Direct synthesis of alkylsilanes by platinum-catalyzed coupling of hydrosilanes and iodoalkanes

Article abstract of DOI:10.1039/c2cc35150a

Alkyl iodides and tertiary silanes were successfully coupled with good functional group tolerance using a Pt(P(tBu)₃)₂/(iPr) ₂EtN/CH₃CN system. The utility of the methodology is demonstrated by the synthesis of silafluofen, a Si-containing insecticide. This journal is The Royal Society of Chemistry.

Full text of DOI:10.1039/c2cc35150a

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Professors Hiroshi Nishihara and
As featured in:
Yoshinori Yamanoi et al. from the Department of
Chemistry, School of Science, The University of Tokyo,
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan.
Direct synthesis of alkylsilanes by platinum-catalyzed coupling of
hydrosilanes and iodoalkanes
A direct and versatile method that employs a Pt-catalyzed system
has been developed for the alkylation of Si−H bonds. With this
approach, iodoalkanes and tertiary silanes were successfully
coupled with good functional group tolerance. The utility of this
methodology is demonstrated by the synthesis of silafluofen,
a Si-containing insecticide.
See Yoshinori Yamanoi,
Hiroshi Nishihara et al.,
Chem. Commun., 2013, 49, 134.
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Direct synthesis of alkylsilanes by platinum-catalyzed
coupling of hydrosilanes and iodoalkanes†
Cite this: Chem. Commun.,
2013, 49, 134
Hikaru Inubushi, Hitoshi Kondo, Aldes Lesbani, Mariko Miyachi,
Yoshinori Yamanoi* and Hiroshi Nishihara*
Received 18th July 2012,
Accepted 1st October 2012
DOI: 10.1039/c2cc35150a
www.rsc.org/chemcomm
Alkyl iodides and tertiary silanes were successfully coupled with
good functional group tolerance using a Pt(P(tBu)3)2/(iPr)2EtN/
CH3CN system. The utility of the methodology is demonstrated
by the synthesis of silafluofen, a Si-containing insecticide.
Alkylsilanes are of great interest in medicinal chemistry owing to
their various biological activities.1 For example, alkylsilanes have
recently been employed as insecticidal protectants,2 anticancer
agents,3 and muscle relaxants.4 As a consequence, significant
research effort is currently being directed toward the development
of reproducible and high-yielding preparative procedures for this
interesting class of compounds. To date, the common approaches
for the preparation of alkylsilanes make use of the Rochow
process, chlorosilanes with organometallic reagents, and hydro-
silylation of olefins,5 the synthetic utility of which is limited.
Hydrosilanes have been widely used as mild reducing agents in
organic synthesis. In general, hydrosilanes do not react sponta-
neously with carbon electrophiles; however, the use of certain
transition metal catalysts can enable fine control of the reduction
process. Reactions of hydrosilanes with alkyl halides in the presence
of suitable transition metal catalysts have been previously shown to
proceed with the liberation of alkanes and halogenated silanes. For
example, Kunai et al., Chatgilialoglu et al., and Srithanakit and
Chavasiri independently reported the syntheses of mono- and
diiodosilanes in high yield (Scheme 1(a)) from reactions of hydro-
silanes with alkyl iodides in the presence of PdCl2 or NiCl2,
respectively.6 Miura et al. also reported alkyl radical mediated
indium-catalyzed reduction of alkyl halides to their corresponding
alkanes with hydrosilanes in high yield.7
Scheme 1 Metal-catalyzed reactions of hydrosilanes and iodoalkanes.
Therefore, we herein report a remarkably convenient and efficient
method for the synthesis of alkylsilanes (Scheme 1(b)) which
proceeds via platinum-catalyzed alkylation of hydrosilanes with
iodoalkanes.
