Hydroboration of nitriles and imines by highly active zinc dihydride catalysts

Article abstract of DOI:10.1039/d0ra09648b

Eco-friendly zinc dihydrides stabilized by N-heterocyclic carbenes were demonstrated to be highly efficient catalysts for the double hydroboration of nitriles with pinacolborane, exhibiting turnover frequencies up to 3000 h-1 at room temperature under sol

Full text of DOI:10.1039/d0ra09648b

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Hydroboration of nitriles and imines by highly
active zinc dihydride catalysts†
Cite this: RSC Adv., 2021, 11, 1128
*
Xiaoming Wang and Xin Xu
Eco-friendly zinc dihydrides stabilized by N-heterocyclic carbenes were demonstrated to be highly efficient
catalysts for the double hydroboration of nitriles with pinacolborane, exhibiting turnover frequencies up to
3000 hꢀ1 at room temperature under solvent-free conditions. The reactions afforded corresponding
diboronated amines with excellent yields and good functional group tolerance. A single Zn–H insertion
product was isolated from a stoichiometric reaction of zinc dihydride with nitrile, and was proved to be
an active species in this transformation. Kinetic studies were performed to give some insights into the
catalytic reactions. In addition, zinc dihydride species also showed high activity for the hydroboration of
imines to boronated amines.
Received 13th November 2020
Accepted 18th December 2020
DOI: 10.1039/d0ra09648b
rsc.li/rsc-advances
hydroboration of organic nitriles. To the best of our knowledge,
only one precedent was reported by Panda et al.15 recently by
Introduction
disclosing that the imidazoline-2-iminato ligand-based zinc alkyl
complexes were active toward hydroboration of nitriles under
The development of efficient synthetic approaches for amines is
of great signicance because this class of compounds has
a widespread application in the ne chemical, agrochemical,
and pharmaceutical industries.1,2 In this context, catalytic
reduction of unsaturated compounds containing e.g. C^N or
C]N bonds with borane has recently emerged and attracted
extensive attention, because it usually affords high reaction selec-
tivity and employs mild reaction conditions.3 Moreover, corre-
sponding hydroboration products, i.e. borylamines, can be used
for the synthesis of a variety of organic compounds.4 The rst
example of nitrile dihydroboration was reported by Nikonov et al.
heating conditions (60 ꢁC), achieving a maximum TOF of 50 hꢀ1
.
In our previous work, we synthesized two molecular zinc(II) dihy-
drides supported by phosphine-functionalized N-heterocyclic car-
bene ligands (Fig. 2), that were found to be efficient for
hydroboration of thermodynamic stable and kinetic inert CO2
under mild conditions.16 Being inspired by this result, we herein
investigated further of these complexes for the catalytic hydro-
boration of nitriles and imines. Gratifyingly, zinc dihydrides 2
exhibit remarkably high activity for hydroboration of a variety of
nitriles and imines, producing boronated amines with a TOF of up
to 3000 hꢀ1 at room temperature in the absence of solvent. A
plausible mechanism is also proposed based on the isolation of
a catalytic intermediate and kinetic studies.
using a molybdenum complex (2,6-iPr2-C6H3N)Mo(H)(Cl)(PMe3)3
(1a, Fig. 1), achieving a turnover frequency (TOF) up to 1.7 hꢀ1
.
5
Since then, many efforts have been made for the development of
catalysts used for this transformation.6 Over the last few years, it
has been expanded to include a variety of metal complexes,
including s-,7 p-,8 d-9 and f-block10 elements, and the representa-
tives of effective complexes for the dihydroboration of nitriles are
shown in Fig. 1. However, most of above complexes show low to
mediate catalytic activity and some of them are unattractive due to
their precious or combustible nature.
Considering the biocompatibility, environmental friendliness
and abundance of zinc ion, it is attractive to use zinc-based cata-
lysts for various organic transformations. Zinc-based complexes
have been successfully used in the hydroboration of a series of
unsaturated molecules, including ketones,11 imines,12 alkynes,13
and N-heterocycles,14 but are rarely seen to be used in the
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry,
Chemical Engineering and Materials Science, Soochow University, Suzhou 215123,
P. R. China. E-mail: xinxu@suda.edu.cn
Fig. 1 Selected metal complexes effective for the hydroboration of
nitriles (reaction temperature and maximum TOF obtained were also
listed).
