Amine radical cations

The mechanism proposed for the production of radicals from the N,N-dimethylaniline/BPO couple179,1 involves reaction of the aniline with BPO by a Sn-2 mechanism to produce an intermediate (44). This thermally decomposes to benzoyloxy radicals and an amine radical cation (46) both of which might, in principle, initiate polymerization (Scheme 3.29). Pryor and Hendrikson181 were able to distinguish this mechanism from a process involving single electron transfer through a study of the kinetic isotope effect. 

It has been suggested that the amine radical cation (46) is not directly involved in initiating chains and that most polymerization is initiated by benzoyloxy radicals.179 However, Sato et a ." employed spin trapping (3.5.2.1) to demonstrate that the anilinomethyl radical (45) was formed from the radical cation (46) by loss of a proton and proposed that the radical 45 also initiates polymerization. Overall efficiencies for initiation by amine-peroxide redox... 

Morkovnik et al. (1989) found experimentally that the addition of an equimolar amount of 4-morpholino- or 4-dimethylaminoaniline to a suspension of nitrosyl perchlorate in 100 % acetic acid, dioxan, or acetonitrile yields a mixture of the diazonium perchlorate and the perchlorate salt of the amine radical cation, with liberation of gaseous nitric oxide. Analogous results in benzene, including evidence for radicals by ESR spectroscopy and by spin trapping experiments, were obtained by Reszka et al. (1990). 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

In piperidine the electron lone-pair can occupy either an axial or an equatorial position in 1-methylpiperidine the axial orientation (lb) is favoured by 99 1 over the equatorial (la). PE spectra and ab initio calculations on methylpiperidines indicate that axial 2-methyl substituents lower the amine lone-pair ionization potential by about 0.26 eV, while equatorial 2-methyl substituents as well as methyl groups on carbon atoms 3 and 4 lower the lone-pair IP by less than 0.1 eV63. This establishes the mechanism of stabilization of the amine radical cation as hyperconjugative electron release, which is larger for CC bonds than for CH bonds. The anti-periplanar orientation of the nitrogen lone-pair and the vicinal C—Me bond (lc) is much more favourable for this type of interaction than the synclinal orientation (Id). 

Since amines generally have low oxidation potentials, they are good electron donors in their ground state, and the donor ability is further enhanced by photoexcitation. The chemical consequence of this single electron transfer (SET) is the generation of the amine radical cations (aminium radicals) and an earlier review on the aminium radicals is available1. 

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... 

Scheme 2 Anodic methoxylation of / -methylstyrene without and with an amine radical cation as mediator.

Scheme 3 Electrochemical Wacker process with amine radical cation-mediated Pd(II) regeneration.

In addition to generating amine radical cation mediators and functionalizing... 

Ingemann, S. Hammerum, S. Derrick, P.J. Fokkens, R.H. Nibbering, N.M.M. Energy-Dependent Reversal of Secondary Isotope Effects on Simple Cleavage Reactions Tertiary Amine Radical Cations With Deuterium at Remote Positions. Org. Mass Spectrom. 1989, 24, 885-889. 

Photoinduced electron transfer promoted cyclization reactions of a-silyl-methyl amines have been described by two groups. The group of Pandey cyclized amines of type 135 obtaining pyrrolidines and piperidines 139 in high yields [148]. The cyclization of the a-silylated amine 140 leads to a 1 1 mixture of the isomers 141 and 142 [149]. The absence of diastereoselectivity in comparison to analogous 3-substituted-5-hexenyl radical carbocyclization stereochemistry [9] supports the notion that a reaction pathway via a free radical is unlikely in this photocyclization. The proposed mechanism involves delocalized a-silylmethyl amine radical cations as reactive intermediates. For stereochemical purposes, Pandey has investigated the cyclization reaction of 143, yielding... 

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. 

Table 6.7. Reversible potentials for amine radical-cation systems in...

G. Pandey, Synlett 1992, 546-552 . .Synthetic Perspectives of Photoinduced Electron Transfer Generated Amine Radical Cations". 

The radical pair generated by proton transfer from tertiary amine radical cations to a,p-unsaturated ketone radical anions (e.g., 71) couple in the p position, forming... 

In analogy to the a-deprotonation of tertiary amine radical cations (see above), amines bearing an a-trimethylsilyl group may undergo heterolytic cleavage of the C—Si bond upon ET, particularly in the presence of nucleophiles. The resulting aminoalkyl radical may couple, for example, to the enone radical anion (71 ) generating 142. " ... 

Using Saveanfs terminology, such a process is called redox catalysis in its proper meaning, while Shono formed the expression homomediatory system . This type of mechanism was already schematically presented in the case of an oxidation in Eqs. (2) to (4). To this category of redox catalysts belong, for example, the radical anions and cations of aromatic and heteroaromatic compounds and some reactions of triaryl amine radical cations. 

The photoreduction of aryl ketones by amines generally occur via a charge transfer interaction between the triplet state of ketone and the amines, as shown in the following scheme. A ketone radical anion and an amine radical cation are formed in the intermediate stage. 

Am+ Amine radical cation (radical center on nitrogen)... 

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