2,6-diphenyl pyrylium

A convenient method leading to pyrans (38) consists in the nucleophilic addition of R anions to 2,6-disubstituted pyrjdium salts, in which the y-position (secondary carbonium ion) is more reactive than the a-positions (tertiary carbonium ions), in opposition to the reactivity of 2,4,6-trisubstituted pyrylium salts.Krohnke and Dickore as well as Dimroth and WolH showed that 2,6-diphenyl-pyrylium salts add the anions R of nitromethane, 1,3-diketones, malonodinitrile, ethyl cyanoacetate, and benzoylacetonitrile. Similar reactions are known in the flavylium series. -Nonactivated R ... 

On the other hand, 1,1,1-trisubstituted alkanes behave similarly to aldehydes, yielding pyrjdium 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-triphenjdpjTylium 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. 

Tetraphenyl-4,4 -bi-4H-pyranyliden wird in einfacher Weise durch Elektro-reduktion und nachfolgende Dehydrierung von 2,6-Diphenyl-pyrylium-perchlorat in Acetonitril/Tetrabutylammoniumperchlorat zu 48% d.Th. erhalten (als Nebenprodukt fallt 15% d.Th. 1,5-Dioxo-l,5-diphenyl-pentan an)8 ... 

There is evidence that the 2H-pyrans are thermodynamically more stable than their 4//-isomers. For example, the addition of methoxide ion to 2,6-diphenyl- and 4-methoxy-2,6-diphenyl-pyrylium perchlorates (46 R = H and R = OMe) was followed by NMR spectroscopy. The formation of the 4H-pyran (47 R = H or OMe) is rapid and at — 30 °C... 

The reaction of trimethyl phosphite with sodium iodide in acetonitrile, when tried on 2,6-diphenylthiopyrylium cation (18), failed to give the desired phosphonate 323 (80JOC2453). It has been reported, however, that 2,6-diphenyl-pyrylium (17) and -thiopyrylium (18) bromides react with triethyl phosphite to give the diethyl phosphonates 322 and 323, respectively (7IDOK600). These can be hydrolyzed with HCl to give the pyranyl-and thiopyranyl-phosphonic acids 326 and 327, which treated with triphe-nylmethyl perchlorate give the pyrylium- and thiopyrylium-4-ylphospho-nic acids 328 and 329. 

Bipyranylidenes may be oxidized in two single-electron steps via bipyryliumyl radicals (47) to bipyrylium ions (48).Conversely, bipyrylium ions prepared by alternative routes may be reduced to bipyryliumyl radi-cals. The radical 47 (R = /Bu) has been characterized by ESR and the electrochemical characteristics of 47 (R = Ph, tBu, and others) have been determined.Decarboxylation of 4-carboxy-2,6-diphenyl-pyrylium ion in the presence of Vaska s compound [(Ph3P)2lrClCO] led to 47 (R = Ph) as the principal product, but whether the reaction involved... 

Eine ahnliche Kondensation soil auch mit 2,6-Diphenyl-pyrylium-perchlorat gelingen... 

Stetter and Reischl239 described the preparation of 2,6-diphenyl-pyrylium perchlorate (57) from l,5-diphenyl-2-penten-4-yn-l-one (56a) and concentrated sulfuric acid in the cold, followed by perchloric acid, in 60% yield. The process may involve hydration to a 1,5-enedione, or an ion 56b related to 52, and valence tautomerism. 

Figure 7.2 depicts simulated voltammetry for the reduction of 2,6-diphenyl pyrylium (DPP" ") in an acetonitrile solution at various scan rates, ranging from 10 to 10 V s This system is described as being an EC2 process. 

Fig. 7.2 The simulated voltammetry for the reduction of 2,6-diphenyl pyrylium at various scan rates.

To establish the experimental achievability of this target the dimerization mechanism of 2,6-diphenyl-pyrylium radicals electrogenerated by reduction of the corresponding cation, in eqn.4, was selected. The mechanism in eqn.4 was first postulated for the zinc powder reduction of pyrylium cations in acetonitrile and further confirmed by... 

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