Ionene-bound borohydrides: Efficient, selective, and versatile polymer-supported reducing agents

Article abstract of DOI:10.1007/s00706-007-0756-5

Reduction of structurally different carbonyl compounds, such as aldehydes, ketones, α,β-unsaturated enals and enones, acid chlorides, and imines was accomplished efficiently using high capacity ionene based borohydride reagents. Aldehydes and imines were reduced to the corresponding alcohols and amines in excellent yields in methanol at ambient temperature while ketones and acid chlorides were reduced in iso-propanol and THF-MeOH at reflux. Chemoselective reduction of aldehydes over ketones was achieved successfully with these reagents. Complete regioselectivity was also observed in the reduction of α,β-unsaturated aldehydes and ketones.

Full text of DOI:10.1007/s00706-007-0756-5

Monatshefte fu¨r Chemie 139, 117–123 (2008)
DOI 10.1007/s00706-007-0756-5
Printed in The Netherlands
Ionene-Bound Borohydrides: Efficient, Selective, and Versatile
Polymer-Supported Reducing Agents
ꢀ
Moslem M. Lakouraj , Mahmood Tajbakhsh, and Majid S. Mahalli
Department of Chemistry, Mazandaran University, Babolsar, Iran
Received June 10, 2007; accepted June 25, 2007; published online November 30, 2007
# Springer-Verlag 2007
Summary. Reduction of structurally different carbonyl com-
Lewis acids, and mixed solvent systems [7, 8], and
pounds, such as aldehydes, ketones, ꢀ,ꢁ-unsaturated enals
and enones, acid chlorides, and imines was accomplished
efficiently using high capacity ionene based borohydride
reagents. Aldehydes and imines were reduced to the corre-
sponding alcohols and amines in excellent yields in methanol
at ambient temperature while ketones and acid chlorides were
reduced in iso-propanol and THF-MeOH at reflux. Chemose-
lective reduction of aldehydes over ketones was achieved suc-
cessfully with these reagents. Complete regioselectivity was
also observed in the reduction of ꢀ,ꢁ-unsaturated aldehydes
and ketones.
finally, (vi) use of polymers and solid beds to stabi-
lize the hydride species [9]. Polymer-supported re-
agents and catalysts have been in use since the
1960s, and the applications of polymer supported
borohydride reagents have been recently reviewed
[10–12]. Polymer bound borohydrides offer several
advantages over the other modified borohydrides.
The primary advantage is the convenience of use
of these materials and the minimal introduction of
ionic species or organic by-product into treated bulk
media. Simplified workup of reaction products is
another advantage. But, one significant limitation of
current polymeric reagents remains their relatively
low loading (0.5–2 mmol=g) for most commercial
reagents, and practically an excess of polymeric
reagents needs to be used to complete reaction.
Due to high price, polymer supported reagents are
not very economical confining the method to small
scale chemistry. High loading resins would increase
the efficiency and the atom economy [13] of these
reagents considerably. In addition, in high loading
reagents, the concentration of the reactant in the
polymer is enhanced, thus reducing the amount of
polymer backbone employed. However, the impor-
tant classical ion exchange resins were based on
anion exchange resins, which loaded with anionic
counter ions show many disadvantages. The classical
ion exchange resins prepared from chloromethylated
styrene and trimethylamine suffer from low loading
because of the weight of polymer backbone, and the
Keywords. Ionene; Borohydride; Reduction; Carbonyl com-
pounds; Imines.
Introduction
Reduction is one of the frequently used reactions in
organic synthesis and a vast variety of reducing
agents has been introduced for this purpose [1].
