| Popis: |
The stereochemistry of the oxymercuration-demercurarion (OM-DM) of olefins related to the cis -bicyclo(3.3.0]octane and endo -2,3-trimethylenenorboniane structures was determined. In the case of cis -bicyclo(3.3.0]oct-2-ene, hydration occurs preferentially at the less hindered 3-position, with little preference shown for exo vs endo . The more hindered 2-product shows a 11:1 preference for the exo -product. The presence of Me groups at positions 2- or 3-, results in the formation of the tertiary alcohols with approximately a 4:1 favoring of the exo -isomer. (The oxymercuration intermediate exhibits a rapid equilibration with time. Consequently, the 4:1 ratios may not represent the true limit for the isomer distribution in the initial kinetic product.) Similarly, 2-methylenebicyclo[3.3.0]octane reveals an 8:1 preferential formation of the exo -alcohol. In the case of endo -trimethylene-norborn-8-ene, the oxymercuration stage is extraordinarily slow and the results do not fit this pattern. Possibly the very slow oxymercuration stage permits equilibration of the initial reaction product. On the other hand, the reaction is fast with 8-methylene- endo -trimethylenenorbornane and the product is 100% of the tertiary exo -alcohol. The same behavior is observed for 2-methylenenorbornane. Surprisingly, 2-methylene- endo -trimethy-lenenorbornane fails to undergo oxymercuration. Consequently, both endo -trimethylenenorborn-8-ene and 2-methylene- endo -trimethylenenorbornane exhibit an exceptional inertness toward oxymercuration, presumably related to the highly rigid U-shaped structure of the parent system. |
