Oxymercuration, anti stereochemistry

Markovnikov regiochemistry and anti stereochemistry are observed, (d) Addition of water by oxymercuration (Section 7.4)... 

One of the features that makes the hydrobora ( ion reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration/oxidation of 1-methylcyclopentene yields trans-2-methylcydopentanol. Boron and hydrogen both add to the alkene from the same face of the double bond—that is, with syn stereochemistry, the opposite of anti—with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an -OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration. 

The stereochemistry of oxymercuration has been examined in a number of systems. Conformationally biased cyclic alkenes such as 4-r-butylcyclohexene and 4-f-butyl-l-methycyclohexene give exclusively the product of anti addition, which is consistent with a mercurinium ion intermediate.17,22... 

The stereochemical outcome is replacement of the C—B bond by a C—O bond with retention of configuration. In combination with stereospecific syn hydroboration, this allows the structure and stereochemistry of the alcohols to be predicted with confidence. The preference for hydroboration at the least-substituted carbon of a double bond results in the alcohol being formed with regiochemistry that is complementary to that observed by direct hydration or oxymercuration, that is, anti-Markovnikov. 

The stereochemical outcome would also be different as the hydroboration adds syn to the alkene, whereas oxymercuration gives the anti product though in this case the stereochemistry is lost in the reduction step. 

Oxymercuration-Reduction (Section 6.3F) Oxymercuration-reduction is used to convert alkenes to alcohols. The mechanism involves reaction of the alkene tt bond with an HgOAc+ to give a bridged mercurinium ion intermediate (a cation) that is, in turn, attacked by HjO from the backside to give a new intermediate, which loses a proton. In a second step, NaBH is added to replace the Hg atom with H.The first step is anti stereoselective because HgOAc and OH add from opposite faces of the alkene. However, the NaBH reduction step scrambles the stereochemistry as H replaces Hg, so the overall process is scrambled... 

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