Sodium Aminodiboranate, a New Reagent for Chemoselective Reduction of Aldehydes and Ketones to Alcohols

Article abstract of DOI:10.1055/a-1479-5008

Sodium aminodiboranate (NaNH ₂(BH ₃) ₂, NaADBH) is a new member of the old borane family, which exhibits superior performance in chemoselective reduction. Experimental results show that NaADBH can rapidly reduce aldehydes and ketones to the corresponding alcohols in high efficiency and selectivity under mild conditions. There are little steric and electronic effects on this reduction.

Full text of DOI:10.1055/a-1479-5008

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Georg Thieme Verlag KG Rüdigerstraße 14, 70469 Stuttgart
2021, 32, A–E
letter
A
en
J. W. et al.
Letter
Synlett
Sodium Aminodiboranate, a New Reagent for Chemoselective
Reduction of Aldehydes and Ketones to Alcohols
Jin Wanga,b
Yu Guoa
Shouhu Lia
O
OH
THF
rt
+
NaNH2(BH3)2
R
R'
R
R'
80–98% yield
39 examples
convenient operation
R = aryl, alkyl
R' = H, aryl, alkyl
Xuenian Chen*a,c
5 min for most aldehydes and simple ketone complexes
a School of Chemistry and Chemical Engineering, Henan Key
Laboratory of Boron Chemistry and Advanced Energy Materials,
Henan Normal University, Xinxiang, Henan 453007,
P. R. of China
xnchen@htu.edu.cn
b College of Chemistry and Chemical Engineering, Mudanjiang
Normal University, Mudanjiang 157011, P. R. of China
c Green Catalysis Center and College of Chemistry, Zhengzhou
University, Zhengzhou 450001, P. R. of China
Corresponding Author
Received: 17.03.2021
Accepted after revision: 10.04.2021
Published online: 10.04.2021
In recent years, considerable effort has been devoted to
the exploration of amine boranes such as ammonia borane
(AB), metal amidoborane (MAB, [M]n+[NH2BH3]n–), which
could be applied as potential hydrogen storage materials
owing to their positively charged hydrogen (H+) and nega-
tively charged hydrogen (H–) are co-existed in the same
molecules, resulting in the hydrogen release under mild
conditions.8 Consequently, they can serve as powerful re-
ductants and are widely used in the reduction of organic
compounds.9 Reductions of aldehydes and ketones using AB
and MAB were first reported in 1980 and 2012.10a–c,11 It is
found that the reaction time of ketones with AB or the de-
rivatives of AB is longer than with THF·BH3 or even no reac-
tion proceeds, suggesting that AB and its derivatives show
much gentler reducibility.
DOI: 10.1055/a-1479-5008; Art ID: st-2021-u0104-l
Abstract Sodium aminodiboranate (NaNH2(BH3)2, NaADBH) is a new
member of the old borane family, which exhibits superior performance
in chemoselective reduction. Experimental results show that NaADBH
can rapidly reduce aldehydes and ketones to the corresponding alcohols
in high efficiency and selectivity under mild conditions. There are little
steric and electronic effects on this reduction.
Key words sodium aminodiboranate, hydrogenation, reduction, alde-
hydes, ketones
Aliphatic alcohols and their derivatives are widely pres-
ent in nature and also are important building blocks in
many synthetic transformations, pharmaceuticals, and ma-
terial syntheses. To directly reduce carbonyl compounds to
alcohols by using borane reagents, such as B2H6, THF·BH3,
and (CH3)2S·BH3, is one of the important ways, which has
been investigated since the middle of the last century.1
However, these reactions are somehow limited because
these boranes are sensitive to air and moisture.2 Thus, met-
al salts of borohydride, for example, NaBH4,3 LiBH4,4 and
While the BH3 Lewis acid–base adducts (L·BH3) and
their derivatives were widely applied as reducing reagents
for the hydroboration reactions, the investigation on
diborane complexes is relatively little. One of the typical
representatives of diborane complexes is aminodiboranate
–
anion (NH2(BH3)2 , ADBH–), whose ammonium salt
([NH4][NH2(BH3)2]) is proposed as one of the possible struc-
tural formulas of the diammoniate of diborane (DADB) in
1936.12 Its sodium salt (NaNH2(BH3)2, NaADBH) was firstly
synthesized through the reaction of NaNH2 or Na with AB,13
then the synthetic methods have been improved and its re-
activity have been explored.8a–c,14 However, the reducing
ability of NaADBH in organic reductions has not been re-
ported yet. Very recently, we have studied the reaction of
NaADBH with carboxylic acid and investigated the reaction
mechanism experimentally and theoretically, as well as
found that the reducing capability of NaADBH is between
that of NaBH4 and LiAlH4.15 Considering six negatively
charged hydrogen atoms involved in NaADBH and its rela-
tive high stability in tetrahydrofuran (THF) solution, it is ex-
pected that NaADBH will exhibit interesting features in the
5
Zn(BH4)2 are widely used as important reductants in or-
ganic chemistry in considering their relative high stability.
