Friedel-Crafts Isomerization. I. Effect of Promoted Aluminum Halides on Halobenzenes

Abstract

Disproportionation (intermolecular isomerization) of fluoro-, chloro-, and bromobenzene was investigated through the effect of water-promoted aluminum chloride and bromide. Investigations were carried out by the use of a gas-liquid chromatographic analytical method. It was observed that ring-bonded fluorine does not undergo intermolecular migration. The decreasing order of activity of halobenzenes in isomerization was found to be Br » Cl. The reaction mechanism of the intermolecular isomerization (disproportionation) of halobenzenes in terms of sigma complex type intermediate state formation is discussed as a consequence of the results obtained. The relative basicity of the halobenzenes determined from spectroscopic data is in agreement with this mechanism. Further proof of the suggested mechanism was obtained by using heavy water as the promoter. Halogen exchange and electrophilic arylations by the halobenzenes accompany the aluminum halide-induced isomerization of halobenzenes. In these reactions the relative order of activity of the halobenzenes was found to be F > Cl > Br, To explain the reversed order of activity as compared to that observed in isomerization and to account for the mechanism, intermediate complexing is suggested through the unshared pairs of the halogens to the acid catalyst (n-complex formation). Evidence for n-complexing of fluorobenzenes with aluminum bromide was obtained by infrared and high resolution nuclear magnetic resonance spectroscopy. Conductivity measurements were also in accordance with the proposed complex formation. Further proof of the n-complex mechanism was obtained by the chlorine exchange occurring between chlorobenzene and AbCV6.

Aluminium-mediated aromatic C–F bond activation: regioswitchable construction of benzene-fused triphenylene frameworks

“…These results indicate that the high efficiency in g-selective cyclisation of 1a can be attributed to the relatively ready generation of the fluorine-stabilised intermediary arenium ions. 7,13 In contrast, cyclisation of an aryl cation intermediate generated from a fluorine-free precursor exhibited opposite regioselectivity (Scheme 4). When 2-(biphenyl-2-yl)naphthalene-1-diazonium chloride, prepared from 2-(biphenyl-2-yl)naphthalen-1-amine (1k), was heated at 60 1C, benzo[g]chrysene (3a) was obtained in 90% yield as the sole product.…”

Regioselective electrophilic borylation of haloarenes

“…The formation of benzene is attributed to the disproportionation of bromobenzene under the action of AlCl 3 , as previously reported. 12 Due to this side reaction and the inferior regioselectivity, these amines were excluded from further borylation studies. The effect of temperature using DMTol was next explored as an alternative route to improve regioselectivity.…”

Decay-Induced Bromination via the 76Kr(EC)76Br and 77Kr (β+, EC)77Br Decay

“…In order to identify possible isomerisation processes we have used para-difluorobenzene as a substrate molecule. Unlike chloro-and bromo-substituents, a migration of the aromatic bonded fluorine is not observed in catalytic isomerisation [40]. In comparison to fluorobenzene the use of 1,4-difluorobenzene should provide additional information on the unusually high Fover-H replacement yield ratio.…”

“…Since the fluorine substituents are positionally stable in halo-arenium ions [40], any isomerisation must be attributed to a Br + or H + migration. This, however, should also lead to the ortho product, at least when the isomerisation proceeds via a π-complex.…”