Lewis acid catalyzed stereoselective carbosilylation. Intramolecular trans-vinylsilylation and trans-arylsilylation of unactivated alkynes

Article abstract of DOI:10.1021/ja011316p

Intramolecular trans-vinylsilylation using silicon-tethered alkynylvinylsilanes 3b-c was catalyzed dramatically by Lewis acids such as EtAlCl₂ to give the corresponding six-membered silacycles 4b-c in high yields. The reaction proceeded via an

Full text of DOI:10.1021/ja011316p

J. Am. Chem. Soc. 2001, 123, 10899-10902
10899
Lewis Acid Catalyzed Stereoselective Carbosilylation. Intramolecular
trans-Vinylsilylation and trans-Arylsilylation of Unactivated Alkynes
Naoki Asao, Tomohiro Shimada, Takashi Shimada, and Yoshinori Yamamoto*
Contribution from the Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Sendai 980-8578, Japan
ReceiVed May 30, 2001
Abstract: Intramolecular trans-vinylsilylation using silicon-tethered alkynylvinylsilanes 3b-c was catalyzed
dramatically by Lewis acids such as EtAlCl2 to give the corresponding six-membered silacycles 4b-c in high
yields. The reaction proceeded Via an exo-mode cyclization. In addition to the vinylsilylation, the intramolecular
trans-arylsilylations using carbon-tethered alkynylarylsilanes 8a-b were also catalyzed by Lewis acids such
as HfCl4 to give six- and seven-membered cyclic (E)-vinylsilanes 9a-b, respectively. The cyclization of silicon-
tethered substrates 13a-d afforded five- and six-membered silacycle products 14a-d in low to high yields.
All arylsilylation reactions proceeded Via an exo-mode fashion exclusively.
version of the Lewis acid catalyzed hydro- and carbometalation9
8
Introduction
produces the corresponding silacycles (eq 3) and carbocycles
The hydro- and carbometalation of a carbon-carbon multiple
(
eq 4), in which the hydro- and carbosilylation take place also
bond is one of the most fundamental and straightforward meth-
7
in a trans manner.
1
odologies for the preparation of new organometallics. Particu-
larly, the hydro- and carbometalation of unactivated alkynes is
a practical method for the preparation of vinylmetals, which
have great versatility as building blocks in organic synthesis.
Although hydroboration and hydroalumination proceed without
any activators, the other hydrometalations such as hydrosily-
lation and hydrostannation are promoted by transition metal
catalysts or by radical initiators. Only a little attention was paid
to the utilization of a Lewis acid as an activator. The allylmeta-
lation of activated alkynes, such as alkynyl ketones (Michael
acceptors) and alkynols (functionally substituted alkynes), in
both intramolecular and intermolecular versions proceeds
smoothly with various allylmetals. However, the allylmetalation
of simple unactivated alkynes is not easy, and only a limited
number of allylmetals can serve for this purpose. Here also, no
attention was paid to the use of a Lewis acid as an activator for
the allylmetalation of unactivated alkynes. We previously
reported that Lewis acids such as AlCl3, EtAlCl2, ZrCl4, HfCl4,
and B(C6F5)3 catalyzed efficiently the intermolecular hydrosi-
2
3
4
5
lylation, hydrostannation, allylsilylation, and allylstannation
of unactivated alkynes to give the corresponding trans-hydro-
and allylmetalation products with very high stereoselectivities
in high chemical yields (eqs 1 and 2).6 The intramolecular
Despite the extensive investigation of the hydro- and carbo-
metalation reactions of alkenes and alkynes, there are very few
reports on vinylmetalation, especially of alkynes. Vinylmeta-
lation of alkynes gives new vinyl organometallics, which may
exhibit a reactivity similar to that of the starting vinyl organo-
metallic compounds. This type of carbometalation results in
oligomerization or polymerization reactions. This is one of the
major reasons for limiting the scope of vinylmetalation of
,7
(1) For reviews, see: (a) Lewis Acids in Organic Synthesis; Yamamoto,
H., Ed.; Wiley-VCH: Weinheim, Germany, 2000; Vols 1 and 2. (b) Marek,
I.; Normant, J. F. In Metal-catalyzed Cross-coupling Reactions; Diederich,
F., Stang, P. J., Eds.; Wiley-VCH: Weinheim, Germany, 1998; pp
2
71-337. (c) Knochel, P. In ComprehensiVe Organic Synthesis; Trost, B.
