Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing organic compound
Patent
1998-08-10
2000-05-16
Gorgos, Kathryn
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing organic compound
205427, 560 76, 549234, 549307, C25B 300, C07C 6976, C07C30777, C07D30512
Patent
active
060632569
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a novel process for preparing phthalides by cathodic reduction of phthalic acid derivatives.
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 solvents at temperatures of up to 65.degree. 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 phthalic anhydride at temperatures of up to 100.degree. C. over 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.degree. 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. Furthermore, 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.sub.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 hydrogen.
In addition, with the processes known hitherto there is a danger that anode corrosion and a poisoning of the cathodes may occur.
It is an object of the present invention to provide a technically simple process for preparing phthalides of high purity and in good yields without the disadvantages of the state of the art and which, in particular, opens up the possibility of exploiting the anode reaction for the preparation of products other than hydrogen.
We have found that this object is achieved by a process for preparing phthalides by cathodic reduction of phthalic acid or phthalic acid derivatives in which the carboxyl groups may be replaced by units which can be derived from carboxyl groups by a condensation reaction and one or more of the hydrogens of the o-phenylene unit of the phthalic acid may be replaced by inert radicals, which comprises carrying out the reduction in an organic solvent containing less than 50% by weight of water in an undivided electrolytic cell.
Starting materials employed for preparing the phthalides are in particular those of the general formula I ##STR1## where the substituents have the following meanings: R.sup.1, R.sup.2, R.sup.3 and R.sup.4 : are each, independently of one another, hydrogen, C.sub.1 - to C.sub.4 -alkyl or halogen C.sub.1 - to C.sub.4 -alkyl, other substituent is CONH.sub.2, where Y is C.sub.1 - to C.sub.4 -alkyl or hydrogen,
Especially preferred are the derivatives of phthalic acid where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are each hydrogen, and amongst those in particular di(C.sub.1 - to C.sub.3 -alkyl) phthalates, especially dimethyl phthalate.
In the compounds of the formula I where R.sup.5 and R.sup.6 are as defined under b), the ammonium salts, and in particular the ammonium salt of phthalamic acid, are particularly preferred.
Suitable electrode materials (for 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 preferably containing l
Hannebaum Heinz
Putter Hermann
BASF - Aktiengesellschaft
Gorgos Kathryn
Wong Edna
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