Bromination of aromatic amines Текст научной статьи по специальности «Химические науки»

DOI: 10.6060/ivkkt.20196204.5673 УДК: 547.551.51

БРОМИРОВАНИЕ АРОМАТИЧЕСКИХ АМИНОВ Ф.А. Мустафаева, Н.Т. Кахраманов

Фатима Алимирза кызы Мустафаева, Наджаф Тофик оглы Кахраманов

Институт Полимерных Материалов НАН Азербайджана, ул. С. Вургуна, 124, Сумгаит, Азербайджанская Республика, AZ5004 E-mail: najaf1946@rambler.ru

Учитывая, что антипирены, биологически активные вещества (противоопухолевые, антибактериальные, противогрибковые, противовирусные), фармакологические препараты, полученные, в частности, на основе бромсодержащих ароматических соединений, широко используются в промышленности и пользуются повышенным спросом, целью данной работы являлось обобщение и систематизация накопленных в этой области знаний. В статье были представлены методы, а также системы реагентов, используемые при бромирова-нии ароматических аминов. Представлено бромирование ароматических аминов агентами галогенирования - бромистым водородом, бромидом натрия, бромидом калия, бромидом аммония, бромидом меди (II), N-бромсукцинимидом, N-бромсахарином, поддерживаемыми полимерами, и различия этих методов с точки зрения используемого окислителя, среды и растворителя, катализатора, региоселективности, количества атомов брома в полученном продукте. Показано влияние растворителей, катализаторов, природы (электронодонорной или электроноакцепторной) и положения (орто-, мета-, пара-) заместителей в ароматическом кольце, условий реакции, молярного соотношения реагентов, температуры реакции и времени проведения реакции бромирования ароматических аминов. Описаны направления реакций бромирования ароматических аминов в различных растворителях, в условиях отсутствия растворителей, в твердых состояниях. Изучено бромирование ароматических аминов в условиях термического, микроволнового, ультрафиолетового излучения. В статье показано региоселективное монобромирование, и также получение ди-, трибромопроизводных ароматических аминов. Также отмечены некоторые экологически безопасные методы бромирования ароматических аминов. Учитывая вышеуказанное, по нашему мнению, данные, представленные в этой статье, будут способствовать оптимизации получения бромпроизвод-ных ароматических аминов, применяемых в промышленности, технике и технологии.

Ключевые слова: ароматические амины, анилины, бромирование, оксибромирование, монобромирование, окислитель, галогенирующий агент, "in situ", региоселективность, катализатор

BROMINATION OF AROMATIC AMINES F.A. Mustafayeva, N.T. Kakhramanov

Fatima A. Mustafayeva, Najaf T. Kakhramanov *

Institute of Polymer Materials, Azerbaijan National Academy of Sciences, Sumgait, S. Vurgun st., 124, Azerbaijan

Republic, AZ5004

E-mail: najaf1946@rambler.ru *

It is known that the antipyrenes, biological active substances (antitumor, antibacterial, an-tifungal, antiviral), pharmacological preparations on the basis of bromine-containing aromatic compounds are widely used in the industry. Considering this and increased demand for these substances the purpose of this work was to summarize and systematize the accumulated knowledge in this area. The article presents methods and reagent systems used in the bromination of aromatic

amines. There have been described the bromination of aromatic amines with hydrogen bromide, sodium bromide, potassium bromide, ammonium bromide, copper (II) bromide, N-bromosuccin-imide, N-bromosaccharin, polymer-supported halogenation agents, and difference of these methods from the point of view of the used oxidizer, the medium and the solvent, the catalyst, the regi-oselectivity, the quantity of bromine atoms in the obtained product. The influence of solvents, catalysts, the nature (electron-donor or electron-acceptor) and position (ortho-, meta-, para-) of the substituents in the aromatic ring, reaction conditions, molar ratio of the reagents, reaction temperature and carrying out time of bromination reaction of aromatic amines has been shown. The bro-mination reactions courses of aromatic amines in different solvents, in solvent free conditions, in solid states has been described. The bromination of aromatic amines under thermal, microwave, ultraviolet radiation conditions has been studied. In the paper the regioselective monobromination and also obtaining of di-, tri- bromo derivatives of aromatic amines has been shown. Taking into account today's priority to environmentally safe methods of bromination of aromatic amines they have been also mentioned. Given the above, in our opinion, the information presented in this article will help to optimize the production of bromo derivatives of aromatic amines used in industry, technics and technology.