M., Fleming, I., Eds.; Pergamon Press: Oxford, U.K., 1991; Vol. 4, pp
8
65-911.
(5) (a) Asao, N.; Matsukawa, Y.; Yamamoto, Y. J. Chem. Soc., Chem.
Commun. 1996, 1513-1514. (b) Matsukawa, Y.; Asao, N.; Kitahara, H.;
Yamamoto, Y. Tetrahedron 1999, 55, 3779-3790.
(6) Yoshikawa, E.; Kasahara, M.; Asao, N.; Yamamoto, Y. Tetrahedron
Lett. 2000, 41, 4499-4502.
(
2) (a) Asao, N.; Sudo, T.; Yamamoto, Y. J. Org. Chem. 1996, 61,
7
654-7655. (b) Sudo, T.; Asao, N.; Gevorgyan, V.; Yamamoto, Y. J. Org.
Chem. 1999, 64, 2494-2499.
(3) (a) Asao, N.; Liu, J.-X.; Sudoh, T.; Yamamoto, Y. J. Chem. Soc.,
Chem. Commun. 1995, 2405-2406. (b) Asao, N.; Liu, J.-X.; Sudoh, T.;
Yamamoto, Y. J. Org. Chem. 1996, 61, 4568-4571. (c) Gevorgyan, V.;
Liu, J.-X.; Yamamoto, Y. J. Chem. Soc., Chem. Commun. 1998, 37-38.
(7) Asao, N.; Yamamoto, Y. Bull. Chem. Soc. Jpn. 2000, 73, 1071-
1087.
(8) Sudo, T.; Asao, N.; Yamamoto, Y. J. Org. Chem. 2000, 65, 8919-
8923.
(4) (a) Asao, N.; Yoshikawa, E.; Yamamoto, Y. J. Org. Chem. 1996,
6
1, 4874-4875. (b) Yoshikawa, E.; Gevorgyan, V.; Asao, N.; Yamamoto,
(9) Imamura, K.; Yoshikawa, E.; Gevorgyan, V.; Yamamoto, Y. J. Am.
Chem. Soc. 1998, 120, 5339-5340.
Y. J. Am. Chem. Soc. 1997, 119, 6781-6786.
1
0.1021/ja011316p CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/10/2001

10900 J. Am. Chem. Soc., Vol. 123, No. 44, 2001
Asao et al.
alkynes. Although it has been known for many years that polar
organometallics (R-M; M ) Li, Mg, Zn, ...) undergo intramo-
lecular carbometalation to alkynes, the carbometalation of
alkynes with organosilicon compounds is not easy due to the
points of view (eq 7).15,16 While no cyclization took place in
the reaction of 3a, the cyclization reactions of 3b,c proceeded
in the presence of a catalytic amount of EtAlCl2 to afford the
silacycles 4b,c in high yields, respectively, which are not easily
10
17
lack of a method for activation of stable carbon-silicon bonds.
available via the previously known methodologies.
Concerning polar organometallics, carbometalation with allyl
and alkyl reagents is more popular than that with vinyl and aryl
1
1
reagents. Recently, we communicated that the intramolecular
trans-vinylsilylation of unactivated alkynes is catalyzed by
certain Lewis acids to give the cyclic vinylsilanes in high yields
1
2,13
(eqs 5 and 6).
