d) Kanda, T.; Engman, L.; Cotgreave, I. A.; Powis, G. J. Org. Chem. 1999, 64, 8161. (3) (a) Back, T. G.; Moussa, Z. J. Am. Chem. Soc. 2002, 124, 12104. (b) Dexter, A. F.; Lakner, F. J.; Campbell, R. A.; Hager, L. P. J. Am. Chem. Soc. 1995, 117, 6412. (c) Allain, E. J.; Hager, L. P. Deng, L.; Jacobsen, E. N. J. Am. Chem. Soc. 1993, 115, 4415. (d) Butler, A.; Walker, J. V. Chem. Rev. 1993, 93, 1937. (4) (a) Detty, M. R.; Zhou, F.; Friedman, A. E. J. Am. Chem. Soc. 1996, 118, 313. (b) Higgs, D.; Nelen, M. I.; Detty, M. R. Org. Lett. 2001, 3, 349. (c) You, Y.; Abe, M.; Detty, M. R. Organometallics 2002, 21, 4546. (5) (a) Detty, M. R.; Friedman, A. E.; Oseroff, A. R. J. Org. Chem. 1994, 59, 8245-8250. (b) You, J.; Ahsan, K. Detty, M. R. J. Am. Chem. Soc. 2003, 125, 4918-4927. (6) For recent reviews: (a) Grayson, S. M.; Fre ´ chet, J. M. M. Chem. Rev. 2001, 101, 3819-3867. (b) Fisher, M.; Vo ¨gtle, F. Angew. Chem., Int. Ed. Engl. 1999, 38, 884. (c) Dendrimers. Top. Curr. Chem. 1998, 197. (d) Gorman, C. Adv. Mater. 1998, 295. (7) Francavilla, C.; Drake, M. D.; Bright, F. V.; Detty, M. R. J. Am. Chem. Soc. 2001, 123, 57. Scheme 1 2883 Organometallics 2003, 22, 2883-2890 10.1021/om030232h CCC: $25.00 © 2003 American Chemical Society Publication on Web 06/10/2003