Key words: aromatic amines, anilines, bromination, oxybrominaton, monobromination, oxidant, halogenation agent, "in situ", regioselectivity, catalyst

Для цитирования:

Мустафаева Ф.А., Кахраманов Н.Т. Бромирование ароматических аминов: обзор. Изв. вузов. Химия и хим. технология. 2019. Т. 62. Вып. 4. С. 47-59

For citation:

Mustafayeva F.A., Kakhramanov N.T. Bromination of aromatic amines: overview. Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol. 2019. V. 62. N 4. P. 47-59

Taking into account that the brominated derivatives of aromatic compounds are widely used and therefore increased demand for these substances, a number of new methods have developed and reported in the literature. These methods differ from the point of view of the used brominating and oxidizing agent, the medium and the solvent, the catalyst, the effect on the environment, the regioselectivity, the number of bromine atoms in the obtained product.

There is information in the literature about the bromination of aromatic amines with different halo-genation agents as HBr, KBr, NaBr, NH4Br, CuBr2, N-bromosuccinimide, N-bromosaccharin and polymer-supported compounds.

The bromination of aromatic amines with a solution of hydrogen bromide and hydrogen peroxide [1, 2] have been carried out without presence of a catalyst (scheme 1) [3] and in the presence of the sodium mo-lybdate catalyst (scheme 2) [4]. The bromine has been formed "in situ" and from the bromination of various substituted anilines the mono-, di-, tribromo- substituted derivatives have been obtained. It was investigated the influence of the reaction time on the course of the reaction. Thus, the oxybromination reactions of 2-nitroaniline (4-nitroaniline) for 15 min and 20 min been carried out and 4-bromo-2-nitroaniline with 94% yield and 2,4-dibromo-6-nitroaniline 97% yield (2-

bromo-4-nitroaniline with 90% yield and 2,6-bromo-4-nitroaniline with 94% yield) been respectively obtained.

NH,

R-

NH,

K^

HBr, H202 CH3OH

R-

Kit

R = H, 0-CH3 p-N02 o-N02 n= 1-3 ' ' '

Scheme 1 Схема 1

f (Br)n

NH,

R-

Ks>

Na2Mo04 HBr + H202

NH,

(Br)n

R = H; o-N02; m-N02; p-N02; p-S02HN2 n= 1-3

Scheme 2 Схема 2

Temperature control in the bromination of anilines with the DMSO/HBr system helped to prepare selectively the mono- or dibrominated products (scheme 3) [5]. The use of ira«s-3,5-dihydroperoxy-3,5-dimethyl-

1,2-dioxolane as a new oxidant in bromination of anilines with HBr have been carried out at room temperature in water [6].

nh,

NH,

NH,

2NaBr +NaBr03 3H+ w

R

R

"(Br)n

R = H, o-N02, p-N02, p-CH3 n=2,3

Scheme 4 Схема 4

NH,

NH.

C6H5CH2(C6H5)3PHS05 / KBr CH3CN,r.t., 75 - 90%

(scheme 6) at room temperature, by use of the I2O5 KBr water system. The bromination of aromatic amines in CH2Cl2/H2O with HJO3/KBr [11] and in water with ortho-periodic acid / sodium bromide (1: 2) (scheme 7) [12] has been carried out at room temperature for a short time.

nh,

Scheme 3 Схема 3

The efficiency of the bromide:bromate reagent at 2:1 ratio has been investigated [7] in bromination of aniline and its derivatives (scheme 4). A molar excess of acid formed a salt with an amine group, after competition of the reaction this mixture was neutralized with sodium bicarbonate. In the bromination of aniline CH3OH was used as the solvent, but in the bromination of the aniline derivatives CH2Q2 was used as the solvent.

