Synthesis of mesoionic n-heterocyclic olefins and catalytic application for hydroboration reactions

Article abstract of DOI:10.1021/acs.joc.0c00257

Mesoionic N-heterocyclic olefins have been developed, which feature high ylidic character. These compounds have been used as efficient catalysts for hydroboration of imines, nitriles, and N-heteroarenes.

Full text of DOI:10.1021/acs.joc.0c00257

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Article
Synthesis of Mesoionic N-Heterocyclic Olefins and
Catalytic Application for Hydroboration Reactions
Zengyu Zhang, Shiqing Huang, Linwei Huang, Xingyu Xu, Hongyan Zhao, and Xiaoyu Yan
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.0c00257 • Publication Date (Web): 09 Sep 2020
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Synthesis of Mesoionic N-Heterocyclic Olefins and Catalytic Application for
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Hydroboration Reactions
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Zengyu Zhang, Shiqing Huang, Linwei Huang, Xingyu Xu, Hongyan Zhao and Xiaoyu Yan*
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Department of Chemistry, Renmin University of China, Beijing 100872, China
TOC Graphic
R
R
H
H
H
H
N
N
N
N
N
N
high ylidic character
catalysts for hydroboration
R
R
R
R
mNHO
R2
N
R1
Y
Y´
R3
+
cat. 2 mol%
C6D6
X
Z
R3
X
´
Z
´
R1
R
N
+ HBpin
R
R
H
R2
or
C
cat. 2 mol%
THF
N
Bpin
N
HBpin
Bpin
X´,Y´,Z´ = C or N-Bpin
Bpin
N
X,Y,Z = C or N
R
N
Bpin
Abstract: Mesoionic N-heterocyclic olefins have been developed, which feature high
ylidic character. These compounds have been used as efficient catalysts for
hydroboration of imines, nitriles and N-heteroarenes.
Introduction
Breslow intermediate was firstly proposed as an active intermediate for the
umpolung of aldehydes by Breslow in 1958.1 Since then, this important species, which
functions as a nucleophilic acyl anion, has held an irreplaceable position in synthetic
and organocatalytic chemistry.2 Outstanding work for isolation and characterization of
Breslow intermediates and their analogous forms A (OR, NR2) were reported in
recent years, despite limited stability of the Breslow intermediates.3 Apart from the
Breslow intermediates A, their deoxy forms B, also called N-heterocyclic olefins
(NHOs), were also well studied.4-10 NHOs have been widely used as alternative and
more σ-donating carbon ligands for low-valent main group elements and transition
metals.11,12 Moreover, NHOs have also been established as organocatalysts among a
wide range of organic transformations, such as reduction,13 transesterification,14 small
molecule activation15 and polymerization.16
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Recently, we have reported the synthesis of Breslow intermediates of mesoionic
carbenes C featuring an elongated length of the C-C bond, which mainly due to the
stronger σ-donating property of mesoionic carbenes compared with Breslow
intermediates A.17 Herein we report the deoxy Breslow intermediates of mesoionic
carbenes D, which are mesoionic N-heterocyclic olefins (mNHOs). These compounds
feature high ylidic character, and can act as efficient catalysts for hydroboration of
imines, nitriles and N-heteroarenes. (Figure 1).18
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R1
R
OH(OR, NR2)
N
R2
R2
R1 = alkyl,aryl
R2 = H, alkyl, aryl
X = N, S
N
X = N, S
R
X
X
Breslow intermediates
NHOs
A
B
R1
R1
R
H
N
H
H
OH
N
N
(OR)
R1 = Dipp, Mes
R2 = CH3, Ph
N
N
N
N
N
N
R1
R1
H
R
R
R2
R2
R
this work
mNHOs
Breslow intermediates of MICs
C
D
Figure 1. Breslow intermediates A, NHOs B, Breslow intermediates of MICs C and
mNHOs D.
Results and discussion
Due to the stronger σ-donating property of mesoionic carbenes, we supposed
mNHOs D would present higher ylidic property in contrast with NHOs B. To confirm
this, DFT calculations were performed at BP86/Def2SVP level of theory (Figure 2).
The highest occupied orbital (HOMO) of mNHO D is mainly located on the exocyclic
carbon atom featuring polarized C=C π bond, which is higher in energy than that of
NHO B, as well as NHC E and MIC F. It is noteworthy that NHOs and mNHOs
feature strong polarization of the exocyclic C=C bond, leading to the hybrid of both
ylidic and olefinic characters. Natural Resonance Theory (NRT) analyses at
M06-2X/6-31G(d) level indicate that ylidic character and olefinic character of mNHO
D are in an about 2:1 ratio, while the ratio is 1:2 for that of NHO B (Figure 3). These
calculations supported the high ylidic character of mNHO D.
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N
N
N
N
N
N
N
N
N
N
CH2
CH2
E/eV
7
E
F
B
D
8
9
0.50
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0.25
-0.17
-0.50
-1.50
-1.10
-1.64
-2.50

-3.00
-3.50
-3.06
-3.28
-4.00
-4.50
-4.24
-4.51
Figure 2. Energy (eV) of the HOMOs and LUMOs of NHO B, mNHO D, NHC E,
MIC F, calculated at BP86/Def2SVP level of theory. The frontier orbitals of
mNHO are given on the right.
N N
N
N N
N
N
H
N
H
N
H
N
H
~1:2
~2:1
H
H
H
H
NHO
mNHO
Figure 3. The ylidic and olefinic character resonance forms of NHO and mNHO.
Natural Resonance Theory (NRT) analyses were carried out at
M06-2X/6-31G(d) level of theory to give the corresponding proportion.
