Generation of silylethynolates via C-Si bond cleavage of disilylketenes induced by t-BuOK

Article abstract of DOI:10.1055/s-2002-32975

Disilylketenes undergo selective mono-desilylation upon treatment of t-BuOK in the presence of HMPA. The resulting silylethynolates are convertible to other disilylketenes in good yields. The intermediary silylethynolate was analyzed by NMR and IR spectroscopy.

Full text of DOI:10.1055/s-2002-32975

LETTER
1329
Generation of Silylethynolates via C–Si Bond Cleavage of Disilylketenes
Induced by t-BuOK1
G
eneration of Silylethy
a
nolate
s
sato Ito,* Eiji Shirakawa, Hidemasa Takaya2
Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Yoshida, Sakyo-ku, Kyoto, 606-8501, Japan
E-mail: mito@o.cc.titech.ac.jp
Received 24 May 2002
O–
Si1
Si2
Abstract: Disilylketenes undergo selective mono-desilylation
upon treatment of t-BuOK in the presence of HMPA. The resulting
silylethynolates are convertible to other disilylketenes in good
yields. The intermediary silylethynolate was analyzed by NMR and
IR spectroscopy.
path (a)
1
Si
Si
Nu
a
b
c
d
e
f
g
h
i
j
Me3Si
Me3Si
Me3Si
Me3Si
Me3Si
Me3Si
t-BuMe2Si
i-Pr3Si
PhMe2Si
Ph2MeSi
Si
O
+
Si
•
–
Nu
Si
path (b)
O–
Key words: disilylketene, silylethynolate, potassium t-butoxide,
carbon-silicon bond cleavage, HMPA
Me3Si
Ph3Si
t-BuMe2Si t-BuMe2Si
t-BuMe2Si PhMe2Si
t-BuMe2Si Ph2MeSi
t-BuMe2Si
Ph3Si
O
Si1
SiNu
•
"silylethynolate"
Si2
Ph3Si
Ph3Si
k
1
Recently we reported that , -disilylenolate intermediates
derived from disilylketenes and organolithiums subse-
quently undergo Peterson-type elimination to give silyl-
acetylenes (Scheme 1).3
Scheme 2
lectivity was not noticeably changed in the case of MeLi
[1b, 32% and 2 (R = Me), 53% yield],9b the formation of
2 (R = t-Bu) was completely suppressed in the case of t-
BuLi, although the yield of 1b was modest (47%). Finally,
we found that the use of t-BuOK instead of organolithi-
ums improved the yield remarkably (1b, 84%) with excel-
lent chemoselectivity. These results clearly show that t-
BuOK prefers the silicon center rather than the ketene car-
bonyl of 1a in the initial step to form a potassium silyl-
ethynolate selectively.10 The synthesis of various
disilylketenes by means of t-BuOK is summarized in
Table 1.11 The silylethynolate derived from 1a and t-
BuOK upon treatment with organosilyl halides such as
i-Pr3SiCl, PhMe2SiCl, Ph2MeSiCl, and Ph3SiCl gave un-
symmetrical disilylketenes 1c–f in good yields (entry 2–
5). When unsymmetrical disilylketenes 1b, 1e, or 1f bear-
ing at least one Me3Si group were employed, the less
bulky Me3Si group was selectively substituted by other
R3Si group to afford the corresponding unsymmetrical
disilylketenes 1g–k in good yields (entry 6, 7, 9–11).
This finding suggests that disilylketenes may be syntheti-
cally equivalent to SiC C+ in organic synthesis.
O–
Peterson-type
elimination
O
RLi
Si
Si
•
Si
R
R
SiO–
Si
Si
Si = triorganosilyl
Scheme 1
In the course of our synthetic exploitation of disil-
ylketenes, we have now found that disilylketenes (1) un-
dergo nucleophilic desilylation with organolithium
reagents at the ketene sp2-carbon [Scheme 2, path (b)] as
well as nucleophilic addition to the ketene sp-carbon
[Scheme 2, path (a)]. The selective nucleophilic desilyl-
ation from 1 may provide a new method for the generation
of silylethynolate intermediates,4,5 which should be useful
for organic synthesis.
First, we examined the reaction of bis(trimethylsil- However, the reaction of unsymmetrical disilylketene 1d
yl)ketene (1a)6 with various nucleophilic reagents in the gave a mixture of 1h and 1b (47% and 32%, respectively)
presence of HMPA7,8 (Scheme 3). The reaction was car- upon treatment with t-BuMe2SiOTf, probably because
ried out by adding organolithiums to a solution of equimo- there is no remarkable difference in the steric congestion
lar amounts of 1a and HMPA in THF at 0 °C. After between two silicon groups in 1d (entry 8).
stirring for 1 h, the reaction mixture was quenched by t-
BuMe2SiOTf at 0 °C and then stirred at r.t. for 2 h. Then,
OSiMe2t-Bu
the product was isolated by an aqueous work-up and then
t-BuMe2SiOTf
Nu
Me3Si
+
1a
1b
R
purified by preparative HPLC. The use of PhLi as a nu-
cleophile resulted in the formation of 1b5a (39% yield) in
addition to 2 (R = Ph, 51% yield).9a While the chemose-
THF
HMPA
SiMe3
2
Nu: RLi (R = Ph, Me, t-Bu), or OKt-Bu
Scheme 3
Synlett 2002, No. 8, Print: 30 07 2002.
