4 tellurapyrylium

Reactions of l-telluracyclohexa-2,5-dienes 21 have been little studied. When heated with HPF6 in acetic acid, compounds 21 eliminate dihydrogen and convert to tellurapyrylium salts 25. The yields (73-83%) are higher than those of the same reaction with the oxygen, sulfur, and selenium analogs of 21 (88MI2). 

Treatment of heterocyclic ketones 22 with ethyl fluorosulfate leads to tellurapyrylium salts 25a (82JOC5235 87JOC2123), and interaction with Meldrum s acid produces compounds 29. 

The first compound containing the tellurapyrylium ring, 10-telluroni-aanthracene perchlorate was synthesized in 1980 (80KGS274). Before long a broad series of the derivatives of tellurapyrylium and tellurachromylium cations was described. 

Recently it was reported that the salts of tellurapyrylium 25 and tellurachromylium 60 cations can be obtained in high yields (60-85%) by heating, respectively, 4//-tellurapyranes 21 and 4//-tellurachromenes 54 with HPF6 in acetic acid (88MI2). 

The reactions of the dibenzo derivative of the tellurapyrylium cation, 10-telluroniaanthracene, were studied in considerable detail. In general, the reactions of tellurapyrylium salts are similar to those of their oxygen, sulfur, and selenium analogs. The peculiar chemical behavior caused by the presence of a tellurium center is most clearly manifested in the reaction of tellurapyrylium cations bearing dimethylamino groups with halogens. 

By coupling with bromine under mild conditions 4-(4 -dimethylamino-phenyl)tellurapyrylium borotetrafluoride 25 affords 10-Te-4 tellurane 115 in 90-97% yield (86MI2). When bromine is taken in excess, an exchange of anions (BF Brj) in 115 takes place. The structure of the compound 115 (X = Brj) has been established by X-ray analysis. 

The Te(IIH>Te(IV) oxidation reactions of tellurapyrylium dyes 126 are much faster than reactions of the carbon 8-backbone. Singlet oxygen reacts with 126 (Y = Se, Te) with a second-order rate of about 10 dm3mol s l. An extremely high rate of oxidation was detected when ozone was used as an oxidant. The large second-order rate constant... 

The reaction serves as an illustrative example of the synthetic utilization of the Te(II)—>Te(IV) two-electron shuttle. Another promising application is seen in photodynamic therapy in which tellurapyrylium dyes 119 can function as photosensitizers to produce either singlet oxygen or superoxide radical-anions (via electron transfer), thereby serving as cytotoxic agents. An important useful property of tellurapyrylium dyes is their absorbance in the near-infrared region where biological tissues are most important. 

The tellurophenopyridazine (M) decomposed under the influence of light and oxygen to 4,5-dibenz-oylpyridazine (66) via the peroxide (65) derived from Diels-Alder reaction of singlet oxygen widi (64 Scheme 24). Tellurapyrylium dyes such as (67) react with singlet oxygen within a few seconds to give dihydroxides (68 Scheme 25). ... 

---