Experimentally,
mNHOs
were
prepared
by
α-deprotonation
of
4-methyl-1,2,3-triazolium 1a-c by potassium bis(trimethylsilyl)amide (KHMDS) in
THF under -78 C.19 After removing the solvent, the remaining solids were extracted
o
with toluene and the corresponding mNHOs 2a-c were obtained in 55%-78% yields
(Scheme 1). Besides, similar products were also formed by using potassium
tert-butoxide as the base, but no reaction occurred when amidines were employed,
which suggested the pKa of C-H in 1 was between alcohol and amidinium.
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R1
R1
R1
-
N N
1 N
N N
1 N
PF6
N N
1 N
KHMDS,THF
-78 oC to rt
R2
R2
R
R2
R
R
7
8
9
H
H
H
H
H
H
H
1
2
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Mes
Dipp
Dipp
Ph
N N
N
N N
N
N N
N
Mes
Me
Dipp
Me
Dipp
H
H
H
H
H
H
2a
55%
2b
78%
2c
61%
Scheme 1. Synthesis of mNHO 2a-c.
It should be mentioned that the 1H NMR of 2a and 2b feature broadened signals,
which we supposed a rapid hydrogen transfer between methylene and methyl (Figure
1
4). However, different from 2a and 2b, H NMR signal of 2c presents two broad
peaks of CH2, with the chemical shifts of 3.68 and 2.85. The single crystals of 2a and
2b were obtained by slow evaporation of the corresponding toluene solution at room
temperature in the glovebox, and the ORTEP of single-crystal X-ray diffraction of 2a
and 2b are showed in SI. X-ray structures of the mNHOs 2a and 2b present an equal
length of the exocyclic C-C bonds, which is due to disorder between the two
–CH3/=CH2 sites in the solid state.
R1
R1
N
N
R1
R1
N
N
N
N
fast proton transfer
H
H3C
H
CH3
H
H
Figure 4. Fast proton transfer of 2a and 2b in solution.
With the property of high HOMO energy and high ylidic character in mind, we
started to investigate the catalytic application of mNHOs. We envisioned that mNHOs
could interact with HBpin to form Lewis adducts, leading to weakening the B-H
bonds and enhancing the hydridic character to promote the hydroboration of
unsaturated substrates.20 Natural bond orbital (NBO) analyses of HBpin-base adducts
were performed at M06-2X/Def2tzvp. Among typical strong Lewis bases, such as
NHC, MIC, NHO, and mNHO, the HBpin-mNHO adducts show the most negative
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charge at H-(Bpin) (Scheme 2). To show the superior catalytic activity of mNHOs,
aldehydes and ketones were employed for hydroboration. The results revealed that
mNHO gave higher yields with low catalyst loading compared with NHO (For details
see SI, Figure S11). And then we started to test more challenging work about
hydroboration of imines and nitriles (Scheme 3). To our delight, the catalytic
hydroboration of aldimines furnished the corresponding product 3a in 87% yield with
only 2 mol% of 2a.21 Hydroboration of ketimines catalyzed by 2a failed which might
be due to the steric hindrance. When arylnitriles were employed as substrates, the
corresponding products 3b-d were obtained in high yields. Moreover, not only
aromatic nitriles but also aliphatic nitriles were tolerated. Hydroboration of
acetonitrile afforded the corresponding product 3e in 94% yield. Unfortunately, ester,
amide and nitro groups were not tolerated and no products were detected (3f-h), of
which was due to decomposition of catalyst by these functional groups. It is
noteworthy that the catalytic hydroboration of imines22 and nitriles23 without metals
have not often been reported. Our protocol not only avoided the use of large amounts
of catalysts but also gave almost full conversion of substrates when compared with
Al, Mg-based catalytic system.23b,d For further comparison, NHO was also employed
as catalyst in hydroboration of imines and nitriles under the same conditions, while
lower yields were obtained (75% for 3a and 72% for 3c, for details see SI, Figure
S11). Besides, excellent intramolecular chemoselective hydroboration on aldimine
rather than nitrile was realized for substrate containing both cyano group and imino
group, the corresponding product (3i) was obtained with high isolated yield (Scheme
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4).
L
O
O
O
O
L
B H
B
more hydridic
N
H
N
weaker
N
N
N
N
N
N
N
H2C
H2C
O
O
O
O
O
O
N
O
O
B
B
B
B
H
H
H
H
-0.095
-0.105
-0.139
-0.143
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Scheme 2. NBO analyses of HBpin-base Adducts. Selected NBO Charges of
H-(Bpin) are Given in the Units. The Optimized Structures of
HBpin-base Complexes at M06-2X/Def2tzvp Level of Theory.
H
R
N
R
R
N
R
2a
2 mol%
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Bpin
+
HBpin
or
R1 C N
12 h, 70 oC
THF
Bpin
R1
N
Bpin
Bpin
Bpin
Bpin
S
N
N
N
N
Bpin
Bpin
Bpin
Bpin
F
3a
3b
83%b
3c
3d
87%a
97%b
95%b
Bpin
Bpin
O
Bpin
N
Bpin
Bpin
N
N
N
O
Bpin
Bpin
Bpin
O2N
N
O
OMe
3e
3f
3g
3h
0%b
94%b
0%b
0%b
Scheme 3. Hydroboration of Imine and Nitriles Catalyzed by 2a. aReaction condition:
o
imine (0.50 mmol), HBpin (0.55 mmol), mNHO 2a (2 mol%), THF (1.5 mL), 70 C
b
in sealed tube. Reaction condition: nitriles (0.50 mmol), HBpin (1.1 mmol), mNHO
2a (2 mol%), THF (1.5 mL), 70 oC in sealed tube.