Art Id.1437-2096,E;2002,0,08,1329,1331,ftx,en;U00702ST.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0936-5214

1330
M. Ito et al.
LETTER
Table 1 Preparation of Various Disilylketenes using t-BuOKa
Table 2 Selected Spectroscopic Data for Ketenes and Alkynyl
Ethers
Entry
Substrate
1a
R3SiX
Product
1b
Yield (%)
compound
13C NMR
(CDCl3)
IR
Ref.
1
2
t-BuMe2SiOTf
i-Pr3SiCl
84
1a
1c5a
60
R(2-n)
C
C
CCO
C C ORn
C=C=O
(ppm) (ppm) (cm–1)
3
1a
PhMe2SiCl
Ph2MeSiCl
Ph3SiCl
1d12a
1e3,13a,b
1f13b
71
Ph2
201.2 47.6 2090 13f
182.6 13.0 2090 13e
4
1a
75
(n-C6H13)(Me3Si) C=C=O
(Me3Si)2 C=C=O
(Me3Si)(Me3Sn) C=C=O
5
1a
76
166.8
1.7 2050 13f
6
1b
t-BuMe2SiOTf
PhMe2SiCl
t-BuMe2SiOTf
t-BuMe2SiOTf
t-BuMe2SiOTf
Ph3SiCl
1g5b,13c
1h5b
75
164.4 –5.8 2080 8a,13f
7
1b
67
(Me3Sn)2
C=C=O
161.7 –13.9 2040– 8a,13f
2060
8
1d
1h (1b)
1i5b
47 (32)b
79
9
1e
t-BuMe2Si
t-BuMe2Si
t-Bu
C C–OSiMe2t-Bu –
C C–Ot-Bu 106.7 37.9 2180 13g
C C–OSiMe2t-Bu 85.5 40.3 2270 13h
C C–Ot-Bu 85.6 40.1 2280 13i
–
2185 13c
10
11
1f
1j5b
81
1f
1k12b
94
a Reaction conditions; substrate: t-BuOK:HMPA:R3SiX = 1:1:1:1,
[substrate] = 0.33 M in THF, 0 °C for 1 h.
n-Bu
b t-BuOSiMe2Ph13d was obtained as a by-product in 31% yield.
the precursors should be beneficial to the development
of the potential utility of silylethynolates in organic syn-
thesis.
The present reaction was not applicable to monosi-
lylketenes. For instance, the reaction of n-hexyl(trime-
thylsilyl)ketene13e with t-BuOK in the presence of HMPA
gave a complex mixture including t-butyl octanoate. This
may be attributable to the difference of thermodynamic
stability between alkynolates and silylethynolates. In oth-
er words, the interaction of the Si–C * orbital with the -
orbital in silylethynolates should stabilize their negative
charge,14 inducing better leaving ability than in the case of
alkynolates.
Acknowledgement
The author (M.I.) appreciates Research Fellowships of the Japan
Society for the Promotion of Science for Young Scientists and
thanks Professor Shigehiro Yamaguchi (Kyoto University) for va-
luable discussions.
References
(1) Presented in part at the 70th Annual Meeting of the
Chemical Society of Japan, Tokyo, March 1996, Abstr.
2J392.
(2) Deceased Oct. 4, 1995. Address all correspondence to Dr.
Masato Ito, Department of Applied Chemistry, Graduate
School of Science and Engineering, Tokyo Institute of
Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-
8552, Japan; E-mail: mito@o.cc.titech.ac.jp.
(3) Ito, M.; Shirakawa, E.; Takaya, H. Synlett 1996, 635.
(4) For reviews on the chemistry of ynolates, see: (a) Stang, P.
J.; Zhdankin, V. V. In The Chemistry of Triple-bonded
Functional Groups; Patai, S., Ed.; John Wiley & Sons:
Chichester, 1994, 1136–1145. (b) Shindo, M. Chem. Soc.
Rev. 1998, 27, 367.
(5) (a) Kai, H.; Iwamoto, K.; Chatani, N.; Murai, S. J. Am.
