Dendrimeric Organotelluride Catalysts for the Activation of Hydrogen Peroxide. Improved Catalytic Activity through Statistical and Stereoelectronic Effects

Abstract

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 M H 2 O 2 in the presence of 1.0 × 10 -5 M catalyst. The rate of appearance of PhSSPh, with a molar extinction coefficient, , 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 diorganotelluride 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 diorganotelluride. 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 derived from substituent changes. However, stereoelectronic effects can only go so far with respect to increasing rates of oxidation of the Te atom. We have shown enhanced catalytic activity in dendrimeric 6 diorganotelluride catalysts 7 in which statistical increases in catalytic activity in two-phase systems were noted by (1) Mohammed, A.; Liebhafsky, ...

Methyl Ester Function: An Intramolecular Electrophilic Trap for the Isolation of Aryltellurenyl Hydroxide and Diorganotellurium Dihydroxide

“…They have also attracted wide interest in biology due to their interesting redox properties . Organotellurium compounds such as ditellurides and monotellurides have been used as glutathione peroxidase (GPx) mimics. , Glutathione peroxidase is a selenoenzyme that catalyzes the reduction of hyroperoxides by using glutathione (GSH). The diorganyl telluroxide 6,6′-telluroxybis(6-deoxy-β-cyclodextrin) (6-TeOdiCD), obtained from 6-TediCD, has been found to show hydrolase mimetic activity .…”

Organocatalysis with dendrimers

“…46 The telluro derivatives 26c-G n were also used for the oxidation of PhSH with H 2 O 2 , leading to the disulfide PhS-SPh. 47 Three generations of polybenzylether dendrons based on a flavin core and oligoethyleneglycol terminal groups were used for mimicking the flavoenzyme activity. The aerobic oxidation of BNAH (N-benzyl-1,4-dihydronicotinamide) as a model for NADH was carried out with these dendrons, as shown in Scheme 13.…”

Synthesis of Ferrocene-Modified Phenylazomethine Dendrimers Possessing Redox Switching

“…Dendrimers, which are spherelike nanosized materials, have a perfectly hyperbranched structure. − This topology facilitates modification of the periphery of the dendrimers with a high density of functional molecules. − Its unique characteristics such as concentration of the chromophores in the uniform and symmetrical structures produce precisely controlled and amplified functions. Numerous dendrimers are now being developed as advanced functional nanomaterials for ion sensors, , catalysts, − drug delivery system (DDS), − etc. For instance, a prodrug for DDS, which is the dendrimer modified with drugs and enzyme substrates at the terminal, and sugar ball, which is the dendrimer modified by a sugar-substrated amino acid and polymerized with them, and many other functions of dendrimers had been reported or under study.…”