Nickel-catalysed reductive amination with hydrosilanes

Article abstract of DOI:10.1002/cctc.201300338

That's just silyl! Selective nickel-catalyzed reductive amination of aldehydes under hydrosilylation conditions is achieved by using a one pot, two step procedure. By using an insitu-generated catalyst (5mol%) from Ni(OAc)₂ and tricyclohexylphosphine with tetramethyldisiloxane (TMDS) as the hydrosilane at 70°C, the corresponding secondary amines were obtained in moderate to good isolated yields.

Full text of DOI:10.1002/cctc.201300338

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DOI: 10.1002/cctc.201300338
Nickel-Catalysed Reductive Amination with Hydrosilanes
Jianxia Zheng,[a] Thierry Roisnel,[b] Christophe Darcel,*[a] and Jean-Baptiste Sortais*[a]
Dedicated to Professor Irina Petrovna Beletskaya
Amines are central building blocks in organic synthesis for the
preparation of natural products, pharmaceutical and agronom-
ical compounds.[1] Among the various synthetic methods to
prepare amines, the catalytic reduction of imines is one of the
most efficient methods developed.[2] The stability of some
imines is, however, rather limited, and their synthesis and pu-
rification can be tedious. On the other hand, direct reductive
amination is an elegant and straightforward alternative to the
synthesis of substituted amines, as the imine is generated
in situ and reduced directly to the corresponding amines. The
reducing agent is usually a borohydride derivative,[3] although
several methods based on transition metal-catalysed hydroge-
nation, hydrogen transfer[4] and hydrosilylation[5] have been
also developed. Hydrosilanes are versatile reducing agents that
allow the use of mild conditions and good chemoselectivities.[6]
In addition, the use of inexpensive silanes such as polymethyl-
hydrosiloxane (PMHS), an abundant and non-toxic by-product
of the silicone industry, or tetramethyldisiloxane (TMDS) offer
useful alternatives for large-scale hydrogenations.[7]
Ni(OAc)2 in the presence of tricyclohexylphosphine could be
an efficient catalytic system for the reduction of aldehydes and
ketones with PMHS as the silane.[16] Here, we report the reduc-
tive amination of aldehydes by hydrosilylation by using an
in situ-generated catalytic system from inexpensive nickel ace-
tate and tricyclohexylphosphine.
To start our investigation, we selected benzaldehyde
(1 equiv.) and p-methoxyaniline (1.5 equiv.) as the model sub-
strates and the combination of Ni(OAc)2 (5 mol%) and PCy3
(10 mol%) as the catalytic system in the presence of 4 ꢀ mo-
lecular sieves (Scheme 1, Table 1).[20,21] At 1008C in toluene
with 4 equiv. of PMHS, the reaction led to 80% conversion into
the amine 3 and 20% of benzyl alcohol 1 resulting from the
The use of earth abundant transition metals has become an
important goal in catalysis.[8] In the field of reduction, particu-
larly in hydrosilylation, many efforts have been devoted to
using inexpensive earth abundant metals such as iron,[9,10]
zinc,[11] titanium[12] or copper.[13] Compared to the former
metals of the first row of the periodic table, nickel has been
employed much less.[14–16] Notably, only two examples of imine
reduction have been reported, one by hydrogen transfer reac-
tion[17] and the other by hydrosilylation.[18] In the field of reduc-
tive amination, catalytic reactions involving nickel are extreme-
ly scarce and mainly nickel nanoparticles have been involved
in the hydrogen transfer reductive amination of aldehydes.[19]
In 2012 and 2013, we have focused our attention on develop-
ing an efficient catalytic system for the reduction of carbonyl
derivatives with nickel.[15–16] We have found that the simple salt
Scheme 1. Nickel-catalysed reductive amination of benzaldehyde with p-ani-
sidine by hydrosilylation.
Table 1. Optimisation of the conditions of the reductive amination
reaction.[a]
Entry
Silane
[equiv.]
Solvent
[mL]
Temp.
