Triphenylpyrylium Tetrafluoroborate

A series of new iV-(pyridylmethyl)azinium salts were synthesized fi om 2,4,6-triphenylpyrylium tetrafluoroborate <95T(51)12425>. 

In a 1-1. three-necked flask equipped with a mechanical stirrer, a reflux condenser, and a dropping funnel are placed 119 g. (0.30 mole) of 2,4,6-triphenylpyrylium tetrafluoroborate (Note 1), 21 ml. (24 g., 0.39 mole) of nitromethane (Note 2), and 350 ml. of absolute ethanol (Note 3). Triethylamine (70 ml., 51 g.) (Note 4) is added rapidly from the dropping funnel to the well-stirred suspension. The reaction mixture becomes reddish brown immediately, and the solid dissolves. After all the triethylamine has been added, the mixture is heated under reflux for 3 hours, cooled, and allowed to stand overnight in a refrigerator. The crystalline product that separates is collected on a Buchner funnel and washed with two 50-ml. portions of ice-cold methanol. The product (75-80 g. m.p. 142-144°) is recrystallized from 200-250 ml. of glacial acetic acid to yield 70-75 g. (67-71%) of 2,4,6-tri-phenylnitrobenzene as slightly yellow crystals, m.p. 144-145° (Note 5). 

The 2,4,6-triphenylpyrylium tetrafluoroborate resulting from this procedure is sufficiently pure for use in the preparation of 2,4,6-triphenylnitrobenzene. ... 

Triphenylnitrobenzene may be prepared by direct nitration of 1,3,5-triphenylbenzene and by the reaction of 2,4,6-triphenylpyrylium tetrafluoroborate with nitromethane. The present procedure is an adaptation of the latter method. 

The present procedure is an improved modification of that described by Balaban for the corresponding perchlorate. 2,4,6-Triphenylpyrylium tetrafluoroborate has also been prepared from the corresponding tetrachloroferrate with fiuoboric acid, from acetophenone and boron trifluoride, and from acetophenone, benzaldehyde, and boron trifluoride etherate. Additional methods for the preparation of pyrylium salts have been reviewed. ... 

Triphenylpyrylium tetrafluoroborate is a versatile and useful stable starting material. Its reaction with nitromethane under basic conditions has made 2,4,6-triphenylnitrobenzene easily available. In addition, pyrylium salts are readily converted to a variety of pyridine derivatives i i . 20 including alkyl- and arylpyridinium salts, to thiopyrylium salts," and to substituted azulenes. ... 

The reaction is thought to proceed by a radical chain mechanism (SRJV1), probably first involving a charge transfer complex between cationic species (3) and the anionic species (4). The illustrative example is the synthesis of 2-phenylnitro-ethane from nitromethane and 1-benzyl-2,4,6-triphenylpyridinium tetrafluoroborate (Expt 5.192) this latter compound is prepared from benzylamine and 2,4,6-triphenylpyrylium tetrafluoroborate (Expt 8.31). 

The retrosynthetic analysis of 2,4,6-triphenylpyrylium tetrafluoroborate (86), involving an initial reduction followed by a disconnection of one carbon-oxygen bond (cf. disconnection of 2,5-dimethylfuran, Section 8.3.1, p. 1146), reveals the substituted 1,5-dicarbonyl compound (89). Further rational disconnection then reveals acetophenone and l,3-diphenylprop-2-en-l-one (chalcone) clearly the latter may originate from acetophenone and benzaldehyde (cf. Section 6.12.2, p. 1032). 

To a mixture of acetophenone (60 g, 0.5 mol) and benzaldehyde (26.5 g, 0.25 mol) is added, with stirring, boron trifluoride-etherate (84 g, 0.65 mol) the mixture becomes coloured and the temperature rises, often to about 70 °C. On maintaining the temperature at 100 °C for 2 hours ether distils from the reaction mixture. The product of the reaction is a viscous brown oil which must be dissolved in acetone (300 ml) and the tetrafluoroborate salt precipitated by the addition of ether (3 litres). 2,4,6-Triphenylpyrylium tetrafluoroborate (yield 40%) is obtained as a yellow crystalline solid by filtration and recrystallisation from acetone m.p. 253-255 °C. 

Nitrosonium terafluoroborate initiates the cation radical-mediated imino-Diels-Alder reaction of IV-arylimines with IV-vinylpyrrolidinones to give cw-4-(2-oxopyr- (g) rolidin-l-yl)tetrahydroquinolines.175 Also, 2,4,6-triphenylpyrylium tetrafluoroborate catalyses the Diels-Alder addition of IV-arylimines with IV-vinylpyrrolidinone and IV-vinylcarbazole to yield the corresponding 2-oxopyrrolidin-l-yl- and carbazole-9-yl-tetrahydroquinolines.176 The tricyclic core (151) of the batzelladine alkaloids has been prepared by a diastereoselective 4 + 2-annelation of the vinylcarbodiimide (150)... 

Significant degradation of compounds can also be obtained in the presence of electron-acceptor sensitizers. In a very recent example, triadimenol, a systemic pesticide widely applied in horticulture and viticulture that is very difficult to degrade by direct UV photolysis, could be significantly decomposed in the presence of electron acceptors such as 9,10-dicyanoanthracene or 2,4,6-triphenylpyrylium tetrafluoroborate. Decomposition was accelerated by the presence of oxygen [17]. 

A Ci-symmetric 1,2,3,4-adduct with a Cs-symmetric bridge102 between C(2) and C(3) (( )-167, Figure 1.39) has been reported as minor product resulting from the reaction between 2-methylpropan-2-ol and C6o +, generated by irradiation in the presence of the PET-(photoinduced electron transfer) sensitizer 2,4,6-triphenylpyrylium tetrafluoroborate.323 Its formation can be understood in terms of a further transformation of the initially formed monoadduct l-([60]fullerenc-l (2//)-yl)-2-mcthylpropan-2-ol. 

Mukai and his co-workers have studied the 2,4,6-triphenylpyrylium tetrafluoroborate-sensitized ring-opening of the epoxides (38) and (39). The reactions exclusively yield products (40) and (41)-the result of C-O bond... 

Direct irradiation of the arylalkenes (11) results in their conversion into the cyclic ethers (12) and (13). These cyclizations are the result of proton transfer within the excited state and cyclization of the resultant zwitterion. Using 2,4,6-triphenylpyrylium tetrafluoroborate as an electron-accepting sensitizer the compounds (11, = 1, R = CH3) and (11, n = 1, R = CH3O) undergo oxidative cleavage even when the reactions are carried out under argon. ... 

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