A Continuously Regenerable Chiral Ammonia Borane for Asymmetric Transfer Hydrogenations

Article abstract of DOI:10.1002/anie.201806877

A novel chiral ammonia borane was designed and developed through the dehydrogenation of ammonia borane with a chiral phosphoric acid, which was highly effective for the asymmetric transfer hydrogenation of imines and β-enamino esters to afford high levels

Full text of DOI:10.1002/anie.201806877

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Accepted Article
Title: A Continuously Regenerable Chiral Ammonia Borane for
Asymmetric Transfer Hydrogenations
Authors: Qiwen Zhou, Wei Meng, Jing Yang, and Haifeng Du
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10.1002/anie.201806877
Angewandte Chemie International Edition
COMMUNICATION
A Continuously Regenerable Chiral Ammonia Borane for
Asymmetric Transfer Hydrogenations
Qiwen Zhou,[a,b,c] Wei Meng,[a,c] Jing Yang,*[b] and Haifeng Du*[a]
Abstract:
A
novel chiral ammonia borane was designed and
Chiral phosphoric acids (CPAs) have been widely used in
the Brønsted acid catalysis.[9] In view of their strong acidity, we
envision that the treatment of CPA 1 with ammonia borane is
likely to lead a rapid release of H2 to generate chiral ammonia
boranes 2, which further react with imines 3 to afford amines 4
and species 5. CPA 1 is possibly regenerated by hydrolysis of
species 5 to complete the reaction cycle (Scheme 1). Herein, we
wish to report our preliminary results on this subject.
developed via the dehydrogenation of ammonia borane with chiral
phosphoric acid, which was highly effective for the asymmetric
transfer hydrogenation of imines and β-enamino esters to afford high
levels of reactivities and enantioselectivies. Significantly, this chiral
ammonia borane can be continuously regenerated during the
transfer hydrogenation with the assistance of water and ammonia
borane, which allowed as low as 0.1 mol % of chiral phosphoric acid
to give satisfactory results. Notably, the role of chiral phosphoric acid
is to produce the chiral ammonia borane.
NH2 C6F5
O
H
N
B
H
H
H
H
H
H
H
H
H
R'
H
S
B
Ph
Ph
O
O
O
O
Ph
Ph
N
tBu
O
C6F5
NH3⋅BH3
Ammonia borane (NH3·BH3) has received substantial attention
due to its unique features including low molecular weight, high
hydrogen capacity, nice stability, and ready availability. Great
progress has been achieved for its application as a solid
hydrogen storage material.[1] In contrast, its use as the direct
hydrogen source for reductions has been relatively less
investigated.[2] Various metal-catalyzed or metal-free transfer
hydrogenations have been reported.[3] Interestingly, Berke and
co-workers discovered a catalyst-free transfer hydrogenation of
H
chiral FLP catalyst
R
O
NH3⋅BH3 (hydrogen source)
catalyst-free
H. Berke
D. J. Williams
our previous work
Figure 1. Examples with Ammonia Borane for Transfer Hydrogenations.
R
R
NH3⋅BH3
O
O
O
O
O
O
P
P
aldimines with ammonia borane via concerted double hydrogen
transfers (Figure 1).
-H2
OH
OBH2⋅NH3
[4]
However, asymmetric transfer
R
R
hydrogenations with ammonia boranes have lagged behind. In
1984, Williams and co-workers utilized a chiral complex of
ammonia borane and 18-crown-6 as a stoichiometric reagent for
asymmetric reductions of ketones to give up to 67% ee (Figure
1).[5] Very recently, our group developed a frustrated Lewis pair
(FLP)[6] of (R)-tert-butylsulfinamide and HB(C6F5)2 for
asymmetric transfer hydrogenations of imines[7] with ammonia
borane as a hydrogen source for regeneration of the chiral FLP
catalyst (Figure 1).[8] The development of highly efficient and
enantioselective transfer hydrogenations is still in great demand.
1
2
imines 3
H2O
R
O
O
O
+
amines 4
P
O[B-N]
R
5
Scheme 1. Strategy for Developing Regenerable Chiral Ammonia Boranes.
