Diels-Alder reactions between cyclopentadiene (CP) and butadiene (BD) give a variety of dimers such as 4-vinyl-cyclohexene (VCH), 5-vinyl-2-norbornene (VNB), cis-3a, 4, 7, 7a-tetrahydroindene (THI) and dicyclopentadiene (DCP), and these dimers contain two double bonds of different reactivity.
In the present investigation, the epoxidation of these dimers with hydrogen peroxide catalyzed by metal oxides such as SeO₂, H₂MoO₄ and H₂WO₄ was carried out under various reaction conditions, and the results obtained were compared with those of epoxidation with organic peracids.
The reaction of VCH (1) with hydrogen peroxide catalyzed by SeO₂ at 40°C for 5hr [Eq. (1)] did not give an epoxide but diol (2) in a 40% yield. In the oxidation of VCH by H₂MoO₄ and H₂WO₄ catalysts, diol (2) was also obtained but in lesser yields than by SeO₂ catalyst.
The epoxidation of VNB (3) with hydrogen peroxide by SeO₂ yielded 33% monoepoxide (4), but H₂MoO₄ and H₂WO₄ catalysts were less selective [Eq. (2)].
When THI (5) was epoxidized with hydrogen peroxide by SeO₂ catalyst, monoepoxide (6) and indene (7) were obtained in 27% and 8% yields, respectively [Eq. (3)], while in the epoxidation by H₂MoO₄ and H₂WO₄ catalysts only (6) resulted in yields from 10 to 15%.
On the other hand, the epoxidation of THI with perbenzoic acid yielded monoepoxides (6) and (8) in a 4:5 molar ratio and diepoxide (9) [Eqs. (4) and (5)].
It was found that the present catalytic epoxidation indicated a higher selectivity for the epoxidation of THI compared with that of epoxidation with perbenzoic acid.
For the formation of indene (7), it seems reasonable to assume that dehydrogenation is generally observed in the oxidation of cyclic compounds using SeO₂.
The epoxidation of DCP (10) occurred readily without any side reactions except when a V₂O₅ catalyst was used to give monoepoxides (11) and (12) as shown in Eq. (6).
Effects of various solvents were examined in the reactions with the SeO₂ catalyst, and it was found that epoxidation proceeded rapidly in a protonic solvent having a lower polarity such as tert-butyl alcohol.
An extremely high ratio of (11)/(12) as compared with that of epoxidation by organic peracid was observed with the catalysts used. It is assumed that such high regioselectivity may be attributable to a steric effect established between the bridge head proton of the norbornene ring in DCP and the epoxidizing agent.