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Original article

Ammonium  persulphate   promoted  synthesis of   polyethylene  glycol

entrapped  potassium  tribromide  and  its  application  in  acylation  and

bromination  of   some  selective  organic  compounds

Rupa Rani Dey, Siddhartha Sankar Dhar*

Department   of   Chemistry,  National  Institute  of   Technology,  Silchar,   Assam  788010,  India

1. 

Introduction

The upsurge in  green   chemistry   has inspired  the fraternity

of   researchers  to design and develop newer environmental

friendly methods  of   synthesis  of   new  or  existing catalysts and/

or  reagents   [1,2]. In  this context, it  may be emphasized  that  the

interest   in  the environmental  friendly synthesis   of   organic [3,4]

and inorganic  tribromides   [5,6] has increased  due to  their

versatile  utility in  organic   transformations. In spite  of   the

availability of   large numbers of   methodologies,   most of   them

are not favorable due to  the   use of    detrimental reagents  (like

liquid  Br2)  forthe  synthesisof   tribromides.   However,   it  is   better   to

use   environmental   friendlyoxidizing agents for the  conversion   of 

bromideto  tribromide   toachieve thesynthesis of   QTBs. Examples

of  

such oxidants include oxone [7], 

persulphate [8], 

KMnO4  [9],andCAN[10].  Over  the  last  fewyears,  ourgroup hasbeeninvolved

in  the   development   of   synthetic protocols  for economical, novel

and environmentally  safe reagents, such as  organic  ammonium

tribromides   (OATBs) [11,12] and catalysts [13]   for important

organic  reactions. We  have already  reported the ammonium

persulphate mediated  synthesis  of   OATBs [8].  Taking cue from

this, the  present paper  reports a  new method of   synthesis of   PEG

KBr3 using ammonium  persulphate  as   oxidant   for the  conversion

of  

Br

to 

Br3

. It 

should 

be 

noted 

here 

that 

potassium 

tribromide(KBr3)  is  an  efficient,   cheap  and  environmentally   benign   reagent.   The

application   of   this  reagent in  organic   transformation   reactions   has

not  been   very   successful   because   KBr3   is  unstable   at  room

temperature.   It  has  been   observed   that   polyethylene   glycol   (PEG)

acts   as  a  good  host  and  captures   the  K+ cation   and  thus,  provides

maximum  stability   to  the  reagent.   Similar   host-guest   chemistry   can

be  observed   in  the  case  of   {[K  18-Crown-6]Br3}  [14]   which   on

recrystallization   form  red crystals.  {[K  18-Crown-6]Br3}  was  also

used  in  the  bromination   of   activated   aromatic  compounds.   The

advantage   of   [{K  PEG}+Br3]  over   {[K  18-Crown-6]Br3}  is  that   PEG  is

less   expensive   and  the  conversion   of   bromide   to  tribromide  is  not

achieved   by  the  addition  of   rather   harmful   liquid  bromine.

Moreover,   the  application   of   PEG.KBr3 has   not  been   well  explored

in 

organic 

transformations 

other 

than 

bromination 

reactions. 

In 

thisarticle   we  wish  to  report  a  new  environmentally   benign   method   of 

synthesis   of   [{K  PEG}+Br3]  and  its  application   as  reagent   in  acylation

and  bromination   reactions.

2.  Experimental

All   the commercial chemicals  are   of analytical  grade  and

used without further purification. The completion  of   the

reaction was monitored  by   TLC. The  synthesized   tribromide

was characterized  with   UV–vis   and   FT-IR spectroscopy.  X-ray

diffraction   (XRD) analysis was  also performed  with

Chinese   Chemical   Letters   24  (2013)   866–868

A  R   T  I  C  L   E  I  N  F   O

 Article history:Received  24   April   2013

Received in revised form 16 May 2013

Accepted  22  May 2013

Available online 1 July 2013

Keywords:

Potassium   bromide

Polyethylene   glycol  (PEG)

Ammonium  per   sulphate

Acylation

Bromination

A  B  S  T  R   A  C  T

In this study,  a new method of synthesis of polyethylene glycol supported potassium tribromide (PEGKBr3) and its application in acylation and bromination reactions are reported. Ammonium persulphate

oxidizesKBrto thecorresponding tribromidewhich is entrappedby polyethyleneglycol leading to stable

PEG KBr3. The reagent is proved to be highly efficient for the acylation of variety of alcohols and

brominationof activated aromatic substrates. Themethod is a mild, onepot reactionandinvolvesno use

of toxic reagents.