Inspired by the work of Williams and Berke, it is highly
probable and desirable to develop a regenerable chiral ammonia
borane for asymmetric transfer hydrogenations. However, the
formidable challenge lies in how to construct the chiral ammonia
borane and how to make this chiral reagent continuously
regenerable during the reaction process.
CPA 1a (10 mol %) was initially subjected to the transfer
hydrogenation of imine 3a with ammonia borane in toluene
(Scheme 2). It was pleased to find that the addition of water (1.0
equiv) could improve both reactivity and enantioselectivity.
Imines 3b-e with different N-protecting groups were further
examined, and 3,5-di(trifluoromethyl)phenyl group (3e) proved to
be the optimal one to give 79% ee (Scheme 3). CPAs 1b-f were
next evaluated for the transfer hydrogenation of imine 3e.
Substituents at the 3,3’-positions of binaphthyl frameworks had a
large impact on the ee values. To our delight, 93% ee with 94%
conversion was obtained with the use of CPA 1f (Scheme 3).
[a]
Q. Zhou, Dr. W. Meng, Prof. H. Du
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Molecular Recognition and Function, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190 (China), and University of Chinese Academy of Sciences,
Beijing 100049 (China).
NPh
1a (10 mol %)
H
NHPh
NH3⋅BH3 (1.0 equiv)
Ph
Ph
E-mail: haifengdu@iccas.ac.cn
toluene, 30 ºC, 12 h
4a
3a
[b]
[c]
Q. Zhou, Prof. J. Yang
SiPh3
without H2O
State Key Laboratory of Chemical Resource Engineering, Beijing
Key Laboratory of Bioprocess, College of Life Sciences and
Technology, Beijing University of Chemical Technology, Beijing
100029 (China)
Email: yangj@mail.buct.edu.cn
Q.Z and W.M. contributed equally.
87% conv.
(33% ee)
O
P
O
OH
O
with H2O (1.0 equiv)
>99% conv.
(39% ee)
SiPh3
Supporting information for this article is given via a link at the end of
the document.
Scheme 2. Initial Studies on Transfer Hydrogenation.
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10.1002/anie.201806877
Angewandte Chemie International Edition
COMMUNICATION
1 (10 mol %)
entries 1-6). Toluene and benzene were more suitable solvents.
Significantly, the amount of water had a large influence on both
reactivity and enantioselectivity (Table 1, entries 1, 7-11).
Without water, only 67% conversion with 81% ee was obtained
(Table 1, entry 7). In contrast, with 1.5 equivalent of water, a
quantitative conversion with 93% ee was afforded (Table 1,
entry 9). Further increasing the amount of water gave similar
results (Table 1, entries 10 and 11). When the CPA loading was
reduced to 5 or 2.5 mol %, slightly lower conversions were
obtained without loss of any enantioselectivities (Table 1, entries
12 and 13). Notably, the CPA loading can be lowered to 0.1
mol % with benzene as solvent at 60 °C to give amine 4e in 89%
conversion with 90% ee (Table 1, entries 14-17). Further raising
the temperature to 80 °C gave a slightly higher conversion
(Table 1, entry 18). Reducing the amount of ammonia borane to
0.5 equivalent gave 48% conversion. It suggests that only one
hydride of chiral ammonia borane is likely utilized in the transfer
hydrogenation (Table 1, entries 16 vs 19).
Ar
Ar
N
HN
NH3⋅BH3 (1.0 equiv)
H2O (1.0 equiv)
toluene, 30 ºC, 12 h
Ph
Ph
*
4
3
3b: Ar = 4-MeOC6H4
3c: Ar = 4-CNC6H4
3d: Ar = 3-NO2C6H4
3e: Ar = 3,5-(CF3)2C6H3
4b: 97% conv. (21% ee)
4c: 94% conv. (69% ee)
4d: 97% conv. (67% ee)
4e: 89% conv. (79% ee)
with CPA 1a
R for CPA 1 with imine 3e as substrate
Cy Cy
F
Ph
Si tBu
Si
Si
3
3
Ph
Cy
1b
1c
1d
1e
1f
93% conv.
(45% ee)
60% conv.
(57% ee)
93% conv.
(47% ee)
94% conv.
(65% ee)
94% conv.
(93% ee)
Scheme 3. Screen of Imines and Chiral Phosphoric Acids.
