Imine radical cations

Explanation of the observed de-oxygenation with the formation of the corresponding imines in the photochemical reaction of a-aryl-A-methylaldonitrones, confirms the intermediate formation of radical cations resulting from PET (441, 467). 

Scheme 29 describes a plausible mechanism for the formation of the products which fit the observed coulometric (n 0.45 F/mol) and preparative results. The intramolecular cyclization process involves a dimerization between a radical cation 52a and the ketene imine 52 to form the intermediate radical cation 52b which then cyclizes to the radical 52c which can abstract a hydrogen atom leading to 54 or can be further oxidized and transformed through a cyclization and deprotonation reaction to 53 which involves 1 F/mol. However, it seems that the [2 -1- 3]-cycloaddition between the parent compound 52 and the cation 52d giving rise to 55 is the fastest reaction as compared with the intramolecular cyclization of 52d to 53. This can also explain the low consumption of electricity. 

The oxidation of 2-phenyl-3-arylaminoindoles has been studied in CH3CN, DMF, and propylene carbonate at a platinum electrode with periodic renewal of the diffusion layer. The oxidation proceeds in two one-electron steps, the first leading to the formation of a radical-cation, which in the second step is oxidized at a more positive potential.424 The main concentration of charge and unpaired spin in the radical-cation are at the amino group. In the presence of base, 2-phenyl-3-arylaminoindoles undergo a two-electron oxidation to the corresponding imines. 

The 1,2-hydrogen shift isomers of neutral (singlet and triplet) thiazole and its radical cation have been investigated87 by a combination of mass spectrometric experiments and hybrid density functional theory calculations. An unexpected isomerization of A-aryl-3-amino-4-nitroisothiazole-5(2//)-imines (71) to 2-(benzothiazol-2-yl)-2-nitro-ethene-1,1-diamines (72) has been reported.88... 

The sole representative of this class of compounds for which there is an experimentally measured enthalpy of formation is CH2CHCH=NH, alternatively called propenaldimine and vinylimine146. The literature value, 125 11 kJ mol 1, was obtained by measurement of the appearance energy for formation of its protonated ion from the radical cation of cyclopentylamine (reaction 56), and then bracketing experiments to deprotonate (reaction 57) this ion to the imine of interest. 

Table 5 Total energies (ET(au)) of imines 43a-c, diazetidines 44a-c and their radical cation (M+ n-1 electrons) as well as the derived first ionization potential IP1V (eV)...

Protonation by acid-base chemistry leads to an internal redox reaction (Fig. 11.19), without change of the number of electrons (Heeger, 2001 MacDiarmic, 2001). The semiconductor (emeraldine base, emeraldine salt, 100 S/cm). Complete protonation of the imine nitrogen atoms in emeraldine base by aqueous HC1 results in the formation of a delocalised polysemiquinone radical cation. This is accompanied by an increase in conductivity of more than 12 orders of magnitude. 

An interesting application of PET mediated bond cleavage reaction from azirine 63 has been reported by Mattay et al. [67] for synthesizing N-substituted imidazoles (65) via the (3 + 2) cycloaddition reaction of resultant 2-azaallenyl radical cations with imines 64. Synthesis of pyrrolophane 3,4-dimethyl ester (68) has been reported recently [68] by the ring opening of 66 followed by inter-molecular cycloaddition with dimethyl acetylene dicarboxylate (67) as shown in Scheme 13. 

For several tautomeric systems ketones/enols, imines/enamin and others) a distinct reversal of the stability order is observed when going from the neutral compounds to the radical cations, the first use of which in a new preparative a-Umpolung reaction has been documented for keto/enol systems. The present review provides a critical evaluation of the chemistry of enol radical cations in solution with a special emphasis on the Umpolung reaction and the intermediates thereof. Other enol type of radical cations are discussed with respect to their potential to provide a-carbonyl radical and a-carbonyl cation intermediates. Hence, this article does not constitute a comprehensive summary on all enol type of radical cation reactions. All potentials in this review are referenced versus SCE, unless noted otherwise. Potentials measured against the ferrocene/ferrocenium couple were converted to SCE by adding 0.334 V. 

