Benzo pyrylium salts dimerization

The unsubstituted benzo[c]pyrylium-4-oxide 19 was formed from iso-chroman-4-one derivatives 17 [76ACS(B)619] or from 18 (84CC702). All attempts to isolate oxide 19, for instance as a fluoroboric acid salt, failed because it reacted further by polymerization and dimerization [76ACS(B)619]. Even in substituted systems 15, in spite of added stabili-... 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

The reaction of 4-1 dimerization is similar to the primary step of recycli-zation of 2-benzopyrylium salts in acidic nucleophilic media (cf. Section III,C,4,b,i), but the reactive electrophile is the initial cation in this case, and not a proton. Probably for this reason, the 4-1 dimerization of 2-benzopyrylium perchlorates is not observed in acidic nucleophilic media, in contrast to a-1 dimerization (cf. Section III,F,2,b). At the same time, the scope of 4-1 dimerizations is less restricted in terms of structural requirements for 2-benzopyrylium salts in comparison with a a-1 dimerization. Thus, in the latter case, the presence of a methyl group in position 1 of benzo[c]pyrylium cation is compulsory, whereas for 4-1 dimerizations, the nature of the substituent in this position may be different, leading to a variety of 4-1 dimers and, as a consequence, to a wide variety of their transformations. 

At the same time, dimerization of one-electron reduction product was indicated by oscillopolarographic studies in acid for l-R-3,4-diphenyl-6,7-dimethoxybenzo[c]pyrylium perchlorates (R = Me, Et, Ph) (76MI1). As was noted in Section III,F, 1, the benzo[c]pyrylium cation 261, unsubstituted in position 1, undergoes l, 1 -dimerization via the postulated intermediate radical, and the bisisochromene 263 thus formed is readily converted into the initial monomeric salt, but not into the corresponding biscation on treatment with chemical electron acceptors (76KGS999). 

Under the action of Ph3P/pyridine in an atmosphere of oxygen, tellura-pyrylium and tellurachromylium salts undergo oxidative dimerization, affording l,l-dioxo(tellurapyranylidene)tellurapyrans 113 and 1,1 -dioxo(benzo[6]pyranilidene)tellurapyrans 114, the yields of each compound approaching 25% (87JOC2123). 

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