A remarkable difference in the deprotonation steps of the Friedel-Crafts acylation and alkylation reactions

Abstract

Friedel-Crafts acylation and alkylation reactions were investigated using density functional theory calculations. The reaction systems studied were (benzene + acetyl chloride + Al2Cl6 (or AlCl3)) and (benzene + 2-chloropropane + Al2Cl6). In the acylation reaction, the acylium ion intermediate is reached either via a Me-C(Cl)-O-Al2Cl6 complex or via direct Cl transfer: Me-C(=O)Cl-Al2Cl6→ Me-C=O]+-Al 2Cl7. The ion adds to benzene electrophilically to form a Wheland intermediate containing a strong C-H-Cl hydrogen bond, which leads to deprotonation and the subsequent formation of acetophenone. The resulting H-Cl-Al2Cl6 fragment is subjected to a nucleophilic attack by the carbonyl oxygen of the acetophenone, and recovery of the Al 2Cl6 bridge is unlikely. Attack of the Al 2Cl6 moiety by Me-C(Cl)=O gives the complex Me-C(Cl)=O-AlCl3, whose reactivity toward acylation is similar to that of the Me-C(Cl)=O-Al2Cl6 complex. In the alkylation reaction, deprotonation does not take place, but rather a [1,2] H-shift from the Wheland intermediate. The resulting α -protonated cumene undergoes deprotonation, with subsequent recovery of the Al2Cl6 bridge. In addition, the Al2Cl6-catalyzed isomerization of the n-propyl to the isopropyl cation was found to be a dyotropic shift.© John Wiley & Sons, Ltd.