Benzene, acylation general reactions

Methods of synthesis for carboxylic acids include (1) oxidation of alkyl-benzenes, (2) oxidative cleavage of alkenes, (3) oxidation of primary alcohols or aldehydes, (4) hydrolysis of nitriles, and (5) reaction of Grignard reagents with CO2 (carboxylation). General reactions of carboxylic acids include (1) loss of the acidic proton, (2) nucleophilic acyl substitution at the carbonyl group, (3) substitution on the a carbon, and (4) reduction. 

A more complex system which involves the photoactivated oxidative addition of benzene to Rh(PMe3)2(CO)Cl is outlined in Scheme 7.20. The system is unusual in having two photoactivated steps and the general reactions shown are based on the observations of Field and Goldman and their co-workers. The oxidative addition yields two isomers the one with the CO and Ph cis to each other is photoactivated to give CO insertion in competition with return to reactants. In the presence of CO, the acyl intermediate reacts by CO addition and reductive elimination to give benzaldehyde and the catalyst. 

The chemistry of pyrrole is similar to that of activated benzene rings. In general, however, the heterocycles are more reactive toward electrophiles than benzene rings are, and low temperatures are often necessary to control the reactions. Halogenation, nitration, sulfonation, and Friedel-Crafts acylation can all be accomplished. For example ... 

A much more general method for acyl silane synthesis involving silyl diazo intermediates is illustrated in Scheme 1688. The lithiated derivative of trimethylsilyl diazomethane reacts smoothly with alkyl halides in THF solution to give a-trimethylsilyl diazoalkanes in good yields. Oxidative cleavage of the diazo moiety is effected using 3-chloroperbenzoic acid in benzene solution, to give access to a wide variety of acyl silanes in yields of up to 71%. A phosphate buffer (pH 7.6) is used to prevent side reactions. Aromatic acyl silanes clearly cannot be prepared by this chemistry since an aromatic nucleophilic substitution reaction would be required. 

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. 

When an O-acyl ester (2) derived from phenylacetic acid is irradiated, a beautiful hyperfine structure of a benzyl radical can be observed by ESR [6]. This can be explained by the fact that the rate constant for the reaction of the formed R and O-acyl esters (2) is about 106 s"1 and it is rather slower than that of decarboxylation of RCO2 (eq. 8.3) [7, 8]. Generally, under photolytic conditions, approximately 100% of the chain reaction occurs, and under benzene refluxing conditions, approximately 80% of the chain reaction and approximately 20% of the caged reaction takes place. 

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