Thiol-promoted

Thiol-promoted addition of a variety of silyl radicals to the aromatic moiety of camptothecin (15) is also reported [32]. The addition occurs predominantly at C7 and C12 positions depending on temperature. At 105 °C, mixture of 7-silyl (favoured) and 12-silyl camptothecins are formed alongside substantial amounts of recovered camptothecin. At 160 °C, 12-silyl isomers are formed preferentially, but the total mass balance is substantially reduced. 

Allyloxysilanes (14) undergo radical chain cyclization in the presence of di-tert-butyl hyponitrite as radical initiator and thiol as a catalyst at ca 60 °C (Reaction 6.3) [5]. The thiol promotes the overall abstraction from the Si—H moiety as shown in Scheme 6.4 and the silyl radical undergoes a rapid 5-endo-trig cyclization. Indeed, EPR studies on the reaction of t-BuO radical with silanes 14 detected only spectra from the cyclized radicals even at — 100°C, which implies that the rate constants for cyclization are > 10 s at this temperature. 

The best evidence for the photolytic decomposition of mercaptans and disulfides into free radicals involves photoinitiation of polymerization of olefins. Thus, photolysis of disulfides initiates the copolymerization of butadiene and styrene,154 as well as the polymerization of styrene207 and of acrylonitrile.19 Thiophenol and other thiols promote polymerization upon ultraviolet irradiation.19 Furthermore, the exchange of RS-groups between disulfides and thiols is greatly accelerated by light. Representative examples are benzothiazolyl disulfide and 2-mercapto-thiazole,90 tolyl disulfide and p-thiocresol, and benzyl disulfide and benzylmercaptan.91 The reaction probably has a free radical mechanism. Similar exchange reactions have been observed of RS-groups of pairs of disulfides have been observed.19... 

Fig. 23.5 Proposed mechanism by which thiol promotes acid-catalyzed bisphenol A production. Reprinted with permission from [16], Copyright 2006 Elsevier...

Another catalyst, commonly used at the beginnings of BPA industrial production, was aqueous hydrochloric acid [19]. However, considerable amounts of water introduced with the acid decreased the effectiveness of the catalyst. Thus, hydrogen chloride gas was started to be used in the process, usually together with the thiol promoters. The mechanism of action of thiol promoters and formation of carbocation intermediate (II) was proposed as given in Figure 7.4 [15]. 

The thiol promoters may be deactivated under typical conditions used to attach them to the catalysts. Nevertheless, the sulphur-protected mercaptan promoters attached to the ion-exchange resins can be used [25]. The other possibility is the modification of the ion-exchange resin with thiazolidines [26]. In the presence of the acidic catalysts thiazolidine is hydrolyzed to 2-aminoethanethiol and acetone. So prepared catalysts show high activity and the high selectivity for BPA synthesis (even more than 95%) and they afford high purity product. Currently, the most often used promoters are 2,2-dimethyl-l,3-thiazolidine and cysteamine (Figure 7.5). The later promoter is the simplest stable aminothiol, naturally formed as a degradation product of the amino acid cysteine [27]. 

Thiol-promoted cyclization of 1,6-dienes and analogous enynes in the presence of radical initiators has been studied. An equimolar amount of ethanethiol, in the presence of diphenyl disulfide as initiator, was added under photolytical conditions to dimethyl diallylmalonate an addition of thiyl radical followed by 5-hex-enyl cyclization took place [48] and cis- and fra 5-cyclopentanes (the ratio being... 

Patterns of ordered molecular islands surrounded by disordered molecules are common in Langmuir layers, where even in zero surface pressure molecules self-organize at the air—water interface. The difference between the two systems is that in SAMs of trichlorosilanes the island is comprised of polymerized surfactants, and therefore the mobihty of individual molecules is restricted. This lack of mobihty is probably the principal reason why SAMs of alkyltrichlorosilanes are less ordered than, for example, fatty acids on AgO, or thiols on gold. The coupling of polymerization and surface anchoring is a primary source of the reproducibihty problems. Small differences in water content and in surface Si—OH group concentration may result in a significant difference in monolayer quahty. Alkyl silanes remain, however, ideal materials for surface modification and functionalization apphcations, eg, as adhesion promoters (166—168) and boundary lubricants (169—171). 

Metal Ion-Promoted Reactions of Thiols. Metal ion-promoted reactions of thiols have been reviewed (53). The bulk of the coverage concerns metal ion promoted aspects of sulfur chemistry. The main topics of interest are the formation of sulfenamides, sulfides, and disulfides using metal-mediated reactions. 

Screen-printed gold electrodes were, firstly, modified with a mixed monolayer of a 25-mer thiol-tethered DNA probe and a spacer thiol, 6-mercapto-1 -hexanol (MCH). The DNA probe sequence was internal to the sequence of the 35S promoter, which sequence is inserted in the genome of GMOs regulating tire transgene expression. 

The acrylic compounds, maleimides, dimaleimides, and thiols are known as direct cross-hnk promoters, since they enter directly into the cross-linking reaction and become interconnecting molecular links [243]. 

Liaw et al. reported that conversions between the neutral sparteine [Fe(NO)2] complex 133 and the anionic Fe(NO)2 [Fe(NO)2(S2C3Hg)] 137 proceed via the cationic sparteine Fe(NO)2 -complex 135 through oxidation by NO" " and transfer of the [Fe(NO)2] -unit to the chelating ligand S-(CH2)3-S 136 (Scheme 35). The resulting anionic complex 137 is stable in contrast to the cationic complex 135. The cationic complex 135 also acts as a [Fe(NO)2] donor in the presence of the DNIC [(PhS)2Fe(NO)2] 138 to yield Roussin s red ester 139. The bidentate thiol ligand S-(CH2)3-S 136 promotes the stability of the anionic DNIC Fe(NO)2 ... 

Chemical stabilizers have been used to reduce the rate of oxygen-promoted degradation of polysaccharides at T>225°F. Methanol and sodium thiosulfate are the most commonly used (86). Sodium dithio-carbamate, alkanolamines, and thiol derivatives of imidazolines, thiazolines, and other heterocyclic compounds have also been tested for this application. Calcined dolomite (B7) and Cu(l) and Cu(ll) salts (88) have been reported to increase the thermal stability of HEC. 

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