Oxidation 2,2 -bipyridinium

In this type of cleavage reaction, it appears that the axial benzoate is the preferred product. If water is excluded from the reaction, a bromo benzoate is obtained.The highly oxidizing medium of 2,2 -bipyridinium chlorochromate and MCPBA in CH2CI2 at rt for 36 h effects a similar conversion of benzylidene acetals to hydroxy benzoates in 25-72% yield. ... 

Organic Molecules It can be seen from our earlier discussion that the presence of a transition metal ion is not always required for an electrochromic effect. Indeed, many organic molecules can yield colored products as a result of reversible reduction or oxidation. 4,4 -Bipyridinium salts are the best known example of such compounds. These compounds can be prepared, stored, and purchased in colorless dicationic form (bipm +). One electron reduction of the dication leads to the intensely colored radical cation (bipm+ ). Such radical cations exist in equilibrium with their dimers (bipm ). In the case of methyl viologen, the radical cation is blue and the dimer is red. By varying the substient group in the molecule, different colors can be obtained. 

By introducing redox-active N-methyl-4,4/-bipyridinium ion (mbpy+) to the oxo-centered triruthenium cores, a series of triruthenium derivatives bearing two or three axially coordinated mbpy+ were prepared by Abe et al. [12, 13]. Electrochemical studies indicated that these mbpy+-containing triruthenium complexes afforded a total of seven to nine reversible or quasi-reversible redox waves in acetonitrile solutions at ambient temperature. Of these redox waves, four or five one-electron redox processes arise from RU3 -based oxidations or reductions involving five or six formal oxidation states, including... 

In a very special system, the photoelectrochemical regeneration of NAD(P)+ has been performed and applied to the oxidation of the model system cyclohexanol using the enzymes HLADH and TBADH. In this case, tris(2,2 -bipyridyl)ruthenium(II) is photochemically excited by visible light [43]. The excited Ru(II) complex acts as electron donor for AT,AT -dimethyl-4,4 -bipyridinium sulfate (MV2+) forming tris(2,2 -bipyridyl)ruthenium(III) and the MV-cation radical. The Ru(III) complex oxidizes NAD(P)H effectively thus... 

The photo-oxidation of the aryl-substituted cycloheptatrienes 7-(/ -methoxy-phenyl)cycloheptatriene and 7-, 1- and 3-(/ -dimethylaminophenyl)cycloheptatrienes to the corresponding radical cations in de-aerated acetonitrile solution was accomplished by electron transfer to the electronically excited acceptors 9,10-dicyanoanthracene, iV-methylquinolinium perchlorate, iV-methylacridinium perchlorate and l,T-dimethyl-4,4-bipyridinium dichloride. In the case of l- p-methoxyphenyl)cycloheptatriene (62), deprotonation of the radical cation occurs successfully, compared with back electron transfer, to give a cycloheptatrienyl radical (63) which undergoes a self-reaction forming a bitropyl. If the photooxidation is done in air-saturated acetonitrile solution containing HBF4 and one of the acceptors, the tropylium cation is formed. Back electron transfer dominates in the / -dimethylaminocycloheptatrienes and the formation of the cycloheptatrienyl radical is prevented. 

Some bipyridinium salts are remarkable herbicides. They rapidly desiccate all green plant tissue with which they come into contact, and they are inactivated by adsorption on to clay minerals in the soil. This potent herbicidal activity is found only in quaternary salts, e.g. diquat (254) and paraquat (255), with redox potentials for the first reduction step between -300 and -500 mV (equations 158 and 159) (B-80MI20504). The first reduction step, which is involved in herbicidal activity, involves a completely reversible, pH independent, one-electron transfer to yield the resonance stabilized radicals (256) and (257). The second reduction step, (256 -> 258) and (257 -> 259), is pH dependent and the p-quinoid species formed are good reducing agents that may readily be oxidized to diquatemary salts. 

Figure 7.9 Electrons travel from the electrode to the ruthenium centers through the bipyridinium dications (a), when the gold voltage (V versus Ag/AgCl) matches the reduction potential of the bipyridinium dications, but cannot return from the ruthenium centers to the electrode (b), even when the voltage is lowered below the oxidation potential of the ruthenium centers.

Bipyridinium dications do not absorb in the visible region of the electromagnetic spectrum.28 After their electrochemical reduction to the corresponding radical cations, however, a broad and intense absorption appears across the visible region. The process is fully reversible and the colorless state is regenerated, after the oxidation of the radical cations back to the original dications. In fact, bipyridinium dications are convenient building blocks for the realization of electrochromic materials, as a result of their spectroscopic response to electrochemical inputs. 

To assess the electrochromic response of the bipyridinium dications embedded into multilayers of 7, we envisaged the possibility of assembling these films on optically transparent platinum electrodes.27d f Specifically, we deposited an ultrathin platinum him on an indium-tin oxide substrate and then immersed the resulting assembly into a chloroform/methanol (2 1, v/v) solution of 7. As observed with the gold electrodes (Fig. 7.5), the corresponding cyclic voltammograms show waves for the reversible reduction of the bipyridinium dications with a significant increase in 2p with the immersion time. In fact, is 0.8,1.5, and 3.1 nmol/cm2 after immersion times of 1, 6, and 72 h, respectively. Furthermore, the correlation between ip and v is linear after 1 h and deviates from linearity after 6 and 72 h. Thus, the bisthiol 7 can indeed form multiple electroactive layers also on platinum substrates. 

The branched rotaxanes 66+, 76+, and 86+, besides the bipyridinium units of the triply branched backbone 56+, contain macrocycle 2 whose two DMB units are oxidized at distinct potentials the first oxidation process practically coincides with that of the model compound /i-dimethoxybenzene, whereas the second one is displaced to a slightly more positive potential. 

The electrochemical behavior of rotaxane 76+ can be straightforwardly explained on the basis of the above discussion. On reduction, a first monoelectronic process, assigned to the first reduction of the free bipyridinium unit, is followed by a bielectronic process, assigned to the first reduction of the two bipyridinium units encircled by the ring. Even the second reduction of the three bipyridinium units, which occurs at more negative potentials, occurs with the same 1 2 pattern. On oxidation, the behavior of rotaxane 76+ is again similar to that of rotaxane 66+, with a more intense process in correspondence of oxidation of the DMB units. 

In rotaxane 86+, the three bipyridinium units are expected to be electrochemically equivalent because each one is encircled by the ring. In agreement with this expectation, it has been found two trielectronic processes corresponding to the first and the second reduction of the bipyridinium units, a situation similar to that observed for compound 56+. In the case of the rotaxane, however, the processes occur at more negative potentials because of the CT interaction. As far as oxidation is concerned, the behavior of 86+ is in line with that of 66+ and 76+. 

Macrocycle 16, containing three equivalent DMN electroactive units, shows three distinct oxidation processes (Fig. 13.17). Such a contrasting behavior between 16 and the tetracationic cyclophanes, in which the two incorporated bipyridinium units undergo simultaneous first and second reductions, can be interpreted considering that, in the cyclophanes, the rigidity of the structure prevents interaction between the two bipyridinium units, whereas the flexible structure of macrocycle 16 allows the three DMN units to approach one another. 

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