[8C]
Time
[h]
Yield 1/2/3
[%]
1
2
3
4
PMHS (4)
TMDS (2.5)
PMHS (4)
TMDS (2.5)
TMDS (2.5)
TMDS (2.5)
TMDS (1.5)
TMDS (1)
toluene (2.5)
toluene (2.5)
THF (1)
THF (1)
THF (1)
THF (1)
THF (1)
THF (1)
THF (1)
100
100
70
70
50
70
70
70
70
70
70
24
24
24
24
24
24
16
16
16
16
24
20/0/80
4/0/96
6/0/94
3/0/97
1/81/18
[a] J. Zheng, Prof. Dr. C. Darcel, Dr. J.-B. Sortais
UMR 6226 CNRS-UR1 “Institut des Sciences Chimiques de Rennes”
Team “Organometallics: Materials and Catalysis”
Centre for Catalysis and Green Chemistry
Universitꢀ Rennes 1
5
6[b]
7
23/0/77
<3/0/>97
0/>97/<3
<3/46/51
8/<3/89
<3/0/>97
8
9[c]
10[d]
11[e]
TMDS (1)
TMDS (1.5)
TMDS (1.5)
263 av. du Gꢀnꢀral Leclerc, 35046 Rennes Cedex (France)
Fax: (+33) 2 23 23 69 39
THF (1)
THF (1)
E-mail: christophe.darcel@univ-rennes1.fr
jean-baptiste.sortais@univ-rennes1.fr
[a] Typical conditions: to a solution of Ni(OAc)2 (5 mol%) and 4 ꢀ molecu-
lar sieves (200 mg), PCy3 (10 mol%), the amine (0.75 mmol, 1.5 equiv.),
the aldehyde (0.5 mmol), and the silane (1–4 equiv.) were added in this
order. The reaction mixture was then heated in an oil bath. After hydroly-
sis and extraction, the yields were determined by 1H NMR and GC.
[b] Without 4 ꢀ molecular sieves, [c] Ni(OAc)2 (2.5 mol%), PCy3 (5 mol%).
[d] 1 equiv. of amine. [e] 1.1 equiv. of amine; the silane was added after
8 h and the reaction mixture then stirred for an additional 16 h.
[b] Dr. T. Roisnel
UMR 6226 CNRS-UR1 “Institut des Sciences Chimiques de Rennes”
Centre de Diffractomꢀtrie X
Universitꢀ Rennes 1
263 av. du Gꢀnꢀral Leclerc, 35046 Rennes Cedex (France)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300338.
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direct reduction of the benzaldehyde (entry 1). By changing
the silane from PMHS to TMDS (2.5 equiv.), the selectivity in-
creased to 96% in favour of amine 3. At 708C in THF, the selec-
tivity towards the formation of the amine increased for both
PMHS and TMDS (respectively 94 and 97%, entries 3 and 4).
On decreasing the temperature to 508C, the imine intermedi-
ate 2 was obtained as the main product (81%, entry 5). We
have also performed the reaction in the absence of 4 ꢀ molec-
ular sieves: the selectivity decreased and a 23:77 molar ratio
mixture of benzyl alcohol 1 and amine 3 was obtained
(entry 6). The quantity of TMDS could be decreased to
1.5 equiv. without any alteration of the selectivity (entry 7). In
contrast, if the catalytic loading of nickel acetate was lowered
to 2.5 mol%, a 1:1 mixture of the imine 2 and the amine 3 was
obtained (entry 9). Finally, if the quantity of aniline was low-
ered from 1.5 to 1 equiv., 8% of benzyl alcohol 1 was detected,
which showed that the formation of the imine was in competi-
tion with the reduction of the aldehyde (entry 10). To reduce
the formation of the undesired alcohol but only use a stoichio-
metric amount of p-anisidine, a sequential procedure was
used; the silane was added after 8 h of reaction and the de-
sired amine then obtained quantitatively (entry 11).
