Polyvinylpyrrolidone-bromine complex: An efficient polymeric reagent for selective preparation of benzyl bromides in the presence of hexamethyldisilane

Article abstract of DOI:10.4314/bcse.v26i2.14

Benzylic bromides were conveniently obtained in high yields via the reaction of the corresponding alcohols with crosslinked polyvinylpyrrolidone- bromine complex (PVPP-Br₂)/hexamethyldisilane in chloroform at reflux condition. Selective conversion of benzyl alcohol to benzyl bromide in the presence of primary aliphatic alcohols, e.g. 2-phenylethanol was also achieved.

Full text of DOI:10.4314/bcse.v26i2.14

Bull. Chem. Soc. Ethiop. 2012, 26(2), 305-309.
Printed in Ethiopia
DOI: http://dx.doi.org/10.4314/bcse.v26i2.14
ISSN 1011-3924
 2012 Chemical Society of Ethiopia
SHORT COMMUNICATION
POLYVINYLPYRROLIDONE-BROMINE COMPLEX: AN EFFICIENT POLYMERIC
REAGENT FOR SELECTIVE PREPARATION OF BENZYL BROMIDES IN THE
PRESENCE OF HEXAMETHYLDISILANE
Masoud Mokhtary1* and Moslem M. Lakouraj2
1Department of Chemistry, Rasht Branch, Islamic Azad University, Rasht, Iran
2Department of Organic-Polymer Chemistry, Mazandaran University, Babolsar, Iran
(Received June 15, 2011; revised January 11, 2012)
ABSTRACT. Benzylic bromides were conveniently obtained in high yields via the reaction of the
corresponding alcohols with crosslinked polyvinylpyrrolidone-bromine complex (PVPP-Br2)/hexamethyldisilane
in chloroform at reflux condition. Selective conversion of benzyl alcohol to benzyl bromide in the presence of
primary aliphatic alcohols, e.g. 2-phenylethanol was also achieved.
KEY WORDS: Polyvinylpyrrolidone-bromine, Benzyl bromide, Hexamethyldisilane, Selective bromination
INTRODUCTION
The transformation of benzyl bromide derivatives into valuable end products in supramolecular
compounds such as fullerene precursor and dendrimer core is often utilized in organic syntheses
[1, 2]. The traditional methods include highly toxic reagent such as HBr gas [3], BBr3 [4, 5] and
PBr3 or combination of reagents like Br2/PPh3 [6], CBr4/PPh3 [7], and high temperatures are
often required.
Some alternative reagents reported for this purpose are bromotrimethylsilane [8],
-
HDMS/PyH+Br3 [9], ClSiMe3/LiBr [9], Br3CCOCBr3/PPh3 or Br3CCO2Et/PPh3 [10] and
silphos/X2[11]. However, most of the reagents introduced for the conversion of alcohols to
bromides gave by-product, and are of limited selectivity.
The application of polymer supports as reagent and catalyst has received special
attention in the past decades, due to the easy work up of reaction products and unique
selectivity which are undoubtly attractive features of this methodology. In addition, most
of the polymer supported reagents can be recycled after use [12, 13]. Among the synthetic
polymeric beds employed for this purpose, poly(vinylpyrrolidone) can act as safe carrier
for a variety of compounds in the blood stream and has been used as a retardant for drugs
and elimination of toxins [14]. It displays a strong binding affinity toward small molecules
[15], its iodine complex, povidon-iodine, is widely used as an anti-infective agent in
clinical treatments [16]. In our previous reports, we introduced crosslinked polyvinyl-
pyrrolidone-bromine complex as an efficient reagent for the bromination of alkenes,
oxidation of alcohols [17], and oxidative deprotection and selective deprotection of silyl
ethers to the corresponding alcohols [18] and bromination of activated aromatic
compounds [19]. To extend the synthetic utility of this reagent as a safe polymeric
brominating agent, our effort has been directed toward conversion of alcohols to the
corresponding bromides using crosslinked polyvinylpyrrolidone-bromine complex (PVPP-
Br2) in the presence of HDMS (Scheme 1).
__________
*Corresponding author. E-mail: mmokhtary@iaurasht.ac.ir

