134977-97-2Relevant articles and documents
Stereochemical studies of the photochemical cycloaddition reaction of alkenes with N-benzoylindole and N-carboethoxyindole; evidence for biradical intermediacy
Hastings, David J.,Weedon, Alan C.
, p. 1171 - 1182 (2007/10/02)
The stereochemistries of the 2 + 2 cycloaddition products obtained from the photochemical addition reaction between N-benzoylindole or N-carboethoxyindole and the alkenes cyclopentene, cyclohexene, cycloheptene, cis- and trans-2-butene, and cis- and trans-4-octene are examined.The structures of the products are shown to be consistent with a photo-cycloaddition mechanism involving the intermediacy of triplet 1,4-biradical species.The quantum yields of adduct formation between N-benzoylindole and both cis- and trans-octene were measured as a function of alkene concentration.The results suggest that cis-octene reacts with the indole derivative's triplet excited state with a rate constant of (1.7 +/- 0.3) * 107 M-1 s-1.The results are also consistent with the immediate products of this reaction being 1,4-biradicals, 98percent of which revert to the ground state indole derivative and alkene, and only 2percent of which proceed to cycloadduct.In marked contrast, the same treatment suggests that trans-octene reacts with the triplet excited state of N-benzoylindole with a rate constant estimated to be in the range of 1 * 106 and 6 * 105 M-1 s-1, and it appears that the 1,4-biradicals formed revert much less efficiency to the starting materials; it is estimated that between 67 and 100percent of the 1,4-biradicals proceed to cycloadducts.In the reaction with cis-octene biradical reversion leads to the formation of trans-octene ("Schenk isomerization"); the quantum yield of this process is determined to be 0.074 +/- 0.004, which may imply that approximately 75percent of the biradicals collapse to cis-alkene and 25percent collapse to the trans-isomer. Key words: indole, photocycloaddition, 1,4-biradicals.
