Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1999-09-24
2001-01-16
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S460000, C568S458000, C568S459000
Reexamination Certificate
active
06175044
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved process for preparing 3,7-dimethyl-2,6-octadien-1-al (citral) by thermal cleavage of 3-methyl-2-buten-1-al diprenyl acetal.
2. Description of the Background
Disregarding the improvements according to the invention, this reaction has already been disclosed in its essential features, Inter alia, by DE 24 11 530 and EP 210 074 B1.
This is a highly complex reaction which proceeds via three stages, as follows from the reaction diagram below.
1. Thermal cleavage of the acetal of the formula II into 3-methyl-2-buten-1-ol (prenol) of the formula III and the cis/trans-prenyl 3-methylbutadienyl ether (dienyl ether) of the formula IV
2. Thermal rearrangement of the dienyl ether of the formula IV in a Claisen rearrangement to give 2,4,4-trimethyl-3-formyl-1,5-hexadiene (formylhexadiene) of the formula V
3. Thermal rearrangement of the dienyl ether of the formula V in a Cope rearrangement to give the desired citral of the formula I
Citral is used as a scent and, furthermore, is of great importance as a starting material for other scents, such as geraniol, and as a starting material for vitamins, for example vitamin A.
Since not only the starting compound of the formula II but also the desired citral, the byproduct prenol and the citral precursors of the formulae IV and V are sensitive substances which can react further in an undesirable manner, the preparation of citral initially succeeded only with yields of from 60 to 70%. Better citral yields by far were obtained, even on an industrial scale, in accordance with the process of EP 0 021 074 B1. This describes a process for preparing citral from the acetal of the formula II, which comprises continuously distilling off from the reaction mixture, during the reaction, the prenol formed as byproduct. The citral and the citral precursors of the formulae IV and V are retained in the reaction mixture in this case by a distillation tower. The continuous removal of the prenol from the reaction mixture increased the citral yield to about from 85 to 90%. To increase the citral yield further, in EP 0 021 074, in addition, the conjoint use of a substance having a boiling point between that of prenol and that of citral and the citral precursors of the formulae IV and V (what is termed the intermediate boiler) was recommended. This measure increased the citral yield to approximately 95% of theory.
A disadvantage of the process described, which is very good per se, is that the process is only suitable for a batchwise procedure and requires long residence times (about 6 hours) and therefore large apparatuses. A further disadvantage of the known process is that an auxiliary, that is to say the intermediate boiler, must be introduced into the system, which means additional resources for metering the auxiliary, for monitoring its concentration and removing it.
SUMMARY OF THE INVENTION
It is an object of the present invention, therefore, to improve the process for preparing citral by thermal cleavage of the acetal of the formula II in such a manner that the abovedescribed disadvantages are avoided. This meant improving the process in such a manner that the use of auxiliaries is not necessary, so that shorter residence times and therefore small apparatuses can be employed, but nevertheless high selectivities for citral are achieved.
We have found that this object is achieved, therefore, by a process for preparing 3,7-dimethyl-2,6-octadien-1-al (citral) of the formula I
by thermal cleavage, in the presence or absence of an acid catalyst, of 3-methyl-2-buten-1-al diprenyl acetal of the formula II
with elimination of 3-methyl-2-buten-1-ol (prenol) of the formula III to give cis/trans-prenyl 3-methylbutadienyl ether of the formula IV
Claisen rearrangement of the resultant butadienyl ether of the formula IV to give 2,4,4-trimethyl-3-formyl-1,5-hexadiene of the formula V
and subsequent Cope rearrangement of the same to give citral of the formula I,
which comprises continuously removing by distillation from the reaction mixture not only the prenol formed of the formula III, but also the intermediates of the formulae IV and V and the citral, even during the reaction, and rearranging the intermediates of the formulae IV and V by heating them to from 100 to 200° C. to form citral before or after removal of prenol and possibly byproducts by distillation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The process according to the invention succeeds particularly advantageously if the thermal cleavage of the acetal of the formula II is carried out in the lower part or in the bottom of a distillation tower which is acting as a cleavage tower and has from 5 to 100 theoretical plates.
In this case, it has proved to be highly expedient if the acetal of the formula II, with or without the acid catalyst, is introduced into the lower part of the tower, into the bottom of the tower or into the tower evaporator (
2
).
One possible method for carrying out the process according to the invention comprises carrying out the thermal cleavage of the acetal of the formula II in the lower part or in the bottom of a distillation tower having from 5 to 100 theoretical plates, the acetal of the formula II being kept by suitable choice of the distillation conditions in the lower part, or in the bottom, of the tower, the citral formed of the formula I, the intermediates formed of the formulae IV and V and the eliminated prenol of the formula III being produced jointly at the top of the tower as overhead stream and the mixture of citral and the intermediates of the formulae IV and V, before or after removing prenol, and possibly byproducts, by distillation, then being passed through a heated delay tube (
7
) in which the intermediates of the formulae IV and V are rearranged at from 100 to 200° C. to give citral. Particularly advantageously, the process according to the invention is arranged in such a manner that the thermal cleavage of the acetal of the formula II is carried out in the lower part (
9
) or in the bottom (
10
) of a distillation tower (
1
) having from 5 to 100 theoretical plates, the acetal of the formula II being kept in the lower part (
9
) or in the bottom (
10
) of the tower (
1
) by suitable choice of the distillation conditions, the citral formed of the formula I and the intermediates formed of the formulae IV and V being taken off together in the liquid state or vapor state at a sidestream takeoff (
5
) disposed in the central or lower part of the tower and the prenol eliminated of the formula III being separated off at the head (
8
) of the tower with the overhead stream.
The mixture produced of citral and the intermediates of the formulae IV and V which is taken off at the sidestream takeoff (
5
) or else from the overhead stream of a tower without a sidestream takeoff is then expediently passed through a heated delay tube (
7
) in which the intermediates of the formulae IV and V are rearranged to form citral at temperatures of from 100 to 200° C.
The acetal required as starting compound can, as described in EP 0 021 074 B1, for example, be prepared very simply by reacting 3-methyl-2-buten-1-ol (prenol) with 3-methyl-2-buten-1-al (prenal). Depending on the acetal preparation conditions, this can comprise from 0.1 to 30% by weight of unreacted prenal and from 0.1 to 60% by weight of unreacted prenol. It is advantageous for the process according to the invention if the concentration of the acetal of the formula II used is over 30% by weight, preferably over 70% by weight in the starting material.
The preferred procedure of the process according to the invention is described below on the basis of the accompanying figure:
BRIEF DESCRIPTION OF THE DRAWINGS
The acetal of the formula II is cleaved to form prenol, citral and the citral precursors of the formulae IV and V in a distillation tower, termed the cleavage tower (
1
), which is equipped with an evaporator (
2
) and a condenser (
3
). Suitable internals (
4
) for the tower are plates, dumped packings and, in particu
Aquila Werner
Fuchs Hartwig
Kaibel Gerd
Therre Jorg
Wegner G{umlaut over (u)}nter
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Padmanabhan Sreeni
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