Lewis acid-catalyzed trans-carbosilylation of simple alkynes

Full text of DOI:10.1021/jo960779o

4
874
J . Org. Chem. 1996, 61, 4874-4875
Ta ble 1. EtAlCl2-Ca ta lyzed Ca r bosilyla tion of
Acetylen es w ith Allyltr im eth ylsila n e
Lew is Acid -Ca ta lyzed tr a n s-Ca r bosilyla tion
of Sim p le Alk yn es
entry
1
R1
R2
2
product yielda (%)
†
Naoki Asao, Eiji Yoshikawa, and
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
Ph
H
H
H
H
H
H
Me
2a
2b
2c
2d
2e
2f
93
95
57
90
85
Yoshinori Yamamoto*
p-CH3C6H4
PhCH2
CH3(CH2)5
CH3(CH2)9
1-cyclohexenyl
Ph
b,c
b
Department of Chemistry, Graduate School of Science,
Tohoku University, Sendai 980-77, J apan
b,d
b,e
73
88
Received April 30, 1996
1g
2g
a
b
Isolated yield, except for where otherwise indicated. Yield
1
determined by H NMR using p-xylene as an internal standard.
PhCH2(CH2dCHCH2)CdCH2 (3c) was produced in 13% yield.
CH3(CH2)9(CH2dCHCH2)CdCH2 (3d ) was produced in 5% yield.
-(1,4-Penadien-2-yl)cyclohexene (3f) was produced in 5% yield.
Since the first carbometallation discovered by Ziegler
c
1
and B a¨ hr in 1927, a number of additions of organome-
d
tallics to carbon-carbon multiple bonds have been re-
ported.2 Unfortunately, however, carbosilylation still
remained unexploited due to the lack of a method for
activation of carbon-silicon bonds.3,4 We wish to report
that allylsilylation of simple unactivated acetylenes 1 is
e
1
that the allylsilylation of 1 proceeded so smoothly in the
presence of Lewis acids (eq 1).
catalyzed dramatically by Lewis acids such as EtAlCl
or AlCl to give the corresponding trans-allylsilylated
2
The results are summarized in Table 1. The reaction
of phenylacetylene 1a with allyltrimethylsilane catalyzed
3
alkynes 2 in good to high yields (eq 1). The allylmeta-
2
by EtAlCl in the presence of 20 equiv of TMSCl gave
the trans-carbosilylation product 2a regio- and stereose-
lectively in 93% yield (Table 1, entry 1). Neither the
stereoisomer of 2a (cis addition product) nor the regio-
isomer of 2a was produced.7 The reaction of 4-ethynyl-
toluene (1b) gave 2b in 95% yield (Table 1, entry 2),
whereas the addition to 3-phenyl-1-propyne (1c) afforded
2
c in 57% yield along with the unsilylated product 3c as
a byproduct (Table 1, entry 3). Very trace amounts of
1
unsilylated products were also detected in H NMR
spectra of the crude products in entries 1 and 2 (Table
lation of activated alkynes, such as alkynyl ketones
1
). The reactions of 1-octyne (1d ) and 1-dodecyne (1e)
(Michael acceptor) and alkynols (functional group sub-
gave 2d and 2e, respectively, in high yields (Table 1,
entries 4 and 5). The trans-allylsilylation of the enyne
stituted alkynes), and/or the intramolecular allylmeta-
lation proceed smoothly with various other allylmetals
than allylsilanes.2 However, allylmetalation of simple
unactivated alkynes 1 is not easy, and only a limited
number of allylmetals are available for this purpose.2
Accordingly, it was rather surprising for us to discover
1
f and internal acetylene 1g also proceed smoothly to give
,5
the corresponding alkenylsilanes 2f and 2g, respectively,
in high yields (Table 1, entries 6 and 7). The use of other
,6
Lewis acids, such as AlCl
reaction of 1-octyne (1d ) gave the allylsilylation product
d in around 60% yield, but the use of EtAlCl afforded
3 3 4
, AlBr , and HfCl , in the
2
2
†
Present address: Department of Chemistry, Graduate School of
the best result with respect to the yield of 2. The trans-
carbosilylation was unambiguously determined by the
stereochemistry of the allylation product 2g; irradiation
of methyl protons attached to double bond of 2g enhanced
the signal of both methylene protons of the allylic position
Science, Hokkaido University.
