Chapter 1

General Introduction

Chapter 1

General Introduction

1.1. Introduction Bromine and organic bromine compounds have traditionally played an important role

in various fields. The bromo organics are very important group of organic halogen

compounds. Even the marine and terrestrial plants, bacteria, fungi, insects, marine animals,

and some higher animals produce naturally occurring bromine containing organic

compounds. However, far more important are synthetic organic bromine compounds.

Elemental bromine is used to manufacture a wide variety of bromine compounds used in

industry and agriculture.1

Bromine - a heavy, volatile, corrosive, reddish-brown, nonmetallic liquid element,

having a highly irritating vapor, is one of the most important and essential member in the

periodic table. It was discovered independently by two chemists Antoine Balard2 and Carl

Jacob Löwig3 in 1825 and 1826.4 Bromine, from Greek brómos, meaning ‘stench’ (of he-

goats)5 is classified as an element in the 'Halogens' section which can be located in group 7 of

the Periodic Table. It is the only non metallic element that is a liquid at room temperature.

The pure chemical element has the physical form of a diatomic molecule Br2. It evaporates

easily at standard temperature and pressure to give a red vapor that has a strong disagreeable

odor resembling that of chlorine. Bromine is highly reactive and is a powerful oxidizing

agent in the presence of water. It reacts vigorously with amines, alkenes and phenols as well

as aliphatic and aromatic hydrocarbons, ketones and acids (these are brominated by either

addition or substitution reactions). With many of the metals and elements, anhydrous bromine

is less reactive than hydrated bromine; however, dry bromine reacts vigorously with

aluminium, titanium, mercury, as well as alkaline earth metals and alkaline metals.6

The industrially produced organic bromine compounds can be divided into two main

groups.1

(1) Organic bromine compounds in which the bromine atom is retained in the final molecular

structure, and where its presence contributes to the properties of the desired products. It

includes mainly flame retardants, biocides, gasoline additives, halons, bromobutyl rubber,

pharmaceuticals, agrochemicals, and dyes.

(2) Organic bromine compounds have traditionally played an important role as intermediates

in the production of agrochemicals, pharmaceuticals and dyes. New process developments

also result in new applications in ultraviolet (UV) sunscreens, high performance polymers,

and others.

1.2. Applications of bromine and bromo compounds

1.2.1. As Flame retardant

One of the most important uses of bromine is to make brominated flame retardants as

they help in the inhibition of combustible materials. These are used as chemical flame

retardants in the plastic industry and in products, such as cables, connectors, plastic covers. It

is also used in the clothing and furniture industries to prevent the products from catching fire.

In the electronics industries, especially in computers, it is used to make printed circuit boards

safe from fire.7 If the material burns the flame retardants produce hydrobromic acid which

interferes in the radical chain reaction of the oxidation reaction of the fire.8-9 Brominated

flame retardants are produced via direct bromination of organic molecules or via addition of

bromine to alkenes. Tetrabromobisphenol A, polybrominated biphenyls, penta-, octa-, deca-

brominated diphenyl ether (oxide) formulation and hexabromocyclododecane are few of the

examples of flame retardants that are made using bromine.7

1.2.2. Gasoline additive Ethylene bromide was an additive in gasolines containing lead anti-engine knocking

agents to raise the octane number. It allows engines to run more smoothly. It scavenges lead

by forming volatile lead bromide, which is exhausted from the engine. 7

1.2.3. Pesticide Bromine is also used to make chemicals that work as pesticides and destroy pests.

Methyl bromide was widely used as pesticide to fumigate soil. Some important biocides are -

bromopropylate (acaricide), bromacil, bromobutide (herbicides), bromofos (insecticides),

benzyl bromoacetate, bis-1,2-(bromoacetoxy)ethane (microbicides (water treatment

biocides)), 1,2-dibromo-3-chloropropane, 1,2,3-tribromopropane (nematocides) etc.1

1.2.4. Medical and veterinary Bromine has been used as a drug, for a long time. Primarily, chemical compounds

derived from bromine are used as oral sedatives, to treat epilepsy and as diuretics. Bromine is

also well known for its central nervous system depression role in bromide poisoning.

Bromide compounds, especially potassium bromide, were frequently used as sedatives in

the19th and early 20th century. Bromides in the form of simple salts are still used as

anticonvulsants in both veterinary and human medicine1,10. Some bromine containing

pharmaceuticals are- bromindione (anticoagulant), bromadoline (analgesic),

bromodiphenhydramine (antihistaminic), zimeldine (antidepressant), halothane, roflurane

(anesthetic (inhalation)), broperamole (antiinflammatory).1

1.2.5. Disinfectant Bromine substances are disinfectants and can be used as an alternative for chlorine. In

swimming pools, bromine is used against the formation and growth of algae, bacteria and

odors in swimming water. Bromine can be used for the disinfection of cooling tower water.1

The most common bromine compounds used in cooling water are 1-bromo-3-chloro-5,5-

dimethylhydantoin and mixtures of sodium bromide with sodium hypochlorous acid.11 It is

also used as a sanitizer, in areas, where there is less or no chances of it coming in contact

with food. Such compounds are used as a sanitizer and a germicide in hospitals, commercial

industries and households to control bacteria, algae and fungi. Bromine is, also, preferred for

disinfection of industrial water.1 The use of bromine compounds is growing in sanitizing

applications in the pulp and paper industry and food-washing applications. The sanitary

preparation field is an area where bromine was found to be safer than its substitutes because

bromine has a higher biocidal activity level for the same volume of product.11

1.2.6. Biological role Bromine has no known essential role in human or mammalian health, but inorganic

bromine and organobromine compounds do occur naturally, and some may be of use to

higher organisms in dealing with parasites. For example, in the presence of H2O2 formed by

the eosinophil, and either chloride or bromide ions, eosinophil peroxidase provides a potent

mechanism by which it kill multicellular parasites (such as, for example, the nematode

worms involved in filariasis). Eosinophil peroxidase is a haloperoxidase that preferentially

uses bromide over chloride for this purpose, generating hypobromite (hypobromous acid).12

