Chapter 8, Problem 74DSP

Oxymercuration

Concerns about mercury’s toxicity have led to decreased use of mercury-based reagents in synthetic organic chemistry. Alternatives exist for many of the transformations formerly carried out with mercury compounds while carrying much less risk. The chemistry of several of the reactions, however, is sufficiently interesting to examine here.

Among the synthetically useful reactions of $\text{Hg}\left(\text{II}\right)$

salts with organic compounds, the most familiar is a two-stage procedure for alkene hydration called oxymercuration–demercuration. Its application in the conversion of $\text{3,3-dimethyl-1-butene}$ to $\text{3,3-dimethy-2-butanol}$ illustrates the procedure.

The reaction is performed in two operations, the first of which is oxymercuration. In this stage the alkene is treated with $\text{mercury}\left(\text{II}\right)\text{ acetate}\text{[Hg}{\left({\text{O}}_2{\text{CCH}}_3\right)}_2$, abbreviated as $\text{Hg}{\left(\text{OAc}\right)}_2\text{]}$. $\text{mercury}\left(\text{II}\right)\text{ acetate}$ is a source of the electrophile $\text{H}\text{gOAc}$, which bonds to $\text{C-1}$

of the alkene. The oxygen of water, one of the components in the ${\text{THF–H}}_2\text{O}$

solvent mixture, bonds to $\text{C-2}$. The demercuration operation uses sodium borohydride ${\text{(NaBH}}_4$, a reducing agent) to convert $\text{C - Hg}$

to $\text{C - H}$.

From the overall reaction, we see that oxymercuration–demercuration

1. accomplishes hydration of the double bond in accordance with Markovnikov’s rule, and

2. carbocation rearrangements do not occur.

Additional information from stereochemical studies with other alkenes has established that

3. anti addition of $\text{HgOAc}$

and $\text{OH}$

characterizes the oxymercuration stage, and

4. the replacement of $\text{HgOAc}$

c by H in the demercuration stage is not stereospecific.

The structure of the intermediate in oxymercuration has received much attention and can be

approached by considering what is likely to happen when the electrophile $\text{H}\text{gOAc}$

reacts with the double bond of an alkene.

Recall from Section $5.9$

that electrons in bonds that are $\text{β}$ to a positively charged carbon stabilize a carbocation by hyperconjugation.

The electrons in a $\text{C - Hg}$σ bond are more loosely held than $\text{C - H}$

or $\text{C - C}$

electrons, making

stabilization by hyperconjugation more effective for $\text{β-C-Hg}$ than for $\text{β-C-H}$

or $\text{β-C - C}$. Hyperconjugative stabilization of the intermediate in oxymercuration is normally shown using dashed lines to represent partial bonds. The intermediate is referred to as a “bridged” mercurinium ion.

The problems that follow explore various synthetic aspects of oxymercuration–demercuration.

Experimental procedures sometimes vary depending on the particular transformation. The source of the electrophile may be a mercury(II) salt other than $\text{Hg}{\left(\text{OAc}\right)}_2$, the nucleophile may be other than ${\text{H}}_2\text{O}$, and the reaction may be intramolecular rather than intermolecular.

Oxymercuration–demercuration of allyl alcohol gives $\text{1,2-propanediol}$

Under the same conditions, however, $\text{4-penten-1-ol}$ yields a compound having the molecular formula ${\text{C}}_5{\text{H}}_{10}\text{O}$.

What is the most reasonable structure for the product of this reaction?