Mild and convenient method for reduction of carbonyl compounds with the NaBH4/charcoal system in wet THF

Article abstract of DOI:10.1515/znb-2006-1014

The NaBH₄/C (charcoal) system reduces a variety of carbonyl compounds such as aldehydes, ketones, acyloins and α-diketones to their corresponding alcohols in high to excellent yields. Reduction reactions were carried out in wet THF at r.t. In addition, regioselective 1,2-reduction of α,β-unsaturated aldehydes and ketones was achieved perfectly with this reducing system. By decreasing the amount of aprotic solvent, all reductions took place fast and efficiently under solid-gel condition.

Full text of DOI:10.1515/znb-2006-1014

Mild and Convenient Method for Reduction of Carbonyl Compounds
with the NaBH4/Charcoal System in Wet THF
Davood Setamdideh and Behzad Zeynizadeh
Department of Chemistry, Faculty of Sciences, Urmia University, Urmia 57159-165, Iran
Reprint requests to Prof. B. Zeynizadeh. E-mail: bzeynizadeh@gmail.com
Z. Naturforsch. 61b, 1275 – 1281 (2006); received April 11, 2006
The NaBH4/C (charcoal) system reduces a variety of carbonyl compounds such as aldehydes,
ketones, acyloins and α-diketones to their corresponding alcohols in high to excellent yields. Reduc-
tion reactions were carried out in wet THF at r. t. In addition, regioselective 1,2-reduction of α,β-
unsaturated aldehydes and ketones was achieved perfectly with this reducing system. By decreasing
the amount of aprotic solvent, all reductions took place fast and efficiently under solid-gel condition.
Key words: Reduction, NaBH4, Charcoal, Carbonyl Compounds
Introduction
fer hydrogenation) have been widely used for the re-
duction of functional groups in organic synthesis [1, 3].
In addition, we found that the application of NaBH4 in
the presence of Pd/C has been only reported for the
reduction of nitro compounds to their corresponding
amines [4]. Although in the reported methods a mix-
ture of transition metals and carbon showed an activity
towards reduction, the literature did not show any ap-
plication for the use of carbon alone as a promoter in
combination with NaBH4. This subject and our contin-
uous efforts to explore the synthetic utilities of mod-
ified borohydride agents [5] encouraged us to inves-
tigate reduction of functional groups by the combina-
tion system of NaBH4/C (charcoal) under wet condi-
tion. Herein, in a preliminary report, we wish to intro-
duce a mild and convenient method for reduction of a
variety of carbonyl compounds such as aldehydes, ke-
tones, α,β-unsaturated carbonyl compounds, acyloins
and α-diketones with the NaBH4/charcoal system in
wet THF at r. t.
Reduction is one of the most fundamental and use-
ful reactions in organic synthesis. The discovery of
NaBH4 in the 1940s has provided an efficient route
for the reduction of functionalized molecules and it
is commonly used in organic laboratory nowadays [1].
However, in spite of its great convenience, this reagent
suffers from some limitations: NaBH4 is a remark-
ably mild reducing agent and reduces only aldehy-
des and ketones in protic solvents. In addition, the
rate of reduction is sometimes slow and the chemo-
and regioselectivity of the reduction of carbonyl com-
pounds is decreased in alcoholic solvents. This sit-
uation made it desirable to develop means for con-
trolling the reducing power of boronate reagents. In
fact, advances have been realized by: (i) substitution
of the hydride(s) with electron releasing or withdraw-
ing groups, (ii) change of the sodium cation to an-
other metal, quaternary ammonium and phosphonium
cations, (iii) concurrent cation and hydride exchange,
(iv) use of amino and phosphanyl ligands to alter the
behavior of the metal hydride, (v) combination of boro-
hydrides with metals, metal salts, Lewis acids, mixed
solvent systems and some other additives, and (vi) fi-
nally use of polymers and solid supports for supporting
the borohydride species. In this context, the applica-
tions of modified borohydride agents in organic syn-
thesis have been reviewed extensively [2].
