Vinylogous Redox Systems

22qx to 30qx have been synthesized as vinylogous series of Weitz systems some members of which have already been discussed under a different point of view. For comparison JiREo is mentioned as an example of the inverse Weitz type and 32qx to represent the open chain type In all cases an all-E-configuration has to be assumed, although rapid E/Z equilibration via their redox equilibria cannot be excluded  

---

Fig. 6. Correlation between Ig Ksem of different vinylogous redox systems and the Coulomb repulsion integral...

Vinylogous Redox Systems Containing Non-quatemized Heterocycles... 

Table 9. Correlation of of redox systems carrying different end groups, with the number of jr-electrons N = 2n + 3 for vinylogous n (n in parantheses excluded)...

Vitamin C is another enol-based redox system like the hydroquinones and tyrosine, but it has no aromatic character. The enediol component is stabilized by a conjugated lactone group. The oxidation (Ej = +58 mV) occurs in two one-electron steps, but the ascorbate radical is not as stable as the semiquinone radical (Bielski, and Richter, 1977). The anion radical disproportionates via an initial dimerization in a similar way as semiquinone radicals (Bielski et al., 1981 Sawyer, et al., 1982). The lifetime of the radical is in the order of microseconds. The acidity of ascorbic acid (pk = 4.2) stems from the OH group in the P position to the lactone carbonyl group. It corresponds to the OH group of a vinylogous carboxylic acid (Scheme 7.2.11). Its UV maximum occurs at 260 nm (e = 1 x 1(T). 

All the vinylogous systems 22-32 exhibit the same pattern of redox properties. The position of the potentials Ei and E2, or better E = (Ei + E2>/2 of a certain vinyl-ogue is mainly determined by the heterocyclic end groups (cf. Table 8). 

If molecules such as these are to be employed in molecular devices, one of the challenges is to learn how to use external stimuli to interact with the system. Such stimuli could be chemical, electrochemical, or photochemical in nature. In view of this, the two [2]-catenanes 14 and 15 were synthesised (Figure 5.8). Related to their characteristic redox properties, the precursor vinylogous viologen building... 

The redox properties of the vinylogous systems 16 and 17 -were studied by cyclic voltammetry <1997JMC381>. They both display two reversible, one-electron oxidations thus, 16 is oxidized at 0.36 and 0.50 V vs. Ag/AgCl in acetonitrile, while 17 is oxidized at 0.29 and 0.44 V vs. Ag/AgCl. 

---