Organometallics 1995, 14, 3641-3648
3641
Oxygen Atom Transfer from Alkylperoxyl Radicals to Aromatic Tellurides Lars Engman* Institute of Chemistry, Department of Organic Chemistry, Uppsala University, Box 531, S-75121 Uppsala, Sweden
Joachim Persson Laboratory of Organic Chemistry, Department of Chemistry, Royal Institute of Technology, S-10044 Stockholm, Sweden
Gabor Merenyi* and Johan Lind* Laboratory of Nuclear Chemistry, Department of Chemistry, Royal Institute of Technology, S-10044 Stockholm, Sweden Received February 14,1995@ The reactions of alkylperoxyl radicals with the diaryl tellurides bis(4-aminophenyl)telluride and bis(4-hydroxyphenyl) telluride were studied by pulse- and y-radiolysis techniques in aqueous media. The diaryl tellurides served predominantly (-70%) as one-electron reductants toward the CC1300' radical (k = 1.5 x lo9 and 2.5 x lo9 dm3 mol-' s-l, respectively), whereas they acted essentially ( L 92%) a s oxygen atom recipients in their reactions with the (CH3)2C(OH)CH200' radical (k = 1.7 x lo8 and 3.7 x lo7 dm3 mol-l s-l, respectively), producing the corresponding diaryl telluroxides. The latter reaction mode was corroborated by the quantitative interception of a reducing a-hydroxylalkyl radical, the product of ,f3-scission of its precursor, the alkoxy1 radical (CH3)2C(OH)CH20'. On the basis of detailed energetic and mechanistic considerations the oxygen transfer is proposed to be a concerted one-step reaction without the intermediacy of a n adduct.
Introduction
diaryl tellurides 2, the most active compounds in the
Oxygen atom transfer from alkylperoxyl radicals has 0 been observed to alkenes' and phosphinedph~sphites.~~~ In both of these cases the rate-determining steps appear to produce intermediate peroxyl radical adducts. With 1 2 the phosphines/phosphites the rate constant of peroxyl radical addition has been found t o be ca. lo4 dm3 mol-l series were shown to possess IC50 values as low as (3s-1 in isopentane solvent at room temperature.2 Later, the CC1300*radical was shown to transfer an oxygen 6) x mol dm-3. This result prompted us t o initiate a study of the reaction of alkylperoxyl radicals with atom to indoles, again via the intermediacy of a peroxide aromatic tellurides. In the following we report on a d d ~ c t .More ~ recently, the CC1300' and CHC1200' reactions of peroxyl radicals with diaryl tellurides radicals were shown5 to participate simultaneously in involving oxygen atom transfer to tellurium. a one-electron as well as a two-electron transfer process with (CH3)zS (DMS), where the latter reaction yielded (CH&SO (DMSO) directly. The reactivity of the pharExperimental Section maceutically interesting organoselenium compound ebPulse radiolysis was performed at room temperature (22selen (1)and structurally related analogues with halo23 " C )utilizing doses of 2-15 Gylpulse corresponding to (1.2genated alkylperoxyl radicals was also studied6recently. 9) x mol dm-3 of radicals. The 7-MeV microtron It was concluded that the organoselenium compounds accelerators and the computerized optical detection systemg served essentially as one-electron reductants in these have been described elsewhere. The cell used had an optical systems. In a study7concerning the antioxidant activity path length of 1 cm. Dosimetry was performed by means of an aerated mol dm-3 KSCN solution takinglo GE= 2.23 in microsomal lipid peroxidation of 4,4'-substituted x 10-4 m2
@Abstractpublished in Advance ACS Abstracts, June 15, 1995. (1) Selby, K.; Waddington, D. J. J. Chem. SOC.Perkin Trans. 2 1980, 65. (2) Furimsky, C.; Howard, J. A. J. Am. Chem. SOC.1973,96, 369. (3) Pomedimski, D. G.; Kirpichnikov, P. A. J. Polym. Sci. 1980,18, 815. (4) Shen, X.;Lind, J.; Eriksen, T. E.; Merenyi, G. J. Chem. SOC., Perkin Trans. 2 1989,555. ( 5 ) Schoneich, C.; Aced, A,; Asmus, K.-D. J.Am. Chem. SOC.1991, 113,375. (6) Schoneich, C.; Narayanaswami, V.; Asmus, K.-D.; Sies, H. Arch. Biochem. Biophys. 1990,282,18.
