Asymmetric Transfer Hydrogenation of N-Unprotected Indoles with Ammonia Borane

Article abstract of DOI:10.1021/acs.orglett.0c01930

A metal-free asymmetric transfer hydrogenation of unprotected indoles was successfully realized using a catalyst derived from HB(C6F5)2 and (S)-tert-butylsulfinamide with ammonia borane as a hydrogen source. A variety of indolines were achieved in 40-78percent yields with up to 90percent ee.

Full text of DOI:10.1021/acs.orglett.0c01930

pubs.acs.org/OrgLett
Letter
Asymmetric Transfer Hydrogenation of N‑Unprotected Indoles with
Ammonia Borane
Weiwei Zhao, Zijia Zhang, Xiangqing Feng,* Jing Yang,* and Haifeng Du*
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c01930
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: A metal-free asymmetric transfer hydrogenation of
unprotected indoles was successfully realized using a catalyst derived
from HB(C6F5)2 and (S)-tert-butylsulfinamide with ammonia borane as a
hydrogen source. A variety of indolines were achieved in 40−78% yields
with up to 90% ee.
hiral indoline skeleton exists widely in natural alkaloids
indolines were achieved in high ee’s with broad functional
Cand pharmaceutically active compounds. Among various group tolerance (Scheme 1, (3)).8 The asymmetric transfer
1
hydrogenation9 was another type of effective reduction
reaction and would avoid the high-pressure and the use of
special devices, which may provide some superiority over
traditional hydrogenations. However, to the best of our
knowledge, there is no report on the straight asymmetric
transfer hydrogenation of indoles so far, which remains an
unsolved problem.10
developed protocols for preparing optically pure indolines,2
there is no doubt that the direct enantioselective reduction of
prochiral indole derivatives is one of the most straightforward
and simplest methods.3 Transition-metal complexes have
proven to be efficient catalysts for the asymmetric hydro-
genation of N-protected indoles;4 however, the hydrogenation
of N-unprotected indoles is relatively less studied (Scheme 1,
(1)).5 In 2010, Zhou and Zhang developed the first highly
enantioselective hydrogenation of N-unprotected indoles using
Pd/(R)-H8-BINAP catalyst with a Brønsted acid as an
activator.5a Subsequently, Vidal-Ferran reported a Brønsted
acid-mediated CC isomerization and Ir/P−OP complexes
catalyzed asymmetric hydrogenation.5b Touge’s and Arai’s
group5c and Chen and Fan’s group5d applied chiral ruthenium
diamine complexes in hydrogenation of N-unprotected indoles
simultaneously and independently to afford excellent enantio-
selectivities. In 2018, Chung and Zhang reported a rhodium/
ZhaoPhos-catalyzed asymmetric hydrogenation of unprotected
indoles which needed preactivated by Brønsted acid HCl
(Scheme 1).5e In addition, Stephan and co-workers developed
a metal-free catalyzed hydrogenation of indoles using 10 mol %
B(C6F5)3 as catalyst in 2011,6 and the N−H of the substrates
needed to be protected.
Despite the significant progress that has been achieved, the
current catalytic systems usually need high-pressure or
preactivation with Brønsted acids. Besides the hydrogenation,
some other reductions of N-unprotected indoles have also
been developed. In 2011, Sun and Chen reported a chiral
Lewis base mediated asymmetric hydrosilylation of unpro-
tected 1H-indoles to afford the corresponding indolines in
moderate to excellent enantioselectivity (Scheme 1, (2)).7
Recently, Yu and Song developed an asymmetric reduction of
C2-aryl-substituted N-unprotected indoles using a chiral
Brønsted acid catalyst, which was derived from a chiral
phosphoric acid boron complex and water, the resulting chiral
As our ongoing research interest in FLP catalysis,11,12 we
developed a novel FLP catalyst13 derived from Piers’ borane14
and (S)-tert-butylsulfinamide,15 which can provide hydride and
proton to the unsaturated substrates and be regenerated under
ammonia borane16,17 as the hydrogen source. This catalytic
system was highly effective for the enantioselective metal-free
transfer hydrogenations for imines,13a 2,3-disubstituted qui-
noxalines,13b and β-N-substituted enamino esters (Scheme
2).13c We envisaged that whether the challenging asymmetric
reduction of indoles could be conducted with this catalyst.