H
2a
2 mol%
HBpin
+
N
N
12 h, 70 oC
THF
Bpin
NC
NC
3i
96%
Scheme 4. Chemoselective Hydroboration of Cyano-substituted Aldimine.
The metal-free catalytic hydroboration of N-heteroarenes24 were also investigated
(Scheme 5). When quinoline, substituted quinolines, isoquinoline and acridine were
employed, the corresponding products, 1,2-addition products and/or 1,4-addition
products, 4a-f were obtained in 73%-98% yields. Besides, we also noticed the
1,2-additon product was the main product when 3-methylquinoline was employed for
hydroboration, which we supposed the steric hindrance affected the selectivity.
Meanwhile, when 2-methylquinoline was employed, no product was detected.
Hydroboration of pyridine afforded the products in low yields (4g/4g´, 33%), which
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was mainly due to the high resonance stabilization of pyridine compounds. Moreover,
pyrazine, pyrimidine, quinazoline and quinoxaline were double-hydroborated in high
yields with only 2 mol% catalyst (4h-l). And 1,3,5-triazine was triple-hydroborated
with almost full conversion (4m). Besides, hydroboration of benzothiazole gave the
corresponding product 4n in 98% yield. For comparison, no product was obtained by
using NHO as catalyst (For details see Figure S11). It should be noted that
NHC-catalyzed 1,2-selective hydroboration of quinolines has been developed
recently.24g
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Y
Y´
2a
2 mol%,
X
Z
X´
Z´
+
HBpin
R
R
H
C6D6
N
N
Bpin
X´,Y´,Z´ = C or N-Bpin
X,Y,Z = C or N
&
&
N
N
Bpin
4a´
N
Bpin
N
N
Bpin
Bpin
Bpin
a
4c
4f
4a
80%
4b
4b´
86%a (1:1)
O
91%a (7:10)
&
O
&
N
N
N
N
N
Bpin
Bpin
4e
Bpin
4e´
Bpin
Bpin
a
98%
4d
4d´
73%a (2:5)
94%a (10:1)
Bpin
Bpin
Bpin
N
N
N
N
&
N
N
N
N
Bpin
4g
Bpin
4g´
Bpin
Bpin
Bpin
c
c
c
4h
4i
4j
98%
85%
97%
33%a,b (2:1)
Bpin
Br
Bpin
Bpin
Bpin
S
N
N
N
N
N
N
N
N
Bpin
Bpin
Bpin
96%
Bpin
a
c
c
d
4n
98%
4k
4l
4m
94%
92%
a
Scheme 5. Hydroboration of N-heteroarenes catalyzed by 2a. N-heteroarenes
b
(0.50 mmol), HBpin (0.55 mmol), mNHO 2a (2 mol%), C6D6 (1.5 mL). The
solvent was replaced by THF. cN-heteroarenes (0.50 mmol), HBpin (1.1 mmol),
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d
mNHO 2a (2 mol%), C6D6 (1.5 mL). 1,3,5-triazine (0.50 mmol), HBpin (1.65
5
6
mmol), mNHO 2a (2 mol%), C6D6 (1.5 mL).
7
8
9
In addition, mNHO-catalyzed hydroboration reaction could be scaled up. For
example, gram-scale hydroboration of p-fluorobenzonitrile and benzothiazole
afforded the corresponding products 3c and 4n in 86% and 92% yields, respectively
(Scheme 6).
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CN
Bpin
Standard condition
N
+ HBpin
3c
F
Bpin
F
86%
5.0 mmol 12.0 mmol
0.605 g
4.3 mmol
1.621 g
S
S
N
Standard condition
+ HBpin
4n
92%
N
Bpin
5.0 mmol
0.675 g
5.5 mmol
4.6 mmol
1.209 g
Scheme 6 Gram-Scale Hydroboration of p-Fluorobenzonitrile and Benzothiazole.
For further investigation of the mechanism, we tried stoichiometric reaction
1
between mNHO and HBpin. However, no obvious adduct was observed by both H
NMR and 11B NMR. DFT calculation showed that the mNHO-HBpin adduct was ~10
kcal/mol higher in energy. We supposed there is an equilibrium between
mNHO/HBpin and their adduct. Based on the experimental results and calculation, we
proposed a plausible mechanism for mNHO catalyzed hydroboration reaction. Firstly,
reaction of HBpin with mNHO afforded the adduct mNHO-HBpin in equilibrium,
which presented increased hydridic character of B-H bond. Then nucleophilic B-H
bond attacked C atom of C=N bond, while the N atom reacted with another HBpin,
leading to the formation of boryl-substituted triazolium 5 and amido-HBpin anion 6.
The transition state was calculated by DFT with an energy barrier of 27.8 kcal/mol
(For details of the calculated reaction pathways, see Figure S8).27 Hydride transfer
between 5 and 6 led to formation of mNHO-HBpin adduct as well as hydroboration
product (Scheme 7).
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N
N
N
N
N
N
H
N
HBpin
N
Bpin
HBpin
N
N
N
H3C
CH2
H3C
CH2
7
B
O
8
9
B
O
O
O
H
H
H3C
CH2
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TS
H
N
N
N
N
Bpin
+
O
O
H
H3C
CH2
B
O
B
N
O
H
6
5
Scheme 7. Plausible Mechanism for mNHO-Catalyzed Hydroboration Reaction.
Conclusion
In summary, mesoionic N-Heterocyclic olefins (mNHOs) have been developed,
which feature high ylidic character caused by the polarization of the exocyclic
carbon-carbon bond, suggesting superior σ-donating property compared with NHOs.
