CA1168655A - 2-[2-HYDROXY-3,5-DI-(.alpha.,.alpha.-DI-METHYLBENZYL) PHENYL]- 2H-BENZOTRIAZOLES AND STABILIZED COMPOSITIONS - Google Patents

Abstract

3-11775/CGS 865/1+2/B

Process for the preparation of nitroazobenzenes Abstract Process for the preparation of compounds of the formula which comprises adding the diazonium salt of an amine of the formula to a strongly alkaline lower alkanol solution, or aqueous solution thereof, which contains a phenol of the formula

Description

7 1 ~ 5 The invention relates to a process for the preparation of o-nitroazobenzene intermediates which can be converted to the corresponding 2-aryl-2H-benzotriazole.
UV abs~rbers of the o-hydroxyphenyl-2H-benzo-triazole type have for a long time enjoyed a considerable economic interest as light stabilisers for organic mater-ial.
Compounds of this type and their preparation and the use of these valuable 2-aryl-2H~benzotria~oles are -known, for example, from the following patent specifica tions: United States 3,004,896, United States 3,055,896, United States 3,072,585, United States 3,074,910, United States 3,189,615 and United States 3,230,194.
However, these stabilisers have poor compatibility in certain substrates. They have a strong tendency to - exude and sublime and/or to volatilise during processing to sheets, films, fibres and the like at high temperatures.
In this way, a disproportionately large proportion of the benzotriazole stabiliser is lost, especially in the case of extremely thin films and coatings. This also àpplies if the films or coatings are exposed to elevated temper~ure during use.

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The process according to the invention differs from the prior art in the use of alkall metal hydroxide in an excess which far exceeds the stoichiometric amount which is necessary to neutralise the resulting diazonium salt and to form a salt with the phenol. Accordingly, the coupling reaction is carried out in the presence of excess hydroxyl ions and a-t a pH value of more than 11.
The teaching of the prior art is that strongly alkaline coupling processes should be avoided. The success with which the coupling process of this specifica--tion has been carried out is therefore particularly sur-prising.
The coupling of phenols wi-th a diazonium salt under weakly alkaline conditions is a known method for the preparation of aromatic azo derivatives. In "Basic Principles of Or~anic__hemistry" by J.D, ~oberts and M,C. Caserio, W,A, Benjamin, Inc, New York (1965), page 893-895, it is emphasised that the coupling process pro-ceeds in an optimum manner at about pH 10 but virtually ceases at higher pH values.
It is also known that o-nitrobenzenediazonium salt solutions are unstable in the presence of alcohol and alkali metal hydroxide and decompose rapidly with the evolution of N2. For this reason, the teaching of the prior art is that coupling reactions in excess alkali metal hydroxide should be avoided, cf. Fierz-David e-t al., "Fundamental Processes of Dye Chemistry", Inter-science, New York (1949), page 239-241, 252-253.
H. Zollinger in "Azo and Diazo Chemistry", Inter-science,New York (1961), page 249-250, reports that when a coupling reaction is carried out under strongly alkaline conditions the equilibrium shifts away from the diazonium ions.

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~ ~6~55 Soviet Union Patent Specification 360,357 describes the coupllng of a sodium phenolate in weakly alkaline solution with excess acid diazonium salt so that the pH
rapidly falls below the value of 11 during the reaction, which is absolutely contrary to the reaction conditi.ons according to the invention.
In contrast to the teaching of the prior ar-t, o-nitroazobenzene is obtained in high yield and in high purity if the reaction is carried out in a strongly alka-line medium and in the presence of a lower alkanol as the solvent.
The invention therefore relates to the pre-paration of a o-nitroazobenzeneintermediate which is used to prepare 2-aryl-2H-benzotriazole light stabilisers.
The invention relates in particular to a process for t.he preparation of compounds of the formula VII
~;)H
T7 ~, N = N ~1 ~T
i1 1 1 1 T8~ 2 ~ ~lo VII

in which T7 is hydrogen or chlorine, T8 is hydrogen, chlorine, Cl 4 alkyl, Cl 4alkoxy9 C2 9 carboalkoxy, carboxyl or -S03H, Tg is Cl_l2 alkyl, Cl_~ alkoxy, phenyl, phenyl substituted by alkyl groups, in which the alkyl groups have 1 to 8 C atoms, cycloalkyl having 5 to 6 C atoms 9 C2 9 carboalkoxy, chlorine carboxyethyl or C7 9 arylalkyl, Tlo is hydrogen, Cl 4 alkyl, Cl ~ alkoxy, chlorine or hydroxyl and Tll is Cl 12 alkyL, chlorinel C5 6 cyclo-alkyl or C7 9 arylalkyl, which comprises adding the diazonium salt of an amine of -the formula VIII

