Intramolecular competition of di-.pi.-methane and oxadi-.pi.-methane rearrangements. Photochemistry of 3,3-dimethylbicyclo[2.2.2]octa-5,7-dien-2-one (dimethylbarrelenone)

Photochemical Reactions of 1,3,3‐Trimethylbicyclo[2.2.2]octa‐5,7‐dien‐2‐ones

“…In the cases of dienones 1d – f , DPM rearrangement products were observed under sensitized conditions. These results corroborate the work of Becker11 and Luibrand12 on their sensitized irradiation of dienones 38 and 11 (vide supra), respectively, in which only DPM products were obtained. The preference for DPM over ODPM rearrangement may be ascribed either to the weaker C=C bond, which can interact more easily than the stronger C=O bond with the triplet biradical during the bridging process11,27 or to the large difference in triplet energy between the sensitizer and the vinyl moiety 28.…”

“…In the triplet‐sensitized irradiation of bicyclo[2.2.2]octadienone 1a , we obtained only the aromatic compound 2a in both solvent systems. This is at variance with the results obtained by Luibrand et al12 in their sensitized irradiation of bicyclic octenone 11 , which generated a DPM photoproduct. The presence of the methyl group at the bridgehead carbon atom may have enhanced the reactivity of the α‐cleavage process that may occur during singlet (S 1 , n,π*) or triplet (T 2 , n,π*) excitation by stabilization of the incipient radical generated at the bridgehead carbon atom 2,20,26b.…”

“…On the basis of this spectral analysis, 1d apparently underwent DPM rearrangement. The assignment of the lower δ value to the C‐8 proton is based on the principle that the chemical shift for a highly strained ring is generally upfield; this observed value is consistent with reported δ values for tricyclic systems possessing structures similar to 1d 12,13. The other conceived rearrangement photoproducts are ruled out as follows: (1) structures 17d , 22d , and 25d should each only exhibit one vinylic proton chemical shift, (2) structure 20d should not exhibit allylic proton coupling, and (3) structure 28d should only exhibit one allylic proton coupling with the vinyl proton.…”

“…The preference for DPM over ODPM rearrangement may be ascribed either to the weaker C=C bond, which can interact more easily than the stronger C=O bond with the triplet biradical during the bridging process11,27 or to the large difference in triplet energy between the sensitizer and the vinyl moiety 28. Nevertheless, the behavior of 1d – f and the dienones 38 and 11 is in line with Zimmerman’s29 suggestions and Luibrand’s12 generalization that vinyl‐vinyl bridging is preferred over vinyl‐keto bridging. It is important to note that when bicyclodienones 1d – f were irradiated in the presence of acetone as sensitizer, the aromatic compounds 2d – f were also formed together with the DPM photoproducts 3d – f , but that when a small amount of acetophenone was added to acetone, only the DPM photoproducts were obtained.…”

“…Barring a few early reports,11–14 the competitive photochemistry of simple bicyclo[2.2.2]octadienones has not been studied in detail. Strikingly, with the exception of the bicyclic system 11 , which bears no substituents on the vinyl carbon atoms,12 all of the simple bicyclo[2.2.2]octadienones irradiated so far feature a vinyl bridge bearing methoxycarbonyl groups on both carbon atoms and all of them have preferentially undergone DPM rearrangements 11,13. This scenario clearly points to a lack of easy access to simple but diversely substituted bicyclo[2.2.2]octadienones.…”