Relative activity of Friedel-Crafts catalysts

Very active Moderately active Weak SECTION 7.1. ELECTROPHILIC 

AROMATIC SUBSTITUTION 

AICI3-CH3NO2, TiCl4, TiBr4,  

The Friedel-Crafts alkylation reaction does not proceed successfully with aromatic substrates having electron-attracting groups. A further limitation is that the alkyl group introduced in the reaction increases the reactivity of the aromatic ring toward further substitution, so that polyalkylation can occur. Polyalkylation is usually minimized in practice by using the aromatic substrate in excess. 

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.  

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Table 11.1. Relative Activity of Friedel-Crafts Catalysts... 

Classification of carbenes on the basis of reactivity toward alkenes 11.1. Relative activity of Friedel-Crafts catalysts... 

Friedel-Crafts catalysts are electron acceptors, ie, Lewis acids. The alkylating ability of ben2yl chloride was selected to evaluate the relative catalytic activity of a large number of Lewis acid haUdes. The results of this study suggest four categories of catalyst activity (200) (Table 1). 

Orientation in Alkylation. An additional factor complicating the usefulness of Friedel-Crafts alkylations is the orientation involved in the introduction of more than one alkyl group. It was discovered at an early date that alkylation with aluminum chloride and alkyl halides yields considerable proportions of m-dialkylbenzenes, as well as the expected o- and jj-isomers. The relative extent of normal and abnormal orientation has been found to be a function of the conditions of alkylation. In general, the more vigorous the conditions with respect to the activity of the catalyst or the alkylating agent or the severity of the time and temperature factors, the greater is the tendency for the forma-... 

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

Phenols are highly activated towards electrophilic attack, which occurs readily at the 2- and 4-positions. For example, phenol reacts with bromine at room temperature in ethanol and in the absence of a catalyst to give 2,4,6-tribromophenol. Other electrophilic substitution reactions such as nitration, sulfonation, Friedel-Crafts, chlorination and nitrosation also proceed readily and hence care is needed to ensure multisubstitution does not occur. Protection of specific ring positions can also prevent unwanted substitution. Relatively mild conditions are usually employed. 

Enantioselective vanadium and niobium catalysts provide chemists with new and powerful tools for the efficient preparation of optically active molecules. Over the past few decades, the use of vanadium and niobium catalysts has been extended to a variety of different and complementaiy asymmetric reactions. These reactions include cyanide additions, oxidative coupling of 2-naphthols, Friedel-Crafts-type reactions, pinacol couplings, Diels-Alder reactions, Mannich-type reactions, desymmetrisation of epoxides and aziridines, hydroaminations, hydroaminoalkylations, sulfoxida-tions, epoxidations, and oxidation of a-hydroxy carbo) lates Thus, their major applications are in Lewis acid-based chemistiy and redox chemistry. In particular, vanadium is attractive as a metal catalyst in organic synthesis because of its natural abundance as well as its relatively low toxicity and moisture sensitivity compared with other metals. The fact that vanadium is present in nature in equal abundance to zinc (albeit in a more widely distributed form and more difficult to access) is not widely appreciated. Inspired by the activation of substrates in nature [e.g. bromoperoxidase. 

Heteropoly acid, aluminum dodecatungstophosphate AIPW12O40, was found to be an effective catalyst for the Friedel-Crafts acylations using carboxylic acid, acetic anhydride, and benzoyl chloride (Scheme 6.9) [11]. Nonhydroscopic nature and relatively high catalytic activity of this heteropoly acid, for example, as compared with Sc(OTf)3 or In(OTf)3, should be noted. Reactions were conducted under solvent-free conditions. 

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