In many cases through the regioselective oxi-dative bromination of aromatic amines with potassium bromide and benzyltriphenylphosphonium peroxymo-nosulfate in a neutral reaction condition [8] the mono-brominated derivatives (scheme 5) have been obtained.

r,n-

✓ \

r,n

Scheme 6 Схема 6

H;IQ6, NaBr^ H20 , r.t.

R= CH3, CjHj Scheme 7 Схема 7

The bromination of various substituted anilines with oxone and sodium bromide (scheme 8) has been carried out [13] while stirring at room temperature, solutions of anilines, oxone and sodium bromide in 1:1 mixture of CH3CN/H2O.

NH,

NH,

R

oxone : NaBr 1:1 .

Br

R

^

R R

R= H, o-NHji p-Cl; p-CN; o-CH3-p-Br

Scheme 5 Схема 5

For anilines brominated in anhydrous solution with the use of potassium bromide in the presence of poly(4-vinylpyridine)-supported bromate [9] a high selectivity at the ortho- and para-positions has been observed.

Hou J. et al. have reported [10] about an ecologically safe method of bromination of 4-nitroaniline

R = p-CN, p-Cl, p-OCH3, o-OCH3

Scheme 8 Схема 8

The use of ammonium molybdate as a catalyst in a selective monobromination of various deactive anilines (scheme 9) with potassium bromide and sodium perborate [14] accelerated the reaction, but did not affect on production of a good yield and high selectivity. While without the presence of the catalyst, the reaction has been completed within 14 h, in the presence of the catalyst the reaction has been completed within 2-3 h. With the ortho-halogenated anilines it has been achieved the excellent results, bromination of ortho-ni-troaniline gives a mixture of mono- and polybromin-ated compounds.

"Sono-halogenation" of aromatic amines with potassium bromide has been carried out under ultrasound in two-phase carbon tetrachloride/water (CQ4/H2O) conditions [15] and nuclear bromination with an electrochemical method has been carried out by two-phase

electrolysis (scheme 10) [16]. An electrolyte bromine source (solution of 5% HBr with 50-60% NaBr) was used as an upper phase. The substrate dissolved in chloroform was used as a low phase. This electrochemical method gave a high yield (70-98%) of monobro-minated compounds and a high regioselectivity at ^ara-position (>95%).

0"NH2 +ВГ

Br

R = o-Br, o-I, o-F, o-N02, p-Br, p-I, p-F, p-N02

Scheme 9 Схема 9

Ос *—*-O-n:

Scheme 10 Схема 10

Halder J. has reported [17] "in situ" formation of bromine by using sodium bromide and 1(M) HCl household bleach and about bromination of active aromatic compounds with the reaction of aromatic electro-philic substitution. The bromination of aniline at 0 °C for 2 h have resulted with a 75% yield at 80:20 ratio of ortho.para bromoaniline.

In oxybromination of aromatic amines with NH4Br-Br2 system (scheme 11), NH4Br was used as a source of bromine and Br2 as an oxidant [18]. The number of bromine atoms in the product depended on the reaction time: from the 2-nitroaniline 4-bromo-2-ni-troaniline (15 min) and 2,4-dibromo-6-nitroaniline (20 min); from the 4-nitroaniline 2-bromo-4-nitroaniline (15 min) and 2,6-dibromo-4-nitroaniline (20 min) were obtained.

NH,

NH,

1.1 eq. NH4Br 1.1 eq. oxone ^

CH30H/H20 (1:1) r.t., 5 min

R = CN, CH302C

Scheme 12 Схема 12

NH4Br, H202 AcOH, r.t.

R = H, CH3, CI, CN, N02, COOH

Scheme 13 Схема 13

NH.

^^ Br

NH.

NH,

NH,

R-

NH4Br, Br2

R-

Br(n)

R= H; o-N02; m-N02; p-N02; p-S02NH2 п = 1-3

Scheme 11 Схема 11

By use of oxone as an oxidizer in oxybromination of aromatic amines with the ammonium bromide [19] within a short period the monobrominated compounds in high yield have been obtained (scheme 12). In oxybromination of anilines (scheme 13) with NH4Br/H2O2 system without a catalyst in acetic acid, the electrophilic substitution of bromine has been formed "in situ" [20].