Nevertheless, reduction of ,-unsaturated aldehydes and
ketones by metal salts of borohydride generally gives three
possible products, such as allylic alcohols, saturated car-
bonyl compounds, and saturated alcohols.6 Recently, pina-
colborane (HBpin) or catecholborane (HBcat) are found effi-
cient for the reduction of C=O double bonds,7 yet the reac-
tion time is longer than other hydroboration. Therefore,
additional reagents are required for these reactions in order
to obtain a higher chemoselectivity.6
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Georg Thieme Verlag KG, Rüdigerstraße 14, 70469 Stuttgart, Germany

B
J. W. et al.
Letter
Synlett
reduction of carbonyl compounds. Furthermore, it is of par-
ticular interests to explore the chemoselectivity of NaADBH
as a reducing reagent.
action yields declined significantly (Table 3, entries 4, 5),
suggesting that reactivity of the negatively charged hydro-
gen atoms in NaADBH can be affected by substrates. It is
mainly due to that the stronger steric hindrance of carbonyl
in ketone prevents the boron atom from interacting with
oxygen atom to induce the reduction. The ratio of 3:1 of
carbonyl group to NaADBH was selected for the reaction ac-
cording to Table 2 and Table 3.
To investigate the reduction of NaADBH to aldehydes
and ketones, cinnamic aldehyde 25 was selected as the
model carbonyl compounds to establish broadly applicable
hydroboration conditions (Table 1). Firstly, we examined
different solvents and found, similarly to the reactions of
NaADBH with carboxylic acids,15 that the reaction proceed-
ed very smoothly in THF, acetonitrile (CH3CN), dioxane, di-
ethyl ether (Et2O), and 1,2-dimethoxyethane (DME) (Table
1, entries 1–5). In these solvents, NaADBH reacted with cin-
namic aldehyde within 5 minutes and achieved good yields.
When methanol (MeOH), N,N-dimethylformamide (DMF),
and water were used as solvents, no product was obtained
due to the decomposition of NaADBH in these solvents (Ta-
ble 1, entries 6–8). Therefore, THF was chosen as solvent for
further investigation.
Table 2 Reduction of NaNH2(BH3)2 with Cinnamyl Aldehyde at Differ-
ent Molar Ratio in THF at Room Temperaturea
THF
rt
OH
O
+
NaNH2(BH3)2
25
25a
Entry
25/NaADBH
Time (min)
Yield (%)b
1
2
3
4
5
6
1:1
2:1
3:1
4:1
5:1
6:1
5
5
98
96
96
95
92
90
Table 1 Reduction of Cinnamyl Aldehyde in Different Solvents at
5
Room Temperaturea
5
120
240
solvent
rt
OH
O
+
NaNH2(BH3)2
a Reaction conditions: 25 (6.0 mmol), NaADBH (x mmol) in THF at room
Entry
Solvent
Time (min)
Yield (%)b
temperature.
b Isolated yield.
1
2
3
4
5
6
7
8
THF
5
5
5
5
5
5
5
5
98
CH3CN
96
Table 3 Reduction of NaADBH with Acetophenone at Different Molar
dioxane
Et2O
90
Ratio in THF at Room Temperaturea
80
OH
O
DME
85
THF
rt
MeOH
DMF
NRc
NRc
NRc
+
NaNH2(BH3)2
18
18a
H2O
Yield (%)b
a Reaction conditions: cinnamyl aldehyde (3.0 mmol), NaADBH (1.0 mmol)
Entry
18/NaADBH
Time (min)
in different solvents (10 mL) at room temperature for 5 min.
b Isolated yield.
c No reaction.
1
2
3
4
5
1:1
2:1
3:1
4:1
5:1
5
5
92
90
92
87
72c
5
NaADBH contains six negatively charged hydrogen at-
oms (BH). To examine the number of active hydrogen atoms
participating in the reduction, the various ratios between
cinnamic aldehyde or acetophenone and NaADBH were
screened for the selective reduction of cinnamic aldehyde
25 and acetophenone 18 (Table 2 and Table 3). In the case of
cinnamic aldehyde, reactions were conducted efficiently at
1:1 and higher molar ratio (Table 2, entries 1–4); even
when 6 equivalents of 25 were applied, reduction product
25a was still obtained in excellent yield although prolonged
time should be given (Table 2, entry 6). These results sug-
gest that all of the six negatively charged hydrogen atoms in
NaADBH can react with aldehydes. While in the case of ace-
tophenone, when 18 was applied at higher molar ratios, re-
60
120
a Reaction conditions: 18 (6.0 mmol), NaADBH (x mmol) in THF at room
temperature.
b Isolated yield.
c Determined by 1H NMR analysis (Figure S1).
