Radicals aminium cation-, generation

In homogeneous solution, the aminium cation radical may generate an olefin cation radical which reacts in cyclic fashion with another olefin before reaccepting an electron. This pathway relies on rapid bond-forming reactions compared with electron exchange. 

The use of /V-chloramines, in principle, allows the facile generation of aminyl radicals upon UV photolysis in neutral media. A radical chain can be envisioned for the formation of 2-chIoromethylpyrrolidines (Scheme 7). In practice, however, there is a slow step in this sequence, step A and/or B, such that other reaction pathways, disproportionation or H-abstraction from the solvent, compete. Surzur has studied the reaction in Scheme 7 in the alcoholic solvents MeOH and /-PrOH, which serve as hydrogen atom sources, and achieved acceptable ratios of cyclic products 25 and 26 to acyclic amine 27 (70TL3107). Other /V-chloroalkenylamines gave similar results (71TL903 80TL287). /8-chloro-substituted amine products such as 25 were the sole products when the reactions were carried out in acetic acid-water mixtures these reactions involve aminium cation radicals and are discussed further in Section III,B. 

Aminyl radicals can be protonated with Br0nsted acids or complexed by Lewis acids to generate an electrophilic nitrogen radical. The pKa of an aminium cation radical in water is =7 as determined from a titrametric... 

Electrophilic radical cyclizations of alkenyl aminium cation radicals have shown synthetic utility. Hofmann-Loffler-Freytag reactions do not compete with 5-exo cyclizations (75BSF1429). The homolytic cyclization of Af-chloroalkenylamines under acidic or Lewis acidic conditions has been studied primarily by Surzur and Stella, and the chemistry of these precursors for electrophilic aminyl radical generation has been reviewed [83AG(E)337]. Radical chain reactions can be initiated by heat, UV photol-... 

Intermolecular addition and addition-cyclization reactions of aminium cation radicals with electron-rich alkenes such as ethyl vinyl ether (EVE) allow an entry into products containing the N—C—C—O moiety of 13-amino ethers 70 or the equivalent of /3-amino aldehydes 71. The mild conditions under which aminium cation radicals are generated from PTOC carbamates makes the reactions described in Scheme 22 possible. In the absence of hydrogen atom donors, the /3-amino ethoxy(2-pyridylthio) acetal 71 was the major product. The mixed acetal can easily be converted... 

A cycloaddition methodology has been exploited in the cation radical-mediated reactions between electron-rich chalcone epoxides 287 and A -aryl imines 286 using tris(4-bromophenyl)aminium hexachloroantimonate (TBPA -SbCle ) as the radical initiator to generate substituted 1,3-oxazolidines 288a and 288b in good yields (Equation 21) <2005SL161>. 

Bicyclo[3.1.0]hex-2-ene radical cation, generated by y-irradiation of the respective hydrocarbon in freon matrices at 77 K, underwent ring opening to the 1,3-cyclohexadiene radical cation. " Tris(/7-bromophenyl)aminium hexachloroantimonate oxidized substituted tricyclo[3.3.0.0 ]-octanes to radical cations, which rearranged to diquinanes. ... 

Homolytic intramolecular amination allows the synthesis of tetrahydroquinoline 28 from the 3-phenyl-propylamine 27 (n = 2). The yield of indoline 30 is lower because the aminium cation radical 29 generated from A-chloro-2-phenethylamine 27 (n = 1) undergoes an easy jff-scission reaction to form the benzyl radical (Scheme 9). 

Laser flash photolysis techniques have been employed to evaluate the dynamics of decarboxylation reactions of cation radicals derived from a-aminocarboxylates. > In one report, variously substituted aminium radicals 47 were generated by laser flash excitation of anilinocarboxylates 46 (Scheme 21) in MeCN solutions containing the acceptor, 1,4-dicyanobenzene. These transients undergo fast, first-order decay by a pathway involving loss of carbon dioxide. The rate constants for decarboxylation were found to be in the range of 8 x 10 to 4 x 10 s . In addition, the rates show the same dependence on nitrogen, a-alkyl and a-phenyl substituents, as do the related a-CH deprotonation, a-desilylation, and retro-Aldol cleavage reactions. 

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. 

Radical cations can be generated by many chemical oxidizing reagents, including Brpnsted and Lewis acids, the halogens, peroxide anions or radical anions, metal ions or oxides, nitrosonium and dioxygenyl ions, stable aminium radical cations, semiconductor surfaces, and suitable zeolites. In principle, it is possible to choose a reagent with a one-electron redox potential sufficient for oxidation-reduction, and a two-electron potential insufficient for oxidation-reduction of the radical ion. 

Cycloadditions can also be catalyzed by cation radicals generated from aminium salts.537-539 In fact, these are cation-radical chain reactions mimicking Diels-Alder reaction. Interestingly, 2,4-dimethyl-1,3-pentadiene undergoes either a Brpnsted acid-catalyzed cycloaddition [Eq. (6.93)] or cation-radical-catalyzed cycloaddition [Eq. (6.94)] to yield different addition products 539... 

