Arenes radical amination

The photolysis of donor-acceptor systems shows a reaction pattern of unique synthetic value. Direct irradiation of the donor-acceptor pairs, such as arene-amine, leads by intramolecular electron transfer, to amine radical cations and arene radical anions. The generated radical cation and radical anion intermediates undergo cyclization reactions providing efficient synthetic routes to N-heterocycles with a variety of ring sizes. 

Removal of an electron generally brings about significant thermochemical C-H bond weakening both in a + n) radical cations (i.e. deprotonation in the benzylic position [152]) and a- -n) radical cations (i.e. deprotonation at the a-carbon of amines [153, 154], ethers [155], and thioethers [156]). The kinetic acidity of benzyl radical cations is rather well understood [152d, 157, 158], although occasionally an addition-elimination route was favored over direct proton transfer [159]. For alkyl arene radical cations [160] a linear relationship has been established between logkdep and thermochemical acidity [157]. 

The exciplexes formed between excited arenes (e.g. phenanthrene, naphthalene, 2,3-dimethylnaphthalene) and the acceptor 1,4-dicyanobenzene in ether solutions containing 1-propylamine are quenched by the addition of tetramethylammonium tetrafluoroborate, and products of addition, (129)-(131), are obtained. The authors suggest that under the relatively non-polar conditions the added salt promotes charge separation in the exciplex so that the amine can attack the arene radical cation. 

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. 

Irradiation of haloarenes in the presence of amines in polar solvents promotes electron transfer from the eunine to the excited arene this can be followed by loss of halide from the resulting arene radical anion. Ultimately dehalogenated arenes are obtained. Two new reports of this process have been published, one for 9,10-dlchloroanthracene and the other for hexabromobenzene. ... 

Irradiation of aromatic hydrocarbons such as phenanthrene, anthracene, naphthalene, and certain substituted naphthalenes in the presence of NaBH4 and m- or p-(NC)2C6H4 promotes a Birch-type photoreduction.The reaction seems to occur by electron transfer from the excited singlet state of the arene to the electron acceptor giving the arene radical cations, which are then reduced by the borohydride. Other reducing agents such as NaBH, NaBHjCN, and NaBH(OMe)3 have been found to be effective and all lead to different isomer ratios. In a mechanistically related reaction, both fluoren-9-ol and the corresponding acetate are reported to be photoreduced to the parent hydrocarbon in the presence of aliphatic amines. The products arise by photoinduced electron transfer followed by proton transfer from the amine. The yield depends on the structure of the amine and increases in the order primary < secondary < tertiary amine. In... 

The PET-generated arene radical cations also undergo nucleophilic substitution via the a-complex. Photocyanation of arenes may be cited in this context as a very early example [139], where hydrogen served as the group undergoing displacement. This concept is further extended [140] for the direct amination of polynuclear aromatic hydrocarbons with ammonia or primary amines via the arene radical cation produced by irradiating arenes in die presence of DCNB. Another potentially useful application of this methodology is... 

Arene radical anions, particularly from polynuclear aromatic hydrocarbons (e.g., phenanthrene, anthracene, and pyrene), generated by ET using amines as the electron donor has been shown [165] to undergo carboxylation reaction (e.g., Phen->189) by the electrophilic addition of CO2, followed by the termination of the resultant radical species by H-abstraction from the solvent (Scheme 39). A laser flash photolysis study [166] has recently confirmed the involvement of arene radical anions in this reaction. 

The initial step in the mechanism outlined in Scheme 2 is electron transfer quenching of the singlet arene by DCNB. Nucleophilic addition of the amine to the arene cation radical followed by proton and electron transfer steps yields the adduct and regenerates the sensitizer. Adduct formation requires the use of polar solvents, and yields are higher in aqueous vs. dry acetonitrile. Adduct formation is observed in moderately polar solvents (ethers) in the presence, but not in the absence, of an added salt, n-Bu4NBF4. The solvent and salt effects were interpreted as evidence for C-N bond formation via the free arene radical cation, rather than via an ion pair (CRIP or SSRIP), However, Nieminen et ah concluded that nucleophilic attack involves a radical ion pair on the basis of their laser flash photolysis investigation. In addition to this unresolved controversy, the timing of the subsequent proton transfer and electron transfer steps remains to be established. 

