Considering the actual cyclobutane ring opening step Pyr'+OPyr ! Pyr+-Pyr' (Scheme 4.5.4), however, where the C(6)-C(6') s bond is broken and a N(1)=C(6) n bond is simultaneously formed, this cleavage should be slower the more the involved orbitals, e.g. the SOMO and the s orbitals of the N(1)-C(6) and C(6)-C(6') bonds, deviate from coplanarity (Scheme 4.5.8).

No geometrical data calculated for dimethylthymine- and dimethyluracil-derived radical cations are, unfortunately, available in the literature. As an approximation, because it is not expected that removal of an electron would result in dramatic geometrical alterations, the X-ray data of the uncharged PyrO Pyr should tentatively reveal the influence of substituents at C(5) and C(5') on the geometry of the dimer radical cation and, especially, of the cyclobutane ring. Assuming that the n orbital at N(1) in PyrO Pyr has a similar orientation as the SOMO in Pyr'+O Pyr, the dihedral angles along n-N(1)-C(6)-C(6') in the various pyrimidine dimers 17 [28], 18 [29], and 19 [30], show that stepwise introduction of substituents at C(5) and C(5') leads to an increase of the torsion at the cyclobutane ring (Scheme 4.5.8). The resulting decreasing overlap of the relevant orbitals could result in a slowdown of the cycloreversion, thus leading to a decrease of the overall splitting rate of PyrO Pyr. It might, therefore, be concluded that the ease of oxidative dimer splitting is governed by both oxidation potential of the dimer and the stereoelectronic requirements of its radical cation [22]. In contrast with this, the second step of the cycloreversion according to Pyr+ -Pyr' ! Pyr + Pyr'+ (Scheme 4.5.4) is not be lieved to be affected by comparable stereoelectronic effects, because any conformation favorable for splitting could be achieved by simple rotation around the C(5)-C(5') s bond.

An analogous stereoelectronic influence on the rate of the reductive repair process was also observed by Carell et al. [28]. The ring opening of the dimer radical anion also proceeds stepwise, but with the C(5)-C(5') bond being broken first. The C(5) and C(5 0)-methyl groups of thymine-derived dimers, which were found to be repaired more slowly than the uracil-derived dimers, lead to distortion of the geometry, which results in deceased overlap of the p * C(4)-O(4) orbital with the s* C(5)-C(5 0) orbital.

Rose et al. [13] also observed slower cleavage of thymine-derived bridged dimers 4 and 5, compared with syn-1 (entry 7). They explained this finding not by stereo-electronic effects but by the possible inability of a thymine radical cation to propagate the chain reaction by oxidizing the corresponding dimer. Because dimers 4 and 5 are linked by a trimethylene bridge, however, interference of the latter on the splitting efficiency of these compounds could not be excluded.

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