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Conformational Analysis of Endoperoxides Grafted onto Bicyclo[2.2.n]alkanes as a Test of the Rigidity of the Bicyclic Skeleton. Photo‐oxidation of [2.2.2] Hericene and 2,3,5,6‐Tetramethylidenebicyclo[2.2.n]alkanes
Author(s) -
Birbaum JeanLuc,
Vogel Pierre
Publication year - 1986
Publication title -
helvetica chimica acta
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.74
H-Index - 82
eISSN - 1522-2675
pISSN - 0018-019X
DOI - 10.1002/hlca.19860690403
Subject(s) - chemistry , bicyclic molecule , conformational isomerism , heptane , octane , reactivity (psychology) , stereochemistry , medicinal chemistry , photochemistry , organic chemistry , molecule , medicine , alternative medicine , pathology
The photo‐oxidation of [2.2.2]hericene ( 6 ) gave successively the endoperoxides 11 (9,10,11,12‐tetramethylidene‐4,5‐dioxatricyclo[6.2.2.0 2,7 ]dodec‐2(7)‐ene), the bis‐endoperoxide 16 (15,16‐dimethylidene‐4,5,11,12‐tetraoxatetracyclo[6.6.2.0 2,7 .o 9,14 ]hexadeca‐2(7),9(14)‐diene), and the tris‐endoperoxide 19 (4,5,11,12,17,18‐hexaoxapentacyclo[6.6.6.0 2,7 .0 9,14 .0 15,20 ]icosa‐2(7),9(14),15(20)‐triene). The endoperoxides 11, 16 , and 19 were formed in the presence or in the absence of a dye sensitizer. The sensitized photo‐oxidations of 2,3,5,6‐tetramethylidenebicyclo[2.2.2]octane ( 4 ), 5,6,7,8‐tetramethylidenebicyclo[2.2.2]oct‐2‐ene ( 5 ), 2,3,5,6‐tetramethylidenebicyclo[2.2.1]‐heptane ( 7 ), and 2,3,5,6‐tetramethylidene‐7‐oxabicyclo[2.2.1]heptane ( 8 ) gave successively the corresponding mono‐endoperoxides 9, 10, 12 , and 13 and the bis‐endoperoxides 14, 15, 17 , and 18 , respectively. Low‐temperature NMR spectra of the bis‐endoperoxides 14 and 16 indicated that their C 2 and C s conformers have the same stability. Similarly, there was no difference in the enthalpy of the D 3 and C 2 conformers of the tris‐endoperoxide 19 . The following reactivity sequence was observed for the sensitized photo‐oxidations of 6–8 and 5,6‐dimethylidene‐7‐oxabicyclo[2.2.1]hept‐2‐ene ( 23 ): 6 + 1 O 2 → 11 > 7 + 1 O 2 → 12 > 8 + 1 O 2 → 13 > 23 + 1 O 2 → 24 , a trend parallel with that reported for the ethylenetetracarbonitrile (TCNE) cycloadditions to the same polyenes. The rate‐constant ratios k 1 / k 2 and k 2 / k 3 for the three successive photo‐oxidations of [2.2.2]hericene ( 6 ) did not differ significantly from unity, in contrast with the Diels ‐ Alder additions of 6 . Similarly, the rate‐constant ratios k 1 / k 2 for the two successive photo‐oxidations of tetraenes 7 and 8 were significantly smaller than those reported for the successive TCNE cycloadditions to 7 to 8 . The endoperoxide formations are not sensitive to the change in the exothermicity of the reactions but they are sensitive to the electronic properties (IP's) of the polyenes.