Organic Syntheses, Coll. Vol. 5, p.517 (1973); Vol. 45, p.39 (1965).
To a saturated solution of 5.5 g. (0.028 mole) of finely powdered cupric acetate monohydrate (Note 1)
in 20 ml. of a 1:1 by volume pyridine-methanol
mixture (Note 2)
, (Note 3)
, (Note 4)
, and (Note 5)
contained in a 50-ml. round-bottomed flask
fitted with a reflux condenser
is added 2.0 g. (0.0196 mole) of phenylacetylene (Note 6)
. The deep-blue suspension becomes green when heated under reflux. After 1 hour of heating, the solution is cooled (Note 7)
and added dropwise to 60 ml. of 18N sulfuric acid
, with stirring and external cooling in an ice-salt freezing mixture (Note 8)
. The resulting white suspension is extracted with three 25-ml. portions of ether
, and the combined ethereal extracts are washed with 15 ml. of aqueous ethanolic silver nitrate
solution (Note 9)
to remove any unchanged phenylacetylene
. The ether
solution is then washed twice with water and dried over anhydrous magnesium sulfate
. When the dried ether
solution is concentrated under reduced pressure, a brown oil (1.81 g.) remains which solidifies on cooling.
The crude solid is purified by dissolving it in 50 ml. of petroleum ether
) and introducing it on a short alumina column (15 g., Brockmann Activity 1 or an equivalent chromatographic alumina)
. The column is then eluted with 300 ml. of a 1:9 mixture of ether-petroleum ether
). Concentration of the eluate leaves a solid which is recrystallized from aqueous ethanol
to give 1.4–1.6 g.
) of diphenyldiacetylene
as large colorless needles, m.p. 87–88° (Note 10)
Commercially available crystalline cupric acetate monohydrate
was used. A large excess of cupric acetate
does not improve the yield. Small catalytic amounts can be used if the cupric salt is continually regenerated by passage of oxygen
through the reaction mixture, but the procedure is much slower.
A good grade of commercial pyridine
was used. The reaction can also be carried out under anhydrous conditions (anhydrous cupric acetate
, anhydrous methanol
); then the pyridine
is distilled from potassium hydroxide
pellets. The yields are similar, and, in fact, water may be added as co-solvent if desired.
The solubility of anhydrous cupric acetate
is ca. 2.3 g. per 100 ml. of pyridine
, and that of the hydrate is ca. 1.6 g. per 100 ml. of pyridine
. The solubility is much improved by the addition of methanol
(solubility ca. 8.6 g. per 100 ml. of a 1:1 mixture of pyridine-methanol
For high-dilution experiments, for example, the cyclization of α,ω-diynes, about 4 volumes of ether
per volume of reagent solution can be added as entraining solvent without precipitation of the copper salt. A lower reaction temperature results.
Commercial grade methanol
was used. Methanol
is best avoided in experiments involving esters, as methanolysis has been encountered.2
It is apparently not essential that all the cupric acetate
be in solution. Large volumes of solvent ensure complete solution but are inconvenient during isolation of the product.
Other solvent systems have been investigated. A base appears to be essential to remove the acetic acid
formed; otherwise insoluble yellow precipitates of the cuprous derivative are obtained, which are only slowly oxidized to the required product.
The reaction can be followed by adding an aliquot to ethanolic silver nitrate
solution (Note 9)
. The reaction is complete when no precipitate of the silver derivative is obtained. Also the disappearance of the infrared absorption band at 3300 cm.−1
(3.03 μ) (ethynyl v
CH) can be followed with carbon tetrachloride
extracts of aliquots.
yield of diphenyldiacetylene
was obtained when the reaction was allowed to proceed for 24 hours at room temperature (20°).
This is more convenient than removing the pyridine
and the methanol
The reagent is made by dissolving 3.5 g. of silver nitrate
in 5 ml. of water and adding 10 ml. of ethanol
4. Merits of the Preparation
The reaction, "Glaser oxidative coupling," is a general one,9
but this particular technique is recommended for the more water-insoluble ethynyl compounds, and also for the cyclization of α,ω-diynes,2,12
where controlled dilution is required.
The cupric acetate
reagent provides a homogenous and basic reaction medium. The yields are high, and there is seldom precipitation of the cuprous derivative which may slow down the cuprous chloride
The rate of oxidative coupling is said to decrease with decreasing acidity of the ethynyl hydrogen.13
underwent only limited reaction after being heated with the reagent under reflux for 24 hours.
It is to be noted that cupric acetate
has been used to oxidize other systems, for example, α-ketols, phenols, thiols, and nitroalkanes.
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