Herein, we wish to report our preliminary efforts in the
asymmetric transfer hydrogenation of N-unprotected indoles.
We commenced our studies with the asymmetric transfer
hydrogenation of 2-methylindole (1a) as a model substrate. To
our pleasure, with (S)-tert-butylsulfinamide (2) (30 mol %)
and HB(C6F5)2 (3) (10 mol %) in combination of NH3·BH3
(4) (2.0 equiv) in cyclohexane (3 mL) at 60 °C for 48 h, the 2-
methylindoline (5a) was afforded in 92% conversion with 52%
ee. When the temperature was lowered to room temperature, a
sharp reduction in conversion and a higher 76% ee were
obtained (Scheme 3).
Received: June 8, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01930
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

Organic Letters
pubs.acs.org/OrgLett
Letter
Scheme 1. Asymmetric Reduction of N-Unprotected Indoles
Scheme 3. Asymmetric Hydrogenation of 2-Methylindole 1a
a
Table 1. Optimization of Reaction Conditions
b
c
entry
2 (mol %) 3 (mol %) temp (°C) conv (%) ee (%)
a
1
2
3
4
5
6
7
8
60
60
60
60
60
50
30
30
30
30
30
30
33
25
92
92
40
91
66
87
92
76
89
88
rac
85
52
55
6
64
82
63
64
59
73
80
ad
,
30
30
30
10
30
30
30
30
30
30
30
30
10
10
10
10
10
10
10
10
20
20
a
b
b
b
b
be
,
bf
,
9
bg
,
10
11
12
bf
,
b
a
All reactions were carried out with 2-methyl-1H-indole (1a) (0.20
mmol), (S)-tert-butylsulfinamide (2) (0.06 mmol, 30 mol %),
HB(C6F5)2 (3) (0.02 mmol, 10 mol %), NH3·BH3 (4) (0.40
mmol), in cyclohexane (3.0 mL) at 60 °C for 48 h according to the
following methods: (A) All materials were subjected in one portion.
(B) 1a, 2 and 3 were subjected and stirred for 12 h at 30 °C before
b
addition of ammonia borane 4 and additive. Determined by 1H
c
NMR spectroscopy of the crude reaction mixture. Determined by
d
e
chiral HPLC. Without NH3·BH3 (4). With Et2NH (10 mol %) as
f
g
an additive. With Et3N (10 mol %) as an additive. With pyridine (10
mol %) as an additive.
reaction existed which may greatly affect the reactivity and
enantioselectivity (Table 1, entry 1). With 30 mol % of 2 and
3, 25% conversion and up to 85% ee were achieved without the
addition of NH3·BH3 (4), and the result was consistent with
the catalyst loading (Table 1, entry 2). When both the catalyst
and hydrogen source were added, 92% conversion and 52% ee
were afforded. To avoid the background reaction, the order of
the material addition was modified. Subjecting NH3·BH3 (4)
12 h later to the reaction mixture of indole 1a, 2, and 3 gave a
slight increase of enantioselectivity with the same conversion as
that of the addition of all materials in one portion (Table 1,
entry 3 vs 4). Lowering the HB(C6F5)2 (3) to 10 mol % led to
a sharp decrease of reactivity and enantioselectivity, and only
40% conversion and 6% ee were obtained (Table 1, entry 5).
The reaction temperature also influenced the reactivity and
enantioselectivity to a large extent, by gradient lowering the
temperature to 30 °C, up to 82% ee was achieved in 66%
conversion (Table 1, entry 6 vs 7). Diethylamine, triethyl-
amine, and pyridine were subjected to the transfer hydro-
genation, respectively, and relative lower ee’s were obtained
(Table 1, entries 8−10). Further optimization was conducted
with HB(C6F5)2 (3) (20 mol %); without addition of
Scheme 2. Asymmetric Transfer Hydrogenation Using an
FLP Catalyst with Ammonia Borane
Subsequently, the reactions conditions were evaluated, and
the results are summarized in Table 1. Without (S)-tert-
butylsulfinamide (2) and HB(C6F5)2 (3), a racemic product
was obtained in 33% conversion, which indicated a background
B
https://dx.doi.org/10.1021/acs.orglett.0c01930
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Moreover, 2,3-dialkyl-substituted indole derivatives 1q−s
were effective substrates to yield mainly cis-indolines 5q−s in
40−66% yields with 72−80% ee’s (Scheme 5). For 2,3-
disubstituted fused ring indoles 1t−1v, it was found that the ee
increased from 27% to 81% accompanied by a ring size of five-
to six- to seven-membered rings. The 3-substituted indole 1w
was also tolerated to give 65% yield with 28% ee.
