DE2429442C3 - Process for the introduction of nucleophiles into the optionally alkyl-substituted nucleus of benzene - Google Patents

Description

The process for the preparation of functional group-containing diluted compounds, e.g. B. ^ o Amino or hydroxy derivatives of certain hydrocarbons are of considerable importance because the Products can be used in a variety of ways. Most of them are the conventional ones Process around multi-stage chemical processes that are relatively complex. So you put z. B. that 4-Amino-phenol from benzene via the nitrobenzene and its reduction in a strong sulfuric acid solution or also by oxidation of 4-nitrosophenol using nitric acid and subsequent reduction of the 4-nitro-phenols using iron powder in dilute hydrochloric acid.

All of these processes proceed in the various stages with low selectivity and thus with moderate Overall yields. In addition, by-products are produced, which as a rule represent an environmental burden. The question of the toxicity of the intermediate products, e.g. B. nitrobenzene, is to be observed.

In contrast to cathodic reductions, electrochemical oxidation reactions take place on the cores of aromatic hydrocarbons so far hardly any technical application. This is mainly the one in comparison to the substrate greater reactivity of the cationic primary products, the i; » usually lower oxidation potentials of the products and the easier oxidizability of the side chains in the case of alkyl aromatics attributable to. The technically interesting formation of ring-substituted derivatives therefore takes place in this one Class of compounds so far often only as a secondary reactant and also with the formation of isomers, especially with acetoxylation (cf. L Eberson and H. Schäfer, "Organic Electrochemistry", Fortschr.chem. Forsch.21 [1971J Springer-Verlag).

The present invention shows a way of overcoming the problems outlined to a great extent. This is done by introducing nucleophiles into the optionally alkylated nucleus of benzene by electrolytic conversion in the presence of non-oxidizing acids, by the conversion in an electroactive aprotic or in a protic solvent in the presence of a strong, non-oxidizing acid and catalytic amounts of a redox system in the form of soluble salts of metal ions It is of course a prerequisite that the used Compounds have a vacancy in the nucleus for the introduction of a nucleophile. As nucleophiles to be introduced In particular, the amino, the hydroxyl or the acylamino and the acyloxy group come into consideration. The appropriate solvent is chosen depending on the type of nucleophile. So when using the Amino group in an aprotic solvent with a suitable electroactive range, e.g. acetonitrile, Methylene chloride, dimethylformamide, propylene carbonate, work. It goes without saying that that the solvent is stable in the electrochemical reaction if not the solvent directly and at the same time as nucleophi).

For the introduction of the hydroxyl group, working in aqueous solution is also present of strong, non-oxidizing acids advantageous. Such Säu.en are mentioned e.g. B. Sulfuric acid, trifluoroacetic acid, but also phosphoric acid, optionally hydrochloric acid. Are the products more sensitive to oxidation than the substrates used (e.g. phenol is more easily oxidized than benzene), so offers the intermediate formation of trifluoroacetates a decisive advantage in so far as the esters of these Acids are more stable to oxidation than the phenols on which they are based and these are usually noticeable higher yields can be obtained after the easily proceeding hydrolysis of the esters. You can also use alcoholic solution or work in low molecular weight aliphatic carboxylic acids.

As metal ions with the required redox properties, for. B. Ions of copper, mercury, Palladium, thallium, lead, cerium and chromium to choose from. Since the nucleophilic substitution with a redox process is connected, the chosen oxidizing metal ion must be easily recoverable by reoxidation. as Examples of salts are the corresponding carbonates, nitrates, acetates or trifluoroacetates.

A particular advantage of the method according to the invention is that, in contrast to the method classical chemistry, which requires the use of stoichiometric amounts, here with low, catalytically effective amounts of salt can be worked. This is e.g. B. in the case of the thallium salt also in Of considerable importance with regard to the well-known thallotoxicosis. It was surprising that the addition of such metal salts in the electrochemical reactions these are more selective than previously known and can proceed with higher, so far not achieved yields.