Among them, sodium borohydride is the most com-
monly used reagent for the reduction of variety of
functional groups in synthesis. To improve its reduc-
ing ability, several substituted borohydrides have
been introduced. In fact, the progress in this field
has been realized by: (i) substitution of the hydride(s)
with other groups [2], (ii) variation in the metal cat-
ion [3, 4], (iii) concurrent cation and hydride ex-
change [5], (iv) complexation with ligands [6], (v)
combination of borohydrides with metal, metal salts,
ꢀ
Corresponding author. E-mail: lakouraj@umz.ac.ir

118
M. M. Lakouraj et al.
higher loading is restricted due to Lewis acid cata-
lyzed cross-linking of chloromethyl groups within
the resin. Furthermore, the molar activity of the sup-
ported reagents can decrease considerably during
storage and can be significantly lower than the orig-
inal loading of counter ion because of the chemical
lability of the ammonium salt, which releases trime-
thylamine. Another general problem of polystyrene
based resins is their chemical liability under oxida-
tion and strong Lewis acidic conditions [14]. In fact,
maximum loading of counter ions is obtained by
a polymer constructed from low molecular weight
functional monomers [15]. Realizing this fact, we
studied linear ionene bromide or chloride as poly
cationic polymer supports to obtain high capacity
linear polymer supported borohydrides, the ionene
borohydrides.
Scheme 2
transformation of aldehydes, ketones, and acid chlo-
rides to the corresponding alcohols, and imines to
their amines. In order to find the optimum condi-
tions, we treated benzaldehyde, acetophenone, ben-
zoyl chloride, and N-benzylideneaniline as model
compounds with ionene borohydrides in different
solvents such as MeOH, t-BuOH, CH2Cl2, CH3Cl,
and THF. Our observation revealed that methanol
is the best solvent for reduction of aldehydes and
imines, but iso-propanol and a mixture of THF and
methanol (1:1) are the best solvents for the reduction
of ketones and acid chlorides (Scheme 2).
Reductions of a variety of structurally different
aromatic and aliphatic aldehydes to their corres-
ponding alcohols are performed efficiently using 1:1
molar ratio of reductants to substrate in methanol at
room temperature. The reactions proceed within 10–
70 min in high to excellent yields (Table 1). Ketones
were also reduced with ionene borohydrides in iso-
propanol at reflux using a 2:1 molar ratio of reducing
agents=ketone and the corresponding alcohols formed
in excellent yields (Table 2). It is noteworthy that
reduction of ꢀ,ꢁ-unsaturated aldehydes and ketones
proceeds in excellent regioselectivity and the corre-
sponding allylic alcohols were obtained exclusively
(Table 1, entry 16 and Table 2, entries 7–10).
Results and Discussion
This report describes the results of utilizing linear
ionene borohydrides as new polymer-supported re-
ducing agents for the reduction of aromatic, aliphatic,
and unsaturated aldehydes, ketones, acid chlorides,
and imines. Ionene borohydrides, 2,4-ionene boro-
hydride (PIBH) and xylylene-ionene borohydride
(PIIBH), are stable non-hygroscopic white powders
which are insoluble in most organic solvents, includ-
ing ether, THF, CHCl3, i-PrOH, and t-BuOH, but
slightly soluble in MeOH. However, they are quite
soluble in water. They are prepared quantitatively by
treatment of aqueous solutions of 2,4-ionene bromide
(PI-Br) [16] or xylylene ionene chloride (PII-Cl)
[17], with alkaline sodium borohydride solution at
room temperature (Scheme 1). The borohydride load-
ing of the ionene borohydrides PIBH and PIIBH
were determined by titration to be 6 and 5 mmol=g
of polymer [18].
The synthesis utility of this new polymer support-
ed borohydrides were studied by examining the
Conversion of acid chlorides to their alcohols is an
important transformation in organic synthesis. In re-
cent years, a few novel reducing agents have been
introduced for this purpose [4a, 19]. Ionene borohy-
drides in a mixture of THF-MeOH converted both
aliphatic and aromatic acid chlorides to the corre-
sponding alcohols in high yields (Table 3).
However, to explore the scope of these polymeric
reducing agents, the reduction of a variety of imines
was also investigated (Table 4). As listed in Table 4,
Scheme 1

Ionene-Bound Borohydrides
119
Table 1. Reduction of Aldehydes with PIBH and PIIBHa
Entry
Substrate
Product
PIBH
PIIBH
Ref.
