Silica-Supported Thiols

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.)...

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... 

FIGURE 1.9 Selection of cinchonan carbamate CSPs that have been prepared in the course of selector optimization studies (type I prototype type II, O-9-linked thiol-silica supported prototype type III, C-ll-linked thiol-silica supported CSPs type IV, dimeric selectors). (Adapted from M. Lammerhofer and W. Lindner, J. Chromatogr. A, 741 33 (1996) W. Lindner et al., PCT/EP97/02888, US 6,313,247 B1 (1997) P. Franco et ah, J. Chromatogr. A, 869 111 (2000) C. Czerwenka et ah. Anal. Chem., 74 5658 (2002).)... 

A potentially useful one-pot procedure for the conversion of suitable aryl thiols to benzo[6]thiophenes or napththo[2,l-6]thiophenes in the presence of silica supported acidic and basic reagents has been devised, as exemplified by the synthesis of the system 9 from 2-naphthalenethiol 10 <05SL2739>. A stepwise variant of the same theme has been employed in a synthesis of 6-bromo-l,2,3,4-tetrahydrodibenzothiophene, a partner for a Suzuki coupling leading to a chromene-4-one containing inhibitor of DNA-dependent protein kinase <05JMC7829>. 

In the recent drive to prepare mesoporous solid acid catalysts for liquid phase processes, a popular approach has been to tether sulfonic acid groups to mesoporous silica supports. The preferred method for functionalising has been to incorporate 3-mercaptopropylsilane in the synthesis gel for the mesoporous silica, and then, after precipitation and isolation, to oxidise the thiol group to sulfonic acid. These materials have exhibited relatively high catalytic activity and the approach is emerging as one of the most successful for preparing acid forms of porous silica. 

The PS-TBD catalyst has been shown to be effective for epoxide ring opening reactions with several nucleophiles such as thiols under solvent free conditions [37,78] (Scheme 6.21). In this case, the reusabihty of the catalyst was also established without a significant loss of reactivity and selectivity. As a related work, the utility of mesoporous silica-supported TBD catalysts was demonstrated in the reaction of propylene oxide with carbon dioxide to prepare the corresponding carbonate derivative under the ultrasonic activation [79]. 

Type IV materials exhibit unique ion-exchange properties supplemented by RP characteristics, so that they may be considered as RP-modified ion-exchange materials. Lammerhofer et al. [59] have developed a number of RP/weak anion exchanger (WAX) materials consisting of a selector immobilized onto thiol-modified silica. In these phases, the WAX site is located at the top of the lipophilic layer and is linked to the silica support via a lipophihe spacer with polar-embedded amide or sulfide groups. These columns, however, are not commercially available. 

Covalent reaction by thiol-disulphide exchange with a macroporous silica support bearing an activated thiol group... 

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]. 

Different types of nucleic acids or their analogues (cDNA, oligonucleotides or peptide nucleic acids), supports (silica, gold, polymeric membranes and gels), surface activation chemistries (organosilanes, thiols) and patterning tools can be used for these purposes and will be described in this review. 

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