Nickel-Catalyzed Synthesis of Silanes from Silyl Ketones

Article abstract of DOI:10.1021/acs.orglett.9b03487

An unprecedented nickel-catalyzed decarbonylative silylation via CO extrusion intramolecular recombination fragment coupling of unstrained and nondirecting group-Assisted silyl ketones is described. The inexpensive and readily available catalyst performs

Full text of DOI:10.1021/acs.orglett.9b03487

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Nickel-Catalyzed Synthesis of Silanes from Silyl Ketones
Watchara Srimontree,† Waranya Lakornwong,† and Magnus Rueping*,†,‡
†Institute of Organic Chemistry, RWTH Aachen University, Landoltweg 1, 52074 Aachen, Germany
‡KAUST Catalysis Center (KCC), King Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi
Arabia
S
* Supporting Information
ABSTRACT: An unprecedented nickel-catalyzed decarbon-
ylative silylation via CO extrusion intramolecular recombina-
tion fragment coupling of unstrained and nondirecting group-
assisted silyl ketones is described. The inexpensive and readily
available catalyst performs under mild reaction conditions and
enables the synthesis of structurally diverse arylsilanes,
including heterocyclic and natural product derivatives.
ransition-metal-catalyzed cross-coupling reactions have
been established as a powerful and straightforward
Scheme 1. Transition-Metal-Catalyzed Silylation Reactions
T
approach for creating C−C or C−heteroatom bonds, which
are common in modern organic synthesis.1 In this context,
over the past few years, the transition-metal-catalyzed
decarbonylative cross-coupling of naturally abundant carbox-
ylic acid derivatives with numerous nucleophiles has been
attracting increased attention, because it avoids classical
electrophilic coupling partners, such as organohalides.2 More-
over, intramolecular decarbonylative coupling reactions
emerged as a more efficient and higher atom-economic
strategy, because of the absence of nucleophiles and bases.3
In this case, the general mechanism consists of oxidative
addition by the transition-metal complex to the C(acyl)−
C(alpha) or C(acyl)−heteroatom bond, subsequent decarbon-
ylation and reductive elimination. Regarding this approach,
transition-metal-mediated decarbonylative intramolecular cou-
plings of aldehydes, ketones, and other carboxylic acid
derivatives have been extensively studied under various
conditions, leading to C−C3,4 and C−heteroatom5−11 bond
constructions. However, to the best of our knowledge, C−Si
bond formation derived from acylsilanes has not been explored
previously.
Organosilicons have been used as key building blocks for
natural product synthesis, drug development, and material
sciences.12 Arylsilanes are traditionally synthesized by the
addition of Grignard or organolithium reagents to chlorosi-
lanes or cyclosiloxanes.13 Subsequently, reactions of aryl
halides with disilanes or hydrosilanes as coupling reagents
have evolved based on transition-metal catalysis (see Scheme
1a).14 Furthermore, recently, our group successfully developed
the nickel/copper-catalyzed decarbonylative silylation of esters
and amides in the presence of silylboranes (Scheme 1b, left).15
However, this process requires the preformation of the phenol
ester or amide and provides as a stoichiometric byproduct the
phenol or imide, respectively. Hence, a procedure that would
start from the acylsilane instead of the activated ester or amide
would only provide CO as a traceless byproduct. We herein
report the first nickel-catalyzed intramolecular decarbonylation
for the transformation of silyl ketones into arylsilanes (Scheme
1b, right).
Our primary investigation in this study focused on the
reaction of (dimethyl(phenyl)silyl)(naphthalen-2-yl)-
methanone (1a) with Ni(cod)2, in the presence of different
phosphine ligands (see Table 1, entries 1−5). PnBu3 was found
as the most effective ligand which provided the decarbonylative
product 2a in 60% NMR yield. The yield increased to 72%
when the reaction was conducted for 36 h (see Table 1, entry
6). Additives such as Mn and Zn were able to slightly improve
the yield, to 64% and 71%, respectively (see Table 1, entries 8
and 9; also see the Supporting Information). Subsequently, we
evaluated the in situ generation of nickel(0) species from
nickel(II) complexes in the presence of a strong reductant;
however, unsatisfactory results were obtained (see Table 1,
entries 11 and 12). In 1,4-dioxane as a solvent, the yield