Our study indicates that mNHOs show efficient catalytic activity for hydroboration of
imines, nitriles and N-heteroarenes. Research in our laboratory is currently underway
to broaden the catalytic application.
EXPERIMENTAL SECTION
General Information. All reagents were obtained from commercial suppliers and
used directly without further purification, unless otherwise noted. Solvents were
obtained directly from solvent purification system to get rid of moisture and oxygen.
For NMR solvents C6D6 and CDCl3 were dried up with activated molecular sieve and
further degassed with nitrogen, then moved to glovebox for further use. All air
sensitive synthetic manipulations were performed in glovebox or carried out in
flame-dried glassware equipped with magnetic agitators under nitrogen atmosphere
using Schlenk techniques. All reactions that required heating were carried out under
oil bath conditions. NMR spectra were recorded on Bruker 400 MHz and 600 MHz
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spectrometers. The chemical shift data for each signal were given in units of δ (ppm)
relative to tetramethylsilane (TMS) where δ (TMS) = 0, and referenced to the residual
solvent resonances. NMR multiplicities were abbreviated as follows: s = singlet, d =
doublet, t = triplet, m = multiplet, br = broad signal. Coupling constants J were given
in Hz. High-resolution mass spectra were acquired on Thermo Q Exactive Focus
Hybrid Quadrupole-Orbitrap mass spectrometer using electrospray ionization mode
(ESI).
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Synthesis of compounds 1a-c.
1,3-dimesityl-4,5-dimethyl-1H-1,2,3-triazolium hexafluorophosphate (1a). A
250 mL Schlenk flask was charged with 1,3-dimesityltriazene19 (2.80 g, 10.0 mmol),
potassium hexafluorophosphate (1.84 g, 10.0 mmol) and 2-butyne (950 μL ,12.0
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mmol). And dichloromethane (20 mL) was added to the flask in the dark at -78 C.
Then tert-butyl hypochlorite (1.30 mL, 12.0 mmol) was added to the mixture. The
mixture was stirred overnight and slowly warmed to room temperature. The contents
were filtered with celite, and the solid was washed with dichloromethane (20 mL × 3).
All the filtrate was collected, and solvent was removed under reduced pressure. Then
residual solid was washed with diethyl ether to obtain white solid (4.01 g, 84%). 1a
was further purified after recrystallization by diffusion of diethyl ether in an acetone
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solution at room temperature. H NMR (CDCl3, 400 MHz): δ 7.11 (s ,4H), 2.40 (s,
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6H), 2.38 (s, 6H), 2.06 (s, 12H). C{1H} NMR (CDCl3, 101 MHz): δ 142.8(trz-C),
140.6(aryl-C),
134.8(aryl-C),
130.1(aryl-C),
129.3(aryl-C),
21.2(ArCH3),
16.8(ArCH3), 8.6(trz-CH3). HRMS (ESI) m/z: [M − PF6]+ Calcd. for C22H28N3
334.2283; Found 334.2270.
1,3-Bis(2,6-diisopropyphenyl)-4,5-dimethyl-1H-1,2,3-triazolium
hexafluorophosphate (1b). A 250 mL Schlenk flask was charged with
1,3-bis(2,6-diisopropyphenyl)triazene19,25 (3.65 g, 10.0 mmol), potassium
hexafluorophosphate (1.84 g, 10.0 mmol) and 2-butyne (950 μL ,12.0 mmol). And
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dried dichloromethane (20 mL) was added to the flask in the dark at -78 C. Then
tert-butyl hypochlorite (1.30 mL, 12.0 mmol) was added to the mixture. The mixture
was stirred overnight and slowly warmed to room temperature. The contents were
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filtered with celite, and the solid was washed with dichloromethane (20 mL × 3). All
the filtrate was collected and solvent was removed under reduced pressure. Then
residual solid was washed with diethyl ether to obtain white solid (5.34 g, 95%). 1b
was further purified after recrystallization by diffusion of diethyl ether in an acetone
solution at room temperature. 1H NMR (CDCl3, 600 MHz): δ 7.66 (t, J = 7.8 Hz, 2H),
7.42 (d, J = 7.9 Hz, 4H), 2.42 (s, 6H), 2.22 (sept, J = 6.8 Hz, 4H), 1.30 (d, J = 6.7 Hz,
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12H), 1.13 (d, J = 6.8 Hz, 12H). C{1H} NMR (CDCl3, 150 MHz): δ 145.8(trz-C),
141.1(Aryl-C), 133.1(Aryl-C), 128.5(Aryl-C), 125.1(Aryl-C), 29.0(CH(CH3)2),
25.1(CH3), 23.1(CH3), 9.1(trz-CH3). HRMS (ESI) m/z: [M − PF6]+ Calcd. for
C28H40N3 418.3217; Found 418.3211.
1,3-Bis(2,6-diisopropyphenyl)-4-phenyl,5-methyl-1H-1,2,3-triazolium
hexafluorophosphate (1c). A 250 mL Schlenk flask was charged with
1,3-bis(2,6-diisopropyphenyl)triazene19,25 (3.65 g, 10.0 mmol), potassium
hexafluorophosphate (1.84 g, 10.0 mmol) and 1-phenylprop-1-yne (1250 μL ,10.0
mmol). And dried dichloromethane (20 mL) was added to the flask in the dark at -78
oC. Then tert-butyl hypochlorite (1.30 mL, 12.0 mmol) was added to the mixture. The
mixture was stirred overnight and slowly warmed to room temperature. The contents
were filtered with celite, and the solid was washed with dichloromethane (20 mL × 3).
