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c,s-2 > c,s-1 > t,s-2

in acetonitrile; radical cation-induced (phenanthrene'+)

21a

a In order of decreasing reactivity.

a In order of decreasing reactivity.

cyclobutane ring has a significant effect on the rate of the cleavage process. Table 4.5.2 presents a compilation of the experimental results reported by various groups.

Because electron transfer is generally believed to be the initial step in the oxida-tive cleavage reaction, it might be expected that a correlation between reactivity and oxidation potential of PyrO Pyr should exist. In Scheme 4.5.2 are listed the irreversible half-peak anodic potentials Ep=2 for the stereoisomeric dimethyluracil- and dimethylthymine-derived cyclobutane dimers 1 and 2, which were taken from Ref. [21a]. The anti-configured dimers clearly have higher oxidation potentials than the corresponding syn dimers. This might be because of considerable perturbation of the HOMO of the syn-configured dihydropyrimidine chromophore by the conjunction of the two chromophores forming the cyclobutane ring; this can be interpreted in terms of through-bond interactions between the n orbitals of N(1) and N(1') involving the C(6)-C(6') bond. In contrast, in the anti dimers no significant perturbation occurs, because the relevant n orbitals are separated by two C-C bonds [21a].

In fact, a faster oxidative splitting of the syn-configured dimers was usually observed. Elad et al. reported that both the unsensitized [11a] and sensitized splitting [11b] of the two syn-configured isomers of 1 is more effective than that of the anti compounds (entries 1 and 2). Analogous behavior was observed in the NO3'-induced splitting of a variety of dimethyluracil- and dimethylthymine-derived dimers (entry 3) [8, 22]. From the findings by Rosenthal et al. [23], who used transition metal salts as redox photosensitizers, it might, however, be concluded that c,a-2 is more easily cleaved than c,s-2 (entries 4 and 5). The reason for this dis crepancy is not clear, but might, perhaps, be because of the different experimental conditions. Whereas the oxidation potentials Ep=2 given in Scheme 4.5.2 were measured in acetonitrile [21a], Rosenthal et al. [23] performed their studies in aqueous solution. No electrochemical data for Pyro Pyr measured under aqueous conditions are yet available in the literature.

Steric repulsion seem to affect the oxidation potential, because the more crowded c,s-1 and c,s-2 commonly have higher oxidation potentials, by ca. 0.1 V, than the corresponding trans,syn-configured isomers (Scheme 4.5.2) [21a]. On the basis of this, of the different isomeric dimethyluracil- and dimethylthymine-derived dimers t,s-1 and t,s-2 are assumed to be oxidatively cleaved more rapidly than their respective stereoisomers. This behavior was indeed observed in several instances, e.g. for the AQS-sensitized splitting of t,s-3 (entry 6) and the Fe(CN)63+-sensitized cleavage of syn-configured 1 (entry 4), both in aqueous solution [23, 24]. Likewise, sensitizer- (entries 2 and 7) or radical-induced oxidative splitting (entry 3) of different Pyro Pyr in organic solvents was occasionally observed to correlate principally with the oxidation potentials Ep=2 of Pyro Pyr [8, 11b, 13, 22].

Despite this, the apparent discrepancies between reactivity and oxidation potential observed for many dimers indicates that the overall rate of the oxidative repair of Pyro Pyr cannot be simply equated with the rate of the initial electron transfer. When isoquinoline radical cations, generated in situ by irradiation of N-ethoxyisoquinolinium hexafluorophosphate, were used as oxidants, c,s-2 was observed to be cleaved faster than t,s-2 (entry 8) [25]. This finding was explained by dipole-dipole repulsion of the two carbonyl groups on the cyclobutane ring in the dimer radical cation of c,s-2, which lead to an enhancement of the splitting efficiency compared with the radical cation of t,s-2, thus exceeding the difference in the rate of the electron transfer between cis,syn and trans,syn dimers. Pac et al. [21a] explained the observed ''reversed'' reactivity with c,s-2 being faster cleaved than c,s-1 and t,s-2 using phenanthrenyl radical cations as oxidants, which were generated in situ by 1,4-dicyanobenzene-sensitized oxidation of phenanthrene, with steric repulsion in an excited state charge-transfer complex between the oxi-dant and the dimer (entry 9). Of the three dimers under investigation, c,s-2 is the most strained molecule, whereas t,s-2 is the less strained. The reactivity of c,s-1 is intermediate, because of the lack of the C(5) and C(5') methyl groups.

In contrast with this, NO3'-induced splitting reveals completely different behavior. It was observed from competition experiments that c,s-1 is cleaved significantly more rapidly than c,s-2 (entry 3) [22]. Because the oxidation potentials of both compounds are virtually identical (see Scheme 4.5.2), the difference in splitting efficiency must be because of the presence or absence of the C(5) and C(5') methyl groups on the cyclobutane ring. NO3' is a strong oxidizing radical (E0 NO3'/ NO3~ = 2.0 V relative to the SCE; in acetonitrile) [26], and it is therefore believed that the electron abstraction should be fast and irreversible (whether this electron transfer proceeds through an inner- or outer-sphere process, is not yet clear). Because both NO3' and the reduced NO3~ are small and planar molecules [27], it seems unlikely that steric interactions between the dimer or its radical cation and NO3' or NO3~, respectively, are responsible for the difference in splitting efficiency.

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