Alkylation of aromatic rings

Unsolvated organomagnesium compounds have been recommended for the synthesis of organometallic derivatives of mercury, boron, aluminum, silicon, germanium, tin, phosphorus, arsenic, and antimony6-8 and have been used in procedures for the alkylation of aromatic rings and for the production of various polymerization catalysts.4 9... 

The alkylation of aromatic rings, called Friedel-Crafts alkylation, is a reaction of... 

Alkylation of aromatic rings with ethylene oxide... 

Zirconium(IV) isopropoxide, 352 Reductive alkylation of aromatic rings Birch reduction, 32 (S)-Prolinol, 261 of carbonyl groups Trityl perchlorate, 339 of other substrates Lithium-Ammonia, 158 Reductive cleavage (see also Reduction of epoxides)... 

Alkylation of Aromatic Rings Th Fried el-Crafts Reaction... 

The alkylation of aromatic rings, called Friedel-Crafts alkylation, is a reaction of very broad scope. The most important reagents are alkyl halides, alkenes, and... 

There is direct evidence, from ir and nmr spectra, that the fert-butyl cation is quantitatively formed when ferf-butyl chloride reacts with AICI3 in anhydrous liquid HCl. In the case of alkenes, Markovnikov s rule (p. 1019) is followed. Carbocation formation is particularly easy from some reagents, because of the stability of the cations. Triphenylmethyl chloride and 1-chloroadamantane alkylate activated aromatic rings (e.g., phenols, amines) with no catalyst or solvent. Ions as stable as this are less reactive than other carbocations and often attack only active substrates. The tropylium ion, for example, alkylates anisole, but not benzene. It was noted on p. 476 that relatively stable vinylic cations can be generated from certain vinylic compounds. These have been used to introduce vinylic groups into aryl substrates. Lewis acids, such as BF3 or AIEta, can also be used to alkylation of aromatic rings with alkene units. 

Bromination of Aromatic Rings 593 Other Aromatic Substitutions 597 Alkylation of Aromatic Rings The Friedel-Crafts Reaction Acylation of Aromatic Rings 604 Substituent Effects in Substituted Aromatic Rings 605 An Explanation of Substituent Effects 610 Trisubstituted Benzenes Additivity of Effects... 

Alkylation of Aromatic Rings The Friedel-Crafts Reaction 603... 

Alkylation of aromatic rings with an olefin or alcohol was thought to be an acid-catalysed process until MgO was found to be an excellent catalyst. In fact, alkylation of phenol with methanol to yield 2.6-xylenol is much more selective on MgO than on SIOt-AItOj. The... 

Further examples of bifunciional acid-basc catalysis are a Idol and Knoevenagel conden.salions 239, 286. 287] or side-chain alkylation of aromatic rings [I48. The better... 

Besides the alkyl halide-Lewis acid combination, two other routes to carbonium ions are quite widely used in synthesis. Alcohols can serve as sources of carbonium ions in strongly acidic media such as sulfuric acid and phosphoric acid. The alkylation of aromatic rings by alcohols is also catalyzed by BF3 and AlQs. ... 

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 nature, the alkylation of aromatic rings most commonly proceeds using 5-adenosyhnethionine ( SAM, 31a) as the source of an electrophilic methyl group. Such alkylations are catalyzed by C-methyltransferase enzymes and are analogous to Friedel-Crafts alkylations. Two C-methyl-transferases, NovO and CouO (from Streptomyces spheroides and Streptomyces rishiriensis, respectively), have been expressed in recombinant E. coli and have been shown to have a synthetically useful substrate scope. Not only are they able to catalyze the regioselective alkylation of various coumarins 32 (X=0), 2-quinolones 32 (X=NH), and naphthalenediols 34-36, but in addition, they are able to accept nonnatural cofactors 31b-f in place of SAM and so can transfer various alkyl groups other than methyl to the substrates (Scheme 32.4) [26]. 

However, transfer reactions are nearly unavoidable side reactions in cationic polymerization and may involve yS-proton transfer even to rather weak bases like the monomer itself, to transfer agents and solvent as well as Friedel-Crafts alkylation of aromatic rings, if monomers like styrene are polymerized. Proton transfer yields unsaturated chains [Eq. (58)], with the double bond in either the endo or exo position. These may lead to branched polymers, if the double bonds are accessible to homo- or copolymerization. 

So, most probably, lower strength of the acid sites on Ni-SAPO-34 rather promotes alkylation of aromatic rings relative to olefin methylafion, oligomerization-cracking transformation. Recently, Khadzhiev with coworkers also found that a modificafion of SAPO-34 with Ni leads to a dramatic increase of the selectivity to ethylene from methanol [128]. They showed 88% of selectivity with / Cj" ratio of 17.6 at 450°C. This observation indirectly confirms that the formation of ethylene is much more related with the alkylation of the coke precursors than for propylene. The presence of Ni favors the dealkylation and avoids the transformation of Cj"... 

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