Reduction of carbonyl compounds with NaBH4 under ultrasound irradiation and aprotic condition

Article abstract of DOI:10.1515/znb-2004-0612

A variety of carbonyl compounds are reduced to their corresponding alcohols with sodium borohydride under ultrasound irradiation and aprotic condition. Reduction reactions are performed in THF at room temperature or under reflux condition. The product alcohols were obtained in good to excellent yields. The chemoselective reduction of aldehydes over ketones was achieved successfully with this system.

Full text of DOI:10.1515/znb-2004-0612

Reduction of Carbonyl Compounds with NaBH4 under Ultrasound
Irradiation and Aprotic Condition
Behzad Zeynizadeh and Saiedeh Yahyaei
Department of Chemistry, Faculty of Sciences, Urmia University, Urmia 57159-165, Iran
Reprint requests to Prof. Dr. B. Zeynizadeh. E-mail: b.zeynizadeh@mail.urmia.ac.ir
Z. Naturforsch. 59b, 704 – 710 (2004); received January 28, 2004
A variety of carbonyl compounds are reduced to their corresponding alcohols with sodium borohy-
dride under ultrasound irradiation and aprotic condition. Reduction reactions are performed in THF at
room temperature or under reflux condition. The product alcohols were obtained in good to excellent
yields. The chemoselective reduction of aldehydes over ketones was achieved successfully with this
system.
Key words: Reduction, Sodium borohydride, Ultrasound, Carbonyl Compounds
Introduction
benzylbicyclo[2.2.2]octylammonium tetrahydroborate
[24] and Ph3PMeBH4 [25]. g) Use of the polymers
or solid supports to supporting hydride species: am-
berlyst, zeolite, alumina, silica gel and polyvinylpyri-
dine are usually used as bed supports for different hy-
dride species [26]. Recently, we have extensively re-
viewed modified hydroborate agents and their applica-
tions in organic synthesis [18, 27]. In addition, several
recent reports extol the virtues of sonication as a well
known tool to facilitate a variety of synthetic method-
ologies [28].
Along the outlined strategies and our continuous
efforts for the preparation and application of modi-
fied hydroborate systems in organic synthesis [17 –
19, 29], we decided to investigate the reducing poten-
tial of NaBH4 for reduction of carbonyl compounds
under ultrasound irradiation and aprotic condition with
the hope that this combination system shows a good
or excellent selectivity and efficiency. Herein, we de-
scribe an efficient and convenient method for the pro-
moted reduction of carbonyl compounds such as alde-
hydes and ketones to their corresponding alcohols with
NaBH4 under ultrasound irradiation.
During the past decades, sodium borohydride has
had a significant impact on modern organic synthe-
sis [1]. This reagent is a relatively mild reducing agent
and mostly used for the reduction of aldehydes and ke-
tones in protic solvents. In order to control the reduc-
ing power of NaBH4, hundreds of substituted boron
hydrides have been introduced in chemical literature
and many of them are now commercially available.
In fact, advances in such a field have been realized
by: a) Substitution of the hydride(s) with other sub-
stituents which may exert marked steric and elec-
tronic influences upon the reactivity of the substituted
complex ion, e.g., NaBH3(OAc) [2], NaBHEt3 [3],
NaBH3CN [4] and NaBH2S3 [5]. b) Variation of alkali-
metal cation and metal cation in the hydride com-
plex, e.g., LiBH4 [6], Cu(BH4)2 [7], Zn(BH4)2 [8].
c) A concurrent cation and hydride exchange, such
as LiBHEt3 (Super Hydride) [9], LiBH(n-butyl) [10],
LiBH(sec-butyl)3 (L-Selectride) [11, 12], KBH(sec-
butyl)3 (K-Selectride) [12], Ca(BH2S3)2 [13a] and
Ba(BH2S3)2 [13b]. d) Use of the ligands to alter be-
havior of the metal hydroborates; (Ph3P)2CuBH4 [14],
(Ph3P)xZn(BH4)2 (x = 1,2) [15], (i-PrO)2TiBH4 [16],
(dabco)Zn(BH4)2 [17], (bpy)Zn(BH4)2 [18], and
(py)Zn(BH4)2 [19] are examples in this area.
e) Combination of tetrahydroborates with Lewis
acids, additives [20] and mixed solvent systems
[21]. f) Changing the cation to quaternary am-
monium and phosphonium tetrahydroborates, such
as Bu4NBH4 [22], PhCH2(Et)3NBH4 [23], 4-aza-N-
Results and Discussion
As mentioned above, sodium borohydride is one of
the most widely utilized reducing agents for the reduc-
tion of aldehydes and ketones in protic solvents espe-
cially methanol or isopropanol [1]. However, it should
be pointed out that in spite of its great convenience,
this reagent generally suffers from limitations such as:
c
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B. Zeynizadeh – S. Yahyaei · Ultrasound Promoted Reduction of Carbonyl Compounds with NaBH4
705
Scheme 2.