Marine organisms are the main source of organobromine compounds. Methyl bromide

produced by marine algae is the most abundant one.13 The essential oil of the Hawaiian algae

Asparagopsis taxiformis consists of 80% methyl bromide.14 Tyrian purple, a bromine

containg organic compound, has been used by humans for a long time.15 Most organobromine

compounds in nature arise via the action of vanadium bromoperoxidase.16 Of the nearly 3200

known naturally occurring organohalogen compounds, more than 1600 contain bromine.

Numerous simple bromoalkanes like CH3Br, CH2Br2, CHBr3, CBr4, CH2ClBr etc. have been

isolated from marine algae. A large number of bromine-containing C15 acetogenins are also

found in marine plants and in sea hares of genus Aplysia that feed on these seaweeds.17 The

wood-rotting fungus Lepista nuda (common wood blewitt) produces the bromocompounds 3-

5 (Fig 1.1).18

CHO

OMe

Br

3

CHO

OMe

Br OMe

4

CHO

Br Cl

OMe

5 Fig 1.1

1.2.7. Photography Film

Another important use of bromine compound is in making photographic films. Silver

bromide is light sensitive and is used in photography. Potassium bromide is used in

photography, to prevent undesired reduction of silver, which causes fogging in a

photograph.19

1.2.8. Dyestuffs

German chemist Paul Friedländer found that the Tyrian purple dye contains bromine

in 1909. Tyrian purple dye is nothing but a chemical 6,6'-dibromoindigo. There are many

applications of bromine in the textile industry. One such dye used is tetrabutylammonium

bromide.1

1.2.9. Drilling fluids

The bromides of calcium, sodium, and zinc form dense solutions in water that are

used as drilling fluids (clear brine fluids).20 About 95% of calcium bromide produced was

used as an oil and gas completion fluid. Use of calcium, sodium, and zinc bromides in oil

well completion fluids has benefitted in recent years from the rebound in world oil prices.11

1.2.10. Brominated vegetable oil

Bromine is used to make brominated vegetable oil, which is used as an emulsifier in

many citrus-flavored soft drinks.21 The compound was extensively used until alternative

emulsifiers were developed.22

1.2.11. Reduce mercury pollution

Bromine is also used to reduce mercury pollution from coal-fired power plants. This

can be achieved either by treating activated carbon with bromine or by injecting bromine

compounds onto the coal prior to combustion.10

1.2.12. As intermediates in organic synthesis It is also used to form intermediates in organic synthesis where it is preferred to

iodine due to its much lower cost. As a rule, the reactivity of bromine compounds in the

nucleophilic substitution is greater than the corresponding chlorine derivatives, owing to the

difference in the bond energies (C-Br 276 kJ/mol vs. C-Cl 328 kJ/mol).1 This reactivity is the

main advantage of using bromine compounds as intermediates. The reaction of aliphatic

bromides with aqueous solutions of bases or with carbonyl compounds gives verities of

alcohols,23-24 the reaction with alcoholates gives ethers,25-28 the reaction with salts of carbonic

acids gives esters,29 and the reaction with sodium cyanide gives nitriles.30 Interaction with

ammonia, both in solution and in the gaseous phase, gives primary, secondary, or tertiary

amines and quaternary ammonium salts, depending on the reaction conditions.29 Aldehydes

and ketones are formed by the hydrolysis of dibromides, RCHBr2 or RCBr2R, respectively,

and the hydrolysis of tribromides, RCBr3, gives carbonic acids.1

1.3. Methods of bromination

1.3.1. Direct use of elemental bromine The elemental bromine can be directly used for bromination of various substrates. In

literature, legion of examples are found. Bromine is most widely used for determination of

organic unsaturation. This involves the reaction of bromine with the unsaturared compound

(Scheme 1.1).31

Br2Br Br

Scheme 1.1

Again α hydrogens of carboxylic acids can be replaced by bromine in presence of

phosphorus halide as catalyst. The reaction is known as Hell -Volhard -Zelinskii reaction.32

Aromatic compounds react with bromine vapor to yield products in which the p-positions of

unsubstituted phenyl groups are brominated and in which olefinic double bonds, not highly

substituted, had added bromine.33 Various types of alkyl bromides can be prepared by the

reaction of equal molar amounts of bromine with the silver salts of carboxylic acids.34 The

bromination of pyrazole by aqueous Br2 is a facile reaction. Treatment of [Co(NH3)5pyzH]3+

with 1mol equiv. of aqueous bromine for 3.5 hour gave the monobrominated complex

[Co(NH3)5 4-Br pyzH]3+.35 Cohen investigated the natural autocatalytic reaction between

acetone and bromine (Scheme 1.2).36

CH3 C

O

CH3 CH3 C

O

CH2Br Scheme 1.2

Again, aromatic compounds can be brominated by treatment with bromine in presence

of a catalyst, most often iron. For active substances, including amines, phenols, naphthalene

and polyalkylbenzenes no catalyst is needed.29 The ortho bromination of phenol can be

achieved using elemental bromine at low temperature.37 Schimid reported a method for Lewis

acid promoted bromination of aromatic compounds.38 A combination of Br2, SbF3 and HF

was used by Jacqesy et al. as a brominating mixture for brominaiton of activated aromatics.39