All the filtrate was collected and solvent was removed under reduced pressure. Then
residual solid was washed with diethyl ether to obtain white solid (5.75 g, 92%). 1c
was further purified after recrystallization by diffusion of diethyl ether in an acetone
solution at room temperature. 1H NMR (600 MHz, CDCl3): δ 7.72 (t, J = 7.9 Hz, 1H),
7.61 (t, J = 7.8 Hz, 1H), 7.48 (d, J = 7.9 Hz, 2H), 7.47 – 7.42 (m, 5H), 7.33 (d, J = 7.9
Hz, 2H), 2.53 (s, 3H), 2.37 – 2.27 (sept, J = 6.8 Hz, 4H), 1.36 (d, J = 6.7 Hz, 6H),
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1.19 (d, J = 6.8 Hz, 6H), 1.15 (d, J = 6.8 Hz, 6H), 1.05 (d, J = 6.7 Hz, 6H). C{1H}
NMR (150 MHz, CDCl3): δ 145.7(trz-C), 145.5(trz-C), 142.8(Aryl-C), 140.7(Aryl-C),
133.4(Aryl-C), 133.2(Aryl-C), 131.9(Aryl-C), 129.7(Aryl-C), 129.6(Aryl-C),
129.1(Aryl-C), 128.4(Aryl-C), 125.3(Aryl-C), 125.1(Aryl-C), 121.2(Aryl-C),
29.4(CH(CH3)2), 29.3(CH(CH3)2), 25.6(CH3), 25.2(CH3), 23.1(CH3), 22.3(CH3),
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9.8(trz-CH3). HRMS (ESI) m/z: [M − PF6]+ Calcd. for C33H42N3 480.3373; Found
480.3372.
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General procedure for the preparation of mNHOs.
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A 100 mL Schlenk flask was charged with a magnetic agitator, triazolium salt (1.0
mmol, 1.0 equiv) and potassium bis(trimethylsilyl)amide (1.1 mmol, 1.1 equiv). Then
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dried THF (10 mL) was added to the flask under -78 C. The mixture was stirred
about 2 h and slowly warmed to room temperature. Then solvent was removed under
reduce pressure, and the residual solid was extracted with dried toluene (20 mL).
Solvent was removed again under reduce pressure to afford mNHOs 2.
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(2a) Brownish red solid (183.3 mg, 55%). H NMR (400 MHz, C6D6): δ 6.72 (br,
4H), 2.25-2.06 (br, 23H). 13C{1H} NMR (101 MHz, C6D6): δ 139.4(trz-C),
135.7(Aryl-C), 134.3(Aryl-C), 132.8(Aryl-C), 129.3(Aryl-C), 128.9(Aryl-C),
20.8(CH3), 17.6(CH3). 16.7(CH3). HRMS (ESI) m/z: [M + H]+ Calcd. for C22H27N3
334.2283; Found 334.2285.
(2b) Brownish solid (325.6 mg, 78%). 1H NMR (400 MHz, C6D6): δ 7.31- 6.88 (m,
13
6H), 3.60-2.27 (br, 4H), 1.38-0.93 (br, 29H). C{1H} NMR (101 MHz, C6D6): δ
130.2(Aryl-C), 127.9(Aryl-C), 124.0(Aryl-C), 28.5(CH(CH3)2), 24.6(CH3), 23.1(CH3),
22.6(CH3). HRMS (ESI) m/z: [M + H]+ Calcd. for C28H40N3 418.3217; Found
418.3219.
(2c) Dark blue solid (292.8 mg, 61%). 1H NMR (400 MHz, C6D6): δ 7.42 (br, 2H),
7.31-7.29 (m, 1H), 7.23 (d, J = 7.4 Hz, 2H), 7.13-7.12 (m, 1H), 6.96 (d, J = 7.7 Hz,
2H), 6.92 (t, J = 7.6 Hz, 2H), 6.77 (t, J = 7.2 Hz, 1H), 3.68 (br, 1H), 3.54 (br, 2H),
3.15 (sept, J = 6.6 Hz, 2H), 2.85 (br, 1H), 1.45 (d, J = 6.6 Hz, 6H), 1.28 (d, J = 6.6 Hz,
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6H), 1.19 (d, J = 6.8 Hz, 6H), 0.89 (d, J = 6.6 Hz, 6H). C{1H} NMR (101 MHz,
C6D6): δ 148.2(trz-C), 147.0(trz-C), 145.7(Aryl-C), 132.9(Aryl-C), 132.7(Aryl-C),
130.7(Aryl-C), 130.1(Aryl-C), 125.7(Aryl-C), 125.4(Aryl-C), 124.5(Aryl-C),
124.2(Aryl-C), 45.5(CH2), 29.8(CH(CH3)2), 28.7(CH(CH3)2), 25.5(CH3), 24.2(CH3),
23.3(CH3), 22.3(CH3). HRMS (ESI) m/z: [M + H]+ Calcd. for C33H42N3 480.3373;
Found 480.3372.
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General procedure for hydroboration of imines and nitriles. A 25 mL Schlenk
flask was charged with a magnetic agitator, aldimine or nitriles (0.50 mmol, 1.0 equiv)
and mNHO 2a (0.0040 mmol, 0.020 equiv). Then dried THF (1.5 mL) was slowly
added to the flask. After that HBpin was added based on the [C=N]/[HBpin]=1:1.1.