Table 1. Optimization of Reaction Conditions.[a]
1f
[mol %]
H2O
[equiv]
Temp.
[°C]
Conv.
[%][b]
Ee
Entry
Solvent
1f (0.1-5 mol %)
H2O (1.5 equiv)
[%][c]
NAr
R'
H
NHAr
R'
R
NH3⋅BH3 (1.0 equiv)
R
4e-u
benzene, 60 ºC
1
2
10
10
10
10
10
10
10
10
10
10
10
5
Toluene
n-Hexane
CH2Cl2
1.0
1.0
1.0
1.0
1.0
1.0
0
30
30
30
30
30
30
30
30
30
30
30
60
60
60
60
60
60
80
60
94
52
93
88
89
92
93
81
81
89
93
93
93
93
93
93
93
93
90
89
93
3e-t: Ar = 3,5-(CF3)2C6H3; 3u: Ar = 3,5-Me2C6H3
H
NHAr
H
NHAr
H
NHAr
R
Ph
3
98
R
S
4m: R = Et
93% (90% ee)(20 h)[b]
4n: R = nPr
4
PhCl
96
4e: R = H, 84% (89% ee)(12 h)[a]
94% (93% ee)(12 h)[b]
4p: 78% (94% ee)
(36 h)[e]
90% (87% ee)(20 h)[b]
4f: R = 4-MeO, 91% (92% ee)(12 h)[b]
4g: R = 4-Me, 87% (91% ee)(15 h)[b]
4h: R = 4-Ph, 95% (93% ee)(14 h)[b]
4i: R = 4-Cl, 88% (93% ee)(19 h)[b]
4j: R = 4-Br, 95% (94% ee)(14 h)[c]
4k: R = 3-Me, 90% (93% ee)(15 h)[b]
4l: R = 3-Br, 96% (94% ee)(14 h)[c]
5
Benzene
THF
>99
58
H
NHAr
H
NHAr
6
O
7
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
67
4o: 89% (87% ee)(12 h)[b]
4q: 89% (87% ee)
(36 h)[e]
8
0.5
1.5
2.0
5.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
62
H
NHAr
H
NHAr
H
NHAr
H
NHAr
N
9
>99
>99
>99
95
Ph
S
Ar = 3,5-Me2C6H3
10
11
12
13
14
15
16
17
18
19[d]
N
4s: 76% (92% ee) 4t: 85% (87% ee)(20 h)[d]
4r: 81% (94% ee)
(36 h)[e]
4u: 84% (87% ee)
(20 h)[c]
0.95 g, 71% (86% ee)(24 h)[c]
(20 h)[d]
(0.56 g, 97% ee, recrystallization)
Scheme 4. Asymmetric Transfer Hydrogenation of Imines, CPA 1f Loading:
[a] 0.1 mol %. [b] 0.5 mol %. [c] 1.0 mol %. [d] 3.0 mol %. [e] 5.0 mol %.
2.5
2.5
1.0
0.5
0.1
0.1
0.5
94
>99
98
A variety of imines 3e-u were subjected to asymmetric
transfer hydrogenations under the optimal reaction conditions
(Scheme 4). All these reactions proceeded smoothly to give the
corresponding products 4e-u in 76-96% yields with 87-94% ee’s.
97
89
The absolute configuration was tentatively assigned as
S
95
according to X-ray structure of compound 4h (see Supporting
Information).[10] 1-Cyclohexylethanone-derived imine 4o gave a
promising 87% ee. Imines 3p-r bearing O- or S-heterocycles
were suitable substrates to give amine products 4p-r in 78-89%
yields with 87-94% ee’s. Notably, the transfer hydrogenation of
imines 3s and 3t containing pyridinyl groups with 3 mol % of
CPA 1f gave amines 4s and 4t in high yields with 92% and 87%
ee, respectively. The reaction of imine 3t can be carried out in
gram-scale with 1 mol % of CPA 1f to afford the desired product
48
[a] All reactions were carried out with imine 3e (0.10 mmol) in solvent (0.5 mL)
for 12 h. [b] Determined by crude 1H NMR. [c] The ee was determined by chiral
HPLC. [d] NH3·BH3 (0.5 equiv) was used.