The copper (II) bromide was an effective reagent for monobromination [21] of aromatic amines at room temperature, the reaction proceeded with selectivity and with production of the corresponding mono-brominated compounds. Using of ionic liquids as a solvent in bromination of unprotected anilines at paraposition with copper (II) bromide (scheme 14) the high yield has been achieved [22].

Br

CuBr2

ionic liquid

R

nh2 nh2

R1= CH3, OCH3, F, CF3, N02; R2= H

R1 = H; R^ = CH3, OCH3, F, CF3

>2 —

Scheme 14 Схема 14

The compatibility of the copper bromide (II) with oxone is presented as a simple and effective reagents for the bromination of aromatic amines (scheme 15),

and primary, secondary, tertiary aromatic amines as the appropriate substrates [23]. Mono- and multi-bromin-ated products have been obtained.

Scheme 15 Схема 15

H X

In regioselective atomic bromination of aromatic compounds by using of brominating agent N-bromosuccinimide (NBS) with ultraviolet radiation (scheme 16) it has been observed the preparation of mono-bromine compounds [24]. The reaction has been carried out at room temperature (30±2 °C) without use of any catalyst. The selectivity of bromination depended on the nature of the substituent on the aromatic ring.

NBS

CH3CN, hv

R = NH2;N(CH3)2

Scheme 16 Схема 16

Br

By using of NBS it has been carried out the para-selective monobromination of anilines in the presence of ammonium acetate as a catalyst at room temperature (scheme 17) [25] and in tetrabutylammonium bromide (TBAB) (scheme 18) [26].

nh2

NBS, CH3COONH4 CH3CN, r.t.

R = H, p-Br, p-CH3, p-N02

Scheme 17 Схема 17

Br

br

NBS

TBAB, 100°C

Scheme 18 Схема 18

Nh2^NO2

Through nuclear bromination of some substituted anilines in the solid state with NBS the experimental and theoretical investigations have been carried

out [27]. In a solid state, the reactivity depended on the reaction time, on the temperature and on the nature of substrate substituents. During carrying out of the reactions in the alloy or in solution with a decrease in the selectivity, the yield of the product has been decreased. The experiments carried out by using NBS in bromination of aromatic amines (scheme 19) at room temperature in carbon tetrachloride with catalytic action of silica gel have been resulted by the production of para-bromo derivatives and ortho-bromo derivatives in a case ofpara- position substituent [28].

NR,

NR,

Br

NBS-silica gel ССЦ rt *

R=H, CH3, C2H5

Scheme 19 Схема 19

It has been reported about bromination of aromatic amines by using N-bromosaccharin (NBSac) in the catalytic amount of Amberlyst-15 (scheme 20) [29] and directed bromination of anilines with NBSac, by use of tungstophosphoric acid as a heterogeneous recyclable catalyst, under solvent-free conditions [30, 31].

NH,

NH,

NBSac, Amberlyst-15 THF, 0°C

R

-¡-Br

R=NH2, OH, Br, N02, OCH3, CI. CH3CHO, CN,COOH

Scheme 20 Схема 20

The aniline and some of its derivatives have been brominated (scheme 21) by use of NBSac and in

the presence of H3PW12O40 catalyst. For show the application and generality of this method the authors have brominated the anilines and phenols. On the basis of the obtained results, it was determined that the functional groups on the aromatic ring did not affect the reactivity.

+ NBSac

H3PW12O40 H2N

NH,

+ NBSac

0°C

H3PW1204l

o°c

X=CH3, N02, Cl, Br, CN Scheme 21 Схема 21

Zarchi M.A.K. et al. have reported [32, 33] about diazotization-bromination of arylamines by using of poly (4-vinylpyridine)-supported ethyl bromide and cross-linked poly(4-vinylpyridine) -supported nitrite ion ([P4-VP]NO2) (scheme 22).

The synthesized complex from the complexation of KBr3 with acyclic poly(ethylene glycol) was shown as a regioselective, selective and effective reagent for monobromination of aromatic amines in the mild reaction condition (scheme 23) [34]. Through the bromin-ation of N,N-alkylated amines at ambient temperature with a polyvinylpolypyrrolidone-bromine complex the monobrominated products with good yield have been obtained [35].

