Reaction Review 5- Oxymercuration of Alkenes to Alcohols PDF

Preview

Summary

Download Reaction Review 5- Oxymercuration of Alkenes to Alcohols PDF

Description

Rxn Review #5 – Oxymercuration of Alkenes to Alcohols The general reaction: Converts alkenes to alcohols by electrophilic addition. OH

1. Hg(OAc)2, H2O, THF 2. NaBH4

O Hg(OAc)2

=

O O-Hg-O

O THF

= an inert solvent

In this reaction, the first step is the electrophilic attack of the +HgOAc on the pi bond to form a bridged carbocation. The evidence of this is the lack of any observed rearrangements. Water then attacks in Markovnikov fashion to give an oxonium at the more substituted C. In the second step, sodium borohydride (NaBH4), a reducing agent, removes and reduces Hg and adds a hydride (H-) to the less substituted C. Proposed bridge mercuric intermediate in mechanism: Hg(OAc)

HgOAc

NaBH4

OH2

OH O

Hgo

H2O

O

Remember alternate reaction. Substitute an alcohol for the water and a Markovnikov ether is produced.

O

1. Hg(OAc)2, CH3CH2OH, THF 2. NaBH4

This reaction is a high yield way to make Markovnikov alcohols and ethers from alkenes with no rearrangement. It’s not used as much any more due to the environmental concern of producing Hg (mercury) waste. If a chiral carbon is produced at the Markovnikov carbon, it is produced racemically. In the example below, note one chiral C already exists and is not involved in the reaction. The second chiral C is produced racemically….yielding diastereomer products overall. Another example: OH

1. Hg(OAc)2, H2O, THF

OH

2. NaBH4

+

diastereomers

Solvents: THF and diglyme are commonly used solvents in this reaction. They may or may not be shown over the reaction arrow. THF

diglyme O

O O

O...