Results and Discussion
Supporting of the transition metals on carbon (char-
coal) and their use as catalysts in conjunction with
molecular hydrogen or ammonium formate in the re-
duction of functional groups motivated us to investi-
gate whether charcoal alone is a good promoter in the
reduction of carbonyl compounds with NaBH4. To in-
The literature review shows that the combination vestigate the influence of charcoal on the rate of reduc-
systems of H2 or HCOONH4 with Pd, Pt, Ru and Rh tions, we performed a set of experiments on the reduc-
supported on carbon (catalytic hydrogenation or trans- tion of benzaldehyde as a model compound by NaBH4
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D. Setamdideh – B. Zeynizadeh · Reduction of Carbonyl Compounds with NaBH4/Charcoal
Table 1. Reduction of ben-
zaldehyde and benzophenone
with the NaBH4/C system un-
der different conditionsa.
Entry Reaction components
Molar ratio
Solvent
Time Conversion
[min]
90
100
5 h
20
[%]
100
100
40
100
100
100
100
30
50
40
100
100
1
2
PhCHO/NaBH4 (1 : 1)
PhCHO/NaBH4 (1 : 1)
THF (2 ml)
CH3CN (2 ml)
CH2Cl2 (2 ml)
THF-H2O (2 : 0.1 ml)
THF (2 ml)
3
PhCHO/NaBH4 (1 : 1)
a
4
5
6
PhCHO/NaBH4 (1 : 0.5)
PhCHO/NaBH4/C (1 : 1 : 1)
PhCHO/NaBH4/C (1 : 0.5 : 1)
All reactions were carried out at
70
2
r. t.
THF-H2O (2 : 0.1 ml)
7
PhCHO/NaBH4 (1 : 0.5) + Charcoal (0.4g) THF-H2O (1 : 0.5 ml)
< 1
6 h
6 h
6 h
75
8
PhCOPh/NaBH4 (1 : 2)
THF (2 ml)
THF-H2O (2 : 0.1 ml)
THF (2 ml)
THF-H2O (2 : 0.1 ml)
THF-H2O (1 : 0.5 ml)
9
PhCOPh/NaBH4 (1 : 2)
10
11
12
PhCOPh/NaBH4/C (1 : 2 : 2)
PhCOPh/NaBH4/C (1 : 2 : 2)
PhCOPh/NaBH4 (1 : 2) + Charcoal (0.4g)
15
(Table 1). Our experiments showed that the reduction efficient solvent for the reduction. To clarify the influ-
of PhCHO with one molar equivalent of NaBH4 in the ence of charcoal under wet conditions, we performed
absence or presence of one molar equivalent of char- reduction of benzophenone with two molar equivalents
coal in THF (the more efficient aprotic solvent) took of NaBH4 and charcoal in THF (2 ml) at r. t. for 6 h
place within 70 – 90 min at r. t. However, we found (Table 1, entry 10). The reaction had 40% conversion
that by adding a few drops of water to the reaction and the optimization experiments showed that the pres-
mixture, the rate of reaction was extremely acceler- ence of charcoal or water alone had little influence
ated with the evolution of hydrogen gas and it was on the rate of reduction. However, the presence of a
completed in 2 min. The reaction conditions were op- small amount of water showed a great synergistic ef-
timized and they resulted in using the molar ratio of fect and completed the reduction in a shorter reaction
PhCHO/NaBH4/charcoal (1 : 0.5 : 1) in a mixture of time (75 min). The synthetic utility of the NaBH4/C
THF-H2O (2 : 0.1 ml) at r. t. as optimal for the com- system in wet THF was further examined with the re-
plete conversion (entry 6).
duction of a variety of aliphatic and aromatic ketones.
All ketones were reduced effectively to their corre-
sponding secondary alcohols within 5 – 200 min at r. t.