Scheme 1.
led us to perform the reduction reactions under reflux
condition in combination with the ultrasound irradia-
tion. THF also is a well suitable solvent for this reac-
tion. To clarify the effect of ultrasound under reflux
condition, we performed the reduction of benzophe-
none with 3 molar equivalents of NaBH4 in absence of
ultrasound irradiation under refluxing THF for 24 h. In
this case, the progress of reduction was 5 – 8% and un-
reacted starting material was recovered from the reac-
tion mixture. When we applied ultrasound irradiation,
the reduction of benzophenone was completed in 2.4 h.
Therefore, to explore the further utility of ultrasound
irradiation for the reduction of ketones, we combined
a variety of aliphatic and aromatic ketones with 2 – 3
molar equivalents of NaBH4 in refluxing THF. The ef-
ficiency of the reactions was good to excellent and only
in the reduction of acetophenone, probably due to its
tendency to enolization, we observed a low efficiency.
The work-up procedure is simple and the same as for
aldehydes. This procedure afforded the product sec-
ondary alcohols which were purified further by short
column chromatography on silica gel (Table 2).
The chemoselective reduction of one functional
group without affecting the other one is a well known
strategy for the preparation of the molecules with ever-
increasing complexity in organic synthesis. This sub-
ject is of great interest [31] and numerous modified hy-
droborate systems have been reported [17– 19, 24, 32].
Since under conditions described above, reduction of
aldehydes and ketones with sodium borohydride and
with ultrasound irradiation is temperature-dependent,
we thought that this system can have a chemoselectiv-
ity towards reduction of aldehydes over ketones. This
fact was demonstrated with the selective reduction of
benzaldehyde in the presence of an equimolar amount
of acetophenone with one equimolar amount of NaBH4
at room temperature for 17 min (Scheme 2).
a) NaBH4 is a mild reducing agent and reduction re-
actions are preferentially carried out in protic solvents,
b) the number of functional groups which are reduced
by this reagent is limited, and c) the rate of reductions
is sometimes slow and low chemoselectivity is accom-
panied with the reductions under protic condition es-
pecially between carbonyl compounds or 1,2- vs. 1,4-
reduction of conjugated enones. This situation made
it desirable to develop means to control the reducing
properties of sodium borohydride. Combination sys-
tems of sodium borohydride have been extensively re-
viewed in the literature, but, as far as we know, for the
use of ultrasound in combination with NaBH4 there is
only one report. This report described the reduction of
two complex pentacyclic compounds of PCUD-8,11-
diones with sodium borohydrideunder protic condition
in MeOH [30].
These facts prompted us to study the reducing po-
tential of NaBH4 for systematic reduction of carbonyl
compounds under ultrasound irradiation and aprotic
condition. Our preliminarily observations and opti-
mization of solvents reveal that performing of the re-
duction of benzaldehyde as a model compound with
NaBH4 in THF at room temperature is strongly accel-
erated under ultrasound irradiation (Scheme 1).
This facilitation with ultrasound waves encouraged
us to subject a variety of structurally different aro-
matic and aliphatic aldehydes with sodium borohy-
dride under the same condition. Reduction reactions
were performed efficiently in THF at room temperature
and completion of the reductions needs to 1 – 2 mo-
lar equivalents of sodium borohydride according to the
nature of substrates. The sonication of reactions was
performed via a micro-tip probe. The efficiency of the
reactions was excellent and the products were obtained
in high to excellent yields. The work-up procedure is
simple and addition of water to the reaction mixture,
then extracting with CH2Cl2 affords the correspond-
ing alcohols for further purification with short column
chromatography on silica gel (Table 1).
The obtained chemoselectivity was excellent and
benzyl alcohol was detected as the sole product of re-
duction besides unchanged acetophenone as an intact
material (Table 3). To further explore the utility, we
examined reduction of benzaldehyde in the presence
We turned our attention into reduction of ketones of other ketones such as benzophenone or cyclohex-
with the experiment of benzophenone as a model com- anone. We observed that the aldehyde was reduced ex-
pound. Low reactivity of ketones relative to aldehydes clusively or nearly so. In another attempt, we applied
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B. Zeynizadeh – S. Yahyaei · Ultrasound Promoted Reduction of Carbonyl Compounds with NaBH4
Table 1. Reduction of aldehydes to their alcohols with NaBH4 under ultrasound irradiationa.
