Dehydrogenation by triphenylmethyl

Indenopyrans (45) which can be obtained after Schroth and Fischer from benzoxalenes (44) and lithium alkyls or aryls, are y-pyrans, and may be dehydrogenated by triphenylmethyl perchlorate to the indenopyrylium perchlorate (46). °... 

Tri- butylpyrylium can be obtained by dehydrogenating the corresponding 1,5-diketone with triphenylmethyl fluoroborate, It was shown by Farcasiu that 1,5-diketones can also he dehydrogenated and dehydrated to pyrylium salts by triphenylmethyl hexachloroantimonate generated in situ from chlorotriphenyl-methane and antimony pentachloride. Even pentaphenylpyrylium may thus be prepared at room temperature. ... 

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Recently it allowed the preparation of bispyrylium salts (where the two pyrylium nuclei with aryl groups in positions 2 and 6 are linked in position 4 by p-phenylene, w-phenylenej or 2,5-thienylene group ) by dehydrogenation with perchloric acid in acetic anhydride or with triphenylmethyl perchlorate in acetic acid. Directly linked dications were obtained similarly (cf. Section II,B,2,a ). 

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... 

Aromatic-aliphatic azo compounds can without exception be prepared by oxidation of the corresponding JV-alkyl-A -arylhydrazines. The oxidant must be one that reacts in neutral or alkaline conditions since the azo product rearranges to arylhydrazone in an acid medium. Mercury(n) oxide, introduced by Emil Fischer341 as oxidant, is almost always used for this reaction. 30% Hydrogen peroxide in hydrogen carbonate solution has been applied to dehydrogenation of A-aryl-A -(triphenylmethyl)hydrazines with good results.342... 

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