Et3B-induced hydrogermylation of alkenes and silyl enol ethers

Article abstract of DOI:10.1021/ol005977n

(Formula presented) Tri-2-furanylgermane in the presence of a catalytic amount of Et₃B adds to internal alkenes as well as terminal alkenes effectively to give the corresponding adducts in good to excellent yields. The addition of tri-2-furanylgermane to silyl enol ethers followed by elimination of germyl and siloxy moieties provides a new route for the conversion of ketones into alkenes.

Full text of DOI:10.1021/ol005977n

ORGANIC
LETTERS
2000
Vol. 2, No. 13
1911-1914
Et3B-Induced Hydrogermylation of
Alkenes and Silyl Enol Ethers
Shinobu Tanaka, Tomoaki Nakamura, Hideki Yorimitsu, Hiroshi Shinokubo, and
Koichiro Oshima*
Department of Material Chemistry, Graduate School of Engineering, Kyoto UniVersity,
Kyoto 606-8501, Japan
oshima@fm1.kuic.kyoto-u.ac.jp
Received April 21, 2000
ABSTRACT
Tri-2-furanylgermane in the presence of a catalytic amount of Et3B adds to internal alkenes as well as terminal alkenes effectively to give the
corresponding adducts in good to excellent yields. The addition of tri-2-furanylgermane to silyl enol ethers followed by elimination of germyl
and siloxy moieties provides a new route for the conversion of ketones into alkenes.
Hydrostannylation1 and hydrosilylation,2 as well as hydro-
germylation,3 of alkynes have been extensively studied and
widely used in organic synthesis. We have previously shown
that in the presence of Et3B, R3SnH, R3GeH, and R3SiH add
to carbon-carbon triple bonds via a radical process.4 Reports
of hydrostannylation and hydrosilylation of alkenes via a
radical mechanism are limited in organic synthesis. Hy-
drostannylation of alkenes via a radical intermediate5 often
gives low yields because the addition of stannyl radicals is
reversible and the equilibrium favors the starting materials.6
Radical-chain hydrosilylation of alkenes is also difficult
because the hydrogen-atom abstraction step is relatively slow
and competing telomerization of alkenes can be a problem.7
Here we describe that tri-2-furanylgermane8 adds to not only
terminal alkenes but also internal alkenes in the presence of
a catalytic amount of Et3B.
A mixture of tri-2-furanylgermane (0.27 g, 1.0 mmol) and
1-octene (0.22 g, 2.0 mmol) was placed in a 20 mL reaction
flask and flushed with argon.9 To this mixture was added a
hexane solution of triethylborane (1.0 M, 0.1 mL, 0.1 mmol)
at 25 °C. After stirring for 1 h at 25 °C, the reaction mixture
was subjected directly to silica gel column chromatography
to give 1-(tri-2-furanyl)germyloctane10 (1) in 97% yield
(Scheme 1).
(1) Davis, A. G. Organotin Chemistry; VCH Verlagsgesellschaft mbH:
Weinheim, 1997; Chapter 3. 4, p 37.
(2) (a) Hiyama, T.; Kusumoto, T. In ComprehensiVe Organic Synthesis:
Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 8,
Chapter 3. 12, p 763. (b) Ojima, I. In The Chemistry of Organosilicon
Compounds; Patai. S., Rappoport. Z., Eds.; Wiley: Chichester, 1989; Part
2, Chapter 25. (c) Corey, J. Y. In AdVances in Silicon Chemistry; Larson,
G. L., Ed.; JAI: Greenwich, 1991; Vol. 1, p 355. (d) Ojima, I.; Li, Z.;
Zhu, J. In The Chemistry of Organosilicon Compounds, Vol. 2; Rappoport,
Z., Apeloig, Y., Eds.; Wiley: Chichester, 1998; Part 2, Chapter 29.
(3) Selected references: (a) Piers, E.; Lemieux, R. J. Chem. Soc., Perkin
Trans 1 1995, 3. (b) Wada, F.; Abe, S.; Yonemaru, N.; Kikukawa, K.;
Matsuda, T. Bull. Chem. Soc. Jpn. 1991, 64, 1701. (c) Lukevics, E.;
Barabanov, D. I.; Ignatovich, L. M. Appl. Organomet. Chem. 1991, 5, 379.
(d) Corriu, R. J. P.; Moreau, J. J. E. J. Organomet. Chem. 1972, 40, 73. (e)
Bernardoni, S.; Lucarini, M.; Pedulli, G. F.; Valgimigli, L.; Gevorgyan,
V.; Chatgilialoglu, C. J. Org. Chem. 1997, 62, 8009.
Scheme 1
(4) (a) Nozaki, K.; Oshima, K.; Utimoto, K. J. Am. Chem. Soc. 1987,
109, 2547. (b) Nozaki, K.; Oshima, K.; Utimoto, K. Tetrahedron 1989, 45,
923. (c) Nozaki, K.; Ichinose, Y.; Wakamatsu, K.; Oshima, K.; Utimoto,
K. Bull. Chem. Soc. Jpn. 1990, 63, 2268. (d) Miura, K.; Oshima, K.;
Utimoto, K. Bull. Chem. Soc. Jpn. 1993, 66, 2356.
(5) Hydrostannylation of alkenes via radical process, see: (a) Miura,
K.; Saito, H.; Uchinokura, S.; Hosomi, A. Chem. Lett. 1999, 659. (b)
Hanessian, S.; Leger, R. J. Am. Chem. Soc. 1992, 114, 3115.
The results are summarized in Table 1. Several comments
are worth noting. (1) Tri-2-furanylgermane adds to not only
(6) Sommer, R.; Kuivila, H. G. J. Org. Chem. 1968, 33, 802.
10.1021/ol005977n CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/31/2000