In this paper, we wish to report the Lewis
acid catalyzed highly stereoselective intramolecular trans-
arylsilylation of unactivated alkynes along with a detailed study
of the intramolecular trans-vinylsilylation.14
These results can be accounted for by the following mecha-
nistic rationale (Scheme 1). The coordination of a Lewis acid
Scheme 1
Results and Discussion
1. Vinylsilylation of Alkynes. As we previously showed, the
intramolecular vinylsilylation reactions using the carbon-
tethered alkynylvinylsilanes 1a,b were catalyzed by Lewis acids
such as EtAlCl2 and AlCl3 to afford six- or seven-membered
cyclization products 2a,b (n ) 1 or 2) in good to high yields,
respectively (eqs 5 and 6).12 The stereostructures of the products
2
clearly showed that the vinylsilylation proceeded in a trans-
fashion stereoselectively: the formation of the stereo and
regioisomers of 2 was not observed. On the basis of these results,
we examined the cyclization reactions of the silicon-tethered
alkynylvinylsilanes 3 for obtaining silacycle compounds, which
have attracted interest from both pharmaceutical and synthetical
7
to the triple bond of 3 would form π-complex 5. The R-carbon
(10) For example, the mean bond dissociation energies D(M-R) of
of vinylsilane moiety would attack the electron deficient triple
bond from the side opposite to the Lewis acid Via an exo-mode
fashion to produce an aluminum ate complex 6 stereoselectively.
The transfer of dimethylsilyl group to the aluminate center and
concomitant shift of electrons to the carbocation center would
afford 4 Via the intermediate 7 and regenerate the Lewis acid.
In the reaction of 3b,c, the developed carbocation â to the
dimethylsilyl group of the intermediate 6 could be stabilized
representative organometallics MRn are as follows: Si-Et, 297 ( 6 kJ/
mol; Li-Et, 209 kJ/mol; Zn-Et, 145 kJ/mol; Al-Et, 242 kJ/mol. See:
Becerra, R.; Walsh, R. In The Chemistry of Organic Silicon Compounds;
Rappoport, Z., Apeloig, Y., Eds.; Wiley: Chichester, U.K., 1998; Vol. 2,
pp 153-180. Skinner, H. A. AdV. Organomet. Chem. 1964, 2, 49-114.
Aylett, B. J. Organometallic Compounds, Vol. I, The Main Group Elements,
Part 2; 1979.
(11) To date very little is known about the intramolecular vinylmetalation
and arylmetalation of unactivated alkynes. For vinyllithiation and aryl-
lithiation, see: (a) Wu, G.; Cederbaum, F. E.; Negishi, E. Tetrahedron Lett.
1
2
by the presence of Ph or Me group at the R and R position,
but in the reaction of 3a, such stabilization is not expected since
both R and R are H. Such substituent effect in the reaction of
1
990, 31, 493-496. (b) Ovaska, T. V.; Warren, R. R.; Lewis, C. E.;
Wachter-Jurcsak, N.; M.; Bailey, W. F. J. Org. Chem. 1994, 59, 5868-
870. (c) Bailey, W. F.; Wachter-Jurcsak, N. M.; Pineau, M. R.; Ovaska,
T. V.; Warren, R. R.; Lewis, C. E. J. Org. Chem. 1996, 61, 8216-8228.
12) Asao, N.; Shimada, T.; Yamamoto, Y. J. Am. Chem. Soc. 1999,
21, 3797-3798.
13) It is well-known that the reactivity of vinylsilanes and arylsilanes
1
2
5
(
(15) (a) Tacke, R.; Wannagat, U. Top. Curr. Chem. 1979, 84, 1-75. (b)
Badger, A. M.; Schwartz, D. A.; Picker, D. H.; Dorman, J. W.; Fontaine,
F. C. J. Med. Chem. 1990, 33, 2963-2970.
(16) (a) Matsumoto, K.; Aoki, Y.; Oshima, K.; Utimoto, K. Tetrahedron
1993, 49, 8487-8502. (b) Tanaka, Y.; Yamashita, H.; Tanaka, M.
Organometallics 1996, 15, 1524-1526. (c) Chauhan, B. P. S.; Tanaka, Y.;
Yamashita, H.; Tanaka, M. Chem. Commun. 1996, 1207. (d) Birot, M.;
Pillot, J.; Dunogues, J. Chem. ReV. 1995, 95, 1443-1477.
