A novel utilization of trifluoromethanide as a base: A convenient synthesis of trimethylsilylacetylene

Article abstract of DOI:10.1039/b101537k

Trimethysilylation of trimethylsilylacetylene was performed using trifluoromethanide as a base. Trifluoromethanide reacted with terminal acetylene and acetylenic selenides as a base or a nucleophile to give trimethylsilylacetylenes in high yields. Results indicated that the phenylseleno group effectively stabilized the intermediate vinyl anion that went through the cyclization reaction.

Full text of DOI:10.1039/b101537k

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A novel utilization of trifluoromethanide as a base: a convenient
synthesis of trimethylsilylacetylene
Mitsuhiro Yoshimatsu* and Masako Kuribayashi
1
Department of Chemistry, Faculty of Education, Gifu University, Yanagido 1-1, Gifu, 501-1193,
Japan. E-mail: yoshimae@cc.gifu-u.ac.jp; Fax: +81-058-293-2207
Received (in Cambridge, UK) 16th February 2001, Accepted 25th April 2001
First published as an Advance Article on the web 11th May 2001
Table 1 Trimethylsilylation of acetylenes with trifluoromethyltri-
Trifluoromethanide generated from CF3SiMe3–Bu4NF
reacted with terminal acetylenes and acetylenic selenides as
a base or a nucleophile to give trimethylsilylacetylenes in
high yields.
methylsilane–Bu4NF
Trifluoromethyltrimethylsilane (Ruppert’s reagent, CF3SiMe3)
has recently attracted attention as a source of the trifluoro-
methyl functional group, and has been widely used for the con-
versions of aldehydes and ketones to trifluoromethyl alcohols,1
of esters to trifluoromethyl ketones2 and of azirines to trifluoro-
methylaziridines.3 In particular, the reactions of acylsilanes
with CF3SiMe3 have produced the versatile intermediate, dif-
luoroenoxysilane, which is readily converted to difluoromethyl
ketones, alcohols and their derivatives.4 These effective trifluoro-
methylations are considered to be due to the strong nucleo-
philicity of the trifluoromethanide (CF3Ϫ) analog formed upon
initiation by the fluoride ion. However, the trifluoromethanide
liberated in these reaction systems is also a strong base (pKa of
HCF3 = 31) and this usually makes it difficult to control these
reactions, which give low yields of the products.5 We have
fortunately found that the reactions of acetylene with CF3-
SiMe3–Bu4NF effectively produce trimethylsilylacetylenes via
the corresponding acetylides. The trimethylsilyl group can be
conveniently used as a good protecting group of terminal acet-
ylenes.6 Furthermore, they can then be converted to other useful
compounds such as the trimethylsilylethynyl ketones,7 ethynyl
sulfones,8 vinyl silanes,9 and allenyl silanes.10 However, the
carbon–silyl bonds are easily cleaved upon usual work-up or
purification of the products. If convenient methods for the
trimethylsilylation of the various types of terminal acetylenes
could be found, they would be useful as a good protection
procedure in the syntheses of various complex targets. We
now report the novel trimethylsilylation of acetylenes using
CF3SiMe3–Bu4NF.
Results and discussion
First, we examined the reaction of propynal diethyl acetal and
CF3SiMe3. To a reaction mixture of CF3SiMe3 (3.00 mmol) and
the propynal acetal (1.00 mmol) was added a drop of Bu4NF
(1 M THF solution) at 0 ЊC. The vigorous formation of gases
quickly ceased and the reaction was complete in less than 1 min.
Trimethylsilylpropynal diethyl acetal 2a was obtained in 60%
yield. The reactions of various acetylenes were examined and
these results are shown in Table 1. Interestingly, acetylenic
selenide 1b also afforded the product in high yield; however, the
corresponding sulfur analog did not give the silylated acetylene
2a. The 1-ethynylcyclohexanols 1d–f provided almost the same
results as those of the propynal acetals. The hydroxy group of
alcohols 1g–i underwent trimethylsilylation to give 2g–i in high
yields. A large number of silylating agents are available for the
introduction of the trimethylsilyl group into various alcohols.
However, in general, it is difficult to incorporate the bulky
trimethylsilyl group in sterically hindered alcohols. Therefore,
this method would also be convenient for the easy silylation
of bulky tertiary alcohols. The 1,6-diyne 1j, -enyne 1k and 1,3-
diyne 1l also gave the silylated products 2j–l. The reactions of
the acetylenic ketone 1m, ester 1n and amide 1o resulted in low
yields or the formation of a complex mixture.
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J. Chem. Soc., Perkin Trans. 1, 2001, 1256–1257
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b101537k