of tri-2-furanylgermane with 2d provided the adduct 3d in
90% yield, the reaction of Ph3GeH, (n-C6H13)3GeH, n-Bu3-
SnH, or (Me3Si)3SiH with 2-methyl-2-butene gave the
corresponding adduct in miserable yield, or failed to give
the desired products (Scheme 2).12 (4) In the case of
Table 1. Addition of Tri-2-furanylgermane to Alkenesa
Scheme 2. Hydrometalation to Trisubstituted Alkene
1-methylcyclohexene (2e), cis-1-(tri-2-furanyl)germyl-2-me-
thylcyclohexane (3e)13 was obtained as a single diastere-
omer.14 (5) Many functional groups are tolerated under the
reaction conditions. For instance, hydrogermylation pro-
ceeded in the presence of carbonyl groups such as ketones
and aldehydes (entries 8 and 9). The reduction of the carbonyl
groups to the corresponding alcohols was not observed. (6)
Even under the concentrated conditions, diallyl ether 2j
afforded tetrahydrofuran derivative 3j by the sequential
addition-cyclization process (entry 10). (7) The reversibility
of the addition reaction was ascertained by the following
experiment. Treatment of an excess amount of (Z)-6-
dodecene (2.0 mmol) with tri-2-furanylgermane (1.0 mmol)
gave the corresponding adduct in 90% yield. The recovered
6-dodecene was determined as a stereoisomeric mixture of
1
E and Z isomer (E/Z ) 84/16) by H NMR (Scheme 3).
The reason for the successful addition of tri-2-furanylger-
mane to alkenes is not clear at this stage. It is conceivable
that the addition of tri-2-furanylgermyl radical to alkenes is
less reversible than the analogous reactions of other trialkyl-
or triarylgermyl radicals. It is also possible that the hydrogen
atom transfer from tri-2-furanylgermane to the initial radical
adduct is more rapid than for other hydrogermanes.15
a
Tri-2-furanylgermane (1.0 mmol), alkene (2.0 mmol), and Et3B (0.1
mmol) were employed unless otherwise noted.
disubstituted olefins but also to tri- and tetrasubstituted
olefins very easily under quite mild conditions to give the
corresponding adducts in good to excellent yields. (2)
Although a small amount of hexane from the solution of
Et3B was present in the reaction mixture, no additional
solvent was necessary.11 (3) A unique feature of tri-2-
furanylgermane was demonstrated by the reaction with
2-methyl-2-butene (2d). Whereas the Et3B-induced reaction
(11) Various solvents were examined for the addition of tri-2-furanylger-
