Styrene substitution, Aromatic electrophilic

Aromatic electrophilic substitution is used commercially to produce styrene polymers with ion-exchange properties by the incorporation of sulfonic acid or quaternary ammonium groups [Brydson, 1999 Lucas et al., 1980 Miller et al., 1963]. Crosslinked styrene-divinyl-benzene copolymers are used as the starting polymer to obtain insoluble final products, usually in the form of beads and also membranes. The use of polystyrene itself would yield soluble ion-exchange products. An anion-exchange product is obtained by chloromethylation followed by reaction with a tertiary amine (Eq. 9-38) while sulfonation yields a cation-exchange product (Eq. 9-39) ... 

Styrene (vinylbenzene) undergoes electrophilic aromatic substitution much faster than benzene, and the products are found to be primarily ortho- and para-substituted styrenes. Use resonance forms of the intermediates to explain these results. 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

It is evident that this type of grafting is restricted to those monomers which are polymerizable by a cationic mechanism moreover, electrophilic substitution on the aromatic nucleus must be possible. Nevertheless, alkylvinyl ethers, isobutene and iV-vinylpyrrolidone could not be grafted on polystyrene, polyvinyltoluene or poly- >-methoxystyrene. The limitation of the method to the system styrene/poly-/j-methoxystyrene is very surprising and not well understood until now. 

There is some spectroscopic evidence that aromatic compounds complex carbenium ions [42]. For example, the complexation equilibrium constant between trityl ions and hexamethylbenzene is K = 68 mol-1 L at 0° C [43]. Complexation should be stronger with more electrophilic carbenium ions such as those derived from styrene and a-methylstyrene. On the other hand, the monoalkyl-substituted phenyl rings attached to the polymer chain are weaker nucleophiles than hexamethylbenzene. A complexation constant K = 4 mol 1 L was reported for trityl cation and styrene [43]. Similar complexes have been proposed to explain the red color observed in inifer systems based on l,4-bis(I-chIoro-l-methyl-ethyl)benzene and BCI3 in CH2C12 at low temperature [44],... 

For the dimerization of 4,4 -dimethoxystilbene, it has been possible to demonstrate spectroelectrochemically [115] and at the rotating ring-disk electrode [116] that the product is formed mainly by radical dimerization of the intermediate radical cations [path B, Eq. (13)]. Fast derivative CV, however, supports for the same olefin a complex ECE pathway [path A, Eq. (13) [117]. Depending on the oxidation potential and the kind of the nucleophiles (acetate, water, or methanol), a tetrahydronaphthalene derivative (Table 6, number 3) [118], a monomer diacetate [118], a tetrahydrofuran [115], or a dimer dimethoxy compound is found. When methanol is replaced by aqueous dichloromethane or by aqueous acetonitrile emulsions as solvent, styrene (Table 6, number 4) [119a] and a-methylstyrene [119b] yield 2,5-diphenyltetrahydrofurans. In some cases cyclization occurs by electrophilic aromatic substitution (analogous to Table 6, number 3). 

Several chain transfer to polymer reactions are possible in cationic polymerization. Transfer to cationic propagating center can occur either by electrophilic aromatic substitution (as in the polymerization of styrene as well as other aromatic monomers) or hydride transfer. Short chain branching found in the polymerizations of 1-alkenes such as propylene may be attributed to intermolecular hydride transfer to polynier. The propagating carbocations are reactive secondary carbocations that can abstract tertiary hydrogens from the polymer ... 

The classical Vilsmeier-Haack reaction - involves electrophilic substitution of a suitable carbon nucleophile with a chloromethyleneiminium salt, for example salt (1). Suitable carbon nucleophiles are generally electron-rich aromatic compounds such as V,N-dimethylaniline (2), alkene derivatives such as styrene (3) or activated methyl or methylene compounds such as 2,4,6-trinitrotoluene (4 Scheme I). These compounds (2-4) react with salt (1) giving, after loss of hydrogen chloride, the corresponding im-inium salts (5-7). Hydrolysis of iminium salt (5) affords aldehyde derivative (8) and this transformation (Ar—H - Ar—CHO) is the well-known Vilsmeier-Haack formylation reaction. Hydrolysis of iminium... 

Homopolymers formed from 2,4,6-trimethoxystyrene, 4-(A,lV-dimethy-lamino)styrene, and A-methyl-2-vinylpyrrole react with MTAD and FTAD. The reaction leads to the incorporation of the TADs into the polymers via ene reaction or electrophilic aromatic substitution (see Section IV,1). The same reaction of these polymers with bis-TADs gives cross-linked polymers insoluble in both polar and nonpolar solvents [89JPS(A)217]... 

The widespread applications of polystyrene derived resins is due to the fact that styrene consists of a chemically inert aUcyl backbone carrying chemically reactive aryl side chains that can be easily modified. As discussed earlier, a wide range of different types of polystyrene resins exhibiting various different physical properties can be easily generated by modification of the crosslinking degree. In addition, many styrene derived monomers are commercially available and fairly cheap. Polystyrene is chemically stable to many reaction conditions while the benzene moiety, however, can be funtionalised in many ways by electrophilic aromatic substitutions or lithiations. As shown in Scheme 1.5.4.1 there are principally two different ways to obtain functionalised polystyrene/DVB-copolymers. 

An aqueous Friedel-Crafts reaction has also been used in polymer synthesis. The acid-catalyzed polymerization of benzylic alcohol and fluoride functionality in monomeric and polymeric fluorenes was investigated in both organic and aqueous reaction media. Polymeric products are consistent with the generation of benzylic cations that participate in electrophilic aromatic substitution reactions. Similar reactions occurred in a water-insoluble Kraft pine lignin by treatment with aqueous acid. A Bisphenol A-type epoxy resin is readily emulsified in aqueous medium with an ethylene oxide adduct to a Friedel-Crafts reaction product of styrene and 4-(4-cumyl)phenol as emulsifier.Electrophilic substitution reaction of indoles with various aldehydes and ketones proceeded smoothly in water using the hexamethylenetetramine-bromine complex to afford the corresponding Z A(indolyl)methanes in excellent yields.InFs-catalyzed electrophilic substitution reactions of indoles with aldehydes and ketones are carried out in water.Enzymatic Friedel-Crafts-type electrophilic substitution reactions have been reported. ... 

What about a nucleophilic carbene, for which negative charge should build up on the olefinic carbon atoms during the carbene addition cf. 5 With ArCH=CH2 substrates, electron-withdrawing aryl substituents would stabilize such a transition state and the p value should be positive. There are several examples of this phenomenon. For example, cycloheptatrienylidene, 10, adds to / -substituted styrenes with p = -t-1.02 - 1.05 (vs. a) consistent with a nucleophilic selectivity that seems to implicate the aromatic resonance form 10a as an important feature of the carbene. [45] It is satisfying to compare this result with p = -0.76 (vs. a) or -0.46 (vs. a" ") for additions to styrenes of cyclopentadienylidene, 11, where contributions of the cyclopentadienide form (11a) would render the carbene electrophilic. [46] However, these conclusions are too facile. There is reason to believe that the chemistry attributed to 10 might in fact be due to its allenic isomer 12. [47] And the electronic structure of 11 is also more complicated than the simple depiction above. [48]... 

The solid support in Merrifield s synthesis of ribonuclease (see Section 25.18) was prepared by incorporating —CH2CI groups into a styrene// -divinylbenzene copolymer by electrophilic aromatic substitution. 

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