Vinyl ethers Friedel-Crafts reactions

Sulfones (55) are generally prepared by the oxidation of the appropriate sulfides (2) (Scheme 26).7 The oxidation is generally performed by treatment with peroxycarboxylic acids, but other oxidants may be used (see Chapter 10, p. 195). Diarylsulfones are often obtained by the Friedel-Crafts reaction, and special methods are available for the synthesis of substituted sulfones such as vinyl, hydroxy and halosulfones (see Chapter 10, p. 197). Sulfonyl ethers (56) can be prepared by reaction of a suitable chloroether (57) with the appropriate - sodium sulfinate (58) (Scheme 27). 

Friedel-Crafts reaction catalysts like anhydrous aluminum chloride are readily soluble in the nitroalkanes. Solutions containing up to 50% aluminum chloride are easily prepared in nitroalkane solvents. These catalytically active complexes, AICI3-RNO2, can be isolated and used in solvents other than the nitroalkane. The reactants in the Friedel-Crafts reaction are often soluble in the nitroalkane reaction medium. Other catalysts like boron trifluoride (BF3), titanium tetrachloride (TiC ), and stannic tetrachloride (SnClj) are also soluble in the nitroalkane solvents. Reaction types which use nitroparaffins as solvents include alkylation of aromatics, acetylation of aromatics, halogenations, nitrations, and the reaction of olefins and hydrogen sulfide to yield mercaptans. Nitroparaffins are used with catalysts such as alkyl-metal (e.g., triethylaluminum, vanadium, or titanium) salts in the polymerization reactions of alkylene oxides, epichlorohydrin, propylene, butylene, vinyl chloride, and vinyl ethers. The nitroparaffin acts as an activator for the catalyst or can serve as the reaction solvent. 

Asymmetric catalysis of Friedel-Crafts reaction with fluoral is established using chiral binaphthol-derived titanium catalysts with or without asymmetric activation to provide a practical synthetic route not only for chiral a-trifluorobenzylalcohols but so for highly enantiopure functionalized jS-trifluoroaldols through the sequential diastereoselective reactions of the resultant vinyl ethers or silyl enol ethers with electrophiles. 

Asymmetric Friedel-Crafts Reactions of Vinyl Ethers... 

Spontaneous ionization requires both good leaving groups and that the resulting carbenium ions are sufficiently stable. For example, although primary triflates are very stable covalent species which do not self-ionize, secondary triflates with phenyl substitents are very reactive and spontaneously ionize. The ionization equilibrium of styryl triflate could not be established because of side reactions such as Friedel-Crafts alkylation [56], On the other hand, methoxymethylium triflate is partially ionized with equilibrium constants Kj = 5-10 4 at 10° C and Kt = 210 4 at -70° C in S02 [57]. In this system, ionization is endothermic. Secondary triflates with alkoxy substituents, such as those in polymerizations of vinyl ethers, are apparently more strongly ionized than their primary counterparts [58,59],... 

Friedel-Crafts-type vinylogous conjugate addition of 2-vinyl pyrroles to enals was achieved site-selectively with the use of diphenylprolinol trimethylsilyl ether 14 as an iminium-enaniine activation catalyst (Scheme 41) [71]. Stepwise, formal [2-1-2] cycloaddition would be a plausible outcome of the reaction for constructing stereochemically enriched cyclobutanes. The polarity of the solvent had a critical impact on the catalytic efficiency. Trace amounts or none of the desired product was formed when less polar toluene or dichloromethane was used. Increasing the polarity of the solvent led to enhancement of the turnover frequency of 14 the polar protic solvent, ethanol, was optimal. 

Cationic polymerizations are started by reaction of electrophilic initiator cations with electron-donating monomer molecules. Catalysts are Lewis acids and Friedel-Crafts catalysts, such as aluminum trichloride (AICI3), and strong acids, such as sulfuric acid (H2SO4). Monomer molecules able to undergo cationic polymerization include electron-rich olefins, such as vinyl aromatics and vinyl ethers, and ring compounds, such as ethylene oxide and tetrahydrofuran. 

Chain Transfer. Chain transfer is the most important chain-breaking reaction in carbocationic polymerization. It can be considered as a subcase of termination, when this latter is accompanied by reinitiation. In chain-transfer reactions the active center is transferred to a new site to imreacted monomer or initiator, the preformed polymer, or the solvent. This can happen by /3-proton elimination from the growing chain, or Friedel-Crafts alkylation of aromatic rings. /8-Hydrogen atoms of the propagating carbenium ions are quite acidic owing to the delocalization of the positive charge. In isobutylene, vinyl ether, and styrene... 

In the second method, the alkoxyamine-ftmctionalized backbone is prepared by a chemical modification of a preformed polymer. Abbasian and Entezami prepared alkoxyamine-functionalized poly(vinyl chloride) (PVC) in a three-step procedure. PVC was first arylated with toluene by Friedel-Crafts acylation followed by a bromination step using N-bromosuccinimide. The bromine atom was finally reacted via nucleophilic substitution by the TEMPO hydro-xylamine anion. PVC-g-PS was finally obtained after TEMPO-mediated polymerization of styrene. A TEMPO-functionalized isotactic poly(l-butene) macroinitiator was synthesized by Jo et al. who used a rhodium-catalyzed activation of the alkane C-H bonds and subsequent transformations of the boronate ester group into an hydroxyl pendant group. This reactive moiety was then used to attach a TEMPO-based alkoxyamine bearing another hydroxy function by an ether linkage. A method to prepare PE-g-PS from a poly(ethylene-co-m,p--methylstyrene) obtained by metallocene-catalyzed polymerization was also reported. The macroalkoxya-mine was synthesized after bromination with N-bromosuccinimide followed by a nucleophilic reaction with the TEMPO hydroxylamine anion. 

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