306
Masoud Mokhtary and Moslem M. Lakouraj
PVPP-Br2/ HMDS
RBr
ROH
CHCl3/ reflux
R = benzyl, naphthylCH2-, (Ph)2CH-, Ph(CH3)CH-, PhCH2(CH3)2-
Scheme 1
EXPERIMENTAL
Apparatus and reagents. Crosslinked polyvinylpyrrolidone (PVPP) was purchased from Fluka.
Other chemicals were purchased from Merck. Melting points were recorded on an electro
thermal melting point apparatus. The NMR spectra were recorded in CDCl3 with TMS as an
internal standard on a Bruker Avance DRX 500 MHz spectrometer. IR spectra were determined
on a SP-1100, P-UV-Com instrument. Purity determination of the products was accomplished
by TLC on silica gel poly gram SIL G/UV 254 plates. Products identified by comparison IR, and
1H NMR spectra, with those reported for authentic samples. In this study, the crosslinked PVPP-
Br2 complex is obtained according to our previous article [19].
General procedure for bromination of benzylic alcohols. To a suspention of PVPP-Br2 (1
mmol, 0.35 g) and HMDS (3 mmol, 0.44 g) in a flask were added chloroform (5 mL) and stirred
about 0.5 h at room temperature. Then a solution of benzyl alcohol (1 mmol) in chloroform (5
mL) was added drop-wise to the reaction mixture and stirred at reflux temperature for 1 h. The
reaction mixture was monitored by TLC in hexane/ethylacetate (4:1) as eluent. Then, the spent
resin was filtered and the residue washed with CH2Cl2 (2 × 25 mL). The filtrate was poured into
a 100 mL separating funnel, washed with water (2 × 25 mL). Finally, the organic layer was
dried over sodium sulfate. Removal of the volatile materials from the dried extracts gave
spectroscopically pure benzyl bromide. A similar procedure was employed in reactions with
other alcohols (Table 1). Further purification performed through silica gel column
chromatography. Caution: benzylic bromides can be powerful lachrymators and should be used
with adequate ventilation and precautions against skin contact or ingestion.
Regeneration of the PVPP-Br2. One of the main aspects in the use of cross-linked
polyvinylpyrrolidone supported bromine complex is the possibility of recycling and reuse of the
polymer. The spent reagent collected from the bromination reactions was washed thoroughly
with diethyl ether to remove any residual organic compounds. The resin was filtered and dried.
For functionalization, the regenerated polymer was treated with dilute bromine solution in
dichloromethane which was added drop wise using a separatory funnel and stirred for 0.5 h at
room temperature. The resulting mixture was filtered and washed with dichloromethane (2 × 20
mL) and dried in desiccator.
3-Methoxybenzyl bromide (Table 1, entry 10). 1H NMR (500 MHz, CDCl3): δ 3.86 (s, 3H),
4.52 (s, 2H), 6.59-7.33 (m, 4H). IR (neat): 3055, 2987, 2870, 1509, 1465, 1421, 1271, 1156,
1155, 1035 cm-1.
1
4-Pri-Benzyl bromide (Table 1, entry 11). H NMR (500 MHz, CDCl3): δ 1.33-1.31 (d, J = 10
Hz, 6H), 2.95-3.01 (m, 1H), 4.56 (s, 2H), 7.27-7.40 (dd, J = 8 Hz, 4H). IR (neat): 3021, 2963,
2870, 1643, 1515, 1494, 1381, 1280, 1265, 1156, 1131, 1036 cm-1.
1
4-Benzyloxybenzyl bromide (Table 1, entry 12). H NMR (500 MHz, CDCl3): δ 4.55 (s, 2H),
5.11 (s, 2H), 6.98-7.49 (m, 9H). IR (neat): 2933, 2867, 1613, 1572, 1456, 1380, 1300, 1224,
1128, 1006 cm-1.
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Short Communication
307
1
1-(2-Bromo-2-methylpropyl)benzene (Table 1, entry 17). H NMR (500 MHz, CDCl3): δ 1.82
(s, 6H), 3.26 (s, 2H), 7.30-7.37 (m, 5H); IR (neat): 3024, 2965, 1653, 1496, 1364, 1208, 1133,
1047 cm-1.
Table 1. Bromination of alcohols to the corresponding bromides with PVPP-Br2/HMDS.
Entry
1
Substrate
Producta
Time
(h)
Yield
(%)b
Ref.
CH2OH
CH2Br
1
98
98
92
96
21-23
21-23
10
2
3
4
Br
Cl
CH2OH
Br
Cl
CH2Br
2
CH2OH
CH2Br
CH2Br
2.5
1.5
Cl
Cl
19-21
CH2OH
Cl
Cl
5
1.5
94
19-21
CH2OH
CH2OH
CH2OH
CH2Br
CH2Br
CH2Br
6
7
8
F
F
2.5
1
98
98
98
19-21
19-21
19-21
Me
Me
Me
CH2OH
Me
1.5
CH2Br
9
MeO
CH2OH
MeO
CH2Br
3
3
90
86
10
-
MeO
MeO
10
CH2OH
CH2Br
11
(CH3)2CH
PhCH2O
CH2OH
(CH3)2CH
PhCH2O
CH2Br
1.5
1.5
98
98
-
-
CH2OH
CH2OH
CH2Br
CH2Br
12
13
14
3.5
3
84
94
19-21
10
HOCH2
CH2OH
BrCH2
CH2Br
Bull. Chem. Soc. Ethiop. 2012, 26(2)