(Table 3) (method A). In order to perform reductions
using less organic solvent, we also accomplished the
reactions in a solid-gel media with two molar equiva-
lents of NaBH4 in the presence of charcoal (0.4 g) in
THF-H2O (1 : 0.5 ml) at r. t. and the product alcohols
were obtained within 2 – 20 min in 83– 98% (Table 3,
method B).
Transformation of acyloins and α-diketones to vic-
inal diols is a subject of interest in organic syn-
thesis. This achievement was examined with the
NaBH4/charcoal system in wet THF. 1.5 – 2 mo-
lar equivalents of NaBH4 in the presence of char-
coal (two molar equivalents) reduce acyloins and
We applied the optimal conditions for the reduction
of a variety of aromatic and aliphatic aldehydes to their
corresponding primary alcohols. As shown in Table 2,
the product alcohols were obtained in high to excellent
yields within 2 – 6 min (method A). In another attempt,
in order to minimize the amount of aprotic solvent,
we performed the reduction of benzaldehyde (Table 1,
entry 7) and other aldehydes (Table 2) in a solid-gel
media with 0.5 molar equivalents of sodium borohy-
dride in the presence of 0.4 g of charcoal in THF-H2O
(1 : 0.5 ml) (method B). The results show that the reac-
tions were completed faster than by method A within
1 – 2 min. In the latter case, the corresponding alco-
hols were also obtained in high to excellent yields (88–
98%).
At the next attempt, we turned our attention towards α-diketones efficiently to their corresponding vici-
reduction of ketones by the NaBH4/charcoal system nal diols in THF-H2O (2 : 0.1 ml) at r. t. (Table 4,
with benzophenone as a model compound. Inherent method A). Under solid-gel condition these reac-
low reactivity of ketones relative to aldehydes led us tions were also carried out, using charcoal (0.4 g)
to perform the reduction with two molar equivalents and 1 – 1.5 molar equivalents of NaBH4 in THF-H2O
of NaBH4 and charcoal in a mixture of THF-H2O (1 : 0.5 ml) at r. t. (Table 4, method B). In both meth-
(2 : 0.1 ml) at r. t. (Table 1, entry 11). The reaction was ods, the vicinal diols were obtained in high to ex-
completed within 75 min and benzhydrol was obtained cellent yields. All attempts to carry out the reduction
in 96% yield. In this case wet THF was also a more of α-diketones into acyloins were unsatisfactory and
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D. Setamdideh – B. Zeynizadeh · Reduction of Carbonyl Compounds with NaBH4/Charcoal
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Table 2. Reduction of aldehydes with the NaBH4/charcoal system in wet THFa.
◦
Substrate
Product
Method
Molar ratio
Time Yield
M. p. or B. p. [ C]
Entry
NaBH4/Subs. [min] [%]b found
reported
(A)
(B)
0.5 : 1
0.5 : 1
2
< 1
94
92
204 – 205
2057
1
2
3
(A)
(B)
0.5 : 1
0.5 : 1
3
< 1
92
94
70 – 71
236
70 – 727
(A)
(B)
0.5 : 1
0.5 : 1
3
95
96
2377
< 1
4
(A)
(B)
0.5 : 1
0.5 : 1
3
< 1
94
96
56 – 58
55 – 587
(A)
(B)
0.5 : 1
0.5 : 1
6
2
93
94
60 – 61
259
59 – 617
2597
5
6
7
8
(A)
(B)
0.5 : 1
0.5 : 1
6
2
92
92
(A)
(B)
0.5 : 1
0.5 : 1
3
94
95
119 – 122
118 – 1227
83 – 857
< 1
(A)
(B)
0.5 : 1
0.5 : 1
6
2
92
90
84 – 85
(A)
(B)
0.5 : 1
0.5 : 1
2
90
93
9
31 – 32
93 – 94
30 – 327
92 – 947
< 1
(A)
(B)
0.5 : 1
0.5 : 1
2
< 1
97
94
10
11
(A)
(B)
0.5 : 1
0.5 : 1
2
98
96
165/16 mmHg 165/16 mmHg7
< 1
(A)
(B)
0.5 : 1
0.5 : 1
6
2
95
94
12
113 – 115
169 – 170
113 – 1157
1707
(A)
(B)
0.5 : 1
0.5 : 1
2
< 1
89
88
13
14
(A)
(B)
0.5 : 1
0.5 : 1
2
< 1
95
98
62 – 63
61 – 637
(A)
(B)
0.5 : 1
0.5 : 1
2
< 1
90
90
225 – 226
176
225 – 2267
1767
15
(A)
(B)
0.5 : 1
0.5 : 1
2
83
88
CH3(CH2)5CHO
CH3(CH2)5CH2OH
16
< 1
a
Method A: the reactions were carried out in the presence of one molar equivalent of charcoal in THF-HO (2 : 0.1 ml) at r. t.; Method B:
2
the reactions were carried out in the presence of 0.4 g of charcoal in THF-H O (1 : 0.5 ml) at r. t. under solid-gel conditions; b yields refer to
2
isolated pure products.
