Norbornene oxymercuration

Formation of mixtures of products in these reactions can be attributed largely to the properties of the acetate group. The reactions of a number of cycloalkenes with thallium(III) salts have been investigated in some detail and the results obtained have served both to elucidate the stereochemistry of oxythallation and to underline the important role assumed by the anion of the metal salt in these oxidations. The most unambiguous evidence as to the stereochemistry of oxythallation comes from studies by Winstein on the oxythallation of norbornene (VII) and norbornadiene (VIII) with thal-lium(III) acetate in chloroform, in which the adducts (IX) and (X) could be precipitated from the reaction mixture by addition of pentane 128) (Scheme 11). Both by chemical means and by analogy with the oxymercuration... 

Unlike the oxymercuration of acyclic olefins, oxymercuration of bicyclic olefins often gives jy -addition products. Norbornenes 93, for example, show exclusive fvo-oxymercuration. In this reaction, the ratio between the isomers depends on the nature of the fvo-substituent (R1) and tro/o-substituent (R2) (Equation (36)). The presence of electron-withdrawing fvo-substituents always leads to a much greater selectivity in favor of 94a-d over 95a-d.116 117 As indicated by extensive theoretical calculations, the charge distribution in the transition states governs the selectivity of these reactions.118... 

Oxymercaration (2, 265-267 3. 194). Brown and Geoghegan have investigated the relative reactivities of a number of olefins in the oxymercuration reaction in a 20 80 (v/v) mixture of water and THF. The following reactivity is observed terminal disubstituted > terminal monosubstituted > internal disubstituted > internal trisub-stituted > internal leirasubstitu ted. Thus steric factors play a major role in the reactivity of olefins. Increased substitution on the double bond and increased steric hindrance at the site of hydroxyl or mercury substituent attachment decrease the rate of reaction. In the case of olefins of the type RCH CHR, cts-oleiins are more reactive than the corre.sponding rra/i.s-olefins. Inclusion of the double bond in ring systems causes a moderate rate increase which varies somewhat with structure cyclohcxenc > cyclo-pentenc cyclooctene norbornene bicyclo[2.2.2]-octene-2. 

These assignments were opposite to those suggested earlier and indicate that oxymercuration in unstrained rings is tram, coinciding with conclusions made from NMR observations. The latter work also established that oxymercuration of norbornene was exo-cis. Table VI shows that indeed... 

Oxymercuration of norbornadiene leads to the norbornene skeletal product in the kinetically controlled step, but this is readily converted in the presence of traces of Cl to the nortricyclene skeletal product 126b). 

The relative reactivities of cyclic olefins, including norbornene and bicyclo-[2,2,2]oct-2-ene, to oxymercuration have been reported. The relative reactivities of (128) and (129) to a variety of reagents showed that additions to... 

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