Recently, bromination of aromatic compounds has been reported using

hexamethylenetetraamine-Br2 in dichloromethane. The reactivity of this reagent was

increased by supporting it to silica gel for bromination of less activated compounds (Scheme

1.3).40

R + HMTA-Br2 or silica supported HMTA-Br2

CH2Cl2

Br

R

Scheme 1.3

A new selective brominating system Br2/SO2Cl2/zeolite, was brought out by Gnaim et

al. for regioselective bromination of aromatic compounds.41 Bromination of 5,6-disubstituted

indane-1-one can be carried out using Br2 under acidic and basic conditions.42 Substituted

bromobenzenes have been synthesized in acceptable yields using a novel Sandmeyer type

reaction with molecular bromine (Scheme 1.4).43 NH2

R

R'

conc. HCl

R'

NH3+Cl-

REtONO

-5-0 oC

N2+Cl-

R

R'

Br

R

R'

Br2

((NH4)2S2)8

Scheme 1.4

Again when aniline was successively treated with n-BuLi, B(OMe)3 and Br2 at -78°C,

4-bromoaniline was formed.44 Selective bromination of alkylbenzenes with bromine in carbon

tetrachloride in the presence of K10- montmorillonite is reported by Pitchumani et al.

(Scheme 1.5).45

CH3

K10-Mont

CH3

Br

+

CH3

Br Scheme 1.5

The bromination reaction of tricyclo [7.2.1.02,7] dodeca-2,4,6,10-tetraene derivatives

was done by using elemental bromine.46 Naae studied the electrophilic bromination of flouro

olefins. The ionic addition of bromine in glacial acetic acid to 4-RC6H4CF=CFX (R = CO2H,

Br, H, CH3, OCH3; X = C1, CF3,) and 4-RC6H4CH=CFCl (R = Br, CH3,) has been studied.

For all olefins, the 1,2-dibromo adduct is the predominant product (Scheme 1.6).47

4-RC6H4CF=CFX + Br2HOAcdark

4-RC6H4CBrFCBrFX

Scheme 1.6

Bromination of petroleum hydrocarbons and their utilization are becoming

increasingly important and have received considerable impetus in recent times. Ijam et al.

reported such a study for a kerosene fraction boiling between 180 and 220°C obtained from a

Kuwait oil field and having a low aromatic content (high furfural miscibility temperature).

This kerosene was brominated from 6.5 to 36% bromine content in steps of approximately

0.2 mol of bromine.48

Choudary et al. reported a novel integrated approach of bromination coupled with

oxybromination of aromatic compounds using LDH-WO4 as a catalyst, aqueous H2O2 as an

oxidant in a biphase comprising dichloroethane and water for the first time (Scheme 1.7).49

CHOCH3

CH3

LDH-WO4 catalyst

H2O2-Br2

OH CHOCH3

CH3

OH

Br

Br

Br

Br Scheme 1.7

Zeolites were also used as catalyst in such bromination reactions. Bromination of

aromatic substrates, namely, toluene, phenol, phenyl acetate and chlorobenzene had resulted

predominantly in the formation of para bromo derivatives.50 Al- Hassan did the bromination

of alkynes and alkynylsilanes with liquid bromine to give 1,2 dibromoalkenes and 1,2

dibromovinylsilanes (Scheme 1.8).51

RC CSiMe31.2 equiv. Br2/CCl4

-12oC/0.5h Br

R Br

SiMe3 Scheme 1.8

Firouzabadi et al. reported that heteropoly acid cesium salt/

cetyltrimethylammonium bromide (a catalytic heterogeneous system) could highly control

regioselective bromination of aromatic compounds with bromine.52 Smith et al. reported

regioselective one pot bromination of aromatic amines treating with n-butyllithium and

trimethyltin chloride to give p-bromoaniline.53 Tanko et al. introduced a new bromination

method with liquid bromine in supercritical CO2. Direct bromination of toluene and

ethylbenzene gave the corresponding benzyl bromides in high yield.54 Amino-substituted

arylmethylketones were selectively brominated in sulfuric acid to afford the corresponding

dibromomethylarylketones (Scheme 1.9).55

R

O

R CHBr2

OBr2

H2SO4

Scheme 1.9

Park et al. used tetrabutylammonium peroxydisulfate in the bromination of the

aromatic compounds substituted with electron donating groups such as methoxy, hydroxy, or

amino groups with Br2.56

There are numbers of such examples where molecular bromine is used for

bromination. However, there is risk in handling bromine because elemental bromine is a

hazardous material and strong irritant. It produces painful blisters on exposed skin and

especially mucous membrane. Even low concentration of bromine vapour can affect

breathing and significant amounts of bromine can damage respiratory system. Bromine on the

skin causes painful burns that heal very slowly. The most important health effects that can be

caused by bromine-containing organic contaminants are malfunctioning of the nervous

system and disturbances in genetic materials.57

So, the use and handling of bromine needs special precautions. Sometimes, this

reagent is not selective. In some cases an extra catalyst has to be added to promote

bromination using bromine. Therefore different brominating reagents are developed to make

bromination procedure more effective and selective.

1.3.2. Use of bromo compounds for bromination Numerous examples of brominating agents are found in literature developed by the

chemists in the passing years. We are disscussing here some of the important brominating

agents.