Ar-NH2

H2S04

[P4 - VP] N02 , 0-5°C

- ArN2+HS04"

Scheme 22 Схема 22

CuBr

HBr (water), r.t.

ArBr + N2

PEQ400 о о о

он

он

_!!_ R = H; o-Cl; р-С1; p-N02, p-CH3, p-OCH3

Scheme 23 Схема 23

In regioselective bromination of aromatic electron rich aromatic compounds [38]. Through the

amines there have been used the polymer-supported or- bromination of 1-(4-aminophenyl)ethanone 1-(4-amino-

ganotin reagents (scheme 24) [36], and N,N,N',N'-tet- 3,5-dibromophenyl)ethanone with 95% yield has been

rabromobenzene-1,3-disulfonylamide (TBBDA) and obtained (scheme 26). poly (N-bromobenzene-1,3-disulfonylamide) (PBBS)

(scheme 25) [37] . nh2

il

r-

1. n-BuLi, Et20, -78°C

nh,

3. Br2, Et20, -78°C r.t.

R-

Br

Scheme 24 Схема 24

Since the tribromoisocyanic acid (TBSA) is the source of three halogen atoms it is presented as a good reagent for the direct solvent-free bromination of

1 eq. TBBDA

CH2CI2 r.t., 30 min

Scheme 25 Схема 25

Scheme 26 Схема 26

1,2-Dipyridine ditribromide-ethane was synthesized and investigated as brominating agent in bro-mination of some aromatic amines (scheme 27). At any stage of reaction no solvent was used and the solid product was obtained [39].

KBr/Oxone

/~Л -

* Y7 Br Yy

r=h,no2,f

Scheme 27 Схема 27

By use of quaternary ammonium tribromide the method of aromatic bromination without a solvent has been developed. In this work [40] it has been reported about some bromination reactions in the thermal and microwave medium without the use of any solvent. In the bromination of the organic substances the various reagents of quaternary ammonium tribromides - tetrabu-tylammonium tribromide, tetraethylammonium tribromide, cetyltrimethylammonium tribromide, tetramethylammonium tribromide have been used and with the aim of determination of the universality of these reagents without solvent the various classes of organic substances have been chosen. The reaction of activated aromatic compounds with these reagents in the thermal and microwave conditions has been resulted in preparation of para--bromine products. The bromination of aniline due to the effect of the cationic part of the tribro-mide reagent has been resulted in preparation of para-bromine products and ortho-substituted side products.

In contrast to the standard methods of bromination through the encapsulated in p-cyclodextrin aniline and N-methylaniline there have been obtained the ortho-bromanilines and para-bromo-N-methylanilines in good yield [41]. It has been reported about the ecologically pure method of bromination of anilines with gaseous bromine and brominated reagent in the solid state [42]. In most cases, the reactions carried out in a solvent-free medium showed a higher yield and selectivity than the reactions occurring in the solvent.

2,4,4,6-Tetrabromocyclohexa-2,5-dienone in dichloromethane or chloro-form has been used as the

halogenation reagent for bromination of aromatic amines, the monobrominated products with superiority at para- position or only at para- position have been obtained [43]. The protection of the amine group was not required, a yield is higher than 90%. The high selectivity for a short reaction time, atomic economy, re-cyclization of the reagent have been shown as the advantages of bromination of aromatic compounds by using of 2,4,4,6-tetrabromo-2,5-cyclohexadienone in conjunction with microwave and ultraviolet radiation [44]. The nuclear bromination of aromatic amines has been carried out with reactants adsorbed on solid support and exposed to microwave radiation (scheme 28).

Scheme 28 Схема 28

By using N-bromophthalimide the bromination method [45] of active aromatic rings has been developed. From the reactions of N,N-dimethylaniline and N,N-diethylaniline with N-bromophthalimide in diethyl ether at room temperature there have been obtained the para-bromo isomers with a good yield (scheme 29).