(
1) Ziegler, K.; B a¨ hr, K. Chem. Ber. 1928, 61, 253.
(2) For reviews, see: (a) Normant, J . F.; Alexakis, A. Synthesis 1981,
8
41 (organo-Li, Mg, Zn, B, Al, and Cu compounds). (b) Oppolzer, W.
Angew. Chem., Int. Ed. Engl. 1989, 28, 38 (stoichiometric organo-Li,
Mg, Zn, and catalytic Ni, Pd, Pt compounds). (c) Negishi, E. Pure Appl.
Chem. 1981, 53, 2333 (organo-Al/Ti and Al/Zr system). (d) Knochel,
P. Comprehensive Organometallic Chemistry II; Able, E. W., Stone, F.
G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, 1995; Vol. 11, p
(5.7% NOE) and a vinyl proton at C-5 position (1.4%
NOE), whereas irradiation of protons of TMS group did
not enhance the signal of those protons at all.
1
59 (organo-Li, Mg, Zn, B, Al, Cu, Hg/Pd, Ni, Mn compounds). (e)
Knochel, P. In Comprehensive Organic Synthesis; Trost, B. M., Fleming,
I., Eds.; Pergamon Press: Oxford, 1991; Vol. 4, p 865. (f) Yamamoto,
Y.; Asao, N. Chem. Rev. 1993, 93, 2207 (organo-Li, Mg, Zn, B, Al
compounds).
Preparation of 2b from 1b is representative. To a
mixture of allyltrimethylsilane (0.19 mL, 1.2 mmol) and
TMSCl (2.54 mL, 20 mmol) was added EtAlCl
2
(0.52 mL,
0.96 M in hexane, 0.5 mmol) at -47 °C. 4-Ethynyltolu-
ene (1b) (0.13 mL, 1.0 mmol) was added, and the reaction
mixture was stirred for 2 h at -47 °C. Diethylamine (2.0
mL, 19.4 mmol) was added, and the mixture was allowed
(3) The net carbosilylation from the three-component coupling
reaction was reported. (a) Chatani, N.; Amishiro, N.; Murai, S. J . Am.
Chem. Soc. 1991, 113, 7778. (b) Obora, Y.; Tsuji, Y.; Kawamura, T. J .
Am. Chem. Soc. 1995, 117, 9814.
(4) For direct C-Si bond cleavages within a transition metal
coordination sphere, see: (a) Lin, W.; Wilson, S. R.; Girolami, G. S. J .
Am. Chem. Soc. 1993, 115, 3022. (b) Horton, A. D.; Orpen, A. G.
Organometallics 1992, 11, 1193. (c) Chang, L. S.; J ohnson, M. P.; Fink,
M. J . Organometallics 1991, 10, 1219 and references cited therein.
3
to warm to 0 °C. Excess aqueous NaHCO was added.
(7) J ung and co-workers reported cis-allylsilylation of phenylacetyl-
ene (ref 6h). However, their stereochemical assignment of 2-phenyl-
1,4-pentadiene is wrong: A proton that appeared at 5.12 ppm was
assigned as Ha and that at 5.42 ppm as Hb. NOE experiments
revealed that the Ha proton appears at 5.42 ppm and the Hb proton
appears at 5.12 ppm. NOEs were observed at aromatic protons when
a proton at 5.42 ppm was irradiated, and those were observed at Hc
and Hd protons when a proton at 5.12 ppm was irradiated.
(
5) Araki, S.; Imai, A.; Shimizu, K.; Yamada, M.; Mori, A.; Butsugan,
Y. J . Org. Chem. 1995, 60, 1841 (allylation of allylindiums).
6) (a) Takai, K.; Yamada, M.; Odaka, H.; Utimoto, K.; Fujii, T.;
(
Furukawa, I. Chem. Lett. 1995, 315 (allyl-Ta). (b) Takahashi, T.;
Kotora, M.; Kasai, K.; Suzuki, N. Tetrahedron Lett. 1994, 35, 5685
(allyl-Zr). (c) Chatani, N.; Amishiro, N.; Morii, T.; Yamashita, T.;
Murai, S. J . Org. Chem. 1995, 60, 1834 (allyl-Zn). (d) Molander, G. A.
