A simple and selective procedure for α-bromination of alkanones using hexamethylenetetramine-bromine complex and basic alumina in solvent-free conditions

Article abstract of DOI:10.1081/SCC-120020205

α-Bromoalkanones 2 were synthesized by the reaction of alkanones 1 with hexamethylenetetramine-bromine complex and basic alumina in solvent-free conditions under microwave irradiation.

Full text of DOI:10.1081/SCC-120020205

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A Simple and Selective Procedure for α-Bromination
of Alkanones Using Hexamethylenetetramine-bromine
Complex and Basic Alumina in Solvent-Free Conditions
a
a
a
Satya Paul , Varinder Gupta & Rajive Gupta
a
Department of Chemistry , University of Jammu , Jammu, India
Published online: 17 Aug 2006.
To cite this article: Satya Paul , Varinder Gupta & Rajive Gupta (2003) A Simple and Selective Procedure for α-Bromination
of Alkanones Using Hexamethylenetetramine-bromine Complex and Basic Alumina in Solvent-Free Conditions, Synthetic
Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry, 33:11, 1917-1922, DOI:
10.1081/SCC-120020205
To link to this article: http://dx.doi.org/10.1081/SCC-120020205
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Õ
SYNTHETIC COMMUNICATIONS
Vol. 33, No. 11, pp. 1917–1922, 2003
A Simple and Selective Procedure for
a-Bromination of Alkanones Using
Hexamethylenetetramine-bromine Complex
and Basic Alumina in Solvent-Free Conditions
Satya Paul,Varinder Gupta,and Rajive Gupta *
Department of Chemistry, University of Jammu,
Jammu, India
ABSTRACT
a-Bromoalkanones 2 were synthesized by the reaction of alkanones 1
with hexamethylenetetramine-bromine complex and basic alumina in
solvent-free conditions under microwave irradiation.
Key Words: a-Bromoalkanones; HMTAB-bromine complex; Basic
alumina; Solvent-free conditions; MW activation.
a-Bromoalkanones are the important synthons used for the synthesis
[
1–3]
of variety of biologically active heterocyclic compounds.
Generally,
a-bromoalkanones have been synthesized by the reaction of alkanone
*Correspondence: Rajive Gupta, Department of Chemistry, University of Jammu,
Jammu 180 006, India; Fax: 91-191-2505086; E-mail: rajgupta5@rediffmail.com.
1
917
DOI: 10.1081/SCC-120020205
Copyright & 2003 by Marcel Dekker, Inc.
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Paul,Gupta,and Gupta
with bromine in an appropriate solvent such as water, chloroform, carbon
[
tetrachloride, acetic acid or N,N-dimethylformamide. The reagents
4]
[5]
[6]
copper(II) bromide, 1,4-dioxane bromooxonium bromide, tribromoa-
[7]
[8]
cetophenone and N-bromosaccharin have been used as alternate bro-
minating agents instead of bromine. Furthermore, the solid organic
[
9]
ammonium tribromides, such as pyridinium,
phenyltrimethyl-
and tetrabutylammonium
have also been used as selective brominating agents.
[10]
[11]
ammonium,
[12]
tribromides
tetramethylammonium
All these reagents are quite expensive and when used make use of
solvents. In these environmentally conscious days, researches have been
directed to develop economic and environmentally friendly experimental
procedures. Thus, there is a need to develop safe and economic method
for a-bromination of alkanones.
Hexamethylenetetramine-bromine complex (HMTAB) has been
[
13]
used for selective oxidation of primary and secondary alcohols to
aldehydes and ketones, regeneration of ketones fromoximes and tosyl-
[14]
[15]
hydrazones,
and selective oxidation of sulfides to sulfoxides.
The
HMTAB is yellow–orange, non-hygroscopic homogeneous solid, very
stable at roomtemperature, not effected by ordinary exposure to light,
air or water and has no offensive odour of bromine. Ease of work-up and
stability of the reagent make it a safe and convenient source of active
[
16]
bromine. (For a review of positive halogens, see Ref. .) To our knowl-
edge, HMTAB/basic alumina has not been used for a-bromination of
alkanones under solvent-free conditions.
In recent years, the use of reagents impregnated over inorganic
[17]
supports has rapidly increased, as these reactions often involve milder
conditions, easier work-up and higher selectivity than similar reactions
using reagents in solution. Recently, an area of intense synthetic
endeavor has emphasized the use and design of reagents without any
solvent to reduce the amount of toxic waste and by-products arising
from the chemical processes prompted by stringent environment
protection laws. In continuation of our ongoing programme to develop
synthetic protocols utilizing microwave irradiation under solvent-free
[
18]
conditions,
alkanones/cycloalkanones (Table 1) with HMTAB/basic alumina in
we wish to report here selective a-bromination of
solvent-free conditions under microwave irradiation.
The reaction in case of Entry 1 has been carried out using HMTAB
under different conditions employing neat conditions as well as basic
supports like K CO , CaCO , neutral alumina and basic alumina in order
2
3
3
to obtain maximum yield in safer conditions. It has been found that under
neat conditions lot of fumes were evolved and hence is not safe to carry out
reaction in open vessel. Using K CO and CaCO , only 20 and 30% con-
2
3
3

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a-Bromination of Alkanones
1919

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Paul,Gupta,and Gupta
Scheme 1.
version takes place in 6 min respectively and rest products were burnt up.
Further, with neutral alumina 50% conversion takes place in 5 min and
rest 50% were the starting material. When basic alumina was used as sup-
port, 75% yield was obtained in 6 min. Thus, basic alumina was found to be
highly efficient support compatible with HMTAB for a-bromination of
alkanones/cycloalkanones (Sch. 1). No dibromination has been observed
(
TLC).
In conclusion, we have developed a rapid and high-yielding protocol
for selective a-bromination of alkanones/cycloalkanones in the presence
of HMTAB/basic alumina in solvent-free conditions under microwave
irradiation. The new procedure is simple, inexpensive and eco-friendly,
making it a useful alternative to existing methods.
EXPERIMENTAL
Melting points (uncorrected) were determined by Toshniwal melting
À1
point apparatus. IR spectra (ꢁmax in cm ) were recorded on Hitachi
2
1
70-30 spectrophotometer using KBr disc and H NMR spectra on
1
3
JNM-PMX 60 NMR (60 MHz) and C NMR on Bruker DPX 200
200 MHz) spectra in CDCl (chemical shifts in ꢀ, ppm) using TMS as
(
3
an internal standard. The mass spectra were performed on Jeol D-300
spectrometer. Microwave irradiation was carried out using a BPL BMO
800T domestic oven having maximum power output of 800 W.
General Procedure for the Synthesis of
a-Bromoalkanones/Cycloalkanones
Alkanone (3 mmol), HMTAB (5 mmol) and basic alumina (5 g) were
grinded in a pestle mortar, when a homogeneous powder was obtained.
This powder was then transferred in a borosil beaker (100-mL) and

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a-Bromination of Alkanones
1921
irradiated in an unmodified domestic microwave oven at 300 watt for an
appropriate time (Table 1, monitored by TLC). After irradiation, the
contents were cooled to room temperature and extracted with methylene
chloride (3 ꢁ 15 mL). The solid inorganic support material was filtered and
the solvent was removed under reduced pressure to afford the product,
ꢀ
which was purified by crystallization frompetroleumether (60–80 C).
1
The structures of the products were confirmed by IR, H NMR, mass
spectral data and comparison with authentic samples prepared according
to literature methods.
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