Organic Syntheses, Coll. Vol. 7, p.229 (1990); Vol. 62, p.24 (1984).
solution is washed with water (2 × 250 mL) and stirred with glacial acetic acid (250 mL)
for 1 hr (Note 6)
. The solution that results is washed with water (2 × 250 mL), and the chloroform
phase is treated with 1.5 L of hexane
and kept at 2°C for 48 hr. The material that precipitates is collected and dried at 0.1 mm in a desiccator (calcium chloride
) for 5 hr to give p-methoxyphenyllead triacetate
) as pale-yellow crystals, mp 138–139°C (Note 7)
. The product may be kept for at least 3 weeks if stored at 2°C in a sealed container
Lead tetraacetate from Merck & Company, Inc.
was used. Acetic acid
was removed from the reagent at 0.1 mm for 24 hr, in the dark, in a desiccator containing potassium hydroxide pellets
from Fluka AG was distilled before use.
A few drops of reaction mixture were shaken with water. A brown precipitate of PbO2
indicates the presence of unreacted lead tetraacetate. For the quantities given, a reaction time of 1 hr at 15–20°C is adequate.
Hexanes, bp 60–69°C
, certified by Fisher Scientific Company were used.
A second metathesis with glacial acetic acid
is carried out to ensure complete conversion of the oligomer into the product.
It has been found that the yield of product is generally higher when the reaction is performed on a smaller scale. Reactions carried out on approximately one-third of the above scale have given yields of approximately 60%
The submitters report that after approximately 1 hr some lead(II) acetate
is deposited as an orange–red gum that may temporarily restrict the motion of the stirring bar; this was not observed by the checkers. The material generally crystallizes after a short period as a white solid.
These washings are extracted separately in order to minimize formation of solid lead(II) sulfate
The procedure described here serves to illustrate a new, general method for effecting the α-arylation of β-dicarbonyl compounds by means of an aryllead triacetate under very mild conditions. Although the first synthesis of an aryllead triacetate was reported relatively recently, a wide range of these compounds can now be readily prepared.2
The most direct route to these compounds is plumbation of an aromatic compound with lead tetraacetate, and in the procedure reported here p-methoxyphenyllead triacetate
has been prepared in this way. It may also be obtained by reaction of the diarylmercury3
or the corresponding aryltrialkylstannane4
with lead tetraacetate
; the latter provides a convenient and very general route to aryllead triacetates.
The first synthesis of p-methoxyphenyllead triacetate
by direct plumbation was reported by Harvey and Norman,5
who obtained the compound in 24%
yield by heating anisole
and lead tetraacetate
in acetic acid
at 80°C for 4 days. Recently it has been found2
that a much faster reaction and higher yield of aryllead compounds can be achieved by use of a haloacetic acid
in place of acetic acid
, and this has allowed the synthesis of a greater range of aryllead triacetates by direct plumbation. The improved reaction rate is presumably due to an increase in electrophilicity of lead when acetate is exchanged for a more electron-withdrawing ligand. The choice of the haloacetic acid depends on the reactivity of the aromatic substrate; thus, while dichloroacetic acid
has been found best for the plumbation of anisole
, trichloroacetic acid
is preferred in the case of toluene
Aryllead tricarboxylates have been shown to be intermediates in two new routes to phenols,6
and to have considerable potential as reagents for the C
-arylation of carbon acids that are more acidic than diethyl malonate
. A study of their reactions with β-diketones,8
derivatives of Meldrum's acid and barbituric acid
has established that such compounds, which contain only one replaceable hydrogen
, undergo smooth arylation in high yield under the conditions outlined in this procedure. Compounds that contain two replaceable hydrogens are less predictable in their behavior. When a 1 : 1 ratio of substrate to aryllead compound is used, dimedone
gave only diarylated product in high yield, while ethyl acetoacetate
gave both mono- and diarylated products in only moderate yield.
Recently it has been shown that triphenylbismuth carbonate13
can be used to achieve a similar arylation of β-dicarbonyl compounds. These reagents also react under very mild conditions and yields are generally high. Prior to the introduction of the organolead and organobismuth reagents, the most promising procedure for arylation of β-dicarbonyl compounds involved reaction of the enolate anion with a diaryliodonium salt, usually at 80–100°C.15
Although only a limited range of substrates has been examined, it would appear that yields are only moderate, and in the case of dimedone
a mixture of mono-, di-, and O
-arylated products is produced. A further method, which has obvious limitations, involves the copper-catalyzed substitution of bromine
in 2-bromobenzoic acids by the enolate anion of a β-dicarbonyl compound.16
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