Lewis acid-catalyzed trans-hydrosilylation of alkynes

Full text of DOI:10.1021/jo961508g

7
654
J . Org. Chem. 1996, 61, 7654-7655
Ta ble 1. Lew is Acid -Ca ta lyzed Hyd r osilyla tion of
Lew is Acid -Ca ta lyzed tr a n s-Hyd r osilyla tion
of Alk yn es
Acetylen es w ith Et3SiHa
Naoki Asao, Tomoko Sudo, and Yoshinori Yamamoto*
Department of Chemistry, Graduate School of Science,
Tohoku University, Sendai 980-77, J apan
Received August 5, 1996
1
Hydrosilylation of alkynes is one of the simplest and
the most straightforward preparative methods for vinyl-
silanes, which have great versatility as building blocks
in organic synthesis.2 It is well known that the hydrosi-
lylation of acetylenes is induced either by radical initia-
tors3 or by transition metal catalysts. The radical
induced procedure often provides a mixture of trans- and
cis-hydrosilylation products. Although the transition
metal-catalyzed reaction proceeds with high stereoselec-
tivity via a cis-hydrosilylation pathway, it usually pro-
duces a mixture of two regioisomers (terminal and
internal adducts) in the reaction with terminal alkynes.
We wish to report that the hydrosilylation of alkynes 1
with trialkylsilanes is catalyzed dramatically by Lewis
1
yield (%)
Lewis acid
(equiv)
R1
R2
entry
2
3
4
1
2
3
4
5
6
ZrCl
HfCl
AlCl
4
(0.2)
(0.2)
(0.2)
CH
CH
CH
3
3
3
3
3
3
(CH
2
2
2
2
2
2
)
9
9
9
9
9
9
H
H
H
H
H
H
H
H
H
H
H
H
H
H
CH
Ph
CH
(1a ) trace
(1a ) 28
(1a ) 93
(1a ) 95
0
4
(CH
(CH
(CH
(CH
(CH
)
)
)
)
)
0
0
0
0
0
0
0
0
0
0
0
0
0
3
EtAlCl
2
(0.2) CH
b
Et AlCl (0.2) CH
2
(1a )
0
c
AlCl
AlCl
AlCl
3
3
3
(0.2)
(0.2)
(0.2)
CH
(1a ) 90
(1b) 85
(1c) 82
(1d ) 89
(1e) 45
(1f) 61
(1g) 86
(1h ) 74
7
8
9
PhCH
t-C
Me Si
2
4
H
9
AlCl (0.2)
3
3
10
AlCl
3
(0.2)
1-cyclohexenyl
(1.0) Ph
(1.2) (i-Pr)
(i-Pr)
d
1
1
1
1
1
2
3
4
EtAlCl
EtAlCl
2
acids such as AlCl
3 2
and EtAlCl , leading to cis-alkenyl-
e
2
3
3
SiO(CH
SiO(CH
O(CH
2
2
)
)
2
4
silanes 2 with very high regio- and stereoselectivities in
good to high yields (eq 1).5 Although most hydrometa-
lations of alkynes proceed in a cis-manner, the Lewis
AlCl
3
(1.2)
,6
EtAlCl
2
(1.2) PhCH
2
2
)
2
(1i)
(1j)
72
73
15
AlCl
AlCl
AlCl
AlCl
3
3
3
3
(0.2)
(0.2)
(0.2)
(0.2)
CH
Ph
Ph
Ph
3
(CH
2
)
4
3
3
2 4
(CH )
1
6
(1k ) 66
(1l) 76
(1m ) 54
acids catalyzed hydrosilylation proceeds in a trans-
manner.7
17
10
26
1
8
C
2
H
5
The results are summarized in Table 1. First, we
examined the hydrosilylation of 1-dodecyne using trieth-
ylsilane with several different kinds of Lewis acids.
Recently, we found that the hydrostannation of alkynes
a
Reactions were conducted in toluene at 0 °C under Ar unless
b
otherwise noted. The starting material 1a was recovered quan-
titatively. c Reaction was carried out without any solvents. 2f was
obtained in 40% yield, if 0.2 equiv of AlCl3 was used as a catalyst.
d
e
Hexane was used as a solvent.
