Alkylaromatic radical cations cleavage

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. 

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... 

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