Nitrobenzene pyrylium salts

This procedure illustrates a general method for converting substituted pyrylium salts to nitrobenzene derivatives. The reaction has been the subject of several reviews. - The yields are generally high, and under these conditions only a single product is formed, in contrast to the nitration of 1,3,5-triphenyl-benzene. The preparation of 2,4,6-triphenylnitrobenzene from the corresponding pyrylium salt eliminates isomer separation problems, which are encountered when the direct nitration procedure is used. Also, labeled compounds can readily be prepared by this method. ... 

Finally, in sect. 2.9, the conversion of 2,4,6- or higher substituted pyrylium salts into the corresponding nitrobenzene derivatives through reaction with nitromethane 27) will be discussed in more detail. Since pyrylium salts of any desired complexity can be easily prepared 28), the conversion of the nitroarenes leads to a large number of phenols of the desired constitution. This method is by far the most versatile in the preparation of a desired arylated phenol. These syntheses, however, involve many steps. 

Triphenyl-pyrylium salts and ring substituted phenylnitromethane derivatives similarly rearrange to the phenols 6 or nitrobenzenes 7. When the ringsubstituted phenylnitromethane is reacted with the 2,4,6-arylated pyrylium salt in t-butanol with one mole of potassium t-butylate, 5 is the suspected intermediate. It rearranges to the phenol 6 when heated in 1,2-dichlorobenzene with one mole of ethyl-diisopropylamine and to the nitro compound 7 with an excess of potassium t-butylation in t-butanol (47-81 % yield l38). Some similar examples are shown in Table 1. 

Table 1. Phenols 3 and 6 and nitrobenzenes 4 and 7 from 2,4,6-arylated pyrylium salts and nitromethyl-arenes... 

P otassium/tert-butanol Nitrobenzenes from pyrylium salts... 

Nitro compounds, ar. Nitrobenzene, m-Dinitrobenzene o-Nitrophenol N-Nitramines Oxonium fluoroborate Trialky loxonium salt Trialky loxonium fluoroborate Pyrylium fluoroborate Carboxylic acid esters HCOOC Hs, p-Nitro-phenyl formate Isopropenyl acetate Salicylic acid esters Ethyl cyanoacetate Chloroformic acid esters Ethyl chloroformate, ClCOOC Hg-i Methyl trichloroacetate Diethyl oxalate Ethyl malonate... 

Nitro compounds, ar. Nitrobenzene, m-Dinitrobenzene o-Nitrophenol N-Nitramines Oxonium fluoroborate Trialky loxonium salt T rialky loxonium fluoroborate Pyrylium fluoroborate Carboxylic acid esters... 

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