Dehydrogenative silylation of terminal alkynes with hydrosilanes under zinc-pyridine catalysis

Article abstract of DOI:10.1002/adsc.201200310

A combination of zinc triflate and pyridine in a nitrile medium was found to act as an effective catalytic system for dehydrogenative silylation with flexible pieces of terminal alkynes and hydrosilanes, thereby producing diverse alkynylsilanes in high to

Full text of DOI:10.1002/adsc.201200310

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DOI: 10.1002/adsc.201200310
Dehydrogenative Silylation of Terminal Alkynes with
Hydrosilanes under Zinc–Pyridine Catalysis
Teruhisa Tsuchimoto,a,* Masakazu Fujii,a Yoshihiko Iketani,a and Masaru Sekinea
a
Department of Applied Chemistry, School of Science and Technology, Meiji University, 1-1-1 Higashimita, Tama-ku,
Kawasaki 214-8571, Japan
Fax : (+81)-44-934-7228; e-mail: tsuchimo@isc.meiji.ac.jp
Received: April 13, 2012; Revised: June 16, 2012; Published online: October 10, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200310.
Abstract: A combination of zinc triflate and pyri-
dine in a nitrile medium was found to act as an ef-
fective catalytic system for dehydrogenative silyla-
tion with flexible pieces of terminal alkynes and hy-
drosilanes, thereby producing diverse alkynylsilanes
in high to excellent yields.
on.[10a,b,d,g,h,k] As a reliable and sustainable process, we
ACHTUNGTRENNUNG
disclose herein the zinc–pyridine-catalyzed D.S. of ter-
À
minal alkynes with H Si. In contrast to the recent ex-
plosive growth of dehydrogenative coupling under
transition metal catalysis,[12] this is the first report of
À
À
coupling between C(sp) H and H Si catalyzed by
a Lewis acid.[13]
Keywords: alkynes; dehydrogenation; Lewis acids;
silanes; zinc
We first examined suitable reaction conditions for
the D.S. of 1-octyne (1a) with methyldiphenylsilane
(2a) (Table 1). Thus, treating 1a and 2a with ZnACHTUNGTRENNUNG(OTf)2
(5 mol%, Tf=SO2CF3) in EtCN at 1008C for 30 h
gave methyl(1-octynyl)diphenylsilane (3a) in 48%
yield, along with 3% yield of isomeric hydrosilylation
Alkynylsilanes are prominent structural motifs, due to products 4a–6a (entry 1). To our great delight, adding
their significance not only as Si-masked synthetic in- a catalytic amount of pyridine contributed to com-
termediates,[1] but also as reagents for carbon– plete the D.S. and to suppress the hydrosilylation
carbon[2] and carbon–heteroatom[3] bond formations. (entry 2); the achievement of the exclusive D.S. is
In view of their unique s–p conjugation properties, worthy of note because the hydrosilylation has been
C C Si units have also received much attention for known to occur under Lewis acid catalysis.[14] In sharp
ꢀ À
optoelectronic applications.[4] The customary way to contrast, other metal triflates and zinc salts were to-
À
construct C(sp) Si bonds includes deprotonation of tally inactive (entries 3–13). As other organic bases, t-
terminal alkynes with strong metallic bases (met– BuPy, Et3N and EtN(i-Pr)2 also assisted the D.S. but
G
base: for example, MeMgBr, BuLi), followed by trap- were less effective; on the contrary, using DMAP or
ping of the resulting alkynylmetals with (pseudo)halo- DBU had a detrimental effect (entries 14–18). The
[5,6]
À
silanes (X Si).
The strategy thus requires handling continuous survey of the solvent effect showed that
À
of the moisture-sensitive reagents, met–base and X
Si, use of which co-produces stoichiometric wastes: the Zn
EtCN is the best solvent (entries 2 and 19–24). With
(OTf)2–pyridine couple in EtCN, lowering of
N
H–base and met–X. Alkynes with base- or nucleo- the reaction temperature to 90, 80, 70 and 608C led to
phile-sensitive functional groups are also incompati- the gradual reduction of the reaction rate (entries 25–
[7]
[8]
À
À
ble. Other approaches with X Si, Me2N Si or 28). The D.S. using 0.80 mmol of 2a gave only 3a in
[9]
À
CH2=CH Si as silylating agents have emerged as >99% yield, showing that an extra amount of hydro-
well, but catalytic dehydrogenative silylation (D.S.) silanes causes no troublesome side-reaction
with hydrosilanes (H Si) appears to be ideal for im- (entry 29). In contrast, decreasing the stoichiometry
proving the above issues: reagent usability, atom- of 2a to 1.2 and 1.0 equiv. resulted in lower yields of
À
economy, waste and functional group compatibility. In 3a (entries 30 and 31).
