Dehydrogenative condensation

Arylation of Aromatic Compounds. In contrast to Friedel-Crafts alkylations, arylations of aromatics are not as well known, and usually require drastic conditions. They iaclude (/) dehydrogenating condensation (SchoU reaction) (2) arylation with aryl haUdes and (J) arylation with dia2onium hahdes. 

Presumably, jS-chloro ketones could also react similarly with methyl(ene) ketones. Another logical extension is the possibility of synthesizing pyrylium salts by dehydrogenative condensation of -chlorovinyl ketones with oleflns like styrene, in the presence of stannic chloride (the olefins so far tested, like isobutene, are not suitable structurally). 

The dehydrogenative condensation of unsaturated ketones with methyl ketones was used for preparing various series of 2,4,6-tri-arylpyrylium salts not only by Dilthey, but also by Wizinger and co-workers (for combinations of phenyl, p-anisyl, and p-dimethyl-aminophenyl substituents), by Amoros-Marin and Carlin (combinations of phenyl and p-chlorophenyl), by Le Fevre and Le Fevre (for combinations of pbenyl and m- or p-nitrophenyl), and by others. ... 

Coke deposits were formed from biphenyl over dealuminated HM only in the presence of propylene, although the amount of coke was less.26,27 Coke deposition occurred in a short period after starting the reaction, and by the contact of 4,4 -DIPB with HM even in the absence of propylene.26 These results suggest that the isopropylated biphenyls produce coke by dehydrogenative condensation at their isopropyl groups on acid sites. Propylene oligomers were formed during the reaction 26 They are alternative precursors of deposited coke. 

Both of these dehydrogenation-condensation ketone syntheses are equally applicable to numerous members of the primary alcohol series. 

Probably the same reaction, the dehydrogenation, condensation, and irreversible adsorption of highly condensed aromatic hydrocarbons, causes the loss of activity of the WS2 catalyst when used at pressures well below 200 atm. As an example (20), in experiments with a paraffin-base petroleum-oil fraction boiling between 200° and 325°C. the results in the accompanying tabulation were obtained ... 

The most important contribution in the field of simultaneous dehydrogenation, condensation, and dehydration made by Russian chemists is the synthesis of butadiene from ethanol over a double oxide catalyst by the method of Lebedev. Much has been published on this process. Lebedev s interest in rubber synthesis began with his researches on conversions of dienes in 1908 and his method of synthesis of butadiene was reported in 1927. An experimental synthetic rubber plant was founded for research in the field and the studies on the mechanism of formation of butadiene and of polymerization were continued after Lebedev s death by his students (103,104,105,188,190,378). A survey of the properties and methods of preparation of butadiene was published by Petrov (289). 

It should, however, be noted that a number of subsequent reactions (oxidation, dehydrogenation, condensation) take place in the later phases of stabilization they will be discussed in detail in Section 3.2. 

A detailed study of this dehydrogenative condensation in the presence of triphenylmethyl perchlorate or fluoroborate was made by Simalty-Siemiatycki and Fugnitto. The reaction is best carried out in refluxing acetic acid nitromethane or acetonitrile give less satisfactory results. Chalcone reacts in these conditions with phenylacet-aldehyde yielding 2,4-diphenylpyrylium with an unsubstituted a-position. This and similar 2-unsubstituted pyrylium salts prepared by this method are so reactive that they do not afford pyridines on treatment with ammonia in the usual conditions this behavior is similar to that of the unsubstituted pyrylium perchlorate. The reaction of... 

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