Structures of the Intermediate Complexes in Friedel-Crafts Acylations

Addition compounds of Lewis acids MX_n and acyl halides R? COX occur as intermediates in Friedel-Crafts acylations. IR and NMR studies on these intermediates have indicated the probable existence of structural isomers. In X-ray structural analysis, it is possible to distinguish two forms, i.e. the molecular form, in which the compounds are present as donor-acceptor complexes R? CXO→MX_n, and the ionic form, in which they can be formulated as oxocarbenium salts R? CO]⁺MX_n+1]^?. The compounds of the donor-acceptor type R? CXO→MX_n are characterized by the formation of a coordinate oxygen-metal bond; the transfer of electrons from the oxygen to the metal of the acceptor is always due to a weak donor-acceptor interaction. The positive charge of the aryloxocarbenium ions is partly delocalized over the aromatic nucleus. The positive charge in alkyloxocarbenium ions, on the other hand, is essentially localized on the carbon atom of the carbonyl group, as is confirmed by electron density distribution calculations.