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 Balard 2 and Carl
Jacob Löwig3 in 1825 and 1826.4 Bromine, from Greek brómos, meaning ‘stench’ (of hegoats)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
1
groups.
(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-, decabrominated 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-5,5dimethylhydantoin 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 35 (Fig 1.1).18
CHO
CHO
CHO
OMe
3
OMe
OMe
Br
Br
OMe
4
Cl
Br
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
Br
Br
Br2
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
O
CH3
O
C
CH3
CH3
C
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
+
CH2Cl2
HMTA-Br2 or
silica supported HMTA-Br2
R
Br
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
NH3+ClR
R
Br
R
R
EtONO
-5-0 oC
conc. HCl
R'
N2+Cl-
R'
Br2
((NH4)2S2)8
R'
Scheme 1.4
R'
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
CH3
K10-Mont
CH3
+
Br
Br
Scheme 1.5
The bromination reaction of tricyclo [7.2.1.0 2,7] 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 + Br2
HOAc
dark
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
CH3
HO
C
CH3
Br
OH LDH-WO4 catalyst
H2O2-Br2
HO
Br
CH3
Br
C
CH3
OH
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
1.2 equiv. Br2/CCl4 R
CSiMe3
o
-12 C/0.5h
Br
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
O
R
O
Br2
H2SO4
R
CHBr2
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
NBS
Br
R
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-(2thienyl)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 Nbromosuccinimide.81 The bromination of carbonyl compounds was done using Nbromosuccinimide 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-7rhamnoglucoside, 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 5vinylindole was brominated regioselectively with this reagent (Scheme 1.11).95
BrH2CXHC
N
N
R
R
X=Br, OH
R=H, CH3, COCH3
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
solid reagent
water suspension
Ph
Scheme 1.12
Br
H
H
Br
Ph
Regiospecific bromination-rearrangement of the 5-substituted 2-alkylidene-4oxothiazolidine 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
CO2Et
PHPB
S
W
O
80-85oC
S
MeCN, 0oC
O
N
H
H
S
W
W
O
N
H
CO2Et
CO2Et
N
H
Br
H
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
O
ArCHO +
OH
HO
PHPB, CH(OEt)3
CH2Cl2, 50oC
Br
Ar
O
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
CH3
R1
Br
BrCN
NHR2
CH3
R1
NHR2
O
Scheme 1.15
Ramalingam et al. used the reagent for the selective bromination of coordinated βdiketones. 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
Br
O
O
SeO2-TMSBr
CCl4
H
H
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 3phenoxybenzyl 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 2,4,6tribromo-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
CH
CBr4/ base/ catalyst (5 mol %)
Benzene/ 20o C
PhC
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,Ndimethylacetamide.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
O
BCDA
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 2and 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-2oxazolidinone (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-2,5dienone.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
O
O
pyrrolidone hydrotribromide
Br
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-3phenyl-2-propenoate, 1,3-diphenyl-2-propen-1-one, 1,4-(4′-methoxy-phenyl)- diphenyl-3propen-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
bromination source for flavone.
good
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 ohydroxyacetophenone 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
OH
O
Br
H+
Pd catalyst, THF/H2O
*
H
CuBr2, LiBr
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 1alkyl-2-pyridones with dioxane dibromide (Scheme 1.21).198
Br
Br
+
N
N
O
O
R
R
N
O
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 chemoselective 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. βdiketones 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,Ndibromobenzenesulfonamide 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 1,4diketones. 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
X
TsNBr2
R''
R'
R''
R'
MeCN:H2O(4:1),rt
or ROH, rt
Br
R'= H, Ar; R''= H, CO2Et, CO2Me
R', 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-2alkenoxylimidic acid ethyl esters (Scheme 1.23).231
R2
Br
R1
R2
.
+
H
TsNBr 2
OEt
OEt
2.0 eq K2CO3
o
-65 C, 15 min
then rt, 30 min
NTs
R1
O
O
R
1
H or naphthyl
R2 Ts
Br
via
N
H
R1
Br
O
OEt
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,Ndibromo-p-toluenesulfonamide as the nitrogen/halogen sources by Han et al. (Scheme
1.24).232
CHBr2
Ph
COPh + TsNBr2
CuI-PPh3 (20 mol %)
N
CH3CN, 4A MS
Ph
Ts
N (+)
_
COPh
Scheme 1.24
Huang et al. also reported a novel electrophilic addition reaction of (Z)-alk-2-en-4ynoates and TsNBr2 providing a facile and highly stereoselective synthesis of densely
functionalized aziridine derivatives (Scheme 1.25).233
H
R2O
Ts
O
H
N
K2CO3 (5.0 equiv)
+ TsNBr2
O (3.0 equiv) CH2Cl2, -78oC, 2h
then rt, 10h
H
R1
OR2
Br
Br
R1
H
NTs
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
OH
R
TsNBr2 (1 equiv.)
R1
O
MeCN, RT
R1
R
R=aryl, alkyl, cycloalkyl
R1=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 Br2
H2O
Stirr
Scheme 1.27
Golden yellow ppt of TsNBr2
(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 Br2
Aq KOH
Golden yellow ppt of TsNBr2
Stirr
Scheme 1.28
However, TsNBr2 prepared from chloramine-T shows better activity than that produced from
p-toluenesulfonamide.
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