Chem. Soc. 1996, 118, 7634. (b) Akai, S.; Kitagaki, S.;
Naka, T.; Yamamoto, K.; Tsuzuki, Y.; Matsumoto, K.; Kita,
Y. J. Chem. Soc., Perkin Trans. 1 1996, 1705. (c) Iwamoto,
K.; Kojima, M.; Chatani, N.; Murai, S. J. Org. Chem. 2001,
66, 169.
To gain deeper insight into the structure of the silyl-
ethynolate, the mixture of 1a and t-BuOK in THF–HMPA
was concentrated in vacuo and the resulting red residue
was analyzed by spectroscopic methods. Its 13C NMR
spectrum in d8-THF showed two signals at 132.8 and 33.4
ppm, in addition to 38.1 and 5.2 ppm which are assignable
to HMPA (3JCP = 3.7 Hz) and Me3Si group, respectively.
Although the absence of any characteristic ketene carbon-
yl signal above 160 ppm may support its structure as ‘O-
ynolate, Me3SiC C–OK’, the observed signals do not fall
within the typical range for the acetylenic ethers (see
Table 2). Moreover, IR spectra (d8-THF) showed a strong
absorption at 2076 cm–1, which is typical for ketenes (see
Table 2). Therefore, the contribution from the ‘C-ynolate,
Me3Si(K)C=C=O’ structure cannot be ruled out. Current-
ly we are trying to get a suitable crystalline for this mate-
rial, to determine its structural property in the solid state
via single-crystal X-ray diffraction analysis.
(6) (a) Sakurai, H.; Shirahata, A.; Sasaki, K.; Hosomi, A.
Synthesis 1979, 740. (b) Efimova, I. V.; Kazankova, M. A.;
Lutsenko, I. F. Zh. Obshch. Khim. 1985, 55, 1647.
(7) Recently, Ishikawa and co-workers reported (Me3Si)3SiLi
undergoes nucleophilic attack onto the silicon center of 1a in
THF, but their attempts to trap the resulting silylethynolate
were unsuccesful: Naka, A.; Ohshita, J.; Kunai, A.; Lee, M.
E.; Ishikawa, M. J. Organomet. Chem. 1999, 574, 50.
In conclusion, we have developed the t-BuOK-induced
C–Si bond cleavage reaction of disilylketenes.15 Our
method may provide a general method for the generation
of silylethynolates efficiently from disilylketenes. Its op-
erational simplicity based on the remarkable stability of
Synlett 2002, No. 8, 1329–1331 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Generation of Silylethynolates via C–Si Bond Cleavage of Disilylketenes
1331
(8) (a) n-BuLi is known to react with 1a at the ketene sp-carbon
in the absence of HMPA: Ponomarev, S. V.; rman, M. B.;
Lebedev, S. A.; Pechurina, S. Y.; Lutsenko, I. F. Zh. Obsh.
Khim. 1971, 41, 127. (b) Also see: Woodbury, R. P.; Long,
N. R.; Rathke, M. W. J. Org. Chem. 1978, 43, 376. (c) For
the reaction of phosphorous ylides with 1a or 1b leading to
mono-silylated allenes, see: Kita, Y.; Tsuzuki, Y.; Kitagaki,
S.; Akai, S. Chem. Pharm. Bull. 1994, 42, 233.
(9) Analytical data for 2: (a) 1,1-Bis(trimethylsilyl)-2-[(1,1-
dimethylethyl)dimethylsilyl]oxy-2-phenylethene (2, R =
Ph). 1H NMR (CDCl3) : –0.31 (s, 9 H), –0.29 (s, 6 H), 0.25
(s, 9 H), 0.86 (s, 9 H), 7.18–7.29 (m, 5 H); 13C NMR (CDCl3)
: –3.1, 2.1, 2.4, 18.6, 26.3, 109.9, 127.8, 128.0, 129.6,
142.0, 165.9. Anal. Calcd for C20H38OSi3: C, 63.42; H,
10.11. Found: C, 63.31; H, 10.35. (b) 1,1-Bis(trimethyl-
silyl)-2-[(1,1-dimethylethyl)dimethylsilyl] oxy-1-propene
(2, R = Me). This compound was not obtained in pure form
and thus only 1H and 13C NMR spectral data were shown.
1H NMR (CDCl3) : 0.12 (s, 9 H), 0.14 (s, 9 H), 0.20 (s, 6 H),
0.95 (s, 9 H), 2.03 (s, 3 H); 13C NMR (CDCl3) : –2.3, 0.1,
2.7, 2.9, 19.1, 26.5, 107.1, 164.5.
conditions. See ref.13c. (c) The use of carbon electrophiles
(MeI, Me2SO4, Me3OBF4, or PhCHO) instead of R3SiX
resulted in the formation of intractable mixtures under
similar conditions.
