Neutral heteroaromatic radicals

The subject of ESR spectroscopy of heterocyclic radicals has been the topic of a previous review (74PMH(6)95). We consider here mainly the results which have appeared subsequently, and also draw attention to a fine review by Hansen (79AHC(25)205). Neutral heteroaromatic radicals require stabilization by delocalization of the odd electron, or they may be generated by continuous in situ photolysis, or trapped in an inert matrix, and examples of these techniques have been discussed (74PMH(6)95). 

These and other homolytic alkylations of neutral heteroaromatics usually proceed in poor yields, but if protonated heteroaromatic bases are used, many of the side reactions are minimized and selectivity is high and yields are good. Selectivity is increased because the alkyl radicals are nucleophilic in character and thus selectively attack the a-position. 

The main direction of decomposition of the cation radical formed at the first stage is a deprotonation leading to a neutral free-radical particle which later oxidizes into a heteroaromatized cation (Scheme 3.90, pathway a). The opposite pathway is rarely observed, i.e., oxidation of the cation radical into a dication foregoing the deprotonation stages (Scheme 3.90, pathway b) [250]. When single-electron transfer occurs along with a cation-radical particle, aromatiza-tion is also observed (Scheme 3.90, pathway c) [292] at the expense of the elimination of a hydrogen atom in the solvent cell , i.e., where repeated collisions of two particles take place. A fourth variation is possible and involves the decomposition of the cation radical and the formation of molecular... 

Heteroatoms also influence the charge carried by heteroaromatic radicals, just as they influence that of diamagnetic aromatic species. Thus, addition of an electron to the aromatic cations 15 and 16 yields radicals which are, respectively, neutral and cationic, albeit isoelectron ic with benzene anion-radicals. 

Aromatic substitution reactions are often complicated and multistep processes. A correlation, however, in many cases can be found between the charged attacking species and the electron density distribution in the molecule attacked during electrophilic and nucleoph c substitution. No such correlation is expected in radical substitution where the attacking particles are neutral, rather a correlation between the reactivities of separate bonds and a free valency index of the bond order. This allows the prediction of the most reactive bonds. Such an approach has been used by researchers who applied quantum calculations to estimate the reactivities of the isomeric thienothiophenes and to compare them with thiophene or naphthalene. " Until recently quantum methods for studying reactivities of aromatics and heteroaromatics were developed mainly in the r-electron approximation (see, for example, Streitwieser and Zahradnik ). The M orbitals of a sulfur atom were shown not to contribute substantially to calculations of dipole moments, polarographic reduction potentials, spin-density distribution, ... 

Finally, an example which exhibits extraordinarily large variations of the HFS constants with solvent should be mentioned. The neutral 4-acetyl-1-methyl-pyridinyl radical shown in Eq. (6-15) exhibits variations of the fl( H) constants, caused by the H-atoms of the heteroaromatic ring and the acetyl group, of the order of 200 to 300%... 

The unsubstituted TTE 16 is nonaromatic, in the Hiickel sense. Oxidation to the cation radical and dication occurs sequentially and reversibly at relatively low potentials ( 1/2 = 0.37 V and 1/2 = 0.67 V vs. SCE in 147t-electron system. In contrast to the neutral TTF, both the cation radical and dication are aromatic as a result of the brt-electron heteroaromaticity of the 1,3-dithiolium cation. The radical cation and dication can be isolated as stable crystalline compounds due to the effective resonance stabilization of the aromatic dithiolium and, to a minor extent, the polarizable sulfur atoms <1996SR1, 1997SL1211, 1999PS99, 2001AGE1372>. 

The major reaction pathway in all cases consists of reduction to the neutral radical followed by coupling, primarily or exclusively in the 4-position with respect to the ring nitrogen. Like the dimeric dianions formed by dimerization of, for example 91 , the neutral dimeric products obtained from positively charged A-heteroaromatic systems are in many cases difficult to isolate since they are easily reoxidized back to the substrate cations. 

Amides have fragmentation patterns similar to their corresponding carboxylic acids. Nitro compounds often have the ions NO (m/z = 30) and NOf (m/z = 46) in their spectra. Aromatic nitro compounds have characteristic peaks at M — 30 and M — 46, due to loss of the radicals NO and NO. Heteroaromatic nitrogen compounds like pyrrole often fragment to lose a neutral HCN molecule this loss of HCN is also seen with aromatic amines such as aniline. 

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