Dilthey

From the same point of view, 4-pyrones (8a) or 2-pyrones are anhydro bases of 4-hydroxypyrylium (9) or 2-hydroxypyrylium salts. Vinylogs (10) and phenylogs (12) (violones, after Dilthey and Burger ) of these systems are known, whose conjugate acids are monocyclic pyrylium salts (11 and 13). 

The first chemists to prepare a monocyclic pyrylium salt were von Kostanecki and Rossbach who in 189 6 described the fluorescence in dilute aqueous solution of the reaction product obtained from l,3,5-triphenjdpontane-l,5-dione (benzylidene-diacetophenone) and sulfuric acid. However, they failed to isolate the compound which caused the fluorescence and did not suspect that it was a pyrylium salt. It was only in 1916-1917 that Dilthey recognized that this fluorescence resulted from 2,4,6-triphenylpyrylium (3). 

For the preparation of larger amounts of p3rrylium salts the method of Baeyer and Villiger was unsuitable. Therefore, the discovery by Dilthey" of new convenient methods for preparing aryl-substituted pyrylium salts in 1916 was an important event. By the studies of Dilthey, and a little later, of Schneider, many reactions of aryl-substituted pyrylium salts were discovered. 

It must be taken into account that in all papers published between 1916 and 1922 the structure of methyldiarylpyrylium salts is erroneously indicated as a symmetrical 2,6-diarjd-4-methylpyrylium structure. The unsymmetrical structure (2,4-diaryl-6-methyl) was recognized by Gastaldi, then by Schneider and Ross, and finally by Dilthey and Fischer. ... 

As mentioned previously, pyrylium salts are dehydration products of the conjugate acids of as-2-ene-l,5-diones, and therefore, most syntheses of pyrylium salts are, in fact, syntheses of 1,5-enediones. Dilthey has devised his new pathways to pyrylium salts by analogy... 

Dilthey wished to adapt, for pyrylium, syntheses which with ammonia were known to yield pyridines. Indeed, there is a marked similarity, and probably a common mechanism between certain syntheses of pyrylium salts and of p5rridines, e g., Chichihabin s and... 

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. ... 

In comparing his three main S5mtheses of pyrylium salts (see Sections II,B,2,f C,2,g and D, l,b) besides his less general syntheses (see Sections II,B,2,e C,2,a and D,2,a), Dilthey stated that the type in Section D, l,b is the most convenient. This is the standard method for preparing 2,4,6-triarylpyTylium salts with identical 2- and 6-substituents, in particular 2,4,0-triphenylpyrylmm. 398,389 vepage yields are SQ% in the presence of BF3-Et.20 they can be raised to 40%. The reaction can be applied to substituted... 

The course of the reaction may involve either the acylation of the ketone to a -diketonic intermediate following thereupon the pathway in Section II, C, 2, a, or alternatively the condensation of two moles of ketone to yield an intermediate dypnone which then undergoes acylation following the pathway in Section II,C, l,a. Dilthey and Fischer thought the first alternative more plausible, on the basis of reaction yields, and this lead them to explore the pathway in Section II,C,2,a. Schneider and Ross and Diels and Alder believed that the second alternative operates. Both views are plausible since acylations of methyl ketones to S-diketones are known to take place in the conditions of this reaction, and dypnone ha.s been isolated from acetophenone on treatment with Friedel-Crafts catalysts, in the absence of an acid anhydride or chloride (an excess of catalyst... 

Catalysts. The methanation of CO and C02 is catalyzed by metals of Group VIII, by molybdenum (Group VI), and by silver (Group I). These catalysts were identified by Fischer, Tropsch, and Dilthey (18) who studied the methanation properties of various metals at temperatures up to 800°C. They found that methanation activity varied with the metal as follows ruthenium > iridium > rhodium > nickel > cobalt > osmium > platinum > iron > molybdenum > palladium > silver. 

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