Thiopyrylium-3-olates

Thiopyran-3(6//)-ones (133 R=H or Me) are converted into 3-hydroxythiopyrylium perchlorates (134 R = H or Me) by triphenylmethyl perchlorate. Deprotonation of these salts (134) by tertiary base did not liberate the thiopyrylium-3-olates (132) but instead gave dimeric products (135 and 136) in high yield. Evidence for the transient formation of the mesomeric betaines (132) is provided by the appearance of a greenish-yellow coloration which rapidly fades. The parent betaine (132 R = H) gave exclusively the endo dimer (136 R = H) (70%). In addition to the endo dimer (136 R = Me) (75%), the 5-methyl derivative (132 R = Me) also gave a small amount of the exo dimer (135 R = Me) (6%). The dimerization (132 136) appears to be rapid and irreversible Attempts to trap the... 

Calculations of the structure of the mesoionic thiopyrylium-3-olate 74 suggest that the C-S bond lengths are similar to those in the pyrylium cation at ca. 168 pm, perhaps supporting the fully charge-separated betaine structure. However, the charge at oxygen is closer to 0.5 than the 1.0 expected for such a structure. Furthermore, the nucleus-independent chemical shift value is appreciably lower than that for the thiopyrylium cation. These data point toward an ylidic structure with an acceptor moiety rather than an aromatic cation and an exocyclic oxyanion <2002IJQ(90)1055>. 

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