Chalcogenopyrylium

The initial step in chalcogenopyrylium salt hydrolysis227 involves the addition of hydroxide/water to the ring as shown in Scheme 26. Hydrolysis of the intermediate thione, selenoketone, and telluroketone irreversibly produces a 2-pentene-1,5 dione derivative. 

Infrared-absorbing dyes 88 have been prepared from croconic acid and 4-methyl-chalcogenopyrylium salts. Red shifts of 7 , are noted as the size of the heteroatom increases and range from 845 nm for (88 X = O) to 1081 nm for (88 X = Te) . 

Fig. 2 Reductive elimination of halogens from chalcogenopyrylium dyes 6-11 containing tellurium(IV) dihalide groups. 

Coulometry studies on cationic dihydroxy tellurium(IV) derivatives 52,53, and 69 indicate that a two-electron reduction occurs (2 F moP to give chalcogenopyrylium... 

Fig. 27 Two-electron reductions of cationic chalcogenopyrylium dyes with tellurium(IV) atoms in the 7r-framework.

The fact that pyrylium salts easily form anhydrobases in water over a large range of pH values, and pseudobases at pH >5, makes such salts incompatible with biomedical applications, but the highly fluorescent 2,4,6-triarylpyrylium salts with phenyl or para-anisyl groups have various applications in reprographic industries. On the other hand, chalcogenopyrylium cations with other atoms (S, Se, Te) are less... 

The bottom line on monocyclic six-membered aromatic compounds is that, so far, only benzene, the azines with one through four nitrogen atoms, phosphabenzene and arsabenzene, pyrylium, azapyrylium and chalcogenopyrylium cations (with or... 

In a spectral UV-VIS study of a series of chalcogenopyrylium dyes it has been shown that the aromaticity of the heterocycles decreases with the size of the chalcogen, whereas the bathochromic effect increases from the oxygen to tellurium [7], The aromaticity index based on data from experimentally determined bond lengths, which yield statistically evaluated bond orders, has been proposed and applied to neutral and ionic five-membered selenaheterocycles. A reasonable parallel also exists between the aromaticity index and resonance energies [37],... 

The chalcogenopyrylium ring is numbered as shown in formula 5. Positions 2 and 6 may also be denoted by a, positions 3 and 5 by jS, and position 4 by y. The anion has been left out in the formula pictures if it has no special influence on the chemical or physical properties of the chalcogenopyrylium ion. In most cases, however, the anion is a nonnucleo-philic one, such as C104 , BF4", or PFj . 

Chalcogenopyrylium ions have a marked carbocationic character illustrated by resonance structures 6b-6c, which suggest pronounced electrophilic reactivity of a and y positions. Indeed most reactions occur through... 

The absorption bands can be correlated to those of benzene (72T5873) the transition to the lowest energy level (band I in Table I) is the equivalent of the Bju - Atg transition of benzene at 256 nm ( Lj, band in Platt notation), and the much greater intensity in chalcogenopyrylium ions must depend on the lower symmetry of these compounds (point group CiJ-This transition in chalcogenopyrylium ions is A, it is allowed in... 

Long-Wavelength Absorption Maximum of Some Substituted Chalcogenopyrylium Ions"... 

Satisfactory linear relationships have been reported between the energy of the longest absorption maximum of a number of chalcogenopyrylium ions and the corresponding difference red thus suggesting that the HOMO-LUMO gap should be directly proportional to the energy of the absorption maximum (88M11). 

Information on IR absorptions of chalcogenopyrylium ions is scarce. IR spectra of unsubstituted pyrylium (1) and thiopyrylium (2) have been reported by Yoshida et al. together with a normal coordinate analysis for the in-plane and out-of-plane vibrations (74T2099). Cation 2 yields lower absorption in wave number than 1 because of the mass effect of the heteroatom. The main reason for the difference in the IR spectra between benzene and heterocycles 1 and 2 is ascribed to the contribution of the carbonium ion structures in the latter cations. This contribution is larger in 1 than in 2 because of the electronegativity of the heteroatom (Section II,A). 

NMR data for the series of 2,6-di-rerr-butyl chalcogenopyrylium ions 25-28 are summarized in Table IV. Noteworthy is the increase of V(H-3,H-4) on changing the heteroatom from selenium to tellurium. 

H Chemical Shifts" (ppm) and H, H Coupling Constants (Hz) of 2,6-Di-ier/-BUTYL Chalcogenopyrylium Hexafluorophosphates ... 

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