Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing organic compound
Reexamination Certificate
2000-03-20
2001-11-13
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing organic compound
C205S441000, C205S431000, C205S434000, C205S435000
Reexamination Certificate
active
06315884
ABSTRACT:
The present invention relates to a novel process for preparing phthalides of particularly high purity by
I. reducing phthalic acid or phthalic acid derivatives, where the carboxyl groups may be replaced by units which can be derived from the carboxyl groups by a condensation reaction and where one or more of the hydrogens of the o-phenylene unit may be substituted by inert radicals, at a cathode in an undivided electrolytic cell and dissolved in an electrolyte,
II. discharging the electrolyte from the electrolytic cell when the reaction has proceeded to the stage where the molar ratio (M), formed by the proportion of phthalide and the sum of the proportions of phthalide and phthalic acid or phthalic acid derivatives in the electrolyte, is from 0.8:1 to 0.995:1, and
III. crystallizing the phthalides from the electrolyte and removing them from the mother liquor, if appropriate after distillative work-up of the electrolyte.
Phthalides are required in particular as intermediates for the synthesis of crop protection agents.
DE A-2 144 419 discloses an electrochemical process for preparing phthalides by cathodic reduction of ammonium phthalamate in an aqueous solution containing up to 50% of organic solvent at temperatures of up to 65° C. on metals having a hydrogen overpotential greater than Cu, for example lead. Under these conditions, the preparation of phthalides is achieved in satisfactory yields if the reduction is carried out in divided electrolytic cells.
The preparation of particularly pure phthalides is described in DE-A-2 510 920. This publication teaches the cathodic reduction of ammoniacal, aqueous solutions of phthalic acid or of phthalic anhydride at temperatures of up to 100° C. on metals having a hydrogen overpotential greater than Cu. Again, the process requires the use of divided electrolytic cells. The phthalide is separated off from the electrolytic mixture by acidifying at from 35 to 100° C., if necessary after removal of excess ammonia, and separating off the precipitated phthalide.
The processes described, however, have the disadvantage of the high expenditure on equipment involved with the use of divided electrolytic cells, since 2 cell circuits are required in this case. In addition, working with 2 cell circuits has the following further disadvantages:
The cell circuits have to be separated by a membrane or a diaphragm; this means an energy loss owing to heat of resistance. Usually, in order to minimize this loss, at least one chamber is charged with an aqueous (>80% H
2
O) solution of supporting electrolytes. In cathodic reductions, this is the anolyte. This considerably reduces the available options for exploiting the anodic reaction. Normally, the sole anodic product formed is oyxgen.
The preparation of phthalides by electrochemical reduction of phthalic acid derivatives in an undivided electrolytic cell is proposed in the non-prior-published DE Patent Application of the reference No. 19618854.7. In this publication, it is also proposed to purify the phthalide by recrystallization. Details of up to which conversion the electrolysis of the phthalic acid derivatives is to be carried out before it is ended are not given in this publication, neither is the conversion implicitly disclosed to the person skilled in the art in the Examples.
The phthalides prepared by the abovementioned methods are of relatively high purity which is sufficient for most applications. However, in some cases the phthalides are required in a degree of purity which cannot be achieved at all, or only with high expenditure, by using the prior art methods.
In particular when using dimethyl phthalates and ring-substituted derivatives thereof, the phthalide and its starting material are hardly separable by distillation. In principle, this purification problem can be bypassed by carrying out the electrolysis until virtually the total amount of starting material is converted. To produce a 98% pure quality, for example, the starting material has to be converted to the point where the weight ratio of product to starting material is at least 98:2 and the molar ratio is 0.98 to 1. When using dimethyl phthalate (MW 194), a weight ratio of product to starting material of 99:1 is therefore to be aimed for. However, this solution to the purification problem has the disadvantage that the selectivity of the reaction is strongly reduced and that useless byproducts are formed in considerable amounts.
It is an object of the present invention to provide a method for preparing phthalides in high purity, good yields and by an economical and simple technical process.
We have found that this object is achieved by the process decribed at the outset.
The starting materials used for preparing the phthalides are in particular those of the formula (I)
where the substituents have the following meanings:
R
1
, R
2
, R
3
and R
4
: are each, independently of one another, hydrogen, C
1
- to C
4
-alkyl or halogen
R
5
, R
6
:
a) are each, independently of one another, —COOH or COOX, where X is C
1
- to C
4
-alkyl,
b) one of the substituents R
5
or R
6
is —COONY
4
and the other substituent is CONH
2
, where Y is C
1
- to C
4
-alkyl or hydrogen,
c) R
5
and R
6
together are —CO—O—CO—.
Particular preference is given to those derivatives of phthalic acid where R
1
, R
2
, R
3
and R
4
are each hydrogen, and among these in particular to the di(C
1
- to C
3
-alkyl) phthalates, especially to dimethyl phthalates.
The electrochemical conversion of these starting materials can be carried out for example by the method described in DE-A-19618854.7.
Electrode materials which are suitable for this process (both as cathode and anode) are in particular commercially available electrodes made of graphite or carbon.
The electrolyte is usually a 2 to 40% by weight strength solution of phthalic acid or a phthalic acid derivative in an organic solvent or a mixture of an organic solvent and water, the mixture generally containing less than 50% by weight, preferably less than 25 and particularly preferably less than 5% by weight of water.
Useful organic solvents are in particular aliphatic C
1
- to C
4
-alcohols, in particular methanol or ethanol, or mixtures of such alcohols with a carboxamide such as dimethylformamide or t-butylformamide.
Suitable conducting salts contained in the electrolytes are, for example, quaternary ammonium salts, such as tetra(C
1
- to C
4
-alkyl)ammonium halides or tetra(C
1
- to C
4
-alkyl)ammonium tetrafluoroborates and preferably methyltributylammonium methylsulfate or methyltriethylammonium methylsulfate, usually in amounts of from 0.4 to 10% by weight, based on the electrolyte.
For the anodic coproduction process, it is advisable to use as anodic depolarizer a conventional organic compound whose suitability for the electrochemical oxidation is generally known to the person skilled in the art. Some of the anodic coproduction processes are preferably carried out in the presence of a mediator. Suitable anodic coproduction processes are described, for example, in D. Kyriakou, Modern Electroorganic Chemistry, Springer, Berlin 1994, Chapter 2.
Useful anodic coproduction processes are in particular the oxidation of C—O or C—N single or double bonds, for example the oxidation of carboxylic acids, or the oxidative C—C coupling in particular of naphthalenes or activated CH groups and the oxidation of methyl groups attached to an aromatic ring to give aldehydes.
The use of methylbenzene or ring-substituted derivatives of methylbenzene where 1 to 3 hydrogens of the phenyl radical may be replaced by C
1
- to C
6
-alkyl radicals or C
1
- to C
4
-alkoxy radicals has been found to be particularly favorable. Examples of such anodic depolarizers include p-xylene and p-tert-butyltoluene.
When preparing aldehydes as coproducts, the use of the abovementioned alcohols as solvents is recommended, since the aldehydes are acetalized and protected against further oxidation.
The other process parameters such as temperature and current density are not crucial as long as they are kept within the conventional limits for the electr
Baumann Dieter
Hannebaum Heinz
Putter Hermann
BASF - Aktiengesellschaft
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Wong Edna
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