Acyl fluorides catalysis

Other reactions not described here are formal [3 -i- 2] cycloadditions of a,p-unsaturated acyl-fluorides with allylsilanes [116], or the desymmetrization of meso epoxides [117]. For many of the reactions shown above, the planar chiral Fe-sandwich complexes are the first catalysts allowing for broad substrate scope in combination with high enantioselectivities and yields. Clearly, these milestones in asymmetric Lewis-base catalysis are stimulating the still ongoing design of improved catalysts. 

Triflates of aluminum, gallium and boron, which are readily available by the reaction of the corresponding chlorides with triflic acid, are effective Fnedel-Crafis catalysis for alkylation and acylation of aromatic compounds [119, 120] Thus alkylation of toluene with various alkyl halides m the presence of these catalysts proceeds rapidly at room temperature 111 methylene chloride or ni-tromethane Favorable properties of the triflates in comparison with the correspond mg fluorides or chlorides are considerably decreased volatility and higher catalytic activity [120]... 

The reaction of sulfur tetrafluoride with carboxylic acids (aliphatic, aromatic, and heterocyclic) is of particular importance as the most general and simplest route to trifluorotnethyl derivatives. The reaction proceeds in two steps, the first step gives acyl or aroyl fluorides 1, and then catalysis in the second step by hydrogen fluoride released in the first step results in the tri-fluoromethyl derivatives 2.41... 

As will be seen later, benzoyl fluoride exhibits Bronsted-acid catalysis in aqueous media86 and was thought to be the only acyl derivative to do so although alkyl fluorides are known to behave in this way also. For benzyl fluoride, the acid-catalysed rate is reported to show dependence on /i0104 105. Satchell87 studied the solvolysis of benzoyl and butyryl fluoride in 60 40 (v/v) dioxan-water mixtures (for which h0 data are known)106. The solvolysis of each compound is catalysed by acid, but does not follow h0. With excdss of solvent the observed first-order loss of acyl derivative obeys the expression... 

Stoichiometry (28) is followed under neutral or in alkaline aqueous conditions and (29) in concentrated mineral acids. In acid solution reaction (28) is powerfully inhibited and in the absence of general acids or bases the rate of hydrolysis is a function of pH. At pH >5.0 the reaction is first-order in OH but below this value there is a region where the rate of hydrolysis is largely independent of pH followed by a region where the rate falls as [H30+] increases. The kinetic data at various temperatures both with pure water and buffer solutions, the solvent isotope effect and the rate increase of the 4-chloro derivative ( 2-fold) are compatible with the interpretation of the hydrolysis in terms of two mechanisms. These are a dominant bimolecular reaction between hydroxide ion and acyl cyanide at pH >5.0 and a dominant water reaction at lower pH, the latter susceptible to general base catalysis and inhibition by acids. The data at pH <5.0 can be rationalised by a carbonyl addition intermediate and are compatible with a two-step, but not one-step, cyclic mechanism for hydration. Benzoyl cyanide is more reactive towards water than benzoyl fluoride, but less reactive than benzoyl chloride and anhydride, an unexpected result since HCN has a smaller dissociation constant than HF or RC02H. There are no grounds, however, to suspect that an ionisation mechanism is involved. 

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