Dioxole radicals

Mattay et al., having discovered exciplex emission from solutions of benzene and 1,3-dioxole [122], continued their investigations with a study on selectivity and charge transfer in photoreactions of a,a,a-trifluorotoluene with 1,3-dioxole and some of its derivatives, and with vinylene carbonate and dimethylvinylene carbonate [15,143,144], a,a,a-Trifluorotoluene and 1,3-dioxole upon irradiation yield three types of products ortho cycloadducts, meta cycloadducts, and so-called substitution products (Scheme 44). The products are formed in the ratio ortho adductsimeta adducts substitution products = 0.8 1.7 0.3. The substitution reaction (which is really an addition of a C—F bond to the double bond of 1,3-dioxole, but named substitution in order to distinguish it from the ortho addition [186] is supposed to start with electron transfer from 1,3-dioxole to excited a,a,a-trifluorotoluene. The radical anion then releases a fluoride ion, which adds to the 1,3-dioxole radical cation. Radical combination then leads to the product. 

The most extensively studied diketone has been biacetyl,24 12g) a detailed mechanistic study of its reactions with a number of olefins has appeared 128). It was suggested that a triplet exciplex is the precursor for formation of 161. The occurrence of electron transfer, presumably subsequent to exciplex formation, has been demonstrated 157,l58) in the reaction of biacetyl with tetramethyl-l,3-dioxole (167). Esr-spectroscopy of irradiated mixtures indicated the presence of biacetyl radical anion and dioxole radical cation. This reaction, which produced a complex mixture of products, was suggested to involve the excited singlet state of biacetyl. 

In some cases the addition of bromide to alkene radical cations is reversible. For example, the addition of bromide to the p-methyl-4-methoxystyrene radical cation occurs reversibly, as demonstrated by the formation of the radical cation when the P-bromo radical is generated independently by photolysis of l-(4-methoxyphenyl)-l,2-dibtomopropane (Eq. 18). An equilibrium constant of 2 x 10 M has been measured for the loss of bromide from this radical in acetonitrile. The apparent lack of reactivity of 1,3-dioxole radical cations with bromide ion in water has also been explained on the basis of reversible addition with rapid loss of bromide from the product radical. However, on the basis of the solvent effects noted above, it is also possible that the lack of reactivity in water is a solvent effect since decreases in reactivity of 4 to 5 orders of magnitude have been observed for reactions of bromide ion with styrene radical cations in largely aqueous solvent mixtures. - ... 

Leismann et al.[182] have recognized this problem in their publication of 1984, in which they describe a thorough and detailed investigation of the kinetics of formation and deactivation of exciplexes of. S) benzene or toluene and 1,3-diox-ole, 2,2-dimethyl-l,3-dioxole, and 2,2,4-trimethyl-l,3-dioxole. The evolution in time of monomer and exciplex fluorescence after excitation using a nanosecond flash lamp was analyzed, and again it was concluded that the formation of exciplexes is diffusion controlled their decay proceeds mainly (>90%) via radiationless routes. The polar solvent acetonitrile enhances radiationless deactivation, possibly by promoting radical ion formation. Because decay of benzene fluorescence is essentially monoexponential, dissociation of the exciplex into Si benzene and dioxole is negligible. 

Photo-induced radical-cation [4+2] cycloadditions between cyclohexadiene 36 and 1,3-dioxole 56a yields 57a and 58a (Sch. 12). The ratio of... 

Arylcyclopropanes and their heterocyclic analogues are liable to electron transfer induced fragmentation of a carbon-carbon bond that in some cases leads to synthetically useful products. Thus, 1,2-diarylcyclopropanes [240-243] as well as 2,3-diaryloxirans [244-246] and -aziridines (in the last case, also 2-monophenyl derivatives) [247,248] are cleaved upon photoinduced electron transfer sensitization. The final result, after back electron transfer, is trans-cis isomerization of the ring. In the presence of a suitable trap, however, a cycloaddition reaction takes place, involving either the radical cation or the ylide. Thus, dioxoles, ozonides or azodioxoles, respectively, are formed in the presence of oxygen and oxazolidines have been obtained from cyclopropanes in the presence of nitrogen oxide (Sch. 23). 

A photodimerization approach has been applied here. Thus photolysis of 4-(4-chlorophenyl)-thiazoline-2-thione forms the head-to-head dimer (37) <86CC1030>. Under radical conditions 1,3-dioxole forms two geometrical isomers of cyclobutabis-l,3-dioxolane <85CC1088>. 

