Pyrylium and Thiopyrylium Ions

Fig. I. Net charges at heteroatom and CH fragments of pyrylium and thiopyrylium ions (see text for further explanations).

The values of of some 2- and 4-(p-phenyl-substituted) pyrylium and thiopyrylium ions have been correlated with Hammett substituent... 

Heteroaromatic cyclic cations like pyrylium and thiopyrylium ions (9.22, X = O or S, respectively R = H or alkyl) react with diazocarbonyl and diazophosphoryl compounds in the 2-, 4- and 6-positions if these are not substituted (R = H). Triethylamine is necessary for removal of the proton in one of these positions, to form the 4/f-pyran or -thiopyran (9.23 9-16), as found by the group of Regitz (Regitz and Khbeis, 1984 Regitz et al, 1985). 

The chemistry of pyrylium and thiopyrylium ions is largely based on the sensitivity to nucleophiles. In this sense, they resemble pyridinium ions. The pyrylium ion is more reactive, because oxygen is more electronegative than nitrogen. The addition of nucleophiles such as alkyl lithiums, NaCN, NaOH, and so on, occurs readily and leads initially to pyran derivatives. Usually, these adducts undergo ring opening to dienals (Scheme 6.29), which are valuable in the synthesis of other compounds. 

The presence of a 4-(4-dimethylaminophenyl) group enhances the visible absorption differences between the pyrylium and thiopyrylium salt analogues. Thus, on reaction of 625 with hydrosulfide ion a change in hue from magenta (540 nm) to blue (580 nm) is observed this color change serves as the basis for a new chemodosimeter selective for hydrosulfide (Scheme 240) <2003JA9000>. 

A positive charge facilitates attack by nucleophilic reagents at positions or to the heteroatom. Amines, hydroxide, alkoxide, sulfide, cyanide and borohydride ions, certain carbanions, and in some cases chloride ions react with pyridinium, pyrylium, and thiopyrylium cations under mild conditions to give initial adducts of types 12 and 13. 

The addition of methoxide ion to pyrylium and thiopyrylium cations has been studied in various solvents (80JOC5160). Kinetic and thermodynamic regioselectivities for the methoxide addition have been obtained in methanol at -40 and 25°C, respectively [86JCS(P2)271]. 

The question whether d-orbitals play an important role in the ground state bonding of thiopyrylium ion has aroused much controversy. Palmer and Findlay, using a nonempirical method involving linear combination of gaussian orbitals, concluded that sulfur 3d orbitals appear to behave as polarization functions rather than bonding orbitals in the normal chemical sense, and therefore, they are used only to a trivial extent (72TL4165). In contrast with this view, Yoshida and co-workers pointed out that the H NMR spectra of thiopyrylium, pyrylium, and N-ethylpyridinium provide clear evidence of 3rf-orbital participation in the former cation, as illustrated by resonance hybrids 7a and 7b. To support their view, they... 

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). 

Wavelengths of the fluorescence maximum as well as fluorescence quantum yields in CHjClj have been reported for the couples of cations 8, 9 and 17, 18 (75MI1). This study also indicates that thiopyrylium ions are less fluorescent, in terms of quantum yield, than pyrylium cations. Fluorescence and phosphorescence spectra of 9 have been discussed also in relation to the formation of charge-transfer complexes (74BCJ442). 

As in five-membered heterocycles, ring strain in six-membered heterocycles is of little or no importance. Pyran and thiine (thiopyran), with an oxygen or sulfur atom, respectively, and pyridine, with a nitrogen atom, are the parent compounds of six-membered neutral heterocycles with one heteroatom and the maximum number of noncumulative double bonds. In contrast to pyran and thiine, pyridine exists as a cyclic conjugated system. However, by (formal) abstraction of a hydride ion, both pyran and thiine can be converted into the corresponding cyclic conjugated cations, i.e. the pyrylium and the thiinium ions (thiopyrylium ion). 

S, or NR. In the transition state to C=C rotation, pyrylium, thiopyrylium, or pyridinium ions are formed, which should cause a considerable lowering of the C=C barriers compared to those in systems lacking this delocalization possibility. This is clearly demonstrated by the fulvenes 64 and (5 (57). The barrier... 

Disproportionation has been observed frequently with thiopyrans and rarely with 4//-pyrans, and all cases involve a tetragonal carbon center (position 2 or 4) bearing at least one C—H bond. Some molecules of the substrate are aromatized to corresponding thiopyrylium or pyrylium ions and others reduced to dihydro or tetrahydro products. The relative abilities of pyrans and thiopyrans to disproportionate were interpreted within a proposed hydride transfer mechanism by a CNDO/2 method.45... 

Early work assigning and contrasting the 13C NMR spectra of thiopyrylium salts with those of seleno- and telluro-pyrylium isosteres has featured in a review <1994AHC(60)65>. 13C NMR data (Table 8) have been recorded for thiopyrylium salts with a range of different counter ions and in several solvents from these data it is evident that C-2 is least influenced by these two variables. 

In phenyl-substituted pyrylium ions, ortho protons of a- and y-phenyl groups resonate at lower fields than meta and para protons in thiopyrylium derivatives the separation between the ortho signals and the meta and para ones is lower and not always appreciable in selenopyrylium derivatives it is decidedly not significant (74UKZ287). 

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). 

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