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra09648b
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conditions (0.5 mol% catalyst loading, room temperature, in
the absence of any solvent; Table 2). Reactions of para- or ortho-
methyl-substituted benzonitriles with HBpin offered the corre-
sponding diboronated amines 5b and 5c in 93% and 86% iso-
lated yields, respectively. Additionally, benzonitriles containing
polar functional groups are tolerated in the reaction. For
example, methoxy, dimethylamino, halogen (F, Cl, and Br)
substituted benzonitriles gave corresponding hydroboration
products 5d–5h in 5–40 min with high isolated yields. Notably,
the reaction was signicantly accelerated when using para-tri-
uoromethyl substituted benzonitrile, leading to a full conver-
sion of substrate only in 2 min. As a result, it afforded
compound 5i with a TOF of 3000 hꢀ1, which is the maximum
value reported to date in nitrile hydroboration.7c,10b For poly-
cyclic 1-naphthonitrile, the quantitative conversion was
observed in 40 min, producing 5j in 90% isolated yield.
Heterocyclic aromatic substrates comprised of a furan, thio-
phene or pyridine framework are applicable in current study to
give anticipated dihydroboration products 5k–5m. The selec-
tivity in the reduction of the nitriles when containing other
unsaturated functional groups was also investigated. In the case
of 4-acetylbenzonitrile, hydroboration of both C]O and C^N
groups occurred at a comparable rate, and only 5n was isolated.
In contrast, methyl 4-cyanobenzoate underwent selective
hydroboration reaction only at C^N moiety even with excess
amount of HBpin, affording compound 5o and keeping ester
Fig. 2 Zinc dihydrides used in this study.
Results and discussion
We initiated our study by using complex 2a (1 mol% based on
Zn) for the reaction of benzonitrile with pinacolborane (HBpin)
at room temperature in various solvents and the results are
summarized in Table 1. The reactions gave low conversions of
benzonitrile in the employed solvents (entries 1–3; 54% in C6D6,
29% in PhF, 26% in THF) in 2 h. It's noteworthy that in all of
these cases, no intermediate species, i.e. PhCH]NBpin, was
detected. Excitingly, a signicant increase in activity was ach-
ieved when reaction was conducted in the absence of any
solvent, which consumed all of the benzonitrile in a short time
(8 min; entry 4), exclusively affording diboronated amine PhCH2-
N(Bpin)2 (5a) with a TOF of 750 hꢀ1. To our knowledge, this
represents the highest activity reported to date for the hydro-
boration of benzonitrile.7c,10b Complex 2b was also applied in this
reaction under similar conditions, which resulted in a lower
conversion of benzonitrile (85%; entry 5) probably due to the
sterically bulky NHC ligand employed for complex 2b. To make
a comparison, a tridentate monoanionic ligand-based zinc mon-
ohydride, i.e. LNNPZnH [LNNP ¼ CH3C(2,6-iPr2C6H3N)CHC(CH3)(-
NCH2CH2PPh2)],17 previously reported by our group, was also
prepared and examined under neat conditions. However, it is
almost ineffective toward the current reaction with 1 mol%
loading in 2 h (entry 6). NHC ligand employed in 2a alone was
found inert for the reaction under the same conditions (entry 7).
