Chemoselective Alkene Hydrosilylation Catalyzed by Nickel Pincer Complexes

Article abstract of DOI:10.1002/anie.201507829

Chemoselective hydrosilylation of functionalized alkenes is difficult to achieve using base-metal catalysts. Reported herein is that well-defined bis(amino)amide nickel pincer complexes are efficient catalysts for anti-Markovnikov hydrosilylation of terminal alkenes with turnover frequencies of up to 83 000 per hour and turnover numbers of up to 10 000. Alkenes containing amino, ester, amido, ketone, and formyl groups are selectively hydrosilylated. A slight modification of reaction conditions allows tandem isomerization/hydrosilylation reactions of internal alkenes using these nickel catalysts.

Full text of DOI:10.1002/anie.201507829

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201507829
German Edition: DOI: 10.1002/ange.201507829
Silanes
Chemoselective Alkene Hydrosilylation Catalyzed by Nickel Pincer
Complexes
Ivan Buslov, Jeanne Becouse, Simona Mazza, Mickael Montandon-Clerc, and Xile Hu*
Abstract: Chemoselective hydrosilylation of functionalized
alkenes is difficult to achieve using base-metal catalysts.
Reported herein is that well-defined bis(amino)amide nickel
pincer complexes are efficient catalysts for anti-Markovnikov
hydrosilylation of terminal alkenes with turnover frequencies
of up to 83000 per hour and turnover numbers of up to 10000.
Alkenes containing amino, ester, amido, ketone, and formyl
groups are selectively hydrosilylated. A slight modification of
reaction conditions allows tandem isomerization/hydrosilyla-
tion reactions of internal alkenes using these nickel catalysts.
PNN ligand, relative to PDI, was exploited to decrease the
oxophilicity of the catalysts and improve their tolerance
towards carbonyl groups. Indeed, in combination with
NaBHEt3 as the activating agent, these complexes catalyzed
anti-Markovnikov hydrosilylation of alkenes containing sev-
eral important functional groups, including esters, tertiary
amides, and ketones.[9] Notwithstanding, internal alkenes and
terminal alkenes containing secondary amide groups could
not be hydrosilylated.
A number of nickel-based catalysts for regioselective
hydrosilylation of alkenes have been reported recently.
[Ni(R-Indenyl)(PPh3)Cl] (R = Me, SiMe3), activated by
NaBPh4, catalyzed hydrosilylation of styrene with PhSiH3 to
give the Markovnikov product.[11] Cationic allyl nickel com-
plexes also catalyzed Markovnikov hydrosilylation of styr-
enes with phenylsilane.[12] In contrast, [(PPh3)2NiBr2]-cata-
lyzed anti-Markovnikov hydrosilylation of styrenes with
Ph2SiH2.[13] A two-coordinate nickel bis(amido) complex
also catalyzed anti-Markovnikov hydrosilylation of 1-octene
with Ph2SiH2.[14] However, broad scope and functional group
compatible hydrosilylation of alkenes has not been demon-
strated by a nickel-based catalytic system. Herein, we report
that nickel(II) bis(amino)amide (N2N) pincer complexes
catalyze chemoselective anti-Markovnikov hydrosilylation
of alkenes. The catalysis has high activity and broad scope.
Not only ester, keto, and NH2 groups, but also formyl and
secondary amide groups, previously only tolerated in precious
metal catalysis, are compatible with this nickel catalysis. The
nickel catalysts also catalyze tandem isomerization and anti-
Markovnikov hydrosilylation of internal alkenes.