J . Org. Chem. 1983, 48, 5409 (allyl-Zn). (e) Miller, J . A.; Negishi, E.
Tetrahedron Lett. 1984, 25, 5863 (allyl-Al). (f) Negishi, E.; Miller, J .
A. J . Am. Chem. Soc. 1983, 105, 6761 (allyl-Zn). (g) Eishi, J . J .;
Boleslawski, M. P. J . Organomet. Chem. 1987, 334, C1 (allyl-Ti). (h)
Yeon, S. H.; Han, J . S.; Hong, E.; Do, Y.; J ung, I. N. J . Organomet.
Chem. 1995, 499, 159 (allyl-Si).
S0022-3263(96)00779-7 CCC: $12.00 © 1996 American Chemical Society

Communications
J . Org. Chem., Vol. 61, No. 15, 1996 4875
Extraction with ether, drying with anhyd Na
2
SO
4
, con-
Sch em e 1
centration, and purification with silica gel column chro-
matography using hexane as eluent gave 2b in 93% yield.
The use of 1 equiv of EtAlCl gave a similar result, but
2
the reaction using 0.1 equiv of EtAlCl
lower yield.
2
afforded 2b in
The present carbosilylation method is applicable to
crotyltrimethylsilane. The reaction of 1.0 equiv of phen-
ylacetylene with 1.2 equiv of crotyltrimethylsilane in the
presence of 0.5 equiv of EtAlCl and 20 equiv of TMSCl
2
gave 3-methyl-2-phenyl-1-(trimethylsilyl)-1,4-pentadiene
in 68% yield (eq 2). Trans-addition was confirmed by
of 2d and 3d in 20% yield. The low yield in the absence
of TMSCl supports the proposed mechanism (Scheme 1),
since regeneration of EtAlCl is diminished due to the
2
side reaction leading to 6. Furthermore, the addition
order of TMSCl was important; the chemical yield of 2
decreased dramatically when TMSCl was added after the
addition of an acetylene (see the above procedure). When
determining the stereochemistry of the products using
NOE experiments. Only the γ-addition product was
produced, and the R-addition product was not detected.
A plausible mechanism for the EtAlCl
allylsilylation is shown in Scheme 1, although it is highly
speculative. The coordination of EtAlCl to 1 would
2
-catalyzed trans-
the reaction of 1a using 0.5 equiv of EtAlCl
out in the absence of TMSCl and quenched with D
the deuterated alkene (Z)-Ph(CH dCHCH )CdCHD was
2
was carried
2
O,
2
2
2
produce the π-complex 4. Allyltrimethylsilane would
attack the electron-deficient triple bond from the side
obtained in 20% yield, clearly indicating that an alkenyl-
aluminum intermediate was involved.8
opposite to EtAlCl
stereoselectively. The coupling between the alkenyl
and silyl group of the ate complex would afford 2 and
EtAlCl , which again would work as a coordinating agent
to 1. On the other hand, the coupling between the chloro
and silyl group would produce Me SiCl and the alkenyl-
2
to produce the aluminum ate complex
Although further investigation is needed to settle this
5
9
unprecedented Lewis acid-catalyzed allylsilylation, the
procedure is useful for the stereocontrolled synthesis of
alkenylsilanes.
2
Su ppor tin g In for m ation Available: Characterization data
for reaction products (10 pages).
3
aluminum derivative 6, which would afford the minor
byproduct 3 upon hydrolysis. Excess amounts of TMSCl
are needed to drive the equilibrium over in favor of
replacing the aluminum with the silicon. Actually, the
J O960779O
(8) Negishi reported that allylaluminums add to unactivated acet-
reaction of 1-octyne in the presence of 0.5 equiv of EtAlCl
and 1 equiv of TMSCl gave a 31:69 mixture of 2d and
d in 30% yield, and the reaction using 0.5 equiv of
EtAlCl in the absence of TMSCl afforded a 5:95 mixture
2
ylenes in a cis-manner in the presence of Zr-catalyst (ref 6e). The
present addition proceeds in trans-manner. Accordingly, it seems that
the allylaluminum species is not involved as a reactive intermediate.
3
(9) For ZrCl
4
-catalyzed hydrostannylation, see: Asao, N.; Liu, J .;
2
Sudoh, T.; Yamamoto, Y. J . Chem. Soc., Chem. Commun. 1995, 2405.