Photocycloadditions of cyclodienes to hexafluorobenzene.

概要

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The stereochemistry of photocycloaddition reactions of various cyclodienes to hexafluorobenzene in cyclohexane solution depends on the structure of the cyclodiene and [2+2]cycloadducts are mainly observed. Photoreaction of 1,5-cyclooctadiene with hexafluorobenzene gives nearly equal amounts of syn and anti[2+2]adducts, which are further transformed to tetracyclic systems, contrary to this, the stereochemistry observed by reactions with 1,4-cyclohexadiene depends on the molar ratio of cyclodiene–hexafluorobenzene and 30–54% of syn addition is observed, the syn adduct, however, being further transformed to 2,3,4,5,6,7-hexafluoropentacyclo[6.4.0.02,7.03,11.06,10]dodec-4-ene. Syn selectivity of [2+2]cycloaddition was enhanced when bicyclic dienes were used and it is interesting that in the case of norbornadiene and benzonorbornadiene only exo attack was observed, and no anti attack occurred in the case of benzonorbornene. The quantum yields of [2+2]photocycloaddition reactions depend on the structure of the cyclodiene and the concentrations of hexafluorobenzene and cyclodiene. The quantum yields for norbornadiene and 1,4-cyclohexadiene are nearly the same, while it is two times higher in a photoreaction with 1,5-cyclooctadiene, when equal amounts of hexafluorobenzene and 1,5-cyclooctadiene were irradiated in cyclohexane at λ=253.7 nm.
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