Stereoselectivity Friedel-Crafts reactions

Stereoselectivity was also observed in the Friedel-Crafts reaction of optically active plienyloxirane with toluene and anisole. The product diarylethanol had an enantiomeric ratio of 60 40 (37). 

Regio- and stereoselective reduction of the non-silylated triple bond, either by partial catalytic hydrogenation,13,1 15 or by lithium aluminum hydride reduction of the propargylic alcohols,11,15,1 afford (after desilylation), respectively, terminal (2)- and (E)-enynes. Furthermore, the remaining trimethylsilyl group in both silylated diynes and enynes may be replaced by another electrophile in a second Friedel-Crafts reaction.18... 

Bandini, M., Melloni, A., Umani-Ronchi, A. New catalytic approaches in the stereoselective Friedel-Crafts alkylation reaction. Angew. Chem., Int. Ed. Engl. 2004, 43, 550-556. 

New catalytic approaches in the stereoselective Friedel-Crafts alkylation reaction 04AG(E)550. 

The application of the aliphatic Friedel-Crafts reaction (chapter 5) to alkynes is a stereoselective approach to vinyl-lithiums that forms a bridge between the Shapiro reaction and the next section-hydrometallation. Acid chlorides 64 react with alkynes under Lewis acid catalysis to give E-P-chloroenones -65 stereoselectively. The chlorine can be replaced by more reactive iodine by conjugate substitution.11... 

New Catalytic Approaches in the Stereoselective Friedel-Crafts Alkylation Reaction ... 

On the other hand, 2-naphthols have been used with different success as Michael donors in conjugate Friedel Crafts reactions with nitroalkenes and related substrates (Scheme 4.53). For example, cinchonine-derived thiourea 72b was identified as an excellent promoter for the reaction of a wide variety of 2-naphthols and nitroolefins, providing excellent yields and enantioselectivities. Remarkably, the more challenging p-alkyl substituted nitroalkenes were also found to undergo the reaction in a highly stereoselective way and with comparable yields to those obtained when nitrostyrene derivatives were employed. 

Subsequently, MBH adducts were successfully utilized as novel stereodefined electrophiles in the Friedel-Crafts reaction with benzene in the presence of a Lewis acid " and sulfuric acid, leading to the stereoselective synthesis of (Z)- and ( )-functionalized trisubstituted alkenes. Notably, MBH adducts obtained from acrylonitrile provide high (Z)-stereoselectivities, while adducts derived from methyl acrylate and aromatic aldehydes give high ( )-stereo-selectivities. When the MBH adducts drived from methyl acrylate and aliphatic aldehydes were involved in the Friedel-Crafts reaction, no significant stereoselectivity was observed (Scheme 3.29). 

Kim and co-workers have reported that the Friedel-Crafts reaction of aza-MBH adducts of A-tosylimine derivatives 93 with arenes catalyzed by sufuric acid provides a stereoselective methodology for the preparation of stereo-chemically defined trisubstituted olefins (Scheme 3.31). They have also investigated the Friedel-Crafts reaction by using chlorobenzene and toluene as substrates however, mixtures of ortho and para isomers were obtained. 

Incorporation of aryl groups at the bridges can be achieved by a solvolytic Friedel-Crafts reaction (in the absence of a Lewis acid) simply by refluxing a mixture of the bromocabxarenes, the aromatic ring and the ionizing solvent (TFE for the electron-rich substrates or hexafluoroisopropanol, Eq. 4.24) [36]. In all case examined, the reaction proceeds in stereoselective fashion affording exclusively the all-cis derivative. 

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