(1) (a) Hiyama, T.; Kusumoto, T. In Comprehensive Organic Syn-
thesis; Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991;
Vol. 8, p 763. (b) Weber, W. P. Silicon Reagents for Organic Synthesis;
Springer-Verlag: Berlin, 1983.
proceeds smoothly in the presence of Lewis acids, such
as ZrCl
kenyltins with high regio- and trans-stereoselectivities
in good yields.8 However, ZrCl
-catalyzed hydrosilylation
gave only a trace amount of 2a (entry 1). A slight
improvement was made by using HfCl as a catalyst, but
the yield was still low (entry 2). Fortunately, however,
the reaction in the presence of 0.2 equiv of AlCl afforded
the trans-hydrosilylation product 2a in 93% yield (entry
). Neither a stereoisomer of 2a (cis addition product)
4 4
and HfCl , producing the corresponding al-
(2) (a) Colvin, E. W. Silicon Reagents in Organic Synthesis; Aca-
demic: London, 1988. (b) Weber, W. P. Silicon Reagents for Organic
Synthesis; Springer: Berlin, 1983. (c) Colvin, E. W. Silicon in Organic
Synthesis; Butterworth: London, 1981.
4
(3) (a) Selin, T. G.; West, R. J . Am. Chem. Soc. 1962, 84, 1860. (b)
4
Benkeser, R. A.; Burrous, M. L.; Nelson, L. E.; Swisher, J . V. J . Am.
Chem. Soc. 1961, 83, 4385.
(4) Transition metal complexes derived from Pt, Ir, Pd, Rh, Ru, Ni,
3
Co, Fe, Re, and Mn metals have been used for hydrosilylation; see
ref 1a.
3
(5) Although AlCl
3
-catalyzed hydrosilylation of alkynes had been
1
reported by Voronkov’s group, detailed information on regio- and
stereoselectivities and generality of the reaction are not available;
see: (a) Voronkov, M. G.; Adamovich, S. N.; Pukhnarevich, V. B. J .
Gen. Chem. USSR (Engl. Transl.) 1982, 52, 2058. (b) Voronkov, M.
G.; Adamovich, S. N.; Sherstyannikova, L. V.; Pukhnarevich, V. B. J .
Gen. Chem. USSR (Engl. Transl.) 1983, 53, 706. (c) W. Ger. Pat. 2,
nor a regioisomer (3a ) was detected in the H-NMR
spectra of the crude reaction product. While EtAlCl was
2
also an efficient catalyst for the trans-hydrosilylation
(entry 4), starting material 1a was recovered quantita-
tively in the Et
seems that the reaction did not proceed due to lower
2
Lewis acidity of Et AlCl, compared to AlCl and EtAlCl .
2
AlCl-catalyzed reaction (entry 5). It
8
(
04, 204 (1979); Chem. Abstr. 1979, 91, 193413x. (d) Br. Pat. 684, 597
1953); Chem. Abstr. 1954, 2761d. (e) U.S. Pat. 2, 555, 589 (1971);
Chem. Abstr. 1951, 8814.
6) AlCl -catalyzed hydrosilylation of alkenes was reported; see: (a)
2
3
(
3
The use of nonpolar solvents such as toluene or hexane
was essential for obtaining high stereoselectivity and
Oertle, K.; Wetter, H. F. Tetrahedron Lett. 1985, 26, 5511. (b) Wetter,
H. F.; Oertle, K. Tetrahedron Lett. 1985, 26, 5515. (c) Yamamoto, K.;
Takemae, M. Synlett 1990, 259.
chemical yield. Unlike EtAlCl
such solvents, and thus the AlCl
tion proceeds in a heterogeneous system. The AlCl
2
, AlCl
3
is not soluble in
(
7) (a) Hydroboration, see: Smith, K.; Pelter, A.; Brown, H. C.