fact, terminal alkynes have reportedly coupled with
With the promising reaction conditions in hand, we
À
H Si. However, it is rather surprising that simple al- next explored the substrate scope of this reaction
kynes with no functional group have been the focus (Table 2). Besides 1a, alkynes 1 with a range of alkyl
of earlier studies.[10,11] Furthermore, hydrosilylation of groups including branched and functionalized ones
the alkyne frequently occurs as
a side-reacti- were silylated successfully in a dehydrogenative
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Table 1. Lewis acid-catalyzed dehydrogenative silylation of
Table 2. Zinc–pyridine-catalyzed dehydrogenative silylation
1-octyne with methyldiphenylsilane.[a]
of terminal alkynes with hydrosilanes.[a]
Entry Lewis acid Solvent
Organic base
Yield [%]
3a[b] 4a–6a[c]
1
2
3
4
5
6
7
8
Zn
Zn
Cu
N
none
48
3
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
t-BuPy[d]
DMAP[e]
Et3N
>99 <1
<1 <1
<1 <1
<1 <1
<1 <1
<1 <1
<1 <1
<1 <1
<1 <1
<1 <1
<1 <1
<1 <1
AgOTf
Fe(OTf)3
In(OTf)3
(OTf)3
(OTf)3
EtCN
EtCN
EtCN
EtCN
EtCN
EtCN
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
Bi
Sc
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
9
YACHTUNGTRENNUNG
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25[i]
26[j]
27[k]
28[l]
ZnF2
ZnCl2
ZnBr2
EtCN
EtCN
EtCN
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
Zn
71
36
91
60
1
2
5
2
EtNACTHNGURETNNU(G i-Pr)2
DBU[f]
<1 <1
73 <1
<1 <1
41 <1
<1 <1
<1 <1
18
89
64
22
9
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
pyridine
1
<1
<1
<1
<1
29[m] Zn
>99 <1
85
76
30[n]
31[o]
Zn
Zn
<1
<1
[a]
Reagents: 1 (0.40 mmol), 2 (0.60–1.6 mmol), Zn
(20–40 mmol), pyridine (0.080–0.20 mmol),
ACHTUNGTRENNUNG
[a]
Reagents: 1a (0.40 mmol), 2a (0.60 mmol), Lewis acid
(20 mmol), organic base (80 mmol), solvent (0.40 mL).
Determined by H NMR.
Determined by GC.
t-BuPy=4-tert-butylpyridine.
DMAP=4-(dimethylamino)pyridine.
(0.40 mL). Yields of isolated 3 based on 1 are shown
here. See Supporting Information for further details.
Bn=benzyl.
[b]
[c]
[d]
[e]
[f]
1
[b]
[c]
c-Hex=cyclohexyl.
DBU=1,8-diazabicycloACTHNUTRGNEUGN[5.4.0]undec-7-ene.
[g]
[h]
[i]
manner (3a–3i). An unsymmetrical disilylacetylene
such as 3j can be adopted as a target structure. Impor-
tantly, the C=C unit also remained untouched without
suffering hydrosilylation (3k).[15] A series of aryl- and
heteroarylacetylenes with different electronic and
steric natures participated in this strategy successfully
(3l–3p). Metallosilylacetylene 3q, which is potentially
useful as a key component for optoelectronic materi-
als,[16] was synthesized in an excellent yield. As shown
thus far, the outstanding tolerance of the functional
DMF=N,N-dimethylformamide.
dioxane=1,4-dioxane.
At 908C.
At 808C.
At 708C.
[j]
[k]
[l]
At 608C.
[m]
[n]
[o]
2a (0.80 mmol) was used.
2a (0.48 mmol) was used.
2a (0.40 mmol) was used.
