Solid electrophilic reagents

Similarly to classical PTC reaction conditions, under solid-liquid PTC conditions with use of microwaves the role of catalyst is very important. On several occasions it has been found that in the absence of a catalyst the reaction proceeds very slowly or not at all. The need to use a phase-transfer catalyst implies also the application of at least one liquid component (i.e. the electrophilic reagent or solvent). It has been shown [9] that ion-pair exchange between the catalyst and nucleophilic anions proceeds efficiently only in the presence of a liquid phase. During investigation of the formation of tetrabutylammonium benzoate from potassium benzoate and tetrabu-tylammonium bromide, and the thermal effects related to it under the action of microwave irradiation, it was shown that potassium benzoate did not absorb micro-waves significantly (Fig. 5.1, curves a and b). Even in the presence of tetrabutylammonium bromide (TBAB) the temperature increase for solid potassium benzoate... 

There is now available a range of stable, easily handled, solid electrophilic fluorinating agents of the N-F type. These remarkable reagents are generally prepared by reaction of a neutral base [108] or a salt [109] with fluorine (Figure 3.16). 

Aromatic iodides (3,287). The definitive paper on the synthesis of aromatic iodides by the reaction of arylthallium dilrifluoroacetates with potassium iodide has been published. Four procedures have been developed. I) Thallalion is carried out as usual and then an aqueous solution of potassium iodide is added directly. 2) The intermediate arylthallium ditrifluoroacctatc is isolated and then treated with potassium iodide. 3) For acid-sensitive. substrates solid TTFA in acetonitrile is used for thallalion. 4)These methods are unsuccessful with highly reactive compounds such as naphthalene and diphenyl. In such cases molecular iodine is used as the electrophilic reagent and TTFA is used as oxidant for the hydrogen iodide formed in the reaction. 

To evaluate conclusively the hypotheses presented here, additional solution and solid-state studies are needed, particularly for trisolvated organolithium compounds, solvent-separated complexes, and unsaturated complexes of the heavier alkali metals. The results do, however, furnish the first glimpse of the stereochemistry of a chemically important class of compounds and should also provide working models for understanding the role of Group la metals in polymerization reactions and in their reactions with electrophilic reagents. 

Use as an Electrophile. The chlorosilane function is highly electrophilic and can react with a variety of nucleophiles, for instance with an aryllithium carbanion, to provide silyl derivatives. The (bromomethyl)chlorodimethylsilane can also be utilized as a bis-electrophilic reagent, thanks to the bromomethylene function. Initial substitution of chlorine by an aryllithium or an aryl Grignard followed by the displacement of bromine by a phenoxide anion provided valuable linkages for solid phase s)m-thesis. Based on the same idea, Martin proposed a new synthesis of unsymmetrical C-aryl glycosides. Orthometallation of a furyl moiety followed by silylation sets a diene on a silicon tether. The masked dienophile (a benzyne) is then introduced by 0-alkylation. The cycloadduct was then converted to various naphthol derivatives (eq 25). 

Tnflic acid is an excellent catalyst for the nitration of aromatic compounds [.S7]. In a mixture with nitnc acid, it forms the highly electrophilic nitronium inflate, which can be isolated as a white crystalline solid Nitronium inflate is a powerful nitrating reagent in inert organie solvents and in tnflic acid or sulfuric acid. It nitrates benzene, toluene, chlorobenzene, nitrobenzene, m-xylene, and benzotn-fluoride quantitatively in the temperature range of-110 to 30 °C with exeeptionally high positional selectivity [87],... 

Kaldor [49 i, 55] demonstrated the advantages of applying solid-supported scavengers to the preparation of parallel arrays in a multi-step fashion. In these studies he examined the clean-up of multiple amine alkylation and acylation reactions using a variety of immobilized electrophilic and nucleophilic scavenger reagents including an amine, isocyanate, aldehyde and acid chloride (Tab. 2.1). 

Recently, the semi-synthesis of Vancomycin (48) on solid supports was accomplished using an allylic linker (Scheme 3.2) [123, 124]. Polymer-bound chiral electrophilic selenium reagents have been developed and applied to stereoselective se-lenylation reactions of various alkenes (Tab. 3.9) [125]. 

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