A further [3 4- 2)-cycloaddition process takes place when imines react with 2-azaallenyl radical cations (23) derived from azirines by photolysis (Scheme 4.2.10) [31, 32]. Yields of (24) are variable, e.g. 2,4,5-triphenyl-1-propyl- (87%), 4-butyl-1,5-dipropyl- (40%), 4-phenyl-1,5-dipropyl- (35%),... 

An exciting cycloaddition which is not pericyclic in nature has been developed on the basis of PET opening of azirines to 2-azaallenyl radical cations [373] these add readily to substrates such as imines and acetylenes. This paves the way for the synthesis of imidazoles [373a, e], heterophanes [373b, e], and even porphyrins (cf. the preparation of 34) [373c, e]. 

Sulfonamides have been studied for cathodic and anodic activity at rotating disk electrodes of gold and platinum in 0.1 M H2SO4 and 0.1 M Na2C03 solutions. All the sulfonamides display anodic activity. The free (protonated) amino group of the sulfonamides is oxidized by a one-electron reaction to the protonated radical cation imine (73). 

Among the cycloaddition reactions which lead to imidazole products is the [3 + 2] cycloaddition of imines to 2-azaallenyl radical cations (derived from azirines by photolysis) which yields 1-substituted imidazoles <91AG(E)1336,93CB543). Similar addition of 2-azallyl anions (made by deprotonation of A-alkylated Schilf bases) to aromatic nitriles can give rise to either 3-imidazolines or imidazoles depending on the substitution pattern in the azaallyl species (229) <83CB492>. These reactions have been reviewed (Scheme 164) <90CHE1>. 

Armesto and his co-workers" have demonstrated that electron-transfer induced cyclization of imines (66) follows the di-jr-methane pathway, via the intermediate radical cation (67), to afford the cyclopropane derivatives (68). The outcome of the reaction is to some extent controlled by the electron-transfer sensitizer used, as can be seen in the reported yields. In the biphenyl substituted example, a cyclopropane derivative (69) is formed in competition with an alter-... 

In contrast to the formation of covalent adducts at flavin or at an amino acid residue by A -cyclopropyl-Af-arylalkylamines and 2-phenylcyclopropylamine, respectively, inactivation of MAO by 1-phenylcyclopropylamine leads to both types of adducts 100). Both inactivation pathways are proposed to originate from an initial one-electron oxidation by flavin to produce a common intermediate, the amine radical cation 33 in Scheme 22 (compounds 33-39). Homolytic cyclopropyl ring opening would lead to the reactive primary alkyl radical 34, which could be captured by the active site radical, either flavin semiquinone or amino acid centered. Subsequent hydrolysis of the imine, pathway (a), forms a... 

Another important application of DCN-sensitized photodissociation of strained ring compoimds has been demonstrated by Muller and Mattay [127] for synthesizing iV-substimted imidazoles (147) by the [3-l-2]-cycloaddition of the 2-azaallenyl radical cation (144), produced by the cleavage of corresponding radical cation from azirine (143), with imines. This strategy is further extended [128] for the synthesis of pyrrolophane 3,4-dimethyl ester (152) by the ring opening cycloaddition reaction of (148) with dimethyl acetylene dicarb-oxylate (Scheme 32). 

Schwarz and coworkers (Goldberg et al., 1994) were able to show very recently that nitrile imine (5.56) is stable in the gas phase. It can be generated in a beam experiment by neutralization-reionization mass spectrometry neutralizing the radical cation HCNNH +. The same authors also calculated the structures and energetics of nitrile imine by a hybrid method of Hartree-Fock and density functional theory (DFT). 

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