(17) (a) Aylett, B. J.; Sullivan, A. C. In ComprehensiVe Organometallic
Chemistry II; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon
Press: Oxford, U.K., 1995; Vol. 2, pp 45-75. (b) Hermanns, J.; Schmidt,
B. J. Chem. Soc., Perkin Trans. 1 1998, 2209-2230. (c) Hermanns, J.;
Schmidt, B. J. Chem. Soc., Perkin Trans. 1 1999, 81-102.
1
(
toward electrophiles is much lower than that of allylsilanes. See: (a) Colvin,
E. W. Silicon in Organic Synthesis; Butterworth: London, 1981. (b) Weber,
W. P. Silicon Reagents for Organic Chemistry; Springer-Verlag: Berlin,
1
5
983. (c) Fleming, I.; Dunogu e` s, J.; Smithers, R. Org. React. 1989, 37,
7-575.
(
14) During our investigation of the present work, Murai and co-workers
reported one example of PtCl2-catalyzed intramolecular arylsilylation of
unactivated alkynes, though both of stereoselectivity and chemical yield
were low, see: Chatani, N.; Inoue, H.; Ikeda, T.; Murai, S. J. Org. Chem.
2
000, 65, 4913-4918.

Lewis Acid Catalyzed Carbosilylation
J. Am. Chem. Soc., Vol. 123, No. 44, 2001 10901
vinylsilane compounds is known,13 and it clearly explains why
the cyclization of 3a did not proceed at all.
triple bond of 8 to a Lewis acid would form π-complex 11.
The carbon at the ipso position of arylsilane moiety would attack
the electron deficient triple bond from the side opposite to the
Lewis acid Via an exo-fashion to produce the ate complex 12
2.1. Exo-Mode Arylsilylation of Alkynes (Type 1). We next
turned our attention to the intramolecular arylsilylation of
unactivated alkynes and found that the Lewis acid catalyzed
reaction of the carbon tethered alkynylarylsilanes 8 gave cyclic
1
9
stereoselectively. The transfer of trimethylsilyl group to the
olefinic part would afford 9 and regenerate the Lewis acid.
2.2. Arylsilylation of Silicon-Tethered Alkynes (Type 2).
The highly stereoselective trans-arylsilylation reactions were
observed again when the cyclization reactions of differently
substituted arylsilanes 13 were examined (eq 9). The corre-
sponding five-membered silacycle product 14a was obtained
in 71% yield when the starting material 13a was treated with
0.5 equiv of HfCl4. No regio- and stereoisomers of 14a were
obtained. The p-tolyl-substituted product 14b was also obtained
in 57% yield. Even the reaction of 13c, bearing a bulky o-tolyl
group as Ar, proceeded to give the corresponding product 14c
though the chemical yield was low (20% yield). While the six-
membered silacycle compounds 14d was obtained regioselec-
tively in the reaction of 13d, the chemical yield was low.2
(E)-vinylsilanes 9 in moderate to good yields (eq 8). The reaction
of 8a, having a tether chain of three methylene groups (n ) 1),
in the presence of 0.1 equiv of HfCl4 in CH2Cl2 gave the trans-
arylsilylation product 9a regio- and stereoselectively in 48%
yield accompanied with a trace amount of 10 and the desilylated
starting material. We could not detect the stereo- (cis-addition
product) and regioisomer (endo-cyclization product) of 9a.
Besides CH2Cl2, toluene was a solvent of choice for the present
arylsilylation. Optimization experiments revealed that the use
of 0.2 equiv of HfCl4 in toluene gave the best result, and 9a
was obtained in 74% yield. Other Lewis acids such as ZrCl4
and AlCl3 gave unsatisfactory results. The seven-membered
product 9b was obtained in 45% yield in the reaction of 8b,
having a tether chain of four methylene groups (n ) 2), with
0,21
0
.2 equiv of EtAlCl2 in CH2Cl2. The stereochemistries of the
olefinic parts of 9a,b were determined unambiguously by NOE
experiments; irradiations of an alkenyl proton attached to the
carbon alpha to trimethylsilyl group of 9a or 9b enhanced the
signal of the benzene proton which was proximal to the alkenyl
group (NOE: 14.7% in 9a; 6.1% in 9b), whereas no enhance-
ments of any signals of methylene protons on the carbocycle
1
8
were observed.
