2.4.6- Triphenylpyrylium perchlorate

Thefree-energy changes (AG) for ET from 90a to DCA, MB and 2,4,6-triphenylpyrylium perchlorate (TPPY+) are —18.4, —10.6 and —32.5 kcalmoD , respectively, indicative of exothermic electron transfer. The rate of disappearance of 90a was enhanced by addition ofMg(C104)2 (Table 20). 

Triphenyl-l,2-oxazepinium perchlorates, a new class of seven-membered cations with eight tt-electrons, have been prepared by the reaction of 2,4,6-triphenylpyrylium perchlorate with nitrones (81ZOR881). 

Caution 1 A serious explosion of the perchlorate has been reported. The perchlorate is almost as sensitive to friction as mercury fulminate. However, the (Corresponding tetrafluoroborate is safe to handle and is chemically similar. 2,4,6-Triphenylpyrylium perchlorate also has explosive properties. 

The distribution of charge in the resonance forms (28)-(31) suggests that nucleophiles may attack at C-2, C-4 or C-6 (or at C-2 or C-4 in 1-benzopyrylium cations, and at C-l, C-3 or C-4a in 2-benzopyrylium ions) but they most commonly add at C-2 for example, attack by cyanide ion gives a 2//-pyran (37) which exists partly or wholly as the acyclic isomer (38). Steric and electronic effects in the reactants probably have a role in determining the course of the reaction of trisubstituted pyrylium salts with nucleophiles. A mixture of both 2H- and 4H-pyrans is sometimes produced, for example, from methoxide ion and 2,4,6-triphenylpyrylium perchlorate (39) no acyclic product was detected in this reaction... 

Table 1 Influence of pH on the Basic Hydrolysis of 2,4,6-Triphenylpyrylium Perchlorate...

The formation of different products is also observed in the diacylation of 2-methyl-1-phenylbut-l-ene (63TL91). When either the alkene or 2-methyl-1-phenylpropan-2-ol is treated with acetic anhydride in perchloric acid, 2,4,6-trimethyl-3-phenylpyrylium perchlorate is formed. However, the use of acetyl chloride and aluminum chloride as the acylating agent leads to the 4-benzyl-2,6-dimethylpyrylium salt, whilst benzoyl chloride gives 4-methyl-2,4,6-triphenylpyrylium perchlorate under similar conditions. 

Mechanistically, the reaction proceeds through Michael addition of the aryl methyl ketone to the enone. Hydride abstraction and cyclization then yield the pyrylium salt. The route is capable of considerable diversification and therefore has much potential. The detailed method for the synthesis of 2,4,6-triphenylpyrylium perchlorate is worthy of mention... 

Treatment of 2,4,6-triphenylpyrylium perchlorate with 3-benzo[i>]-thienylmethylmagnesium bromide affords the 4//-pyran (155),460 which undergoes allylic rearrangement in diethylene glycol in the presence of sodium to give the 2//-pyran (156).461 Under the same reaction conditions 156 is further converted by an intramolecular aldol condensation into 3-(2,4,6-triphenylphenyl)benzo[6]thiophene. 

Triphenylpyrylium perchlorate reacts with fuming nitric acid at 0°C to produce a tris(nitrophenyl) derivative in which the 2- and 6-phenyl groups are nitrated in the 3-position and the 4-phenyl group is nitrated in the 4-position. The positions of the nitration were established by nitration of the unambiguously synthesized mononitro derivatives (32JCS2894). 

On the other hand, 1,1,1-trisubstituted alkanes behave similarly to aldehydes, yielding pjn ylium salts (128) with identical substituents in positions 2 and 6. Thus, Dorofeenko and co-workers condensed 2 moles of acetophenone with 1 mole of benzotrichloride in the presence of perchloric acid obtaining 2,4,6-triphenylpyrylium perchlorate with 1 mole of ethyl orthoformate, they obtained 2,6-diphenyl-pyrylium perchlorate (57) from o-hydroxyacetophenone, ortho-formic ester and perchloric acid, 4-ethoxybenzopyrylium perchlorate was formed. ... 

Devise a mechanism to explain the formation of 1,3,5-triphenylbenzene from reaction of 2,4,6-triphenylpyrylium perchlorate on reaction with 2 mol equivalents of PhsP = CH2. 

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