After determination of solvents and molar ratio, we in-
vestigated the reactions in a wide range of aromatic and ali-
phatic carbonyl compounds with different functional
groups (Scheme 1).16 The reactions for all the carbonyl
compounds underwent smoothly at room temperature and
afforded the corresponding primary alcohols in good to ex-
cellent yields.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, A–E

C
J. W. et al.
Letter
Synlett
converted into substituted phenol after hydrolysis (16a).
The protonic H of carboxyl group in 3-benzoylpropionic
acid was also believed to react with NaADBH first to form
the corresponding sodium salt but this did not slow the re-
duction of carbonyl group, which was still completed with-
in 5 minutes (27a). The amine substituent also prolonged
the reaction time probably because its coordinated ability
with borane (17a). It is similar to the reduction of carboxyl-
ic acids with THFBH3.17 (3) Ketones with large steric hin-
drance took longer time to be reduced due to steric hin-
drance (35a–39a), but the reduction rate was still much
faster than that of AB10 and NaAB11 reported in the litera-
ture. The longest reaction time for 1-naphthalenone was
only 1.5 hours (38a) when NaADBH was used, suggesting
that NaADBH is the most efficient hydroboration reagent in
comparison with AB and NaAB. (4) In the reduction of ke-
tone substrates bearing the C=C double bond (25a, 35a,
36a) or C=O carbonyl in carboxyl group (27a), 1,2-reduction
products were obtained in good to excellent yields (25a,
35a, 36a) and both functional groups were intact, showing
the good tolerance and chemoselectivity of NaADBH. (5) It
is noted that the carboxyl group in 3-benzoylpropionic acid
was not reduced (within 5 min) (27a), which was different
from the hydroboration by using diborane or THFBH3.18 It
was found that ketone was reduced first if NaADBH was
used, in contrast, carboxylic acid was reduced first if THF-
BH3 was used.18 But both ketone and carboxylic acid moiety
can be reduced by prolonged reaction time. This reaction
indicated that NaADBH exhibits an excellent chemoselec-
tivity to the carbonyl groups.
OH
O
THF
NaNH2(BH3)2
+
R'
R
R'
R
rt
OH
1a, R = H, 5 min, 93%; 2a, R = Me, 5 min, 90%; 3a, R = OMe, 5 min, 90%;
4a, R = NO2, 5 min, 96%
R
R
OH
OH
5a, R = Me, 5 min, 90%; 6a, R = OMe, 5 min, 90%; 7a, R = NO2, 5 min, 96%
8a, R = F, 5 min, 90%; 9a, R = Cl, 5 min, 90%; 10a, R = Br, 5 min, 96%;
11a, R = I, 5 min, 90%; 12a, R = Me, 5 min, 90%; 13a, R = Bu, 5 min, 90%;
14a, R = OMe, 5 min, 90%; 15a, R = NO2, 5 min, 90%;
16a, R = OH, 15 min, 88%; 17a, R = Me2N, 15 min, 85%
t
R
OH
18a, R = H, 5 min, 92%; 19a, R = Me, 5 min, 90%; 20a, R = OMe, 95%;
21a, R = Cl, 5 min, 82% ; 22a, R = Br, 5 min, 80%; 23a, R = I, 5 min, 87%;
24a, R = NO2, 5 min, 89%
R
OH
OH
25a, 5 min, 98%
27a, 5 min, 80%
29a, 5 min, 92%
26a, 5 min, 92%
28a, 5 min, 94%
OH
OH
OH
OH
OH
O
OH
30a, 5 min, 86%
31a, 5 min, 88%
32a, 5 min, 84%
OH
33a, 5 min, 82%
35a, 30 min, 88%
37a, 25 min, 91%
OH
N
O
OH
OH
34a, 5 min, 87%
36a, 25 min, 96%
OH
OH
OH
OH
38a, 90 min, 90%
39a, 30 min, 92%
To study the mechanism of the reaction, the different
ratios between cinnamic aldehyde and NaADBH were
screened, and the reaction was monitored by 11B NMR spec-
troscopy (Figure 1). When the reagent ratio between cin-
namic aldehyde and NaADBH was increased from 1:1 to 3:1,
the formed NaAB signal was observed, as well as unknown
boron signals at  = 2.65–3.8 ppm.19 No gas or precipitate
were formed. When the reagent ratio was increased to 4:1,
a lot of precipitate was formed and only the signal of NaAB
appeared in the 11B NMR spectrum of the solution. This ob-
servation led us to propose the mechanism as shown in
Scheme 2. First, the negatively charged hydrogen atoms in-
teract with the C atom in the carbonyl group and at the
same time the B atom interacts with the O atom of the alde-
hyde, e.g., the insertion of the carbonyl group into a B-H
bond.9 After three B-H bonds of the BH3 group repeatedly
participate in the reaction, intermediate I is formed and
then converts in to NaAB and boric ester. The latter hydro-
lyzes to the alcohol, and the former, the formed NaAB, fur-
ther reduces the aldehyde (red part in scheme 2) following
the process as described in literature.11
Scheme 1 Reduction of carbonyl compounds to primary alcohols with
NaADBH. Reagents and conditions: carbonyl compound (3.0 mmol),
NaADBH (1 mmol) in THF (10 mL) at room temperature. Isolated yields
are given.