J-1
at 500 nm. y-radiolysis was carried out in a
(7) Andersson, C.-M.; Brattsand, R.; Hallberg, A,; Engman, L.; Persson, J.; Moldbus, P.; Cotgreave, I. A. Free Radical Res. 1994,20, 401. See also: Cotgreave, I. A.; Moldbus, P.; Engman, L.; Hallberg, A. Biochem. Pharmacol. 1991,42, 1481. ( 8 ) Rosander, S. Thesis, The Royal Institute of Technology, Stockholm, Sweden, 1974; TRITAEEP-74-16,p 28. (9) Eriksen, T. E.; Lind, J.; Reitberger, T.Chem. Scr. 1976, 10, 5. (10)Fielden, E. M. In The Study of Fast Processes and Transient Species by Electron Pulse Radiolysis; Baxendale, J. H., Busi, F., Eds.; Reidel: Dordrecht, Holland, 1982: NATO Advanced Study Institutes Series pp 49-62.
0276-733319512314-3641$09.00/0 0 1995 American Chemical Society
3642 Organometallics, Vol. 14, No. 8, 1995 6oCo y-source at a dose rate of 0.27 Gy/s as determined by the Fricke dosimeter.1° The solutions were made up in Milliporedeionized water. Melting points (uncorrected) were determined by using a Buchi 510 melting point apparatus. 'H NMR spectra were obtained with a Bruker AC-F 250 instrument operating at 250 MHz and recorded for CD30D solutions containing tetramethylsilane as the internal standard. Elemental analyses were performed by Analytical Laboratories, Engelskirchen, Germany. Chemicals. 2-Methyl-2-propanol(Merck, p.A. ),2-propanol (Aldrich, HPLC grade), NaN3 (Aldrich, 99%), NaBr (Aldrich, 99%),HCOzNa (Aldrich, 99%+), NaOH (Aldrich, 99.99%),HzSO4 (Merck, supra pure), and NaB40rlOH20, KH2P04, and KHPO4 (Merck, p.A.) were used as received. Bis(4-aminophenyl) telluride and bis(4-hydroxyphenyl) telluride were prepared according to a literature procedure." Bis(4-aminophenyl) Telluroxide. To a solution of bis(4-aminophenyl)telluride (0.25 g, 0.80 mmol) in methanol (2 mL) at 0 "C was added tert-butyl hydroperoxide (110 pL, 70%; 0.81 mmol). After 1 h the solvent was evaporated and the residue crystallized from CHzCld'hexane. The yield of bis(4aminophenyl) telluroxide was 0.22 g (84%); mp 195-200 "C dec. 'H NMR: 6 6.78 (d, 2H), 7.51 (d, 2H). The analytical sample was obtained after heating a t reduced pressure for 3 h (60 "C/10-2mmHg). Anal. Calcd for C12HlzN2TeO: C, 43.96; H, 3.69. Found: C, 43.81; H, 3.81. Bis(4-hydroxyphenyl)Telluroxide. To a boiling solution of bis(4-hydroxyphenyl) telluride (0.20 g, 0.64 mmol) in methanol (10mL) was added tert-butyl hydroperoxide (90 pL, 70%; 0.66 mmol). After 15 min hexane was added to cloudiness and the reaction mixture cooled to precipitate 0.12 g of bis(4-hydroxyphenyl) telluroxide. A second crop of the material (0.058 g) was obtained after further precipitation with hexane, corresponding to a total isolated yield of 85%. lH NMR: 6 6.94 (d, 2H), 7.66 (d, 2H). The analytical sample (mp 197-200 "C dec) was obtained after heating at reduced pressure for 3 h (60 oC/10-2mmHg). Anal. Calcd for CIzH1003Te: C, 43.70; H, 3.06. Found: C, 43.78; H, 2.93. Bis(4hydroxyphenyl)telluroxide has previously been reported twice in the literature: mp 93 aC12and mp 228 'C.13 Both reports are probably in error.
