Combined KOH/BEt3Catalyst for Selective Deaminative Hydroboration of Aromatic Carboxamides for Construction of Luminophores

Article abstract of DOI:10.1021/acs.orglett.0c03033

The selective catalytic C-N bond cleavage of amides into value-added amine products is a desirable but challenging transformation. Molecules containing iminodibenzyl motifs are prevalent in pharmaceutical molecules and functional materials. Here we established a combined KOH/BEt3 catalyst for deaminative hydroboration of acyl-iminodibenzyl derivatives, including nonheterocyclic carboxamides, to the corresponding amines. This novel transition-metal-free methodology was also applied to the construction of Clomipramine and luminophores.

Full text of DOI:10.1021/acs.orglett.0c03033

pubs.acs.org/OrgLett
Letter
Combined KOH/BEt3 Catalyst for Selective Deaminative
Hydroboration of Aromatic Carboxamides for Construction of
Luminophores
Wubing Yao,* Jiali Wang, Aiguo Zhong, Jinshan Li, and Jianguo Yang
Cite This: https://dx.doi.org/10.1021/acs.orglett.0c03033
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ABSTRACT: The selective catalytic C−N bond cleavage of amides into value-added
amine products is a desirable but challenging transformation. Molecules containing
iminodibenzyl motifs are prevalent in pharmaceutical molecules and functional
materials. Here we established a combined KOH/BEt3 catalyst for deaminative
hydroboration of acyl-iminodibenzyl derivatives, including nonheterocyclic carbox-
amides, to the corresponding amines. This novel transition-metal-free methodology
was also applied to the construction of Clomipramine and luminophores.
minodibenzyl and its derivatives have emerged as promising
alternatives to heterocycles for use as emissive or host
Catalytic hydrogenation is a significant protocol for the C−
I
N bond cleavage of amides (Scheme 1a). During the past
materials for organic light-emitting diodes (Figure 1).1 At
Scheme 1. Selective Cleavage of Amide C−N Bonds
Figure 1. Representative iminodibenzyl derivatives.
present, the dominant methods for synthesis of these value-
added iminodibenzyl compounds include two-component
cyclizations and one-pot cascade protocols.2 Unfortunately,
the reported cyclization methods require nontrivial starting
materials, whereas the cascade approaches are generally
narrower in substrate scope and functional group compati-
bility. These limitations seriously restrict the application of
these two methodologies.
As an alternative, the controlled reduction of amides to
amines (hydrogenolysis) represents a valuable transformation
for the synthesis of iminodibenzyl cores. However, this route
remains challenging because of the relatively low electro-
philicity of amide carbonyl groups and the high bond
dissociation energy of C−N bonds (95−104 kcal mol−1).3
The traditional methods for the dissociation of C−N bonds in
amides often suffer from terrible selectivity (C−N vs C−O
cleavage), high-cost chemicals, and excess waste products.4
Obviously, for precise and predictable control of C−N bond
cleavage in catalytic amide reduction, it is of interest and
necessary to continue development of synthetic strategies that
are highly selective and more ecofriendly.
decade, in work pioneered by Milstein,5 Ikariya,6 and Bergens,7
elegant transition-metal catalysts, dominated by the noble
metal Ru8 and involving the reductive cleavage of amide C−N
bonds, have been reported. Recently, some ecofriendly and
low-cost base-metal catalysts, including Fe,9 Co,10 Mn,11 and
Mo12 complexes, were developed for effective catalysis in the
deaminative hydrogenation of amides. While each approach
Received: September 11, 2020
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© XXXX American Chemical Society
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A

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a
has its own merits, it is noteworthy that many hydrogenation
methods involve high-pressure reactors and high temperatures,
which may affect the application of these processes.
Table 1. Optimization of Reaction Conditions
Complementary to the direct hydrogenolysis of amide
derivatives is the use of hydrosilanes as hydrogen donors,
providing a promising alternative method because of its safety
HBpin
yield of 2a
yield of 3
and operational simplicity (Scheme 1b). The elegant works of
entry
catalyst
(equiv)
solute
(%)
(%)
14
Enthaler13 and Fernandez-Alvarez using Mo-catalyzed or Ir-
́
1
2
KOH/BEt3
KOTMS/
BEt3
CH3ONa/
BEt3
NaOH/BE3
KOAc/BEt3
BEt3
KOH
B(C6F5)3
BPh3
KOH/BEt3
KOH/BEt3
KOH/BEt3
KOH/BEt3
KOH/BEt3
2.5
2.5
MTBE
MTBE
31
15
2
2
catalyzed hydrosilylation of amides allow access to the
hydrolyzed products. However, current hydrosilylation ap-
proaches frequently suffer from using complex metal catalysts,
noble/air-sensitive silanes (Ph2SiH2 and HSiMe2Ph), and non-
ideal selectivity.