-catalyzed hydrosilyla-
3
Borane Reagents; Academic Press: London, 1988. Smith, K.; Pelter,
A. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press: Oxford, 1991; Vol. 8, p 703. (b) Hydroalumination,
see: Sato, F. J anssen Chim. Acta 1990, 8, 3. Eisch, J . J . In Compre-
hensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon
Press: Oxford, 1991; Vol. 8, p 733. (c) Hydromagnesation, see: Sato,
F. J . Organomet. Chem. 1985, 285, 53. (d) Hydrostannation, see:
Pereyre, M.; Quintard, J . -P.; Rahm, A. Tin in Organic Synthesis;
Butterworths: London, 1987. (e) Hydrozirconation, see: Labinger, J .
A. In Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
Pergamon Press, Oxford, 1991, 8, 667. (f) Hydrozincation, see: Gao,
Y.; Harada, K.; Hata, T.; Urabe, H.; Sato, F. J . Org. Chem. 1995, 60,
3
-
catalyzed hydrosilylation proceeded smoothly even with-
out a solvent to give 2a in 90% yield (entry 6).
The AlCl - or EtAlCl -catalyzed hydrosilylation was
3 2
examined with several other alkynes. The reactions of
3-phenyl-1-propyne (1b), 3,3-dimethyl-1-butyne (1c) hav-
(8) Asao, N.; Liu, J .-X.; Sudoh, T.; Yamamoto, Y. J . Chem. Soc.,
Chem. Commun. 1995, 2405. Asao, N.; Liu, J -. X.; Sudoh, T.; Yama-
moto, Y. J . Org. Chem. 1996, 61, 4568.
2
90.
S0022-3263(96)01508-3 CCC: $12.00 © 1996 American Chemical Society

Communications
J . Org. Chem., Vol. 61, No. 22, 1996 7655
Ta ble 2. Lew is Acid -Ca ta lyzed Hyd r osilyla tion of
Sch em e 1
1
-Dod ecyn e 1a w ith R3SiH
Lewis acid
(equiv)
entry
R3SiH
yield (%)
1
2
3
4
5
AlCl3 (0.2)
AlCl3 (0.2)
EtAlCl2(0.2)
EtAlCl2(0.2)
EtAlCl2 (0.2)
AlCl3 (0.2)
t-BuMe2SiH
(c-C6H11)Me2SiH
Ph3SiH
Ph2MeSiH
PhMe2SiH
(EtO)3SiH
(2n )
(2o)
(2p )
(2q)
(2r )
(2s)
78
73
40
45
60
0
a
column chromatography using hexane as an eluent to
afford 2a (263 mg, 0.93 mmol) in 93% yield.
The Lewis acid-catalyzed hydrosilylation of diyne
6
a
The starting material 1a was recovered quantitatively.
compounds was examined. Very interestingly, the AlCl
0.2 equiv)-catalyzed hydrosilylation of 1,6-heptadiyne 4a
with 4 equiv of Et SiH gave the six-membered cyclization
product 5 in 60% yield, whereas that of 1,7-octadiyne 4b
afforded bis-hydrosilylation product 6 in 47% yield. The
assingment of the stereochemistry of 5 was made by NOE
experiments (see the supporting information). No cy-
clization product was obtained from 4b under various
reaction conditions: for example, the use of less than 4
3
ing a bulky tert-butyl group, and (trimethylsilyl)acetylene
(
(1d ) proceeded smoothly to produce 2b, 2c, and 2d ,
3
respectively, in high yields (entries 7-9), whereas the
hydrosilylation of a conjugated enyne, 1-ethynyl-1-cyclo-
hexene (1e), gave 2e in moderate yield (entry 10). The
reaction of phenylacetylene (1f) afforded 2f in 61% yield
when a stoichiometric amount of EtAlCl
2
was used (entry
as a catalyst gave 2f in
The hydrosilylation of alkynes bearing sily-
1
4
1). The use of 0.2 equiv AlCl
0% yield
3
.
