Radical cations epoxidation

In order to rationalize the complex reaction mixtures in these slurry reactions the authors suggested that irradiations of the oxygen CT complexes resulted in simultaneous formation of an epoxide and dioxetane36 (Fig. 34). The epoxide products were isolated only when pyridine was co-included in the zeolite during the reaction. Collapse of the 1,1-diarylethylene radical cation superoxide ion pair provides a reasonable explanation for the formation of the dioxetane, however, epoxide formation is more difficult to rationalize. However, we do point out that photochemical formation of oxygen atoms has previously been observed in other systems.141 All the other products were formed either thermally or photochemically from these two primary photoproducts (Fig. 34). The thermal (acid catalyzed) formation of 1,1-diphenylacetaldehyde from the epoxide during photooxygenation of 30 (Fig. 34) was independently verified by addition of an authentic sample of the epoxide to NaY. The formation of diphenylmethane in the reaction of 30 but not 31 is also consistent with the well-established facile (at 254 nm but not 366 or 420 nm) Norrish Type I... 

Electrochemical oxidation of epoxides in absence of nucleophiles, catalyses a rearrangement to the carbonyl compound. The electrolyte for this process is dichlo-romethane with tetrabutylammonium perchlorate. Reaction, illustrated in Scheme 8.7, involves the initial formation of a radical-cation, then rearrangement to the ketone radical-cation, which oxidises a molecule of the substrate epoxide. The process is catalytic and requires only a small charge of electricity [73]. 

These results indicate that vacuum curing occurs through a radical reaction mechanism and is terminated by reaction of the ring-opened epoxy group with the azide group (not nitrene) under exposure. There is a possibility that polymerization initiated by an exposure-induced radical cation may occur. Furthermore, it is thought that reaction products from both the azide and epoxide serve as dissolution inhibitors, because the sensitivity of EAP is almost the same as that of EP, as shown in Figures 1 and 2. 

C-C Cleavage in Epoxides. Radical cations generated by photoinduced electron transfer from epoxides (130) or aziridlnes (131) also ring open, giving oxidative products in the presence of oxygen. For example, dicyanoanthracene sensitizes the conversion of aryl epoxides to ozonides, eq. 48,... 

Independent rate studies by Bruice and Castellino143 and by the Bauld group144 concluded that any alkene radical cation must have a lifetime less than about 10 12 s, ruling out any meaningful role in the reaction for the free species. More recent work has generally concluded that no alkene cation radical is involved in epoxidations catalyzed by iron(III)... 

Epoxidation of cyclooctene with hydrogen peroxide, catalysed by the methoxide-ligated form of iron(III) tetrakispentafluorophenyl [F20TPPFe(III)] porphyrin, is proposed to involve a reaction of F20TPPFe(in) with hydrogen peroxide to form an iron(III) hydroperoxide species, which then undergoes both heterolytic and homolytic cleavage to form iron(IV) n -radical cations and iron(IV) oxo species, respectively. 

The iron(IV) r-radical cations are responsible for the epoxidation of cyclooctene, whereas the iron(IV) oxo species are responsible for the decomposition of hydrogen peroxide (Scheme 7).175... 

Isomerization of epoxides to ketones.3 This radical cation [as well as trityl hexachlorostibnate, (C6H5)3C+SbCl6 ] effects this reaction. 

The secondary electron-transfer processes, often used in mechanistic in estiga-tions and in preparative applications of electron-transfer photochemistry, enhance the quantum yields of product formation [167], In fact, as we have already pointed out in a previous section, the efficiency of separation of the geminate pair is strictly dependent on the redox potentials (see also indirect photooxygenation processes) [43, 50, 80-83, 135], Anyway, although in the present case the subsequent electron-transfer from epoxide to biphenyl radical cation BP is endothermic enough, in the absence of very fast competing reactions this primary radical cation may still undergo an endothermic electron-transfer process with epoxides. 

An important factor in the oxidation of epoxides by BP" to their corresponding radical cations would certainly be the much longer lifetime of BP compared to... 

However, Schaap s results are not consistent with a mechanism involving attack of superoxide ion on the opened radical cations. In fact, the striking stereochemical course could be accommodated only by admitting the formation of the most stable E-E conformation of epoxide radical cations, and by showing that the ring closure of the zwitterion intermediates could occur faster than the bond rotation. 

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