2013 Siddhartha Sankar Dhar. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All

rights reserved.

*  Corresponding   author.

E-mail  address:  ssd_iitg@hotmail.com   (S.S.  Dhar).

Contents  lists  available  at  SciVerse  ScienceDirect

Chinese Chemical Letters

jou rn al  h omepage:  www.els evier .co  m/locat  e/cc   le t

1001-8417/$   –  see   front  matter    2013  Siddhartha   Sankar   Dhar.   Published   by  Elsevier   B.V.   on   behalf   of   Chinese   Chemical   Society.  All   rights   reserved.

http://dx.doi.org/10.1016/j.cclet.2013.05.036

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diffractometer system-XPERT-PRO equipped  with Cu  Ka

radiation. IR   spectra were recorded  on  KBr with  MAGNA  550

FTIR spectrometer. 1H  NMR   spectra were recorded in  DMSO-d6 /

CDCl3   on  a  Bruker  Ultra  Shield 400 plus spectrometer.

PEGKBr3   was  synthesized  by  mixing  1  mmol   of   PEG-4000

(4.0  g)  to  the  solution  of   3  mmol   of   KBr  (0.36  g)  in  15  mL   of   2  mol/L 

H2SO4 and  the  mixture  was  stirred  for  ca. one  hour.  To  the  resulting

solution,  ammonium  persulphate  (3  mmol,  0.68  g)  solution  was

added   and  stirred  for  another  30  min.  An  orange-red  viscous  liquid

of   [{K  PEG}+Br3]  appeared  which  was  extracted  with  Et2O  and

dried  under  vacuum  (yield  81%).  The  formation  of   tribromide

(Scheme  1) was  confirmed  by  electronic  absorption  spectroscopy

(Fig.  S1  in  Supporting  information)  and  FT-IR   technique.  An  intense

band  observed  at  274  nm  is  characteristic  for  tribromide   anion

(Br3)  [15].  The  IR   spectrum  of   PEG  KBr3   (Fig.  S2  in  Supporting

information)  exhibits  characteristic  (Br3)  bands  at  52  (n1)  and  189

(n2)  cm1 for  bending  and  asymmetric  stretching,  respectively

[16].  The  XRD  pattern  of   PEG  KBr3 at   room  temperature  indicates

the  formation  of   the  crystalline  structure.  The  X-ray  peaks  in  therange  of   2u (108  <  2u   <  608)  show  some  weak  low-angle  peaks  and

one  high  angle  peak  at  278. The  XRD  pattern  of   PEG  (Fig.  S3  in

Supporting  information)  shows  two  sharp  peaks  at  19.38 and  23.58

which  are  not  observed  in  the  XRD  pattern  of   PEG  KBr3,  and

consistent  with  literature  information  [17,18]. The  disappearance

of   peaks  suggests  that  PEG  strongly  interacts  with  KBr3  [19].  The

broad  peak  appearing  at  228 (d  =  4  A )   is  a  result  of   inter-chain

interactions  from  London   dispersion  forces  [20,21].  Much  infor-

mation   could  not  be  inferred  (resolved)  regarding  the  packing  and

lattice  structure  of   the  compound  due  to  lack  of   literature  on  the

powder   XRD  pattern  of   this  compound.

A  representative  acylation  reaction  was  conducted  by  adding

PEGKBr3 (1   mmol,  4.3  g)  to  the  stirred  reaction  mixture  of   alcohol

(1  mmol)   and  acetic  acid  (5  mL).  The  mixture  was  refluxed  for  ca.half   an  hour  with  the  progress  of   the  reaction  monitored  by  TLC

(10%  ethyl  acetate/hexane).  After  completion  of   the  reaction,  the

entire  mixture  was  poured   into  a  saturated  solution  of   NaHCO3

(20  mL).   The  product  was  extracted  with  5  mL   of   ethyl  acetate  and

dried  with  anhydrous  sodium  sulphate  and  evaporated  under

vacuum  to  obtain  the  pure   product.

The  bromination  reactions  were  carried  out  in  solvent  free

manner.  In  a  typical  reaction,  PEGKBr3 (1   mmol,  4.3  g)  was  added

to  the  aromatic  substrate  (1  mmol)  in  a  mortar   and  was  ground  for

the  desired  reaction  time.  The  progress  of   the  reaction  was

monitored  by  TLC  (10%  ethyl  acetate/hexane).  After  completion  of 

the  reaction,  the  product  was  extracted  with  ethyl  acetate  and

evaporated  under  vacuum  to  obtain  the  pure   brominated  product.