Table 2. Scope
of
the
reductive
amination
reaction.[a]
Entry[a]
Product
R
Yield[b,c]
[%]
1
OMe
H
>97 (97)
91 (90)
93 (79)
2[d]
3[e]
F
4
OMe
H
>97 (97)
>95 (81)
82 (82)
5[d,f]
6[e]
F
7[d,e]
OH
87 (80)
91 (46)
>97 (62)
41
8[d,e]
CO2Me
NHCOMe
CN
9[d,e]
10[d,e]
11[d,e]
NO2
14
12
13
14
H
>97 (95)
91 (89)
91 (85)
OMe
CF3
With the optimised conditions in hand (1.1 equiv. amine,
1 equiv. aldehyde, 708C, 8 h, then 1.5 equiv. TMDS, 708C, 16 h),
we focused on the scope of the reaction. At first, various
amines were tested with aromatic aldehydes such as benzalde-
hyde and p-anisaldehyde (Table 2, entries 1–6): with the acti-
vated p-methoxyaniline, the corresponding amines were ob-
tained with 97% isolated yields. In contrast, with non-activated
aniline, the reduction of the imine required a longer reaction
time of 40 h to reach completion and, with the electron-defi-
cient 4-fluoroaniline, 4 equiv. of TMDS were needed to obtain
the secondary amine in good yields (79–82%). The electronics
of the amine partners seemed to have a strong influence on
the reaction because, with an electron-rich amine, the reaction
was performed in short time, whereas the presence of elec-
tron-withdrawing group induced longer reaction times (8 vs.
40 h). The group tolerance was also studied on the aldehyde
partner by reaction with 1.1 equiv. of p-anisidine. Thus, the tol-
erance towards acidic phenol moieties was also demonstrated,
as was the selectivity towards esters and amides functional
groups; the resulting reactions led to high conversions of
amines (87–97%) and isolated amines with non-altered func-
tional groups isolated in moderate to good yields (46–80%;
entries 7–9). In contrast, with p-cyano- and p-nitro-benzalde-
hyde derivatives, the conversions towards the desired amine
were low or moderate (41 and 14%, respectively).
15[e]
16[e]
H
OMe
>97 (97)
>97 (97)
17
18[c]
CH2Ph
CH3À(CH2)11À
>97 (71)
>97 (67)
19[e]
20[e]
Ph
>97
>97 (97)
CH3À(CH2)11À
21
22[d,h]
p-OMeÀC6H4
0[g]
58
PhCH2ÀCH2
[a] Typical conditions: To a solution of Ni(OAc)2 (5 mol%) and 4 ꢀ molecu-
lar sieves (400 mg), PCy3 (10 mol%), amine (1.1 mmol, 1.1 equiv.) and al-
dehyde (1 mmol) were added in that order and stirred at 708C for 8 h.
Then TMDS (1.5 mmol, 1.5 equiv.) was added and the reaction stirred for
16 h, after which hydrolysis was performed by adding MeOH (2 mL) and
NaOH (2 mL), followed by stirring overnight. [b] Yields were determined
by using 1H NMR spectroscopy. [c] Isolated yields after purification by
column chromatography in parentheses. [d] t=40 h instead of 16 h,
[e] 4 equiv. TMDS, [f] 1.5 equiv. amine. [g] p-Methoxybenzyl alcohol was
observed. [h] t=48 h instead of 8 h.
If using secondary amine such as the dipropylamine, the re-
action was more difficult to perform. If reacting with p-methox-
ybenzaldehyde, no resulting product of reductive amination
was detected and the sole product obtained was the corre-
sponding alcohol (entry 21). Notably, the dipropylamine could
be condensed with the hydrocinnamaldehyde with moderate
conversion (58%; entry 22). The transformation of ketones
under similar reductive amination conditions was unsuccessful,
with only low amounts of amine obtained. In comparison to
other transition metal catalysts (iron[5l] and zinc[5h]), the activity
of this nickel catalytic system was competitive in the reductive
amination of aldehydes with siloxanes as the reducing reagent.
To try to develop better insights into the reaction conditions,
several stoichiometric experiments were conducted in order to
identify which type of active catalysts could be generated
The reaction of aldehyde with benzylamine proceeded well
and the products were isolated with 85–95% yields, irrespec-
tive of the nature of the substituent on the aromatic ring of
the aldehyde (entries 12–14). If the reaction was performed
with aliphatic aldehydes such as hydrocinnamaldehyde and oc-
tanal, the corresponding secondary amines were obtained in
good to excellent conversions and yields (entries 17–20). Nota-
bly, aliphatic amines such as dodecylamine were also suitable
for the reductive amination (97% yield, entries 15, 16, 18 and
20).