mane to 4-octene. In hexane (5 mL), tetrafuranylgermane was obtained in
23% yield as a byproduct in addition to the adduct 3a (60% yield).
Tetrafuranylgermane might be formed by the attack of germyl radical to
the carbon having a germyl group. The yields of 3a and tetrafuranylgermane
in other solvents were as follows: benzene, 52% and 20%; THF, 46% and
21%; EtOH, 55% and 20%; MeOH, 58% and 16%; H2O, 80% and 8%.
The best results were obtained without solvent.
(7) For hydrosilylation of alkenes via radical process, see: Kopping, B.;
Chatgilialoglu, C.; Zehnder, M.; Giese, B. J. Org. Chem. 1992, 57, 3994
and references therein.
(8) Tri-2-furanylgermane was easily prepared according to the literature.
Nakamura, T.; Yorimitsu, H.; Shinokubo, H.; Oshima, K. Synlett 1999, 1415.
(9) Ethyl radicals were formed from the reaction of Et3B with trace
amounts of O2 remaining in the reaction flask. Additional oxygen was not
necessary.
(12) The Et3B-mediated radical reaction of terminal olefin such as
1-octene with Ph3GeH, (Me3Si)3SiH, or n-Bu3SnH afforded the correspond-
ing adduct in 82%, 94%, or <20% yield, respectively.
(13) Compound data for 3e: 1H NMR (300 MHz, CDCl3) δ (ppm) 1.00
(d, J ) 6.9 Hz, 3H), 1.30-1.46 (m, 3H), 1.50-1.60 (m, 2H), 1.62-1.88
(m, 3H), 2.10-2.26 (m, 2H), 6.45 (dd, J ) 3.0 Hz, 1.5 Hz, 3H), 6.75 (d,
J ) 3.0 Hz, 3H), 7.73 (d, J ) 1.5 Hz, 3H); 13C NMR (75 MHz, CDCl3) δ
(ppm) 17.35, 21.79, 24.32, 27.30, 31.01, 33.78, 34.79, 109.74, 121.20,
147.19, 154.03; IR (neat, cm-1) 2920, 1549, 1453, 1361, 1205, 1149, 1099,
1063, 1003, 895, 884, 814, 740, 595. Anal. Calcd for C19H22GeO3: C, 61.52;
H, 5.98. Found: C, 61.77; H, 6.09.
(10) Compound data for 1: 1H NMR (300 MHz, CDCl3) δ (ppm) 0.88
(t, J ) 6.9 Hz, 3H), 1.18-1.40 (m, 10H), 1.50-1.65 (m, 4H), 6.46 (dd, J
) 3.3 Hz, 1.5 Hz, 3H), 6.75 (d, J ) 3.3 Hz, 3H), 7.73 (d, J ) 1.5 Hz, 3H);
13C NMR (75 MHz, CDCl3) δ (ppm) 13.97, 14.42, 22.53, 24.31, 28.95,
29.06, 31.74, 32.57, 109.75, 120.98, 147.33, 153.69; IR (neat, cm-1) 2920,
1550, 1458, 1361, 1206, 1150, 1100, 1003, 896, 884, 814, 741. Anal. Calcd
for C20H26GeO3: C, 62.07; H, 6.77. Found: C, 61.83; H, 6.77.
(14) The stereochemical outcome, which is tentative, might be explained
by the steric hindrance of tri-2-furanylgermyl group. Tri-2-furanylgermane
would approach the intermediary carbon radical from the opposite side of
germyl moiety.
1912
Org. Lett., Vol. 2, No. 13, 2000

