Pyrylium 2.4.6- triphenyl

Pyrylium perchlorate, 2,4,6-triphenyl-hydrolysis, 3, 741 nitration, 3, 649 reactions with alkali, 3, 652 with methoxides, 3, 652 with piperidine, 3, 655 synthesis, 3, 869... 

A 2,4)6-trisubstituted 2H or 4/f) pyran (38, R = R = Ph) was reported to result in low yield by catalytic reduction of 2,4,6-triphenyl-pyrylium salts by oxidation or by treatment with concentrated sulfuric acid it regenerated the triphenylpyrylium cation. There was no subsequent confirmation of this reaction. The reduction of pyrylium salts with sodium borohydride affords 1,5-diones by way of 4H-pyrans and 2,4-dien-l-ones by way of 2H-pyrans. ... 

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. ... 

Although cyclic azoalkanes are well known as biradical precursors [159] they have been used as 1,2- and 1,3-radical cation precursors only recently [160-164]. Apart from the rearrangement products bicyclopentane 161 and cyclopentene 163, the PET-oxidation of bicyclic azoalkane 158 yields mostly unsaturated spirocyclic products [165]. Common sensitizers are triphenyl-pyrylium tetrafluoroborate and 9,10-dicyanoanthracene with biphenyl as a cosensitizer. The ethers 164 and 165 represent trapping products of the proposed 1,2-radical cation 162. Comparison of the PET chemistry of the azoalkane 158 and the corresponding bicyclopentane 161 additionally supports the notion that the non-rearranged diazenyl radical cation 159 is involved (Scheme 31). 

However, radical intermediates cannot be definitely excluded, at least not in the reaction of pyiylium salts in pyridine. Steuber showed that such pyrylium salts as 2.4.6-triphenyl-pyrylium-or 2.4.6-tri-tert-butyl-pyrylium-tetrafluoroborate can be reduced to stable pyryl radicals 32 by pyridine this reduction proceeds particularly smoothly if traces of copper powder are added. 

These three methods can be used to prepare a large number of 2.4.6-tri-or higher substituted X -phosphorins (Table 4). Markl describes only a single case (no details given) in which a pyrylium salt with an unsubstituted a-position 2.4,5-triphenylpyrylium salt is used to form a X -phosphorin which has no substituent at the 6-position 2.4.5-triphenyl-X -phosphorin (Table 2, no. 27). 

In contrast to the unsubstituted ring compounds 118 and 120,1,1-diaryl-, 1,1-dialkyl- or l-aryl-l-alkyl-X -phosphorins with three phenyl groups in positions 2,4 and 6 of the X -phosphorin ring 122 are much easier to prepare and to handle. They can be obtained either from 2.4.6-triphenyl-pyrylium salts or from 2.4.6-triphenyl-X -phosphorins. Most of these 2.4.6-tri-substituted X -phosphorins are very stable and can be isolated as well-defined crystalline compounds. They do not react with the above-mentioned cations. However, reversible protonation-deprotonation does take place in the presence of acids. 

The betaines 297 are easily prepared from a pyrylium salt and the appropriate 4-aminophenol followed by deprotonation. Typically, 2,4,6-triphenyl-pyridinium iodide and 4-aminophenol give the blue-black betaine (297 R = R = Ph, R = H) which also occurs as a red hexahydrate. With methyl iodide this compound gives 7V-(p-methoxyphenyl)-2,4,6-triphenyl-pyridinium iodide. 10,221... 

In contrast to 4//-pyrans, the 2//-pyrans have been rarely reported to be aromatized to pyrylium salts. 2,6-Dimethyl-4-methoxy-2//-pyran (323) was easily converted to the corresponding perchlorate 373 with perchloric acid.318,319 A similar oxidation was reported for 2,4,6-triphenyl-2//-pyran.217 The formation of pyrylium salts 375 and 377 from 374 and hydrogen chloride261 or from 376 and perchloric acid181 are not oxidations. 

An attempt to prepare 2-(2-nitrophenyl)-4,6-diphenylpyrylium from l,3-diphenylprop-2-en-1 -one and 2-nitroacetophenone gave only 2,4,6-triphenylpyrylium (58BSF1458). Similarly, substantial formation of this symmetrical pyrylium salt was observed during syntheses of unsymmetrically substituted salts. Thus, pinacolone and chalcone afforded both 2-f-butyl-4,6-diphenylpyrylium and the 2,4,6-triphenyl derivative. The latter product is considered to arise from a retro-aldol reaction of the enone into a mixture of benzaldehyde and acetophenone the latter reacts with unchanged chalcone to give the unrequired salt (80T679). 

The outline procedure involves the initial reaction of the 2,4,6-triphenyl-pyrylium halide with the primary amine to yield the corresponding 2,4,6-triphenylpyridinium halide (see Section 8.4.1, and also Section 5.15.3, p. 768) this reaction proceeds either at room temperature in a suitable solvent, or more efficiently under reflux in benzene with azeotropic removal of water. Pyrolysis of the pyridinium halide under controlled conditions then yields the alkyl (or aralkyl) halide in good yield. The mechanism of the reaction in this case is probably of the Sn2 type. 

