Aldehydes aromatic, chemical activity

Friedel-Crafts acylation of aromatics is of considerable practical value owing to the importance of aryl ketones and aldehydes as chemical intermediates.346 Whereas alkylation of aromatics with alkyl halides requires only catalytic amounts of catalysts, acylation to ketones generally necessitates equimolar or even some excess of the Friedel-Crafts catalysts. Usually one molar equivalent of catalyst combines with an acyl halide, giving a 1 1 addition compound, which then acts as the active acylating agent [Eq. (5.137)]. 

CJFormic acid and its chemically activated forms have become valuable one-carbon " C]formyl synthons for a variety of carbon-14-labeled heterocyclic compounds, mainly via A- or S-[ " C]formyl species, as indicated in Figure 5.26. In addition they have become reagents of choice for the preparation of aromatic, heteroaromatic and a-keto[carbonyl- " C]aldehydes. 

The Horner-Wittig reaction of a-phosphoryl sulphoxides 442, which are chemically stable, results in the formation of a, -unsaturated sulphoxides 443 in high yields (equation 264). The reaction has been found to be non-stereoselective, mixtures of E and Z isomers being formed from aldehydes and unsymmetrical ketones . In the case of aromatic aldehydes this reaction can also be advantageously performed in a two-phase catalytic system even without the usual PTC catalysts (Table 24). Intramolecular Horner-Wittig reaction of a-phosphoryl-5-oxosulphoxides 444 leads to a, -unsaturated cyclic sulphoxides 445 (equation 265). Starting from optically active 0,0-... 

Single electron oxidation of the non-activated carbonyl group, e.g. in aliphatic or aromatic aldehydes, ketones and carboxylic acid derivatives, is, on the other hand, much less feasible and only a handful of methods and synthetic applications are known. Useful methods for synthetic applications are chemical modifications to lower the oxidation potentials by peripheral donor substitution and a-silylation, or redox umpolung via oxidation of the corresponding carbonyl enols or enol ethers. 

Sonochemistry has been applied to acceleration of the Reformatsky reaction, Diels-Alder reactions, the arylation of active methylene compounds nucleophilic aromatic substitution of haloarenes, and to hydrostannation and tin hydride reduction. " Other sonochemical applications involve the reaction of benzyl chloride and nitrobenzene, a Sr I reaction in liquid ammonia at room temperature, and Knoevenagel condensation of aromatic aldehydes. lodination of aliphatic hydrocarbons can be accelerated, and oxyallyl cations have been prepared from ot,ot -diiodoketones using sonochemistry. Sonochemistry has been applied to the preparation of carbohydrate compounds.When sonochemistry is an important feature of a chemical reaction, this fact will be noted in the reactions presented in Chapters 10-19. 

From the results presented in Table 13.1, one can see that TBAB attacks quinones more rapidly than any other chromophores. TBAB also selectively attacks aldehydes over ketones. The selectivity is based on the reaction rates, and the yield is based on the observed final absorbance at the end of the reaction time. More so, aromatic ketones must be activated to react with TBAB by the presence of some electron attracting functional group near the carbonyl group. Finally, alcohol groups are totally inert toward the action of TBAB. Since hydrogen peroxide is known to attack ketones faster than aldehydes [68], the two bleaching chemicals do not compete for the same chromophores and are complementary to one another. 

Direct hydrogenation of aromatic carboxylic acids to the corresponding aldehydes has been industrialized by use of modified ZrC>2 catalyst by Mitsubishi Chemical Co.[16]. Although the reaction mechanisms are not clear at present, the hydrogenation and dehydration abilities of Zr02, which are associated with basic properties, seem to be important for promoting the reaction. By modification of Zr02 with metal ions such as Cr and Mn, the activity is increased, crystallization is suppressed and the coke formation is avoided. 

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