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The reaction mixture was stirred at 70 C for 12 h under oil bath conditions. After
being allowed to cool to room temperature, solvent was removed under reduce
pressure, and the products were washed with pentane (2  0.5 mL) for further
purification. (Besides, the blank control experiments were tried under the same
conditions in the absence of 2a. The results showed imines were hydroborated in low
yields, and the corresponding products 3a and 3i were obtained in 37% and 45%
yields. But no products were detected for nitriles, for details see Table S7)
(3a)22b, e Colorless oil (134.4 mg, 87% yield). 1H NMR (400 MHz, C6D6): δ 7.49 (d,
J = 7.8 Hz, 2H), 7.24 (d, J = 7.5 Hz, 2H), 7.14-7.00 (m, 6H), 4.78 (s, 2H), 1.08 (s,
12H). 13C{1H} NMR (101 MHz, C6D6): δ 146.5(Aryl-C), 140.7(Aryl-C),
126.4(Aryl-C), 121.5(Aryl-C), 120.7(Aryl-C), 82.7(C(CH3)2), 51.2(N(CH2)),
24.4(C(CH3)2).
(3b)23d Colorless solid (148.9 mg, 83% yield). 1H NMR (400 MHz, C6D6): δ 7.6 (d,
J = 7.4 Hz, 2H), 7.26 (t, J = 7.6 Hz, 2H), 7.14-7.10 (m, 1H), 4.62 (s, 2H), 1.03 (s,
24H). 13C{1H} NMR (101 MHz, C6D6): δ 143.5(Aryl-C), 131.8(Aryl-C),
131.7(Aryl-C), 82.3(C(CH3)2), 47.6(N(CH2)), 24.4(C(CH3)2).
(3c)23c Colorless solid (355.5 mg, 97% yield). H NMR (400 MHz, CDCl3): δ
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7.28-7.26 (m, 2H), 6.93-6.90 (m, 2H), 4.17 (s, 2H), 1.19 (s, 24H). 13C{1H} NMR (101
MHz, CDCl3): δ 161.5 (d, 1JC-F = 243.0 Hz, Aryl-C), 138.8(Aryl-C), 129.2 (d, 3JC-F
=
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7.9 Hz, Aryl-C), 114.5 (d, JC-F = 21.0 Hz, Aryl-C), 82.3(C(CH3)2), 46.5(N(CH2)),
24.5(C(CH3)2).
(3d)23c Colorless oil (173.3 mg, 95% yield). 1H NMR (400 MHz, CDCl3): δ 7.10 (d,
J = 4.9 Hz, 1H), 6.92 (d, J = 3.2 Hz, 1H), 6.87 (dd, J = 4.9, 3.6 Hz, 1H), 4.37 (s, 2H),
1.22 (s, 12H). 13C{1H} NMR (101 MHz, CDCl3): δ 146.8(Aryl-C), 126.1(Aryl-C),
124.5(Aryl-C), 123.5(Aryl-C), 82.5(C(CH3)2), 42.1(N(CH2)), 24.5(C(CH3)2).
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(3e)23c Colorless solid (139.5 mg, 94% yield). 1H NMR (400 MHz, CDCl3): δ 3.04
(q, J = 7.0 Hz, 2H), 1.21 (s, 24H), 1.02 (t, J = 7.0 Hz, 3H). 13C{1H} NMR (101 MHz,
CDCl3): δ 81.9(C(CH3)2), 38.5(N(CH2)), 24.5(C(CH3)2), 18.6(CH2(CH3)).
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(3i) Colorless solid (160.3 mg, 96% yield). 1H NMR (400 MHz, C6D6): δ 7.29 (d, J
= 8.0 Hz, 2H), 7.13-7.09 (m, 2H), 6.96 (d, J = 8.5 Hz, 2H), 6.84-6.82 (m, 3H), 4.52 (s,
2H), 1.06 (s, 12H). 13C{1H} NMR (101 MHz, C6D6): δ = 145.8(Aryl-C),
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131.9(Aryl-C),
128.8(Aryl-C),
126.8(Aryl-C),
121.9(Aryl-C),
118.7(CN),
112.8(Aryl-C), 110.7(Aryl-C), 83.0(C(CH3)2), 50.8(N(CH2)), 24.3(C(CH3)2).
General procedure for the hydroboration of N-heteroarenes. General procedure
for the hydroboration products 4a-4e: A 25 mL dried Schlenk flask was charged with
a magnetic agitator, N-heteroarenes (0.50 mmol, 1.0 equiv) and mNHO 2a (0.0040
mmol, 0.020 equiv). Then dried C6D6 (1.5 mL) was slowly added to the flask. After
that HBpin was added based on the [C=N]/[HBpin] = 1:1.1. The reaction mixture was
stirred at room temperature for 12 h. Then solvent was removed under reduce pressure,
and the products were washed with pentane (2  0.5 mL) for further purification.
(Besides, the blank control experiments for N-heteroarenes were tried under the same
conditions in the absence of 2a, and no products were detected, for details see Table
S7).
The mixture of 4a and 4a´ were obtained as yellow oil (110.5 mg, 86% yield).
(4a)26a: 1H NMR (400 MHz, C6D6): δ 7.82 (d, J = 8.2 Hz, 1H), 7.14-6.75 (m, 3H),
6.26 (d, J = 9.6 Hz, 1H), 5.57 (dt, J = 9.4, 4.2 Hz, 1H), 4.16 (dd, J = 4.2 Hz, 1.7 Hz
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2H), 1.04 (s, 12H). (4a´)26a: H NMR (400 MHz, C6D6): δ 8.17 (d, J = 8.3 Hz, 1H),
7.14-6.75 (m, 4H), 4.82 (m,1H), 3.32 (m, 2H), 1.02 (s, 14H).