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Dehydrogenative Silylation of Terminal Alkynes with Hydrosilanes under Zinc–Pyridine Catalysis
Scheme 2. Zirconocene-mediated transformation of 3m.
quential one-pot treatment of 3m with the “Cp2Zr”
species derived from Cp2ZrCl2/2EtMgBr in THF,[19]
followed by the addition of ethyl chloroformate and
the final quenching with the aqueous HCl gave 11 in
79% yield in a regio- and stereospecific fashion.[20]
The use of 3m with the SiMePh2 group has an advant-
age that the transformation proceeds in a higher
yield, compared to the original process carried out
ꢀ
with PhC CSi(Me)2Bn (53% yield).
Some experimental results are available that pro-
vide insight into the mechanistic aspects. First, on
1H NMR monitoring of a solution obtained after
À
mixing of ZnACHTNUTRGNE(UNG OTf)2 (Zn) and HSiMePh2 (2a, H Si) in
a 1:1 ratio at room temperature for 30 min in CD3CN,
Scheme 1. Zinc–pyridine-catalyzed dehydrogenative silyla-
tion of terminal alkynes with silanes (alkynes:silanes=2.2 or
3.5:1).
a new broad singlet appeared at d=4.64 (0.35H) as
À
a part of the hydrogen atom of H Si [(b) in Figure 1].
This implies a Lewis acid–base interaction between
[21]
À
Zn and H Si, as previously reported. The loss of
3
ꢀ
groups, Cl, OCOMe, phthalimidoyl, HexOCH2C C, the J
(SiH,CH3) coupling in the signal may be attrib-
CN, t-Bu(Me)2SiO, C=C, CF3 and ferrocenyl, is note- uted to an H Si bond weakened or cleaved in com-
worthy. The D.S. can be equally well applied to hydro- pensation for Zn H bond formation. Interestingly,
À
À
silanes other than 2a. Thus, as monohydrosilanes, treating HSiEt3 with Zn
(OTf)2 also led to the appear-
HSiMe2Ph, as well as trialkyl analogues coupled dehy- ance of the same signal (0.54H) [(d) in Figure 1], de-
À
drogenatively with alkynes to give 3r–3u in high spite the fact that the original H Si signals of 2a (d=
yields. Unlike the silyl unit handled so far, the SiOSi 4.88) and HSiEt3 (d=3.60) were observed at different
moiety can be also introduced into the product (3v). positions far from each other [(a) and (c) in Figure 1].
With H2SiMePh and H2SiPh2, double dehydrogenative These results suggest existence of the same active spe-
coupling proceeded to afford 7a and 7b, respectively, cies in the cases of (b) and of (d). Therefore, consideÀr-
À
with the Si-tethered diyne skeleton, which is a useful ing generation of a zinc hydride species {[Zn H] }
precursor for Si-containing p-systems[17] and also ni- apart from the Si moiety, rather than partially activat-
trogen-containing heteroarenes[18] (Scheme 1). With ed ZndÀ···H···Sid+ species, one can understand the re-
the use of H3SiPh, product 8 was obtained through sults in Figure 1.[22,23]
À
triple dehydrogenative coupling. The present method
Alkyne 1 other than H Si 2 is also a potential can-
can be utilized to construct the extended s–p conjuga- didate to undergo activation by Zn. One possibility is
tion system 9 and also SiOSi-tethered diyne 10 in its base-assisted deprotozincation to form an alkynyl-
a single operation. In all the cases, the achievements zinc species, which has been known to react with alde-
of chemical yields of over 70% and the absence of hydes[24] and silyl triflates.[7b] We therefore first at-
alkyne-hydrosilylation confirm the high validity and tempted trapping of 12 with PhCHO under the stan-
reliability of this strategy. Furthermore, a practical ad- dard D.S. conditions, but no expected product 13 was
vantage can be demonstrated by carrying out the syn- obtained at all (Scheme 3). Using TfOSiEt3 in the re-
thesis on a preparative scale. For example, D.S. of 1a action of 4-phenyl-1-butyne gave a small amount of
or phenylacetylene with 2a on a 10-mmol scale pro- alkynylsilane 3t. However, 3t was again produced in
vided us with 2.94 g (95% yield) of 3a or 2.81 g (94% a comparable yield even without Zn
ACHTUNGRTEN(NUNG OTf)2, showing
yield) of 3m, respectively. that the involvement of 12 should be unlikely here.
