TELLURAPYRYLIUM DYES AS CATALYSTS FOR OXIDATIONS WITH HYDROGEN-PEROXIDE AND AS SCAVENGERS OF SINGLET OXYGEN - DIHYDROXYTELLURANES AS MILD OXIDIZING-AGENTS

Dihydroxytelluranes derived from tellurapyrylium dyes via oxidative addition of hydrogen peroxide or via scavenging of singlet oxygen in the presence of water are useful as mild oxidants. These materials undergo two-electron reductions with values of E-degrees near 0.0 V (vs SCE). The dihydroxytellurane 5c oxidizes leucodye 3a to 4a and leucodye 3b to 4b with second-order rate constants of >1 X 10(3) M-1 s-1 at 298.0 +/- 0.1 K based on the pseudo-first-order appearance of reduced tellurapyrylium dye 7c. These values are much larger than the second-order rate constants for the oxidation of leucodyes 3 to 4 (<2 X 10(-1) M-1 s-1 at 298.0 +/- 0.1 K) with hydrogen peroxide. The dihydroxy tellurane 5c (10(-5) M) oxidizes thiophenol (10(-3) M) to phenyl disulfide in minutes in 75% methanol, based on the pseudo-first-order loss of 5c at 298.0 +/- 0.1 K. This rate is much faster than the uncatalyzed oxidation of thiophenol to phenyl disulfide with hydrogen peroxide in 75% methanol-d4 (half-life of approximately 1 day). The second-order rate constants for the oxidative addition of hydrogen peroxide to tellurapyrylium dyes 7 and 13 range between 0.139 M-1 s-1 for 13 and 2.26 M-1 s-1 for 7c in distilled water. Consequently, tellurapyrylium dye 7c and other tellurapyrylium dyes serve as catalysts for hydrogen peroxide oxidations of leucodyes 3 and aromatic thiols via the formation of dihydroxytelluranes as intermediates. Values of k(cat) of up to 2.25 X 10(4) M-2 s-1 for the oxidation of 3a with turnover numbers of greater than 300 were observed. Neutral tellurium(II) compounds to not catalyze these oxidations. Thiapyrylium dye 20 and selenapyrylium dye 21 did not catalyze these reactions. The second-order rate constants for oxidative addition of hydrogen peroxide are 4.62 X 10(-4) and 4.68 X 10-4 M-1 s-1 in water and 6.98 X 10(-6) and 1.09 X 10(-5) M-1 s-1 in 90% methanol for 20 and 2 1, respectively. The quantum yield for self-sensitized scavenging of singlet oxygen by tellurapyrylium dyes is solvent dependent, varying from 9.5 X 10(-5) for 7c (1.0 X 10(-5) M) in 99% methanol, 1.2 X 10(-3) in 50% methanol, to 5.5 X 10(-3) in water. The largest contributing factor to this variation is the rate of reaction of singlet oxygen with tellurapyrylium dyes in various solvents. For 7c, k(ox) increases from 9 X 10(6) M-1 s-1 in 99% methanol, 1.8 X 10(8) M-1 s-1 in 50% methanol, to 8 X 10(8) M-1 s-1 in water. The dihydroxytelluranes formed from scavenging of singlet oxygen can be utilized directly for the oxidation of leucodyes 3 with turnover numbers of >50.