Small and Medium Rings, 76. Photorearrangement of Polycyclic Quinoxalines, Isomerisations of Isodrin-Type and Aza-Di-π-Methane Chromophores
✍ Scribed by Behr, Jürgen ;Braun, Ralf ;Martin, Hans-Dieter ;Steigel, Alois ;Rubin, Mordecai B.
- Book ID
- 102789035
- Publisher
- Wiley (John Wiley & Sons)
- Year
- 1991
- Tongue
- English
- Weight
- 579 KB
- Volume
- 124
- Category
- Article
- ISSN
- 0009-2940
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✦ Synopsis
Quinoxalines, multichromophoric / Sensitization, intramolecular / Cycloaddition [2n + 2x1 / Quinoxaline di-x-methane rearrangement / PE Spectra The multichromophoric polycyclic quinoxaline 11 has been mixing of energy levels in these two fragments of 11, as evisynthesized starting with diketone 7. Irrespective of whether denced by the PE spectrum of quinoxaline 13. The direct inquinoxaline 11 is irradiated in its nn* or n ~* band, the pho-teraction of a quinoxaline chromophore with a nonconjugated toproduct formed in high yield is invariably the cage com-double bond, i.e. an aza-di-n-methane rearrangement, is pound 14. This excitation energy transfer from the quinoxaline shown to occur photochemically in bicyclic quinoxaline 12. to the isodrin subchromophore of 11 is facilitated by a strong Much work has been devoted to the studyexperimentally and theoretically -of proximal, non-conjugated chromophores, especially double bonds. Among the various combinations of chromophores that have been investigated, two special ones have attracted much attention. Isodrin-type molecules (e. g. 1) contain two face-to-face x-bonds reminiscent of the electronic situation in norbornadiene (2). But, in contrast to the latter diene, a stronger spatial x overlap is conducive to a more intense through-space interaction and, additionally, cooperative through-bond coupling results in a particularly highlying x-HOMO in 1 Photoisomerization to yield the cage compound 3 in observed with high quantum yields2b). 1 2 3 4 5 6
A second chromophore of photochemical interest is the di-x-methane moiety of P,y-unsaturated systems (cf. 4). The structural requirements for an efficient photorearrangement demand good interaction (homoconjugation) between centers 2 and 4 of a diene 43). Interestingly, a nitrogen atom may replace an olefinic carbon, as in 5, without impairing the course of the reaction that is believed to lead to the product via a 1,4-diradical 64) formed first.