(1) N - Bromo Succinimide (NBS)

NBS is most popular brominating agent. So many reactions have already been done

using this reagent. Different aromatic substitution resulting in excellent yields of product

takes place under easily attainable experimental conditions with different reagent systems and

with various kinds of catalysts (Scheme 1.10).58-68

R RBrNBS

Scheme 1.10

Kharasch et al. observed the bromination of 1-octene with NBS.69 Monobromination

of pyrrole and 1-methyl-, 1-benzyl-, and 1-phenylpyrrole with 1 mol of N-bromosuccinimide

in tetrahydrofuran results in the regiospecific formation of the 2-bromopyrroles was observed

by Gilow et al.70 Shusherina et al. brominate 1-alkyl-2-pyridones to the corresponding

monobromides with the help of NBS.71 Bromination of 4-(2-thienyl)thiazoles and 2-(2-

thienyl)quinoline was done by Smirnov et al. using NBS in glacial acetic acid or acetic

anhydride.72 Bromination of 8R-4-phenyl- 2,3-dihydro-1H-l,5-benzodiazepinones-2

containing substituents of different types in the benzene ring of the heterocycle was studied

by Sobol et al.73 Zhu et al. treated apogossypol hexamethyl ether with NBS in DMF at room

temperature.74 Shaw et al. selectively brominated α-chloro and α-bromo carboxylic acid

derivatives.75 Liquid phase bromination of chlorobenzene, toluene and xylenes using zeolite

as catalyst and N-bromosuccinimide as the brominating agent was also done.76 Kruglenko

studied the bromination of substituted 5 H- imidazo[1,2-b]-1,2,4-triazepin-4-ones and thiones

with the help of NBS.77 Heropoulos and coworkers did bromination with NBS assisted by

ultrasound and microwave. Various alkylaryls with N-bromosuccinimide, either neat or in

water, showed diverse chemoselectivity.78

Benzylic bromination takes place with NBS when radical initiator is present.

Numerous examples are found in literature.79-80 Horman and coworkers did benzylic

bromination in the reactions of benzyl methyl ether and related compounds with N-

bromosuccinimide.81 The bromination of carbonyl compounds was done using N-

bromosuccinimide in the presence of different kind catalyst and reagent system.82-87 The

reaction of olefins with N-bromosuccinimide to form bromohydrins was also done by

different workers.88-89

(2) Pyridine hydrobromide perbromide, C5H5N.HBr.Br2

Pyridine hydrobromide perbromide can be considered as a general brominating agent,

which may be used in brominations ordinarily performed with molecular bromine, such as

substitution on aromatic rings and additions to double bonds.90 It is a useful reagent for

brominations of ketones. Monobromination and dibromination of steroid ketones were done

successfully.91 Vona and Merker brominated various organic substrates like β naphthol,

styrenes, phenols and anilines using this reagent.92 Lorette et al. used this reagent in the

flavone chemistry to convert acetylated hesperidin and naringin into diosmin and apigenin-7-

rhamnoglucoside, respectively.93 The bromination of tetraphenylcyclobutadiene-nickel

bromide complex was achieved by the action of pyridinium hydrobromide perbromide. The

product was 3,4-dibromotetraphenylcyclobutene and 2,5-di-p-bromotetraphenylfuran.94 5-

vinylindole was brominated regioselectively with this reagent (Scheme 1.11).95

N

R

N

R

BrH2CXHC

R=H, CH3, COCH3 X=Br, OH Scheme 1.11

Samath et al. tried this reagent as brominating agent for the bromination of many

metal acetylacetones.96 Reeves and his coworkers selectively mono- and dibrominated

anisole, p-methylanisole and p-dimethoxybenzene.97 Tanaka et al. introduced a new method

of stereoselective bromination with this reagent for stilbene and chalcone in water suspension

medium (Scheme 1.12).98

Ph

Ph

Ph

Ph

H Br

Br H

solid reagentwater suspension

Scheme 1.12

Regiospecific bromination-rearrangement of the 5-substituted 2-alkylidene-4-

oxothiazolidine derivatives induced by pyridinium hydrobromide perbromide (PHBP)

provides a new synthetic approach to the corresponding push-pull thiazolidines with two

exocyclic double bonds (Scheme 1.13).99

N

S

CO2Et

OW

HH

N

S

CO2Et

OW

BrH

N

S

CO2Et

OW

HH

PHPBMeCN, 0oC

80-85oC

CONHPh, COPh, CO2Et, CN Scheme: 1.13

Pyridinium hydrobromide perbromide (PyHBr3) was used by Raman et al. for the

bromination of the Schiff base chelates of copper(II), nickel(II), cobalt(II) and zinc(II)

derived from acetylacetone and p-anisidine under acidic and ammoniacal conditions at room

temperature.100 A convenient and practical method for the one-pot synthesis of ω-bromoesters

from aromatic aldehydes and diols in the presence of PHPB as brominating reagent and

triethoxymethane as dehydrating agent was developed by Aoyama et al. (Scheme 1.14).101

ArCHO + HOOH

PHPB, CH(OEt)3

CH2Cl2, 50oC Ar

O

OBr

Scheme 1.14

(3) Cyanogen bromide

In very earlier days, von Braun studied the action of cyanogen bromide on benzyl

cyanide in presence of sodium ethoxide. The brominated benzyl cyanide, C6H5CHBr.CN was

isolated.102 Steinkopf and his coworkers did some experiments regarding BrCN as

brominating agent.103 Bunce et al. found that the photolysis reaction took place between

cyanogen bromide and cyclohexane to give cyclohexyl bromide with small amount of

gaseous product.104 Again, in the synthesis of α-Bromo-β- aminoenones, this reagent was

used as a brominating agent (Scheme 1.15).105

R1

CH3

NHR2 R1

O

CH3

NHR2

BrBrCN

Scheme 1.15

Ramalingam et al. used the reagent for the selective bromination of coordinated β-

diketones. Actually cyanogen bromide was used as a new twin purpose reagent for

bromination as well as cyanation of β-diketonates.106

(4) Selenium oxybromide

The oxybromide of selenium, SeOBr2, was originally prepared by Schneider and later

it was described as a powerful solvent, as an oxidizing and brominating agent.107 In 1995, Lee

