Chemistry: electrical and wave energy – Processes and products
Patent
1986-03-20
1987-06-02
Niebling, John F.
Chemistry: electrical and wave energy
Processes and products
C25B 300
Patent
active
046701125
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a process for the preparation of p-amino phenols of the general formula I set forth in the introductory protion of claim 1, by electrolytic reduction of p-phenylazophenols in an aqueous medium, and the process of the invention is characterized by performing the electrolytic reduction in a basic medium at a pH value at least equal to the pKa value of the p-phenylazophenol and at an elevated temperature of at least 50.degree. C. and preferably about 70.degree. to 100.degree. C. In this process, e.g. the compound 5-aminosalicylic acid may be conveniently obtained, said compound being a valuable active component in certain medicaments, cf. the PCT Application No. 81/02671, for the treatment of colitis ulcerose and Crohn's disease.
Arylazophenols of the general formula produced by coupling a diazoted aromatic amine (an arylidiazonium compound) with a phenol in a basic medium (H. E. Fierz-David & L. Blangley: Grundlegende Operationen der Farbenchemie, 5th ed., Vienna 1943). This known coupling reaction has been used for many years in the production of dyes. The reaction is as follows: (2)
Arylazophenols can be reduced electrolytically in an acid medium to amines and amino phenols. The reaction can either take place directly (see e.g. Chem. Abstr., 13, 843 (1919) and Chem. Abstr., 15, 839 (1921)) or indirectly (see J. F. Norris & F. O. Cummings, Ind. Eng. Chem., 17, 305 (1925) and the U.S. Pat. No. 1,542,265). However, such a reaction is difficult to carry out with a good yield in practice since the arylazophenol is sparingly soluble in an aqueous acid, unless it contains an HSO.sub.3 group or an NR.sub.2 group in which the two R groups are the same or different and represent hydrogen or alkyl. It has been attempted to use an alcoholic hydrochloric acid solution (E. Puxeddu, Gazz. Chim. Ital. 48 (II), 25 (1919)), and it has been proposed to add organic solvents, providing for some, but frequently not sufficient improvement in solubility. Moreover, purification and recovery of the solvent pose problems.
Also the U.S. Pat. No. 3,645,864 describes electrolytic reduction in an acid medium. In this case, the starting material is nitrobenzene which is reduced to p-amino phenol and its derivatives at 60.degree. to 150.degree. C. and at a cathode potential of -0.25 to -0.35 V with respect to a saturated calomel electrode.
According to the DE Offenlegungsschrift No. 2 256 003, electrolytic preparation of amino phenols proceeds in a basic medium, the electrolyte solution being an alkali metal hydroxide solution. However, the starting materials are nitrosophenols which must be synthesized beforehand in an inert atmosphere, and to obtain reasonable results it is necessary to use a large number of electrolysis cells in series connection.
In view of polarographic studies (see T. M. Florence, Austr. J. Chem., 18, 609 (1965); T. M. Florence, J. Electroanal. Chem., 52, 115 (1974); H. A. Laitinen & T. J. Kneip, J. Am. Chem. Soc., 78, 736 (1956) and Chem. Abstr. 48, 4333 (1954) the following mechanism has been proposed for the reductive cleavage of p-arylazophenols (here shown with p-phenylzophenol): ##STR3##
It will be seen that the reaction outlined above involves a total of 4 electrons (n=4). The slow step in the reaction sequence is step (4), and the polarographic results show in fact that the reaction (4) proceeds so slowly in a basic liquid that it cannot be observed at all under such circumstances. Thus, the final step (5) is not observed either, and, in practice, only n=2 is obtained by polarography in a sufficiently basic liquid, for a number of compounds already at pH values of 5.0 and higher. Accordingly, Puxeddu (Gazz. Chim. Ital. 50 (II), 149 (1920)) found no p-amino phenol by reduction of hydroxyazobenzene in a basic liquid.
Some heterocyclic compounds, e.g. 4-pyridylazophenol, can be cleaved by electrolytic reduction in a basic liquid (T. M. Florence, J. Electroanal. Chem., 52, 115 (1974)), the mechanism being presumably as follows (and not as shown on p. 124 in the reference): ##STR4##
REFERENCES:
patent: 1542265 (1925-06-01), Norris et al.
patent: 1882758 (1932-10-01), Britton
patent: 3645864 (1972-02-01), Lawson et al.
D. E. Danly, Emerging Opportunities for Electroorganic Processes, Marcel Dekker, Inc., New York, 1984.
Farmaceutisk Laboratorium Ferring A/S
Hsing Ben
Niebling John F.
LandOfFree
Process for preparing p-amino phenols by electrolysis does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for preparing p-amino phenols by electrolysis, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for preparing p-amino phenols by electrolysis will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-611498