All these various pyrylium salts are converted by ammonia, or alkyl, arylkyl or aryl primary amines into the corresponding pyridinium salts.25 The probable reaction sequence is given in the illustrative example 1-benzyl-2,4,6-triphenyl-pyridinium tetrafluoroborate26 (Expt 8.31). 

NMA+) and 2,4,6-triphenyl-pyrylium tetrafluoroborate (TPP+) in the presence of biphenyl as cosensitizer were suitable for this reaction [174], The assumed mechanism of formation of do by this cosensitization is shown in Scheme 7. Reaction of do with H-donors such as te/t-butylmethylether, propionaldehyde and alcohols results in the formation of 1 1 adducts, the 1-substituted 1,2-dihydro-[60]fullerenes. Product structure support a H-abstraction process [212,213] rather than nucleophilic addition. In Scheme 8, the general formation of 1-substituted l,2-dihydro-[60]fullerenes is shown. Selected examples of the products obtained by this method are summarized in Table 10. 

As expected on the basis of the higher stability of sulfonium salts as compared with oxonium salts, thiopyrylium cations are more stable and less reactive than pyrylium cations hydride-accepting ability decreases in the order pyrylium> selenopyrylium> thiopyrylium [95], 2,4,6-Triphenyl-thiopyrylium salts react with ammonia and primary alkylamines forming the corresponding pyridine and pyridinium salts, respectively, but they do not react with aniline or its derivatives [96-99], As described below, the Se- or Te-analogs are less stable than the thiopyrylium salts. 

Zhang, W., Guo, Y., Liu, Z., Jin,X., Yang, L., and Liu, Z.-L. (2005) Photochemically catalysed Diels-Alder reaction of arylimines with Y-vi nyl pyrrol idi none and N-vinylcarbazole by 2,4,6-triphenyl-pyrylium salt synthesis of 4-heterocycle-... 

When R is primary alkyl, the second-order rate constant k2 is obtained by taking the slope of kobs vs. concentration of the nucleophile. The plot passes through the origin, indicating a pure SN2 mechanism without SN1 participation. The reference pyridinium ion is the 2,4,6-triphenyl derivative (because pyrylium precursors with phenyl substituents are more easily prepared) (82AHC(Suppl 2)1) but numerous other substituents have been introduced into the ring. Rate constant values reported in Table XIX, where release of steric strain has a major influence, are in agreement with the role of structural factors discussed in Section IV,A. 

The electron-impact mass spectra of bromides, iodides, and fluorobo-rates of the 2,4,6-triphenyl-substituted cations 8 and 9 have the base peak at the mass number of the cation (74OMS80). No molecular ion peak of an adduct between the cation and the anion has been found the fluoroborates show also weak peaks with the elemental composition of an adduct between the cation and F". On the contrary, the spectra of perchlorates do not show the peaks at the mass number of the cation but peaks indicating the addition of an oxygen atom and the removal of a hydrogen atom. From ionization potential measurements it has been shown that the bromides, iodides, and fluoroborates of 8 and 9 are thermally reduced in the mass spectrometer to volatile free radicals 50 and 51 prior to evaporation, presumably with concomitant oxidation of the anion. In the presence of a nonoxidizable anion, e.g., perchlorate, reduction of the cations to free radicals does not take place. Interestingly, the order of ionization potentials of the radicals, 50 < 51, indicates that the LUMO energy level of pyrylium is higher than that of thiopyrylium, consistent with electrochemical studies (Section II,D). 

Besides saturated 1,5-diketones, unsaturated 1,5-diketones can also, in some cases, be converted into thiopyrylium salts. The reaction of aryl substituted 2,4-dichloro-2-pentene-1,5-diones (87) with HjS and HCIO4 in a mixture of AcOH and AC2O leads to the formation of 3-chlorothiopyrylium perchlorates 88-90. It should be noted that under the same conditions l,3,5-triphenyl-2-pentene-l,5-dione is converted quantitatively into the corresponding pyrylium salt 8. The pentenediones not containing chlorine atoms in the molecule evidently do not react with H2S under the conditions of acid catalysis as a result of the fact that the cyclization rate for them significantly exceeds the addition rate of H2S (90ZOR1904). 

Nitriles can condense cyclically with aldehydes, in the presence of acid, to yield hexahydro-.s-triazines e.g., propionitrile and formaldehyde give the hexahydro-l,3,5-tripropionyl-s-triazine (25b).62 When benzo-nitrile (1 mole) and benzoyl chloride (2 moles) were heated to 150° with zinc (or stannic) chloride, a high yield of 2,4,6-triphenyl-3,5-diaza-pyrylium salt (46) was formed,623 which was converted into 2,4,6-triphenyl-s-triazine by ammonia. 

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. 

The pyrylium-3-olates (65) form salts under acidic conditions. The triphenyl compound (65 R = R = R" = Ph, R = H) is oxidized by air to the butenolide 73, which is also formed when a solution of the same precursor is photolyzed (A = 3660 A) in the presence of a benzophenone sensitizer. This oxidative rearrangement (65 R = R = R" = Ph, R = H - 73) has been rationalized in terms of the triketone intermediate 71 cyclizing to the zwitterion 72 followed by rearrangement. ... 

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