Scheme 4. Substrate Scope for Asymmetric Hydrogenation
of 2-Substituted N-Unprotected Indoles
In summary, the asymmetric transfer hydrogenation of N-
unprotected indole derivatives was successfully realized using a
catalyst derived from HB(C6F5)2 and (S)-tert-butylsulfinamide
with ammonia borane as hydrogen source. Various indolines
were prepared in 40−78% yields and up to 90% ee. Further
efforts to search for efficient and universal chiral FLPs and
expand the substrate scope in asymmetric reactions are
underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01930.
Procedure for the asymmetric transfer hydrogenation,
characterization of products, and data for the determi-
nation of enantiomeric excesses along with the NMR
spectra (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
Scheme 5. Substrate Scope for Asymmetric Hydrogenation
of 2,3-Disubstituted N-Unprotected Indoles
Xiangqing Feng − Beijing National Laboratory for Molecular
Sciences, CAS Key Laboratory of Molecular Recognition and
Function, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China; University of Chinese Academy of
Sciences, Beijing 100049, China; orcid.org/0000-0003-
1739-925X; Email: fxq@iccas.ac.cn
Jing Yang − State Key Laboratory of Chemical Resource, Beijing
Key Laboratory of Bioprocess, College of Life Science and
Technology, Beijing University of Chemical Technology, Beijing
100029, China; orcid.org/0000-0002-1832-9619;
Email: yangj@mail.buct.edu.cn
Haifeng Du − Beijing National Laboratory for Molecular
Sciences, CAS Key Laboratory of Molecular Recognition and
Function, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China; University of Chinese Academy of
Sciences, Beijing 100049, China; orcid.org/0000-0003-
0122-3735; Email: haifengdu@iccas.ac.cn
Authors
Weiwei Zhao − Beijing National Laboratory for Molecular
Sciences, CAS Key Laboratory of Molecular Recognition and
Function, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China; State Key Laboratory of Chemical
Resource, Beijing Key Laboratory of Bioprocess, College of Life
Science and Technology, Beijing University of Chemical
Technology, Beijing 100029, China
Zijia Zhang − Beijing National Laboratory for Molecular
Sciences, CAS Key Laboratory of Molecular Recognition and
Function, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China; State Key Laboratory of Chemical
Resource, Beijing Key Laboratory of Bioprocess, College of Life
Science and Technology, Beijing University of Chemical
Technology, Beijing 100029, China
triethylamine, 88% conversion and 80% ee were achieved
(Table 1, entry 12).
The substrate scope of the transfer hydrogenation of
unprotected indoles were examined under the optimal reaction
conditions (Scheme 4). For various 2-alkylsubstituted N-
unprotected indoles 1a−m and 1o−p, the reactions proceeded
smoothly, and the corresponding indolines 5a−m and 5o−p
were obtained in 50−78% yields with 64−90% ee’s. For 2-
arylsubstituted indole 1n, a racemic product was afforded in
50% yield, which suggested that a reversible process of
protonation and deprotonation was involved in the mecha-
nism. However, when N-Me-protected indole 1x was subjected
in the reaction, the product 5x was obtained in moderate yield
with poor enantioselectivity.
Complete contact information is available at:
C
https://dx.doi.org/10.1021/acs.orglett.0c01930
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
of Transition Metal and Anion Binding. Org. Lett. 2018, 20, 2143−
2147.
https://pubs.acs.org/10.1021/acs.orglett.0c01930
(6) Stephan, D. W.; Greenberg, S.; Graham, T. W.; Chase, P.;
Hastie, J. J.; Geier, S. J.; Farrell, J. M.; Brown, C. C.; Heiden, Z. M.;
Welch, G. C.; Ullrich, M. Metal-Free Catalytic Hydrogenation of
Polar Substrates by Frustrated Lewis Pairs. Inorg. Chem. 2011, 50,
12338−12348.
