Peroxides photochemical decomposition

Thermal and photochemical decomposition of peroxides (4) and (5) lacking a-hydrogens (those derived from ketones) produces macrocycHc hydrocarbons andlactones (119,152,153). For example, 7,8,15,16,23,24-hexaoxatrispiro [5.2.5.2.5.2] tetracosane (see Table 5) yields cyclopentadecane and oxacycloheptadecan-2-one. 

The reactions of /-butoxy radicals are amongst the most studied of all radical processes. These radicals are generated by thermal or photochemical decomposition of peroxides or hyponitrites (Scheme 3.75). 

Aroyloxy radicals are formed by thermal or photochemical decomposition of diaroyl peroxides (see 3.3.2.1) and aromatic peroxyesters (3.3.2.3) (Scheme 3.78) alkoxycarbonyloxy radicals are similarly produced from peroxydicarbonates (33.2.2). 

Diacyl peroxides undergo thermal and photochemical decomposition to give radical intermediates (for a recent review, see Hiatt, 1971). Mechanistically the reactions are well understood as a result of the many investigations of products and kinetics of thermal decomposition (reviewed by DeTar, 1967 Cubbon, 1970). Not surprisingly, therefore, one of the earliest reports of CIDNP concerned the thermal decomposition of benzoyl peroxide (Bargon et al., 1967 Bargon and Fischer, 1967) and peroxide decompositions have been used more widely than any other class of reaction in testing theories of the phenomenon. 

CIDNP studies of the decomposition have centred mainly on thermal decompositions photochemical decomposition has generally been less intensively investigated. While most reports of polarization refer to n.m.r. spectra, a number of papers have described polarization of other nuclei, (Kaptein, 1971b Kaptein et al., 1972), (Lippmaa el al., 1970a, b, 1971 Kaptem, 1971b Kaptein et al., 1972 Kessenikh et al., 1971), and F (Kobrina et al., 1972) contained in the peroxide reactant. Additionally, polarization of P has been reported in the products of decomposition of benzoyl peroxide in phosphorus-containing solvents (Levin et al., 1970). 

Another interesting question is whether excited singlet states of oxygen are released during the photochemical decomposition of the Co-0-0-Co5+ chromophore. We explored this question by decomposing 2-p in the presence of 1,3-cyclohexadiene which is known to react with excited singlet states of 02 to give the endo peroxide.82 Some endo peroxide may have been formed, and other peroxidic products were formed, but the nature of this reaction is not yet clear. 

Highly selective formation of phenyl acetate was observed in the oxidation of benzene with palladium promoted by heteropoly acids.694 Lead tatraacetate, in contrast, usually produces acetoxylated aromatics in low yields due to side reac-tions. Electrochemical acetoxylation of benzene and its derivatives and alkoxylation of polycyclic aromatics789 790 are also possible. Thermal or photochemical decomposition of diacyl peroxides, when carried out in the presence of polycyclic aromatic compounds, results in ring acyloxylation.688 The less reactive... 

Consideration of the absorption spectra of these two gases leads to the conclusion that the primary photochemical decomposition in nitrogen peroxide, which consists of nitrogen peroxide and nitrogen tetraoxide, is due to the tetraoxide constituent. 

The ability of diacyl peroxides to undergo photochemical decomposition with concomitant C-C bond formation was elegantly exploited by Vederas and coworkers for the preparation of functionalized amino acids (Scheme 20) [27,28]. Symmetrical and unsymmetrical diacyl peroxides of amino acids were conveniently prepared from suitably protected aspartic and glutamic acids in a few steps. Photolysis of the peroxides with a 254-nm UV lamp at low temperature and in the absence of solvent led to decarboxylation... 

Scheme 20 Modified amino acid synthesis via the photochemical decomposition of diacyl peroxides...

Thermal or Photochemical Decomposition of Hydrogen Peroxide Hydrogen peroxide decomposition can occur by homolytic or heterolytic scission... 

The classical methods of generating authentic free radical intermediates include thermal and photochemical decomposition of diacyl peroxides (or peresters) and dialkyldiazenes. Both these types of precursor have been successfully used to produce cyclopropylmethyl-type radicals but, because neither compound can readily be incorporated into a ehain reaction, most applications have been in mechanistic or spectroscopic work. Bis(cyclopropylacetyl) peroxide (1) and /err-butyl cyclopropaneperoxyacetate (2) have frequently been used in EPR spectroscopy. ... 

With the aim to study PA transformations Makarov [40] used method of free radicals initiation by thermal and photochemical decomposition of peroxides. The author succeeded in finding high efficiency of PA maeromolecules breakages under the action of free radicals at the temperature 20-98 ° C (within the limits of operational temperatures). He also succeeded in determination of reactions sequence and revealing the phase directly responsible for the acts of polymer chains destruction. It is shown that in the conditions of thermal initiation transformations of peroxides are caused by macroradicals and photochemical - by own radicals of peroxides. 

J.W. Moffett, O.C. Zafiriou (1993). The photochemical decomposition of hydrogen peroxide in surface waters of the Eastern Caribbean and Orinoco River. J. Geophys. Res., 98,2307-2313. 

J.P. Hunt, H. Taube (1952). The photochemical decomposition of hydrogen peroxide. Quantum yields, tracer and fractionation effects. Nature, 74, 5999-6002. 

IR spectra of IA and N-methyl IA (24) during thermal decomposition show bands of 1810-1830 cm-1, which are in accordance with the proposed /J-lactam 48.121 Analogous decomposition of 30 (R = phenyl) shows bands at 1830 cm-1 106,108 The concentration of 47 in this thermolysis reaction was too small for detection in IR, but UV shows absorption at 575 nm, indicating the presence of 47.106 Photochemical decomposition of phthaloyl peroxide yields 47 and 48 (X = O), showing IR frequencies at 1904 cm-1 (48) and 2139 cm-1 (47),109 which could also be observed in the thermal decomposition of salicylic acid chloride (49) with 1930 cm-1 and 2070 cm-1.121... 

However, although the Haber-Willstatter chain reactions have been assumed to occur in certain catalytic systems, notably the ferrous-ferric ion system (4), more recent evidence to be described subsequently, does not support this assumption. On the other hand, such reactions appear to offer the most plausible explanation for the photochemical decomposition of hydrogen peroxide, although even here a satisfactory analysis of the kinetics has yet to be made. 

It is well known that hydrogen peroxide which absorbs fairly strongly in the region below 3100 A. undergoes photochemical decomposition. 

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