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under our catalytic conditions. Recently Ananikov and Bele-
skaya have shown that Ni(acac)2 and PMe2Ph were leading to
Ni0-phosphines complexes, with the concomitant oxidation of
one phosphine in the presence of traces of water.[22] As we
have shown that Ni(acac)2 could also be used as nickel precur-
sor instead of Ni(OAc)2 and that the basic dimethylphenylphos-
phine could be used instead of tricyclohexylphosphine for the
reduction of carbonyl derivatives,[16a] we have investigated the
reaction Ni(acac)2 with Ph3SiH in the presence of PMe2Ph in
a molar ratio of 1:1:3 in toluene to try to identify any product
resulting from an oxidative addition of the silane onto the
nickel. The mixture was stirred for a five minutes at room tem-
perature until all solid dissolved, then layered with pentane
and stored at À188C, which allowed the isolation of large
blue-purple crystals. (Scheme 2) The molecular structure of 4,
(acac)2(PCy3)], with only one phosphine coordinated, evidenc-
ing an 18-electron, five-coordinated square-pyramidal structure
(see Figure 1).[24]
Finally, by mixing Ni(OAc)2, PCy3 and Ph2SiH2, the mixture
turned orange-red after 3.5 h at 1008C. The reaction solution
was then transferred into an NMR tube for analysis. The main
compound revealed a signal at À2.04 ppm in the 1H NMR
spectrum for Ni(HÀSi) and a signal at +45.9 ppm in the
31P NMR spectrum, which was in accordance with the complex
[{Ni(PCy3)}2(m-SiHPh2)2], as described by Osakada but synthes-
ised from Ni(cod)2, PCy3 and Ph2SiH2.[25] These spectrometric
data could confirm that the first step of the catalytic cycle was
the reduction of NiII to Ni0 species.[22,25]
In conclusion, we have developed an efficient nickel-cata-
lysed reductive amination of aldehydes by using an in situ-gen-
erated catalyst from Ni(OAc)2 and tricyclohexylphosphine. At
708C, with tetramethyldisiloxane as the hydrosilane, the corre-
sponding secondary amines were obtained in moderate to
good isolated yields. This in situ-generated nickel system from
Ni(OAc)2 and phosphine provides
Ni(cod)2 in catalysis.
a good alternative to
Scheme 2. Stoichiometric reactions of Ni(acac)2 with phosphines. Cy=cyclo-
hexyl.
Experimental Section
A 10 mL oven dried Schlenk tube containing a stirrer bar was
charged with 4 ꢀ molecular sieves (200 mg) and Ni(OAc)2 (10 mg,
0.05 mmol). After purging with argon (3 cycles), THF (2 mL) was
added, followed by PCy3 (168 mL, 0.1 mmol), amine (1.1 mmol) and
aldehyde (1.0 mmol). The reaction mixture was stirred in a preheat-
ed oil bath at 708C for 8 h. TMDS (264 mL, 1.5 mmol) was then
added and the reaction mixture stirred at 708C for an additional
16 h. After cooling to RT, MeOH (2 mL) was added followed by 2m
NaOH (2 mL) under vigorous stirring. The reaction mixture was
stirred further for 16 h at RT, then extracted with diethyl ether (3ꢂ
10 mL). The combined organic layers were dried over anhydrous
MgSO4, filtered and concentrated in vacuo. The residue was puri-
fied by silica gel column chromatography by using an ethyl ace-
tate/petroleum ether mixture (5:95 to 20:80) to achieve the desired
product.
which was identified by X-ray diffraction, revealed that only
the complex trans-[Ni(acac)2(PMe2Ph)2] already described by
Ananikov could be isolated. Notably, if the same reaction was
performed at 708C for 8 h, despite a colour change from
yellow to orange-red, no identifiable species could be detect-
ed.[23] Surprisingly, performing the same reaction with PCy3,
large green crystals of 5 were formed, the molecular structure
of which, as determined by X-ray diffraction analysis, was [Ni-
Acknowledgements
We are grateful to the Universitꢀ de Rennes 1, CNRS, Rennes Mꢀt-
ropole and Ministꢁre de l’Enseignement Supꢀrieur et de la Recher-
che for support, and to Axa Research Fund for a grant to J.Z.
Keywords: aldehydes · amines · hydrosilylation · nickel ·
reductive amination
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Received: May 2, 2013
Revised: June 7, 2013
Published online on July 24, 2013
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 2861 – 2864 2864