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T7 ~- ,'NH2 i T8~ 2 VIII

in which T7 and T8 are as defined above, in stoichiometric amounts or in a small excess, based on the phenol to be coupled, to a strongly alkaline lower alkanol solu-tion, or aqueous solution thereof, which contains a phenol of the formula IX.
(~H
\ / 11 ~ IX.
,~ 10 -in which Tg, Tlo and Tll are as defined above, the phenol solution containing an alkali metal hydroxide in amounts such that a pH value of above ll is ensured and that excess hydroxyl ions are present even after neu-tralising the acid diazonium sal-t and the reac-tion temperature being -15C to +30C. Preferably, the tempera-ture is -2C
to +5C.
After the reaction has endedj the reaction mixture is advantageously acidified with acetic acid or with a mineral acid and the product is isolated by filtration T8 can be Cl 4 alkyl 9 for example methyl, ethyl or n-butyl. T8 can also be Cl 4 alkoxy, for example methoxy, ethoxy or n-butoxy T8 can also be C2 9 carbo-alkoxy, for example carbomethoxy, carboethoxy or carbo-n-octoxy.
Tg can be Cl 12 alkyl, for example methyl, ethyl, sec.-butyl, tert.-butyl, amyl, tert -octyl or n-dodecyl, : - .
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Tg can be Cl_4 alkoxy, for example methoxy, ethoxy or n-butoxy. Tg can be phenyl substitu-ted by Cl 8 alkyl groups, and the alkyl groups can be, for example, methyl, tert.-butyl, tert -amyl or -ter-t.-octyl. Tg can be C5 6 cycloalkyl 9 for example cyclopentyl or cyclohexyl.
Tg can be C2 9 carboalkoxy, for example carbomethoxy, carboethoxy, carbo-n-butoxy or carbo-n-octoxy. Tg can be C7 9 arylalkyl, for example benzyl, a-methylbenzyl or a,a-dimethylbenzyl.
Tlo can be Cl 4 alkyl, for example methyl, ethyl or n-butyl.
Tlo can be Cl 4 alkoxy, for example methoxy~
ethoxy or n-butoxy.
Tll can be Cl 8 alkyl, for example methylg sec.-butyl, tert~-butyl, tert.-amyl or tert.-octyl.
T11 can be C5 6 cycloalkyl, for example cyclo-pentyl or cyclohexylO Tll can be C7 9 arylalkyl, for example benzyl, a-methylbenzyl or a,a-dimethylbenzyl, Preferably, T7 is hydrogen.
Preferably, T8 is hydrogeng Cl 2 alkyl, methoxy or carboxyl.
Preferably, Tg is Cl_l2 alkyl, cycloalkyl, phenyl, a-methylbenzyl, a,a-dimethylbenzyl or carboxyethyl, Preferably, Tlo is hydrogen, hydroxyl or methyl.
Preferably, Tll is Cl_l2 alkyl, cyclohexyl, benzyl, a-methylbenzyl or a,a-dimethylbenzyl.
If Tg and Tll are both alkyl radicals, the sum of the C atoms in these radicals is preferably 4.
Particularly preferen-tially, T8 is hydrogen or chlorine, Particularly preferentially, Tg is methyl, tert.-butyl, tert.-amyl, tert -octyl, sec -butyl, cyclohexyl, carboxyethyl, a-methylbenzyl or a,a-dimethylbenzyl.
Particularly preferentially, Tlo is hydrogen, Particularly preferentially, Tll is methyl, sec.-butyl, tert.-butyl, tert.-amyl, -tert.-octyl, a-methyl benzyl or a,a-dimethylbenzyl.

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The alkaline coupling reaction according to the invention is very suitable for the preparation of the o-nitroazobenzene intermediate, which is obtainable only in low yield by conventional acid coupling, The process according to the invention is therefore particularly suit-able for the preparation of o-nitroazobenzenes in which Tg and Tl1 are aralkyl radicals, for example a-methylbenzyl, a,a-dimethylbenzyl or tert.-octyl.
The starting materials, for example phenols, o-nitroaniline, 4-chloro-2-nitroaniline or ~-methylstyrene, are available commercially or can be prepared in a simple manner by known me-thods The o-nitrobenzenediazonium compound, in turn, is prepared by conventional diazotisation processes, in which sodium nitrite acts in acid solution on a o-nitroaniline of -the formula VIII.

