Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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7.7-Dichlorobicyclo- [3.2.0] -hept-2-en-6-one (subsequently
called "the compound") is a valuable intermediate, especially for syntheses
of tropolone and of pesticides (cf. Belgian Patent 706,795)o
It is prepared according to a known process (Tetrahedron, 27
(1971), pages 615 - 633) by reacting cyclopentadiene with dichloroketene
according to the following equation:
~r C - Clz ~ ~ Cl
This reaction is difficult to handle because both reactants are
unstable compounds. Cyclopentadiene can be stored only at low temperatures
because of its pronounced dimerization tendency. Dichloroketene is still
unknown as an isolated substance and is obtainable only in dilute solution;
it must be prepared in situ~ conveniently by dehydrochlorination of
dichloroacetyl chloride by means of tertiary amines.
In the known processes for preparing the compound, cyclopentadiene
and one of the components of the dichloroketene formation, i.e. either
dichloroacetyl chloride or tertiary amine, are introduced into the reaction
ve99el in high dilution with inert solvent and the other component of the
dichloroketene formation is added dropwise. The reaation temperature is not
allowed to exceed 50 C. The molar ratio of cyclopentadiene to dichloroketene
is at least about 3:1, i.e. a considerable excess of cyclopentadiene is always
used.
This process is technically expensive and wasteful because of the
large quantities of solvent as well as the excess
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of cyclopentadiene required.
Moreover the isolation of the reaction product is difficult,
since bicycloheptenones, especially halogenated, are thermally unstable.
Attempts to isolate or to purify larger quantities of said product by conven
tional distillation methods frequently result in explosive decompositions,
e9pecially if impurities are present.
It is therefore an object of the present invention to prepare
the compound in a more economical manner with a higher space-time yield and
to obtain a product of higher purity without the danger of decomposition.
According to the present invention there is provided a process
for preparing 7.7-dichlorobicyclo- [3.~.] -hept-2-en-6-one by reaction in
an inert solvent between cyclopentadiene and dichloroketene prepared in situ
by reaction between dichloroacetyl chloride and a tertiary amine, wherein
one of the dichloroacetyl chloride and tertiary amine is dissolved in the
inert solvent and the other of the dichloroacetyl chloride and tertiary amine
is added with the cyclopentadiene to the inert solvent~ the process being
carried out at a temperature between about 20 and about 120&, preferably
about 40 to about 100 C. The reaction mixture is then worked up in
usual manner.
Both components of the dichloroketene preparation are
advantageously used in about stoichiometric quantities and cyclopentadiene
is used in a molar excess of about 5 to 25%.
The reaction mixture is advantageously worked up by steam
distillation.
In contrast to the known process where cyclopentadiene together
; with one of the components of the dichloroketene
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formation is given first into the reaction vessel only one component of the
dichloroketene preparation is initially present. This saves considerable
quantities of solvent required in the known process because of the instabi-
lity of cyclopentadiene. Only a minimum of solvent is necessary to assure
stirring of the reaction mixture, which in the course of the reaction be-
comes increasingly difficult because of the precipitation of aminochloro-
hydrate. The preferred amount of solvent generally is in the range of about
2 to 4 parts by volume per part by weight of cyclopentadiene.
A further important improvement of the process according to the in-
vention over the state of the art is that a large excess of cyclopentadiene
can be dispensed with. It is not required, consequently, to recover non-
reacted cyclopentadiene in an expensive and wasteful manner.
Since cyclopentadiene is only added later in the process of the in-
vention, it is possible to maintain a higher reaction temperature than in
the known process and thus to obtain a higher reaction velocity. Surpris-
ingly no or practically no di-or polymerization of the cyclopentadiene occurs
even at temperatures around or above 100C, since the reaction of cyclo-
pentadiene with dichloroketene proceeds faster that the polymerization re-
action.
The yield of pure substance in the process according to the in-
vention is in the same range as in the known process (about 70 to 80%).
The space-time yield, however, is moreover considerably improved because of
the advantages mentioned before.
External heating is not required in the process of the invention.
The desired reaction temperature adjusts itself according to the input
velocity of the mixture of cyclopentadiene and second dichloroketene-com-
ponent as a consequence of the reaction heat and may be maintained at the
desired level optionally by reflux or exterior cooling. It is likewise
possible to start feeding at a higher temperature, optionally at the reflux
temperature of the mixture firstly introduced into the reaction vessel, which
may be advantageous in some cases.
All liquids inert with regard to the reactants may be used as solvents
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for the first component of dichloroketene formation. Especially useful are
aliphatic and aromatic hydrocarbons having boiling points preferably below
150C, particularly in the range of from 50 to 100C, such as (C6 to C10)
paraffines, cyclohexane, benzene, toluene, xylene, chlorinated aromatic
agents such as chlorobenzene, dichlorobenzene, but also ethers such as di-
isopropyl ethers, tetrahydrofurane etc..
The reaction mixture can be worked up in known manner, i.e. by
vacuum distillation, or preferably by steam distillation with normally heated
or overheated steam, whereby first the solvent and optionally a small excess
of dicyclopentadiene and then the bicycloheptenone is distilled of. The com-
pound obtained in this way is very pure and contains only traces of substances
favouring decomposition of the product, especially at higher temperatures.
The yields of the product obtained in the distillation and in the working up
are higher than that of the product only isolated by vacuum distillation.
The improved thermal stability permits to obtain higher reaction temperatures
in further reactions. It is moreover quite surprising that the steam dis-
tillation may be carried out with the strongly acidic reaction mixture, the
acid reaction being caused by aminohydrochloride, although the bicyclohepte-
none hydrolizes in weakly alcaline solution tof a pH of 9) at high tempera-
tures and is converted into tropolone by weak acidic medium.
The following examples illustrate the invention:
E X A M P L E 1:
295 g (2 moles) of dichloroacetyl chloride in 800 ml of toluene were
introduced into a 2 liter flask and, beginning at room temperatures, a
cooled mixture of 203 g of triethylamine (2 moles) and 165 g of cyclopenta-
diene (2.5 moles) was added thereto. During the time of addition of 40
minutes the temperature rose to about 90C and was maintained at this level
for 30 minutes after the addition had been stopped. The components of the
mixture were then distilled off with overheated steam, until distillation was
3Q fini$hed. 258 g of pure 7.7-dichloro-bicy~1O-[3.2.0]-hept-2-en-6-one (73%
of the theory) were obtained by vacuum distillation from the toluene phase
of the distillate.
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The aqueous distillation residue was rendered alkaline with sodium
hydroxide solution and triethylamine was stem-distilled off.
E X A M P L E 2:
135 kg of technical heptane and 69 kg of triethylamine were introduced
into a 500 liter vessel. A mixture of 100 kg of dichloroacetyl chloride and
52 kg of cyclopentadiene (of 95%) obtained at a temperature of -10C was
added within 45 minutes, whereby the inner temperature rose to 86C and the
mixture began to reflux. After having continued stirring for lO minutes
at 90C, heptane, some dicyclopentadiene and the reaction product were
steam-distilled. After separation of the phases the organic phase was
dehydrated by slightly distilling it and 7.7-dichlorobicylco-[3.2.01-hept-
2-en-6-one was obtained therefrom by fractional distillation in a yield of
95 kg ~80% of the theory).