et al. brominated aldehydes and ketones by selenium oxybromide which was generated in situ

from selenium dioxide and bromotrimethylsilanes (Scheme 1.16).108

H

O

H

O

Br

CCl4SeO2-TMSBr

Scheme 1.16

(5) Bromide – bromate couple

(a) Aqueous solution of potassium bromide and bromate, KBr + KBrO3

In 1903, Bogert and Hand used aqueous solution of potassium bromide and bromate

to brominate anthranilic nitrile.109

(b) Mixture of Sodium bromide and bromate

Adimurthy et al. use an alternative method for the regio and stereoselective

bromination of alkenes, alkynes, toluene derivatives and ketones using sodium bromide/

bromate couple.110 Again the same bromide/ bromate couple was used by Patil et al. for the

synthesis of α-bromoketones, where bromide/ bromate couple act as a non hazardous

brominating agent.111

(6) (a) 3-Bromo-5,5-dimethylhydantoin

It can brominate cyclohexene to 3-bromocyclohexene and cyclohexanone to its 2-Br

derivative. Like NBS, it can brominate in benzylic position. For example, the reagent gives

benzyl bromide with toluene.112

(b) 1,3-Dibromo-5,5-dimethylhydantoin

Orazi et al reported that higher yields with shorter reaction time were obtained with

this reagent than 3-bromo-5,5-dimethylhydantoin in aromatic bromination.113 This reagent is

capable of doing at least three types of bromination reaction.

(1) Allylic Bromination (2) Hydrobromination (3) Substitution of active H.114

Again, this reagent was used as a regioselective brominating agent for the preparation of 3-

phenoxybenzyl alcohol.115

(7) HOBr

When HOBr was added to a H2SO4 solution of C6H6 in water, immediately PhBr was

separated. Similarly benzoic acid was converted to m-bromobenzoic acid in fairly strong

HNO3 or H2SO4. This reactivity accords with the view that Br+ or (H2O)Br+ is present as a

result of equilibrium.116

(8) Tribromo-m-cresol bromide

Goldhahn reported that tribromo-m-cresol bromide could be used as a brominating

agent. He showed that tribromo-m-cresol bromide on boiling with m cresol gave 2,4,6-

tribromo-m-cresol. On the other hand, MePh, xylene or EtPh were brominated in the side

chain.117

(9) Iodine monobromide

In 1938, Militzer reported that IBr is mild brominating agent for substitution on the

aromatic ring.118 Bennett and Sharpe repeated Militzer's experiments with iodine

monobromide with nitrobenzene as solvent and phenol and salicylic acid as substances to be

brominated.119 IBr is an effective reagent in the asymmetric monobromination of steroid

aldehydes. Yanuka and coworker reported that iodine bromide affected the monobromination

cholanal, norcholanal and 3α-acetoxynorcholanal in high yields.120

(10) Carbon tetrabromide:

Hunter and Edgar, in 1932 investigated the possibility of the use of carbon

tetrabromide as a brominating agent. Bromination took place with majority of the compounds

they studied with a side product bromoform. The reagent brominates selectively the side

chain of the benzene hydrocarbons instead of attacking the ring.121 In 1985, Camino and

Guaita carried out bromination with carbon tetrabromide to low molecular weight

polybutadienes and studied the fire returdant action of the brominated product.122 Lukevics et

al. developed a new, simple and selective phase transfer catalysed bromination method for a

variety of terminal acetylenes. They used carbon tetrabromide as the brominating agent

(Scheme 1.17).123

PhC CHCBr4/ base/ catalyst (5 mol %)

Benzene/ 20o CPhC CBr

Scheme 1.17

The catalytic bromination of alkanes, cycloalkanes and arylalkanes with near 100%

yields was performed using CBr4 as a brominating agent in the presence of copper- and

nickel-containing catalysts.124 Smirnov et al. described catalytic bromination of alkanes

(decane and dodecane), cycloalkanes (cyclohexane and adamantane), haloalkanes (butyl

chloride) and arylalkanes (toluene, p- and m-xylenes, and mesitylene) by the action of CBr4

in the presence of metal-complex catalyst.125

(11) 2,2-Dibromo-2-cyano-N,N-dimethylacetamide

Vyas et al. described a new brominating agent named 2,2-dibromo-2-cyano- N,N-

dimethylacetamide.126 It is an effective and preferential brominating agent for

monobromination at α carbon atoms of ketones.

(12) 2-Bromo-2-cyano-N,N-dimethylacetamide

It was found that 2-bromo-2-cyano-N,N-dimethylacetamide (BCDA) was an effective

for monobromination at α carbon of ketones with high selectivity (Scheme 1.18).127

O

BCDA

O

CH2Br

Scheme 1.18

(13) Potassium bromate

Kraft first reported potassium bromate in sulfuric acid to brominate benzene to

bromobenzene.128 Again it was reported that potassium bromate brominated benzene in acetic

acid in the presence of a catalytic amount of sulfuric acid.129 Harrison et al. reported that

potassium bromate in sulfuric acid was a convenient and powerful brominating agent capable

of brominating aromatic rings containing deactivating groups.130

(14) 5,5-Dibromo-2,2-dimethyl-4,6-dioxo-1,3-dioxane

It is a brominating agent for bromination of saturated and α, β unsaturated carbonyl

compounds.131 The brominations were caried out at room temperature without using any

catalyst.

(15) Phosphorus tribromide – DMF

Yazima and Munakata used the brominating agent PBr3- DMF for the synthesis of 2-

and 4- bromoquinolines from the corresponding methoxyquinolines.132

(16) 3- Bromo-4,4-dimethyl-2-oxazolidinone

Bodor et al. prepared a new brominating agent, 3-bromo-4,4-dimethyl-2-

oxazolidinone (NBDMO).133 They investigated the reagent and compared to that of NBS and

found that it had a better or equivalent brominating and oxidising propery than NBS.