R — R/— CHj¡ C2H5

Scheme 29 Схема 29

The reaction of anilines, ortho- and meta-substi-tuted anilines and N,N-dialkyl-anilines in sulfuric acid with bromine and nitrosonium hydrogen sulphate has been resulted in preparation of 4-bromoanilines. The bromination process did not proceed without the presence of the nitrosonium ion [46]. As a result of the reacting anilines and benzyltriphenyl-phosphonium tri-bromide in condition of dichloromethane and methanol in the presence of sodium bicarbonate at room temperature for several minutes the polybromoanilines were obtained [47].

Through the bromination of anilines with 1-ben-zyl-4-aza-1-azoniabicyclo [2.2.2]octane tribromide in the presence of a small amount of methanol and CaCO3 the brominated aromatic amines have been obtained [48]. 1-benzyl-4-aza-1-azoniabicyclo [2.2.2]octane tribromide is shown as an effective reagent for the bromination of aniline derivatives, the active and non-active aromatic amines have been brominated (scheme 30).

O J?

Ntt

CaC03/CH30H

CsH5

R= p-Br, o-Cl, p-Cl, o-CH3, p-CH3, o-N02, m-N02, p-N02, o-C02H n=2,3

Scheme 30 Схема 30

1-Butyl-3-methylpyridinium tribromide ionic liquid was synthesized and used as a regioselective reagent/solvent for the nucleophilic bromination of various anilines (scheme 31) at room temperature, in most cases the only solvent for the extract was the water [49].

H,N

Л^Чг-Вг

CH3 ВгГ

N I

Bu

Br2

Scheme 31 Схема 31

сн3

*) I Br" + HBr

n I

Bu

N,N-Dibromo-p-toluenesulfonamide (TsNBr2) was shown [50] as a new brominating reagent for the bromination of aromatic compounds, the reaction was rapid and with application of this method to various aniline (scheme 32), phenol, anisoles at room temperature there have been obtained the polybrominated aromatic products.

nh,

nh,

TsNBr2

CH3CN, r.t. instantaneous

Scheme 32 Схема 32

Kumar L. et al. have reported [51, 52] about the selective bromination of various substituted aromatic compounds of industrial and pharmacological significance with the use of an aqueous solution of CaBr2-Br2. The derivatives of anilines and phenols with such strong electron-acceptor groups as carbonyl, nitro showed the good reactivity with the brominating reagent, and in a short reaction time were brominated with good yield (92-98%) and purity (>99%). The resulting by-product (HBr) was effectively used and with renewal of the brominating reagent without loss of selectivity the following bromination was reused. As a result of regioselective direct bromination of the aniline in the presence of the tetrabutylammonium peroxysulphate (scheme 33) for 1 h with the Br2 the mono-brominated product with 84 % yields was has been obtained [53].

CH2C12

25°C

О

Scheme 33 Схема 33

H2N-

/ \

-Br

Cerichelli G. et al. have reported about the bro-mination of some N,N-substituted anilines in a homogeneous solvent and in the presence of a suspension of cetyltrimethylammonium bromide (CTAB), cetyltrime-thylammonium tribromide, cetylpyridinium tribrimide, cetylquinuclidium tribromide, cetyltributylammonium tribromide, cetyldimethyl-2-hydroxyethylammonium tri-bromide, 1,2-bis(cetyl-N,N-dimethylammonium)ethane tribromide, 1 -hexadecylpyridibium tribromide surfactants in water [54-57]. In contrast to the homogeneous

condition in the presence of surfactants it has been observed an increase of regioselectivity ortho/para in the high ratio (scheme 34). In authors' opinion, the regi-oselectivity depends on nature of the substituents bonded to the nitrogen atom in the aniline and on the temperature.

The reaction of the aromatic amines with the benzyltrimethylammonium tribromide in the dichloro-methane-methanol (in the composition was a calcium carbonate) for 0.5 h led to the formation of bromine-substituted aromatic amines in good yield [58].

nrr'

nrr'

S/Br,

Вг

1.R = R'=H

2.R = H,R' = CH3

3. R = R' = CH,

NH,

NH,

Br2, La(NQ3)3- 6H20

CH3CN, r.t. R = p-CH3; o-Cl; p-Cl;o-Br

Scheme 35 Схема 35

The regioselective bromination of anilines in the presence of the heterogeneous catalytic system Cs2.5Ho.5PWi2O4o/cetyltrimethylammonium bromide gave an effective result (scheme 36) [62].

a) S= solvent, ortho/para<0,l 1

b) S= CTAB/H20, ortho/paiaX),66

Scheme 34 Схема 34

It has been reported about a regioselective method of bromination of the aniline derivatives at room temperature in dichloromethane with ethylenebis(N-methylimidazolium) ditribromide and the possibility of using this isolated reagent several times [59].