(7) Xiao, Y.-C.; Wang, C.; Yao, Y.; Sun, J.; Chen, Y.-C. Direct
Asymmetric Hydrosilylation of Indoles: Combined Lewis Base and
Brønsted Acid Activation. Angew. Chem., Int. Ed. 2011, 50, 10661−
10664.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for the financial support by the National
Natural Science Foundation of China (91856103, 21521002,
and 21825108).
(8) Yang, K.; Lou, Y.; Wang, C.; Qi, L.-W.; Fang, T.; Zhang, F.; Xu,
H.; Zhou, L.; Li, W.; Zhang, G.; Yu, P.; Song, Q. Chiral Brønsted Acid
from Chiral Phosphoric Acid Boron Complex and Water: Asymmetric
Reduction of Indoles. Angew. Chem., Int. Ed. 2020, 59, 3294−3299.
(9) (a) Noyori, R.; Hashiguchi, S. Asymmetric Transfer Hydro-
genation Catalyzed by Chiral Ruthenium Complexes. Acc. Chem. Res.
1997, 30, 97−102. (b) Gladiali, S.; Alberico, E. Asymmetric Transfer
Hydrogenation: Chiral Ligands and Applications. Chem. Soc. Rev.
2006, 35, 226−236. (c) Wu, X.; Xiao, J. Aqueous-Phase Asymmetric
Transfer Hydrogenation of Ketones-a Greener Approach to Chiral
Alcohols. Chem. Commun. 2007, 2449−2466. (d) You, S.-L. Recent
Developments in Asymmetric Transfer Hydrogenation with Hantzsch
Esters: A Biomimetic Approach. Chem. - Asian J. 2007, 2, 820−827.
(e) Ikariya, T.; Blacker, A. J. Asymmetric Transfer Hydrogenation of
Ketones with Bifunctional Transition Metal-Based Molecular
Catalysts. Acc. Chem. Res. 2007, 40, 1300−1308. (f) Ouellet, S. G.;
Walji, A. M.; MacMillan, D. W. C. Enantioselective Organocatalytic
Transfer Hydrogenation Reactions Using Hantzsch Esters. Acc. Chem.
Res. 2007, 40, 1327−1339. (g) Zheng, C.; You, S.-L. Transfer
Hydrogenation with Hantzsch Esters and Related Organic Hydride
Donors. Chem. Soc. Rev. 2012, 41, 2498−2518. (h) Wang, D.; Astruc,
D. The Golden Age of Transfer Hydrogenation. Chem. Rev. 2015,
115, 6621−6686.
REFERENCES
■
(1) (a) Rakhit, A.; Hurley, M. E.; Tipnis, V.; Coleman, J.; Rommel,
A.; Brunner, H. R. Pharmacokinetics and Pharmacodynamics of
Pentopril, a New Angiotensin-Converting-Enzyme Inhibitor in
Humans. J. Clin. Pharmacol. 1986, 26, 156−164. (b) Cragg, G. M.;
Kingstom, D. G. I.; Newman, D. J. Anticancer Agents from Natural
Products; CRC: Boca Raton, 2005. (c) Fattorusso, E.; Taglialatela-
Scafati, O. Modern Alkaloids: Structure, Isolation, Synthesis, and Biology;
Wiley-VCH: Weinheim, 2008. (d) Kochanowska-Karamyan, A. J.;
Hamann, M. T. Marine Indole Alkaloids: Potential New Drug Leads
for the Control of Depression and Anxiety. Chem. Rev. 2010, 110,
4489−4497. (e) Ishikura, M.; Abe, T.; Choshi, T.; Hibino, S. Simple
Indole Alkaloids and Those with a Non-Rearranged Monoterpenoid
Unit. Nat. Prod. Rep. 2013, 30, 694−752.
(2) Liu, D.; Zhao, G.; Xiang, L. Diverse Strategies for the Synthesis
of the Indoline Scaffold. Eur. J. Org. Chem. 2010, 2010, 3975−3984.
