Substitution reactions radical cations

This preliminary study represents the first example of nucleophilic substitution at the periphery of porphyrin nuclei, and will later appear as a convenient route for the synthesis of a variety of wc o-substituted porphyrins. Indeed, a similar reactivity has been reported in several publications. In particular. Smith and coworkers investigated the nucleophilic substitution of nitrite, chloride, pyridine, imidazole, cyanide, triphenylphosphane, thiocyanate, acetate, and azide to P-octaaUc-ylporphyrin mero-positions [103-106]. To perform such substitution reactions, radical cations of the parent unsubstituted porphyrins were always obtained by chemical processes, using oxidants such as iodine, thallium nitrate, triarylammo-niumyl salts, and A -chlorobenzotriazole. 

The overall conclusion from the reaction of BP and 6-substituted BP radical cations with nucleophiles of various strengths is that weak nucleophiles display higher selectivity toward the position of highest charge localization. Thus another important factor in the chemical reactivity of radical cations is represented by the strength of the nucleophile. 

Photo-induced electron-transfer decarboxylation reactions have been reviewed. A variety of methyl- and methoxy-substituted phenol radical cations have been generated by either photo-induced electron transfer or photo-ionization in dry solvents such as acetonitrile. In the presence of small amounts of water the radical cations are not detected and the phenoxyl radical is the only transient species observed. The 2-methoxyphenol radical cation was found to be more reactive than the 4-methoxy radical cation. 

A review considering the generation and characterization of radical ions, their reactions, formation of species with three-electron bonds, and radical cations of strained systems has been published." The redox and acidity properties of a number of substituted benzene radical cations were smdied by pulse radiolysis. ... 

Bimolecular Reaction of / -Substituted Triphenylphosphines Radical Cation with Water... 

Scheme 13. Reactions of a substituted trimethylenemethane radical cation.

Generation of substituted aryl radical cations in the presence of nucleophiles can lead to products of side-chain substitution (processes such as anodic benzylic substitution of toluenes, which are dealt with in a separate chapter) or to products of addition to the aromatic ring itself. Nuclear addition products in /j /m-substituted systems have been proposed to form in essentially one of two ways, depending on substitution pattern and reaction conditions. Radical cations formed by electrochemical reaction (E) may be trapped by chemical reaction (C) with a nucleophile (or its anion). Repeating this sequence leads to nuclear addition products (LXV), formed by what is referred to as the ECEC mechanism [Eq. (31)] [74]. An analogous pattern may be inferred for or / (9-substituted systems. 

The reaction of a series of substituted styrene radical cations with anions has recently been studied in detail by laser flash photolysis. Representative kinetic data are summarized in Tables 3 and 4 and demonstrate that most of the anions studied react with styrene radical cations with diffusion controlled rate constants. These reactions can involve either addition to the p-carbon to give a benzyl radical (Eq, 15) as discussed above or electron transfer to regenerate the precursor alkene plus the oxidized nucleophile (NU , Eq. 16). Transient absorption spectra have been used to distinguish between these two possibilities. For example, reaction of the radical cation with either bromide or chloride leads to the formation of a transient that is identified... 

The data for the reactions of four substituted styrene radical cations with selected dienes are summarized in Table 8. - As discussed above for the reaction of styrene radical cations with nucleophiles, the interpretation of these data is complicated by the possibility that two competing reactions are responsible for the observed quenching of the radical cation. One of these is electron transfer from the alkene to the styrene radical cation to generate the neutral styrene and the radical cation of the alkene (Eq. 29). In this case, the quenching rate constant is that for electron transfer, and does not provide any information on the kinetics for the initial addition, although the secondary radical calion/neutral pair may in some cases lead to adduct formation. The other reaction is addition of the alkene to the radical cation to generate an adduct radical cation that is the precursor of the final cyclobutanation and Diels-Alder products (Eq. 30). 

The neat resin preparation for PPS is quite compHcated, despite the fact that the overall polymerization reaction appears to be simple. Several commercial PPS polymerization processes that feature some steps in common have been described (1,2). At least three different mechanisms have been pubUshed in an attempt to describe the basic reaction of a sodium sulfide equivalent and -dichlorobenzene these are S Ar (13,16,19), radical cation (20,21), and Buimett s (22) Sj l radical anion (23—25) mechanisms. The benzyne mechanism was ruled out (16) based on the observation that the para-substitution pattern of the monomer, -dichlorobenzene, is retained in the repeating unit of the polymer. Demonstration that the step-growth polymerization of sodium sulfide and /)-dichlorohenzene proceeds via the S Ar mechanism is fairly recent (1991) (26). Eurther complexity in the polymerization is the incorporation of comonomers that alter the polymer stmcture, thereby modifying the properties of the polymer. Additionally, post-polymerization treatments can be utilized, which modify the properties of the polymer. Preparation of the neat resin is an area of significant latitude and extreme importance for the end user. 

An interesting method for the substitution of a hydrogen atom in rr-electron deficient heterocycles was reported some years ago, in the possibility of homolytic aromatic displacement (74AHC(16)123). The nucleophilic character of radicals and the important role of polar factors in this type of substitution are the essentials for a successful reaction with six-membered nitrogen heterocycles in general. No paper has yet been published describing homolytic substitution reactions of pteridines with nucleophilic radicals such as alkyl, carbamoyl, a-oxyalkyl and a-A-alkyl radicals or with amino radical cations. 

The reaction proceeds by an ET pathway giving the I9e organoiron radical cation and the organic radical R which couple in the cage after escape ofX. The cationic Fe1 intermediate is noted at low temperature by its characteristic purple color and the classical spectrum of Fe1 species with rhombic distortion (g = 2.091, 2.012, 2.003 at —140 °C in acetone) before collapse to the orange substituted cyclohexa-dienyl Fe11 complexes. 

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