T7\ ~ H2 ~ ! '! VIII
T8 ' ~2 The compounds of the formula VIII and IX are illustrated below by some examples. They are all available commercially.
Phenols

2,4-Di-tert.-butylphenol, 2,4 di--tert.-amylphenol, 2,4-di-tert,-octylphenol, 2-tert.-butyl-4-methylphenol, 2,4-di-(~,~-dimethylbenzyl)-phenol, 2,4-di-n-octylphenol, 2,4-dimethyl-phenol, 3,4-dime-thyl-phenol, 2-methyl-4-(a,~-dimethylbenzyl)-phenol, 2,4-dichloro-phenol, 3,4-dichloro-phenol, 2-(~,~-dimethyl-benzyl)-4-methylphenol, 2-(~
dimethyl-benzyl)-4-tert.-butylphenol, 2-(~,~-dimethyl-benzyl)--4-tert.-octylphenol, 2-tert.-octyl-4-(a,~-dime~thyl-benzyl)-phenol, 2-(~-methylbenzyl)-4-me-thylphenol, 2-cyclo hexyl-4-methylphenol, 2-sec.-butyl-4--tert -butylphenol, 2-tert.-butyl-4-sec.-bu-tylphenol and 2-methyl-4-carboxy-ethylphenol Preferred phenols are: 2,4-di-tert,-. " ' ' :

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butylphenol, 2,4-di-ter-t.-amylphenol, 2,4-di-tert -octyl-phenol, 2-tert.-butyl-4-methylphenol, 2-sec -butyl-L~--tert.-butylphenol, 2-(a-methylbenzyl~-4-methylphenol, 2?4-di-(a,a-dimethyl-benzyl)-phenol, 2-(a,a-dimethyl-benzyl)-L~-tert -octylphenol and 2-tert ~octyl-4-(~,a-di-methylbenzyl)-phenol.
o-Nitroanilines ~ = . .
o-Nitroaniline 7 4-chloro-2-nitroaniline, 4,5-di-chloro-2-nitroaniline, 4-me-thoxy-2-nitroaniline, 4-methyl-2-nitroaniline, 4-ethyl-2-nitroaniline, n-butyl

3-nitro-4-aminobenzoate, n-octyl 3-nitro-~-aminobenzoate,

4-n-butoxy-2-nitroaniline, 3-nitro-4-aminobenzoic acid and 3-nitro-4-aminobenzenesulfonic acid.
Preferred o-ni-troanilines of the inven-tion are o-nitroaniline and 4-chloro-2-nitroaniline, The reaction is carried out by adding a dilute acid solution, preferably acidified with HCl, which con-tains the diazonium salt of the corresponding o-nitro-aniline to a strongly alkaline solution of the corres-ponding phenol~ Suitable lower alkanols are alkanols having 1 to 4 C atoms, for example methanol, ethanol, isopropanol and n-butanol,but methanol, ethanol and iso-propanol arepreferred, andmethanol isparticularlypreferred.
As lit-tle water as possible should be present during the coupling reaction. It is advantageous to use a highly concentrated diazonium salt solution and also to employ an alkaline phenol solution which con-tains only a lower alkanol or a dilute alkanol with a minimum water content. The solvent mixture consists to -the extent of at leas-t 50% by weight of alkanol and preferably to the extent of 75% by weigh-t of alkanol.
The alkaline alkanol solution contains an alkali metal hydroxide, for example NaOH and KOH, preferably NaOH. The amount of the alkali metal hydroxide is so chosen that the concentration of hydroxyl ions in the reaction mixture after neutralisa-tion of the acid diazon-ium salt is still above pH 11.

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fi S 5 In principle, a stoichiome-tric amount of the diazonium salt, based on the phenol, can be employed, but a small excess (of up to 20%) is advantageous. On the one hand it facilitates isola-tion of the product and on the other hand it increases the yield, The reductive cyclisation of VII to 2-aryl-2H-benzotriazole can be effected by one of the known reduction methods, for example zinc and NaOH, hydrazine or catalytic hydrogenation with a noble metal catal~st.
- The benzotriazoles produced according to the invention are effective light stabilisers for a large number of organic polymers.