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D. Setamdideh – B. Zeynizadeh · Reduction of Carbonyl Compounds with NaBH4/Charcoal
Table 3. Reduction of ketones with the NaBH4/charcoal system in wet THFa.
◦
Entry
1
Substrate
Product
Method
Molar ratio Time Yield
M. p. or B. p. [ C]
NaBH4/Subs. [min] [%]b found
reported
(A)
(B)
2 : 1
2 : 1
75
15
96
92
66 – 67
65 – 677
(A)
(B)
(A)
(B)
(A)
(B)
(A)
(B)
(A)
(B)
(A)
(B)
(A)
(B)
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
1.5 : 1
2 : 1
2 : 1
100
10
95
93
95
96
98
94
93
95
94
94
92
92
95
98
2
3
4
5
6
7
8
204/745 mmHg 204/745 mmHg7
36 – 38
38 – 388
95 – 96/1 mmHg 94 – 96/1 mmHg9
30
10
200
18
160
15
219 – 220
68 – 69
–
218 – 2208
67 – 699
–
200
20
25
5
100
10
80 – 82/1 mmHg 80 – 82/1 mmHg8
(A)
(B)
2 : 1
2 : 1
180
20
95
95
9
10
11
52 – 54
50 – 547
(A)
(B)
2 : 1
2 : 1
20
10
96
97
153 – 154
163 – 166
153 – 1547
163 – 1667
(A)
(B)
2 : 1
1 : 1
15
2
88
91
(A)
(B)
2 : 1
1 : 1
10
2
85
83
12
13
14
160 – 161
57 – 58
160 – 1617
5810
(A)
(B)
(A)
(B)
(A)
(B)
2 : 1
1 : 1
2 : 1
1 : 1
2 : 1
1 : 1
15
2
5
94
93
90
91
84
87
115/749 mmHg 115/749 mmHg7
132
1327
< 1
5
15
< 1
a
Method A: the reactions were carried out in the presence of two molar equivalent of charcoal in THF-HO (2 : 0.1 ml) at r. t.; Method B:
2
the reactions were carried out in the presence of 0.4 g of charcoal in THF-H O (1 : 0.5 ml) at r. t. under solid-gel condition; b yields refer to
2
isolated pure products.
only vicinal diols were recovered from the reaction THF. The reduction was carried out with the molar
mixture.