3
2.5
MTBE
7
3
4
5
6
7
8
9
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.0
2.0
MTBE
MTBE
MTBE
MTBE
MTBE
MTBE
THF
Et2O
PhMe
THF
17
3
0
0
<1
<1
50
30
19
5
<1
0
As a reducing agent, pinacolborane (HBpin) is a relatively
low-cost, bench-stable hydrogen source that exhibits high
functional group tolerance.15 Therefore, catalytic reduction of
carboxamide moieties using HBpin is highly desirable.
Surprisingly, the hydroboration reduction of amide substrates
(C−O cleavage) is very rare.15,16 Moreover, the reductive
cleavage of C−N bonds in amide functions by employing
hydroboration protocols was disclosed by Findlater only using
a lanthanum catalyst.16a To date, no transition-metal-free
catalyst has been realized for the selective C−N bond cleavage
of amides utilizing hydroborons.
0
<1
<1
4
2
3
10
11
12
13
51
1
<1
b
14
THF
82 (80)
a
Reaction conditions: 1a (1.0 mmol), HBpin (2.5 mmol), catalyst
(5.0 mol %), solvent (2 mL), rt, 16 h; then 1 M NaOH/H2O, rt, 1 h.
Hence, the development of non-transition-metal catalysts for
selective deaminative hydroboration of amides, especially the
iminodibenzyl amide derivatives of the broad-spectrum amine
luminophores, is of great interest. Among the various non-
transition-metal catalysts investigated, boron Lewis acids are
especially attractive because of their high Lewis acid strength
and low environmental impact, and significant progress has
been made in recent years. Notably, since the ground-breaking
finding that the strong boron Lewis acid B(C6F5)3 can activate
Si−H bonds through η1-coordination, this Lewis acid has
emerged as a powerful tool for cleavage of carbon−heteroatom
bonds.17 However, as a catalyst, B(C6F5)3 is relatively
expensive. In contrast, BEt3 is more air-stable and at least
100 times less expensive than B(C6F5)3. Inspired by previous
works,18,19 we herein report KOH/BEt3-catalyzed hydro-
genolysis of C−N bonds in acyl-iminodibenzyl derivatives to
produce iminodibenzyl luminophores in good yield and high
selectivity (Scheme 1c), high Lewis acid strength, and low
environmental impact, and significant progress has been made
in recent years. Notably, since the ground-breaking finding that
the strong boron Lewis acid B(C6F5)3 can activate Si−H bonds
through η1-coordination, this Lewis acid has emerged as a
powerful tool for cleavage of carbon−heteroatom bonds.17
However, as a catalyst, B(C6F5)3 is relatively expensive. In
contrast, BEt3 is more air-stable and at least 100 times less
expensive than B(C6F5)3. Inspired by previous works,18,19 we
herein report KOH/BEt3-catalyzed hydrogenolysis of C−N
bonds in acyl-iminodibenzyl derivatives to produce iminodi-
benzyl luminophores in good yield and high selectivity
(Scheme 1c). 3-Chloro-iminodibenzyl (2a) is an important
precursor for the construction of highly valuable Clomipr-
amine, which is used as a treatment for depression. Thus, we
commenced the selective cleavage of the C−N bond in 3-
chloro-iminodibenzyl amide (1a) with HBpin by evaluating
the catalytic activities of non-transition-metal catalysts. As
shown in Table 1, the reaction in the presence of 5.0 mol %
KOH/BEt3 and 2.5 equiv of HBpin at room temperature
selectively formed 2a in the best yield after 16 h (entry 1).
Notably, the same reaction using BEt3 or KOH alone did not
The GC yield is based on 1a with 1,2-dimethoxybenzene as an
internal standard, and the isolated yield is in parentheses. At 100 °C
for 24 h.
b
proceed (entry 6 or 7, respectively). To our surprise, the use of
electrophilic B(C6F5)3 or Lewis acidic BPh3 also failed to
generate any desired product under standard conditions
despite having enabled various reductions of amides (entries
8 and 9).17,20 Subsequent optimization revealed that use of a
tetrahydrofuran (THF) solvent, which has a moderate
dielectric constant, suppressed the formation of 3 and provided
2a as the primary product in 50% yield (entry 10). Pleasingly,
the selectivity of the C−N bond cleavage was efficiently
improved using 2.0 equiv of HBpin (entry 13). Remarkably,
extending the temperature and reaction time further improved
the yield and selectivity of 2a (entry 14).