Subsequently, reactions of a variety of nitriles with HBpin
promoted by complex 2a were examined under our typical
Table 2 Hydroboration of various organic nitriles with HBpin cata-
lyzed by 2aa
Table 1 Zinc hydride-catalyzed hydroboration of benzonitrilea
Entry Catalyst Loading (mol%) Solvent Time (min) Conv.b (%)
1
2
3
4
5
6
7
2a
2a
2a
2a
2b
0.5
0.5
0.5
0.5
0.5
1
C6D6
C6H5F 120
120
54
29
26
99
85
Trace
Trace
THF
Neat
Neat
Neat
Neat
120
8
8
120
120
L
NHC
NNPZnH
1
a
Reaction conditions: [2a] ¼ 0.0015 mmol, [HBpin] ¼ 0.63 mmol, [R-
a
Reaction conditions: nitrile (0.30 mmol), pinacolborane (0.63 mmol), CN] ¼ 0.30 mmol, room temperature. Conversions were determined
room temperature. Conversions were determined by 1H NMR by 1H NMR spectroscopy (integration of residual 3 vs. 5). Isolated
spectroscopy (integration of residual benzonitrile vs. diborylamine yields were shown in the parentheses. 3.1 eq. HBpin was used.
product).
b
b
c
Reaction was conducted at 80 ꢁC.
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motif intact. Moreover, an intermolecular competition reaction
between benzonitrile, styrene, and HBpin (1 : 1:2 molar ratio)
was examined under above conditions. The hydroboration
product 5a was exclusively formed while the styrene remained
intact, thus indicating a preference of nitrile reduction over the
alkene in the reaction. Furthermore, the scope of the substrates
was expanded to the more challenging aliphatic nitriles. As
shown in Table 2, all three selected nitriles were also success-
fully converted to the target borylamines 5p–5r, although the
ꢁ
reactions required an elevated temperature (80 C) and a pro-
Scheme 1 Synthesis of complex 8 and its use in the hydroboration.
longed reaction time (240 min) to achieve a complete conver-
sion of nitriles.
Inspired by the excellent performance of complex 2a in the
hydroboration of C^N bond, we next examined its application
in the imine hydroboration, which provides an efficient method
for the synthesis of secondary amines.18 It was found that 2a
also showed high activity toward reduction of a variety of imines
with HBpin under neat condition, affording hydroborated
product 7 at room temperature (Table 3). Reaction of N-benzy-
lideneaniline with 1.1 equiv. of HBpin gave 7a in the presence of
0.5 mol% of zinc dihydride (2a), with complete consumption of
imine in 15 min (TOF ¼ 400 hꢀ1).19 Subsequently, a wide range
of C- and N-substituted imines were subjected to the reactions.
For the imines bearing diverse substituents at C-phenyl ring,
the expected hydroborated amines 7b–7h could be obtained in
excellent isolated yields (82–94%) from 10 to 40 min. Among
them, substrates with electron-withdrawing groups (–Cl, –Br,
–CF3 or –NO2) proceeded rapidly in comparison to those with
electron-donating groups (–OMe or –NMe2). Introduction of the
substitute at N-phenyl of imine, either p-Cl or p-Me, slightly
suppressed the rate of hydroboration reactions as in the cases of
generation of 7i and 7j. Aliphatic N-substituted imines are also
tolerated with current catalytic system and produced corre-
sponding products 7k–7m, albeit requiring an elevated
temperature (80 ꢁC) and a prolonged reaction time (6–16 h).
Heterocyclic C-substituted imine was applied under the given
conditions, providing boronated aniline 7n in 40 min. When
introducing an ester group to C-phenyl of imine, hydroboration
reaction still occurred at C]N moiety even with excess amount
of HBpin, affording compound 7o with 82% isolated yield.
To gain more insights into the reaction mechanism, we
conducted a stoichiometric reaction between the zinc dihy-
drides 2a and two equimolar amounts of p-MePhCN in toluene.
The reaction rapidly occurred and gave complex 8 in 93% iso-
lated yield (Scheme 1) through the insertion of one Zn–H into
the –C^N moiety. It was fully characterized by multinuclear
NMR spectroscopy and elemental analysis. Addition of an
excessive amount of the nitrile did not lead to a further inser-
tion reaction probably due to the steric hindrance. The 1H NMR
spectrum showed a sharp signal at d 9.39 ppm which was
identied as a characteristic imine N]CH-R, while the signal of
remaining Zn–H was located at d 4.81 ppm. This was further
conrmed by a control experiment using deuterium labeling
complex 2a-D as a starting materiel (see the ESI for details†).