(3) (a) Glorius, F. Asymmetric Hydrogenation of Aromatic
Compounds. Org. Biomol. Chem. 2005, 3, 4171−4175. (b) Zhou,
Y.-G. Asymmetric Hydrogenation of Heteroaromatic Compounds.
Acc. Chem. Res. 2007, 40, 1357−1366. (c) Kuwano, R. Catalytic
Asymmetric Hydrogenation of 5-Membered Heteroaromatics. Hetero-
cycles 2008, 76, 909−922. (d) Wang, D.-S.; Chen, Q.-A.; Lu, S.-M.;
Zhou, Y.-G. Asymmetric Hydrogenation of Heteroarenes and Arenes.
Chem. Rev. 2012, 112, 2557−2590.
(4) (a) Kuwano, R.; Sato, K.; Kurokawa, T.; Karube, D.; Ito, Y.
Catalytic Asymmetric Hydrogenation of Heteroaromatic Compounds.
J. Am. Chem. Soc. 2000, 122, 7614−7615. (b) Kuwano, R.; Kaneda,
K.; Ito, T.; Sato, K.; Kurokawa, T.; Ito, Y. Highly Enantioselective
Synthesis of Chiral 3-Substituted Indolines by Catalytic Asymmetric
Hydrogenation of Indoles. Org. Lett. 2004, 6, 2213−2215.
(c) Kuwano, R.; Kashiwabara, M.; Sato, K.; Ito, T.; Kaneda, K.; Ito,
Y. Catalytic Asymmetric Hydrogenation of Indoles Using a Rhodium
Complex with A Chiral Bisphosphine Ligand PhTRAP. Tetrahedron:
Asymmetry 2006, 17, 521−535. (d) Chen, Z.; Zhou, Y. Asymmetric
Hydrogenation of Heteroarenes with Multiple Heteroatoms. Synthesis
2016, 48, 1769−1781. (e) Kuwano, R.; Kashiwabara, M. Ruthenium-
Catalyzed Asymmetric Hydrogenation of N-Boc-Indoles. Org. Lett.
2006, 8, 2653−2655. (f) Baeza, A.; Pfaltz, A. Iridium-Catalyzed
Asymmetric Hydrogenation of N-Protected Indoles. Chem. - Eur. J.
2010, 16, 2036−2039.
(5) (a) Wang, D.-S.; Chen, Q.-A.; Li, W.; Yu, C.-B.; Zhou, Y.-G. Pd-
Catalyzed Asymmetric Hydrogenation of Unprotected Indoles
Activated by Brønsted Acids. J. Am. Chem. Soc. 2010, 132, 8909−
8911. (b) Nunez-Rico, J. L.; Fernandez-Perez, H.; Vidal-Ferran, A.
Asymmetric Hydrogenation of Unprotected Indoles Using Iridium
Complexes Derived from P−OP Ligands and (Reusable) Brønsted
Acids. Green Chem. 2014, 16, 1153−1157. (c) Touge, T.; Arai, T.
Asymmetric Hydrogenation of Unprotected Indoles Catalyzed by η6-
Arene/N-Me-Sulfonyldiamine−Ru(II) Complexes. J. Am. Chem. Soc.
2016, 138, 11299−11305. (d) Yang, Z.; Chen, F.; He, Y.; Yang, N.;
Fan, Q.-H. Highly Enantioselective Synthesis of Indolines: Asym-
metric Hydrogenation at Ambient Temperature and Pressure with
Cationic Ruthenium Diamine Catalysts. Angew. Chem., Int. Ed. 2016,
55, 13863−13866. (e) Wen, J.; Fan, X.; Tan, R.; Chien, H.-C.; Zhou,
Q.; Chung, L. W.; Zhang, X. Brønsted-Acid-Promoted Rh-Catalyzed
Asymmetric Hydrogenation of N-Unprotected Indoles: A Cocatalysis
(10) Related papers: (a) Rueping, M.; Brinkmann, C.; Antonchick,
A. P.; Atodiresei, I. Asymmetric Synthesis of Indolines by Catalytic
Enantioselective Reduction of 3H-Indoles. Org. Lett. 2010, 12, 4604−
4607. (b) Luo, Z.; Sun, G.; Zhou, Z.; Liu, G.; Luan, B.; Lin, Y.;
Zhang, L.; Wang, Z. Stereogenic cis-2-Substituted-N-Acetyl-3-
Hydroxy-Indolines via Ruthenium(II)-Catalyzed Dynamic Kinetic
Resolution-Asymmetric Transfer Hydrogenation. Chem. Commun.
