Reaction of Singlet Oxygen with 2-Methylnorborn-2-ene, 2-Methylidenenorbornane, and their 7,7-Dimethyl Derivatives. The transition state geometry for hydroperoxidation
✍ Scribed by Charles W. Jefford; André F. Boschung
- Publisher
- John Wiley and Sons
- Year
- 1974
- Tongue
- German
- Weight
- 963 KB
- Volume
- 57
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
The dye‐sensitized photo‐oxygenation of 2‐methylnorbonr‐2‐ene (3), 2‐methylidene‐norbornane (4) and their 7,7‐dimethyl derivatives (5 and 6) has been studied. In all cases allylically rearranged hydroperoxides were formed, except that 4 also gave a little norbornanone (presumably from the dioxetane) and 5 gave some endo‐3,7,7‐trimethylnorbornan‐2‐one as a secondary photo‐product. It was found that the exo/endo attack ratios by singlet oxygen on 3 and 5 are 66 and 0.19. By exploiting the C (3) monodeuteriated derivatives, 4 and 6 showed ratios of 28 and 0.67.
Rates of reactivity of the olefins 3 and 4 were compared to methylidenecyclopentane, 1‐methylcyclopentene and 1‐methylcyclohexene as monocyclic standards. Additionally, comparative rates between the 7,7‐dimethyl olefins and their parents were measured. When further comparison was made of the rate ratios partitioned for exo and endo attack, it was seen that oxygen experienced a 500‐ to 1000‐fold rate retardation on approach to the endo side of 3 compared to that for its monocyclic analogue. Exo rates between the parent norbornene 3 and its 7,7‐dimethyl derivative 5 showed a 250‐fold decrease. Although four times smaller than the difference reported for epoxidation, the evidence clearly pointed to a one‐step cyclic process as the rate determining step for photo‐oxygenation. The steric evidence, taken with the low values found for the intermolecular isotope effects of 1.14 ± 0.01 and 1.02 ± 0.01 observed for exo and endo‐3‐deuterio‐2‐methylidene‐norbornanes, permits the deduction that the transition state is largely dipolar. In the early stages of the addition bonding between one end of the oxygen molecule and the terminal vinyl carbon is advanced. At the same time positive charge is dispersed by hyperconjugation between the central carbon atom and the allylic carbon‐hydrogen bond. At a later stage the anionic oxygen atom abstracts the loosened allylic hydrogen atom to create the hydroperoxide. No evidence for the formation of a discrete perepoxide intermediate was obtained.