The model studies of our catalytic system were carried out using
dimethylphenylsilane and 1-iodobutane (Table 1). Various palla-
dium and platinum complexes were selected as the alkylating
catalysts because both Si–H11,8a,d,f–h and alkyl–I12 bond activation
have been demonstrated for each metal. Contrary to our expecta-
tions, the palladium complexes employed did not show any
catalytic activity even under the same conditions previously devel-
oped for the arylation of hydrosilanes (entries 1 and 2). On the
other hand, the selected platinum complexes successfully catalyzed
the desired alkylation, which is the first example of catalytic
alkylation of hydrosilanes. Successful results were first obtained
with Pt(PPh3)2(CH2QCH2) employed as catalyst (entry 4).13
Among the platinum catalysts considered, Pt(P(tBu)3)2 exhibited
the best performance (55% yield, entry 5). An investigation into
the solvent effects on the reaction system of interest revealed
that the alkylation notably proceeded in polar and coordinating
solvents such as ethers or nitriles, with acetonitrile being the
most favorable (entry 11). Following a brief screening of bases,
(iPr)2EtN was found to provide the shortest reaction time and the
highest yield (60%) (entry 11). Although iodoalkanes were the
most efficient substrates, the corresponding bromoalkanes
could also be employed with this system, albeit with the desired
products being obtained in reduced yields.14
Throughout the course of our studies on the synthetic use of
hydrosilanes, we have demonstrated a number of examples of
transition metal-catalyzed arylation of hydrosilanes.8–10 However,
our method was limited in that it could only be applied to aryl
iodides. In fact, silylation of iodoalkanes has yet to be shown.
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-0033, Japan. E-mail: yamanoi@chem.s.u-tokyo.ac.jp,
nisihara@chem.s.u-tokyo.ac.jp; Fax: +81 3 5841 8063
Fig. 1 summarizes the scope of the alkylation reaction using
various hydrosilanes and iodoalkanes under the optimized
reaction conditions. The cross-coupling proceeds smoothly over
† Electronic supplementary information (ESI) available: Compound
characterization data and copies of 1H and 13C NMR. See DOI: 10.1039/c2cc35150a
c
134 Chem. Commun., 2013, 49, 134--136
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Table 1 Screening of reaction conditionsa
The generality of this alkylation reaction was explored by
employing different iodoalkanes. The reaction proceeded
smoothly with other considered 1-iodoalkanes (7 and 8). Inter-
estingly, iodomethane was also a suitable substrate for this
alkylation process (9). Similarly, 1,3-diiodopropane could couple
twice with two equivalents of hydrosilane in low yield (10). The
introduction of a trifluoromethyl substituent at the terminus of
an iodoalkane had no significant influence on reactivity (11). In
addition, 1-iodoalkanes with reactive functional groups, such as
an ester and a nitrile, which frequently require protection in
conventional methods that make use of organometallic reagents,
were well tolerated in this reaction system (12 and 13). Further-
more, the reaction of 1-chloro-3-iodopropane (14) indicated
that this catalytic system exhibited halogen-selectivity. The
catalytic system also proved to be effective in the case of
trialkylsilane (15). In all reactions considered, reduced com-
pounds (alkanes) were obtained in ca. 10–50% as side products.
As compared to primary alkyl iodides, secondary or tertiary
alkyl iodides were much less reactive. For example, the reaction
of phenyldimethylsilane and 2-iodobutane under optimized
conditions afforded only a trace amount of the expected alky-
lated silane product.
Entry Catalyst
Base
Solvent
Time (d) Yield (%)
b
1
2
3
Pd(PPh3)4
Pd(P(tBu)3)2
Pt(PPh3)4
(iPr)2EtN THF
(iPr)2EtN THF
(iPr)2EtN THF
3
3
3
3
2
—
—
—
b
b
4
5
6
7
8
9
10
11
12
13
14
Pt(PPh3)2(CH2QCH2) (iPr)2EtN THF
19
55
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
Pt(P(tBu)3)2
(iPr)2EtN THF
(iPr)2EtN 1,4-Dioxane 1
41
(iPr)2EtN CH2Cl2
(iPr)2EtN CH3NO2
(iPr)2EtN Toluene
(iPr)2EtN PhCN
(iPr)2EtN CH3CN
Et3N
Cy2MeN CH3CN
K3PO4 CH3CN
1
3
1
3
1
3
3
3
o1
o1
o1
34
60
46
CH3CN
47
28
a
Reaction conditions: all reactions consisted of 1-iodobutane
(1.0 mmol), dimethylphenylsilane (3.0 mmol), base (1.0 mmol), catalyst
(0.05 mmol) in 2 mL of solvent and were performed under Ar at 50 1C.
b
Not detected.