The following examples illustra-te the invention, Percentages are by weight, A. Preparation of the precursor: 2,4-di-(a,a-dimethyl-benz~ henol This intermediate was prepared in accordance with - the description in U,S, Patent Specification 2~714,120, 705,8 g (7.5 mols) of phenol are mixed with 1,772,7 g (15 mols) of a-methylstyrene in the presence of 25,7 g (0.135 mol) of p-toluenesulfonic acid monohydrate as the catalyst and the mi~-ture is heated at 140C ~or 2,5 hours under N2. After the reaction has ended, the mi~ture is cooled to 110C and 1,125 ml of toluene are added.
The resulting solution is then washed at 80C with 750 ml of an aqueous solution consisting of 37,5 g of NaC03 and 75 g of NaCl, The organic phase is was,hed three times with 1,000 ml of aqueous NaCl solution and dried over ànhydrous sodium sulfate and the product is -then isolated by filtration and vacuum distillation, The main fraction contains 1,229.8 g (L~9,6% of theory) of the product, ~oiling poin-t 172-175/0.15-0.18 mm Hg. Melting point 63-65C.

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B. Comparison Example: Preparation of the intermediate 2-nitro-2 ? -hydroxy-3~5l-di~ -dimethyl-benzyl)-a benzene ~y~ ~cid co~ i3~ reaction 90.6 g of a 260/o aqueous solution of naphthalene-sulfonic acid, 1.9 g of Triton~X-207 (non-ionic surfactant),

5.6 g of Conoco~AAS-9OF (sodium dodecylbenzene sulfonate) and 90 ml of water are introduced in-to a 2 1 three-necked round-bottomed flask fitted wi-th a stirrer and a thermo-meter. The mix-ture is warmed to 40C, 116,5 g of 2,4 di-(~,a-dimethyl-benzyl)-phenol, which has been preheated to 90C, are then added and the mix-ture is kept at 40C, with vigorous stirring, A cold solution of o-nitrobenzenediazonium chlor-ide is prepared from 49.8 g (0.36 mol) of o-nitroaniline and from 24,9 g (0,36 mol) of sodium nitrite in concen-trated HCl at -5C to 0C, This solution is added dropwise in the course of 3 hours to the reaction mix-ture and the resulting dark red to black solution is kept at 40C overnight. The -temperature is then raised to 65C
for one hour and to 95C for a further 30 minutes.
After cooling to 85C, the crude product, which is a deep red viscous mass, is isolated.
The comminuted crude product is mixed with 200 ml of hot water (75C) and the mix-ture is then left to stand with 400 ml of methanol overnight. The mixture is stirred with a further 400 ml of methanol to produce fine granules~ 81.9 g (48.4% of theory) of the intermediate were obtained, Melting point: 139-141C, The thin layer chromatogram shows a homogeneous product with Rf = 0,61 on silica gel in 3 parts of cyclo-hexane and 1 part of ethyl acetate as the solvent.
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Example 1: 2-C2-Hydroxy-3,5-di-(~,a-dimethyl-ben~yl)-phenyl]-2H-benzotriazole, 386 g (0.805 mol) o~ o-nitroazobenzene, prepared in Example A, and 1,200 ml of toluene are introduced into a 5 1 three-necked round-bottomed flask fitted with a gas inlet, a stirrer and a reflux condenser, 240 ml of isopropanol and 240 ml of water are added to this solution, 160 ml of 50,1% sodium hydroxide solution are added to the solution, whilst passing in nitrogen, A flask contain-ing 158,2 g (2,42 gram atoms) of zinc is connected to the reaction flask, The zinc dus-t is fed in portions, in the course of 90 minutes, into the reaction mixture and this is carried out in such a way that the reaction tem-perature remains between 40 and 45C. After all of the zinc has been added, the reac-tion mixture is kep-t at 40C
for 1 hour and is then warmed to 70C for 3 hoursO The mixture is cooled to room temperature and acidified with 600 ml of concentrated hydrochloric acid, The residual zinc is removed by filtration, The organic phase, which con-tains the product, is washed four times with 340 ml of dilute hydrochloric acid, After removing the solvent by vacuum distillation, -the viscous crude product is obtained and this crystallises on standing, In order to purify the crude product, it is first recrystallised from 750 ml of e-thyl acetate and then from 1,000 ml of an acetoni-trile/ethyl acetate mixture (4 : 1) and is then dissolved in 1,250 ml of toluene, The -toluene solu-tion is extracted with 70% aqueous sulfuric acid in order to remove coloured impurities, 219.3 g (60,9% of theory) of yellowish-whi-te crystals are obtained, Melting point 140-141C, Analysis: C30H29N30 calculated C: 80,51 H: 6,53 N: 9.39 found C: 80,53 H: 6,54 N: 9,51 ,~

