Preparation of Supported Reagents

Sol-gel synthesis Direct method can use to introduce functional group, e.g. via (MeO), Si-R resulting materials can have high surface areas may be difficult to control becoming more popular 

Post-modification of support material Most commonly using silane, but may be less stable than analogous sol-gel materials due to only partial surface reaction alternative methods include via chlorination (e.g. Si-Cl (Si-R) 

Impregnation (pore filling followed by evaporation of solvent) Requires a suitable solvent enables good control over dispersion and loading likely to give physisorbed reagent only 

Precipitation/ co-precipitation Useful for poorly soluble reagents but may be difficult to control 

Adsorption from solution Easy to carry out but loading may be low 

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In many cases, the comparison of a reaction accelerated by microwave irradiation has been made with the same reaction in an oil bath at the same bulk temperature. Unfortunately, there have been quite a few reports in the chemical literature that have not been conducted with such proper control of conditions and consequently a fair comparison is not possible. Nevertheless, using this MW approach, the problems associated with waste disposal of solvents that are used several fold in chemical reactions, and excess usage of chemicals are avoided or minimized. The discussion pertaining to the preparation of supported reagents or catalysts has not been included in this chapter because numerous review articles are available on this theme [14—22],... 

Amines can be linked to polymeric alcohols as carbamates. Carbamate attachment of amines can be achieved by reaction of isocyanates with alcohol linkers, or by treatment of alcohol linkers with phosgene [339,427,428] or a synthetic equivalent thereof, followed by exposure to the amine (Figure 3.26). The reagents most commonly used for the activation of alcohol linkers are 4-nitrophenyl chloroformate [69,429-436] and carbonyl diimidazole [427,437-440], The preparation of support-bound carbamates is discussed in Section 14.6. 

As alternatives to 4-nitrophenyl chloroformate, carbonyl diimidazole [100-102] or di-A-succinimidyl carbonate [103,104] can be used to convert polymeric alcohols into alkoxycarbonylating reagents suitable for the preparation of support-bound carbamates. Polystyrene-bound alkoxycarbonyl imidazole is less reactive than the corresponding 4-nitrophenyl carbonate, and sometimes requires heating to undergo reaction with amines. Additional activation of these imidazolides can be achieved by N-methylation (Entry 9, Table 14.7). 

Solid-supported reagents which have found utility include Nafion-scandium Lewis acid catalyst (allyl additions to aldehydes) [62], HOBt (medium-ring lactamization) [63], EDC (preparation of active esters) [64], and thiazolium hydrotribromide (brominations) [65], A review has also appeared describing the use of supported reagents in separation science, primarily for the selective sequestration of metal ions [66],... 

The use of supported reagents offers an attractive option for improving the quality of products prepared using solution-phase chemistry. Additionally, liquid-phase synthesis, for example using PEG, provides opportunities to combine some of the benefits of solid-phase approaches with the versatility of solution-phase synthesis. Smart methods such as resin capture for isolating specific compounds from mixtures of products will also help to increase the utility of solution-based approaches. This chapter encompasses developments in each of these areas. 

Oligonucleotide synthesis. - 1.1.1 DNA Synthesis. There have been few publications on improvements to DNA synthesis, and the main area of development has been in the development of modified oligonucleotides (ODNs). There have, however, been some improvements in the use of supports, reagents and protecting groups. Two new thymidine modified solid supports have been prepared suitable for oligonucleotide synthesis in which the support is attached via the thymine N3 position. These modified supports allow for ODN synthesis... 

The development of methodologies for the covalent bonding of ligands to polymeric supports is important for the preparation of novel reagents to be used in ionic and molecular separations which are important to the environment and for the preparation of chemical sensors which can be used in the monitoring of environmentally sensitive areas. Our research focuses on identifying the mechanisms by which immobilized ligands... 

Polymer-supported Wittig reagents were first prepared more than 20 years ago [32]. It has been shown that the success of the reaction depends strongly upon (i) the preparation of the reagent by bromination and phosphination of cross-linked polystyrene rather than by co-polymerization using styryldi-phenyl phosphine, and (ii) the generation of the phosphorane with a base/ solvent system that swells the phosphonium sites in the polymer network (Scheme 6) [33]. Thus, bromination of polystyrene 1 yielded phenyl bromide 32, and this was followed by phosphination with n-butyUithium and chlor-odiphenylphosphine or with Hthium diphenylphosphide to give 33, a compound which is commercially available (Scheme 6). 

Preparation of the reagent [70] A solution of PEG monomethyl ether 89 (MW = 750 5.88 g, 7.8 mmol) in benzene (20 mL) was dried azeotropically for 24 h in an apparatus fitted with a Dean-Stark trap and subsequently added dropwise to a solution of chlorosulfonyl isocyanate (88) (1.10 g, 7.8 mmol) in dry benzene (20 mL). The mixture was stirred at room temperature for 1 h, then concentrated to dryness. A solution of this residue in benzene (35 mL) was added dropwise to a solution of triethylamine (2.5 mL, 17.3 mmol) in benzene (15 mL). The mixture was stirred for 30 min at room temperature, then filtered, and the solid was dried to yield polymer-supported Burgess reagent 91 (6.2 g, 82%). 

Scheme 7.20 Preparation of supported triazine coupling reagent 59 and its use in amidation reactions.

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