major stereoisomer of 5a18 gave (Z)-6-tridecene selectively
in quantitative yield upon treatment with TMSOTf. On the
other hand, sequential treatment of the major stereoisomer
of 5a with K2CO3/methanol and then KH/18-Crown-6/
HMPA provided (E)-6-tridecene exclusively (Scheme 4). On
Scheme 3
Scheme 4
The addition of tri-2-furanylgermane to silyl enol ethers
also proceeds efficiently to afford â-siloxygermanes with
high regioselectivity. The regioisomer 6 could not be detected
in the reaction mixture. The results are shown in Table 2.
Table 2. Et3B-Induced Addition of Tri-2-furanylgermane to
Silyl Enol Ethersa
the basis of similar arguments to â-hydroxysilanes, these
results indicate that the major stereoisomer is the syn isomer.
The transformation of ketones to alkenes has received much
attention in organic synthesis and many methods have been
published.19 Because silyl enol ethers are readily available
from ketones, this new method will provide an alternative
route for the formation of alkenes starting from ketones
(Scheme 5).
a Tri-2-furanylgermane (1.0 mmol), silyl enol ether (2.0 mmol), and Et3B
(0.1 mmol) were employed unless otherwise noted. b E/Z ratio of 4a was
35/65.
Scheme 5
Trimethylsilyl enol ether 4b, generated from cyclohexanone,
gave cis-1-(trimethylsiloxy)-2-(tri-2-furanyl)germylcyclohex-
ane (5b) as a single stereoisomer.14 In the case of the tert-
butyldimethylsilyl ether 4e, the addition reaction was slug-
gish. The product was obtained in only 26% yield along with
recovered starting material 4e (69%) after stirring the reaction
mixture for 5 h at 25 °C.
Then, the conversion of â-siloxygermanes to alkenes was
examined. Stereospecific 1,2-elimination of â-hydroxysilanes
under acidic or basic conditions is well-known for the
synthesis of alkenes.16 However, there have been few reports
on the related â-hydroxy- or â-siloxygermanes.17 Treatment
of 5c with a catalytic amount of trimethylsilyl triflate
provided 1-dodecene in 92% yield. In a similar fashion, the
In summary, radical-mediated hydrogermylation of internal
alkenes can be achieved by using tri-2-furanylgermane. This
method can provide a new method for stereoselective
conversion of silyl enol ethers into alkenes. The procedure
for this reaction is extremely simple and avoids the use of
excess reaction solvent.
(15) For a mechanistic discussion of the addition of germyl radicals to
alkenes, see ref 3e.
(16) (a) Ager, D. J. Org. React. 1990, 38, 1. (b) Kelly, S. F. In
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon
Press: Oxford, 1991; Vol. 1, Chapter 3.1, p 729.
(17) Kawashima, T.; Iwama, N.; Tokitoh, N.; Okazaki, R. J. Org. Chem.
1994, 59, 491-493.
(18) The major stereoisomer of 5a can be separated by silica gel column
chromatography.
(19) Shapiro, R. H. Org. React. 1976, 23, 405.
Org. Lett., Vol. 2, No. 13, 2000
1913

Acknowledgment. This work was supported by Grant-
in Aid for Scientific Research (Nos. 09450341 and 10208208)
from the Ministry of Education, Science, Sports and Culture,
Government of Japan. Financial support by Banyu Pharma-
ceutical Co. Ltd. is also acknowledged. T.N. and H.Y. thank
the JSPS Research Fellowship for Young Scientists.
OL005977N
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Org. Lett., Vol. 2, No. 13, 2000