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Ex_mple 2: 2-~itro-2~-hydroxy-3',5~-di-(~,~-dimethyl-benzyl)-azobenzene 13.5 g (0.21 mol) of solid potassium hydroxide and 10 ml of water are introduced into a 500 ml three-necked flask fitted with a s-tirrer, a gas inlet and a pressure relief device. The resulting hot solu-tion is cooled with 80 ml of methanol.
16,5 g (0,05 mol) of 2,4-di-( a, a- dimethyl-benzyl)-phenol and 85 ml of methanol are added under nitrogen and the mixture is cooled to -4C, A cooled solution con-taining 42,9 g (o.06 mol) of o-nitrobenzenediazonium chloride in concen-trated HCl is added in the course of 15 minutes, ~ith stirring, and the temperature is kept at -2C to 0C, A deep red colorationj caused by the azo dye phenoxide formed, develops immediately, The mix-ture is stirred for a further 10 minutes at -1C to +1C
and is then acidified slowly (in the course of 2 minutes) with 20 ml of glacial acetic acid and the temperature is kept at +1 to +3C, The brick-red suspension is stirred for a further 15 minutes, during which time a rise in -temperature is possible, and is then filtered, The filter residue is washed with a solution of 40 g of ice and 160 ml of methanol and finally with 1,800 ml of water, The pale red crude product is dried in vacuo at 60C and 75 mm Hg for 16 hours, The yield is 22.7 g, 82% of theory, Analysis by spectrophotometry indi-cates a purity of 86,7%, Melting point: 135 - 140C, The crude product is recrystallised from hot n-butanol (5 ml per gram), Melting point: 147 - 148C, 1:16~

Example 3: 2-Nitro-2'-hydroxy-3',5'-di-(a,a-dimethyl-benzyl)-azobenzene Using the process of Example 2, 13,6 g (0,34 mol) of solld sodium hydroxide are dissolved in 145 ml of methanol, and 16,5 g (0,05 mol) of 2,4-di-(a,a-dimethyl-benzyl)-phenol and 20 ml of methanol are addedO The solution is cooled to +2C, In the meantime, a o-nitrobenzenediazonium chloride solution is prepared from 8,3 g (o,06 mol) of o-nitroaniline and 17,3 g of concen-trated HCl and 6 ml of water and from 4,3 g of sodium nitrite in 8 ml of wa-ter, The diazonium solution is added in the course of 2 hours at -~2C to the alkaline solution, The deep red mixture is stirred for 30 minutes at +2C and then acidified with 20 ml of glacial acetic acid and the pale red precipitate is isolated by filtration. The product is washed with 3 x 50 ml of methanol and then with 4 x 75 ml of water and is then dried at 75C/ mm Hg, The yield is 21,6 g (89%) with 97%
purity, Melting point: 140-142C.
Examples 4 - 16: Alkaline coupling reaction The procedure is as in Example 2, However, other phenols are used in place of 2,4-di-(a,a-dimethyl-benzyl)-phenol, The yield varies in accordance with the substituents on the phenol, , .

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Example Phenols Yield of No. o-nitroazo-benzene in %
, 4 2,4-di-tert,-amyl 66 2,4-dl-tert.-octyl 70(50)