(12) Analytical data for new disilylketenes (silicon-attached
quarternary carbon was not observed in 13C NMR spectra):
(a) (Dimethylphenylsilyl)(trimethylsilyl)ketene(1d). 1H
NMR (CDCl3) : 0.14 (s, 9 H), 0.52 (s, 6 H), 7.40–7.43 (m,
3 H), 7.59–7.63 (m, 2 H); 13C NMR (CDCl3) : –0.2, 1.2,
127.8, 129.4, 133.6, 138.3, 166.9; IR(neat) 2084 cm–1
(CCO); MS (70 eV) m/z 248 (M+); bp 62–70 °C (0.05
mmHg). Anal. Calcd for C13H20OSi2: C, 62.84; H, 8.11.
Found: C, 63.10; H, 8.28. (b) Bis(triphenylsilyl)ketene(1k).
1H NMR (CDCl3) : 7.21-7.49 (m, 30 H); 13C NMR (CDCl3)
: 127.7, 127.8, 129.7, 129.8, 133.7, 135.2, 135.4, 135.9,
166.2; IR(nujol) 2080 cm–1 (CCO); MS (70 eV) m/z 558
(M+); mp 165-166 °C. Anal. Calcd for C38H30OSi2: C, 81.67;
H, 5.41. Found: C, 81.40; H, 5.32.
(13) (a) Ponomarev, S. V.; Zolotareva, A. S.; Ezhov, R. N.;
Kuznetsov, Y. V.; Petrosyan, V. S. Russ. Chem. Bull. 2001,
50, 1093. (b) Ponomarev, S. V.; Zolotareva, A. S.; Leont’ev,
A. S.; Kuznetsov, Y. V.; Petrosyan, V. S. Russ. Chem. Bull.
2001, 50, 1088. (c) Groh, B. L.; Magrum, G. R.; Barton, T.
J. J. Am. Chem. Soc. 1987, 109, 7568. (d) Buncel, E.;
Edlund, T. K. V. U. J. Organomet. Chem. 1992, 437, 85.
(e) Pons, J.-M.; Kocienski, P. Tetrahedron Lett. 1989, 30,
1833. (f) Grishin, Y. K.; Ponomarev, S. V.; Lebedev, S. A.
Zh. Org. Khim. 1974, 10, 404. (g) Valent’, E.; Perics, M. A.;
Serratosa, F. J. Org. Chem. 1990, 55, 395. (h) Stang, P. J.;
Roberts, K. A. J. Am. Chem. Soc. 1986, 108, 7125.
(i) Pericàs, M. A.; Serratosa, F.; Valent’, E. Tetrahedron
1987, 43, 2311.
(10) Rathke’s report on the isolation of 1a by warming a THF
solution of lithio t-butyl bis(trimethylsilyl)acetate to 25 °C
may indicate that the eliminated t-BuOLi does not undergo
C–Si bond cleavage of 1a under their condition: Sullivan, D.
F.; Woodbury, R. P.; Rathke, M. W. J. Org. Chem. 1977, 42,
2038.
(11) (a) General Procedure for the Preparation of
Disilylketenes using t-BuOK: Preparation of [(1,1-
dimethylethyl)dimethylsilyl](trimethylsilyl)ketene(1b)
from 1a is representative. Disilyllketene 1a (296.5 mg, 1.59
mmol) was dissolved in THF (1.6 mL) and HMPA (0.28 mL,
1.61 mmol). To the resulting yellow solution was added a
solution of t-BuOK (180.0 mg, 1.60 mmol) in THF (3.2 mL)
at 0 °C. The mixture was stirred for 1 h at the same
temperature and quenched by t-BuMeSiOTf (0.37 mL, 1.61
mmol). After stirring the resulting solution for 2 h at room
temperature, the reaction mixture was diluted with pentane
and washed with water. Then the organic layer was dried by
Na2SO4 and filtered through Florisil. Evaporation of the
solvent followed by silica-gel chromatography (Wakogel C-
200) afforded 1b as a colorless oil (305.4 mg, 84% yield).
(b) We were unable to detect the formation of any
(14) Bassindale, A. R.; Glynn, S. J.; Taylor, P. G. In The
Chemistry of Organic Silicon Compounds, Part 1, Vol. 2;
Rappoport, Z.; Apeloig, Y., Eds.; John Wiley & Sons:
Chichester, 1998, Chap. 7.
(15) (a) Examples for a related alkali metal alkoxide-induced C–
Si bond cleavage reaction: Sakurai, H.; Nishiwaki, K.; Kira,
M. Tetrahedron Lett. 1973, 42, 4193; see also ref. 13d.
(b) We suppose that HMPA may coordinate onto the smaller
silicon center of the disilylketene to form a penta-coordinate
silicate, and facilitate the alkoxide-induced Si–C bond
cleavage.
O-silylated product (silyl silylethynyl ether) under these
Synlett 2002, No. 8, 1329–1331 ISSN 0936-5214 © Thieme Stuttgart · New York