The reaction conditions for transfer hydrogenations of imine
3e with CPA 1f were subsequently optimized. Various solvents
gave moderate to high conversions with 81-93% ee’s (Table 1,
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10.1002/anie.201806877
Angewandte Chemie International Edition
COMMUNICATION
4t in 71% yield with 86% ee. After a recrystallization in the
mixture of water and methanol, the ee was further improved to
97%. Moreover, imine 3u with 3,5-dimethylphenyl N-protecting
group was an effective substrate to give amine 4u in 84% yield
with 87% ee. Remarkably, a reductive amination of aceto-
phenone (6) and 3,5-dimethylaniline (7) using 5 mol % of CPA 1f
with 4Å molecular sieves and isopropanol instead of water, can
be successfully realized in a sequent one pot manner to give the
desired product 4u in 62% yield with 84% ee (Scheme 5).
2f cleanly (Figure 2b). A stoichiometric reaction of 2f and imine
3e resulted in the appearance of several P-signals which was
attributed to the production of different [N-B] species (Figure 2c).
Treating this mixture with water, most of these signals were
converted to that of CPA 1f, but some weak signals still left
(Figure 2d). Subsequent addition of ammonia borane led a
simple signal of chiral ammonia borane 2f (Figure 2e). These
results indicate that chiral ammonia borane is a reactive species,
and both water and ammonia borane can efficiently promote its
complete regeneration from a messy mixture.
O
Me
Ph
6
+
a) CPA 1f
NH3⋅BH3 (1.0 equiv)
1f (5.0 mol %), 4Å MS
benzene, 60 °C, 12 h
NH2
H
N
Me
iPrOH (1.5 equiv)
benzene, 60 °C, 12 h
Ph
b) Chiral ammonia borane 2f
4u
62% (84% ee)
Me
Me
7
c) 2f + imine 3e
Scheme 5. A Reductive Amination in Sequent One Pot Manner.
CPA 1f
d) Addtion of water (10 eq)
2f
To extend the usage of this regenerable chiral ammonia
borane，a variety of β-enamino esters 8a-o were examined for
the asymmetric transfer hydrogenation under a slightly modified
reaction conditions (Scheme 6).[11] To our pleasure, all these
reactions went smoothly to afford β-amino esters 9a-o in 55-
96% yields with 66-94% ee’s. N-protecting groups had little
impact on the enantioselectivity (9a-c). Both electron-
withdrawing and donating substituents on the phenyl group were
well tolerated for this reaction (9d-k). β-enamino esters 8n-o
bearing benzyl and cyclohexyl group were also effective
substrates to give high yields but relatively lower ee’s. Moreover,
β-enamino cyanide 10 was a highly reactive substrate for this
reaction to afford β-amino cyanide 11 in 98% yield with 90% ee.
e) Addtion of NH3⋅BH3 (1.0 eq)
31P NMR (ppm)
Figure 2. 31P NMR Studies on the Transfer Hydrogenation (in benzene at
room temperature).
The hydrogen transfer between chiral ammonia borane 2f
and imine 3e was investigated by DFT calculations. Concerted
6-membered ring transition states TS-(R) and TS-(S) were
located at the M06-2X/6-31G(d,p) level (Figure 2).
A 1.7
Kcal/mol activation enthalpy difference in the gas phase predicts
a 93% ee favored for (S)-isomer. It is highly consistent with the
experimental result. For imine 3u, the corresponding transition
states were also located, which predicts a similar ee to the
obtained one (see Supporting Information). Moreover, DFT
calculation suggests that both water and ammonia borane can
cleavage the O-B bond to regenerate CPA 1f (see Supporting
1f (5 mol %)
H2O (1.5 equiv)
NHAr
ArHN
R
H
CO2Et
CO2Et
NH3⋅BH3 (1.1 equiv)
toluene, 60 ºC, 24 h
R
H
8a-o
9a-o
PMPHN
H
ArHN
Ph
PMPHN
H
CO2Et
CO2Et
CO2Et
Information). But water seems to be
a
major contributor
9a: Ar = Ph, 78% (92% ee)
9b: Ar = p-tolyl, 83% (92% ee)
9c: Ar = PMP, 87% (92% ee)
S
9m
79% (94% ee)
according to the obtained experimental results.
