Structures of the Intermediate Complexes in Friedel‐Crafts Acylations

Abstract

Addition compounds of Lewis acids MX, 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,.and the ionic form, in which they can be formulated as oxocarbenium salts [R-CO]+ [MX, + ,I--. The compounds of the donor-acceptor type R-CXO+MX, 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.

Effect of the composition of the initiating complex acyl halides/aluminium bromide on the cationic polymerization of isobutylene

“…It is known2, 4, 5, 11 that adducts formed by interaction of acyl halides with Lewis acids (for example, aluminium bromide) can exist in solution as donor–acceptor complexes (DAC) or acyl salts (AS): where X is Cl or Br and n = 1 or 2.…”

“…Assignment of the signals was performed according to data published in reviews or in original papers 2. 4, 5, 11–13 Taking into account the low polarity of the media (hexane, methylene chloride) employed subsequently for IB polymerizations, the solutions for NMR studies were prepared in CH 2 Br 2 because of its relatively low polarity. Table 1 lists chemical shifts only for the most characteristic signals (C in carbonyls and H for carbon atoms in a β position relative to carbonyl).…”

“…Unexpected spectra are recorded in solutions of (1:1 and 1:2) mesitoyl complexes both of which are purely ionic. The absence of the DAC form for these adducts has been attributed11 to steric hindrance, whilst three methyl electron‐donating substituents on the benzene ring favour the formation of the (CH 3 ) 3 C 6 H 2 CO + carbocation.…”

“…In this case, the k el / k p value is even higher than that for the system with the CH 3 COBr·AlBr 3 complex. The situation may be understood when taking into account the fact that the steric restrictions prevent the formation of DAC between the carbonyl group of the mesitoyl bromide and an electron acceptor (for instance, aluminium bromide11) should be maintained after addition of the mesitoyl cation to an IB molecule during initiation. As a consequence, the ability of such head carbonyl groups to participate in the ‘carbocationic stabilization’ becomes rather poor.…”

Computational Studies on the Reactivity of Substituted 1,2-Dihydro-1,2-azaborines

“…The reaction mechanism of metal chlorides-assisted Friedel-Crafts acylation of aromatic compounds is known to be quite complex: Lewis acid complexation to either the carbonyl 34,35 or the halogen 36,37 of the acyl chloride has been described, as well as additional complexation of a second…”