Telluropyrylium

Doddi, G., Ercolani, G., Thiopyrylium, Selenopyrylium, and Telluropyrylium Salts, 60, 65. 

Sterically hindered seleno- and telluropyrylium salts 40 react with squaric acid 41 <1993H(35)1149> and also with 4,5-dihydroxy-4-cyclopentene-l,2,3-trione (croconic acid) 42 <2000JOC2236> in the presence of base to afford squarylium and croconate dyes 43 and 44, respectively (Equations 11 and 12). Dyes of this type have important industrial applications (Section 7.11.8.2). 

The key step in the hydrolysis of seleno- and telluropyrylium dyes involves nucleophilic addition of water to the substrate <1997JOC4692, 1998JOC5716>. This hydrolysis is discussed in Section 7.11.5.5. 

Telluroxides derived from telluropyrylium dyes can be reduced by glutathione to the parent dye (Equation 16). The reaction occurs via a two-step mechanism <1994JOC8245>. This reduction has important implications for photodynamic therapy and related oxidative chemotherapy (see Section 7.11.8). 

The hydrolysis of seleno- and telluropyrylium dyes involves nucleophilic addition of water to the substrate. a,f3-LJnsaturated selones and -tellones 60 are intermediates in this reaction (Equation 22) <1997JOC4692, 1998JOC5716>. Diketones are the primary hydrolysis products isolated in high yields from these reactions. The tellurium derivative hydrolyzes most rapidly over the pH range 3-12, with the sulfur analogue least reactive under these conditions. Seleno- and telluropyrylium dyes are important both for their biological activities and potential industrial applications (see Section 7.11.8). 

Seleno- and telluropyrylium trimethyne dyes 61 are prepared by treatment of seleno- or telluropyrylium dyes with acetic anhydride (Equation 23) <1990JA3845, 1990JME1108> [also see CHEC-II, 5.11.8], These reactions work well for the preparation of symmetrical dyes, however in attempts at the synthesis of unsymmetrical dyes, symmetrical... 

Isoseleno- and isotellurochromenes 85 can be readily converted to the corresponding 2-benzoseleno- and 2-benzo-telluropyrylium tetrafluoroborates 86 by treatment with triphenylcarbenium tetrafluoroborate (Equation 35) <2002J(P1)606>. An analogous reaction has also been used for the conversion of tellurochromenes 87 to the corresponding 1-benzotelluropyrylium salts 88 (Equation 36) <1999J(P1)1665>. When the ring substituent was methyl or isopropyl, the desired benzotelluropyrylium salts formed but could not be isolated due to elimination of a proton from the substituent. 

Pyrylium (1) and its chalcogen analogs, thiopyrylium (2), selenopyrylium (3), and telluropyrylium (4), are the parent structures of an important class of six-membered heteroaromatic cations. 

Fig. 3. Selected bond lengths (in A) and bond angles (in degrees) for a thiopyrylium-thiopyrylium (A) and a telluropyrylium-pyrylium (B) monomethine dye.

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