H
ydrosilylation of alkenes is widely used for the production
of numerous consumer goods and fine chemicals.[1,2] Platinum
catalysts, such as Speierꢀs[3] catalyst and Karstedtꢀs[4] complex,
are most often used for this reaction because of their high
activity and selectivity. However, the high cost and low
abundance of platinum motivates the development of alter-
native catalysts based on more-abundant and economical
metals. A number of iron and nickel catalysts have shown
good efficiency and activity for alkene hydrosilylation.[1a,5] In
pioneering work, Chirik and co-workers reported that bis-
(imino)pyridine (PDI) iron(0) bis(dinitrogen) complexes
catalyzed anti-Markovnikov hydrosilylation of terminal
alkenes.[6] Modification of the steric properties of the PDI
ligands allows addition of tertiary silanes.[7a,b] Related terpyr-
idine and bis(imino)pyridine iron(II) dialkyl complexes also
served as catalysts for the hydrosilylation of alkenes and
alkynes.[7c] These iron complexes exhibit high activity with
turnover frequencies (TOFs) of up to 100000 hÀ1, but they are
not compatible with carbonyl groups, which compete favor-
ably with alkenes for hydrosilylation. The limited stability of
these complexes is another constraint. Several groups had
then used stable iron(II) complexes as precatalysts and either
NaBHEt3 or an organometallic reagent as an activating
reagent to generate active iron catalysts for anti-Markovnikov
alkene hydrosilylation.[8–10] While this modification improved
the stability of the catalysts, functional-group compatibility
remains to be improved. In a notable example, Huang and co-
workers developed phosphinite iminopyridine (PNN) iron(II)
complexes, where the more-electron-rich character of the
Our group reported earlier that the bis(amino)amide
nickel chloride complex [(MeN2N)Ni-Cl] (1) was an excellent
catalyst for cross-couplings of non-activated alkyl halides and
direct C H alkylation.[15] Reaction of 1 with NaOMe gave the
À
methoxide complex [(MeN2N)Ni-OMe] (2), which reacted
with Ph2SiH2 to yield the hydride complex [(MeN2N)Ni-H] (3;
Scheme 1).[16] The complex 3 reacted with acetone and
ethylene to give [(MeN2N)Ni-OiPr] (4) and [(MeN2N)Ni-Et]
(5), respectively. The reaction with acetone was slow and took
12 hours to complete. However, the reaction with ethylene
was much faster and was complete within minutes. Thus,
supported by the MeN2N pincer ligand, the nickel(II) hydride
species, the proposed intermediate in many nickel-catalyzed
[*] I. Buslov, J. Becouse, S. Mazza, M. Montandon-Clerc, Prof. Dr. X. Hu
Laboratory of Inorganic Synthesis and Catalysis, Institute of
Chemical Sciences and Engineering, Ecole Polytechnique FØdØrale de
Lausanne (EPFL), ISCI-LSCI
=
hydrosilylation reactions, reacts preferentially with the C C
bond over that of the carbonyl group. This result suggested
that the N2N pincer/Ni complexes might be potential catalysts
for chemoselective hydrosilylation of alkenes containing
carbonyl groups.
BCH 3305, 1015 Lausanne (Switzerland)
E-mail: xile.hu@epfl.ch
Homepage: http://lsci.epfl.ch
Indeed, in the presence of 1 equivalent of NaOiPr,
10 mol% of 1 catalyzed hydrosilylation of 1-octene (6a)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507829.
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Table 1: Nickel-catalyzed hydrosilylation of terminal and cyclic alkenes.[a]
Scheme 1. Structure and transformations of N2N pincer/Ni complexes.
with Ph2SiH2 at À708C to give octyldiphenylsilane (7a) in
93% yield. The NaOiPr base was used to convert 1 into
[(MeN2N)Ni-OiPr], which presumably activated the silane to
give 3. To conduct the reactions under base-free conditions,
which may provide better group tolerance, the alkoxide
complexes 2 and 4 were tested as catalysts. Both complexes
catalyze the hydrosilylation of 6a with Ph2SiH2 in high yields
at room temperature. To estimate the activity, a solvent-free
reaction of 6a with Ph2SiH2 was performed at room temper-
ature on a 10 mmol scale. Using 0.025 mol% of the catalyst
[(MeN2N)Ni-OMe], 98% conversion was reached within 2.5–
3 minutes, thus giving an average TOF of about 83000 hÀ1.
The catalyst loading for this reaction could be lowered to
0.01 mol%, thus giving a TON of about 10000. This reactivity
is among the highest for nonprecious metals in alkene
hydrosilylation reactions.
[a] Reaction conditions: alkene (0.5 mmol), Ph2SiH2 (1.2 equiv),
[(MeN2N)Ni-OMe] (1 mol%), THF (1 mL), 6 h, RT. [b] Yields of isolated
products are reported. [c] 5% of [(MeN2N)Ni-OMe] and 24 h. [d] Alkene
(0.55 mmol), Ph2SiH2 (0.5 mmol). [e] 2 mmol scale, 8% of 2-(diphenyl-
silyl)propanamine was also produced. THF=tetrahydrofuran.
The scope and tolerance of hydrosilylation were then
explored using 2 as the catalyst (Table 1). For convenience of
handling, the catalyst loading was 1 mol% on these lab-scale
reactions (0.5 mmol). Secondary silanes worked best, while
primary and tertiary silanes gave lower yields (about 30%).[17]
A large number of functionalized alkenes were hydrosilylated
with anti-Markovnikov selectivity. Not only terminal, but also
cyclic alkenes (7d, 7q) were reactive. The reaction was faster
for a terminal alkene than for a cyclic alkene, so a substrate
containing both groups was selectively hydrosilylated at the
terminal alkene moiety (7e). Substitution at the b-position of
the olefin was allowed (7b,c). Excellent functional-group
tolerance was achieved. Epoxide (7 f), aryl bromide (7g),
alkyl bromide (7h), ester (7i, 7s), and tertiary amine (7m)
were readily tolerated. Moreover, primary amines (7j–l),[18]
and NH-containing amides such as NH-COCF3 (7n,o),
carbamate NH-Boc (7p), and sulfonylamide NH-SO2Me
(7r), which were challenging for previous iron and nickel
catalysts,[9] were compatible with the current method.
Unfortunately, as for previous nonprecious catalysts, alcohols,
carboxylic acids, and allylhalides remained incompatible.
