Radical cations of amines

Photoinduced electron-transfer reaction of aromatic compounds with amines is one of the most fundamental reactions in the electron-donor-acceptor systems, which was recently reviewed by Lewis [35], Because of the low oxidation potentials of the amines, the photoinduced one-electron transfer from the amines to the excited singlet states of aromatic hydrocarbons ( Aril ) readily occurs to give the radical cations of amines and the radical anions of aromatic compounds even in the less polar solvents. 

The radical anions of electron-deficient aromatic compounds and aromatic hydrocarbons, which are generated by photoinduced electron transfer, can be proto-nated by protic solvents or by the radical cations of amines to produce their neutral radicals [Eq. (7)]. Dispropotionation of the radicals yields the reduction products. The radical anions of 1,1-diphenylethene in the presence of 1,4-dimethoxynaphthalene as an electron donor is also protonated by protic solvents to give Markownikoff adducts [402,403] (Scheme 118). 

Photoinduced electron transfer between amines and aromatic hydrocarbons occurs to generate radical cations of amines and radical anions of aromatic hydrocarbons. Pac and Sakurai reported the photoaddition of N,N-dimethylaniline to anthracene via photoinduced electron transfer [60]. In benzene, the 4n + 4n) photocyclodimer of anthracene is produced as a sole isolable product, although an emission due to the exciplex formed from anthracene and JV,N-dimethylaniline is observed. In acetonitrile, the addition of dimethylaniline to anthracene occurs via their radical ions to give 9,10- dihydro-9-(4 -dimethylaminophenyl)anthracene as the major product. However, the photoamination on anthracene takes place even in benzene when iV-methylani-line is used as an electron donor. Sugimoto and his coworkers reported the intramolecular photoaddition of anilines to aromatic hydrocarbons to give cyclic amino compounds (Scheme 16) [61-63]. 

The photoreactions of aliphatic amines with aromatic hydrocarbons have also been reported by several groups. With tertiary amines, deprotonation occurs from the radical cations of amines at the a-carbon to generate carbon radicals which react with the radical anion of aromatic hydrocarbons. With secondary amines, deprotonation from the radical cations of amines occurs both at the a-carbon and at the nitrogen atom, so that the reaction becomes complicated [64-65]. 

Photoinduced electron transfer from amines to the excited ketones generates the radical cations of amines and the radical anions of ketones [140]. The radical anions of ketones are usually reduced to the corresponding alcohols. However, in some cases the coupling reaction between the radical anions and... 

This statement does not mean, however, that the mechanism of diazotization was completely elucidated with that breakthrough. More recently it was possible to test the hypothesis that, in the reaction between the nitrosyl ion and an aromatic amine, a radical cation and the nitric oxide radical (NO ) are first formed by a one-electron transfer from the amine to NO+. Stability considerations imply that such a primary step is feasible, because NO is a stable radical and an aromatic amine will form a radical cation relatively easily, especially if electron-donating substituents are present. As discussed briefly in Section 2.6, Morkovnik et al. (1988) found that the radical cations of 4-dimethylamino- and 4-7V-morpholinoaniline form the corresponding diazonium ions with the nitric oxide radical (Scheme 2-39). 

FDMR has also been used to detect the transient radical cations formed from secondary amines by pulse radiolysis. As mentioned earlier this technique has been used to study a variety of systems such as the radical cation of triethylamine. The radical cations of diethylamine, n-propyl amine and f-butylamine, have also been studied25. The results have shown that the FDMR signal is enhanced with increasing alkyl substitution of the amine as in the pyrrolidines (18) and the piperidines (19)25. 

Other important aromatic amines such as chlorpromazine (26) have also been subjected to oxidation studies using oxidants produced by pulse radiolysis. Typical among these is the use of chloroalkylperoxyl radicals formed by pulse radiolysis in a variety of solvents. These oxidants yield the corresponding radical cation. The rate constants (Table 3) for these reactions were determined42. Other studies have determined the reactivity between chlorpromazine and BiV- in H2O/DMSO in varying proportions. The rate constants for the formation of the radical cation of chlorpromazine were similar in value to those obtained from the peroxy radical reactions4. 

A variety of amines have been used as mediators for electrochemical oxidation reactions. In these reactions, the amine is oxidized at an anode surface to form a radical cation. The amine radical cation then oxidizes a second substrate triggering a reaction of synthetic interest. The regenerated amine is then reoxidized at the anode... 

