Friedel-Crafts Chemistry at Halogenated Surfaces

[摘要] Friedel-Crafts reactions play an important role in the synthesis of fine chemicals on an industrial scale. Aluminium trichloride has emerged as the catalyst of choice as it possesses strong Lewis acidity and is relatively inexpensive. Environmental considerations, in particular waste minimisation, dictate that aluminium trichloride be replaced as soon as possible. Strong interactions between the reagents and the AICI3 are problematic and result in complexation of the reagents to the Lewis acid catalyst, effectively deactivating the catalytic sites. The hydrolytic destruction of the spent catalyst thereafter and disposal of the associated waste poses a considerable burden on the environment. A study has been made to identify materials to replace AICl3 which have comparable activity but are not prone to substrate deactivation. The Friedel-Crafts catalytic activity at room temperature of the solids; SF4 fluorinated gamma-alumina, CCl4 chlorinated gamma-alumina, beta-AIF3 and supported organic layer catalysts derived from treatment of chlorinated and fluorinated gamma-alumina with CH3CCl3, were examined as they exhibit Lewis acidic properties in halogen exchange reactions. Catalytic performance of the solids was studied in the Friedel-Crafts alkylation of toluene and benzene with t-butyl chloride. Chlorination of gamma-alumina with CCl4 results in a solid with catalytic activity comparable to the archetypal Lewis acid catalyst AlCl3, in the model Friedel-Crafts alkylation reactions. Interestingly, the SF4 fluorinated gamma-alumina was catalytically inactive in the model systems, instead interacting with the reactants to form a highly coloured deposit on the surface of the solid. In-situ infrared analysis showed that (CH3)3CCl vapour was completely deposited on both the CCl4 chlorinated gamma-alumina and SF4 fluorinated gamma-alumina, notably with release of HCl(g) over CCl4 chlorinated gamma-alumina. Uptake and lability of [36Cl]-chlorine labelled alkyl and acyl halides on the solid surfaces, were examined via the direct monitoring Geiger Muller counting technique. Interaction of [36Cl]-chlorine labelled (CH3)3CC1 with CCL4 chlorinated gamma-alumina resulted in an appreciable [36Cl]-chlorine surface count on the solid, 65-70% of which was displaced/exchanged on introduction of unlabelled HCl. Similar displacement/exchange of the [36Cl]-chlorine associated with the solid surface was observed when the CCl4 chlorinated gamma-alumina was treated with [36Cl]- chlorine labelled CH3CHCICH3 and CH3COCl. It has been shown previously that treatment of gamma-alumina with CCl4 results in two types of surface chlorine species, Al-Cl terminal chlorine species, associated with Bronsted acid sites and readily exchangeable, and Al-Cl-Al bridging chlorine species, associated with Lewis acid sites and inert to exchange. The Friedel-Crafts catalytic activity of the chlorinated solid is the result of activation of the alkyl halide species, to form a transient carbenium ion, via interaction of the alkyl halide with the surface of the solid. It is very difficult to determine the exact nature of the interaction between the Lewis basic reagent and solid surface; it may involve simple adsorption of the alkyl halide at Bronsted acid sites on the solid or a more complex equilibrium involving dehydrochlorination of the alkyl halide at Lewis acid sites and protonation of the resultant alkene via surface Bronsted acidity. Failure of the CCl4 chlorinated gamma-alumina to catalyse consecutive Friedel- Crafts alkylation reactions may be the result of inert chlorine species associated with Lewis acid sites, effectively deactivating the active sites on the solid, analogous to AICI3. In addition, the hygroscopic nature of gamma-alumina makes it susceptible to moisture, resulting in deactivation of the catalytic activity via coordination of the water molecules to the surface sites on the solid. Efforts were made to eliminate moisture, by working in an inert atmosphere with dry reagents, but there was always the potential that residual moisture was present. Interestingly, in preliminary studies, the CCl4 chlorinated gamma-alumina was not catalytic in a model Friedel-Crafts acylation reaction (i.e. benzoylation of benzene with benzoyl chloride). The formation of hydrolysis products may indicate that the lack of catalytic activity was due to deactivation of the solid via coordination of water molecules to the active sites on the solid surface, rather than an inherent lack of acidity of the solid. Solid gamma-alumina fluorinated with SF4 was not an active Friedel-Crafts alkylation catalyst, due to preferential interaction of the alkyl halide starting material with the solid, forming a highly coloured supported organic layer. Analysis of the supported organic layer by GCMS indicated it was based on a C4 hydrocarbon unit, consistent with the (CH3)3CCl dehydrochlorination product, (CH3)2C=CH2.

[发布机构] University:University of Glasgow

[关键词] Organic chemistry