Radical cations selectivity

Thianthrene radical cation is also an excellent one-electron oxidant of iron porphyrin complexes. Such oxidation of Fem(0Cl03)(TPP), where TPP is meso-tetraphenylporphyrin, provides the corresponding porphyrin 7r-cation radical analytically pure [32]. Similar oxidation of the AT-methyl porphyrin complex (N-MeTPP)FenCl, where AT-MeTPP is AT-methyl-meso-tetraphenylporphyrin, afforded [N-MeTPPFemCl]+ which was not further oxidized [33]. Thus thianthrene radical cation selectively oxidized the aromatic porphyrin ligand in one case and the metal center in the other. Ligand oxidation at a phenolic moiety has also been reported [34] on treatment of a 1,4,7-triazacyclononane appended with one or two phenol moieties ligated to Cu(II) complex with thianthrene radical cation. 

Theoretical and Experimental Studies of the Benzene Radical Cation Effects of Selective... 

Enolether radical cation 7, generated in a double strand, oxidizes selectively a nearby guanine base (G) in the same double strand [3, 4]. This elec-... 

The guanine radical cations (G +) are detected by their reactions with water, which leads after treatment with piperidine or ammonia to selective strand cleavage [14]. A similar charge detection method was used by J.K. Barton, G.B. Schuster and I. Saito as described in their articles in this volume. The cleavage products were separated and quantified by gel electrophoresis. A typical example is shown in Fig. 7 where the GGG unit acts as a thermodynamic sink for the positive charge, and the efficiency of the charge transfer can be measured by the product ratio Pggg/Pg-... 

The anthraquinone group of the UAQ sensitizer is intercalated on the 3 -side of its linkage site [15]. Use of UAQ permits assessment of the directionality of long-range radical cation migration. Both AQ and UAQ enable the selective and efficient introduction of a radical cation in duplex DNA, whose lifetime is controlled by its relatively slow bimolecular reaction primarily with H20. 

Exceeding the limitation of molecular dynamics, the steric requirement of trimethylsilyl groups can cause drastic changes both in structure and of molecular properties of organosilicon compounds. For illustration, the so-called "Wurster s-Blue11 radical ions are selected On one-electron oxidation of tetramethyl-p-phenylenediamine, its dark-blue radical cation, detected as early as 1879 [11a], is gene-... 

The overall conclusion from the reaction of BP and 6-substituted BP radical cations with nucleophiles of various strengths is that weak nucleophiles display higher selectivity toward the position of highest charge localization. Thus another important factor in the chemical reactivity of radical cations is represented by the strength of the nucleophile. 

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

In the early 1980s, one of the authors of this chapter began to study argon matrix isolation of radical cations235 by applying the radiolytic techniques elaborated by Hamill and Shida. A central factor was the addition of an electron scavenger to the matrix which was expected to increase the yield of radical cations and the selectivity of the method. For practical reasons, X-rays replaced y-rays as a radiolytic source and argon was chosen as a matrix material because of its substantial cross section for interaction with keV photons (which presumably effect resonant core ionization of Ar). Due to the temporal separation of the process of matrix isolation of the neutral molecules and their ionization, it was possible to obtain difference spectra which show exclusively the bands of the radical cations. 

TABLE 4. Change in cr and A/v between neutral and cation geometry for a series of selected non-conjugated diene radical cations... 

Diels-Alder catalysis.1 This radical cation can increase the endo-selectivity of Diels-Alder reactions when the dienophile is a styrene or electron-rich alkene. This endo-selectivity obtains even in intramolecular Diels-Alder reactions. Thus the triene 2, a mixture of (Z)- and (E)-isomers, cyclizes in the presence of 1 to 0° to the hydroindanes 3 and 4 in the ratio 97 3. Similar cyclization of (E)-2 results in 3 and 4 in the ratio 98 2 therefore, the catalyst can effect isomerization of (Z)-2 to (E)-2. Even higher stereoselectivity is observed when the styrene group of 2 is replaced by a vinyl sulfide group (SC6H5 in place of QHtOCT ). 

---