9l
79% (92% ee)
PMPHN
H
PMPHN
H
PMPHN
H
CO2Et
CO2Et
CO2Et
R
Ph
9n
82% (66% ee)
9o
9d: R = 2-F, 55% (92% ee)
9e: R = 2-Me, 57% (81% ee)
9f: R = 3-F, 62% (91% ee)
9g: R = 3-MeO, 93% (93% ee)
9h: R = 4-F, 87% (93% ee)
9i: R = 4-Cl, 84% (91% ee)
9j: R = 4-MeO, 96% (89% ee)
9k: R = 3,4-(MeO)2, 96% (91% ee)
88% (69% ee)
N
NHPMP
CN
PMPHN
Ph
H
O
P
CN
H
P
O
B
H
H
B
N
Ph
N
H
C
10
11
98% (90% ee)
C
Si
N
F
H
Scheme 6. Asymmetric Transfer Hydrogenation of β-Enamino Esters and
Cyanide.
F
TS-(R)
TS-(S)
≠
≠
ΔΔHact = 26.6 Kcal/mol
ΔΔHact = 28.3 Kcal/mol
Preliminary studies were conducted for a better insight into
this reaction. H2 was released rapidly when CPA 1f was treated
with ammonia borane (1.0 equiv), to give chiral ammonia borane
Figure 3. DFT Calculation for the Hydrogen Transfer Process.
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10.1002/anie.201806877
Angewandte Chemie International Edition
COMMUNICATION
2007, 46, 8116-8118; Angew. Chem. 2007, 119, 8262-8264; c) C. W.
Hamilton, R. T. Baker, A. Staubitz, I. Manners, Chem. Soc. Rev. 2009,
38, 279-293; d) A. Staubitz, A. P. M. Robertson, I. Manners, Chem. Rev.
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2012, 5, 9257-9268; f) A. Rossin, M. Peruzzini, Chem. Rev. 2016, 116,
8848-8872.
Based on the experimental and theoretical mechanistic
results, a plausible reaction cycle is outlined in Figure 4. CPA 1
quickly reacts with ammonia borane to release H2 and form
chiral ammonia borane 2. Double hydrogen transfers occur via a
concerted 6-membered ring transition state TS-(S) to produce
amine 4 and species 5 (several [N-B] species). A followed
hydrolysis of species 5 through a 4-membered ring transition
state TS1 regenerates CPA 1, which further reacts with
ammonia borane to reform the reactive chiral ammonia borane.
[2]
[3]
a) R. O. Hutchins, K. Learn, B. Nazer, D. Pytlewski, A. Pelter, Org. Prep.
Proced. Int. 1984, 16, 335-372; b) A. Staubitz, A. P. M. Robertson, M. E.
Sloan, I. Manners, Chem. Rev. 2010, 110, 4023-4078; c) O. Songis, C.
S. J. Cazin, Synlett 2013, 24, 1877-1881.
For selected examples, see: a) Y. Jiang, O. Blacque, T. Fox, C. M.
Frech, H. Berke, Organometallics 2009, 28, 5493-5504; b) G. Ménard,
D. W. Stephan, J. Am. Chem. Soc. 2010, 132, 1796-1797; c) C. C.
Chong, H. Hirao, R. Kinjo, Angew. Chem. Int. Ed. 2014, 53, 3342-3346;
Angew. Chem. 2014, 126, 3410-3414; d) S. Fu, N.-Y. Chen, X. Liu, Z.
Shao, S.-P. Luo, Q. Liu, J. Am. Chem. Soc. 2016, 138, 8588-8594; e) Q.
Zhou, L. Zhang, W. Meng, X. Feng, J. Yang, H. Du, Org. Lett. 2016, 18,
5189-5191.
O
O
O
P
NH3⋅BH3
OBH2⋅NH3
2
imine 3
H2
O
O
O
P
OH
1
O
O
HO-[B-N]
P
[4]
a) X. Yang, L. Zhao, T. Fox, Z.-X. Wang, H. Berke, Angew. Chem. Int.
Ed. 2010, 49, 2058-2062; Angew. Chem. 2010, 122, 2102-2106; b) X.
Yang, T. Fox, H. Berke, Chem. Commun. 2011, 47, 2053-2055; c) X.
