Nitrile hydroboration reactions catalysed by simple nickel salts, bis(acetylacetonato)nickel(ii) and its derivatives

Article abstract of DOI:10.1039/c7cy00944e

Simple nickel salts, bis(acetylacetonato)nickel(ii) and its derivatives, catalysed the hydroboration reactions of aryl and alkyl nitriles with catechol borane. The reaction smoothly proceeded for nitriles with various substituents to form the corresponding bis(boryl)amines in good to excellent yields.

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Nitrile hydroboration reactions catalysed by simple nickel salts,
bis(acetylacetonato)nickel(II) and its derivatives
Received 00th January 20xx,
Accepted 00th January 20xx
Go Nakamura, Yumiko Nakajima,* Kazuhiro Matsumoto, Venu Srinivas and Shigeru Shimada*
DOI: 10.1039/x0xx00000x
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Simple nickel salts, bis(acetylacetonato)nickel(II) and its presence of 1 mol% of [RuCl2(p-cymene)]2. On the other hand,
derivatives, catalysed hydroboration reactions of aryl and alkyl nitrile hydroboration reactions using unactivated boranes,
nitiles with catechol borane. The reaction smoothly proceeded for such as HBcat (cat = catecholato) and HBpin (pin = pinacolato)
nitriles with various substituents to form the corresponding are still rare, although the catalytic hydroboration reactions of
bis(boryl)amines in good to excellent yields.
a wide range of substrates bearing multiple C−C, C−O, or C−N
bonds have been intensively developed.10-14
Introduction
We have thus far engaged in the development of various
nickel hydrosilylation catalysts.15,16 Among these studies, it
Efficient and versatile synthesis of primary amines has been
intensively studied due to the significant importance of the
amines as key intermediates in the chemical industry.1
Reduction of nitriles is one of the most powerful methods for
the production of primary amines and has been efficiently
achieved using stoichiometric amounts of metal hydrides.2
However, these methods suffer from several drawbacks, such
as high temperatures, low functional group tolerance, low
reaction selectivity, etc. Hydrosilylation3,4 and hydrogenation5
of nitriles have also been reported as alternative routes for the
production of amines in the last few decades. In the former
examples, transition metal complexes as well as a non-metallic
Lewis acid are revealed to efficiently catalyse the reactions
under ambient conditions. More recently, hydroboration
catalysed by metal complexes has emerged as a new approach
for the nitrile reductions.6-8 For example, the reactions are
reported to be catalysed by metal complexes supported by a
unique ligand system, such as imido-hydrido complex
was
revealed
that
simple
nickel
) and its derivatives
and bis(2,2,6,6-
acted as
selective hydrosilylation catalysts of olefins with various
functional groups in the presence of NaEt3BH.15 Since
are
salts,
bis(acetylacetonato)nickel(II) Ni(acac)2
bis(hexafluoroacetylacetonato)nickel(II)
(1
(2)
tetramethyl-3,5-heptanedionato)nickel(II)
(3)
1-3
cheap and readily available from commercial sources and thus
are expected to be a good candidate as precious metal
surrogates, we became interested in the further reactivity
investigation of 1-3. In this study, it was revealed that 1-3
serve as good hydroboration catalysts of various nitriles.
Different from the hydrosilylation reactions, the hydroboration
reactions were successufully catalysed by
additives such as metal hydride reagents.