3
equiv of Et SiH gave a mixture of 6 and a monohydrosi-
lylation product. The results are in marked contrast to
loxy and benzyloxy groups gave the corresponding trans-
hydrosilylation products in good to high yields (entries
the cyclization reaction of diyne compounds by Ni-
1
2-14). Very interestingly, the use of 1.2 equiv of Lewis
acids was essential for obtaining good chemical yields
entries 12-14). Most probably, 1 equiv of Lewis acid
catalyzed hydrosilylation.9
(
would be needed to coordinate to an oxygen atom of the
silyloxy or benzyloxy group, and the remaining 0.2 equiv
of Lewis acid would act as a catalyst. The reaction of
symmetrical internal acetylenes, such as 6-dodecyne (1j)
and tolan (1k ), also proceeded smoothly (entries 15 and
1
6), whereas the reaction of unsymmetrical acetylenes,
such as 1-phenyl-1-propyne (1l) and 1-phenyl-1-butyne
1m ), afforded a mixture of regioisomers, 2l and 3l and
m and 3m , respectively (entries 17 and 18). The
A plausible mechanism for the AlCl
catalyzed trans-hydrosilylation is shown in Scheme 1,
although it is speculative. The AlCl or EtAlCl (AlX
would coordinate to the acetylenic bond of 1 to produce
complex 7.10 A hydride from R
SiH would attack an
electron-deficient triple bond from the side opposite to
AlX to produce an aluminum ate-complex 8. The
intermediate 8 would undergo coupling between the
trialkylsilyl cation and the vinyl group with retention of
3
geometry to give 2 and AlX . The above mechanism is
3 2
- or EtAlCl -
(
2
3
2
3
)
regioisomeric ratio depended on the steric bulk of the
substituent R . The methyl substituent gave a ratio of
7
2
3
6:10, whereas the ethyl group afforded that of 54:26.
3
Next, we carried out the AlCl - or EtAlCl -catalyzed
3
2
hydrosilylation of 1a with various silanes, and the results
are shown in Table 2. The reaction of tert-butyldimeth-
ylsilane and cyclohexyldimethlysilane proceeded smoothly
to give the corresponding trans-hydrosilylation products
very much related to the mechanism of Lewis acid-
8
11a
catalyzed hydrostannation, allylstannation,
and
2
n and 2o, respectively, in good yields (entries 1 and 2).
11b
allylsilylation, developed recently in our laboratory.
The addition of phenyl-substituted silanes, such as
triphenyl-, diphenylmethyl-, and phenyldimethylsilane,
gave lower yields in comparison with that of alkyl-
substituted silanes (entries 3-5 vs 1-2). The reaction
of triethoxysilane did not proceed at all (entry 6). These
results suggest that the silanes having an electron-
donating substituent, such as alkyl groups, are more
suitable to the Lewis acid-catalyzed hydrosilylation.
Su p p or tin g In for m a tion Ava ila ble: Full spectroscopic
and analytical characterization for compounds 2a ,b,d ,e,g-r ,
3l,m , 5, and 6 (26 pages).
J O961508G
(9) Tamao, K.; Kobayashi, K.; Ito, Y. J . Am. Chem. Soc. 1989, 111,
6478.
(10) The addition order of reagent, substrate, and Lewis acid is
Preparation of 2a from 1a is representative. To a
suspension of AlCl
3
(27 mg, 0.2 mmol) in toluene (1.0 mL)
important to obtain higher yields of the desired product. The order
mentioned in the synthesis of 2a is essential. If the acetylene 1a
was added triethylsilane (0.19 mL, 1.2 mmol) at 0 °C
under an Ar atmosphere. The mixture was stirred for 5
min, and then 1a (0.21 mL, 1.0 mmol) was added. The
mixture was stirred for 1 h at 0 °C, and excess amounts
of triethylamine (0.2 mL, 1.5 mmol) were added. After
3 3
was added to AlCl prior to the addition of Et SiH, significant
amounts of the trimerization product of 1a (aromatic compounds)
were obtained as a byproduct. Accordingly, it seems that coexistence
3 3
of Et SiH is needed, when the coordination of AlCl to the acetylene
takes place, in order to capture instantaneously an activated triple
bond.
(11) (a) Asao, N.; Matsukawa, Y.; Yamamoto, Y. J . Chem. Soc.,
addition of aqueous NaHCO
3
solution, the usual workup
Chem. Commun. 1996, 1513. (b) Asao, N.; Yoshikawa, E.; Yamamoto,
Y. J . Org. Chem. 1996, 61, 4784.
gave crude product, which was purified by silica gel