摘要:

Mendeleev Communications Electronic Version Issue 6 1997 (pp. 213–252) Radical formation during ozonation of triphenylmethane on silica gel Qadir K. Timerghazin Nikolai M. Shishlov Natal’ya N. Kabalnova Sergey L. Khursan and Valerii V. Shereshovets* Institute of Organic Chemistry Ufa Scientific Centre of the Russian Academy of Sciences 450054 Ufa Russian Federation. Fax +7 347 235 6066; e-mail qadir@chemlum.ufanet.ru Low-temperature ozonation of triphenylmethane on silica gel is accompanied by generation of peroxy radicals and proceeds via formation of ozone–triphenylmethane complexes; without silica the reaction does not proceed at the same temperatures. Low-temperature ozonation of tertiary hydrocarbons adsorbed on a silica surface (dry ozonation) leads to the formation of tertiary alcohols with remarkably high selectivity and rate.1 There are hardly any data on the mechanism intermediates and kinetics of this process.Here we report on the observation of free-radical intermediates during the dry ozonation of triphenylmethane used as a model substrate. Silica gel (surface area 540 m2 g–1) precalcined at 300 °C for 5 h was used in this work. Triphenylmethane was purified by recrystallisation from ethanol. Triphenylmethane (0.04–0.80 g) dissolved in pentane was added to the silica gel (1 g) with intense stirring then the solvent was removed by rotary evaporation. A glass ampoule charged with silica gel bearing adsorbed triphenylmethane was placed into a cooling bath (–90 °C). A mixture of ozone and oxygen (1–2% O3) was supplied to the ampoule by glass capillary for 5–10 min.A sample was kept in liquid nitrogen. A SE/X 2544 spectrometer supplied with an optional modulating unit (100 kHz) was used to detect the radicals. The EPR spectra were recorded in the temperature range –196 to –30 °C. The g-factors of the spectra were determined by comparison with those of the spectrum of a,a'-diphenyl-b-picrylhydrazyl (DPPH; g = 2.0037). During the low-temperature ozonation of triphenylmethane on silica gel the sample acquired a green colour. Apparently this is due to the formation of a complex between ozone and aromatic hydrocarbon. The existence of such complexes for a variety of arenes and their derivatives in the liquid phase is well established.2–4 Without triphenylmethane the silica gel turned blue from accumulating ozone. The EPR spectrum of the silica–triphenylmethane–ozone system recorded at liquid nitrogen temperature consisted of a single line without hyperfine structure with a pronounced asymmetrical character Figure 1(a).From this spectrum the principal g-factors were determined g|| = 2.0353 and g^ = 2.0082. The isotropic g-factor was calculated as g = (g|| + 2g^)/3 = 2.0172. At higher temperatures the anisotropy of the signal decreases slightly but still exists in the temperature range over which the signal is observed. With a temperature rise its intensity decreases and at –40 °C the EPR signal almost disappears completely Figure 1(b). The shape of the EPR signal is an asymmetrical singlet and is typical of alkylperoxy radicals trapped in solid matrices5 or adsorbed on silica gel at low temperatures.6 Comparison of the g-factor obtained in this work (g = 2.0172) with literature data for peroxy radicals (g = 2.0120–2.0154)5,7 confirms that the spectra observed in our experiments do indeed correspond to peroxy radicals 1.Conservation of signal anisotropy at fairly high temperatures confirms that radicals are unable to rotate freely on the silica surface. Their translational motion is probably also limited. It should be noted that during low-temperature ozonation of crystalline triphenylmethane (without silica) neither a change in sample colour nor the EPR signal was observed. Pure silica gel and that with adsorbed ozone or triphenylmethane also do not show signals in the EPR spectra. The results of this study make it possible to conclude that dry ozonation of triphenylmethane proceeds via formation of ozone–triphenylmethane complexes and is accompanied by generation of peroxy radicals.We are grateful to Dr. B. I. Kutepov for surface area measurements and scientific discussions. References 1 Z. Cohen E. Keinan Y. Mazur and T. H. Varkony J. Org. Chem. 1975 40 2141. 2 P. S. Bailey J. W. Ward T. P. Carter J. E. Nieth and C. M. Fischer J. Am. Chem. Soc. 1974 96 6136. 3 V. V. Shereshovets L. G. Galimova and V. D. Komissarov Izv. Akad. Nauk SSSR Ser. Khim. 1981 2488 (Bull. Acad. Sci. USSR Div. Chem. Sci. 1981 30 2055). 4 E. V. Avzyanova N. N. Kabalnova and V. V. Shereshovets Izv. Akad. Nauk Ser. Khim. 1996 371 (Russ. Chem. Bull. 1996 45 360). 5 J. E. Bennet D. M. Brown and B. Mile Trans. Farad. Soc. 1970 66 386. 6 W. J. Maguire and D. M. Pink Trans. Farad. Soc. 1967 63 1097. 7 W. A. Pryor N. Ohto and D. F. Church J. Am. Chem. Soc. 1982 104 5813. O C O 1 (a) (b) 324 326 328 330 332 334 H/mT 1 mT –90 °C –40 °C Figure 1 EPR spectra of the silica–triphenylmethane–ozone system (a) in liquid nitrogen; (b) at –90 °C and –40 °C. Received Moscow 20th August 1997 Cambridge 13th October 1997; Com. 7/06174I

ISSN:0959-9436     

出版商:RSC    

年代:1997

数据来源: RSC