A rich variety of chemical oxidizing reagents have been applied for the generation of radical cations. The principal reagent types include Bronsted and Lewis acids the halogens certain peroxide anions or radical anions numerous metal ions or oxides nitrosonium and dioxygenyl ions stable organic (aminium) radical cations ... 

As for halogens as oxidizing reagents, bromine has proved more useful than its homologs chlorine and iodine. It was employed as early as 1879 on di- and tetra-methyl-p-phenylenediamine [27-29] and early in this century, Wieland used it to generate the aminium salts of triarylamines and tetraarylhydrazines [30, 31]. Since bromine adds readily to unsaturated as well as to some strained ring compounds, it is not expected to be very useful in the context of the radical cations discussed here. 

The generation of cr-radical cations from saturated hydrocarbons requires very strong SET oxidizers. The oxidation reactions can be accomplished by chemical electron transfer (CET), photochemical electron transfer (PET), and anodic oxidation. The oxidation potentials of stable, organic CET oxidants, e.g., commercially available tris(4-bromophenyl)aminium hexachloroantimonate (TBA +SbCI<,) or tris(2,4-dibromo-phenyl)aminium hexachloroantimonate (TDA +SbCl6 ), are too low (1.06 and 1.50 V... 

When the cation radical of this alkyne is generated by y radiolysis in a solid matrix at 77 K and then warmed to 150 K, the ESR spectrum of the 1,2,3,4-tetramethyl-1,3-butadiene cation radical is observed. An analogous intramolecular reaction was also observed even in a rigid matrix at 77 K. The feasibility of the cycloaddition step itself is therefore indicated, but little work has yet been done in respect of the aminium salt or PET induced cycloadditions of alkynes in solution at ambient or near-ambient temperatures. Whether a chain or catalytic alkyne cyclodimerization can be effected is yet unclear, as is the potential fate of the cyclobutadiene products. 

Evidence for the operation of cation radical mechanisms for cycloaddition has often been provided by means of a comparison of the results obtained for various methods of generating cation radicals. For example, in the Diels-Alder cycloaddition of phenyl vinyl sulfide to 1,3-cyclopentadiene (Scheme 36) the same adducts are formed whether the cation radicals are generated by chemical ionization (aminium salt), photochemical ionization (the PET method), or electrochemical ionization (anodic oxidation) [65]. 

No other adducts are formed, and the endo/exo diastereomeric ratio is essentially the same for all of these methods. Further, the existence of an acid catalyzed mechanism for cycloaddition can be explicitly excluded by using an excess of a hindered amine base (2,6-di-tert-butylpyridine, DTBP) in the aminium salt induced reaction and by examining the results of an authentic acid catalyzed reaction (using, for example, triflic acid). In the former case, the same endo and exo adducts are formed in virtually the same relative amounts, but in the latter case neither of these adducts is formed. It is worth noting that acid catalyzed reactions have indeed sometimes been observed under typical aminium salt conditions [70], but these have never been observed, nor would they be expected, under PET conditions. Finally, in the instance where cation radicals are generated by the aminium salt method, the intervention of substrate cation radicals can usually be verified by the addition of the reduced form of the catalyst, i.e., the neutral triarylamine, to the reaction mixture. 

The cation radical VCB rearrangement has been found, at least in several cases, to occur intramolecularly, rather than by dissociation/recombination [86]. An especially interesting case is the rearrangement of the CB dimers of 1,3-cyclohexadiene generated by triplet sensitized photochemistry. In the presence of the usual aminium salt catalyst (3+ ), these CB dimers are quite stable, but if the more powerful hex-abromo aminium salt, 4+, is used, these individual dimers rearrange to the DA dimers via a stereospecific VCB rearrangement (Scheme 46). 

Synthesis of oxazabicycloalkanes and related products was achieved by a one pot electron-proton-electron (EPE) transfer mediated reactions of the amine moiety [317]. Here the iminium cation is formed from the second electron oxidation of the a-aminoalkyl radical, generated via the a-deprotonation of the planar aminium radical owing to their low ionization potential. The iminium cation thus formed can... 

J.K. Cha et al. developed a stereocontrolled synthesis of bicyclo[5.3.0]decan-3-ones from readily available acyclic substrates. Acyclic olefin-tethered amides were first subjected to the intramolecular Kulinkovich reaction to prepare bicyclic aminocyclopropanes. This was followed by a tandem ring-expansion-cyclization sequence triggered by aerobic oxidation. The reactive intermediates in this tandem process were aminium radicals (radical cations). The p-anisidine group was chosen to lower the amine oxidation potential. This substituent was crucial for the generation of the aminium radical (if Ar = phenyl, the ring aerobic oxidation is not feasible). 

---