2018, 54, 13503−13506.
(11) For a seminal work, see: (a) Welch, G. C.; San Juan, R. R.;
Masuda, J. D.; Stephan, D. W. Reversible, Metal-Free Hydrogen
Activation. Science 2006, 314, 1124−1126. For leading reviews, see:
(b) Stephan, D. W.; Erker, G. Frustrated Lewis Pairs: Metal-Free
Hydrogen Activationand More. Angew. Chem., Int. Ed. 2010, 49, 46−
́
76. (c) Soos, T. Design of Frustrated Lewis Pair Catalysts for
MetalFree and Selective Hydrogenation. Pure Appl. Chem. 2011, 83,
667−675. (d) Erker, G. Frustrated Lewis Pairs: Some Recent
Developments. Pure Appl. Chem. 2012, 84, 2203−2217. (e) Paradies,
J. Metal-Free Hydrogenation of Unsaturated Hydrocarbons Employ-
ing Molecular Hydrogen. Angew. Chem., Int. Ed. 2014, 53, 3552−
3557. (f) Stephan, D. W. Frustrated Lewis Pairs: From Concept to
Catalysis. Acc. Chem. Res. 2015, 48, 306−316. (g) Stephan, D. W.;
Erker, G. Metal-Free Hydrogenation of Unsaturated Hydrocarbons
Employing Molecular Hydrogen. Angew. Chem., Int. Ed. 2015, 54,
6400−6441. (h) Stephan, D. W. Frustrated Lewis Pairs. J. Am. Chem.
Soc. 2015, 137, 10018−10032. (i) Stephan, D. W. The Broadening
Reach of Frustrated Lewis Pair Chemistry. Science 2016, 354, aaf7229.
For leading reviews on asymmetric metal-free hydrogenations, see:
(j) Liu, Y.; Du, H. Frustrated Lewis Pair Catalyzed Asymmetric
Hydrogenation. Huaxue Xuebao 2014, 72, 771−777. (k) Feng, X.; Du,
H. Metal-Free Asymmetric Hydrogenation and Hydrosilylation
Catalyzed by Frustrated Lewis Pairs. Tetrahedron Lett. 2014, 55,
6959−6964. (l) Shi, L.; Zhou, Y.-G. Enantioselective Metal-Free
Hydrogenation Catalyzed by Chiral Frustrated Lewis Pairs.
ChemCatChem 2015, 7, 54−56. (m) Meng, W.; Feng, X.; Du, H.
D
https://dx.doi.org/10.1021/acs.orglett.0c01930
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
Frustrated Lewis Pairs Catalyzed Asymmetric Metal-Free Hydro-
genations and Hydrosilylations. Acc. Chem. Res. 2018, 51, 191−201.
(12) (a) Liu, Y.; Du, H. Chiral Dienes as “Ligands” for Borane-
Catalyzed Metal-Free Asymmetric Hydrogenation of Imines. J. Am.
Chem. Soc. 2013, 135, 6810−6813. (b) Liu, Y.; Du, H. Metal-Free
Borane-Catalyzed Highly Stereoselective Hydrogenation of Pyridines.
J. Am. Chem. Soc. 2013, 135, 12968−12971. (c) Wei, S.; Du, H. A
Highly Enantioselective Hydrogenation of Silyl Enol Ethers Catalyzed
by Chiral Frustrated Lewis Pairs. J. Am. Chem. Soc. 2014, 136, 12261−
12264. (d) Zhang, Z.; Du, H. A Highly cis-Selective and
Enantioselective Metal-Free Hydrogenation of 2,3-Disubstituted
Quinoxalines. Angew. Chem., Int. Ed. 2015, 54, 623−626. (e) Gao,
B.; Feng, X.; Meng, W.; Du, H. Asymmetric Hydrogenations of
Ketones and Enones with Chiral Lewis Base-Led Frustrated Lewis
Pairs. Angew. Chem., Int. Ed. 2020, 59, 4498−4504.
