Radical-cations from arenes alkylation

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

However, it is difficult to reconcile the observed relative reactivities of hydrocarbons with a mechanism involving electron transfer as the rate-determining process. For example, n-butane is more reactive than isobutane despite its higher ionization potential (see Table VII). Similarly, cyclohexane undergoes facile oxidation by Co(III) acetate under conditions in which benzene, which has a significantly lower ionization potential (Table VII), is completely inert. Perhaps the answer to these apparent anomalies is to be found in the reversibility of the electron transfer step. Thus, k-j may be much larger than k2 for substrates, such as benzene, that cannot form a stable radical by proton loss from the radical cation [Eqs. (224) and (225)]. With alkanes and alkyl-substituted arenes, on the other hand, proton loss in Eq. (225) is expected to be fast. 

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. 

Photoamination of a variety of aromatic compounds with ammonia and alkyl-amines under photosensitized electron transfer mediated reaction conditions has been extensively investigated by Yasuda and coworkers (Scheme 70) [320-324]. The photoamination reactions of stilbene derivatives with ammonia have been utilized for the synthesis of a variety of isoquinoline derivatives [324]. The photoamination is initiated by photochemical electron transfer from the arenes to the electron acceptor followed by nucleophilic attack of ammonia or primary amines on the aromatic radical cations (Scheme 70). 

Ultra-violet light irradiation of tetramethylphenylene diamine (152) in aqueous or methanolic solutions of alkyl nitriles yields the ortho-amino substituted product (153). ° With benzyl nitriles the ortho-benzyl product (154) is obtained. It is proposed that both products are produced by electron transfer from the excited state of the diamine to the nitrile. With alkyl nitriles the products are derived from solvolysis of the adduct formed by attack of the nitrile nitrogen on the arene radical cation, while with benzyl nitrile electron transfer from the excited amine gives the benzyl nitrile radical anion which ionises by loss of cyanide the resulting benzyl radical then attacks the diamine. 

In view of the selectivity and predictability of both the cleavage reaction of the cation radical and the addition of benzyl radicals to the anion radical, this process can be considered a useful method for aromatic alkylation, although, as it appears from the foregoing discussion, it is limited to arene polynitriles. Provided that the thermodynamic requirements for the various steps are met, chemical yields are often satisfactory since, as... 

Apart from titanocene initiators, iron arene complexes have also been applied for light-induced radical polymerization, namely of acrylates [202-204]. Iron arene complexes were originally developed for cationic polymerization because they release acid upon irradiation. However, light-induced reactions include the formation of alkyl radicals, which may be utilized for radical initiation. 

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