The Ph2MeSi-installed alkynylsilane that has been On the basis of these results, there is no reason to
prepared here is seldom reported in the literature.[1–10] assume a route starting with the activation of 1 by
Accordingly, in order to demonstrate its utility in or- Zn.
ganic synthesis, we preliminarily performed the
Taking all of the above observations into consider-
carbon–carbon bond forming transformation of 3m as ation, we show a possible reaction mechanism in
a representative reaction (Scheme 2). Thus, the se- Scheme 4. Thus, silylium ion 14[25,26] and zinc hydride
Adv. Synth. Catal. 2012, 354, 2959 – 2964
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1
Figure 1. 500 MHz H NMR spectra in CD3CN of (a) HSiMePh2, (b) a 1:1 mixture of Zn
N
of mixing at room temperature, (c) HSiEt3, and (d) a 1:1 mixture of Zn
ACHTUNGTERN(NUNG OTf)2 and HSiEt3 after 30 min of mixing at room
temperature.
Scheme 4. A possible reaction mechanism.
Scheme 3. Trial for trapping of alkynylzinc species.
ZnACTHNUTRGEN(UGN OTf)2 would have been necessary to address the
reaction triggered by the activation not of alkynes but
À
of H Si, and the nitrile solvent with good coordinat-
15 would be formed first through the encounter of Zn ing ability might have played a crucial role to stabilize
and 2. Subsequent deprotosilylation between 14 and the resulting Si+ by solvation.[27]
1 would afford alkynylsilane 3. Since the activation of
In summary, we have developed, for the first time,
2 by Zn readily occurs even at room temperature the Lewis acid-catalyzed dehydrogenative silylation of
without an organic base (see Figure 1), pyridine may terminal alkynes with hydrosilanes. Our strategy fea-
participate in the stage of the deprotosilylation, as tures a wide range of substrate coverages with the
shown in 16, for enhancing the nucleophilicity of 1. In high functional group tolerance and with the excellent
the case that pyridine is absent, the hydride in 15 may level of reaction efficiency, and thus would be the
work as a base. Finally, 15 and [H–pyridine]+ would most promising avenue in synthesizing alkynylsilanes
react to regenerate Zn and pyridine.
under a variety of situations. In terms of the promo-
We now consider that the finding of this reaction is tion of sustainable chemistry, using a base metal such
likely to be ascribed to the appropriate choices of the as zinc for the catalyst is also a distinct advantage of
Lewis acid as well as of the reaction medium. Thus, this methodology.
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Dehydrogenative Silylation of Terminal Alkynes with Hydrosilanes under Zinc–Pyridine Catalysis
14039–14041; c) J. Ohshita, D. Tanaka, J. Matsukawa, T.
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reactions, see: d) M. Yamaguchi, A. Hayashi, T.
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General Procedure
ZnACHTUNGTRENNUNG(OTf)2 [(7.27 mg, 20.0 mmol) or (14.5 mg, 40.0 mmol)] was
placed in a 20-mL Schlenk tube, which was heated at 1508C
under vacuum for 2 h. The tube was cooled down to room
temperature and filled with argon. EtCN (0.40 mL) was
added to the tube and then the mixture was stirred at room
temperature for 3 min. To this were added alkyne
1
(0.400 mmol), hydrosilane 2 (0.600, 0.800, 1.20 or
1.60 mmol) and pyridine [(6.3 mg, 0.080 mmol) or (15.8 mg,
0.200 mmol)] successively, and the resulting mixture was
stirred at 1008C. After the time specified in Table 2, a satu-
rated aqueous NH4Cl solution (0.5 mL) was added to the
mixture, and the aqueous phase was extracted with EtOAc
(5 mLꢁ3). The combined organic layer was washed with
brine (1 mL) and then dried over anhydrous sodium sulfate.
Filtration and evaporation of the solvent followed by
column chromatography on silica gel gave the corresponding
product (3).
Acknowledgements
Partial financial support by a Grant-in-Aid for Scientific Re-
search (No. 24550123) from the Ministry of Education, Cul-
ture, Sports, Science and Technology is highly acknowledged.
We greatly appreciate Dr. Go Hirai as well as Prof. Dr.
Mikiko Sodeoka at RIKEN, Japan, for their helpful support
in HR-MS measurements.
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À
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Adv. Synth. Catal. 2012, 354, 2959 – 2964