Thiopyrylium ions

A number of examples of the typical thiopyran transformations to corresponding thiopyrylium ions and dihydro or tetrahydro derivatives have been described in the last decade. Similar behavior was found with a selenopyran. 

MO calculations were performed rarely for thiopyrans except for an MNDO study of 2 [84ZN(A)267], Charge distribution and orbital interaction concepts were explored in an interpretation of model reactions of thiopyrylium ions with azides giving 68 and the corresponding 3,5-unsub-stituted thiopyrans (84T3549) as well as for the equilibria between 1 and 2 or 167 and 168, respectively (92JOC4431). 

Thiopyrylium ions have been shown to behave as effective dienophiles. Shimizu and co-workers demonstrated that 2-benzothiopyrylium salts undergo [4+2+]-type cationic polar cycloadditions with various 1,3-dienes yield benzo-fused bicyclic sulfonium salts <1994J(P1)3129> (see Section 8.32.2.1.2 of CHEC-II(1996)). However, in this study the authors observed that cycloaddition of 2-benzothiopyrylium salt 40 with 2,3-dimethylbuta-l,3-diene 41a with prolonged reaction times caused a decrease in yield of the cycloadduct 22a (see entries 1 and 2 in Table 5 and Equation 10). In fact, stirring of the reaction mixture for 30 h did not afford the cycloadduct at all, see entry 3, but gave an undetermined complex mixture. 

Simple 4//-thiopyran (7)5,90,36 or its monosubstituted derivatives7 were oxidized to corresponding thiopyrylium ions of type 221 with several agents. A remarkable difference was observed in the behavior of 7 toward halogens chlorine and iodine caused aromatization, whereas bromine only added to the substrate.361... 

Analogous fragmentations to thiopyrylium ions constitute a typical model for various 2//-thiopyran derivatives155,304,345,464 along with other ions.155,299,304,345,465 A chromium complex was also measured.4542... 

Equally interesting is the situation in the second class of compounds studied (analogues of non-alternant hydrocarbons), which is best divided into two sub-groups analogues of the tropylium ion and analogues of azulene. The empirical correlation of experimental and theoretical excitation energies studied requires a further subdivision into compounds with one heteroatom (e.g. thiopyrylium ion) and two heteroatoms, either adjacent (e.g., 1,2-dithiolium ion) or non-adjacent (e.g., 1,3-dithiolium ion). Experimental and theoretical data are presented in Table VII. Table VIII summarizes data for the derivatives of dithiolia. Figure 15 shows the absorption curves of 1-benzo-... 

JA5630). The apparent increased stability of the sulfur-containing cations has been ascribed to a lower stability of their corresponding alcohols. The obtained pKr+ value of the unsubstituted thiopyrylium ion, however, has been questioned (Section IV,C,3). 

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... 

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... 

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). 

The unsubstituted thiopyrylium ion (2) has been found to form CT complexes in CHjCN with both olefins and aromatic hydrocarbons (72CL17 75BCJ1519). Two CT absorption bands have been observed in the former case, and one in the latter. The slope obtained by the plot of the CT transition energies vs the ionization potentials of donors is 0.27 for the olefin complexes and 1.04 for the aromatic hydrocarbon complexes. These slopes suggest that 2 interacts with olefins more strongly than with aromatic hydrocarbons. Strong interactions in the olefin complexes would manifest themselves also in the appearance of two CT bands. These have been ascribed to electronic transitions from the HOMO of the olefin donor to the lowest and the second lowest vacant orbital of 2. The CT absorption frequencies of the complexes of 2 with olefins and aromatic hydrocarbons have been used to calculate their heat of formation by an empirical relation (81MI4). 

Electrochemical oxidation of thiopyrans 74 and 75 to the corresponding thiopyrylium ions proceeds by successive losses of an electron, a proton, and another electron (84KGS318). The same behavior has been shown by 1,2,3,4,5,6,7,8-octahydrothio- and seleno-xanthenes (91KGS47). Electrochemical reduction of 1,2,3,4,5,6,7,8-octahydrothio- and seleno-xanthylium cations has been also investigated (9IKGS47). 

A useful reaction is the treatment of thiopyran-2-thiones with peracetic acid to form 2-unsubstituted thiopyrylium ions. Although the reaction actually involves reduction of the ring system, the exocyclic sulfur atom is oxidized and eliminated as sulfate (70CJC3388 74CJC3021). For example, 2,4-diphenylthiopyrylium ion (154) has been prepared from the thiopyran-2-thione 164. The reaction does not succeed with thiopyran-4-thiones thus reaction of 2,6-diphenyl-4//-thiopyran-4-thione [122 (R = = Ph)] with peracetic acid gives the thiopyran-4-one 140... 

The first preparation of the unsubstituted thiopyrylium ion (2), illustrated in Scheme 11, has been developed by Pettit starting from thiophene (60TL11). 

Although, normally, )8-alkyl-substituted thiopyrylium salts do not condense with either aldehydes or chalcogenopyranones, it has been reported that the 5//-indeno[2,l-h]thiopyrylium ion 237 reacts with 2,6-diphenyl-4//-pyran-4-one to yield the condensation product 238 (77JHC119). 

Diphenyl-4-methylthiopyrylium (204) and 4,6-diphenyl-2-methyl-thiopyrylium (109) ions have been brominated in AcOH containing Hg(OAc)2 affording the corresponding thiopyrylium ions in which the methyl group is converted into the dibromomethyl group (77URP546614). 

Thiols react with halogenothiopyrylium salts to give the product of substitution. Thus 4-chlorothiopyrylium reacts with thiophenol to give the 4-thiophenoxythiopyrylium ion (75T2669). The reactivity of 4-chlorothiopyrylium and N-methyl-4-chloropyridinium toward thiophenol has been compared by a competitive experiment. The thiopyrylium ion was demonstrated to be ca. 4 times more reactive than the pyridin-ium ion. 

Some intramolecular additions to thiopyrylium ions involving sulfur as nucleophilic center have been reported in Sections IV,B,1 and 2. 

A number of 2,4,6-triarylthiopyrylium salts reacts with dialkylamines in diethyl ether to give stable crystalline 2-dialkylamino-2//-thiopyrans (83ZC144 84EGP212964 86JPR567). Reactions of these with a number of nucleophiles and electrophiles can be easily accounted for by considering them in equilibrium with the parent thiopyrylium ion and amine (86JPR567). 

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