Electrophilic Addition and Substitution

This section deals with Bronsted acid and Lewis acid catalyzed reactions, excluding Friedel-Crafts reactions, but including reactions such as nitrations, halogenations, and Claisen rearrangements. Friedel-Crafts reactions are discussed in the subsequent Sections 5.1.2.2 and 5.1.2.3. 

Wilkes and co-workers have investigated the chlorination of benzene in both acidic and basic chloroaluminate(III) ionic liquids [66]. In the acidic ionic liquid [EMIM]C1/A1C13 (X(A1C13) 0.5), the chlorination reaction initially gave chlorobenzene, which in turn reacted with a second molecule of chlorine to give dichlorobenzenes. In the basic ionic liquid, the reaction was more complex. In addition to the 

7 Stoichiometric Organic Reactions and Acid-Catalyzed Reactions in Ionic Liquids 

Fee et al. have investigated the Fewis acid-catalyzed three-component synthesis of a-amino phosphonates [73]. This was carried out in the ionic liquids [BMIM][PFg], 

Lee found that the reaction gave good yields (70-99 %) in the ionic Hquids [BMlM][PFg], [BMIM][OTf, and [BMlM][SbFg] with Lewis acids such as Yb(OTf)3, Sc(OTf)3, Dy(OTf)3, Sm(OTf)3, and InCl3. The reaction was also performed in [BM1M][PF6] or dichloromethane with Sm(OTf)3 as the catalyst The ionic liquid reaction gave a yield of 99 %, compared with 70 % for the reaction in dichloromethane [73]. 

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Thus, in spite of its lack of reactivity, iodine reacts chemically with unsaturated compounds, whereby the silica gel of the TLC layer can sometimes be assigned a catalytic role [11, 12]. Irreversible oxidations and electrophilic substitution and addition reactions have been observed on the interaction of iodine with tertiary nitrogen compounds such reactions possibly depend on particular steric relationships or are favored by particular functional groups [13, 14]. 

Pyridine. Pyridine and its methyl substituted derivatives (picolines and lutidines) were found to polymerize electrochemically and, under certain circumstances, catalytically. This behavior was not expected because usually pyridine undergoes electrophilic substitution and addition slowly, behaving like a deactivated benzene ring. The interaction of pyridine with a Ni(100) surface did not indicate any catalytic polymerization. Adsorption of pyridine below 200 K resulted in pyridine adsorbing with the ring parallel to the surface. The infrared spectrum of pyridine adsorbed at 200 K showed no evidence of either ring vibrations or CH stretches (Figure 5). Desorption of molecular pyridine occurred at 250 K, and above 300 K pyridine underwent a... 

Free halogens are generally inconvenient to use, owing to their toxic and corrosive nature, but can be replaced by quaternary ammonium polyhalides. Quaternary ammonium tribromides are well established [e.g. 1] as solid, readily handled and relatively non-toxic alternatives for electrophilic bromine. More recently, other quaternary ammonium polyhalides have been produced, which together with the tribromides, have wide application as catalysts or in stoichiometric quantities in electrophilic substitution and addition reactions, oxidations, etc. 

Substituent effects observed for this reaction are entirely consistent with those described for electrophilic substitution and addition —only reversed. That is, the reactivity of an arene in metal reductions is increased by electron-withdrawing groups and decreased by electron-donating groups. Substituents that can stabilize the anion-radical intermediate facilitate the reduction (see Exercise 22-35). 

Hetarenium ions as reagents in electrophilic substitution and addition reactions... 

Through this path, electrophilic substitution and addition can be achieved on aromatic and unsaturated substrates. It appears to be a good way to introduce the MeS group in a fairly regioselective manner. 

V-Chlorosuccinimide is a convenient reagent for the electrophilic substitution and addition of chlorine to organic compounds. Other chlorinating agents of use include Chlorine, Sulfuryl Chloride, Chloramine-T, tert-Butyl Hypochlorite, and Trichloroiso-cyanuric Acid. The primary advantages of using NCS include the ease in handling, the mild conditions under which chlorination proceeds, and the ease of removal of the inoffensive byproduct succinimide. 

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