Organic Syntheses, CV 6, 766
Submitted by J. M. Jerkunica and T. G. Traylor
1.
Checked by A. K. Willard and R. E. Ireland.
1. Procedure
A
3-l, three-necked flask fitted with a
thermometer and a
mechanical stirrer is charged with
95.7 g. (0.300 mole) of mercury(II) acetate (Note
1) and 300 ml. of water. After the acetate dissolves,
300 ml. of diethyl ether is added. While this suspension is stirred vigorously
28.8 g. (0.300 mole) of 1-methylcyclohexene (Note
2) and (Note
3) is added, and stirring is continued for 30 minutes at room temperature (Note
4). A solution of
150 ml. of 6 N sodium hydroxide is added followed by
300 ml. of 0.5 M sodium borohydride in 3 N sodium hydroxide. The
borohydride solution is added at a rate such that the reaction mixture can be maintained at or below 25° with an
ice bath.
The reaction mixture is stirred at room temperature for 2 hours, after which time the
mercury is found as a shiny liquid. The supernatant liquid is separated from the
mercury (Note
5), the
ether layer is separated, and the aqueous solution is extracted with two
100-ml. portions of ether. The combined
ether solutions are dried over
magnesium sulfate and distilled, giving
24.1–25.8 g. (
70.5–75.4%) of
1-methylcyclohexanol, b.p.
154.5–156°;
n21D 1.4596 (Note
6).
2. Notes
2.
1-Methylcyclohexene was purchased from K & K Laboratories and used without further purification.
3. Sometimes a yellow color [
mercury(II) oxide] appears at this point and disappears as the reaction proceeds. If the yellow color does not disappear in about 10 minutes,
1.5 ml. of 70% perchloric acid per mole of
mercury(II) acetate may be added to accelerate the reaction. Under these conditions even unreactive olefins are completely oxymercurated in about an hour.
4. The checkers found that extending the time of oxymercuration to 2 hours did not improve the yield.
5. The checkers found that the reaction mixture could be decanted from the
mercury only if the mixture was allowed to stand for at least one hour after stirring was stopped. An alternate procedure, which proved quite satisfactory, was filtration of the entire reaction mixture through a Celite pad immediately after the stirring was stopped.
6. The distilled product slowly deposits
mercury. In an effort to determine whether extent of this deposition is reduced by extending the time of reduction, the checkers found that stirring the crude alcohol with Celite for 15 hours, followed by filtration and distillation, did not diminish the amount of
mercury deposited. However, in a typical run where the yield of distilled alcohol was
24.6 g. (
72.0%), after standing for 24 hours, the distillate was decanted from the deposited
mercury and redistilled to give
21.4 g. of
1-methylcyclohexanol, which did not deposit
mercury upon standing for one week at room temperature. The yield of twice-distilled alcohol was
62.6%.
3. Discussion
Although the reaction proceeds faster in tetrahydrofuran–water
4,5,6 or in
acetone–water,
ether was used as solvent in this reaction for convenience of product separation and purification. However, the oxymercuration is acid catalyzed;
9 oxymercuration of unreactive olefins such as
cis-cycloöctene can be accelerated by adding acid (Note
3). Rapid stirring also accelerates the reaction. An additional advantage of using ether is that less olefin is produced during the reduction using this solvent than when
tetrahydrofuran is used. Elimination can be a serious side reaction during the reduction, amounting to 30% of total demercurated product when the oxymercurial from
cis-cycloöctene is reduced in tetrahydrofuran–water. In ether–water, however, less than 10% olefin is produced.
As a general procedure, if the olefin is impure, the oxymercuration-reduction process may include an olefin purification step. Alternatively, this process may be used to purify the olefin for other purposes.
2,6 In such cases,
acetone is substituted for
ether, and after oxymercuration for the same length of time as suggested above, the solution is poured with stirring into two volumes of water containing one equivalent each of
sodium hydrogen carbonate and
sodium chloride. The mercury derivative is filtered, recrystallized from
ethanol–water,
ether,
dioxane, or
ethyl acetate–heptane,
10 then either reduced, as described above (in 70–80% yield), producing pure alcohol, or deoxymercurated with cold
6 N hydrochloric acid,
2 ethereal
lithium aluminum hydride11 (added cautiously), or high concentrations of alkali halides,
6,11,12 producing the pure olefin.
Oxymercuration may also be used to prepare ethers, acetates, amines, or amides (Markownikoff adducts). Several excellent procedures for these syntheses have been published by Brown and co-workers.
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