Methanol radical cation

Cationic radicals are much less stable and noticed prominently in mass spectroscopy. When a molecule in gas phase is subjected to electron ionization, one electron is abstracted by the electron beam to create a radical cation. This species represents the molecular ion or parent ion, which on fragmentation gives a complex mixture of ions and uncharged radical species. For example, the methanol radical cation fragments into a methyl cation CFl and a hydroxyl radical. Secondary species are also generated by proton gain (M -F 1) and proton loss (M — 1). 

Calculated geometry of methanol radical cation. (Adapted from reference 295.)... 

Moreover, one should mention that in spite of similar electronic structures, PBN and the isoquinoline nitrone (278) react in a different way. Under no circumstances does PBN give an oxidative methoxylation product, whereas nitrone (278) reacts readily to form a,a-dialkoxy-substituted nitroxyl radical (280) (517). Perhaps this difference might be due to the ability to form a complex with methanol in aldo-nitrones with -configuration. This seems favorable for a fast nucleophilic addition of methanol to the radical cation (RC), formed in the oxidation step. The a-methoxy nitrone (279), obtained in the initial methoxylation, has a lower oxidation potential than the initial aldo-nitrone (see Section 2.4). Its oxidation to the radical cation and subsequent reaction with methanol results in the formation of the a,a-dimethoxy-substituted nitroxyl radical (280) (Scheme 2.105). 

In a rare example of the use of phenylselenides as radical precursors in the generation of alkene radical cations by the fragmentation approach, Giese and coworkers generated a thymidine C3/,C4/ radical cation by expulsion of diethyl phosphate. Trapping experiments were conducted with methanol and with allyl alcohol (Scheme 16), when nucleophilic attack was followed by radical cyclization [66]. 

One of the interesting molecules that has been studied in considerable detail is nor-bomadiene (80). Much of this interest has been associated with the interactions between the double bonds of the system. Thus irradiation affords quadricyclane (81). This area of study will be discussed later in this chapter. The radical cation 82 can also be formed from both norbomadiene and quadricyclane by irradiation in acetonitrile/methanol solution with the DCB/phenanthrene sensitizer system. Several products (Scheme 2) are formed in low yield and it should be noted that there is little difference in the yields of products obtained from either starting material. However, it is evident that attack by methanol occurs from the exo face32. 

Olefins with electron-donating substituents as the aUcoxy, acylamino, phenyl, or vinyl group can be coupled in methanol to give 1,4-dimethoxy dimers and/or dienes (Scheme 2). The first intermediate in this coupling reaction is a radical cation, which either by electrophilic addition to the olefin and subsequent le-oxidation (path A) [49] or by radical dimerization (path B) [50, 51] leads to a dimer dication that undergoes methanolysis or deprotonation. Representative examples of this coupling reaction are summarized in Table 7. 

Another heterocyclization is presented by Panifilow et al. Cyclic acetals and ethers are obtained by electrochemical oxidation of the terpenoid alcohol linalool 57 in methanol containing alkaline and sodium methoxide as electrolyt [102]. Anodic oxidation of the C(6)-C 7) double bond of linalool leads to the radical cation 58. In addition to direct methoxylation of the radical cation an attack on the hydroxyl group takes place. After a second one-electron oxidation and following methoxylation the regioisomeric cyclic acetal and a subsequent 1,2-hydride shift, the cyclic acetal 60 and the cyclic ether 61 are finally formed in yields of 16 and 24%, respectively (Scheme 13). As shown by Utley and co-workers bicyclic lactones 65 and 66 can be synthesized by anodic oxidation... 

Recent studies conducted by the same group revealed that the radical cation of toluene generated by photoinduced electron transfer can be deprotonated in a protic cosolvent and thus efficient trapping by electrophilic alkenes is feasible, yielding benzylation products. Secondary hydrogen abstraction by the benzyl radical from methanol generates hydroxymethyl radicals, which can also be used for preparative hydroxymethylation of alkenes (Scheme 18) [24],... 

Among these are the factors controlling the regiochemistry of the reaction, the employed diene compounds, and the possible intramolecular cyclization of various alk-4-enol and ene-diene radical cations (Scheme 49) [70-72], Also, by changing the nucleophile from the generally used methanol to, for instance, acetonitrile can yield photo-NOCAS products in good yields [73,74],... 

Anodic nuclear substitution by methoxide or cyanide ions gives acceptable yields only for methoxybenzenes and methoxynaphthalenes. The nucleophile is attached to the point of highest positive charge density in the radical-cation and for many examples this leads to ipio-substitution. Oxidation of 1,4-dimethoxybenzene in methanol containing potassium hydroxide leads to the quinone diketal 8 [69]. The reaction is a general one for 1,4-dimethoxybenzenes [70, 71] and 1,4-... 

Anodic oxidation of methoxybenzenes in aqueous sulphuric acid also leads to loss of the methoxy substituent, this time through jju o-substitution on the radical-cation by water. Anisole and 4-niethoxyphenol are both converted to quinone [81]. The elimination of methanol is catalysed by protons by the mechanism illustrated in Scheme 6.8. Diphenyl derivatives have also been isolated from oxidation of some methoxybenzenes. They arise through the competitive reaction involving a... 

In methanol, the radical-cation intermediates from oxidation of thiophenes and N-methylpyrroles can be trapped to give low molecular weight products. Reactivity resembles that of furan but with additional consequences because of the properties of thioethers and amines. 

Formation of low molecular weight products from oxidation of N-methylpyrroIe is most successfully achieved with methanol and sodium cyanide as electrolyte. Tire radical-cation is captured by cyanide ion and 2-cyanopyrroles are formed in good yields when a 2-position in the substrate is vacant. In this reaction, a carbon-carbon bond is formed at the site of highest charge density. When both 2- and 5-positions are blocked by a methyl group, the intermediate radical-cation loses a proton to give the benzylic-type radical. Further reaction leads to cyanation on the 2-methyl group as in 61 [200]. 

Electron transfer-induced nucleophilic addition to several otho cyclopropane compounds was also studied. The nucleophilic addition of methanol to quadricy-clane radical cation 8 produces the two methanol adducts 53 and 54. The stereochemistry of the methoxy groups in these structures identifies the preferred direction of nucleophilic attack upon the intermediate radical cations 8. Detailed NOE experiments delineate the structure of 53 and establish conclusively that the norbomene derivative 54 contains a 7-fl ri-methoxy group. The stereochemistry of both is compatible with stereospecific nucleophilic attack exclusively firom the exo-position. 7-Methylenequadricyclane also is attacked exclusively from the exo-face.These results can be explained via backside attack with inversion of configuration. 

The radical cations 15 + and 16 + add methanol exclusively from the exo face forming exo-methoxynorbornyl and nortricyclyl free radicals, which undergo rapid cyclopropylcarbinyl-butenyl interconversions (119 120 121 see... 

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