The 

products 

were 

characterized 

by 

IR  

and 

NMR  

spectra 

(seeSupporting  information).

3.  Results  and  discussion

The  PEG  supported  tribromide  is  a  viscous  liquid  which  is  found

to  be  highly  stable  and  could  be  stored  at  room  temperature

(NH4)2S2O8

PEG +  KBr 

2mol/L H2SO4, r.t.PEG KBr 3.

Scheme  1.  Synthesis   of   PEGKBr3.

R    R 

Br 

PEG·KBr 3

r.t.Where R= -NH2, -NHR,-OH,-OMe

Scheme  2.  Representative   bromination   of   aromatic   substrate   by  PEGKBr3.

O

OH

  H

OH

OH

OH

R 

R'

OH

O

O

H

HR 

R'

- H2OOH

O

R 

R'

- H+

+ H+

CH3COOH

R'

OH

R 

HBr 

+ H2O

CH3COOHHBr PEG.KBr 3   +   Br 2PEG.KBr 

+

O

O

R 

R'

Scheme  3.  Plausible   mechanism   of   acylation  of   alcohols   mediated  by  PEGKBr3.

 Table  1

Acylation  of   selectively   chosen   alcohols   promoted  by  PEGKBr3.a

Entry  Substrate   Product  Time  (min)  Yield  (%)b

1OH OAc

20  90

2OH

n=7  OAc

n=7

15  85

3OH

n=9   OAcn=9

15  92

4   OH   OAc   20  89

5

Ph

OH

Ph

OAc   20  88

6OH   OAc

  20  93

7OH

  OAc  25  91

a PEGKBr3  (1   mmol),  alcohol   (1  mmol),  acetic  acid  (5  mL).b Isolated  yield.

 Table  2

Bromination   of   selectively   chosen   activated  aromatic   compounds   by  PEGKBr3.a

Entry  Substrate   Product   Time  (min)  Yield  (%)b

1OH   NH2Br 

10  93

2 NH2Br 

10  87

3 N   NBr 

15   90

4OMe   OMeBr 

12   81

5

 NH

O

Me   NH

O

Me

Br  13   85

6   OH   Br 

OH

10  89

7   OH   OH

Br 

12   84

a PEGKBr3  (1   mmol),  aromatic   substrate   (1  mmol).b

Isolated 

yield.

R.R.  Dey,  S.S.   Dhar   /   Chinese  Chemical  Letters   24  (2013)  866–868  867

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without  significant  decomposition.  To  study  the  utility  of 

PEGKBr3,  acylation  and  bromination  reactions  were   conducted

with  some  chosen  organic  substrates.  A  representative  bromina-

tion   is  shown  in  Scheme  2.  The  plausible  mechanism  of   acylation

by  PEGKBr3   is  depicted  in  Scheme  3.  Fundamentally,  PEG  KBr3liberates  Br2 which  in  turn  forms  HBr  in  the  presence  of   acetic  acid.

Then  HBr  reacts  with  acetic  acid  and  alcohols  to  generate  the  pure

products   (Scheme  3).  Products  were  further  characterized  by

comparing  their  melting  point  and  boiling  point  with  authentic

pure   samples  [22].  The  results  have  been  summarized  in  Tables  1

and  2.

4.  Conclusion

The  present  protocol  emphasizes  the  development  of   economic

and  environmentally  safe  synthesis  of   polyethylene  glycol

supported  potassium  tribromide  and  its  useful  application  in

the  acylation  of   alcohols  and  the  bromination  of   aromatic

substrates.  The  advantages  of   this  reagent  are  stability,  high

efficiency,  reusability  and  non-hazardous  nature.  In  spite  of   the

availability  of   numerous  reagents  in  the  literature  for  the  above

mentioned  organic  reactions,  the  advantages  represented  herein

will 

serve 

as 

an 

alternative 

protocol 

for 

acylation 

of  

alcohols 

andbromination  of   organic  substrates.

 Acknowledgment

S.S.D.  acknowledges  the  Department   of   Science  and  Technolo-

gy,  DST,   New  Delhi,  India,  for  financial  assistance  received  through

a  SERC  fast  track  project  (No.  SR/FTP/CS-100/2007).

 Appendix    A.  Supplementary   data

Supplementary  data  associated  with  this  article  can  be  found,  in

the  online  version,  at  http://dx.doi.org/10.1016/j.cclet.2013.05.036.

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R.R.  Dey,  S.S.   Dhar   /   Chinese  Chemical  Letters   24  (2013)  866–868868