1-3 without any
Results and Discussion
Catalytic activities of various metal salts including 1-3 were
(ArN)Mo(H)(Cl)(PMe3)3,6
β-diketiminato n-butylmagnesium
initially investigated for the hydroboration of benzonitrile
(PhCN) with HBcat at room temperature. As shown in Table 1,
complex7 and bis(2’-hydroxy-6’-iminopyridyl)isoindoline Ru(II)
complex8 at a catalyst loading of 5-10 mol%. Mechanistic
studies supported that all these reactions proceed via agostic
borylamino and/or borylimino intermediates. It is to be noted
that the utility of commercial available [RuCl2(p-cymene)]2
was reported as the most efficient catalyst in 2016.9 Thus,
hydroboration of various nitriles proceeds at 60 °C in the
it was revealed that 1-3 exhibited excellent performance in
benzene to quantitatively give 1,1-bis(boryl)amine,
PhCH2N(Bcat)2 (Table 1, entries 1-3). The use of polar solvents
such as tetrahydrofuran and 1,2-dichloroethane resulted in the
drastic decrease in the product yields (trace amounts). The
reaction also proceeded at 1 mol% catalyst loading of
slower rate, yielding PhCH2N(Bcat)2 in 49, 19, and 78% yields,
respectively. Thus, the highest activity of among three was
confirmed. NiCl2 did not catalyse the reaction, whereas
Ni(cod)2 (cod cyclooctadiene) exhibited good catalytic
1-3 at
3
=
activity (vide infra) (Table 1, entries 4 and 5). Among other
metal
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Table 1. Hydroboration of PhCN with HBcat catalysed by metal salts[a]
completely consumed on the treatment with 4 equiv of HBcat
after 15 min
Table 2. Hydroboration of various nitriles with HBcat catalysed by 3[a]
entry
catalyst
(mol%)
1
yield[b]
(%)
1
2
3
4
5
6
7
8
9
>99 (49)[c]
>99 (19)[c]
>99 (78)[c]
ND
2
entry
1
R
Ph
3 (mol%)
Product yields%[b]
>99 (93)
40 (36)
3
0.5
5
NiCl2
2
o-CH3C6H4
m-CH3C6H4
p-CH3C6H4
p-MeOC6H4
p-CF3C6H4
m-ClC6H4
p-ClC6H4
2-furyl
2-thienyl
PhCH2
Me
Ni(cod)2
Co(acac)2
Cu(acac)2
Mn(acac)2
Pd(acac)2
87
3
0.5
0.5
0.5
0.5
5
>99 (92)
>99 (92)
>99 (95)
>99 (85)
81 (64)
37
4
ND
5
ND
6
ND
7
[a] Catalyst (0.01 mmol), PhCN (0.20 mmol), HBcat (0.44 mmol) in C6H6
5 mL at room temperature for 12 h. [b] NMR yield based on an internal
standard (PhSiMe3). [c] The values in parenthesis represent the yields at
a 1 mol% catalyst loading.
8
5
78 (62)
9
5
>99 (95)
47 (29)
10
11
12
13
14
15
16
5
3
>99 (88)
>99 (84)
>99 (90)
>99 (94)
>99 (90)
>99 (90)
3
acetylacetonates, only Co(acac)2 showed moderate catalytic
activity (Table 1, entries 6-9).
Et
3
iPr
3
Next, the scope of the reaction was investigated using the
tBu
5
most active catalyst
Hydroboration of benzonitrile was achieved at lower catalyst
loading, 0.5 mol% of , resulting in the quantitative formation
3. The results are summarized in Table 2.
Cy
5
[a] Nitriles (0.20 mmol), HBcat (0.44 mmol) and catalyst (0.5-5 mol% vs
nitriles) in C6H6 5 mL at room temperature for 18 h. [b] NMR yield (isolated
yield).
3
of PhCH2N(Bcat)2 (Table 2, entry 1). Similarly, those of m-
tolunitrile and p-tolunitrile smoothly proceeded to
quantitatively form ArCH2N(Bcat)2 (Ar = m-CH3C6H4, p-CH3C6H4)
under the same reaction conditions (Table 2, entries 3, 4). In
contrast, hydroboration of o-tolunitrile resulted in the
formation of (o-CH3C6H4)CH2N(Bcat)2 in 40% yield even at 5
to form black precipitates, which was confirmed to be
catalytically inactive. In this reaction, dissociation of the ligand
was evidenced by the formation of [tBuCOCHC(tBu)O-
1
κ
O,
κ
O’]Bcat (
4
), which was confirmed by H and 11B NMR as
mol% catalyst loading of
3, indicating the induced steric
well as by a single crystal X-ray diffraction analysis. No signal
assignable to the NiH group was observed in the hydride
region (δ 0 to −30 ppm) of the 1H NMR spectra. Based on these
hindrance by the o-methyl group (Table 2, entry 2). It is to be
mentioned that the reactions of benzonitriles with both
electron-donating OMe and -withdrawing CF3 groups smoothly
proceeded at 0.5 mol% catalyst loading (Table 2, entries 5, 6).
