1- alkylaromatic radical cations

At pH <4 both 2"+ and 3,+ undergo C-H deprotonation as the exclusive reaction while in basic solution they behave as oxygen acids undergoing OH-induced OH deprotonation in a diffusion controlled process. Results indicate that in alkylaromatic radical cations, overlap between the scissile bond and the jt-system containing the unpaired electron is a fundamental requirement for the bond cleavage. The observation that for both 2 and 3, the 1,2-H atom shift occurs more rapidly than C-C ft-scission whereas the radical derived from OH de-... 

Although a previous chapter in this volume provides a broader perspective on the reactivity of radical cations, in this section we will examine intramolecular electron-transfer reactions coupled with or followed by cleavage of a bond in odd electron species such as radical cations, radical zwitterions and radical anions. In particular, this paragraph will be divided in oxidative and reductive bond-cleavage processes. Because this field is however too large to be covered extensively here, the discussion will be limited to selected examples—for oxidative cleavages, side-chain fragmentation reactions of alkylaromatic radical cations and decarboxylation reactions of radical zwitterions derived from benzoic and arylalkanoic acids, and for reductive... 

Oxidative Bond-cleavage Processes Alkylaromatic radical cations... 

The rate of side-chain fragmentation of an alkylaromatic radical cation can be influenced by the relative orientation of the scissible bond and the aromatic n system (stereoelectronic effect). The orientation most suited for cleavage is that where the dihedral angle between the plane of the n system and the plane defined by the scissible bond and the atom of the n system to which this bond is connected is 90°. Scheme 27 shows the conformation most suited for C-H bond cleavage. 

The transition state for the deprotonation of an alkylaromatic radical cation might be described in terms of the mesomeric structures in Scheme 31. In structure I the positive charge is delocalized on the aromatic ring whereas structures II and III represent the homolytic and heterolytic C-H bond cleavage modes, respectively. 

If the homolytic bond-dissociation energy (BDE) is the main factor governing the dynamics of proton transfer, the transition state is better represented by structures I and II. In an alkylaromatic radical cation the build-up of positive charge at the a-carbon requires extensive charge delocalization from the aromatic ring and this is likely to occur with greater efficiency in 4-Me rather than 4-MeO substituted radical... 

The intramolecular selectivity in the deprotonation of alkylaromatic radical cations has been investigated in a series of 5-X-l,2,3-trimethylbenzene radical cations the results showed that spin and/or charge density at the scissible C H bond can strongly influence the kinetic acidity, which is, accordingly, very sensitive to the nature and position meta or para) of ring substituents (Scheme 32) [149]. 

As remarked previously, the deprotonation rate of an alkylaromatic radical cation can be influenced by the stereoelectronic effect (Scheme 27). In this respect, Tolbert provided convincing evidence for the operation of stereoelectronic effects in the deprotonation of 9-alkylanthracene and 9,10-dialkylanthracene radical cations... 

The mechanism discussed above for the deprotonation of alkylaromatic radical cations, involving a bimolecular reaction between the radical cation and the base (B), leading to a carbon centered neutral radical and the conjugated acid of the base (BH" ") as described in Scheme 28, has been recently questioned by Parker who provided evidence for an alternative mechanism in proton-transfer reactions between methylanthracene radical cations and pyridine bases [154] this involved reversible covalent adduct formation between the radical cation and the base followed by elimination of BH+ (Scheme 36). 

Thermodynamically cleavage of a C-H bond in an alkylaromatic radical cation is strongly favored over C-C fragmentation due to the much higher solvation free energy of the proton as compared to a carbocation. However, the former process is characterized by significantly higher intrinsic barriers (0.5-0.6 eV for C-H... 

In contrast with these findings are however the similar rates of C-C bond cleavage calculated for the radical cations of PhCH2CH20H and 2-indanol [185, 186], indicating that further studies are needed in order to assess the importance of stereoelectronic eflfects on C-C bond fragmentation reactions of alkylaromatic radical cations. 

The importance of through bond delocalization in C-C bond cleavage reactions of alkylaromatic radical cations has been established in both theoretical and experi-... 

Taken together, these studies suggest that the transition state for C-C bond cleavage in alkylaromatic radical cations can be conveniently described by the following mesomeric structures (Scheme 48), where the contribution of structure I is... 

In addition, both I and the radical cation can trap O, leading to new radical species and complication of the overall mechanistic picture [29]. In case of alkylaromatics, radical cations unda-go side-chain fragmentation to benzyl radicals, giving rise to side-chain oxidation products [15, 28] (see Section 14.2.4). 

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