Time=
min
Yield=
b,c
%
Time=
min
Yield=
b,c
%
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Ph–CHO
Ph–CH2OH
30
60
50
10
15
10
10
40
10
10
55
10
10
70
40
55
25
35
96
30
45
40
10
10
10
10
30
10
10
35
10
10
55
30
40
15
30
98
[23]
[23]
[23]
[23]
[23]
[23]
[23]
[23]
[24]
[23]
[23]
[24]
[23]
–
4-HO–Ph–CHO
4-MeO–Ph–CHO
4-Cl–Ph–CHO
4-HO–Ph–CH2OH
4-MeO–Ph–CH2OH
4-Cl–Ph–CH2OH
4-Br–Ph–CH2OH
4-NC–Ph–CH2OH
4-O2N–Ph–CH2OH
3-MeO–Ph–CH2OH
3-Cl–Ph–CH2OH
3-O2N–Ph–CH2OH
2-HO–Ph–CH2OH
2-Cl–Ph–CH2OH
2-O2N–Ph–CH2OH
2,3-HO–Ph–CH2OH
2-(4-Cl–Ph–S)–Ph–CH2OH
Ph–CH¼CHCH2OH
Ph–CH2CH2OH
93
93
98
95
92
93
93
95
96
95
97
96
91
92
90
95
90
95
92
96
95
95
95
90
95
93
95
96
95
92
95
92
96
91
4-Br–Ph–CHO
4-NC–Ph–CHO
4-O2N–Ph–CHO
3-MeO–Ph–CHO
3-Cl–Ph–CHO
3-O2N–Ph–CHO
2-HO–Ph–CHO
2-Cl–Ph–CHO
2-O2N–Ph–CHO
2,3-HO–Ph–CHO
2-(4-Cl–Ph–S)–Ph–CHO
Ph–CH¼CHCHO
Ph–CH2CHO
(CH3)2CH(CH2)3CH(CH3)–
CH2CHO
CH3(CH2)5CHO
–
[23]
[23]
[23]
(CH3)2CH(CH2)3CH(CH3)–
CH2CH2OH
CH3(CH2)5CH2OH
19
25
88
20
85
[23]
a
All reactions were carried out in methanol at room temperature using 1=1 molar ratio of BH4ꢁ=substrate
b
All products were known compounds and characterized spectroscopically (1H NMR and IR) and showed physical and spectral
data in accordance with their expected structure and by comparison with authentic samples
c
Yields refer to pure isolated products
Table 2. Reduction of ketones with PIBH and PIIBHa
Entry
Substrate
Product
PIBH
PIIBH
Ref.
Time=
h
Yield=
b,c
%
Time=
h
Yield=
b,c
%
1
2
3
4
5
6
7
8
Ph–COCH3
Ph–CH(OH)CH3
Ph–CH(OH)CH2CH3
Ph–CH2CH(OH)CH3
Ph–CH(OH)–Ph
ꢀ-tetralol
1.1
1.5
1.5
3
1.8
3
96
1
97
[23]
[23]
[23]
[23]
[23]
–
Ph–COCH2CH3
Ph–CH2COCH3
Ph–CO–Ph
ꢀ-tetralone
9-flurenone
Ph–CH¼CHCOCH3
4-Me–Ph–CH¼
CHCO–Ph–4-Me
4-Br–Ph–COCH¼CH–Ph
2-cyclohexen-1-one
3-acetylpyridine
cyclohexanone
2-heptanone
95
98
96
97
93
92
90
1.2
1.5
2.5
1.5
2.7
2
95
97
97
96
93
92
90
9-flurenol
Ph–CH¼CHCH(OH)CH3
2.5
3
[24]
–
4-Me–Ph–CH¼
2.5
CHCH(OH)–Ph–4-Me
4-Br–Ph–CH(OH)CH¼CH–Ph
2-cyclohexen-1-ol
1-(3-pyridyl)ethanol
cyclohexanol
9
10
11
12
13
2.5
2.2
0.8
2
92
88
97
88
87
2.5
2
0.8
2
93
90
95
90
85
–
[23]
[24]
[23]
[23]
2-heptanol
2
1.5
a
All reactions were carried out in i-PrOH at reflux using 2=1 molar ratio of BH4ꢁ=substrate
b
All products were characterized spectroscopically (1H NMR and IR) and showed physical and spectral data in accordance with
their expected structure and by comparison with authentic samples
c
Yields refer to pure isolated products