Subsequently, complex 8 was directly employed as a catalyst for
the reaction of p-MePhCN with pinacolborane under standard
conditions. As expected, it efficiently provided the diboronated
amine p-MePhCH2N(Bpin)2 (5b) in 88% isolated yield (Scheme
Table 3 Hydroboration of various imines with HBpin catalyzed by 2aa
a
Reaction conditions: [2a] ¼ 0.0015 mmol, [HBpin] ¼ 0.33 mmol,
[imine]
¼
0.30 mmol, room temperature. Conversions were
determined by 1H NMR spectroscopy (integration of residual 6 vs. 7).
b
Fig. 3 Plots of ln[(p-MePhCN)t/(p-MePhCN)0] versus time for the
hydroboration of p-MePhCN by catalyst 2a. Reaction conditions: [p-
MePhCN]0 ¼ 0.40 M, [HBpin]0 ¼ 0.84 M, [2a]0 ¼ 0.0146–0.0232 M.
Isolated yields were shown in the parentheses.
Reaction was
conducted at 80 ꢁC.
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conducted and showed a KIE value of 3.1 (ESI, Fig. S119†). The
experimental evidence herein implied that the B–H bond breaking
processes might be the rate-determining step.7a,10b
Based on our observations and previous literature report-
s,7a,8c,15 a plausible mechanistic framework for zinc dihydrides-
catalyzed nitrile hydroboration is depicted in Scheme 2. The
reaction of zinc dihydride 2 with nitrile gave single insertion
product 8, which then reacted with one molecule of HBpin to
possibly give a zinc mixed hydride/amino 9. Finally, complex 9
underwent a s-bond metathesis reaction with another molecule
of HBpin to afford nal product 5 with the regeneration of
catalyst 2.
Fig. 4 Plots of ln kobs vs. ln[2a] for the hydroboration of p-MePhCN by
catalyst 2a.
Conclusions
In summary, the double hydroboration of nitriles to the corre-
sponding diboronated amines have been achieved with broad
1), suggesting that complex 8 behaved as an active intermediate substrate scope by using a molecular zinc dihydride based on
in the hydroboration of nitriles. Attempt to isolate other inter- NHC ligand at room temperature under solvent-free conditions.
mediate species by a stoichiometric reaction of complex 8 with Complex 2a showed extremely high activity for this trans-
pinacolborane failed because of the immediate generation of formation, affording TOFs of up to 3000 hꢀ1. In addition,
the corresponding diboronated amine 5b.
complex 2a was also effective for the hydroboration of imines to
Kinetic studies of the catalytic reactions were also performed boronated amines. The stoichiometric reaction of 2a with
in the presence of catalyst 2a and monitored by 1H NMR spectra a nitrile led to the isolation of a single Zn–H insertion product 8,
(see the ESI for details†). Initial reactions were carried out using which was proved to be an active intermediate in the catalysis.
incremental catalyst starting concentrations in conjunction Kinetic studies revealed a pseudo second-order dependence on
with a 1 : 2.1 molar ratio of p-MePhCN to HBpin. As shown in the catalyst concentration. Further mechanistic studies and
Fig. 3, plots of ln[(p-MePhCN)t/(p-MePhCN)0] versus time for all application to other catalytic reactions of this molecular zinc
trials exhibited good straight lines, displaying a rst-order dihydride are underway in our laboratory.
kinetic behavior. As a result, pseudo-second order kinetic
behavior on catalyst 2a was obtained (Fig. 4) which is distinct
with that in the zinc alkyl-catalyzed hydroboration of nitrile.15
Conflicts of interest
When large excess of p-MePhCN (5 equivalents) was used, There are no conicts to declare.
a
rst-order dependence on HBpin was observed (ESI,
Fig. S115†). A kinetic isotope effect (KIE, kH/kD) of 2.0 was obtained
when replacement of HBpin by DBpin as the reductant in the
Acknowledgements
reaction (ESI, Fig. S118†). In addition, a competitive deuterium This work was supported by the National Natural Science
labeling experiment using p-MePhCN, HBpin, and DBpin in Foundation of China (21871204).
a 1 : 2.1 : 2.1 molar ratio under our typical conditions was also
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© 2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 1128–1133 | 1133