Utilizing the optimized conditions, we explored the substrate
scope concerning amide starting materials for the selective C−
N bond cleavage (Scheme 2). Pleasingly, a variety of acyl-
iminodibenzyl and acyl-iminostilbene derivatives bearing
halogen (1a, 1j, 1k, and 1v), alkenyl (1f), alkoxy (1h and
1y), trifluoromethyl (1p), or sulfhydryl (1q) substituents were
all well tolerated in hydroboration reactions, selectively
providing the valuable iminodibenzyl analogues in moderate
to excellent yields. However, substrates bearing steric
substituents such as isopropyl (1d), tert-butyl (1t), and phenyl
(1u and 1v) groups decreased the reactivities. To our delight,
acetyl-protected carbazole (1i−1l) and 9,10-dihydroacridine
derivatives (1m−1q) similarly gave ideal yields of the nitrogen-
containing luminophores at room temperature.
Encouraged by the success of C−N bond cleavage of acyl-
iminodibenzyl analogues, we continued to evaluate the
deaminative hydroboration of nonheterocyclic amides. Pleas-
ingly, when the reactions were performed at room temperature,
the diarylamine carbonyl substrates, including alkyl (1r−1t and
1w−1ab) and aryl amides (1u and 1v), were all smoothly
transformed to give the corresponding amine luminophores in
moderate to excellent yields with exclusive selectivity.
B
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To disclose a plausible reaction mechanism, several control
tests were performed. First, addition of typical radical
scavengers, such as TEMPO and 9,10-dihydroanthracene, did
not obviously affect the hydroboration transformations
(Scheme 4a), rendering a free radical mechanism unlikely.
Scheme 2. KOH/BEt3-Catalyzed Selective Hydrogenolysis
of Acyl-iminodibenzyl Derivatives
a
Scheme 4. Control Tests
Moreover, addition of commonly used heterogeneous catalyst
poisons such as PMe3 or Hg showed no adverse effect on the
yield of 2a (Scheme 4b), which indicated that the combined
KOH/BEt3 catalyst was likely to be homogeneous under the
described conditions.
Subsequently, the control experiment with 5 mol % KHBEt3
afforded the desired product 2a in a result that was very similar
to the reaction catalyzed by the combined KOH/BEt3 catalyst
(Scheme 4c). This result suggested that KHBEt3 itself may
serve as a catalyst for amide C−N bond cleavage. This result is
consistent with the original mechanism unveiled by Huang for
the BEt3-catalyzed reduction of carbonyl compounds to
alcohols.19
a
Reaction conditions: 1 (1.0 mmol), HBpin (2.0 mmol), BEt3 (5.0
mol %), KOH (5.0 mol %), THF (2 mL), rt, 24 h; then 1 M NaOH/
H2O, rt, 1 h. Yields are of isolated products. At 100 °C for 24 h.
b
In addition, the treatment of various aldehydes with 1.0
equiv of HBpin using 5 mol % catalyst at room temperature
conveniently gave alcohol products in high yields (Scheme 5),
indicating aldehydes as possible intermediates in the current
hydroboration protocol.
Next, we studied the generation of 2a and 3 on the basis of
the amount of HBpin. A presentation of HBpin-dependent
yields for the catalytic C−N bond cleavage of 1a in THF is
given in Figure 2. We found that hydrogenolysis of 1a by
treatment with 2.0 equiv of HBpin offered the best selectivity
and yield of 2a. This result partly contributed to the
explanation of the catalytic cycle reported below.
However, the attempted C−N bond cleavage of N-aryl-N-alkyl
amides or N,N-dialkyl amides did not proceed (see page S4 of
the Supporting Information).
The transition-metal-free-catalyzed C−N cleavage of amides
was further applied in the synthesis of Clomipramine 4
(Scheme 3a), while compound 5, which can provide
derivatives as promising emissive or host materials for organic
light-emitting diodes, was also smoothly prepared from
hydrolyzed product 2b (Scheme 3b).
Scheme 3. Synthetic Applications
Simultaneously, we performed kinetic studies to explore the
roles of the catalyst, amide, and HBpin at the RDS (see pages
S7−S14 of the Supporting Information). Measurements of the
initial rates (kin) for the protocols of different concentrations of
substrates and KOH/BEt3 catalyst exhibited a corresponding
increase in the rates of the reactions. Plots of kin versus the
concentrations of amide, HBpin, and the combined catalyst
offered linear curves (slopes of 3.05 × 10−4, 2.03 × 10−4, and
5.94 × 10−3 M s−1), indicating a first-order rate dependence on
both amide, HBpin, and the catalyst.
On the basis of previous reports15,19,21 and our control tests
and kinetic studies, we proposed a plausible mechanism for the
C
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Scheme 5. KOH/BEt3-Catalyzed Hydroboration Reduction
of Aldehydes
Scheme 6. Plausible Mechanism
a
a
Reaction conditions: aldehyde (1.0 mmol), HBpin (1.0 mmol), BEt3
is released with concomitant regeneration of KBEt3H after
repeating the catalytic cycle likewise. The subsequent
hydrolysis of ammonia borane and alkoxyborane allows the
facile formation of amine and alcohol.
