Friedel-Crafts catalysts, hydrogen phosphoric acid

Since the Friedel-Crafts catalysts are usually thought of as halides of a few metals, a natural question to raise might be If these halides are really acids differing in no fundamental manner from H-acids in their behavior, why do not H-acids catalyze reactions of the Friedel-Crafts type also The answer is that they do. Hydrogen fluoride,phosphoric acid, and sulfuric acid have... 

Alkylation of Aromatics. Aromatic hydrocarbons containing a replaceable hydrogen can be alkylated unless steric effects prevent introduction of the alkyl group (61,78-82). The reaction is called the Friedel-Crafts alkylation, first realized in the presence of aluminum chloride, which is the catalyst still the most frequently used and studied in Friedel-Crafts reactions. In addition, many other acid catalysts are effective (80,82-84). These include other Lewis acids (other aluminum halides, gallium chloride, boron trifluoride, ferric chloride, zinc chloride, stannous and stannic chloride, antimony chloride) and protic acids (hydrogen fluoride, concentrated sulfuric acid, phosphoric acid, polyphosphoric acid, trifluo-romethanesulfonic acid, and alkane- and arenesulfonic acids). 

The chiral phosphoric acid catalyst 121 forms hydrogen bonds with enethiourea 203 and the imine, formed in situ from amine 201 and aldehyde 202. The enethiourea 203 attacks then that imine in a pseudointramolecular manner from the si-face to form iminium ion depicted in TS-5. After intramolecular aza-Friedel-Crafts reaction, hexahydropyrroloquinolines 204 were obtained. 

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