A plausible mechanism of these reactions is shown in Scheme
3
. After the formation of π-complex 15 from the starting
material 13 with Lewis acid, the ipso carbon of arylsilane of
5 would attack the internal acetylenic carbon to give an ate
1
1
9
complex 16. The cleavage of the carbon-silicon bond
followed by the cyclization through the transfer of silyl group
to olefinic carbon would give the cyclization product 14. The
decreased yield in the reaction of 13c would be explained by
the steric interaction of bulky o-tolyl group and alkyne moiety
in the step from 15 to 16.
Scheme 3
A plausible mechanism for the Lewis acid catalyzed trans-
arylsilylation is shown in Scheme 2. The coordination of the
Scheme 2
Conclusion
We are now in a position to efficiently prepare various types
of silylated carbocycles and silacycles regio- and stereoselec-
(
19) Armitage, D. A. In ComprehensiVe Organometallic Chemistry;
Wilkinson, S. G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, U.K., 1982; Vol. 2, pp 47-51.
(20) The starting materials decomposed easily under the reaction
conditions, and the products other than 14c,d were not obtained.
(21) The palladium-catalyzed cycloaddition of silacyclobutane with
acetylene was carried out to give 14d in low yield, see: Sakurai, H.; Imai,
T. Chem. Lett. 1975, 891-894.
(
18) While the preparation of 9a by the titanium-mediated carbonyl
olefinations has been reported, the stereoselectivity was quite low (Z/E )
-1.5:1), see: Petasis, N. A.; Akritopoulou, I. Synlett 1992, 665-667.
1

10902 J. Am. Chem. Soc., Vol. 123, No. 44, 2001
Asao et al.
tively Via the Lewis acid catalyzed intramolecular vinylsilylation
and arylsilylation of unactivated alkynes. The present reaction
provides an easy access to a variety of five-, six-, and seven-
membered cyclic compounds, which are not easily available
Via the previously known methodologies and may be useful as
building blocks in organic synthesis. As observed in the Lewis
acid catalyzed hydro- and allylsilylation, here also the coordina-
tion of Lewis acids to alkynes followed by the nucleophilic
attack of the alkenyl and aryl groups to the electron-deficient
alkynyl carbon (or vinyl cation) is a key for the present un-
precedented transformation. Further studies to elucidate the
precise mechanism of this reaction and to extend the scope of
synthetic utility are in progress in our laboratory.
successively at 0 °C. The mixture was extracted with pentane three
2 4
times. The combined extracts were dried (Na SO ) and evaporated to
leave the crude product, which was purified by column chromatography
(basic silica gel, hexane eluent) to give 4b (0.46 mmol) in 92% yield.
Lewis Acid Catalyzed Intramolecular Arylsilylation. The prepara-
tion of 9a is representative. To a suspension of HfCl (64 mg, 0.2 mmol)
4
in toluene (6 mL) was added 8a (0.22 mL, 1.0 mmol) at -78 °C. The
reaction mixture was allowed to warm to 10 °C over a period of 1 h.
After the mixture was stirred for 5 min at 10 °C, excess amounts of
diethylamine (0.6 mL) and saturated aqueous solution of NaHCO
added successively at 0 °C. The mixture was extracted with ether. The
combined ether extracts were dried (Na SO ) and evaporated to leave
3
were
2
4
the crude product, which was purified by column chromatography (basic
silica gel, hexane eluent) to give 9a (0.74 mmol) in 74% yield.
Experimental Section
Supporting Information Available: A table listing opti-
mization study data on the reactions of 8a,b and text and figures
providing identification data for all products including copies
of the NMR spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
Lewis Acid Catalyzed Intramolecular Vinylsilylation. The prepa-
ration of 4b is representative. To a solution of 3b (0.5 mmol) in
CH Cl (5 mL) was added EtAlCl (0.1 mmol, 1 M in hexane) at 0 °C.
2 2 2
The mixture was allowed to warm to room temperature gradually. After
the mixture was stirred for 3 h, excess amounts of diethylamine (0.5
mL) and a saturated aqueous solution of NaHCO
3
were added
JA011316P