Acylating agents cation

Acylation (Section 25 22) Esterifi cation of the available hydroxyl groups occurs when carbohydrates are treated with acylating agents... 

Pyridinium bromide, N-phenacyl-NMR, 2, 121 reactions, 2, 336 Pyridinium cations metabolism, 1, 234 reactivity, 2, 167 Pyridinium chloride hydrogenation, 2, 284 Pyridinium chloride, N-acyl-as acylating agent, 2, 180 Pyridinium chloride, cetyl-as antiseptic, 2, 519... 

The anhydride or acyl chloride and the catalyst (proton acid or Lewis acid) interact leading to the acylating agent [formulated here for brevity as an acyl cation (83)]. ... 

Various classes of cationic initiators have been used to polymerize lactones protic acids, Lewis acids, acylating agents, and alkylating agents. These initiators are often difficult to handle experimentally... 

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. 

Methyl-1,2-dithiolium cations also react with dithio esters which serve as acylating agents (Eq. 23).41... 

The vapor-phase acylation of pyrrole was carried out over zeolite catalysts such as HZSM-5(19.7), HZSM-5(30), HZSM-5(280), HY, and cation-promoted modified zeolites like CeHZSM-5(30), LaHZSM-5(30), and CeHY, in a fixed-bed reactor at atmospheric pressure using acetic anhydride as an acylating agent <1998CAL95>. The catalytic activity of the zeolite catalysts was dependent on the reaction temperature and the type of cation polymer used in the modification of the zeolite surface. The acylation was found to be more active on Br0nsted acidic sites available over zeolite systems. The yield of l-(l//-pyrrol-2-yl)-l-ethanone 376 with respect to the conversion of pyrrole on HZSM-5(280) at 250 °C was 75.5% (Equation 87). 

Covalent compounds, which are strong alkylating or acylating agents may initiate the cationic polymerization of heterocycles. Again, as in the case of acids, classification of these agents is relative for example, alkyl bromide will efficiently alkylate strongly nucleophilic monomers like cyclic amines or oxazolines and thus initiate their polymerization whereas it will be uneffective in the polymerization of cyclic ethers. 

The catalysts were examined in the acetylation of toluene, one of the difficult acylations because of the difficulty in the formation of an intermediate acetyl cation (MeCO ) from alkyl chain acid anhydrides and hydrides. The reactivities increase as the length of the hydrocarbon chain of the acylating agent is lengthened [210], which shows acetylation to be the most difficult acylation. The heterogeneous acetylation of toluene using solid acidic promoters has not been reported in detail. In fact, to complete acetylation with the anhydride is quite difficult under reflux condihons [211]. 

Several detailed reaction paths have been proposed for the addition process (117). The most powerful acylating agent present is the proton-ated lactam (XXIX), p. 437, and the strongest nucleophile is the neutral amine group so that the fastest addition should be reaction (92). Since the equilibrium (91) results in a low concentration of lactam cations and neutral amine groups, most amine groups are present as ammonium ions or undissociated salt (—COONH3—). Therefore, a two-step or concerted mechanism could also be operative in which the iVI-protonation of the lactam by an ammonium ion is accompanied or followed by acylation of the amine and intramolecular proton transfer [220], viz. 

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