Supported Thiols

For quantitative analysis of solid supported thiol residues on free macroporous or PEG grafts, Ellman s reagent has been used [5,5 -dithio-hfr-(2-nitrobenzoic acid]. However only qualitative information can be gained using lightly crosslinked polystyrene resins [Badyal et al. Tetrahedron Lett 42 8531 2007]. 

Figure 3.11 The orange colour of Pd(AcO)2 disappears from a solution five minutes after adding silica-supported thiol. (Image courtesy of SiliCycle.)...

An alternative approach is the cleavage of a UV-active protecting group from the resin, such as the widely used Fmoc Test. The quantitation of the 9-fluorenyl-methyloxycarbonyl (Fmoc) protecting group for amines is used in SPPS as an indirect method to determine the extent of a peptide coupling reaction. Similar approaches have also been recently reported for the quantitation of supported thiols [151, 154] and have also been the subject of an excellent review [148]. 

The most important characteristics that make fluorous synthesis superior to solid-supported synthesis is the favorable reaction kinetics associated with the solution-phase reactions. Comparison reactions using fluorous vs. solid-supported thiols to scavenge a bromide are shown in Fig. 1 [16]. Using 1.5equiv of F-thiol 1, more than 95% bromide was quenched in less than 40 min (top line). Under the same conditions and using 1.5 equiv PS-thiol 2, only 50% of the halide was quenched after 80 min (bottom line). By doub-... 

The efficiency of this approach was first studied through the radical reduction of 1-bromoadamantane by Et3SiH in presence of the polyHIPE-supported thiol catalyst (table l).The conversion of 1-bromoadamantane is complete after 1 hour at 80°C with a catalytic amount of dodecanethiol and two equivalents of triethylsilane. In the same conditions, 70% of the bromide were reduced in the presence of the supported thiol, showing a lower reactivity of this last one. The absence of reduction without thiol proved the role of the supported thiol. 

The polyHIPE supported thiol is a good catalyst for radical reduction of alkyl halides and reductive cyclisations of e-bromoalkenes by triethylsilane. 

The Integral Electrosorption Valency of Metal-Supported Thiol... 

Electrosorption Valency of Three Mercury-Supported Thiol Monolayers... 

After making an approximate estimate of the absolute potential difference y/ across the mercury/aqueous solution interphase, let us detennine the electrosorption valency of three mercury-supported thiol monolayers for which uM was measured by potential-step chronocoulometry. As already reported in Sec. IV.1, n-octadecanthiol... 

To verify whether the Pd(OH)2/C system was truly heterogeneous or not, a series of three-phase tests were carried out (Scheme 14.2). In the first case, the aryl halide was anchored to a Wang resin via an amide linker. Under standard conditions for 4.5 h, 100% conversion of supported substrate to arylated product was observed. In addition, in the presence of a silica-supported thiol-based scavenger resin (thiols are known to have a very high affinity for palladium), less than 5% of the desired product formed. These results led to the determination of... 

In the context of diagenesis in recent anoxic sediments, reduced carotenoids, steroids, and hopanoids have been identified, and it has been suggested that reduction by sulhde, produced for example, by the reduction of sulfate could play an important part (Hebting et al. 2006). The partial reduction of carotenoids by sulfide has been observed as a result of the addition of sulfide to selected allylic double bonds, followed by reductive desulfurization. This is supported by the finding that the thiol in allylic thiols could be reductively removed by sulhde to produce unsaturated products from free-radical reactions (Hebting et al. 2003). 

A specific example where heterogeneous supports provide nanoparticle size-control is the immobilization of homogeneous silver nanoparticles on polystyrene [366]. This work was extended later to the development of a one-pot method for the size-selective precipitation of silver nanoparticles on PVP-protected thiol-functionalized silica. During the immobilization of very small silver nanoclusters both the size of the silver nanoclusters and the thickness of the silver layer on the support could be controlled directly by the reaction parameters applied (Fi re 16) [367]. 

The S-methyldithiocarbazates of both 2-formylquinoline, 18, and 1-formyliso-quinoline, 19, when mixed with cobalt(II) salts in ethanol-dimethoxypropane, yield [Co(L-H)2]N03 and [Co(L-H)2]2[CoA4], where A = Cl, Br, I, NCS, or NCSe [145]. Magnetic and spectral data support these stoichiometries and bonding is via the ring nitrogen, azomethine nitrogen, and thiol sulfur. More recently [146], [Co(19-H)2]2[CoCl4] has been shown to have d-d bands consistent with the presence of both cobalt centers. 

We have studied the hydrogenolysis of 2-(perfluorohexyl)ethane thiocyanate to 2-(perfluorohexyl)ethane thiol. It was discovered that perfluoroalkyl thiocyanates can be reduced to thiols and co-product hydrogen cyanide with molecular hydrogen in the presence of a carbon-supported palladium-tin catalyst. This result is surprising since it is known that palladium and other gronps 8 to 10 metal catalysts are poisoned by the product thiol, traces of hydrogen snlfide byprodnct, and the hydrogen cyanide co-product. For that reason, we characterized the catalyst to understand why it was so robust under conditions that would normally poison snch a catalyst. 

It is apparent that a new synthetic methodology, preferably catalytic, is needed for the synthesis of this important class of 2-(perfinoroalkyl)ethane thiols. In this context, a variety of catalysts was examined to determine if they wonld catalyze the hydrogenolysis of 2-(perfinorohexyl)ethane thiocyanate. In the conrse of this study, much to our surprise, it was discovered that a carbon supported Pd-Sn would catalyze the reaction. It is known that palladium and other group Vtll metal catalysts are poisoned by the product thiol, traces of hydrogen sulfide byproduct, and the hydrogen cyanide co-prodnct (6), but our observations are that this catalyst is surprisingly robust in the reaction medium. 

Polymer-supported catalysts often have lower activities than the soluble catalysts because of the intraparticle diffusion resistance. In this case the immobilization of the complexes on colloidal polymers can increase the catalytic activity. Catalysts bound to polymer latexes were used in oxidation reactions, such as the Cu-catalyzed oxidation of ascorbic acid,12 the Co-catalyzed oxidation of tetralin,13 and the CoPc-catalyzed oxidation of butylphenol14 and thiols.1516 Mn(III)-porphyrin bound to colloidal anion exchange resin was... 

The thioester hypothesis can be summed up as follows the formation of thiols was possible, for example, in volcanic environments (either above ground or submarine). Carboxylic acids and their derivatives were either formed in abiotic syntheses or arrived on Earth from outer space. The carboxylic acids reacted under favourable conditions with thiols (i.e., Fe redox processes due to the sun s influence, at optimal temperatures and pH values) to give energy-rich thioesters, from which polymers were formed these in turn (in part) formed membranes. Some of the thioesters then reacted with inorganic phosphate (Pi) to give diphosphate (PPi). Transphosphorylations led to various phosphate esters. AMP and other nucleoside monophosphates reacted with diphosphate to give the nucleoside triphosphates, and thus the RNA world (de Duve, 1998). In contrast to Gilbert s RNA world, the de Duve model represents an RNA world which was either supported by the thioester world, or even only made possible by it. 

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