a wide range of substrates in moderate to good yield. Firstly, we
To highlight the practical utility of the developed catalytic
considered the reactivity of a variety of hydrosilanes in a model system, it was employed in a simplified synthesis of the insecticide
reaction (1). Interestingly, ethylmethylphenylsilane could be alkyl- silafluofen, 16.2b–d Silafluofen has been used as an agricultural
ated by this reaction (2). Hydrosilanes with two or three phenyl insecticide since 1995 in Japan owing to its high insecticidal activity,
groups displayed opposite results. Methyldiphenylsilane (3) gave low toxicity, and chemical stability under various environmental
almost the same yield as that of the model reaction, whereas conditions. Under the optimum reaction conditions described
triphenylsilane was unreactive owing to significant steric effects. above, dimethyl(4-ethoxyphenyl)silane was successfully coupled
Phenyl ring substituents at the 4-position of phenyldimethylsilane with 1-iodo-3-(4-fluoro-3-phenoxyphenyl)propane15 to afford sila-
had little effect on the silane reactivity (4 and 5). Notably, fluofen 16 in 42% yield. Because the market for household
2-phenyldimethylsilane could be readily alkylated by the pre- insecticides is estimated to be about 100 billion yen per year,
sented method (6).
this new alkylation reaction is promising from a practical
Fig. 1 Substrate scope of the Pt-catalyzed alkylation of tertiary silanes. Yields are isolated yields. Numbers in parentheses represent the ratio of alkylsilane : alkane, reaction
temperature, and reaction time. The ratios were determined by GC-MS of crude reaction mixture. a The ratios could not be determined due to high volatility of reduced alkanes.
c
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organosilane insecticide bearing a pyrethroid structure.
Although the mechanistic details of the demonstrated catalysis
are not presently clear, NMR experiments have been carried out to
verify the operative catalytic pathways. The components of the
1
initial step were elucidated by carefully monitoring the H NMR
of the reaction between dimethylphenylsilane and iodomethane,
which revealed that P(tBu)3 initially dissociates from Pt(P(tBu)3)2 in
the presence of iodomethane to afford ((tBu)3PCH3)+IÀ and
PtP(tBu)3. The doublet peaks observed in the reaction 1H NMR
(((tBu)3PCH3)+I–: d 1.74 (d, J = 12.0 Hz), 1.55 (d, J = 14.5 Hz);
7 K. Miura, M. Tomita, Y. Yamada and A. Hosomi, J. Org. Chem., 2007,
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PtP(tBu)3:
d 1.50 (d, J = 13.0 Hz)) are characteristic of
((tBu)3PCH3)+IÀ and PtP(tBu)3.16,17 The aforementioned potentially
explains why Pt(P(tBu)3)2 could not successfully catalyze the reac-
tions of much bulkier secondary or tertiary iodoalkanes. Here,
PtP(tBu)3, which is a highly reactive species, is most likely coordi-
nated by acetonitrile and thus, present in solution as the more
stable complex, PtP(tBu)3(CD3CN)n (n = 0–2). This notion is sup-
ported by the fact that the reaction proceeds more readily in
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1
coordinating solvents (Table 1). H NMR of the reaction did not
contain any signals derived from Pt–H or Pt–CH3 during the course
of the synthesis and therefore, it was not possible to determine
which species, hydrosilane or iodomethane, underwent oxidative
addition first.
On the basis of the above results, we propose a plausible
mechanism as shown in Fig. S2 (ESI†) for the Pt-catalyzed alkylation
of tertiary silanes. Initially, dissociation of one P(tBu)3 molecule
from Pt(P(tBu)3)2 occurs smoothly in the presence of the alkyl
iodide. Coordination of CH3CN stabilizes the formed active inter-
mediate, PtP(tBu)3, to which oxidative addition occurred.11,12c
Followed by additional oxidative addition18 or s-bond metathesis,19
reductive elimination affords the desired alkylsilane in the
presence of base with concomitant regeneration of PtP(tBu)3.
We thank Ms Kimiyo Saeki and Dr Aiko Sakamoto, the
Elemental Analysis Center of The University of Tokyo, for the
elemental analysis measurements. The present work was financially
supported by Grant-in-Aids for Scientific Research (C) (No.
24550221) and Scientific Research on Innovative Areas ‘‘Coordina-
tion Programming’’ (area 2107, No. 21108002) from the Ministry of
Education, Culture, Sports, Science, and Technology, Japan.
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´
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´
S. Lopez, Chem.–Eur. J., 2012, 18, 4401.
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c
136 Chem. Commun., 2013, 49, 134--136
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