The interactions of a-olefins or styrene with sulfur dioxide (16) or a-olefins (24, 58, 78), frans-stilbene (64), styrene (1,63), p-dioxene (52), 2,2-dimethyl-l,3-dioxole (17), or alkyl vinyl ethers (1, 63) with maleic anhydride yield charge transfer complexes which are stable and generally readily detectable either visually or by their ultraviolet absorption spectra. However, under the influence of a sufficiently energetic attack in the form of heat or free radicals, the diradical complexes open, and alternating copolymers are formed. 

Continuing work 158) on photoreactions of electron-rich olefins with biacetyl shows that the complexity of product mixtures obtains in these reactions also. Effects of solvent polarity provide further support for the importance of ionic intermediates in these reactions. The reactions of biacetyl with 1,1-diethoxyethylene are proposed to proceed via the triplet state (in contrast to reactions with dioxoles). The reversal of regiospecifity between thermal and photochemical cycloaddition of this olefin with biacetyl is nicely explained by the assumption of excited state electron transfer from olefin to dione to give the corresponding radical ions. 

In the present context, the term electron rich alkenes refers primarily to enol ethers, enol sulfides, and A-vinylamides or A-vinylamines. Such alkenes are typically much more readily ionizable than are simple alkenes. The conversion of these substrates to the corresponding (highly electron deficient) cation radicals represents a sharp Umpolung. The Diels-Alder additions of tra j -anethole, phenyl vinyl ether, phenyl vinyl sulfide, 1,3-dioxole, and A-methylindole to 1,3-cyclohexadiene have been reported (Scheme 22) [49, 52]. 

Other electron-rich systems are 1,3-dioxols and dioxenes. With the former, OH adds to the 4,5-C-C bond to give (x-alkoxy-y9-hydroxyalkyl radicals, Scheme 19. With the fully methylated system (left side of Scheme 19), at pH 2 the OH adduct is quantitatively converted (by H+-induced dehydration) to the radical cation, as judged by ESR, whereas with only hydrogens as substituents (right side), at pH 2 dehydration is too slow to lead to a visible decrease of the (stationary) concentration of the OH adduct (Scheme 19) [45]. 

Even a small increase in the electron density of the system, by introducing a methylene group, i.e., by going from the 1,3-dioxol to the 1,4-dioxene system, leads, however, to complete H+-induced conversion of the OH adduct into the radical cation (Figure 7 and Scheme 20) [45]. 

Because of possible relevance to mutagenesis, considerable effort has been devoted to study of the photochemical transformations of oxypyrimidines uracil, for example, takes part in a [2 -i- 2] cycloaddition with itself, or with vinylene carbonate (l,3-dioxol-2-one). Uracils undergo radical additions these too are of possible relevance to mutagenesis mechanisms. 

Oxirane formation can also occur via free radical mechanisms, as in the reaction of certain fluoroalkenes with oxygen. Under pressure at elevated temperatures, oxygen alone can suffice, but activation is frequently provided in the form of radical initiators (e.g., tribromofluoromethane) and ultraviolet light. Thermolysis of dioxole 5, comonomer from which DuPont s Teflon-AF is made, offers an unusual route to an oxirane. Rearrangement of the heterocycle presumably takes place via a biradical intermediate. ... 

It should be noted that these dioxoles are extremely reactive in free radical polymerizations. Dioxoles are the hrst fluorinated monomers containing an internal olehnic structure that homopolymerize and possess reactivity similar to tetrafluoroethylene. This high reactivity is believed to a result of the steric accessibility of the double bond. 

Diels-Alder reaction of pyran-2-ones. Diels-Alder reaction of 2-pyrones, if successful, can provide unusual cyclohexenecarboxylic acids, but thermally promoted cycloadditions with these electron-deficient dienes usually result in decarboxylation and aromatization of the adducts as a result of the required high temperatures (6,291-292). Successful Diels-Alder reactions of 3-bromo-2-pyrone (1) with the electron-rich dioxole 2 can be effected with a catalytic amount of ethyldiisopropylamine at 90° (4 days) to give the major adduct (endo-3) in 63% yield. The adduct is hydrolyzed by p-toluenesulfonic acid in methanol to 4 as the only diastereomer. The trisilyl ether of 4 was transformed to the a,/8-unsaturated ester 5 by radical debromination and DBU isomerization. ... 

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