From pyrylium cations

Phosphorus. Substituted phosphorus analogues of pyridine (phosphinines, A3-phosphabenzenes, also called phosphonins or phosphorins) were first prepared by Markl starting from pyrylium cations their chemical properties suggest that their aromaticity is lower than that of pyridine (e.g. phosphinine 186, Scheme 72).230-232 Molecular calculations for other six-membered jr-systems with planar tricoordinate phosphorus, such as phospininines 186 and 187, have evidenced their aromaticity (Scheme 72).156... 

Related reactions include preparations of furans from pyrylium cations (Section 3.2.1.6.3.v). 1,4-Dithiins (56JA850) and 1,2-dithiins (67AG(E)698) readily lose sulfur on heating, yielding the corresponding thiophene (191 — 192 193 — 194). 

Nonempirical MO calculations were also used to predict the IR absorption spectrum of unsubstituted 2//-pyran (1) (90CCC10). Some MNDO and AMI studies were carried out to obtain a thermodynamic interpretation of the regioselectivity of the formation of pyrans from pyrylium cations... 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

The pyrylocyanine obtained by Strzelecka and Simalty from 2,6-diphenyl-4-phenacylpyrylium and orthoformic ester (cf." Section II, B, 1, a) has the structure of a pseudo base. Accordingly, its protonation is accompanied by dehydration leading to a triple pyrylium cation (see Scheme 4). 

The two features of interest in the structure of 6-(2-hydroxyprop-l-enyl)-2,4-dimethyl-pyrylium cation (179) are the enolic nature of the 6-substituent and the shortness of the C(6)—C(l ) bond. The latter is considered to indicate a significant contribution from the methylenepyran structure (180) (75CC284). 

For explanation of experimental results and for correlation of charge densities with NMR data, semiempirical quantum-chemical calculations of benzo[c]pyrylium cation have been employed. Interestingly, the first calculation of 1,3-dimethyl-benzo[r]pyrylium cation by the simple linear combination of atomic orbitals/molecular orbital (LCAO/MO) method (70KGS1308) revealed a preference for the resonance from a in which the value of the charge density at C was three times as much as at C3. 

One hydrogen atom has to be removed from the C4 position of pyran 9.11 to produce the pyrylium cation, but it is important to realise that the hydrogen atom is lost not as a proton but as a negatively-charged hydride ion. The process is therefore an oxidation of pyran 9.11. 

The presence of 1 equivalent of TPP1 OTf- with the chromium anion TpCr(CO)3- as the tetrabutylammonium salt in dichloromethane results in the loss of the carbonyl bands of the anion at 1890 and 1740 cm-1. Their complete replacement by the sharps, band at 2018 cm-1 and the broad E band (1898 and 1838 cm-1) of the 17-electron radical TpCr(CO)3- indicates that the ion-pair annihilation proceeds to completion. Variation of the pyrylium cation, by the replacement of TPP+ with a weaker acceptor such as tri-p-anisylpyrylium triflate (TAP+ OTf-), consistently results in lower conversions of the carbonylmetal anions. For example, the treatment of TpMo(CO)3 with the TAP+ salt leads to a light red solution of TAP (Am 560 nm) (92) and a greatly diminished concentration of TpMo(CO)3- as judged by the reduced carbonyl absorbances in comparison with that obtained from TPP+ at the same concentration. Even with this weaker acceptor cation, however, the strong chromium anionic donor TpCr(CO)3- is completely oxidized by 1 equivalent of TAP+ to form TpCr(CO)3- in essentially quantitative yields. 

Although the constant K includes a contribution from the reversible dimerization of the pyranyl radical in Eq. (41), the trend generally shows an increase with acceptor strength ( %) of the pyrylium cation and donor strength ( °,) of the oiganometalhc anion. 

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