The mixture of 4b and 4b´ were obtained as yellow oil (123 mg, 91% yield). (4b)26a:
1H NMR (400 MHz, C6D6): δ 7.71 (d, J = 8.2 Hz, 1H), 6.66-6.64 (m, 2H), 6.27 (d, J =
9.5 Hz, 1H), 5.64-5.59 (m, 1H), 4.16 (dd, J = 4.2, 1.7 Hz, 2H), 1.10 (s, 3H), 1.05 (s,
12H). (4b´)26a: 1H NMR (400 MHz, C6D6): δ 8.07 (d, J = 8.4 Hz, 1H), 6.95-6.92 (m,
3H), 4.86-4.83 (m, 1H), 3.34 (d, J = 3.3 Hz, 2H), 1.10 (s, 3H), 1.02 (s, 12H).
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(4c)26a Colorless solid (102 mg, 80% yield): H NMR (400 MHz, C6D6): δ
7.02-6.98 (m, 1H), 6.91-6.81 (m, 3H), 6.73 (d, J = 7.4 Hz, 1H), 5.63 (d, J = 7.5 Hz,
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1H), 4.64 (s, 2H), 1.03 (s, 12H). 13C{1H} NMR (101 MHz, C6D6): δ 133.3(N(CH)),
132.6(Aryl-C), 132.0(Aryl-C), 128.7(Aryl-C), 125.9(Aryl-C), 125.2(Aryl-C),
123.5(Aryl-C), 105.9(CH(CH)), 83.1(C(CH3)2), 46.0(N(CH2)), 24.4(C(CH3)2).
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4d and 4d´ were obtained as yellow oil (104.7 mg, 73% yield). (4d)26a: H NMR
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(400 MHz, C6D6): δ 7.69 (d, J = 8.2 Hz, 1H), 6.88-6.85 (m, 1H), 6.55 (d, J = 2.9 Hz,
1H), 6.23 (d, J = 9.5 Hz, 1H), 5.66-5.61 (m, 1H), 4.14 (dd, J = 4.2, 1.7 Hz, 2H), 3.34
(s, 3H), 1.06 (s, 12H). (4d´)26a: 1H NMR (400 MHz, C6D6): δ 8.09 (d, J = 9.2 Hz, 1H),
6.95 (dt, J = 8.1, 1.7 Hz, 1H), 6.73 (dd, J = 9.0, 3.0 Hz, 1H), 6.49 (d, J = 3.1 Hz, 1H),
4.80 (dt, J = 8.1, 1.7 Hz, 1H), 3.35 (s, 2H), 3.34 (s, 3H), 1.07 (s, 12H).
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(400 MHz, C6D6): δ 7.86 (d, J = 8.1 Hz, 1H), 7.12-7.08 (m, 1H), 6.90-6.82 (m, 2H),
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6.01 (s, 1H), 4.09 (s, 2H), 1.50 (s, 3H), 1.06 (s, 12H). (4e´)26b: H NMR (400 MHz,
C6D6): δ 8.20 (d, J = 8.3 Hz, 1H), 7.12-7.08 (m, 1H), 6.90-6.82 (m, 3H), 3.20 (s, 2H),
1.53 (s, 3H), 1.01 (s, 12H).
General procedure for the hydroboration products 4f-4n: A 25 mL Schlenk flask
was charged with a magnetic agitator, N-heteroarenes (0.50 mmol, 1.0 equiv) and
mNHO 2a (0.0040 mmol, 0.020 equiv). Then dried C6D6 or THF (1.5 mL) was slowly
added to the flask. After that HBpin was added based on the [C=N]/[HBpin]=1:1.1.
The reaction mixture was stirred at 70 oC (oil bath) for 12 h (4g-4n) or 18 h (4f). After
being allowed to cool to room temperature, solvent was removed under reduce
pressure. The products were washed with pentane (2  0.5 mL) for further
purification.
(4f)26b Colorless solid (150.4 mg, 98% yield): 1H NMR (400 MHz, C6D6): δ 7.85 (d,
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J = 8.0 Hz, 2H), 7.18-6.93 (m, 6H), 3.54 (s, 2H), 1.06 (s, 12H). C{1H} NMR (101
MHz, C6D6): δ 142.4(Aryl-C), 130.3(Aryl-C), 127.2(Aryl-C), 126.2(Aryl-C),
123.4(Aryl-C), 122.6(Aryl-C), 83.0(C(CH3)2), 33.8(CH2), 24.4(C(CH3)2).
4g and 4g´were obtained as yellow oil (NMR yield 33%, CH2Br2 was used as
internal standard). (4g)26c: 1H NMR (400 MHz, C6D6): δ 6.56 (dt, J = 8.4, 1.6 Hz, 2H),
4.55-4.60 (m, 2H), 2.82 (tt, J = 3.2, 1.6 Hz, 2H), 0.96 (s, 12H). (4g´)26c: 1H NMR (400
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MHz, C6D6): δ 6.74 (d, J = 7.6 Hz, 1H), 5.77-5.83 (m, 1H), 5.04-5.14 (m, 2H), 4.18
(dd, J = 4.0, 1.6 Hz, 2H), 1.00 (s, 12H).
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(4h)26d Colorless solid (164.6 mg, 98% yield): 1H NMR (400 MHz, C6D6): δ 6.72
(d, J =8.2 Hz, 1H), 4.72 (s, 2H), 4.62-4.58 (m, 1H), 3.75 (s, 2H), 1.04 (s, 12H), 1.01
(s, 12H). 13C{1H} NMR (101 MHz, C6D6): δ 129.3(N(CH)), 102.2(CH(CH)CH2),
82.5(C(CH3)2), 82.0(C(CH3)2), 55.8(N(CH2)N)), 41.8(N(CH2)), 24.6(C(CH3)2),
24.4(C(CH3)2).