According to the experimental results, it was found that
(1) for both aliphatic and aromatic aldehydes, the hydrobo-
ration reaction can be completed within 5 minutes to form
the corresponding alcohols (1a–15a, 25a, 26a, 28a–33a)
due to the higher activity of aldehydes than ketones. All ali-
phatic aldehydes, including straight chain-, branched chain-
, and ring-forming aldehydes, can be reduced instantly to
primary alcohols (26a, 30a, 31a). Neither the electron-do-
nating or electron-withdrawing groups nor their relative
position on the aromatic ring in aromatic aldehyde have
any effect on the reduction (2a–15a, 19a–24a). (2) The ac-
tivity of protic H of the hydroxyl group reacted with the
formed NaAB to form AB and the corresponding sodium
salt, which took longer reaction time because the sodium
salt is slightly soluble in organic solvents. This sodium salt
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D
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References and Notes
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Figure 1 11B NMR spectra of the reaction mixture between NaADBH
and cinnamyl aldehyde at different molar ratios in THF at room tem-
perature, referenced to BF3·OEt2 in C6D6.
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NaNH2(BH3)2
RCHO
NaBH3NH2B(OCH2R)3 (I)
B(OCH2R)3
H2O
NaNH2BH3
RCHO
+
H2O
NaNH2B(OCH2R)3
RCH2OH
Scheme 2 The possible mechanism for NaADBH with aldehydes
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In summary, it was found that NaADBH is an efficient
reductant towards aldehydes and ketones in good to excel-
lent yields, with high chemoselectivity and tolerance on
functional groups. This work describes a facile and conve-
nient procedure for the selective reduction of aldehydes and
ketones to the corresponding alcohols.
Conflict of Interest
The authors declare no conflict of interest.
Funding Information
This work is financially supported by the National Natural Science
Foundation of China (Grant Numbers 21771057 and U1804253).Natio
n
alNatural
S
cienceFo
u
n
d
ati
o
n
o
f
C
h
ina(2
1
7
7
1
0
5
7)Natio
n
alNatural
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cienceFo
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ati
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o
f
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h
ina(U
1
8
0
4
2
5
3)
Acknowledgment
We thank Dr. Jiemin Jiao for assistance in the preparation of this man-
uscript.
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Supporting Information
Supporting information for this article is available online at
https://doi.org/10.1055/a-1479-5008.
S
u
p
p
ortingInformationS
u
p
p
ortingInformatio
n
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progress of the reaction was monitored by TLC. Upon comple-
tion, the hydrolysis of borate esters was carried out by addition
of excess water, forming the corresponding alcohols and
trimethylborate. The reaction mixture was extracted with
EtOAc (3 × 20 mL) and CH2Cl2 (3 × 20 mL). The combined organic
extracts were dried over anhydrous Na2SO4 and concentrated by
rotary evaporation. The residue was purified by silica gel
column chromatography to obtain the product 1a as a colorless
liquid (301 mg, 2.79 mmol, 93%). 1H NMR (600 MHz, CDCl3):  =
7.26–7.20 (m, 2 H), 7.20–7.14 (m, 3 H), 4.40 (s, 2 H), 4.04 (s, 1
H). 13C NMR (151 MHz, CDCl3):  = 140.9, 128.6, 127.7, 127.1,
65.3.
(17) Yoon, N. M.; Pak, C. S.; Brown, H. C.; Krishnamurthy, S.; Stocky,
T. P. J. Org. Chem. 1973, 38, 2786.
(16) Typical Experimental Procedure
A solution of NaADBH (1 mmol, 1.0 equiv) in THF (5 mL) was
added into the solution of benzaldehyde (3.0 mmol, 3.0 equiv)
in THF (5 mL) at room temperature under air atmosphere. The
(18) Subba Rao, B. C.; Thakar, G. P. Curr. Sci. 1963, 32, 404.
(19) Daly, S. R.; Bellott, B. J.; Kim, D. Y.; Girolami, G. S. J. Am. Chem.
Soc. 2010, 132, 7254.
© 2021. Thieme. All rights reserved. Synlett 2021, 32, A–E