Platinum anode at 1.5 mm electrode spacing at 30 ° ± 5 ° C and a constant current density of 1 A · dm ² were comparative measurements in a Durchströmzelle from Eberson-type stainless steel cathode, conducted at a cell voltage of 3-4 volts. The electrolyte was composed as follows: 20 ml (0.25 mol) CF3COOH, 5 ml (0.04 mol) (CF ₃ CO) ₂ 0.0.2 mol of aromatic hydrocarbon dissolved in 300 ml of dry acetone, the 0, 6molar of CF ₃ COONa and possibly 10- ⁴ molar was of metal salt. The electrolyses were terminated after each 10.7 Ah had passed. The resulting esters were distilled

separated off and hydrolyzed to the phenols by boiling water and determined by gas chromatography. The mean current yields (from 3 batches each) result, for example, from the addition of TiCCh as follows (they are not optimized for the individual cases):

OH

Slromau> hcutc [%] t 3%

TKO,

flour
kaiulvsicri + 25th 46 K) 16 13th 20th 18th 25th 52 66

κ T-o I

JC ₃ H ₇

1-C ₄ H,

The electrochemical conversion can be carried out under the usual process conditions take place, if desired, for. B. also at elevated temperatures depending on the selected solvent. It can also be used with the usual cells as well as the known electrodes (combinations), e.g. B. Coal / Iron, platinum / iron, platinum / platinum, are worked, with a membrane or diaphragm separation in general between anode and cathode is not required.

example 1

1: In a conventional electrolytic cell, a mixture of benzene, trifluoroacetic acid and water in a volume ratio of 1 2, 10- ⁴ molar to thallium (I) carbonate was, while introducing ammonia gas at 40 ° C with 2.5 V 0.5 A · dm- ^{2 converted} to carbon / iron electrodes. After neutralizing the electrolyte with NaOH, ether was extracted and the ether was drawn off; colorless 4-aminophenol was obtained from the residue by sublimation. In order to avoid this isolation of the 4-aminophenol, which is sensitive to oxidation in air, it was chemically converted from the electrolyte into tri-aminodimethoxyphenol.2 CF ₃ COOH by adding ether and methanol and determined as such (analogously in example 2 and 3 procedure). The yield calculated on 4-aminophenol was 250 g ^{· m ~ 2} · h- '.

Example 2

The mixture described in Example 2 except that ^10-4 molar with respect to mercuric acetate was reacted to platinum / electrodes at 1 A · dm. ² The yield of 4-aminophenols in the form of derivatives was ίο 75 g - m ~ ² · h " ¹ .

Example 3

A mixture of benzene, water, conc. Sulfuric acid and saturated ammonia solution in the volume ratio is 2: 1: 1: 1, and 10- ⁴ molar to thallium (I) acetate was at 45 ° C with 2.6 V, 0.5 A · dm ² of platinum / iron Electrodes implemented. 4-Amino-phenol was obtained after work-up in the form of derivatives with a yield of 150 g · m ~ ² · h " ¹ .

Example 4

A mixture of acetonitrile, trifluoroacetic acid and benzene in a volume ratio of 15: 2: 1 with an addition of thallium (I) carbonate (0.004 molar) was at 60 ⁰ C with 2.8 V and 0.4 A ■ dm- ² of graphite / Iron electrodes implemented. The yield of acetanilide achieved after hydrolysis was 25 g · m ~ ² · h- '.

Example 5

w A mixture of toluene, sodium trifluoroacetate, trifluoroacetic acid and anhydride in a molar ratio of 4: 2: 1: 1 with an addition of thallium (I) carbonate (0.01 molar) was dissolved in acetone and at 35 ° C. with 3.6 V and 1 A · dm- ^{2 implemented} on platinum / iron electrodes. After ester hydrolysis, p-cresol was obtained with a yield of 60 g · m ~ ² - h ~ '.

Example 6

A mixture of trifluoroacetic acid, anhydride and tert. Butylbenzene in a volume ratio of 4: 1: 6 was dissolved in 300 ml of dry acetone, the 0.6 molar of sodium trifluoroacetate and 10-molar of a) sodium chromate (VI) or b) thallium (I) carbonate or c) cerium (III) carbonate was. Electrolysis on platinum electrodes was carried out at 35 ° C. with 3.6 V and IA · dm- ^2. After hydrolysis of the trifluoroacetates, 4-tert-butylphenol was obtained with current yields of a) 71% or b) 66% or c) 72%.

Claims (4)

Patent claims: 1. Process for the introduction of nucleophiles into the optionally alkyl-substituted nucleus of the Benzene, by electrolytic conversion in the presence of non-oxidizing acids, thereby characterized in that the reaction is carried out in the presence of a strong, non-oxidizing agent Acid and catalytic amounts of a redox system in the form of soluble salts of metal ions carries out in electroactive aprotic or in protic solvents. 2. The method according to claim 1, characterized in that in the presence of a thallium (l) salt, a cerium (III) salt or a chromate works as a metal ion donor. 3. Process according to Claims 1 and 2, characterized in that one is used as a strong, non-oxidizing Acid sulfuric acid or trifluoroacetic acid is used. 4. Process according to Claims 1-3, characterized in that the nucleophilic component is used Ammonia or water is used.