Charge transfer complexes from electrophilic

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,... 

Brown (1959) has presented a charge transfer model of the transition state for electrophilic reactions which differs appreciably from that proposed by Fukui and his collaborators and leads to the definition of a new reactivity index termed the Z value . The model is based on a more conventional formulation of the charge transfer mechanism, which avoids the complete transfer of electrons associated with v = 0,1,2 in Fukui s model. There is no dependence on the formation of a pseudo tt orbital in the transition state, nor is hyperconjugation invoked. A wave function for a charge transfer complex is written as a linear combination of a wave function < o describing the unperturbed ground state of the molecule under attack, and a function which differs from (Pq in the replacement... 

The early stage of electrophilic bromination of tetraisobutylethylene (TTBE) has been examined to detect the formation constant (K[) for the 1 1 charge-transfer complex (in CH2CI2, AcOH and MeOH). Based on the Kf values, the thermodynamic parameters for CTC formation from TTBE + Br2 are AH = —4.2( 0.2) kcalmol-1 and AS = —9.5( 0.7) e.u. TTBE reacts with Br2 to afford substitution products, two of which are an initially formed allylic bromide and a more slowly formed diene bromide127. 

In Solution. During the 1940s and 1950s, proposals were made by separate groups of workers that charge-transfer complexes had a role in electrophilic aromatic substitution (8-12). The idea was that in a reaction of an aromatic compound (ArH) with an electrophile (E), covalent bonding was preceded by transfer of charge from ArH to E ... 

Despite intense study of the chemical reactivity of the inorganic NO donor SNP with a number of electrophiles and nucleophiles (in particular thiols), the mechanism of NO release from this drug also remains incompletely understood. In biological systems, both enzymatic and non-enzymatic pathways appear to be involved [28]. Nitric oxide release is thought to be preceded by a one-electron reduction step followed by release of cyanide, and an inner-sphere charge transfer reaction between the ni-trosonium ion (NO+) and the ferrous iron (Fe2+). Upon addition of SNP to tissues, formation of iron nitrosyl complexes, which are in equilibrium with S-nitrosothiols, has been observed. A membrane-bound enzyme may be involved in the generation of NO from SNP in vascular tissue [35], but the exact nature of this reducing activity is unknown. 

Isomeric product distributions. Isomeric product distributions obtained from toluene and anisole have been the subject of considerable mechanistic discussion in electrophilic aromatic nitration (Schofield, 1980 Olah et al., 1989). As applied to nitrations with iV-nitropyridinium ion, the yellow colour of the EDA complex immediately attendant upon the mixing of toluene and PyN02 in acetonitrile persists for about a day (in the dark), whereas the charge-transfer colour of toluene and Me2PyNOj is discharged within 10 min at 25°C. Both bleached solutions afford an identical product mixture (81), consisting of o- (62%), m- (4%) and p-nitrotoluenes (34%)... 

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