Radical cation generation

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

Radical cations generated in a mass spectrometer from aldehydes and ketones with y hydrogens undergo a rearrangement in which a y hydrogen is first transferred and a carbon-carbon bond is then cleaved, e.g. 

Obviously, in the ground state of the radical cations generated, the positive charge delocalizes energetically favorably into the half-shells Si(SiR3)3, which due to the low effective nuclear charge of their Si centers [2,5a,8,9] are excellent electron donor substituents. 

Other radical cyclization approaches to the synthesis of piperidines include a CAN-mediated stereoselective cyclization of epoxypropyl cinnamyl amines <06TL705> and a cyclization of (-trimethylsilylmethylamine radical cation, generated via a photoinduced electron transfer reaction to a tethered -functionality <06JOC8481>. 

At present a variety of studies with PAH, as well as other chemicals, suggest that metabolic activation in target tissues can occur by one-electron oxidation (6,7). The electrophilic intermediate radical cations generated by thTs mechanism can react directly with various cellular nucleophiles. In this paper, we will discuss chemical, biochemical and biological evidence which indicates that one-electron oxidation plays an important role in the metabolic activation of PAH. 

Although cycloaddition reactions have yet to be observed for alkene radical cations generated by the fragmentation method, there is a very substantial literature covering this aspect of alkene radical cation chemistry when obtained by one-electron oxidation of alkenes [2-16,18-26,28-31]. Rate constants have been measured for cycloadditions of alkene and diene radical cations, generated oxidatively, in both the intra- and intermolecular modes and some examples are given in Table 4 [91,92]. 

Using alkene radical cations generated under photostimulated electron-transfer conditions, Arnold and coworkers showed that the addition of an-... 

Scheme 17 Radical cation generation and trapping by Norrish type I photofragmentation...

Scheme 20 Radical cation generation by 1,5-hydrogen abstraction and fragmentation...

A y-lactone was formed in excellent yield by the nucleophilic cyclization of a carboxylic acid onto an alkene radical cation generated from a (i-nilrophosphale under tin hydride conditions (Scheme 21) [139]. Related experiments employing the acetate group and an internal carboxylate nucleophile failed, emphasizing the very rapid collapse of the alkene radical cation/acetate ion pair [127]. 

The first evidence that the radical cation generated by a single-electron transfer (SET) of an unsymmetrical 1,5-diene 408 can undergo a [3,3]-sigmatropic shift (Cope reaction)... 

The radical cations generated at the anode surface can dimerize before they diffuse into solution. The dimeric compound formed can then be cyclized in solution via formation of a carbon-heteroatom bond. The anodic oxidation of enaminones reported earlier [46] and also recently [47] could serve as an illustration of this type of intermolecular cyclization. 

A number of papers have reported studies on pyrimidine radical cations. 1-Methylthymine radical cations generated via a triplet-sensitized electron transfer to anthraquinone-2,6-disulfonic acid were detected by Fourier transform electron paramagnetic resonance (FTEPR). The parent 1-methylthymine radical cation, and its transformation to the N(3)-deprotonated radical cation, were observed. Radical cations formed by addition of HO and POs" at C(6) were also detected depending on the pH. Similarly, pyrimidine radical cations deprotonated at N(l) and C(5)-OH were detected from the reaction of 804 with various methylated pyrimidines." These radicals are derived from the initial SO4 adducts of the pyrimidines. Radical cations of methylated uracils and thymines, generated by electron transfer to parent ions of... 

In addition to the former example, Pandey et al. achieved efficient a-aryla-tion of ketones by the reaction of silyl enol ethers with arene radical cations generated by photoinduced electron transfer from 1,4-dicyanonaphthalene. Using this strategy various five-, six-, seven-, and eight-membered benzannulated compounds are accessible in yields in the range 60-70% [39],... 

Radical-cations generated in this way are characterised by their uv spectra (Table 6.2) and their esr spectra. The oxygen lone-pairs in meftoxybenzene radical-cations participate in delocalization of the positive charge so that, on the esr time-scale, phenomena due to hindered rotation about the benzene-oxygen bond appear. The two ortAo-hydrogens in anisole radical-cation have different esr coupling constants. 1,4-Dimethoxybenzene radical-cation is found as a mixture of cis- and fra j-isomers due to this hindered rotation [22],... 

The second important reaction of radical-cations is attachment of a nucleophile to one of the ring carbon atoms. In solutions at pH > 4 radical-cations generated by... 

The Jovian moon, lo, shows an orange hue (Fig. 6.1), which may be due to long-chain alkane radical cations. The atmosphere of lo consists mostly of methane deep UV photolysis proceeds with electron ejection thus, the molecular ion of methane was perhaps the earliest organic radical cation, generated by solar irradiation aeons ago. 

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,... 

Organic radical cations generated by electron transfer photosensitization are species with greatly enhanced pK a s compared with their closed shell precursors (142). If a C-H bond is located adjacent to the site of predominant charge density, rapid deprotonation will ordinarily ensue, producing a radical whose fate determines the identity of the isolated products. ... 

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