(13) (a) Li, S.; Li, G.; Meng, W.; Du, H. A Frustrated Lewis Pair
Catalyzed Asymmetric Transfer Hydrogenation of Imines Using
Ammonia Borane. J. Am. Chem. Soc. 2016, 138, 12956−12962. (b) Li,
S.; Meng, W.; Du, H. Asymmetric Transfer Hydrogenations of 2,3-
Disubstituted Quinoxalines with Ammonia Borane. Org. Lett. 2017,
19, 2604−2606. (c) Zhao, W.; Feng, X.; Yang, J.; Du, H. Asymmetric
Transfer Hydrogenations of β-N-Substituted Enamino Esters with
Ammonia Borane. Tetrahedron Lett. 2019, 60, 1193−1196.
(14) (a) Parks, D. J.; von H. Spence, R. E.; Piers, W. E.
Bis(pentafluoropheny1)borane: Synthesis, Properties, and Hydro-
boration Chemistry of a Highly Electrophilic Borane Reagent. Angew.
Chem., Int. Ed. Engl. 1995, 34, 809−811. (b) Parks, D. J.; Piers, W. E.;
Yap, G. P. A. Synthesis, Properties, and Hydroboration Activity of the
Highly Electrophilic Borane Bis(pentafluorophenyl)borane, HB-
(C6F5)2. Organometallics 1998, 17, 5492−5503.
(15) (a) Ellman, J. A.; Owens, T. D.; Tang, T. P. N-tert-
Butanesulfinyl Imines: Versatile Intermediates for the Asymmetric
Synthesis of Amines. Acc. Chem. Res. 2002, 35, 984−995. (b) Ferreira,
́
F.; Botuha, C.; Chemla, F.; Perez-Luna, A. tert-Butanesulfinimines:
structure, synthesis and synthetic applications. Chem. Soc. Rev. 2009,
38, 1162−1186. (c) Robak, M. T.; Herbage, M. A.; Ellman, J. A.
Synthesis and Applications of tert-Butanesulfinamide. Chem. Rev.
2010, 110, 3600−3740.
(16) (a) Stephens, F. H.; Pons, V.; Baker, R. T. Ammonia-Borane:
the Hydrogen Source par Excellence? Dalton Trans. 2007, 2613−2626.
(b) Marder, T. B. Will We Soon Be Fueling our Automobiles with
Ammonia−Borane? Angew. Chem., Int. Ed. 2007, 46, 8116−8118.
(c) Hamilton, C. W.; Baker, R. T.; Staubitz, A.; Manners, I. B−N
Compounds for Chemical Hydrogen Storage. Chem. Soc. Rev. 2009,
38, 279−293. (d) Staubitz, A.; Robertson, A. P. M.; Manners, I.
Ammonia-Borane and Related Compounds as Dihydrogen Sources.
Chem. Rev. 2010, 110, 4079−4124. (e) Huang, Z.; Autrey, T. Boron−
Nitrogen−Hydrogen (BNH) Compounds: Recent Developments in
Hydrogen Storage, Applications in Hydrogenation and Catalysis, and
New Syntheses. Energy Environ. Sci. 2012, 5, 9257−9268. (f) Johnson,
H. C.; Hooper, T. N.; Weller, A. S. The Catalytic Dehydrocoupling of
Amine−Boranes and Phosphine−Boranes. Top. Organomet. Chem.
2015, 49, 153−220. (g) Rossin, A.; Peruzzini, M. Ammonia−Borane
and Amine−Borane Dehydrogenation Mediated by Complex Metal
Hydrides. Chem. Rev. 2016, 116, 8848−8872.
(17) Ding, F.; Zhang, Y.; Zhao, R.; Jiang, Y.; Bao, R. L.-Y.; Lin, K.;
Shi, L. B(C6F5)3-Promoted Hydrogenations of N-Heterocycles with
Ammonia Borane. Chem. Commun. 2017, 53, 9262−9264.
E
https://dx.doi.org/10.1021/acs.orglett.0c01930
Org. Lett. XXXX, XXX, XXX−XXX