Entry
Substrate
Product
Molar ratio
NaBH4/Subs.
Time
(min)
Yield
(%)b
M.p. or B.p. (◦C)
found
calcd.
1
2
3
4
5
1:1
1:1
17
22
60
94
25
97
98
98
96
95
204 – 205
70 – 71
205 [33a]
70 – 72 [33a]
59 – 61 [33a]
23 – 25 [33a]
118 – 122 [33a]
1.7:1
1.7:1
2:1
60 – 61
23 – 25
119 – 121
6
7
8
9
1:1
1:1
1:1
1:1
20
24
15
30
99
95
94
95
62 – 63
30 – 31
70 – 71
64 – 65
61 – 63 [33a]
30 – 32 [33a]
70 – 72 [33a]
63 – 65 [33a]
10
11
12
13
1:5:1
1:1
22
35
18
6
97
96
99
94
–
–
250/723
236
250/723 [33a]
237 [33a]
110 – 112 [33a]
1:1
1:1
109 – 111
14
15
1.2:1
8
94
–
–
1:1
2:1
5
97
93
–
–
16
17
30
115 – 116
114 – 115 [33b]
1:1
1:1
15
22
93
92
113/12
113/12 [33a]
18
135 – 137
136 – 138 [33a]
a
All reactions were irradiated with ultrasound waves in THF at room temperature;b yields refer to isolated pure products.
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B. Zeynizadeh – S. Yahyaei · Ultrasound Promoted Reduction of Carbonyl Compounds with NaBH4
707
Table 2. Reduction of ketones to their alcohols with NaBH4 under ultrasound irradiationa.
Entry
1
Substrate
Product
Molar ratio
NaBH4/Subs.
Time
(h)
Yield
(%)b
M.p. or B.p. (◦C)
found
calcd.
3:1
2.4
99
66 – 67
65 – 67 [33a]
2
3
3:1
3:1
3
97
99
–
–
–
–
1.8
4
5
2:1
2:1
1.5
2
98
95
154 – 155
153 – 154 [33a]
–
–
6
7
8
2:1
2:1
3:1
0.4
0.9
6
96
93
30
116 – 118
161
118 – 119 [33b,c]
160 – 161 [33a]
204/745 [33a]
203/745
9
2:1
3:1
2
4
99
80
122/14
–
121 – 123/14 [33d]
–
10
11
2:1
2:1
3
97
92
129/16
128 – 130/16 [33e]
115/749 [33a]
12
0.6
114/749
a
All reactions were irradiated with ultrasound waves in refluxing THF;b yields refer to isolated pure products.
Table 3. Competitive reduction of aldehydes and ketones
with NaBH4 under ultrasound irradiationa.
this procedure to the reduction of two ketones such as
9-fluorenone and acetophenone and it was found that
9-fluorenone was reduced exclusively (Table 3).
Entry Substrate 1 Substrate 2 Condi- Molar Time Conv.1/
tion ratiob (min) Conv.2 (%)c
In order to highlight and showing the advan-
tages and limitations of our system, we compared
our results with those achieved by other systems
such as NaBH4/Dowex1-x8 [29], (bpy)Zn(BH4)2 [18],
(py)Zn(BH4)2 [19a], (Ph3P)2Zn(BH4)2 [15], Ph3PMe
BH4 [25], and 4-aza-N-benzylbicyclo[2.2.2] octylam-
monium tetrahydroborate [24] (Table 4). The compari-
son shows that in most cases our system is comparable
or more efficient.
1
2
3
RT 1:1:1 17
RT 1:1:1 18
RT 1:1:1 20
100:0
100:0
100:5
4
Reflux 2:1:1 1.5 h 100:0
a
All reactions were irradiated with ultrasound waves in THF;b mo-
lar ratio as: NaBH4/Subs.1/Subs.2; c conversions refer to TLC mon-
itoring and isolated pure products.
Conclusion
the reduction of a variety of aromatic and aliphatic car-
bonyl compounds with NaBH4. Reduction reactions
Under the defined conditions, we observed that ir-
radiation with ultrasound waves efficiently promoted
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B. Zeynizadeh – S. Yahyaei · Ultrasound Promoted Reduction of Carbonyl Compounds with NaBH4
Table 4. Comparison of reductions of aldehydes an ketones with NaBH4/ultrasound system and other reported reagents.
