Allylic halides Friedel-Crafts reaction

A short review in Chinese covers Friedel-Crafts reactions of alkenyl halides with aromatic hydrocarbons.49 High yields of the allylated aromatic products have been... 

This alternative to the Friedel-Crafts reaction, extensively developed by Stille and coworkers, is particularly important, since the reaction conditions are essentially neutral, and so provides a method for acylation of compounds containing an acid-sensitive functionality which would preclude the use of the Friedel-Crafts reaction. Reaction temperatures are often below 100 C, and high (1000-fold) turnovers of the catalyst have been achieved. Solvents employed include chloroform, toluene, and, on occasions, HMPA. Some reactions have been carried out under an atmosphere of carbon monoxide to prevent excessive decarbonylation of the acyl palladium intermediate. Indeed, carbonylative coupling of alkenylstannanes with allyl halides in the presence of carbon monoxide ca. 3 atm or greater 1 atm =101 kPa) offers an alternative to the Friedel-Crafts acylation, ketones being formed by the reaction of the stannane with the acyl species formed by carbon monoxide insertion into the allyl palladium intermediate. ... 

Catalytic amounts of I fCl4-AgC104 and Hf(OTf)4 are used for activation of acid halides and acid anhydrides for Friedel -Crafts acylation (Scheme 42) 178 the reactions of both reactive and unreactive aromatic substrates proceed smoothly in the presence of Hf(OTf)4. Furthermore, the Fries rearrangement179,180 and direct C-acylation of phenolic compounds181,182 take place using Hf(OTf)4. Formation of esters and Mannich-type reactions and allylation of imines have been also reported.152... 

With Friedel-Crafts halides (usually A1C13 and BF3) it is necessary to use equimolar or excess catalyst when alcohols are the alkylating agents. With primary alcohols usually 2 mol of catalyst per mole of alcohol must be used. Complexing the alcohol, as well as binding the water formed in the reaction explains the experimental findings. With secondary and tertiary alcohols, lesser amounts of catalyst are needed. The reactivity of different alcohols follows the order methyl < primary < secondary < tertiary, allyl, benzyl. The ease of carbocation formation according to Eq. (5.51) is most probably responsible for this reactivity order ... 

C-Alkylations have been performed with both support-bound carbon nucleophiles and support-bound carbon electrophiles. Benzyl, allyl, and aryl halides or triflates have generally been used as the carbon electrophiles. Suitable carbon nucleophiles are boranes, organozinc and organomagnesium compounds. C-Alkylations have also been accomplished by the addition of radicals to alkenes. Polystyrene can also be alkylated under harsh conditions, e.g. by Friedel-Crafts alkylation [11-16] in the presence of strong acids. This type of reaction is incompatible with most linkers and is generally only suitable for the preparation of functionalized supports. Few examples have been reported of the preparation of alkanes by C-C bond formation on solid phase, and general methodologies for such preparations are still scarce. 

Despite the lack of examples of Friedel-Crafts acylations catalyzed by Lewis acids, - reactions of stannanes with acyl halides catalyzed by palladium species have found considerable use for the preparation of ketones. Since alkyl groups are only transferred slowly from tin, more rapid transfer to the acyl chloride is observed for alkynyl, alkenyl and allyl, as well as aryl and benzyl, groups. This leads to a versatile synthesis of ketones.Acylations of alkenylstannanes are both regio- and stereo-specific. 

Furthermore, the decomposition of the zinc enolate to allylic halide is favored by substitution in the allyl portion. The halide then undergoes Friedel Crafts condensation with the aromatic solvent catalyzed by zinc bromide present in the reaction. 

Step It Lewis acid-Lewis base reaction. Friedel-Crafts acylation begins with the donation of a pair of electrons from the halogen of the acyl halide to aluminum chloride to form a molecular complex similar to what we drew for Friedel-Crafts allylations. In this complete halogen has a positive formal charge and aluminum has a negative formal charge. 

Friedel-Crafts and Other Alkylation Reactions. A variety of r-butyl-substituted aromatics have been prepared with isobutene in the presence of acids (eqs 19-21). Due to steric factors, the regiochemistry is enhanced. Isobutene can be alkylated by benzylic and allylic halides in the presence of Zinc Chloride (eqs 22-24). ... 

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