Interestingly, no hydrosilylation was obtained for the reac-
tions of Ph2SiH2 with terminal (1-octyne and phenylacety-
lene) and internal (2-octyne) alkynes.
(8a) was hydrosilylated with Ph2SiH2, thus giving 6-(diphe-
nylsilyl)hexan-2-one (9a) in 53% yield and 9:1 selectivity for
alkene versus ketone hydrosilylation. Changing the solvent
from THF to DMA improved the selectivity to as high as 15:1.
The yields for carbonyl-containing alkenes were further
increased using a loading of 2 mol%. Thus, both ketone and
formyl groups were tolerated (8a–d; Scheme 2). To the best
of our knowledge it is the first example of selective hydro-
silylation of formyl-containing alkenes by a base-metal
catalyst.
To illustrate the synthetic utility of the hydrosilylation
products, two alkylsilanes (7c and 9b) were oxidized using
hydrogen peroxide, and the corresponding alcohols were
obtained in good yields (Scheme 3).
When internal alkenes such as 2-pentene, 2-octene, or
1-ethylcyclohexene were used as substrates, certain platinum-
and rhodium-based catalysts can catalyze tandem isomer-
ization and hydrosilylation to give terminal alkylsilanes.[3a,20]
Recently, analogous tandem isomerization/hydroboration of
internal alkenes was developed.[21] Related dehydrogenative
silylation of internal olefins to yield allylsilanes was also
reported.[22] However, there are few prior reports of base-
metal-catalyzed isomerization/hydrosilylation reactions.[6]
It was found that in the presence of 10 mol% of 2,
2-octene reacted with Ph2SiH2 to give octyldiphenylsilane
(7a) in 77% yield (Table 2). The reaction is proposed to occur
by the isomerization of 2-octene to 1-octene followed by anti-
Nickel-catalyzed hydrosilylation of ketones and alde-
hydes is well established.[19] Therefore, selective hydrosilyla-
tion of alkenes in the presence of ketones and aldehydes is
challenging. Using the protocol in Table 1, 5-hexen-2-one
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Table 2: Nickel-catalyzed tandem isomerization/hydrosilylation of
alkenes.
Entry Method[a]
Substrate
Product
Yield
[%][b]
1
2
A
A
77[c]
63[c]
3
4
A
A
64[c]
92
42[c]
24[d]
Scheme 2. Nickel-catalyzed hydrosilylation of ketone- and formyl-con-
taining alkenes. Yields of the isolated products are reported. DMA=
dimethylacetamide.
5
6
A
A
7
8
B
B
65
59
[a] Method A: alkene (0.5 mmol), Ph2SiH2 (2.0 equiv), [(MeN2N)Ni-OMe]
(10 mol%), THF (3 mL), 24 h, RT. Method B: alkene (0.5 mmol),
Ph2SiH2 (2.0 equiv), NaOtBu (1.0 equiv), [(MeN2N)Ni-Cl] (10 mol%),
THF (3 mL), 24 h, À708C. [b] Yields of isolated products are reported.
[c] 6–9% of internal hydrosilylation product was also obtained.
[d] 1-Methylcyclohexene was used as the solvent, and GC-MS yield
reported.
Scheme 3. Oxidation of alkylsilanes to alcohols.
Acknowledgements
Markovnikov hydrosilylation. The tandem isomerization and
hydrosilylation also occurred with other such alkenes, having
various functional groups (13a–d), with good selectivity for
the formation of terminal alkylsilanes. For a cyclic substrate,
such as 1-methylcyclohex-1-ene, the yield of the isomer-
ization/hydrosilylation was low. For similar reactions of 3- and
4-octenes, a modification of reaction conditions was benefi-
cial. Thus, 1 was used as the precatalyst, 1 equivalent of
NaOtBu was used as the base, and the reactions were
conducted at À708C (Method B). Under these reaction
conditions, 3- and 4-octenes were isomerized and hydro-
silylated to give 7a in good yields. It is noted that these
tandem reactions are slower than hydrosilylation reactions of
terminal alkenes, thus suggesting that the slow step is the
isomerization.
In summary, we have developed an efficient protocol for
nickel-catalyzed anti-Markovnikov hydrosilylation of func-
tionalized alkenes. While several nickel bis(amino)amide
pincer complexes are competent catalysts, the methoxide
complex [(MeN2N)Ni-OMe] (2) is a highly active and selective
catalyst under base-fee conditions. Ester, amido, sulfonyla-
mido, amino, ketone, and even formyl groups were tolerated.
Furthermore, the nickel pincer complexes catalyze isomer-
ization/hydrosilylation of internal functionalized alkenes.
This work is supported by the EPFL and the Swiss National
Science Foundation.
Keywords: alkenes · chemoselectivity · nickel · pincer ligands ·
silanes
How to cite: Angew. Chem. Int. Ed. 2015, 54, 14523–14526
Angew. Chem. 2015, 127, 14731–14734
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Received: August 21, 2015
Revised: September 14, 2015
Published online: October 2, 2015
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