6 2,4-di-tert.-octyl 47

7 2,4-di-(a,a-dimethyl-benzyl) 82

8 2,4-di-(a,a-dimethyl-benzyl) 89 c

9 2 (1-phenylethyl)-4-methyl 84.3 4-methyl 56 d 11 2,4-di-n-octyl 56 e 12 2-methyl-4-(a,a-dimethyl-benzyl) ~5 ~
13 2-(~,a-dimethyl-benzyl)-4-methyl 66 f 14 2-(~,a-dimethyl-benzyl)-4-ter-t,-butyl 54 2-(~,~-dimethyl-benzyl)-4--tert.-oc-tyl 62 16 2-tert.-octyl-4-(a,~-dimethyl-benzyl) 73 control without phenol 0 g a) The process was as in Example 3 with a higher concen-tration of alkali. The yield after puri~ication is 50%. Evolution of N2 gas took place. (- ~i8% of theory).
b) Evolution o~ N2 = 60% of theoryO
c) Process was in accordance with Example 3, d) Evolution of N2 gas = 29% of theory.
e) The acid coupling reaction gave a yield of only 16 -19%.
~) Evolution of N2 gas = 33% of theory.
g) When no phenol is present,about 67% o~ the N2 theore-tically present is evolved from the decomposed diazonium solution.
The yield is 47% when 4-chloro-2-nitrobenzenediazonium chloride is coupled with this phenol, Example 17: 2-Nitro-2'-hydroxy-3',5'-di-(a,a-dimethyl benzyl)-azobenzene When Example 2 is repeated using ethanol in place of methanol, evolution of N2 (5~/o of theory) lS observed.
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, . ~ :, , : , A low yield (50.5~0) of the abovementioned product is obtained The same resul-t is obtained when isopropanol is used in place of methanol.
These data show that methanol is -the most suitable solvent for the alkaline coupling reaction Example 18: 4-Chloro-2-nitro-2'-hydroxy-3',5'-di-(~,a-dimethyl-benzyl)-azobenzene ~ hen Comparison Example B is repea-ted using, in place of the diazonium solu-tion prepared from 2-nitro-aniline, a diazonium solution prepared from 4-chloro-2-nitroaniline in an equivalent amount, 47,3% of dark red product are obtained.
Example 19: 2-Nitro-2'-hydroxy-3'-(a,~-dimethylbenzyl)-5-tert octylazobenzene, This compound is obtained, in the form of a red crystalline solid with a melting point of 133-134C, by the general process of Example 3, by a coupling reaction of o-nitrobenzene-diazonium chloride wi-th 2-(~,a-dimethyl-benzyl)-4-tert -octyl-phenol : .
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Claims (3)

WHAT IS CLAIMED IS:

1. A process for the preparation of a compound of the formula VII
VII
in which T7 is hydrogen or chlorine, T8 is hydrogen, chlorine, C1-4 alkyl, C1-4 alkoxy, C2-9 carboalkoxy, carboxyl or -SO3H, T9 is C1-12 alkyl, C1-4 alkoxy, phenyl, phenyl substituted by alkyl groups, in which the alkyl groups have 1 to 8 C atoms, cycloalkyl having 5 to 6 C
atoms, C2-9 carboalkoxy, chlorine, carboxyethyl or C7-9 arylalkyl, T10 is hydrogen, C1-4 alkyl, C1-4 alkoxy, chlorine or hydroxyl and T11 is C1-12 alkyl, chlorine, C5-6 cycloalkyl or C7-9 arylalkyl, which comprises adding the diazonium salt of an amine of the formula VIII
VIII
in which T7 and T8 are as defined above, in stoichiometric amounts or in a small excess, based on the phenol to be coupled, to a strongly alkaline lower alkanol solution, or an aqueous solution thereof, which contains a phenol of the formula IX
IX

in which T9, T10 and T11 are as defined above, the phenol solution containing an alkali metal hydroxide in amounts such that a pH value of above 11 is ensured and that excess hydroxyl ions are present even after neutralising the acid diazonium salt, and the reaction temperature being -15°C to +30°C, and isolating the product after acidifying the reaction mixture.

2. A process according to claim 1, for the preparation of a compound of formula VII in which T7 is hydrogen, T8 is hydrogen, C1-C2 alkyl, methoxy or carboxyl, T9 is C1-C12 alkyl, cyclohexyl, phenyl, .alpha.-methylbenzyl, .alpha.,.alpha.-dimethyl-benzyl or carboxyethyl, T10 is hydrogen, hydroxyl or methyl and T11 is C1-C12 alkyl, cyclohexyl, benzyl, .alpha.-methylbenzyl or .alpha.,.alpha.-dimethylbenzyl, which comprises adding the diazonium salt of an amine of the formula VIII in which T7 and T8 are as defined herein, in stoichiometric amounts or in a small excess, based on the phenol to be coupled, to a strongly alkaline lower alkanol solution, or an aqueous solution, or an aqueous solution thereof, which contains a phenol of the formula IX, in which T9, T10 and T11 are as defined herein.

3. A process according to claim l, characterized in that the nitroazobenzene is cyclisized by reductive cyclisation to a 2-aryl-2H-benzotriazole.