Pyrylium hexachloroantimonate

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. ... 

A general method for the synthesis of pyrylium salts is the cyclodehydration of 1,5-dicarbonylalkanes (Scheme 4.4). Acetic anhydride is commonly used as both solvent and reagent, but since the initial product is a 4/f-pyran, an oxidant such as the triphenylmethyl (trityl) cation in the form of triphenylmethyl hexachloroantimonate is added (Ph3C + [H ] -> Ph3CH). In certain cases, however, it is advantageous to isolate the pyran and to oxidize it in a separate step. 

Much of the NMR work on pyrylium compounds has been carried out on the perchlorate salts, but measurements on the tetrafluoroborate and hexachloroantimonates show that the nature of the anion has little effect on the chemical shifts of the ring protons. A paramagnetic anion, such as FeCLT, may cause line broadening. 

This ring system is represented by the 1,3,5-oxadiazinium salts, which are diaza analogs of pyrylium salts. The parent compound is not known, nor are simple alkyl derivatives, but triaryl-and bis(dialkylamino)oxadiazinium salts are stable compounds. The salts contain a complex counterion, such as pentachlorostannate, hexachloroantimonate, tetrachloroferrate, tetrafluo-roborate or perchlorate. 

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