ratio of NaBH4/C of 0.5 : 1 in THF-H2O (2 : 0.1 ml)
Regioselective 1,2-reduction of α,β-unsaturated at r. t. (Table 5, entry 1, method A). The result shows
aldehydes and ketones to give allylic alcohols is that the product, cinnamyl alcohol, was obtained per-
a widely used transformation in organic synthesis fectly by this reducing system. This protocol was also
which is achieved with various hydride transferring successful in the reduction of citral and methacrolein
agents [1]. Reduction of conjugated carbonyl com- within 2 – 10 min at r. t. (Table 5, entries 4, 6). The
pounds with sodium borohydride is highly solvent corresponding allyl alcohols were obtained regios-
dependent and generally does not result in a use- electively in 85 – 92% yields. Reduction of conju-
ful regioselectivity [6]. The regioselectivity of the gated enones took place with two molar equivalents
NaBH4/charcoal system was examined with the reduc- of NaBH4 and charcoal in a mixture of THF-H2O
tion of cinnamaldehyde as a model compound in wet (2 : 0.1 ml) at r. t. Benzalacetone, benzalacetophenone,
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D. Setamdideh – B. Zeynizadeh · Reduction of Carbonyl Compounds with NaBH4/Charcoal
Table 4. Reduction of acyloins and α-diketones with the NaBH4/charcoal system in wet THFa.
1279
◦
M. p. or B. p. [ C]
Entry
Substrate
Product
Method
Molar ratio
NaBH4/Subs. [min]
Time
Yield
[%]b
found
reported
(A)
(B)
2 : 1
10
5
95
95
1
2
135 – 137
135 – 137
134 – 1377
1.5 : 1
(A)
(B)
1.5 : 1
1 : 1
7
3
93
92
134 – 1377
(A)
(B)
2 : 1
1.5 : 1
12
6
92
93
3
4
–
–
(A)
(B)
1.5 : 1
1 : 1
5
3
94
94
–
–
(A)
(B)
2 : 1
1.5 : 1
20
5
95
98
5
63 – 64
63 – 6411
a
Method A: the reactions were carried out in the presence of two molar equivalents of charcoal in THF-HO (2 : 0.1 ml) at r. t.; Method B:
2
the reactions were carried out in the presence of 0.4 g of charcoal in THF-H O (1 : 0.5 ml) at r. t. under solid-gel conditions; b yields refer to
2
isolated pure products.
Table 5. Reduction of conjugated carbonyl compounds with the NaBH4/charcoal system in wet THFa.
◦
Entry
Substrate
Product
Method
Molar ratio
Time Yield
M. p. or B. p. [ C]
reported
NaBH4/Subs. [min] [%]b found
(A)
(B)
0.5 : 1
0.5 : 1
2
< 1
92
93
1
2
3
249 – 250
25012
(A)
(B)
2 : 1
2 : 1
40
10
96
96
39 – 41
56 – 57
39 – 4111
55 – 5712
(A)
(B)
2 : 1
2 : 1
25
10
98
95
(A)
(B)
0.5 : 1
0.5 : 1
10
2
92
92
4
5
231 – 232
231 – 23212
(A)
(B)
2 : 1
30
10
91
94
107/3 mmHg 107/3 mmHg12
1.5 : 1
(A)
(B)
0.5 : 1
0.5 : 1
2
85
85
6
7
113 – 115
96 – 97
113 – 1157
96 – 977
< 1
(A)
(B)
2 : 1
2
85
87
1.5 : 1
< 1
a Method A: the reactions were carried out with 1 and 2 molar equivalents of charcoal in THF-HO (2 : 0.1 ml) at r. t.; Method B: the reactions
2
were carried out in the presence of 0.4 g of charcoal in THF-H O (1 : 0.5 ml) at r. t. under solid-gel conditions; b yields refer to isolated pure
2
products.
β-ionone and methyl vinyl ketone were the examples
The exact mechanism of this protocol is not clear,
which were reduced by the NaBH4/C system with per- however, we think that the two following factors may
fect regioselectivity and efficiency in wet THF. Under play a role in the reductions: (i) Charcoal is a very
solid-gel conditions, the reductions also took place fast finely porous material and the use of sodium borohy-
and regioselectively with high yields of products (Ta- dride in the presence of water makes it to finely dis-
ble 5, method B).
perse on the surface of charcoal and subsequently more
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D. Setamdideh – B. Zeynizadeh · Reduction of Carbonyl Compounds with NaBH4/Charcoal
interaction with the substrate shows higher efficiency ples (melting or boiling points). Organic layers were dried
over anhydrous sodium sulfate. All yields refer to isolated
pure products. TLC (silica gel 60 F254 aluminum sheet) was
applied for the purity determination of substrates, products
and reaction monitoring.
in the reductions. (ii) The hydrolysis of the alkoxy-
borate intermediate produced in the reaction mixture
in the presence of water could promote the reductions.