(17) Bromine chloride

Schulek and Burger reported bromine chloride exclusively as a brominating agent.134

It can brominate most of the aromatic compounds like benzene, phenols, anisole, toluene,

cresols, salicylic acids etc.135 Boudakian et al. did the vapour phase bromination of pyridine

with bromine chloride to give 2-bromopyridine (75%) as the major product.136

(18) 2,4,4,6-Tetrabromocyclohexa-2,5-dienone

Schmidt et al. studied the reaction of 2,4,4,6-tetrabromocyclohexa-2,5-dienone with

cyclohexene and methanol and reported the formation of 1-bromo-2-methoxycyclohexane.137

Tsubota and coworkers reported various reactions with alkenes in presence of different

bases.138 The reagent was also investigated for aromatic bromination139 and selective α

bromination of α, β unsaturated ketones.140 In 1974 Calo et al. studied the solvent effect on

the ortho: para ratio in the bromination of phenol with 2,4,4,6-tetrabromocyclohexa-2,5-

dienone.141 Hallas et al. used this reagent for p- bromination of aromatic and heteroaromatic

amines with a very good yield.142 Then this reagent was used as a useful reagent for

brominative cyclization of polyene143 and the cation radical bromination of a broad range of

organic compounds and for the electrophilic bromination of phenols and dialkylanilines.144

Again, this tetrabromide was also successfully used in the nucleophilic substitution of

bromine for hydroxyl, i.e., served as a source of nucleophilic halogen in presence of

triphenylphosphine.145 Again, the reaction of flavones with this reagent gave the

corresponding 3-bromoflavones under the mild reaction condition.146

(19) Bromo derivative of isocyanuric acid

(a) Sodium monobromoisocyanurate (SMBI)

Sodium monobromoisocyanurate (SMBI) was developed as a new brominating

reagent by Kaminski.147 SMBI brings about ionic and regioselective bromination in acidic

conditions.148 Imafuku et al. reported bromination of azulenic compounds in neutral

conditions.149

(b) Dibromoisocyanuric acid

Gottardi did bromination reactions with dibromoisocyanuric acid under ionic

conditions.150 Gottardi studied dibromo isocyanuric acid for bromination of perbrominations

of aromatic compounds.151 Brown et al. monobrominated isoquinoline regioselectively in

CF3SO3H using N,N'-dibromoisocyanuric acid (DBI) to give 5-bromoisoquinoline.152

(c) Tribromoisocyanuric acid

An efficient and highly regioselective bromination of activated aromatic rings

promoted by tribromoisocyanuric acid by in situ generation of Br+ was developed by

Almeida et al.153 Tozetti et al. utilized this reagent to perform the vicinal vicinal

dihalogenation of alkenes.154

(20) Pyrrolidinone hydrotribromide

Awang et al. used this reagent for selective bromination of ketones. It shows keto

selectivity (Scheme 1.19).155

OO

Brpyrrolidone hydrotribromide

Scheme 1.19

(21) Organic ammonium bromides

(a) Tetramethylammonium tribromide (TMATB)

In 1923, Cattaway and coworkers synthesised and studied the properties of

tetramethylammonium tribromide.156 Avramoff investigated the effect of this reagent on

several aromatic hydrocarbons, which could undergo either nuclear or benzylic

bromination.157

(b) Tetraethylammonium tribromide

Borah and Thakur did various reactions using this reagent. At first they have

synthesised alkyl ammonium tribromides with a green procedure using ceric ammonium

nitrate (CAN) as oxidant.158

(c) Phenyltrimethylammonium tribromide (PTATB)

It is used for brominating the α-position of carbonyl compounds159-164 α׳ bromination

of α,β-unsaturated ketones165 and for the addition of bromine to alkenes.166

(d) Cetyltrimethylammonium tribromide (CetTMATB)

Borah et al. use this reagent to brominate variety of aromatics and some other

substances like aniline, phenol, anthracene, imidazole, cresols, phluroglucinol, methyl-3-

phenyl-2-propenoate, 1,3-diphenyl-2-propen-1-one, 1,4-(4′-methoxy-phenyl)- diphenyl-3-

propen-1-one, 16-dehydro pregnonalone acetate etc.167

(e) Tetrabutylammonium tribromide (TBATB)

Bromination of substituted alkenes with tetrabutylammonium tribromide is one of the

easiest method for the preparation of vicinal dibromoalkanes.168 This reagent is used to

brominate various alkenes.169-170 Various acyclic α-bromo enones as well as cyclic α-bromo

enones were prepared from the corresponding acyclic cyclic enones respectively by Bose et

al.171 Bose et al. prepared a wide range of aurones exclusively from 2'-acetoxychalcones.172

Wu et al. used this reagent to synthesised 6-bromocoumarins.173

(f) Benzyltrimethylammonium tribromide

Those involve electrophilic bromination of aromatic compounds such as phenols, aromatic

amines, aromatic ethers, acetanilides, arenes, and thiophene, α-bromination of arenes and

acetophenones, and also bromo-addition to alkenes were done by the use of this reagent.174-177

(g) Tetraethylammonium bromide

Recently, Kirschning reported that a combination of Et4NBr and PhI(OAc)2 was used

in a bromoacetoxylation of olefin.178-179 R4NBr/PhI(OAc)2 system is also a very good

bromination source for flavone.180

(h) Tetrabutylammonium bromide (TBAB)

The regioselective bromination of activated aromatics was done using this reagent.181-182