Entry Substrate
Molar ratio (reag./subs.), Time (h), Yield (%)
I
II [29]
III [18]
IV [19a]
V [15]
VI [25]
VII [24]
1
2
3
1(0.28)(97)
1a(0.05)(96) 0.25(0.02)(95) 1(0.5)(91)
–
1(c)(90)
1(0.25)(90)
1(0.36)(98)
1a(0.15)(99) 0.25(0.08)(98) 1(0.2)(99)
1(c)(88)
1(c)(86)
1(0.23)(90)
2(0.8)(85)
1.7(1.56)(96) 1.5a(3)(99)
0.35(0.17)(99) 1(1.3)(96)
0.25(0.13)(99) 1(0.8)(95)
1(0.17)(89) 1(c)(83)
1.5(c)(100) 1(c)(100)
4
1(0.33)(99)
–
1(0.25)(90)
5
6
3(2.4)(99)
2(0.9)(93)
3b(3.2)(98)
1(0.75)(99)
2(4.3)(97)
–
–
2(21.5)(90)
–
1.5b(1.25)(90) 0.5(0.15)(88) 2(2)(89)
1(1)(95)
1(10)(95)
7
2(1.5)(99)
2b(1.8)(94)
1(1.5)(94)
2(5.3)(98)
2(0.33)(85) 1.6(18)(80)
–
I
NaBH4/Ultrasound; II NaBH4/Dowex1-x8; III [(bpy)Zn(BH4)2]; IV [(py)Zn(BH4)2]; V [(Ph3P)2Zn(BH4)2]; VI Ph3PMeBH4; VII [PhCH2-
(dabco)]BH4; a,b referred to using of 10 and 20 mg of Dowex1-x8 per one mmol of substrate respectively;c immediately.
were performed in THF at room temperature or un-
der reflux condition. The product alcohols were ob-
tained in good to excellent yields. The chemoselective
reduction of aldehydes over ketones was achieved suc-
cessfully with this system. In addition to these advan-
tages, the simple work-up procedure and convenient
reduction reactions under aprotic condition are the rea-
sons that could be making this system an attractive and
additional bench-top reducing system to the present
methodologies.
A typical procedure for reduction of aldehydes to alco-
hols with NaBH4 under ultrasound irradiation: In a round-
bottom flask (10 ml) equipped with magnetic stirrer, NaBH4
(0.037 g, 1 mmol) was added, to the solution of benzaldehyde
(0.106 g, 1 mmol) in THF (5 ml). The stirring reaction mix-
ture was irradiated with ultrasound waves at room tempera-
ture. Sonication was continued for 17 min and the progress
of the reaction (eluent: CCl4/Et2O = 5/2) was monitored by
TLC. At the end of the reaction, distilled water (5 ml) was
added and the reaction mixture was stirred for additional
5 min. The mixture was extracted with CH2Cl2 (3 × 10 ml)
and dried over anhydrous sodium sulfate. Evaporation of the
solvent and short column chromatography of the resulting
crude material over silica gel (elution with CCl4/Et2O = 5/2)
afforded pure liquid benzyl alcohol (0.l05 g, 97% yield, Ta-
ble 1).
Experimental Section
General: Sonication was performed by using a Cole
Palmer high intensity ultrasonic processor (600 W, 20 KHz)
via a micro-tip probe and 30% amplitude. All reagents and
substrates were obtained from commercial sources and used
A typical procedure for reduction of ketones to alcohols
without further purification. THF was dried prior to use with NaBH4 under ultrasound irradiation: In a two necked
by a standard method. The products were characterized by round-bottom flask (10 ml) equipped with a magnetic stir-
a comparison with authentic samples (melting or boiling rer and condenser, to the solution of benzophenone (0.182 g,
points) and their 1H NMR or IR spectra. Organic layers were l mmol) in THF (5 ml), NaBH4 (0.113 g, 3 mmol) was added.
dried with anhydrous sodium sulfate before concentration in The reaction mixture was stirred and irradiated with ultra-
vacuo. All yields refer to isolated pure products. The purity sound waves under reflux condition. Sonication was contin-
determination of the substrates, products and reactions was ued for 2.4 h and TLC monitored the progress of the reaction
accomplished by TLC on silica gel PolyGram SILG/UV 254 (eluent: CCl4/Et2O = 5/2). After completion of the reaction,
plates. Products were purified by column chromatography distilled water (5 ml) was added and the reaction mixture was
packed with silica gel.
stirred for additional 5 min. The mixture was extracted with
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B. Zeynizadeh – S. Yahyaei · Ultrasound Promoted Reduction of Carbonyl Compounds with NaBH4
709
CH2Cl2 (3 × 10 ml) and dried over anhydrous sodium sul- Acknowledgement
fate. Evaporation of the solvent and short column chromatog-
The authors gratefully acknowledge the Research Council
of Urmia University (RCUU) for the support of this work.
raphy of the resulting crude material over silica gel (elution
with CCl4/Et2O = 5/2) afforded pure crystals of benzhydrol
(0.l82 g, 99% yield, Table 2).
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