120
M. M. Lakouraj et al.
Table 3. Reduction of acid chlorides to alcohols with PIBH and PIIBHa
Entry
Substrate
Product
PIBH
Yield=%b,c
PIIBH
Ref.
Time=min
Time=min
Yield=%b,c
1
2
3
4
5
Ph–COCl
Ph–CH2OH
30
20
20
60
40
88
90
95
88
90
20
20
15
45
30
90
91
94
90
92
[23]
[23]
[24]
[23]
[24]
4-Cl–Ph–COCl
2-Cl–Ph–COCl
Ph–CH2COCl
Ph–CHClCOCl
4-Cl–Ph–CH2OH
2-Cl–Ph–CH2OH
Ph–CH2CH2OH
Ph–CHClCH2OH
a
All reactions were carried out in THF-MeOH at reflux using 4=1 molar ratio of BH4ꢁ=substrate
b
All products were characterized spectroscopically (1H NMR and IR) and showed physical and spectral data in accordance with
their expected structure and by comparison with authentic samples
c
Yields refer to pure isolated products
Table 4. Reduction of imines to amines with PIBH and PIIBHa
Entry
Substrate
Product
PIBH
Yield=%b,c
PIIBH
Yield=%b,c
Ref.
Time=h
Time=h
1
2
3
4
5
6
7
8
9
10
Ph–CH¼N–Ph
Ph–CH2NH–Ph
4
3.5
5
3.5
2.7
3.5
4
4
4
9
87
89
85
89
87
85
82
88
85
84
3.5
3.2
4
3
2.3
3
3
4
4
7
88
91
85
90
88
86
84
86
86
82
[25]
[26]
[26]
[27]
[28]
[23]
[24]
[29]
[30]
[31]
4-Cl–Ph–CH¼N–Ph
4-MeO–Ph–CH¼N–Ph
4-NC–Ph–CH¼N–Ph
4-O2N–Ph–CH¼N–Ph
Ph–CH¼NCH2–Ph
CH3CH2CH2CH¼N–Ph
Ph–CH¼NC(CH3)3
Ph–CH¼NCH2CH2CH3
Ph–(CH3)C¼N–Ph
4-Cl–Ph–CH2NH–Ph
4-MeO–Ph–CH2NH–Ph
4-NC–Ph–CH2NH–Ph
4-O2N–Ph–CH2NH–Ph
Ph–CH2NH–CH2Ph
CH3CH2CH2CH2NH–Ph
Ph–CH2NHC(CH3)3
Ph–CH2NHCH2CH2CH3
Ph–(CH3)CHNH–Ph
a
All reactions were carried out in MeOH at room temperature using 3=1 molar ratio of BH4ꢁ=substrate
b
All products were characterized spectroscopically (1H NMR and IR) and showed physical and spectral data in accordance with
their expected structure and by comparison with authentic samples
c
Yields refer to pure isolated products
ionene borohydride reagents could reduce a variety
of imines in high yields. Ketimine was also reduced
in excellent yield (entry 10). Imines bearing a chloro,
nitro, or a nitrile group were chemoselectively re-
duced to the corresponding amines (entries 2, 4
and 5).