Results
Engman et al.
r
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Figure 1. UV spectra of bis(Caminopheny1)telluride(I1) and telluroxide(IV)compounds in water: ( 0 )(4-NHZc6&)2Te, pH 7; (W) (4-NH2C6H&TeOH+,pH 5 ; ( 0 )(4-NHzCsHJ2Te(OH)2,pH 8. The spectra were recorded with a wavelength step of 1nm. The symbols are added for identification purposes only. toxic and foul-smelling. In addition, most compounds are not sufficiently water soluble to be studied in aqueous solution, where the production of radical species in a pulse radiolysis experiment is fully controlled. The diaryl tellurides and diaryl telluroxides used in this study are all crystalline odorless materials with water solubilities in the range of 10-4-10-3 mol dm-3. Furthermore, the ultraviolet spectra of the diaryl tellurides are different from those of the corresponding diaryl telluroxides (see Figure 1). Thus, the solubility and W properties of the selected organotellurium compounds make pulse radiolysis the method of choice for studying their interaction with peroxyl radicals. Thermodynamic Properties of Aromatic Telluroxides. When dissolved in water, diorganyltellurium(IV) oxides (R2TeO) probably form the dihydroxides,16J7and their reported16J8basicity can be explained by equilibrium 1. The pK1 values of (4-NHz-CsH412-
+ H20== R,Te(OH), + H+
Preparation and Properties of Diaryl Tellurides and Diaryl Telluroxides. Diaryl telluroxides are available by several methods, the most common ones involving oxidation (air,sodium periodate, N-haloimines, m-chloroperoxybenzoicacid, tert-butyl hypochlorite) of the corresponding diaryl telluride or hydrolysis of a suitable diaryltellurium(IV) derivative (dihalide, sulfonamide).14 We found it convenient to prepare bis(4aminophenyl) telluroxide (84%) and bis(4-hydroxyphenyl) telluroxide (85%) by oxidation of the respective diaryl tellurides with tert-butyl hydroperoxide in methanol. The required diaryl tellurides were previously prepared in our 1aboratories.ll The study of organotellurium compounds by pulse radiolysis has received very little attention15 t o date. This is probably because organotellurium compounds have a reputation of being
TeOH+ and (4-HO-C6H&TeOH+ are 5.85 and 5.75, re~pective1y.l~At pH values below 3.5 protonation occurs at the amino group of the (4-NH2-CsH&TeOH+ compound, but the exact value of this pKa was not determined. Similarly, (4-NH2-CsH4)zTeis protonated below pH 4. To investigate the formation of the tellurium dihydroxide more quantitatively, dry Ar2TeO was dissolved in water-free tetrahydrofuran (THF). The W spectra of such solutions containing 3 x low5mol dm-3 (4-NH2-CsH4)2TeOor (4-HO-C&)2TeO were found t o change with the amount of added water, until they attained a final form above 0.1 mol dm-3 of H2O. The final spectra were very similar to the corresponding aqueous spectra of AraTe(0H)z. The equilibrium con-
(11)Engman, L.; Persson, J.; Andersson, C. M.; Berglund, M. J . Chem. Soc., Perkin Trans. 2 1992,1309. See also: Engman, L.; Persson, J. Organometallics 1993, 12, 1068. (12) Reichel, L.; Kirschbaum, E. Justus Liebigs Ann. Chem. 1936, 523, 211. (13) Irgolic, K. J. In Houben-Weyl, Methods of Organic Chemistry; Klamann, D., Ed.; Georg Thieme Verlag: Stuttgart, Germany, 1990; Vol. E 12b, p 645. (14) Reference 13, p 640. (15) Bergman, J.; Eklund, N.; Eriksen, T. E.; Lind, J. Acta Chem. Scund. 1978, A32, 455.
(16)Lederer, K. Justus Liebigs Ann. Chem. 1912, 391, 326. (17) Compounds reported as diorganyl telluroxides were sometimes formulated as hydrates, RZTeO.HZ0, and sometimes as diorganyltellurium dihydroxides, RzTe(OHl2: Detty, M. R. J . Org. Chem. 1980, 45,274. Uemura, S.; Fukuzawa, S. J.Am. Chem. SOC.1983,105,2748. (18)The basic character of diaryl telluroxides has recently found synthetic use in aldol condensations: Engman, L.; Cava, M. P. Tetrahedron Lett. 1981,22,5251.Akiba, M.; Lakshmikantham, M. V.; Jen, K.-Y.; Cava, M. P. J . Org. Chem. 1984, 49, 4819. (19) Engman, L.; Lind, J.; Merenyi, G. J . Phys. Chem. 1994, 98, 3174.
R2TeOH+
(1)
0 Atom Transfer from Alkylperoxyl Radicals
Organometallics, Vol. 14, No. 8, 1995 3643
Table 1. Rate Constants Determined in This Work
3.7
107 107 1.5 x 109 2.5 x 109 ~ 1 . x5 lo7 4 x 103 (s-1) 1
4.5 103 (8-1) (1.3f 0.2) x lo2 (4.7 < pH
