Benzene, acylation stability

An important difference between Fnedel-Crafts alkylations and acylations is that acyl cations do not rearrange The acyl group of the acyl chloride or acid anhydride is transferred to the benzene ring unchanged The reason for this is that an acyl cation is so strongly stabilized by resonance that it is more stable than any ion that could con ceivably arise from it by a hydride or alkyl group shift... 

Sometimes acylium ions lose carbon monoxide to generate an ordinary carbonium ion. It will be recalled that free acyl radicals exhibit similar behavior at high temperatures. Whether or not the loss of carbon monoxide takes place seems to depend on the stability of the resulting carbonium ion and on the speed with which the acylium ion is removed by competing reactions. Thus no decarbonylation is observed in Friedel-Crafts reactions of benzoyl chloride, the phenyl cation being rather unstable. But attempts to make pivaloyl benzene by the Friedel-Crafts reaction produce tert-butyl benzene instead. With compound XLIV cyclization competes with decarbonylation, but this competition is not successful in the case of compound XLV in which the ring is deactivated.263... 

Another important path of research, especially to organic chemists, resulted from a discovery by Woodward, Rosenblum, and Whiting at Harvard University in 1952 (128). These investigators noted the failure of ferrocene to undergo Diels-Alder reactions and its resistance to catalytic hydrogenation. They reasoned that because of its remarkable stability, ferrocene might behave like an aromatic substance. These suppositions proved to be the case, as ferrocene was readily acylated under Friedel-Crafts conditions to form acyl derivatives. Indeed, the name ferrocene was given to biscyclopentadienyliron because of its chemical similarity to benzene (128). 

Related classes of gitonic superelectrophiles are the previously mentioned protoacetyl dications and activated acyl cationic electrophiles. The acyl cations themselves have been extensively studied by theoretical and experimental methods,22 as they are intermediates in many Friedel-Crafts reactions. Several types of acyl cations have been directly observed by spectroscopic methods and even were characterized by X-ray crystal structure analysis. Acyl cations are relative weak electrophiles as they are effectively stabilized by resonance. They are capable of reacting with aromatics such as benzene and activated arenes, but do not generally react with weaker nucleophiles such as deactivated arenes or saturated alkanes. 

Benzofurazans show greater thermal stability but may be cleaved photochemically. Irradiation of benzofurazan in benzene and in methanol gives the azepine (26) and the urethane (27), respectively in the presence of triethyl phosphite (Z,Z)-l,4-dicyanobuta-1,3-diene is formed. The proposed mechanism (Scheme 3) involves nitrile oxide, oxazirene, acyl nitrene and isocyanate intermediates, and is supported by spectrophotometric studies (76HCA2727) and by trapping of the nitrile oxide as its isoxazole cycloadduct with DMAD (75JOC2880). 

The mechanism of Friedel-Crafts acylation (shown next) resembles that for alkylation, except that the electrophile is a resonance-stabilized acylium ion. The acylium ion reacts with benzene or an activated benzene derivative via an electrophilic aromatic substitution to form an acylbenzene. 

Compound 220 in solution gives rise to a dynamic equilibrium between the enaminoketone (i )-220 and iV-acyl forms (Z)-221 in the ground state (Scheme 14). In nonpolar solvents, such as hexane, benzene, and toluene, the equilibrium is displaced toward isomer ( )-220, which is stabilized by intramolecular hydrogen bond it absorbs in the region of 470 nm. In polar solvents like DMSO, the equilibrium shifts almost completely toward the N-acylated form (Z)-221. 

Acylation of bisamidrazones followed by thermal cyclization of the linear polymer (Scheme 148) has provided valuable polymers of high thermal stability benzene and pyridine... 

In Friedel-Crafts acylation, the Lewis acid AICI3 ionizes the carbon-halogen bond of the acid chloride, thus forming a positively charged carbon electrophile called an acylium ion, which is resonance stabilized (Mechanism 18.7). The po.sitively charged carbon atom of the acylium ion then goes on to react with benzene in the two-step mechanism of electrophilic aromatic substitution. 

Indoles have been prepared from reactions of o-aminophenylketones with reactive , or stable " arsonium ylides. Oxo-stabilized ylides reacted with 2-chloro-oximes to give trans-5-acyl-A -isoxazolines, and isoxazoles have been obtained from reactive arsonium ylides and a-isonitrosoketones, and from triphenylarsonium methylide and nitrile oxides The latter ylide reacts similarly with nitrile imines to give pyrazoles. With triphenylarsonium benzylides and benzoylylides,benzene diazonium salts give 1,3,4,6-substituted 1,4-dihydro-1,2,4,5-tetrazines in a reaction in which initial coupling of the reagents is followed by a dimerisation. ... 

Instead, these heterocycles and their derivatives most commonly undergo electrophilic substitution nitration, sulfonation, halogenation. Friedel-Crafts acylation, even the Reimer-Tiemann reaction and coupling with diazonium salts. Heats of combustion indicate resonance stabilization to the extent of 22-28 kcal/ mole somewhat less than the resonance energy of benzene (36 kcal/mde), but much greater than that of most conjugateci dienes (about Tlccal/mole). On the basis of these properties, pyrrole, furan, and thiophene must be considered aromatic. Clearly, formulas I, II, and III do not adequately represent the structures of these compounds. 

When SO3 itself is employed, the concentration is at a maximum and the initial stage of the sulfonation is completed easily and rapidly. However, the sulfonic acid formed in the initial stage easily reacts with a second mole of SO3 to form a complex which may be much less reactive than SO3 itself. Thus, when reacting a hydrocarbon with SO3 on an equimolar basis, one half of the hydrocarbon is sulfonated with SO3 and the other half is sulfonated by the less reactive complex (see the section on Kinetics). Likewise, when sulfonating organic acids with SO3, the initial reaction product is an acyl sulfate which is next converted to the desired sulfonate under considerably more drastic conditions. The second half of these reactions is the slower, and the rate and reaction conditions are determined by the stability of the initial complex, which varies greatly. The benzene-sulfonic acid-S03 complex is quite reactive, while that from naphthalene-disulfonic acid is comparatively unreactive. When SO3 is used for sulfation, the reaction appears simpler ... 

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