Yang, T. Fox, H. Berke, Org. Biomol. Chem. 2012, 10, 852-860; d) X.
Yang, T. Fox, H. Berke, Tetrahedron 2011, 67, 7121-7127; For a review,
see: e) X. Yang, Z. Xie, J. He, L. Yu, Chin. J. Org. Chem. 2015, 35,
603-609.
H
B
R
O
O
H
H
Ar
H2N
N
Ar'
O
O
O
H
OH
[B-N]
P
O
TS-(S)
TS1
O
O
O
amine 4
P
H2O
[5]
[6]
B. L. Allwood, H. Shahriari-Zavareh, J. F. Stoddart, D. J. Williams, J.
Chem. Soc., Chem. Commun. 1984, 1461-1464.
O[B−N]
5
For leading reviews, see: a) D. W. Stephan, G. Erker, Angew. Chem.
Int. Ed. 2010, 49, 46-76; Angew. Chem. 2010, 122, 50-81; b) T. Soós,
Pure Appl. Chem. 2011, 83, 667-675; b) J. Paradies, Angew. Chem. Int.
Ed. 2014, 53, 3552-3557; Angew. Chem. 2014, 126, 3624-3629; c) L.
Shi, Y.-G. Zhou, ChemCatChem 2015, 7, 54-56; d) D. W. Stephan, G.
Erker, Angew. Chem. Int. Ed. 2015, 54, 6400-6441; Angew. Chem.
2015, 127, 6498-6541; e) D. W. Stephan, J. Am. Chem. Soc. 2015, 137,
10018-10032; f) D. W. Stephan, Acc. Chem. Res. 2015, 48, 306-316; g)
D. W. Stephan, Science 2016, 354, aaf7229; h) W. Meng, X. Feng, H.
Du, Acc. Chem. Res. 2018, 51, 191-201.
Figure 4. Plausible Reaction Cycle.
In summary, a novel and regenerable chiral ammonia
borane was developed via a H2 releasing reaction of ammonia
borane and chiral phosphoric acid, which was a highly effective
reductive reagent for asymmetric transfer hydrogenations. A
wide range of amines and β-amino esters were obtained in 55-
96% yields with 66-94% ee’s. Significantly, this chiral ammonia
borane can be efficiently and continuously regenerated during
the transfer hydrogenation with the assistance of water and
ammonia borane, which allowed as low as 0.1 mol % of chiral
phosphoric acid to give satisfactory reactivities and
enantioselectivities. Notably, herein, chiral phosphoric acid
played a role as a simple Brønsted acid to generate the reactive
chiral ammonia borane. Mechanistic studies suggest that the
hydrogen transfer between chiral ammonia borane and imine
occurs via a concerted 6-membered ring transition state. The
unique features of this chiral ammonia borane make it a
potentially useful chiral reagent for other asymmetric transfer
hydrogenations.
[7]
For selected reviews on asymmetric reduction of imines, see: a) J.-H.
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[8]
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Acknowledgements
We are grateful for the generous financial support from the
National Natural Science Foundation of China (21572231,
21521002, and U1663227).
[10] CCDC-1813490 contains the supplementary crystallographic data for
4h. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Keywords: asymmetric transfer hydrogenation • ammonia
borane • chiral phosphoric acid • chiral ammonia borane • water
[1]
For leading reviews, see: a) F. H. Stephens, V. Pons, R. T. Baker,
Dalton Trans. 2007, 2613-2626; b) T. B. Marder, Angew. Chem. Int. Ed.
This article is protected by copyright. All rights reserved.

10.1002/anie.201806877
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A
novel chiral ammonia borane was
Qiwen Zhou, Wei Meng, Jing Yang*,
and Haifeng Du*
developed for asymmetric transfer
hydrogenations to give high levels of
yields and ee’s. Significantly, this chiral
ammonia borane can be continuously
regenerated during the reaction with the
assistance of water and ammonia
borane, which allowed as low as 0.1
mol % of chiral phosphoric acid to give
satisfactory results.
Page No. – Page No.
A Continuously Regenerable Chiral
Ammonia Borane for Asymmetric
Transfer Hydrogenations
This article is protected by copyright. All rights reserved.