By using alkyl bromide and sodium hydride in DMSO, the various aromatic systems with different substitutents were brominated [60]. By selecting the corresponding alkyl bromide and controlling its amount a large range of mono- brominated substrates has been obtained. Initially for the bromination of aromatic compounds it has been chosen 2-bromopentane, for increase of the reactivity of the bromination 2-bromo-pentane was replaced with the ethyl 2-bromopropio-nate. From the bromination of N,N-dimethylaniline and N,N,3,5-tetramethylaniline with the 2-bromopen-tane there have been respectively obtained 4-bromo-N,N-dimethylaniline and 4-bromo-N,N,3,5-tetrame-thylaniline; and from the bromination of N,N-dime-thylaniline and N,N-dimethyl-3-nitroaniline with the 2-bromopropionate - 2,4-dibromo-N,N-dimethylaniline and 4-bromo-N,N-dimethyl-3-nitroaniline respectively.

The bromination reaction has been studied with a number of aromatic amines with electron-donor and electron-acceptor substituents at room temperature with La(NO3)3-6H2O catalyst (scheme 35) [61]. It has been observed a higher yield of mono-brominated products in anilines with electron-donor groups than anilines with electron-acceptor groups. The reaction has been carried out in the various proton and aprotic solvents and the acetonitrile (CH3CN) was the most appropriate for this reaction. The efficiency of La(NO3)3-6H2O catalyst has been investigated by the aniline reaction with elemental bromine, without and in the presence of the catalyst.

NH,

Br2 (1.1 mmol) CTAB (0.025 mmol) Cs2.5H0.5PW (0.05 mmol)

CH2C12, r.t., 43-98% R=H, Cl

Scheme 36 Схема 36

NH,

Environmentally friendly catalytic monobromina-tion of anilines by use of sodium bromide, hydrogen peroxide (35%), cerium (III) chloride as a catalyst, at room temperature or under reflux conditions in water was the regioselective at para-position [63]. The regioselective oxybromination of anilines with the ammonium molybdate catalyst [64] was possible with the electrophilic substitution of bromine obtained "in situ" by use of H2O2 as an oxidant and KBr as a brominating reagent.

The regioselective bromination of aromatic amines has been carried out (scheme 37) by using of boric acid as a catalyst (reusable), KBr as a source of bromine and H2O2 as an oxidant agent [65]. The perox-oborate obtained from the reaction of boric acid with H2O2 is shown as the effective catalyst in the selective bromination of organic compounds at room temperature. The high selectivity, mild reaction conditions, the use of H2O or C2H5OH as a solvent have been noted as advantages of this synthetic method.

NH,

NH,

Rl R.

H3BO3, H2O2J H~

KBr, H,0

R j=R2=R3=R4=R5=H

Rj=RJ=CHJJ R2=R3=R4=H Rj=RJ=CHJ| R!=R2=R4=H Rj=FJ R2=R3=R4=R5=H

Rj=R2=R4=Rj=Hj R3=Br R]=Rj=CHjJ R2=R^=Hj R3=Br R3=R5=CH3; R2=R4=H; R|=Br R,=F; R2=R4=R5=H; R3=Br

Scheme 37 Схема 37

The bromination of the anilines (scheme 38) with high selectivity at para- position has been carried out under mild conditions by use of tetraethylammonium chloride/methanol system as a cocatalyst [66].

