Amine-derived radicals

These results also confirm that, when the amine function is linked to the polymer, the grafting efficiency is greatly enhanced, the amine-derived radical being a much better initiating species than the semipinacol radical. 

Taking into account that for poIy(VBP-co-DMAS) a pronounced tendency to exciplex formation is observed by phosphorescence measurements [54-57], the above results can be interpreted [20,22] by assuming the occurrence of cage selfcombination of the amine-derived radicals (formed by hydrogen transfer within the radical ion pairs) due to their high local concentration along the polymer... 

Scheme 18. A, amine A, the amine-derived radical on the a-carbon M, monomer molecule. According to Encinas et al. [155], both radicals A and MH are able to initiate polymerization of vinyl monomers.

When one of the substituents on the amine nitrogen atom is ready to trap a radical formed by treatment of iV-(a-aminoalkyl)benzotriazole with Sml2, cyclization may occur. Such a situation is depicted in Scheme 102. Thus, (4-penten-l-yl)amine derivative 652 is reduced to radical 653 that is then rapidly trapped by the alkenyl group and... 

The photolysis of donor-acceptor systems provides unique synthetic opportunities. Direct irradiation of the donor-acceptor systems, such as systems containing arene and amine components, leads to intramolecular electron transfer, that is, to amine cation-radical and arene anion-radical moieties. After generation, these moieties undergo cyclization reactions providing efficient synthetic routes to fV-heterocycles with a variety of ring sizes. Thus, direct irradiation of secondary amino-ethyl and aminopropyl stilbenes leads to benzazepines in improved yields (Hintz et al. 1996). As known, benzazepines are used in medicine as antidepressants. Scheme 7.44 illustrates ion-radical cyclization with the formation of benzazepine derivative (65% yield). 

Using this method, the dimerization reactions of short lived methyl-diphenylamine and diphenylamine radical cations are successfully investigated61. In this study tris(4-bromophenyl)amine cation radical (TBPA,+) was used as 1 . Oyama et al.,94 used tris(2,4-dibromophenyl)amine (TDBPA) as the reaction initiator (M,+) and spectroscopically detected anthracene derivative cation radicals in acetonitrile using the ESTF method. This approach holds good potential for evaluation of the reactivities of short lived cation radicals. 

The most conspicuous property of aliphatic amines, apart from their fishy smell, is their high basicity, which usually precludes N-alkylations under acidic reaction conditions (last reaction, Scheme 6.3). Hence, alkylation of amines with tertiary alkyl groups is not usually possible without the use of highly stabilized carbocations which can be formed under basic reaction conditions. Rare exceptions are N-alkyla-tions of amines via radicals (Scheme 4.2), copper-catalyzed propargylations (Scheme 6.3), and the addition of amines to some Michael acceptors and allyl palladium or iridium complexes. Better strategies for the preparation of tert-alkylamines include the addition of Grignard reagents to ketone-derived imines [13] or the reduction of tert-alkyl nitro compounds. 

In summary, these results constitute strong evidence for the two-step reaction sequence. They require that the deprotonation of the aminium radical cation be competitive on the CIDNP timescale i.e. surprisingly fast since it involves a carbon acid. The results delineate the fate of the amine derived intermediates with particular clarity, since they are observed directly for amine derived products. The conclusions based on the above CIDNP results were confirmed by time resolved optical spectroscopy in a variety of systems [179-182]. However, in essentially all these systems the reaction progress is monitored by following the complementary spectra of the acceptor derived radical intermediates, such as ketyl, semiquinone, stilbene, or thioindigo radical anions. 

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]. 

We have limited our discussion of n-donor derived radical cations to a single structure type. Other n-donors include hydrazines, polycyclic amines, ethers, phosphines, and thioethers. Several dioxetane radical cations were mentioned in Section 4.1. A detailed or even cursory discussion of these donors exceeds the scope of this article. 

In most cases, no significant interaction between ketyl type radicals and the monomer was observed. This suggests that the amine derivative species A- is the only initiating species. In contrast, the ketyl radicals terminate the chain reaction of many monomers. 

It was reported [231] that phenolic and aromatic aminic AO are able to form bound-in species in EPM cured with peroxides. This principle was exploited in NR doped with 2,6-di-terr-butyl-4-methylphenol and tert-butyl peroxide. Other phenolic AO bearing methyl groups may take place in a similar process too [232]. The extent of the coupling of radicals derived from phenolic antioxidants with macroalkyls is influenced by the concentration of phenols. A competitive process, autocoupling of phenol derived radicals, increases with increasing concentration of the phenolic antioxidant [17]. 

As for the amine moiety, the rearrangement has been demonstrated (although not unequivocally in all cases) for mono-A -substituted glycosyl-amines derived from aromatic amines and aralkyl- and alkyl-amines and for di-A-substituted glycosylamines containing dialkyl, alkyl-aryl, aralkyl, isocyclic, and heterocyclic alkyl radicals. The rearrangement has been reported for A-glucosyl derivatives of amino acids, but not as yet... 

N -Disulfonimides, RN(S02R )2, have been advanced as amine derivatives suitable for Sn2 processes, at least in those cases where R is a primary radical. N-(l-Decyl)-A(,lV-di(p-toluene)sulfonimide... 

Benthoeyanin A is a powerful radical scavenger from the mycelium of Streptomyces prunicolor. A new route to phenazine is described via the Pd(II)-catalyzed intramolecular amination of aryl bromide [9]. Treatment of 2-bromo-3-nitro-methylbenzoate (230) and 6-aminobenzofuran derivatives 231 with Pd(OAc)2 gave nitrodiphenylamine 232 in 99%. Catalytic hydrogenation of 232 followed by bromination gave the phenylenedi-amine derivative 233 in 91% yield. Cyclization of the diamine 233 provided the key intermediate phenazine 234. 

Corresponding reactions to those noted above with alcohols are found to occur with amines, the radical produced being derived by hydrogen abstraction from the carbon linked to the heteroatom. Radical generation can be effected with y-irradiation or UV light (> 250 nm or > 290 nm) using either liquid amines or aqueous solutions. The tendency to lose the heteroatom from the radical moiety is more pronounced than with the alcohol derivatives. The extreme case is found with 6-unsubstituted purines, in which no 6-aminoalkylpurine is obtained concomitant deamination occurs giving the 6-alkylpurine as product (Scheme 7). 

According to this mechanism, hindered-amine derivatives terminate propagating reactions (eqs. 5 and 6) by trapping both the alkyl and peroxy radicals. In effect, NO competes with 02, and NOR competes with RH. Since the nitroxyl radicals are not consumed in the overall reactions, they are effective at low concentrations. 

The next example of comparatively simplicity, this time nonaqueous, is the crystalline alanine. There are several products of irradiation of that solid crystalline amino acid. In this state it occurs as zwitterion as NMR shows, i.e. the amine group is protonated -N+H3. Single ionization spurs, of a low energy, cause deamination which leads to detachment of ammonia and formation of a free radical. Pulse radiolysis of single crystals of L-alanine shows, that the alanine derived radical CH3-C H-C02-, which shows the spectrum with maximum at 348 nm [9], stabilizes during 5 milliseconds [10], It is usually observed not spectroscopically but by the EPR method [11] it shows extreme stability, being applied as reference dosimeter. 

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