Di-tert-butyl peroxyoxalate

The formation of isobutyric acid in the presence of the additives studied, and the results of additional studies (di-tert.-butyl peroxyoxalate/isobutyroal-dehyde/amine), point to the intermediate formation of acyl peroxy radicals. 

Separate experiments in which tert.-butoxy radicals were produced thermally in benzene from di-tert.-butyl peroxyoxalate failed to reveal any direct reaction of these radicals with amine II. Even at higher temperatures (A/ 150°C, dichlorobenzene, +00+ decomposition), the +0 radicals attacked neither amine II nor nitroxide I. The earlier described experiments of ketone photooxidation showed additionally that amine II displays no specially marked reactivity towards peroxy radicals. 

Of considerable practical importance is the concentration of substrate. Even fairly reactive compounds do not autoxidize readily at sub-millimolar concentrations [7a]. The rate of initiation is also crucial in promoting autoxidation. Although many autoxidation reactions are auto-initiated, the use of initiators such as d -tert-butyl peroxyoxalate (DBPO) [12a] or di-tcr -butyl hyponitrite (DTBN) [12b] can be beneficial. DBPO and DTBN are convenient sources of tcrt-butoxyl radicals in the temperature ranges of 20-60 and 20-90 °C, respectively (Scheme 2). They are especially effective when used in conjunction with an excess of tcrt-butyl hydroperoxide (TBHP), whose role is to trap product peroxyl radicals as hydroperoxides, thereby preventing side-reactions [13a,c. In contrast to auto-initiated autoxidation, the autoxidation of simple alkenes (e.g. 1-methylcyclohexene, Scheme 2) by the DBPO-TBHP procedure is of comparable preparative value to the widely used singlet oxygen route [13a,b]. 

Yamamura comprebensively reviewed stoicbiometric oxidations of phenols with both metal (VjOg, VOClj, MnOj, CrOj, MoOCl, Tl O, Ag O) and nonmetal compounds (hypervalent iodobenzenes, 2,3-dichloro-5,6-dicyano-/)-benzoquinone, dioxirane, di-tert-butyl peroxide, di-ferf-butyl peroxyoxalate) [57]. Some of these reagents, for example, metal-free PhI(OAc)j and PhI(OCOCF3)j, seem to be rather safe and are especially useful in the low-scale synthesis of complex molecules of natural products. However, one should remember that none of the stoichiometric oxidations is compatible with the concept of green chemistry because such processes produce vast amounts of effluents, which are difficult to dispose of. For example, the active oxygen content in bis(trifluoroacetoxy)iodobenzene is only 3.8%, and a typical E-factor for oxidations with this oxidant lies in the range of 15-25. 

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