On the other hand, it is often necessary in organic
synthesis to reduce one particular carbonyl group
without affecting other carbonyl groups in a mole-
cule. In the case of hydride reagents, aldehydes
generally are more susceptible to reduction than ke-
tones. The most commonly employed reducing agents
are too reactive under normal conditions to differ-
entiate between aldehydes and ketones. A number of
modified tetrahydroborate reagents have been re-
ported to show the discrimination ability between
aldehydes and ketones [20]. Along the outlined strat-
egy, and to explore the chemoselectivity of ionene
borohydrides in reduction of aldehydes over ketones,
we carried out competition experiments of a variety
of structurally different aldehydes and ketones. In
the preceding section, we noticed that reduction of
aldehydes and ketones were both temperature and
solvent dependent. Therefore, this goal could be eas-
ily achieved by selective reduction of aldehyde in the
presence of an equimolar amount of ketone using
one equivalent of ionene borohydrides at room tem-
perature. The general trend of this discrimination
ability for reduction of aldehydes in the presence
of ketones is shown in Table 5.
The reduction of the nitro, nitrile, and carboxylate
groups by ionene borohydrides does not occur to any
appreciable extent under the conditions described
above for the reduction of carbonyl compounds and

Ionene-Bound Borohydrides
121
Table 5. Chemoselective reductions of aldehydes versus ketones with (PIBH and PIIBH) in methanol
Entry
Substrate
Product
Time=min
Yield=%a
PIBH
PIIBH
1
2
Ph–CHO
Ph–COCH3
Ph–CH2OH
Ph–CH(OH)CH3
40
60
40
70
40
96
8
98
8
Ph–CH¼CHCHO
Ph–CH¼CHCH2OH
93
5
92
6
Ph–CH¼CHCOCH3
Ph–CH2CHO
Ph–CH2COCH3
Ph–CH¼CHCH(OH)CH3
Ph–CH2CH2OH
Ph–CH2CH(OH)CH3
3
96
6
97
9
4
Ph–CHO
Ph–CO–Ph
Ph–CH2OH
Ph–CH(OH)–Ph
98
4
98
7
5
Ph–CH2CHO
cyclohexanone
Ph–CH2CH2OH
cyclohexanol
96
6
97
7
a
Yield determined by GC
dehydes and amines at ambient temperature under solvent-
free conditions [21]. N-(ꢀ-Methylbenzylidene)benzylamine
was prepared from reaction of ketone and amine according
to Ref. [22].
The reactions were monitored by TLC using silica gel plates
and the products were purified by flash column chromatogra-
phy on silica gel (Merck; 230–400 mesh) and were identified
1
by comparison of their IR and H NMR spectra and physical
1
data with those of authentic samples. H NMR spectra were
measured at 90MHz with a JEOL JNM-EX 90 spectrometer
with tetramethylsilane as an internal reference and CDCl3 as
solvent. IR spectra were recorded with a Pye-unicam SP 1100
spectrophotometer. Gas chromatography was recorded on a
Perkin Elmer clarus 500 GC instrument. Elemental analyses
were performed on a LECO 250 instrument; their results
agreed favorably with the calculated values.
Scheme 3
imines. Another interesting feature of this method
is that ionene borohydrides could be regenerated to
their initial activity. For this purpose, the collected
spent polymers were neutralized with HCl (10%)
and then treated with a fresh alkaline solution of
NaBH4 (Scheme 3).
In conclusion we developed new and efficient high
loading linear polymer-supported reducing agents,
2,4-ionene and xylylene ionene borohydrides, for re-
duction of aromatic and aliphatic aldehydes, ketones,
acid chlorides, and imines. The advantages of the
present reagent in terms of ease of preparation and
reusability of supported polymers, work-up proce-
dure, and high capacity of borohydride make this
protocol a valuable alternative to the existing poly-
meric reductants.
Preparation of Ionene-Bound Borohydrides
An alkaline solution of sodium borohydride was prepared
from 1.51g NaBH4 (0.04 mol) in 10cm3 5 M NaOH solution.
Then, an aqueous solution of 2,4-ionene bromide [17] (5.00 g
in 10cm3 H2O) or xylylene ionene chloride [18] (4.00 g in
20cm3 H2O) was added drop-wise to the above solution at
room temperature and stirred for 2 h. The resulting white solid
was filtered off, washed with ethanol:ether (1:1), and dried in a
vacuum deꢁsiccator over CaCl2. The content of active reducing
agent BH4 was determined by the titrimetric method [16].