Hydroborations of m- and p-chlorobenzonitriles required a
higher catalyst loading, 5 mol%, to attain good yields, 81% and
78%, respectively (Table 2, entries 7, 8). It was revealed that p-
XC6H4CN (X = Br, I) did not undergo hydroboration under the
same reaction conditions, probably due to the occurrence of
experiments, it is presumable that
3 is reduced on the
treatment with HBcat to form catalytically active Ni(0) species,
which could be easily converted to black precipitate.19
There are several potential mechanisms for hydroboration
catalysed by
following the previously reported Ni-catalyzed hydroborations
(Scheme 1).13,20 The reaction starts with the reduction of
by
HBcat to form an active Ni(0) species accompanied by the
formation of . The Ni(0) species undergoes oxidative
addition of HBcat, leading to the formation of boryl hydride
1-3. One possible mechanism is postulated,
3
C–X bond cleavage.17
A
heteroaromatic nitrile, 2-
A
furancarbonitrile, was successfully hydroborated (Table 2,
entry 9), whereas the reaction of 2-thiophenecarbonitrile
slowly proceeded to give the corresponding hydroborated
product in 47% yield (Table 2, entry 10). Hydroborations of
various alkyl nitriles were also achieved at a catalyst loading of
3-5 mol% giving the products quantitatively. (Table 2, entries
11-16).
4
A
intermediate
B
. Complex
B
undergoes insertion of a nitrile into
After the occurrence of reductive
the Ni–H bond to give C 21
.
elimination, borylimine RCH2=NBcat is formed as the
monohydroborylated product and reproduce . RCH2=NBcat
A
further reacts with HBcat to give the final product
RCH2N(Bcat)2. It was confirmed that the reaction of N-
benzylidenebenzylamine with HBcat resulted in the
quantitative formation of (PhCH2)2NBcat at room temperature
without a catalyst. This result strongly supports the occurrence
of the non-catalysed final hydroboration step.
To obtain information on the reaction mechanism,
reactions of
1H NMR spectroscopy. The addition of PhCN (2 equiv) to
resulted in no observable spectroscopic change. It is likely that
3 with PhCN or HBcat in C6D6 were monitored by
3
there is a fast coordination equilibrium between 3 and PhCN,
as previously reported by Henneike et al.18 In contrast,
3 was
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Scheme 1. Possible mechanism of hydroboration of nitrile catalysed by
3
The proposed mechanism including the active Ni(0) species is
consistent with the comparable activity of Ni(cod)2 as . On
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8
9
1-3
the other hand, it was also confirmed that the product yield
significantly decreased to 38% when the hydroboration of
PhCN was performed in the presence of excess Hg with
vigorous stirring.22 The result supports the in situ formation of
metallic nickel(0) particles as an active catalyst.23 Thus, it is
possible that the reaction is catalysed by both homogeneous
and heterogeneous Ni(0) species.
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Conclusion
In this study, we have shown one of rare examples of
hydroboration reactions of various nitriles using commercially
available simple nickel salts
1-3. It is to be noted that the
catalysts 1-3 are completely free from the complexity in the
preparations, which are often troublesome in the previously
reported systems. In addition, the catalyst loading could be
reduced to 0.5 mol% in the hydroboration of benzonitriles.
Studies to elucidate the mechanistic details and further
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applications of 1-3 as catalysts are to be performed.
This work was supported by MEXT Grant-in-Aid for
Scientific Research on Innovative Areas “Stimuli-responsive
Chemical Species” (no. 15H00967) from JSPS. We also thank
the Collaborative Research Program of Institute for Chemical
Research, Kyoto University (grant #2016-15).
and C. Jones, ACS Catal., 2017, 7, 1853.
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22 The hydroboration reaction of PhCN was similarly performed
following the procedure of Table 2, entry 1 in the presence
of 0.98 g of Hg to give PhCH2N(Bcat)2 in 38% yield.
23 J. A. Widegren and R. G. Finke, J. Mol. Catal. A: Chem., 2003,
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Graphical and textual abstract for the Table of Contents
Simple Nickel salts, bis(acethylacetonato)nickel(II) abd its derivatives,
catalyzed hydroboration reactions of aryl and alkyl nitriles with
catechol borane
.
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