308
Masoud Mokhtary and Moslem M. Lakouraj
CH(CH3)Br
(Ph)2CHBr
CH2C(CH3)2Br
15
CH(CH3)OH
(Ph)2CHOH
3
75
19-21
16
17
3.5
4
70
47
10
-
CH2C(CH3)2OH
aAll of the products were identified by comparing melting point and 1H NMR with those of authentic simples
reported in the literature. bYields refer to isolated products.
RESULTS AND DISCUSSION
The PVPP-Br2 complex is insoluble, non-volatile polymeric reagent and retains its activity
several months after its preparation. As shown in Table 1, a variety of benzylic alcohols
was treated with PVPP-Br2/HDMS in chloroform as a solvent under reflux condition.
Primary benzylic alcohols (Table 1, entries 1-14) were easily converted to the
corresponding benzyl bromides in good to excellent yields. Secondary benzylic alcohols
(Table 1, entries 15, 16) were converted to the corresponding bromides in good yields. In
the case of 2-methyl-1-phenyl-2-propanol (entry 17) as
a tertiary alcohol, the
corresponding bromide is obtained in 47% yield. Surprisingly, the aromatic ring of the
alcohols are not affected under the reaction conditions. Attempts to brominate primary and
secondary aliphatic alcohols with addition of more reagent were unsuccessful even at
longer time. Therefore, the selective conversion of benzylic alcohols to the corresponding
bromides was achieved in the presence of primary and secondary aliphatic alcohols. In a
typical experiment when equimolar amount of benzyl alcohol and 2-phenylethanol were
treated with one equivalent of the polymeric reagent for 3 h in chlorform under reflux
conditions, only benzyl alcohol was selectively converted to benzyl bromide and 2-
phenylethanol was remained unchanged (Scheme 2).
PhCH2Br
PhCH2OH
(95%)
PVPP-Br2/ HMDS
CHCl3/ reflux (3h)
+
+
(0%)
PhCH2CH2OH
PhCH2CH2Br
Scheme 2
HMDS
2(CH3)3Si-Br
R
CH2OH
CH2Br
n
-
Br
+
N
O
Br
R
spent reagent
+
2(CH3)3Si-OH
Br2/CH2Cl2
Scheme 3
Bull. Chem. Soc. Ethiop. 2012, 26(2)

Short Communication
309
It is reasonable to assume that the present reaction is initiated by the oxidative cleavage of
the silicon-silicon bond in hexamethyldisilane to the corresponding bromotrimethysilane in the
presence of PVPP-Br2 complex [20]. The in situ generated bromotrimethylsilane reacted with
benzylic alcohols to afford benzyl bromide derivatives. The spent resin could be converted back
to the starting polyvinylpyrrolidone complex after washing with organic solvent as depicted in
(Scheme 3).
CONCLUSIONS
In conclusion, crosslinked polyvinylpyrrolidone-bromine complex is stable, non-hygroscopic,
easy to prepare and handle, and represent an efficient and selective brominating reagent for
preparation of benzyl bromide derivatives in the presence of hexamethyldisilane.
ACKNOWLEDGEMENTS
We are grateful to Islamic Azad University, Rasht branch and University of Mazandaran for
financial assistance of this work.
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