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(4i)26d Colorless solid (142.8 mg, 85% yield): 1H NMR (400 MHz, C6D6): δ 6.06 (s,
2H), 3.44 (s, 4H), 1.04 (s, 24H). 13C{1H} NMR (101 MHz, C6D6): δ 110.8(N(CH)),
82.4(C(CH3)2), 42.1(N(CH2)), 24.5(C(CH3)2).
(4j) Colorless solid (187.2 mg, 97% yield): 1H NMR (400 MHz, C6D6): δ 8.14-8.11
(m, 2H), 6.96-6.94 (m, 2H), 3.61 (s, 4H), 1.06 (s, 24H). 13C{1H} NMR (101 MHz,
C6D6):
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133.0((Aryl-C)), 121.2(Aryl-C), 120.8(Aryl-C), 82.4(C(CH3)2),
43.9(N(CH2)), 24.4(C(CH3)2). HRMS (ESI) m/z: [M+H+2H2O-2HOBpin] Calcd. for
C8H11N2 135.0917; Found 135.0912.
(4k)26d Colorless solid (198.7 mg, 96% yield): 1H NMR (400 MHz, C6D6): δ 7.03 (s,
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1H), 4.54 (s, 2H), 3.96 (s, 2H), 1.00 (s, 24H). C{1H} NMR (101 MHz, C6D6): δ
130.4(N(CH)), 96.4(BrC), 83.0(C(CH3)2), 82.5(C(CH3)2), 55.1(N(CH2)N),
48.8(N(CH2)), 24.5(C(CH3)2), 24.3(C(CH3)2).
1
(4l)26d Colorless solid (181.4 mg, 94% yield): H NMR (400 MHz, C6D6): δ
8.00-7.98 (m, 1H), 7.11-7.06 (m, 1H), 6.78-6.73 (m, 2H), 4.86 (s, 2H), 4.44 (s, 2H),
1.09 (s, 24H). 13C{1H} NMR (101 MHz, C6D6): δ 141.9(Aryl-C), 126.5(Aryl-C),
124.3(Aryl-C), 121.0(Aryl-C), 120.8(Aryl-C), 82.5(C(CH3)2), 82.2(C(CH3)2),
57.5(N(CH2)N), 45.9(N(CH2)), 24.6(C(CH3)2), 24.4(C(CH3)2).
(4m)26d Colorless solid (213.9 mg, 92% yield): 1H NMR (400 MHz, C6D6): δ 4.62
(s, 6H), 1.07 (s, 36H). 13C{1H} NMR (101 MHz, C6D6): δ 81.9(C(CH3)2),
58.7(N(CH2)), 24.5(C(CH3)2).
(4n)26d Colorless solid (128.8 mg, 98% yield): 1H NMR (400 MHz, C6D6): δ 7.61
(d, J = 8.0 Hz, 1H), 6.98 (d, J = 7.8 Hz, 1H), 6.90 (t, J = 7.7 Hz, 1H), 6.65 (t, J = 7.5
Hz, 1H), 4.92 (s, 2H), 1.01 (s, 12H). 13C{1H} NMR (101 MHz, C6D6): δ
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144.5(Aryl-C), 130.2(Aryl-C), 125.2(Aryl-C), 122.0(Aryl-C), 121.8(Aryl-C),
114.3(Aryl-C), 83.1(C(CH3)2), 52.6(N(CH2)), 24.4(C(CH3)2).
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Gram scale hydroboration of p-fluorobenzonitrile and benzothiazole
A 25 mL Schlenk flask was charged with a magnetic agitator, p-fluorobenzonitrile
(5.0 mmol, 1.0 equiv) and mNHO 2a (0.0040 mmol, 0.020 equiv). Then dried THF
(5.0 mL) was added to the flask. After that HBpin was slowly added to the mixture
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(12 mmol, 2.4 equiv). The reaction mixture was stirred at 70 C (oil bath) for 12 h.
After being allowed to cool to room temperature, solvent was removed under reduce
pressure. The product was washed with pentane (2  1.0 mL) for further purification
and 3c was obtained as colorless solid (1.621 g, 4.3 mmol).
A 25 mL Schlenk flask was charged with a magnetic agitator, benzothiazole (5.0
mmol, 1.0 equiv) and mNHO 2a (0.0040 mmol, 0.020 equiv). Then dried C6D6 (5.0
mL) was added to the flask. After that HBpin was slowly added to the mixture (5.5
mmol, 1.1 equiv). The reaction mixture was stirred at 70 oC (oil bath) for 12 h. After
being allowed to cool to room temperature, solvent was removed under reduce
pressure. The product was washed with pentane (2  1.0 mL) for further purification
and 4n was obtained as colorless solid (1.209 g, 4.6 mmol).
ASSOCIATED CONTENT
The Supporting Information is available free of charge via the Internet at
http://pubs.acs.org.
Copies of 1H and 13C{1H} NMR spectra for all compounds, the calculation details and
X-ray crystallography details of compound 2a, 2b (CIF).
AUTHOR INFORMATION
Corresponding Author
*E-mail for X.Y.: yanxy@ruc.edu.cn.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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This work was supported by the National Natural Science Foundation of China
(21602249), the Fundamental Research Funds for the Central Universities and the
Research Funds of Renmin University of China (Program 20XNLG20), and Public
Computing Cloud Platform, Renmin University of China. We also thank professor
Guy Bertrand and professor Jean Bouffard for sharing their unpublished works.
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(27) Similar transition state has been proposed previously, see ref 13.
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