Acyl cations, exchange

Treatment of lithium enolate species, such as 7, with a variety of metal halide species produces enolates with different reactivities in particular, diethylaluminum(IH) and copper(I) species have been found to profoundly alter stereodifferentiation in reactions of iron acyl enolates (see Section D.1.3.4.2.5.1.). It has not been established whether complex formation or discrete ti ansmetalation occurs usually, a temperature increase from — 78 °C to — 42 °C is required for maximum effect, suggesting that cation exchange is responsible. In some cases, such additives exert an influence at —78 °C13, and this has been attributed to simple Lewis acid-type interactions with the substrate instead of transmetalation of the enolate species. For simplicity, when such additives are allowed to react with enolate species at temperatures of — 42 =C and above prior to the addition of other reagents, the process shall be referred to as transmetalation. 

As reported in the literature, the acylation of aromatic hydrocarbons can be carried out by using zeolites as catalysts and carboxylic acids or acyl chlorides as acylating agents. Thus toluene can be acylated by carboxylic acids in the liquid phase in the presence of cation exchanged Y-zeolites (ref. 1). The acylation of phenol or phenol derivatives is also reported. The acylation of anisole by carboxylic acids and acyl chlorides was obtained in the presence of various zeolites in the liquid phase (ref. 2). The acylation of phenol by acetic acid was also carried out with silicalite (ref. 3) or HZSM5 (ref. 4). The para isomer has been generally favoured except in the latter case in which ortho-hydroxyacetophenone was obtained preferentially. One possible explanation for the high ortho-selectivity in the case of the acylation of phenol by acetic acid is that phenylacetate could be an intermediate from which ortho-hydroxyacetophenone would be formed intramolecularly. 

The pioneer work in this field was carried out on polystyrene-supported acid catalysts [161]. Thereafter, several works on the use of sulfonic, strong acidic cation exchangers as acid catalysts were reported for alkylation, hydration, etherification, esterification, cleavage of ether bonds, dehydration, and aldol condensation [162,168-171], Besides, industrial applications of these materials were evaluated with reactions related to the chemistry of alkenes, that is, alkylation, isomerization, oligomerization, and acylation. [163,169], Also, Nation, an acid resin which has an acid strength equivalent to concentrated sulfuric acid, can be applied as an acid catalyst. It is used for the alkylation of aromatics with olefins in the liquid or gas phases and other reactions however, due to its low surface area, the Nation resin has relatively low catalytic activity in gas-phase reactions or liquid-phase processes where a nonpolar reactant or solvent is employed [166],... 

Allylic alcohols [e.g. citronellol, geraniol (8), and nerol (18)] exhibit strong shielding at the y-carbon and deshielding at the 5-carbon in the 13C n.m.r. upon acylation.127 Cationic exchange resins separate acyclic [e.g. myrcene (12)] from cyclic (e.g. limonene) monoterpenoids.128... 

Similar studies were also performed with acyl cations. Whereas no exchange was observed in the acetyl cation, exchange did take place in the... 

The neutral and acid-catalysed hydrolysis of mesitoyl chloride in acetonitrile containing 1% 018-enriched water is not accompanied by O18 exchange (Bender and Chen, 1963). It is suggested that in both cases unimolecular heterolytic bond fission occurs with the formation of an acyl cation. In alkaline solution a tetracovalent intermediate is postulated on the basis of a comparison of the effect of substituents on the rate of hydrolysis of the corresponding benzoate esters. The exchange of O18 in alkaline solution was not determined experimentally. 

In mesitoic acid the rate of exchange (in aqueous dioxan) is proportional to h0. The positive entropy of activation ( + 9 e.u.) and the large energy of activation (33 kcal. mole-1) are consistent with the formation of a transition state not involving water. It was suggested that exchange occurs by an A-l mechanism, in which an acyl cation is formed. 

In a series of valuable studies, the activity of various cation-exchanged Y zeolites in the acylation of toluene and xylenes with aliphatic carboxylic acids was investigated In a model reaction between toluene and octanoic acid, the activity of rare earth-, transition metal-, and alkaline earth-cation-exchanged Y zeolites was considered. CeY zeolite exhibits the highest activity (3 yield = 75%) in the para-acylation (Scheme 4.3) in agreement with the results published in an early study qj., contrary, unmodified Y zeolite shows a lower activity (3 yield < 40%), and transition metal and alkaline-earth-exchanged Y are nearly inactive. 

Metal-exchanged KIO clays were studied as catalysts in the acylation of benzo crown ethers 36 with AC (Scheme 4.25). The best catalyst is SnKlO, which affords product 37 in 90% yield after 1 h. The activity of SnKlO is about three times higher than that of clayfen. Product 37 is accompanied by a minor by-product ( 5%) due to the opening of the polyether ring followed by O-acetylation. The cafalysf acfivify is lowered upon recycling. The redox mechanism depicfed in Scheme 4.26 can represenf an alternative pathway for the formation of an acyl cation fhaf can accounf for the high activity of SnKlO. 

As sulfur-containing organic molecules are known catalyst poisons because of strong adsorption, acylation of thioethers is difficult to obtain. The reaction between thioanisole and AAN can, however, be performed in the presence of ion-exchange resins that are more robust to deactiva-tion. The process is carried out in a Parr autoclave in 1,2-dichloro-ethane at 70°C. Under these conditions, other acid catalysts such as sulfated zirconia and KIO clay do not show any noticeable activity. Only the cation exchange resin catalysts, which contain Bronsted sites, are effective. Among these, Amberlyst-15 shows maximum conversion because it... 

Sheemol, V. N., Tyagi, B., and Jasra, R. V. 2004. Acylation of toluene using rare earth cation exchanged zeolite (3 as solid acid catalyst. J. Mol. Catal. A Chem. 215 201-208. 

Chiche, B., Finiels, A., Gauthier, C., Geneste, R, Graille, J., and Pioch, D. 1987. Acylation over cation-exchanged montmorillonite. /. Mol. Catd. 42 229-235. 

Acylation of aromatic ethers with acid anhydrides in the presence of cation-exchanged clays. Appl. Catal. A Gen. 171 155-160. 

Gauthier et al. (1989) studied the activity of various cation-exchanged Y-type zeolites in the acylation of toluene with octanoic acid, obtaining selectivities to the para isomer of 94% at 75% yield of acylated product. The most efficient catalysts were rare-earth-exchanged zeolites (70% exchange), the following order of activity being observed Cr3+, Zr4+ < M2+, Cu2+, Co2+ <11 < Pr3+, La3+, Gd3+, Yb3+, Ce3+. 

Phenylalanine, synthesized from [ C]cyanide by the Bucherer modification of the Strecker synthesis, was resolved into its l- and D-iso-mers by the action of the d- and L-amino acid oxidases, respectively. The optically active amino acid was separated from phenylpyruvic acid by cation exchange chromatography (3). Similarly, DL-[ F]acyl-p-fluoro-phenylalanine has been subjected to stereospecific deacylation with the fungal enz)nne, L-amino acylase enz)nnatically generated L-[ F]p-fluoro-phenylalanine was separated from the D-acyl amino acid by column chromatography (4). 

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