Homolytic aromatic substitution can also be performed with electrophihc het-eroatom-centered radicals. The radical amination of arenes with electrophilic diaUcyl aminyl radical cations represents a valuable method for the preparation of aniUne derivatives [2e, 111a]. The radical cations (R2HN ) are generally prepared from the corresponding N-chloroamines in an acidic medium, using catalytic amounts of a metal salt [Fe(II)-, Ti(III)-, Cu(I)- and Cr(II)-salts]. The reaction func-... 

Ammonia or amines were likewise used as nucleophiles in place of alcohols [52]. Here, PET between an arene S (e.g. triphenylbenzenes) and 1,4-dicyanoben-zene (DCB) led to the arene radical cation S and the radical anion DCB. Secondary ET from a donor (D) to S produced the radical cation of the former (D ) that added ammonia. After deprotonation, coupling of the resulting neutral radical with DCB and cyanide loss led to the corresponding NOCAS product Suitable donors were arylcyclopropanes, quadricyclane and dienes that gave 4-(l-aryl-... 

Support-bound triazenes, which can be prepared from resin-bound secondary aliphatic amines and aromatic diazonium salts [455], undergo cleavage upon treatment with acids, leading to regeneration of the aromatic diazonium salts. In cross-linked polystyrene, these decompose to yield nitrogen and, preferentially, radical-derived products. If the acidolysis of polystyrene-bound triazenes is conducted in the presence of hydrogen-atom donors (e.g. THF), unsubstituted arenes can be obtained (Entries 8 and 9, Table 3.47). In the presence of alkenes or alkynes and Pd(OAc)2, the initially formed diazonium salts undergo Heck reaction to yield vinylated or alkynylated arenes (Entry 10, Table 3.47). Similarly, unsubstituted arenes can be obtained by oxida-... 

As with arene-amine radical ion pairs, the ion pairs formed between ketones and amines can also suffer a-deprotona-tion. When triplet benzophenone is intercepted by amino acids, the aminium cation radical can be detected at acidic pH, but only the radical formed by aminium deprotonation is detectable in base (178). In the interaction of thioxanthone with trialky lamines, the triplet quenching rate constant correlates with amine oxidation potential, implicating rate determining radical ion pair formation which can also be observed spectroscopically. That the efficiency of electron exchange controls the overall reaction efficiency is consistent with the absence of an appreciable isotope effect when t-butylamine is used as an electron donor (179). 

In 1990, Porta and co-workers [6] reported the first nucleophilic radical addition to aldimines promoted by aqueous TiCl3, based on a one-pot tricomponent reaction involving an aromatic amine, a generic aldehyde, and an arene-diazonium salt (Equation 14.15). 

Titanium plays a key role in this process owing to its reducing power in the Ti(III) state, it acts as radical initiator generating Ar- by decomposition of arene-diazonium salts (Equation 14.16) owing to its Lewis acid character in the Ti(IV) state, it promotes the formation of the inline [29] from a generic aldehyde and a primary aromatic amine in the aqueous medium (Equation 14.17) and increases... 

The studies related to the interactions of electronically excited arene molecules with tertiary amines have provided a basis for the present understanding of exciplexes and radical ion-pair phenomena [41,82], PET reactions of amines yield planar amine radical cations (Eq.20) which are deprotonated to give a-amino radicals (Eq.21) and usually cross-coupling (Eq.22) between radical pairs of donor-acceptor terminates the photoreaction [32a, 83]. Mechanistic studies revealed contact ion pair (CIP) intermediate for these reactions [84, 85]. 

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