By use of sodium perborate as an oxidant [67] the oxybromination of aromatic amines has been investigated. The sodium perborate with glacial acetic acid-acetic anhydride, potassium bromide and sodium

volphramate as the catalyst has been shown as a new system for the bromination of aromatic amines. Some reactions have been carried out at room temperature, but the most efficient procedures have been undergone with the heating of the solution at 75 °C. The most reactions were the regioselective, when ortho- andpara-substitution is possible, in both cases the para-substituted product was only separated isomer.

ГЛ

Br2/CH2C12

x>

cat.: (C2H5)4NC1

CH3OH

Scheme 38 Схема 38

Sharma S.K. has reported [68] about ecologically safe, regioselectivity of the use of AlBr3-Br2 system for bromination of aromatic amines (scheme 39).

nh,

NH,

R-n-

AlBr, - Br? (n mol) || * r

solvent, 5-3 min , r.t. yield 85-99%

Br,

were the regioselective, when ortho- and para-substitution is possible, in both cases the para-substituted product was only isolated isomer. para-Substituted aromatic compounds have been brominated at the orthoposition. In the presence of electron-acceptor groups in the aromatic ring, the bromination rate decreased, and the presence of electron-donor groups increased.

NH.

R= o-N02; m-N02; p-N02; p-S02NH2 n=l,2

Scheme 39 Схема 39

The bromination of economically significant aromatic compounds with AlBr3-Br2 aqueous system without use of the catalyst and oxidant has been carried out. The substituted anilines, phenols, aldehydes and anilides were brominated. The AlBr3-Br2 aqueous system has been obtained by addition of molecular bromine Br2 to an aqueous solution of aluminum tribro-mide at room temperature (25±10 °C). The regioselectivity of the reaction depended on the substituent functional groups. The presence of electron-acceptor groups in the aromatic ring significantly reduced the bromination rate of the rings. Depending on the ratio of the starting materials, the various compounds have been obtained. At 1:1:1 ratio of 4-nitroaniline:AlBr3:Br2 2-bromo-4-ni-troaniline, at a 1:2:2 ratio 2,6-dibromo-4-nitroaniline; at 1: 1: 1 ratio of 2-nitroaniline:AlBr3:Br2 4-bromo-2-nitroaniline and at 1: 2: 2 ratio 2,4-dibromo-6-nitroan-iline have been obtained.

The benzyl triethyl ammonium tribromide (BTATE) has been shown as a regioselective brominating agent for aromatic compounds in the presence of a mixture of methanol/dichloromethane solvent, calcium carbonate at room temperature [69]. Among the compounds used in this method there are also anilines (scheme 40), some of which have been immediately converted into new compounds. Most of the reactions

C6H5CH2N(C2H5)3Br3

CaC03/CH30H/CH2Cl2 HN03/2KBr

© e

C6H5CH2N(C2H5)3Br

R= CH3, CI, N02, C6H5

Scheme 40 Схема 40

The silica-supported quinolinium tribromide (SXTB) has been synthesized and used in regioselective bromination of aromatic amines (scheme 41) as an effective solid brominating reagent [70]. It has been investigated the effect of a solvent on the bromination reaction with SXTB. In CH2Q2, C2H5OH, DMF, 1,4-dioxane, DMSO and CS2 the reaction did not occur, in CH3CN the products with high selectivity have been obtained. The compounds with electron-donor groups are brominated faster than compounds with electron-acceptor groups. In the presence of substituents at ^ara-position of the anilines, the substituent reaction proceeded at ortho-position with mono-bromine selectivity.

NH-

NH,

" HBr

R = CH3) N02, CI, СООН, OCH3

Scheme 41 Схема 41

It has been reported [71] about a regioselective method for the bromination of aromatic compounds in dichloromethane with hexamethylenetetramine-bromine (HMTAB). The selectivity depends on the temperature and on the nature of the substituent. When the substituent in the aniline molecule in the 2-position the bromine atom enters the 4-position, in the 4-position - the 2-position (scheme 42).

NH,

R

ЧЧ

HMTA-Bromine or silica supported HMTA-Bromine

CH2CI2

NH2

R= H, p-Br, o-Cl, p-Cl, o-N02> p-N02

Scheme 42 Схема 42

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Поступила в редакцию 19.07.2017 Принята к опубликованию 29.10.2018

Received 19.07.2017 Accepted 29.10.2018

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