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

摘要

Friedel-Crafts acylation and alkylation reactions were investigated using density functional theory calculations. Thereaction systems studied were (benzene + acetyl chloride + Al2C16 (or AICI3)) and (benzene + 2-chioropropane +Al2C16). In the acylation reaction, the acylium ion intermediate is reached either via a Me—C(C1)=0—Al2C16 complexor via direct CI transfer: Me—C(=0)C1—Al2C16 Me—C=01+—Al2W. The ion adds to benzene electrophilically toform a Wheland intermediate containing a strong C—H—CI hydrogen bond, which leads to deprotonation and thesubsequent formation of acetophenone. The resulting H—CI—Al2C16 fragment is subjected to a nucleophilic attack bythe carbonyl oxygen of the acetophenone, and recovery of the Al2C16 bridge is unlikely. Attack of the AI2CI6 moiety byMe—C(CI)=0 gives the complex Me—C(CI)=O-AICI3, whose reactivity toward acylation is similar to that of theMe—C(CI)=0-Al2C16 complex. In the alkylation reaction, deprotonation does not take place, but rather a [1,2] H-shiftfrom the Wheland intermediate. The resulting a-protonated cumene undergoes deprotonation, with subsequentrecovery of the Al2C16 bridge. In addition, the Al2C16-catalyzed isomerization of the n-propyl to the isopropyl cationwas found to be a dyotropic shift.