(i) Tetramethylammonium bromide

Farkas et al. did bromination with tetramethylammonium bromide-bromine

mixtures.this mixture was used for the convenient preparation of dibromopropanol from allyl

alcohol.183

(22) Cupric bromide

Unsaturated aldehydes184 have been successfully brominated with cupric bromide in

methanol, aqueous methanol, or toluene at the α-carbon atom. Fort reported that

bromination of isophorone, 3,4,5-trimethylanisole, β-naphthol, and 5,5-

dimethylcyclohexane-1,3 dione by using cupric bromide in methanol or methanol-water at

room temperature.185 Doifode studied the reaction between cupric bromide and either

chalcones or 7-hydroxycoumarins.186 Again cupric bromide can be used for selective

bromination like brominate steroidal ketones in the presence of an isolated double bond.187

Doifode and Marathey studied the action of cupric bromide in dioxane on o-

hydroxyacetophenone and on some of its derivatives.188 Cupric bromide when refluxed in

methanol and higher alcohols, α-bromination of carbonyl-conjugated vinyl groups,

bromination of isolated double bonds, trans bromination of internal acetylenes, and

tribromination of terminal acetylenes were observed.189 Mosnaim et al. found that 9-alkyl

and 9-arylanthracenes underwent halogenation with cupric halides to give good yields of

their 10-halogenated derivatives.190 Bair et al. investigated a synthetic technique, which was

based on the reaction of olefins with copper (II) halides in presence of strong coordinating

agents. Olefins reacted readily with copper (II) bromide in the presence of a variety of

ligands to produce exclusively vicinal dibromoalkanes.191 Chaintreau et al. studied cupric

bromide as benzylic bromination reagent in polar media. The reaction in acetonitrile with

diisopropyl-peroxydicarbonate as oxidising agent yielded 18% of the product, however

without solvent and oxidising agent the yield was 2.5%.192 Rodebaugh et al. reported a new

method for dibromination of n-pentenyl glycosides. They used the reagent combination

CuBr2/LiBr.193 El-Qisairi reported a direct, regioselective and enantioselective approaches

for the synthesis of chiral α-bromo carbonyl compounds in moderate to high ee’s and good

yields in presence of cupric bromide (Scheme 1.20).194

O

H+

OH

Pd catalyst, THF/H2O

CuBr2, LiBr

O

H

Br

*

Scheme 1.20

Bhatt and Nayak investigate the use of CuBr2 as a brominating agent for electron-rich

aromatics.195

(23) Dioxane dibromide

Pasaribu and Williams did some selective bromination of the side chain of substituted

acetophenones with this mild brominating agent.196 Kotlyar et al. studied the kinetics and

mechanism of the bromination of cyclic acetals with 1,4-dioxane dibromide.197 Shusherina et

al. reported that the corresponding monobromides were obtained in the bromination of 1-

alkyl-2-pyridones with dioxane dibromide (Scheme 1.21).198

N O N

+

N

BrBr

O O

R R R

R= CH3, C2H5, C4H9 Scheme 1.21

Again, selective synthesis of α-bromo and α,α-dibromoalkanones using dioxane–

dibromide and silica gel was reported by Paul and coworkers.199 Chaudhuri et al. reported a

mild, solvent-free protocol using dioxane dibromide for highly regioselective aromatic

electrophilic bromination.200

(24) Pyridinium bromochromate (PBC)

Muathen studied this reagent as a new stable brominating agent for aromatic

compounds.201 Again pyridinium bromochromate was used to brominate various substituted

hydroxy-acetophenones, aldehydes and phenols for efficient and selective nuclear

bromination.202

(25) Trimethylsilylbromide

The possible value of trimethylsilyl halide for the preparation of anomeric halides was

suggested by Jung and Lyster.203-204 Gillard et al. used this brominating agent under mild

condition in presence of various protecting groups. They utilised this reagent to

Stereoselective bromination of anomeric glycosyl acetates.205 Again, for stereo and

chemoselective dibromination of alkenes this reagent can be used. Hazra et al. prepared

tetradecyltrimethylammonium permanganate (TDTAP), a reagent which in combination with

trimethylbromosilane (TMBS) provided a simple and mild method for stereo- and chemo-

selective transdibromination of alkenes.206

(26) 2,4-Diamino-1,3-thiazole hydrotribromide

This solid reagent was prepared by Forlani and efficiently applied for bromination of

aryl and styryl methylene ketones to give the corresponding 1-bromoalkyl ketones in high

yield.207

(27) 1,8-diazobicyclo [5. 4. 0] undec-7-ene hydrobromide per bromide

Muathen reported the brominating properties of DBUHBr3, with a number of

aromatics and heteroaromatics. Four different general procedures were employed for

aromatic bromination with this reagent depending on the nature of the aromatic ring.208

(28) Benzimidazolium bromochromate

Özgün and Degirmenbaşi introduced this reagent for bromination of various aromatic

compounds in acetic acid solution of the reagent.209

(29) Tetraalkyl ammonium dichlorobromate

Negero et al. utilised this reagent to brominate phenols.210 Then they tried

bromination of some aromatic hydrocarbons also.211

(30) Quinolinium bromochromate (QBC)

The new reagent, quinolinium bromochromate (QBC) functions as a good

brominating agent for aromatic compounds.212

(31) N,N - Dibromo benzene sulfonamide (Dibromoamine - T)

In 1971, Robinson tried to brominate norbornene and in this regard, he used this

reagent as a brominating agent.213 Tajbakhsh et al. used this reagent as a regenarable

brominating agent for bromination of carbanionic substrates under mild conditions. β-

diketones and β-ketoesters were brominated by this reagent without using any bases.214

