Monoelectronic oxidation

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

TABLE 3. Redox data for the monoelectronic oxidation of aminoxyl radicals... 

We have seen that the generation of an aminoxyi radical from the hydroxylamine precursor (cf. Scheme 2) is viable through the use of a monoelectronic oxidant having appropriate redox potential, such as CAN (i.e. Ce" +) or by the use of the inner-sphere Pb(OAc)4 oxidant the case of PINO and BTNO from HPI and HBT, respectively, has been underpinned above. It is of no wonder, then, that the generation of PINO and BTNO, and of additional aminoxyi radicals as well has been achieved through... 

As anticipated in Section II.B (Scheme 6), monoelectronic oxidation of R2N—O may afford a stable oxoammonium ion (R2N=0+), the case of TEMPO being exemplary. 

In the electrochemical experiments performed in argon-purged dichloromethane solution, [Os(bpy)2(l)]2+ undergoes a reversible monoelectronic oxidation at +1.03 V vs SCE (Figure 7, Table 2), that can be assigned to the oxidation of Os(II)[6c,e]. The potential value at which the metal ion is oxidized is very close to those obtained for the metal oxidation in the model compounds [Os(bpy)2(5)]2+ and [Os(bpy)3]2+ (Table 2). On reduction, two reversible and monoelectronic processes are observed in the accessible potential window (Figure 7, Table 2). 

Cyclic voltammetry shows that the [2]rotaxane 22 1 h can also undergo electrochemical switching (Figure 16) by a monoelectronic oxidation process to give the radical pentacationic species 225+. On oxidation, the benzidine unit is converted to its monocationic radical state, generating an electrostatic repulsion, which causes the tetracationic cyclophane to move to the biphenol unit in [2]rotaxane 224+. This redox procedure is completely reversible. 

Monoelectronic oxidation of the TTF unit is accompanied by the circumrotation of the desymmetrized ring through the cavity of the tetracationic cyclophane. Indeed, after oxidation, the newly formed monocationic tetrathiafulvalene unit... 

The reactions catalyzed by laccases proceed by the monoelectronic oxidation of a suitable substrate molecule (phenols and aromatic or aliphatic amines) to the corresponding reactive radical (Riva, 2006). The redox process takes place with the assistance of a cluster of four copper atoms that form the catalytic core of the enzyme they also confer the typical blue color to these enzymes because of the intense electronic absorption of the Cu-Cu linkages (Piontek et al., 2002). The overall outcome of the catalytic cycle is the reduction of one molecule of oxygen to two molecules of water and the concomitant oxidation of four substrate molecules to produce four... 

The work of Kaifer and colleagues [82a-c] and other researchers [83a,b] on different ferrocene derivatives complexed with jff-CD illustrates a similar behavior the monoelectronic oxidation drastically lowers the host guest affinity (see Scheme 3). 

A particular emphasis is to be made on these systems the recognition of ferrocene-based substrates can be switched off by monoelectronic oxidation of the substrate itself, whereas in the case of positively charged guests the binding interaction can be electrochemically activated by either a mono- (for cobaltocenium) or bi-electronic (for viologen) reduction process [86]. 

The electrochemical behavior of ferrocene is relatively simple, giving rise to a reversible monoelectronic oxidation process at a very accessible potential. Most commonly, ferrocene has been used to functionalize the periphery of dendrimers, along the scheme illustrated in Figure 2b. Dendrimers 1 [42] and 2 [55] exemplify the commonly observed electrochemical behavior ... 

Scheme 19 a Electrochemical monoelectronic reduction, b Electrochemical monoelectronic oxidation... 

The use of 1 equivalent of CuCl2 allowed the isolation of the Co(III), 10, and Cu(III), 11, derivatives. Further oxidation of 10 and 11 with one mole of CuCl2 led to 7 and 9, respectively, which have also been obtained (see reaction 5) from the direct oxidation of 2 and 4 using / -benzoquinone. In the case of cobalt and copper, the cyclopropane formation is a clean two step monoelectronic oxidation. 

Pseudo-octahedral frans-iron(II)dichloro bis-(l,2-(dimethylphosphino)ethane is easily oxidized to iron(IIl) in a fully reversible fashion at the rather mild potential of —0.09 V versus SCE in dichloromethane. The second oxidation (1.33 V) is less chemically reversible at the voltampero-metric timescale (0.2 Vs ) but testifies that iron(IV) can be electrogenerated, although not very stable in this case [93]. It is possible that the bisphosphine chelator becomes more labile in the iron(IV) state. Indeed, monoelectronic oxidation of the iron(II) species causes a lengthening of the equatorial Fe—P bonds and a shortening of the axial Fe—Cl bonds. 

The addition of anthracene to maleic anhydride (Fig. 11) was reported to be accelerated by sonication. From a mechanistic study in the presence of electron carriers, an electron transfer process was ruled out. These results could not be reproduced, and no difference between the sonochemical and thermal rates and yields was observed (adduct formation in 30% after 1 h, 50% after 3 h, with or without sonication). In the presence of monoelectronic oxidizers such as ferric chloride,or tris(4-bromophenyl)aminyl hexachloroantimonate (TBPA),28a,c change was noted in these figures, although the radical cation of anthracene was formed.43 This radical cation is not involved in the reaction pathway. 

At this time possible transients have not been observed yet. Consequently monoelectronic oxidations have been carried out using pulse radiolysis technique (Febetron 708) and fast kinetics absorption spectrophotometry. 

In the absence of imidazole, compounds 12 and 13 exhibited a monoelectronic oxidation process at 0.46 (12) and 0.48 V (13), ascribed to the ferrocenyl oxidation (Fig. 47.13). Upon addition of excesses of imidazole, the intensities of the oxidation waves of both compounds increased, a fact that supports the occurrence of the classical oxidation sequence of ferrociphenols, depicted above for compounds 4 and 7. 

Finally, some electrophiles can behave as monoelectronic oxidants towards 18-electron metal-alkyl complexes, which leads to decomposition of these complexes by heterolytic cleavage of metal-carbon bond in the resulting 17-electron species ... 

Likewise, monoelectronic oxidation of a neutral 18-electron metal complex to a 17-electron cationic complex before insertion also considerably facilitates this reaction. Whereas the 18-electron complex p eCp(CO)2Me] only very slowly undergoes CO insertion into the Fe-CH3 bond at ambient temperature, the 17-electron cation rapidly gives the acetylated complex in MeCN at -78°C, the solvent playing the role of the nucleophile that coordinates to the iron center. ... 

NO is isoelectronic to CO. The salt NO PFe in a very good monoelectronic oxidant, but NO can also coordinate to transition metals by substitution of a carbonyl ligand ... 

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