Nucleophilic addition donor radical cations

A further variation of these functionalizations of cyanoarenes is the NOCAS process [14, 15]. As shown in Scheme 14.2, path g, this involves the addition of a nucleophile (which is often the solvent) to the donor radical cation. The thus-formed neutral radical adds to the acceptor radical anion, while rearomatization by the loss of an anion leads again to an overall ipso-substitution. AUenes could be used as the donors in these reactions, as shown recently by Arnold [50]. Accordingly, the irradiation of TCB in the presence of tetramethylaUene (15) in a 3 1 MeCN/MeOH mixture afforded 1 1 1 arene-allene-methanol adduct 16 in 48% yield (Scheme 14.9, central part). Interestingly, the addition of methanol took place exclusively at the central allene carbon, while aromatic substitution occurred through the terminal carbons. co-Alkenols, in which an O-nucleophile and an easily oxidized moiety are both present, could also be used. In the latter case, the initial ET was followed by a cyclization, yielding aryl-substituted tetra-hydrofurans or tetrahydropyrans as the final products via a tandem Ar—C, C—O bond formation [51]. 

Contrary to path e, the other paths are based upon the stability of the electron-withdrawing substituted aromatic radical anions. Indeed such extensively delocalized species are often weak bases and nucleophiles and may be quite persistent (in the case of polycyanoaromatics, indefinitely persistent in solution under appropriate conditions). In this case, the following reaction depends on the formation of a reactive intermediate from the cation radical of the donor. Typical examples are as follows. A first possibility is fragmentation of the radical cation, yielding a radical that couples with the aromatic radical anion (path/). In a variation of this mechanism, the radical is first trapped by a radicophile and it is the radical adduct that couples with the radical anion (path g). A further possibility is addition of a nucleophile to the radical cation, and coupling of the resulting radical with the aromatic radical anion (path h). 

Owing to the existence of two centers for nucleophilic attack (at C2 and C5) in radical cations (220) obtained from the oxidation of 4-H -imidazole-1,3-dioxides (219), the formation of two products of methoxy group addition was observed, namely NNR (221) and NR of 3-imidazoline-3-oxide (222). The ratio of the products depends on the electronic nature of substitutes R1 and R2. Both, the donor character of R1 and acceptor character of R2 facilitate the formation of nitroxyl radicals (222) with the yield of (221) increasing with the inverted effect of the substituents. As was mentioned in Section 2.4, the results of preparative electrochemical oxidative methoxylation of 4H -imidazole-1,3-dioxides are similar to the results of chemical oxidation. 

Radical cations exhibit a wide variety of reactions, including unimolecular reactions such as rearrangement, fragmentation, and intramolecular bond formation as well as bimolecular reactions with ionic, radical, or ground state species. Notable processes include reaction with nucleophiles to produce radicals, reaction with radicals to produce cations, reaction with electron donors to produce biradicals, and reaction with ground state molecules to give addition products. Often the products of reactions of radical cations with neutral species are different from those observed by reaction of the corresponding carbocation with the same reactant... 

With benzene and biphenyl the nucleophilicity of the solvent was particularly critical, due to the high reactivity of the radical-cation. By testing different solvents such as DCM, NBACN or NM (considered as acidic, according to the Lewis concept), PC (neutral), MeOH, THF, DMSO and DMF (basic) it was found that the electropolymerization of benzene and biphenyl only occurred with strongly anhydrous ((H2O) < 5.10 M) solvents having a donor number (DN) less than 15 or a pKbh+ of less than —10 [113]. Addition of a strong acid to the solvent was also favourable, and it was found that the addition of triflic acid in NM increased the current efficiency of benzene electropolymerization considerably [107]. 

Photoamination of 6 proceeds via the nucleophilic addition to the radical ion pairs between 6 and m-DCB followed by one-electron reduction of the aminated radicals with m-DCB" and protonation to give 26 according to Scheme 6.8. However, the highly reactive localized cation radicals of 6, in general, tends to cause side reactions involving dimerization, deprotonation, and isomerization, resulting in the amination in lower yields. It is well known that aromatic hydrocarbons (ArH) work as tr-donor that can interact with a cation radical [49]. The additive effect of 1,3,5-TPB or m-TP appears to come from the formation of Ji-complex with 6". The Jt-complex formation with ArH would lower the reactivity of the cation radicals and suppress the side reactions, resulting in the effective photoamination of 6. 

The DCNB-sensitized addition reactions of 1,1-diarylethylenes with ammonia or primary amines yield the a ri-Markovnikov adducts. The mechanism is analogous to that shown in Scheme 7 for addition to sthbene. The regioselectivity is determined by nucleophilic attack of the amine on the alkene cation radical to yield the more stable benzyl radical intermediate. The mechanism and dynamics of the reactions of p-methoxystyryl radical cations with amines have been investigated. Anihne and EtjN are found to react as electron donors with rate constants near the diffusional Hmit. Primary amines react as nucleophiles, with somewhat slower rate constants. 

The photo-NOCAS reaction was first described by McMahon and Arnold and is a photonucleophilic Sfj2Ar aromatic substitution between dicyanobenzene and an olefin in the presence of electron donor photosensitizers (phenanthrene or biphenyl) in acetonitrile-methanol solutions. This reaction system has been researched extensively in recent times. As shown in Scheme 6, the single electron transfer from olefin to photo-excited electron-deficient dicyanobenzene forms the cation radical of the olefin, which initiates a quenching reaction with nucleophile solvent methanol molecules and forms the methoxyalkyl radical. Addition of an electron transfer photosensitizer (phenanthrene or biphenyl) to the reaction mixture increases the efficiency of the reaction simply by absorbing more Hght. The excited state of the photosensitizer donates an electron to dicyanobenzene to give the photosensitizer radical cation and dicyanobenzene radical anion. The photosensitizer radical cation then oxidizes the olefin. 

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