General Procedure for Reduction of Aldehydes
In a 25 cm3 round-bottomed flask equipped with a magnetic
stirrer, a solution of 1 mmol aldehyde in 10cm3 methanol was
prepared. The ionene borohydride agent (1mmol) was added
and the mixture was stirred at room temperature. Progress of
the reaction was monitored by TLC. On completion of the
reaction, the spent polymer was removed by filtration. The
solvent was evaporated and the resulting crude material puri-
fied by column chromatography on silica gel (n-hexane:ethyl
acetate ¼ 5:1) to afford the pure alcohol.
Experimental
Materials were purchased from Fluka and Merck companies.
Aldimines were prepared from stirring a 1:1 mixture of al-

122
M. M. Lakouraj et al.
General Procedure for Reduction of Ketones
References
In a 25 cm3 round-bottomed flask equipped with a magnetic
stirrer and condenser, a solution of 1 mmol ketone in 10cm3
i-PrOH was prepared. The polymeric reducing agent (2mmol)
was added and the reaction mixture was stirred magnetically
under reflux condition for appropriate times (Table 2). Prog-
ress of the reaction was monitored by TLC. On completion of
the reaction, the spent polymer was removed by filtration. The
solvent was evaporated and the resulting crude material puri-
fied by column chromatography on silica gel to (n-hexa-
ne:ethyl acetate ¼ 5:1) to afford the pure alcohol.
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General Procedure for Reduction of Acid Chlorides
Ionene borohydride (4mmol) was suspended in THF in pres-
ence of the respective acid chloride (1mmol) during a period
of 15 min under reflux and stirring. Then 8 cm3 methanol were
added and stirred at reflux for specified time (Table 3). The
reactions were monitored by TLC using mixtures of ethyl
acetate=n-hexane as eluent. On completion of the reaction,
the spent polymer was removed by filtration. The solvent
was evaporated and the resulting crude material was purified
by column chromatography on silica gel (n-hexane:ethyl
acetate ¼ 8:1) to give the pure alcohols in 88–95% yields.
General Procedure for Reduction of Imines
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A mixture of 1 mmol imine, 10 cm3 methanol, and 3 mmol
ionene borohydride was placed in a 25cm3 round bottomed
flask and stirred at room temperature. After completion of the
reaction (monitored by TLC), the spent polymer was removed
by filtration. The solvent was evaporated and the resulting crude
material was purified by column chromatography on silica gel
(n-hexane:ethyl acetate ¼ 10:1) to give the pure products.
General Procedure for Competitive Reduction
of Aldehydes and Ketones
In a 25 cm3 round-bottomed flask, a solution of 1 mmol alde-
hyde and 1 mmol ketone in 10 cm3 methanol was prepared.
The polymeric reducing agent (1mmol) was added and the
mixture stirred magnetically at room temperature for specified
times (Table 5). Progress of the reaction was monitored by
TLC. After completion of the reaction, the spent polymer was
removed by filtration. The solvent was evaporated and the
resulting crude materials were analyzed by GC.
Regeneration Procedure for Ionene Borohydrides
After completion of the reduction reaction and isolation of the
product, the spent polymer was separated and dissolved in
water, and neutralized with HCl (10%). Upon addition of
freshly prepared alkaline solution of sodium borohydride to
this solution at room temperature and stirring for 2 h, ionene
borohydride precipitated. The resulting solid was filtered off,
washed with ethanol:ether (1:1), and dried in a vacuum desic-
cator over CaCl2.
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Chem 151: 207
Acknowledgements
[18] Lattle DA, Jensen EH, Struck WA (1952) Anal Chem 24:
1843
Financial support of this work from the Research Council of
Mazandaran University is gratefully acknowledged.

Ionene-Bound Borohydrides
123
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