Conclusions
Reduction of carbonyl compounds with the NaBH4/charcoal
system in wet THF; typical procedure
In this investigation, we have shown that the combi-
nation system of NaBH4/charcoal in wet THF reduces
a variety of carbonyl compounds to their correspond-
ing alcohols in high to excellent yields. Reduction re-
actions were carried out with 0.5 – 2 molar equivalents
of NaBH4 in the presence of 1 – 2 molar equivalents
of charcoal in THF-H2O (2 : 0.1 ml) (method A). By
lowering the amount of aprotic solvent and increasing
that for charcoal (0.4 g) (method B) and performing the
reactions in a solid-gel media, we observed faster re-
duction rates. The combination system of NaBH4/C in
the case of conjugated aldehydes and ketones showed
a perfect regioselectivity and efficiency. Reduction of
acyloins and α-diketones by the NaBH4/C system also
produced efficiently the corresponding vicinal diols
in wet THF. All reductions were accomplished at r. t.
and therefore, under the aspects of the availability and
cheapness of the reagents, high efficiency and regiose-
lectivity of the reductions, shorter reaction times, easy
work-up procedure, this new protocol for NaBH4 re-
duction of carbonyl compounds could be a useful ad-
dition to the present methodologies.
Method A: In a round-bottomed flask (10 ml) equipped
with a magnetic stirrer, a solution of benzaldehyde (0.106 g,
1 mmol) in THF (2 ml) was prepared. To this solution,
NaBH4 (0.019 g, 0.5 mmol) and then charcoal (0.012 g,
1 mmol) was added and the mixture was stirred at r. t.
for 30 sec. Afterwards, H2O (0.1 ml) was added and the
mixture was stirred at r. t. for 2 min. Completion of the re-
action was monitored by TLC (eluent; CCl4/Et2O: 5/2). Wa-
ter (5 ml) was then added to the reaction mixture and it
was stirred for another 5 min. The mixture was extracted
with CH2Cl2 (3 × 6 ml) and dried over anhydrous Na2SO4.
Evaporation of the solvent and a short column chromatog-
raphy of the resulting crude material over silica gel (elu-
ent; CCl4/Et2O: 5/3) afforded the pure liquid benzyl alcohol
(0.102 g, 94%, Table 2).
Method B: In a round-bottomed flask (10 ml) equipped
with a magnetic stirrer, a solution of benzophenone (0.182 g,
1 mmol) in THF (1 ml) was prepared. To this solution,
H2O (0.5 ml) and charcoal (0.4 g) was added and the pre-
pared solid-gel mixture was stirred at r. t. for 1 min. After-
wards, NaBH4 (0.076 g, 2 mmol) was added and the mix-
ture was stirred for another 15 min. The progress of the re-
action was monitored by TLC (eluent; CCl4/Et2O: 5/2). At
the end of the reaction, the mixture was filtered and washed
with CH2Cl2 (3 × 6 ml). The filtrate was dried over anhy-
drous Na2SO4; evaporation of the solvent and a short column
chromatography of the resulting crude material over silica
gel (eluent; CCl4/Et2O: 5/3) gave white crystalline benzhy-
drol (0.169 g, 92%, Table 3).
Experimental Section
General
All substrates and reagents were purchased from com-
mercially sources with the best quality and used without
further purification. Charcoal was used in activated form
(Merck). IR and 1H NMR spectra were recorded on Thermo
Nicolet Nexus 670 FT-IR and Bruker 300 MHz spectrome-
ters, respectively. The products were characterized by their
1H NMR or IR spectra and comparison with authentic sam-
Acknowledgement
The authors gratefully appreciated the financial support of
this work by the research council of Urmia University.
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Unauthenticated
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