(5.0 mol %), KOH (5.0 mol %), THF (2 mL), rt, 4 h; then 1 M
NaOH/H2O, rt, 1 h. Yields are of isolated products. The yields in
parentheses are GC yields.
In conclusion, a transition-metal-free approach for the
selective catalytic cleavage of inert C−N bonds in acyl-
iminodibenzyl molecules with HBpin, using a low-cost and
abundant combined KOH/BEt3 catalyst, has been disclosed.
The notable advantages of the developed method are high
efficiency, mild conditions, operational simplicity, and excellent
selectivity.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c03033.
General considerations, typical reaction procedures,
DFT calculations, kinetic studies, NMR spectra, and
spectra for pure products (PDF)
AUTHOR INFORMATION
Corresponding Author
Figure 2. Time course for hydrogenolysis of 1a (1.0 mmol), HBpin
(1.0−3.5 mmol), BEt3 (5.0 mol %), KOH (5.0 mol %), and THF (2
mL) at 100 °C for 24 h and then 1 M NaOH/H2O at rt for 1 h. The
GC yield based on 1a with 1,2-dimethoxybenzene as an internal
standard.
■
Wubing Yao − School of Pharmaceutical and Materials
Engineering, Taizhou University, Jiaojiang 318000, P. R.
China; Department of Chemistry, Zhejiang Sci-Tech University,
Hangzhou 310018, P. R. China; orcid.org/0000-0002-
4244-4609; Email: icyyw2010@yeah.net
selective hydroboration reduction (Scheme 6). Initially, BEt3 is
converted by KOH to form a borate that rapidly reacts with
HBpin to generate KBEt3H.19 Subsequently, KBEt3H reacts
with the tert-amide to generate an intermediate ion pair A.
Reactions of hydrides with HBpin through ion pair A
analogues have been described by Shao,15 Crabtree,21a and
Findlater.21b The further reaction of this ion pair with 1 equiv
of HBpin gives borate B and regenerates the catalyst.
Importantly, the density functional theory (DFT)-calculated
enthalpy and entropy of reactions using DFT-B3LYP/3-
21+G* also demonstrated the possible generation of species
A and B (page S14 of the Supporting Information and Tables
S8 and S9). Indeed, with the electron density of the N atom
being decreased by the conjugation effect, borate B undergoes
facile conversion to aldehyde and ammonia borane. Thereafter,
upon addition of an additional 1 equiv of HBpin, alkoxyborane
Authors
Jiali Wang − School of Pharmaceutical and Materials
Engineering, Taizhou University, Jiaojiang 318000, P. R.
China; Department of Chemistry, Zhejiang Sci-Tech University,
Hangzhou 310018, P. R. China
Aiguo Zhong − School of Pharmaceutical and Materials
Engineering, Taizhou University, Jiaojiang 318000, P. R. China
Jinshan Li − School of Pharmaceutical and Materials
Engineering, Taizhou University, Jiaojiang 318000, P. R. China
Jianguo Yang − School of Pharmaceutical and Materials
Engineering, Taizhou University, Jiaojiang 318000, P. R.
China; Department of Chemistry, Zhejiang Sci-Tech University,
Hangzhou 310018, P. R. China; orcid.org/0000-0003-
4268-7384
D
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́
̀
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H.; Hazari, N.; Jaraiz, M.; Nova, A. Chem. Sci. 2020, 11, 2225−2230.
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(b) Das, U. K.; Kumar, A.; Ben-David, Y.; Iron, M. A.; Milstein, D. J.
Am. Chem. Soc. 2019, 141, 12962−12966. (c) Das, U. K.; Janes, T.;
Kumar, A.; Milstein, D. Green Chem. 2020, 22, 3079−3082.
Complete contact information is available at:
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Notes
The authors declare no competing financial interest.
́
(12) Leischner, T.; Artus Suarez, L.; Spannenberg, A.; Junge, K.;
ACKNOWLEDGMENTS
■
Nova, A.; Beller, M. Chem. Sci. 2019, 10, 10566−10576.
(13) Krackl, S.; Someya, C. I.; Enthaler, S. Chem. - Eur. J. 2012, 18,
15267−15271.
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China
(22001188), the Natural Science Foundation of Zhejiang
Province (LQ19B020001), the Project of Taizhou University
Fund for Excellent Young Scholars, and the Science and
Technology Plan Project of Taizhou (1803gy04). The DFT
data were provided by Prof. Aiguo Zhong (Taizhou
University).
́
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A. A.; Glogau, K.; McDonald, C.; Unruh, D. K.; Findlater, M. Nat.
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DEDICATION
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Dedicated to Professor Qizhong Zhou on the occasion of his
48th birthday.
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