Specifically, the mediators (Med ) used were the radical cations of tris-(4-bromophenyl)amine and 2,3-dihydro-2,2-dimethylphenothiazine-6(17/)-one [59], The results of the oxidative cyclizations under the homogeneous oxidation conditions are parallel to those obtained by direct anodic oxidation. 

The 1,4-photoaddition of aliphatic amines with benzene via photoinduced electron transfer was first reported by Bryce-Smith more than 30 years ago [375-378], In the photoreaction of triethylamine with benzene, the proton transfer from the radical cation of triethylamine to the radical anion of benzene is proposed as a probable pathway (Scheme 113). In the case of tertiary amines, the photoaddition is accelerated by the addition of methanol or acetic acid as a proton source. Similar photoaddition of diethyl ether to benzene takes place assisted by trifluoroacetic acid, where methanol is not affective [379], In these photoreactions, a-hydrogen next to the heteroatom moves to the radical anion of benzene as a proton, followed by radical ccoupling to give 1,4-addition products. Similar photoaddition of amines to the benzene ring has been reported by Ohashi et al. [380,381],... 

Nucleophilic attack of amines to the radical cations of aromatic compounds [Eq. (6)] is much more favorable than the direct attack to the aromatic rings bearing electron-donating substituents or unsubstituted aromatic hydrocarbons [Eq. 

Radical Cations of n-Donors — Electron Transfer Reactions of Amines 169... 

The deprotonation step, either by the sensitizer radical anion or by some adventitious base, is essential for the formation of any amine derived products. This step can be prevented if the a-hydrogens are arranged in a plane orthogonal to the singly occupied nitrogen n-orbital a requirement which is met for the radical cation of l,4-diazabicyclo[2.2.2]octane (DABCO). The low oxidation potential, due to the interaction of the pair of transannular nitrogens, makes this an excellent electron transfer quencher. Yet, no product formation is observed as a result of these interactions, with the possible exception of the zwitterionic adducts formed with highly electrophilic ketones [193]. 

With some of these quenchers, formation of UO2 and/or of the radical cation of the quencher was also observed in flash experiments216,217> 22°). However, it is not clear214) how much of this proceeds from an intermolecular route, since UOl+ is known to complex, for example, with aromatic amines. 

Protonation is another common reaction in this category and reduction of the acceptor via protonation of the radical anion (e.g. by moisture present in the solvent) is often observed as a side process. It may become the main path either under acidic conditions or if the radical cation of the donor is a good proton donor (Scheme 3). As an example, the 1,2-dihydro derivative is the main product from 1,4-dicyanonaphthalene (DON) when irradiated in the presence of a donor and trifluoroacetic acid [33]. Such a reaction is more important when the radical cation is the proton source. Typically, the irradiation of naphthalene with tertiary amines leads to a mixture of 1,4-dihydro- and 1,2,3,4-tetrahydro-naphthalene, as well as tetrahydrobinaphthyl and l(l-diethylamino)ethyl-l,4-... 

Benzyl ethers are not cleaved by 1. Removal of the benzyl ether function can, however, be achieved with the cation radicals of amines 2 or 3. Thus selective deprotection of primary and secondary hydroxyl groups is possible. These deprotections can be performed in the presence of primary or secondary alkyl bromides, which are unaffected by 1 or the radical cations of 2 or 3. 

The unrestricted and free electron transfer (FET) from donor molecules to solvent radical cations of alkanes and alkyl chlorides has been studied by electron pulse radiolysis in the nanosecond time range. In the presence of arenes with hetero-atom-centered substituents, such as phenols, aromatic amines, benzylsilanes, and aromatic sulfides as electron donors, this electron transfer leads to the practically simultaneous formation of two distinguishable products, namely donor radical cations and fragment radicals, in comparable amounts. 

As was noted in Scheme 12, distonic radical cations obtained from cyclopropane bond cleavages add oxygen rapidly, producing products with two CO bonds. So do some alkene radical cations. Addition of O2 to an alkene radical cation is formally a nucleophilic attack by the single alkene n electron on O2, and oxidizes both carbon atoms (an alkene radical cations has formally two -f carbons, and the adduct a 1+ and an oxygen-bound carbon). The oxygenation of the radical cation of bia-damantylidine (96) leads to dioxetanes such as 98 in chain reactions (see Scheme 21) [110]. The reactions may be initiated electrochemically or photochemically, but tris(o,p-dibromophenyl)amine hexafluoroantimonate, 97, is a superior catalyst for the dark reaction of certain tetraalkylalkenes, with turnovers up to ca. 800 at... 

The amine radical cation of triethylamine formed in these reactions can exist in an acid-base equilibrium, as shown in Scheme 6. 

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