Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acid esters
Reexamination Certificate
2000-01-28
2001-11-13
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acid esters
Reexamination Certificate
active
06316661
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based on German Application DE 199 03 269.6, filed Jan. 28, 1999, which disclosure is incorporated herein by reference.
FIELD OF THE INVENTION
This invention relates to a process for the production of 2,3,5-trimethylhydroquinone diesters by aromatization of 2,6,6-trimethyl-2-cyclohexene-1,4-dione (4-oxoisophorone, ketoisophorone, KIP) with an acylating agent and an acidic soluble or insoluble catalyst, which process may be operated either continuously or discontinuously or in a combination of these modes.
BACKGROUND OF THE INVENTION
2,3,5-Trimethylhydroquinone diesters are not only valuable intermediates in the synthesis of vitamin E and other pharmaceutically active chromane compounds, but are also widely used as antioxidants.
The formation of 2,3,5-trimethylhydroquinone diesters from KIP in the presence of a strongly acidic catalyst and an acylating agent such as carboxylic anhydrides, acyl halides or enol esters is known and has been described in several patents (for example DE 26 46 172 C2, EP 808 815 A2 and EP 0 850 910 A1).
It is furthermore known that while this aromatization may indeed be performed at up to 100% conversion, primarily depending upon the catalyst, selectivity is between 80 and 95%. The secondary product which is mainly formed is the isomeric 3,4,5-trimethylpyrocatechol diester. This secondary reaction, which always occurs, is described in EP 0 850 912 A1.
Isolating the 2,3,5-trimethylhydroquinone diester from the reaction solution and purifying it is complex and entails considerable losses. In examples using dissolved catalysts, the mixture of 2,3,5-trimethylhydroquinone diester and the 3,4,5-trimethylpyrocatechol diester is precipitated by addition of water or basic aqueous solutions and purified by recrystallization from organic solvents. Recovering the catalyst from the filtrate is complex and the yield is greatly reduced on recrystallization. The acylating agent, which is introduced into the aromatization reaction in excess, hydrolyzes and is unusable for subsequent reactions.
When undissolved catalysts are used, such as strongly acidic ion exchangers or H-Y zeolites, separation and reuse of the catalyst is simple, but the isolation and high-loss purification of the 2,3,5-trimethylhydroquinone diester are no different from the reaction solutions using dissolved catalysts.
Due to the low isolation yield and the large quantities of waste, as an industrial production process, this method is associated with serious economic and environmental disadvantages.
SUMMARY OF THE INVENTION
The object of the invention is to provide a process by means of which 2,3,5-trimethylhydroquinone diesters may be produced with solid or dissolved catalysts with minimal amounts of waste in a continuous or discontinuous mode of operation or in a combination of these modes.
The present invention provides a process for the production of 2,3,5-trimethylhydroquinone diesters by catalytic reaction of ketoisophorone (KIP) with an acylating agent, characterized in that,
a) a carboxylic anhydride having a C
1
to C
4
alkyl residue is used as the acylating agent;
b) after the acylating reaction a proportion of the resultant carboxylic acid is optionally removed by distillation
c) the reaction solution is cooled to a temperature of −10° to 35° C., preferably of 0 to 30° C.;
d) the product which has crystallized out is separated and washed; and
e) the filtrate is returned to step a), optionally after removing a proportion of the resultant carboxylic acid by distillation and optionally after discharging a proportion of the filtrate.
The reaction of KIP with a carboxylic anhydride in the presence of a strongly acidic catalyst results in the formation of 2,3,5-trimethylhydroquinone diester, the isomeric trimethylpyrocatechol diester and carboxylic acid. 2 Mol of carboxylic acid are formed per mol of reacted KIP.
It has been found that the carboxylic acid formed during the reaction is well suited to separating the 2,3,5-trimethylhydroquinone diester and 3,4,5-trimethylpyrocatechol diester. The two aromatic compounds may be separated by simple crystallization from carboxylic acids, preferably acetic acid. Despite the similarity of the molecules, no mixed crystals are formed.
After the reaction is complete, the reaction solution is cooled and 2,3,5-trimethylhydroquinone diester is crystallized. After the crystals have been separated and the optionally used solid catalyst has previously been removed by filtration, a filtrate remains, corresponding to the proportion of the 2,3,5-trimethylhydroquinone diester which has not been filtered out, the 3,4,5-trimethylpyrocatechol diester formed as secondary product, the catalyst optionally used in dissolved form together with the carboxylic anhydride introduced in excess. This filtrate, denoted the recycle solution, comprising a saturated product solution with all secondary products, unreacted educts and optionally the catalyst, is completely recycled, wherein no new catalyst need be added in the case of a homogeneous reaction. The acylating agent may now be introduced in a stoichiometric quantity as the excess required for complete conversion of KIP is present in the recycled filtrate.
On each recirculation, the volume of the solution increases by the formation of 2 mol of carboxylic acid per mol of converted KIP, wherein due to the good solubilizing power only a proportion of the diester formed crystallizes out. A proportion of the carboxylic acid is accordingly preferably removed by distillation, so maintaining the quantity of the recycle solution at a constant volume. The distillation step may be performed alternatively before or after separation of the crystals.
The trimethylpyrocatechol diester and, to some slight extent, also other secondary products, accumulate in the recycle solution. The concentration of trimethylhydroquinone diester may rise to close to the saturation concentration without disadvantage. This saturation concentration value is obtained from the solubility curve and crystallization temperature. In the case of the diacetate, at a crystallization temperature of 20° C., the concentration of trimethylpyrocatechol diacetate in the recycle solution may rise to up to 20 wt. %, wherein the isolated product remains pure. The content of secondary products in the solution may be maintained at a tolerable level by discharging a sub-stream of the recycle solution. The quantities of catalyst and acylating agent contained therein are made up by appropriate additions to the cycle.
The process according to the invention for the production of trimethylhydroquinone diesters generates another advantage. As the concentration of trimethylpyrocatechol diacetate in the recycle solution rises, selectivity for the formation of further quantities of this secondary product decreases. When the concentration of the trimethylpyrocatechol diacetate rises from 0 to 15 wt % in the recycle solution, selectivity for the formation of further quantities thereof is virtually halved.
The crystals of the desired product are washed using the corresponding carboxylic acid, a saturated product solution in the carboxylic acid or also using an inert solvent such as, for example, hydrocarbons or ethers, which solvent is recovered by distillation.
In the process according to the invention, all the process steps may optionally be designed such that the operation proceeds continuously, partially continuously, batchwise or in a combination thereof.
The process consists of the sub-steps: chemical reaction (I), distillation (II), crystallization (III) and separation of solids (IV). Sub-steps II and III may be combined into a single step. The sequence may also be selected such that, after the chemical reaction (I), crystallization (III) and separation of solids (IV) proceed first and then distillation (II).
The educts, ketoisophorone and an acylating agent, preferably acetic anhydride, and the recycle solution are introduced into the reactor for the chemical reaction. The type of reactor is influenced by the properti
Hafner Volker
Huthmacher Klaus
Krill Steffen
Markowz Georg
Shi Nongyuan
Degussa -Huls AG
Killos Paul J.
Pillsbury & Winthrop LLP
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