Revesz et al prepared the scaffold of 3,7,9-triazabicyclo[3.3.1]nonane from N,N-

dibromobenzenesulfonamide as brominating agent and ethyl acrylate.215

(32) Sodium bromate

Masuda et al. used sodium bromate as a brominating agent to synthesised

bromohydrins from alkenes.216 Kikuchi et al. again used this reagent for selective selective

side chain and ring brominations of alkylbenzenes.217 Groweiss used sodium bromate as a

powerful brominating agent for aromatic compounds that contain deactivating substituents.218

(33) HBr In 1942, Couper et al. studied the action of HBr in acetic acid on unsaturated 1,4-

diketones. They found that HBr in acetic acid not only acted as brominating agent.219 Davis et

al. used HBr to prepare brominated pentaerythritols.220 Later it was found that combination of

aqueous tert-butylhydroperoxide or hydrogen peroxide and hydrobromic acid as an effective

reagent for bromination of aromatic compounds with or without a catalyst in various solvent

systems.221-226 A new method of bromination was reported by Gavara et al. with iso-amyl

nitrite/HBr system. The reaction succeeded with slightly activated arenes and heterocyclic

compounds.227

1.4. N,N-Dibromo-p-toluenesulfonamide: The titled brominating agent

1.4.1. Review of literature N,N-Dibromo-p-toluene sulfonamide is a brominating agent which was first used by

Kharasch for the synthesis of 1-phenyl-2-(p-toluenesulfonamido)-1 bromoethane.228 The

same reagent was also used by Paul et al. for the synthesis of 3,5-dihydroxy-1-substituted

piperidines from 1,4-pentadiene.229 In both the cases, the reagent was not studied

expeditiously. Recently, this reagent attracts much attention from the chemists. Phukan et al.

synthesized bromohydrins using this reagent. A very rapid and efficient method has been

developed for the synthesis of vicinal bromohydrins and alkoxybromides directly from an

olefin without any catalyst. The reaction was performed in CH3CN water (4:1) or alcohol.

Bromohydrins are formed instantaneously, whereas formation of alkoxybromides takes 30-60

min (Scheme 1.22).230

R'R''

TsNBr2

MeCN:H2O(4:1),rtor ROH, rt

R'R''

X

Br

R'= H, Ar; R''= H, CO2Et, CO2MeR', R''= -(CH2)4 -

X=OH or OR

Scheme 1.22

Huang et al. proposed a novel reaction pathway of 2,3-allenoates with TsNBr2 as an

electrophile in the presence of K2CO3 to produce (1E,2E)- 3-bromo-4-oxo-N-tosyl-2-

alkenoxylimidic acid ethyl esters (Scheme 1.23).231

R1

H

R2

OEtO

R1 H or naphthyl

+ TsNBr2

2.0 eq K2CO3

-65oC, 15 minthen rt, 30 min

via

ONTs

OEt

R2

Br

R1

O

Br R2

N

OEtR1

HBr

Ts

.

Scheme 1.23

Again an efficient diamination reaction of alkenes has been developed for the

synthesis of bromoalkylbranched imidazolines by using CuI–PPh3 as the catalyst and N,N-

dibromo-p-toluenesulfonamide as the nitrogen/halogen sources by Han et al. (Scheme

1.24).232

PhCOPh + TsNBr2

CuI-PPh3 (20 mol %)

CH3CN, 4A MSNN

CHBr2

Ts

COPhPh

(+)_

Scheme 1.24

Huang et al. also reported a novel electrophilic addition reaction of (Z)-alk-2-en-4-

ynoates and TsNBr2 providing a facile and highly stereoselective synthesis of densely

functionalized aziridine derivatives (Scheme 1.25).233

R1

HH

R2O

O+ TsNBr2

(3.0 equiv)

K2CO3 (5.0 equiv)

CH2Cl2, -78oC, 2hthen rt, 10h

R1

O

Br

N

OR2

NTsBr H

H

Ts

Scheme 1.25

Recently, it is proved that this reagent has oxidizing power. It can oxidize varieties of

alcohols to the corresponding aldehydes without using any catalyst (Scheme 1.26).234

R

OH

R1

TsNBr2 (1 equiv.)

MeCN, RTR

O

R1

R=aryl, alkyl, cycloalkylR1=H, alkyl, cycloalkyl

Scheme 1.26

It can be stated that there is much more scope in this reagent as a brominating agent.

That’s why; we have thought to explore some new organic transformation using the same.

1.4.2 Preparation of the reagent The reagent can be prepared by following literature procedure. It can be prepared

from chloramine-T235 as well as p- toluene sulfonamide.236

(a) From Chloramine -T

Recrystalised chloroamine-T (10 g) was dissolved in water (200 ml) and liquid

bromine (2 ml) was added dropwise from burette with constant stirring at the solution. The

golden yellow precipitate of N,N-dibromo-p-toluene sulfonamide was thoroughly washed

with water, filtered under suction and dried in a desiccator for 24 hours (Scheme 1.27).

Chloramine-T + Liq Br2H2O

StirrGolden yellow ppt of TsNBr2

Scheme 1.27

(b) From p-Toluene sulfonamide

In a 250 ml three necked flask were placed with 5 g of p-toluenesulfonamide in aq

KOH solution (3.6 g of KOH in 25 ml water). Then 10g of bromine were added with

vigorous stirring. The resulting precipitate of N,N-dibromo-p-toluenesulfonamide was

filtered, washed with water and dried (yield 9.8 g, 98%) (Scheme 1.28).

p- Toluene sulfonamide + Liq Br2Aq KOH

StirrGolden yellow ppt of TsNBr2

Scheme 1.28

However, TsNBr2 prepared from chloramine-T shows better activity than that produced from

p-toluenesulfonamide.

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