Nitrosonium/arene complex

Fig. 4. Correlation of the experimentally determined values of the degrees of charge-transfer (Z) in nitrosonium/arene complexes with the calculated mixing coefficients ct2 based on HDA and Ada- Data from ref. [28].

Fig. 12 Typical time-resolved absorption spectrum following the charge-transfer excitation of nitrosonium-EDA complexes with arene (hexamethylbenzene) showing the bleaching of charge-transfer absorption and growth of the donor cation radical...

Productive bimolecular reactions of the ion radicals in the contact ion pair can effectively compete with the back electron transfer if either the cation radical or the anion radical undergoes a rapid reaction with an additive that is present during electron-transfer activation. For example, the [D, A] complex of an arene donor with nitrosonium cation exists in the equilibrium with a low steady-state concentration of the radical pair, which persists indefinitely. However, the introduction of oxygen rapidly oxidizes even small amounts of nitric oxide to compete with back electron transfer and thus successfully effects aromatic nitration80 (Scheme 16). 

The nitrosonium cation can serve effectively either as an oxidant or as an electrophile towards different aromatic substrates. Thus the electron-rich polynuclear arenes suffer electron transfer with NO+BF to afford stable arene cation radicals (Bandlish and Shine, 1977 Musker et al., 1978). Other activated aromatic compounds such as phenols, anilines and indoles undergo nuclear substitution with nitrosonium species that are usually generated in situ from the treatment of nitrites with acid. It is less well known, but nonetheless experimentally established (Hunziker et al., 1971 Brownstein et al., 1984), that NO+ forms intensely coloured charge-transfer complexes with a wide variety of common arenes (30). For example, benzene, toluene,... 

E.K. Kim u. J.K. Kochi, Oxidative Aromatic Nitration with Charge-Transfer Complexes of Arenes and Nitrosonium Salts, J. Org. Chem. 54, 1692 (1989). 

If fcf > feBET- the overall transformation can occur rapidly despite unfavorable driving forces for the electron transfer itself. Only follow-up reactions with high kf can compete with back electron transfer. Different kinds of such unimolecular processes can drive the equilibria toward the final product. A representative example is the mesolytic cleavage of the C-Sn bond in the radical cation resulting from the oxidation of benzylstannane by photoexcited 9,10-dicyanoanthracene (DCA). This is followed by the addition of the benzyl radical and the tributyltin cation to the reduced acceptor DCA [59]. In the arene/nitrosonium system, [ArH, NO+] complexes can exist in solution in equilibrium with a low steady-state concentration of the ion-radical pair. However, the facile deprotonation or fragmentation of the arene cation radical in the case of bifunctional donors such as octamethyl(diphenyl)methane and bicumene can result in an effective (ET) transformation of the arene donor [28, 59]. Another pathway involves collapse of the contact ion pair [D+, A- ] by rapid formation of a bond between the cation radical and anion radical (which effectively competes with the back electron transfer), as illustrated by the examples in Chart 5 [59]. 

Another study by Kochi et al. examined the chemistry of nitrosation [52]. Several [NO, arene]" Jt-complexes were characterized by UV-visible, NMR, and IR spectroscopy. X-ray quality crystals were also obtained at -78°C with mesitylene, hexamethylbenzene, and other arenas. The highly colored nitroso jt-complexes were prepared directly from nitrosonium salts (i.e., NO+ SbCl ) and the aromatic compound. Structural studies revealed the distance between the ring and the electrophile is roughly 1A less than the sum of the van der Waals radii and the N—O bond length is significantly lengthened as a result of strong donor-acceptor interaction. 

INO,C >, and this results in only one broad minimum on the PES, the ic-complex. The results show that the nitrosonium reaction has a different mechanism compared to nitration and that eare should be taken when interpreting other S Ar reactions based on structures of complexes between arenes and NO. ... 

---