decreased to 24% (see Table 1, entry 14). The effect of
temperature was investigated and 150 °C was found to be
Received: October 2, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b03487
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 2. Substrate Scope with Respect to Aryl and Silyl
Substitution
a
entry
[Ni] catalyst
ligand
additive
yield (%)
b
1
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(cod)2
Ni(acac)2
Ni(OAc)2·4H2O
−
dcype
dppe
−
−
−
−
−
−
−
Mn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
21
26
trace
41
60
72
0
64
71
80
0
21
0
24
86 (83)
70
b
2
3
4
5
PCy3
PPh3
PnBu3
PnBu3
−
c
6
7
8
9
10
11
12
13
14
15
PnBu3
PnBu3
PnBu3
PnBu3
PnBu3
PnBu3
PnBu3
PnBu3
PnBu3
c
c
c
c
c d
,
Ni(cod)2
Ni(cod)2
Ni(cod)2
c e
,
f
c g
,
16
a
Reaction conditions: 1a (0.20 mmol), Ni(cod)2 (10 mol %, 0.02
mmol), ligand (20 mol %, 0.04 mmol), toluene (1 mL), 160 °C, 16 h,
yield determined by 1H NMR spectroscopy using 1,3,5-(OMe)3C6H3
b
c
as an internal standard. Ligand (10 mol %, 0.02 mmol). Running for
d
e
f
36 h. Using 1,4-dioxane. At 150 °C. Yield after column
chromatography. At 140 °C.
g
optimal, the desired product 2a being obtained in high yield
after isolation (83%; see Table 1, entry 15). Furthermore,
control experiments indicated that Ni(cod)2 and PnBu3 are
required for this process (see Table 1, entries 7 and 13).
With the optimized reaction conditions in hand, the
substrate scope of the nickel-catalyzed intramolecular decar-
bonylative silylation of acylsilanes was investigated. Silyl
ketones 1a−1c lacking substituents on the aromatic ring,
were smoothly converted to the corresponding products 2a−
2c in good to high yields. Next, the effect of different electron-
donating and electron-withdrawing substituents in the para-
position of the phenyl ring was investigated. Various functional
groups were tolerated under the optimal conditions, affording
the corresponding products 2d−2i and 2l−2n in good to high
yields.
Substituents in the meta and ortho position were also
tolerated, and the products 2j and 2k were obtained in
satisfactory yields. Furthermore, heterocyclic silyl ketones 1o
and 1p were converted to the decarbonylated products 2o and
2p in gratifying yields. Because of the chemoselectivity of the
decarbonylation reaction, applicable specifically to the silyl
ketone functional group (1n), these conditions allowed the
late-stage functionalization of cholesterol and menthol
derivatives 1q and 1r in high yields. In addition, the scope
of different silyl groups was evaluated and the expected
products 2s and 2t were isolated in good to moderate yields
(see Scheme 2 bottom). Under identical reaction conditions,
naphthalen-2-yl(triethylsilyl)-methanone (1u) was tested;
however, the analogous product 2u was obtained in lower
yield.
a
Reaction conditions: 1 (0.20 mmol), Ni(cod)2 (10 mol %, 0.02
mmol), PnBu3 (20 mol %, 0.04 mmol), Zn (2.0 equiv), toluene (1
mL), 150 °C, 36 h, yield after isolation. Without Zn. cReaction on 1
b
d
mmol scale, 48 h. NMR yield.
on the Si atom, exhibited the slowest rate, most probably due
to the weak coordinating ability to the nickel complex (Figure
1; also see the Supporting Information).
Based on our results, a plausible catalytic pathway is
illustrated in Scheme 3. The insertion of nickel(0) species into
Si−C(O) bond favors the formation of acylnickel(II)
intermediate A,16 which can undergo decarbonylation to
generate arylnickel(II) silane species B. Lastly, reductive
To investigate the influence of the steric properties of the
silicon residue further, we conducted a series of parallel
reactions with substrates 1a, 1s, and 1t and noticed that the
sterically encumbered substrate 1t, bearing three phenyl rings
B
DOI: 10.1021/acs.orglett.9b03487
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
W.L. was supported by the Thailand Research Fund through
the Royal Golden Jubilee Ph.D. Program for the one-year
exchange fellowship to RWTH-Aachen University. We thank
Tengfei Ji for performing the large scale experiment.
Figure 1. Initial reactions with substrates bearing different
substituents on the α-Si atom.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b03487.
Detailed experimental procedures, spectral data for all
compounds, and copies of 1H, 13C, and 19F NMR
spectra (PDF)
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: magnus.rueping@rwth-aachen.de.
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DOI: 10.1021/acs.orglett.9b03487
Org. Lett. XXXX, XXX, XXX−XXX