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A plausible mechanism for the fluoride-catalyzed trimethyl-
silylation of acetylenes is given in Scheme 1. Trifluoromethanide
stabilizes the intermediate vinyl anion that undergoes the
cyclization reaction.
Acknowledgements
The support of part of this work by the Ministry of Education,
Science and Culture, Japan, is gratefully acknowledged.
Notes and references
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Flynn, Synlett, 1996, 57; S. Kaneko, N. Nakajima, T. Katoh and
S. Terashima, Chem. Pharm. Bull., 1997, 45, 43; asymmetric trifluoro-
methylation: K. Iseki, T. Nagai and Y. Kobayashi, Tetrahedron
Lett., 1994, 35, 3137; M. W. Walter, R. M. Adlington, J. E. Baldwin,
J. Chuhan and C. J. Schofield, Tetrahedron Lett., 1995, 36, 7761.
2 J. Wiedemann, T. Heiner, G. Mloston, G. K. S. Prakash and
G. A. Olah, Angew. Chem., Int. Ed., 1998, 37, 820; R. P. Singh,
G. Cao, R. L. Kirchmeier and J. M. Shreeve, J. Org. Chem., 1999,
64, 2873.
3 C. P. Felix, N. Khatimi and A. J. Laurent, Tetrahedron Lett., 1994,
35, 3303.
4 T. Brigaud, P. Doussot and C. Portella, J. Chem. Soc., Chem.
Commun., 1994, 2117; T. Brigaud, O. Lefebvre, R. Plantier-Royon
and C. Portella, Tetrahedron Lett., 1996, 37, 6115.
5 D. J. Adams, J. H. Clark, L. B. Hansen, V. C. Sanders and
S. J. Tavener, J. Chem. Soc., Perkin Trans. 1, 1998, 3081.
6 D. R. M. Walton, Protecting Groups in Organic Chemistry,
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3280.
Scheme 1
4, initially formed from the reaction of CF3SiMe3 3 with
Bu4NF, abstracts the proton of the terminal acetylene to give
trifluoromethane 5 and an acetylide 6. The reaction of the
acetylide 6 with CF3SiMe3 3 gives the trimethylsilylacetylene
7 and trifluoromethanide 4. The trifluoromethanide ion is
then involved in the trimethylsilylation catalytic cycle of
the acetylenes using CF3SiMe3. Treatment of the acetylenic
selenides with a strong base is well known to give the corre-
sponding acetylides, which were trapped with a variety of
electrophiles to afford the acetylenes.
The acetylenic selenides are easily converted to the trimethyl-
silylacetylenes under the given conditions, CF3SiMe3–Bu4NF,
and, therefore, we next examined the reactions of other
selenides and found that the reaction of the sulfonamide 8
with CF3SiMe3 involves cyclization to give the 2-(phenylseleno-
methylene)pyrrolidine 9 in 94% yield (Scheme 2).11 We also
8 L. Waykole and L. A. Paquette, Org. Synth., 1993, Coll. Vol. VII,
281.
9 T. K. Jones and S. E. Denmark, Org. Synth., 1990, Coll. Vol. VII,
524.
10 K. J. H. Kruithof, R. F. Schmitz and G. W. Klumpp, Tetrahedron,
1983, 39, 3073.
11 Compound 9: colorless prisms; mp 136–140 ЊC; IR ν(KBr)/cmϪ1
1340, 1160 (SO2); δH (400 MHz; CDCl3) 1.57–1.66 (2H, m, CH2),
2 08 (2H, t, J = 7 Hz, CH2), 2.43 (3H, s, Me), 3.58 (2H, t, J = 7 Hz,
CH2), 6.12 (1H, d, J = 1 Hz, olefinic H), 7.23–7.32 (5H, m, ArH),
7.51–7.57 (2H, m, ArH), 7.75 (2H, d, J = 8 Hz, ArH); δC (100 MHz;
CDCl3) 21.78 (q), 22.03 (t), 33.46 (t), 51.44 (t), 108.56 (d), 127.19 (d),
128.03 (d × 2), 129.17 (d × 2), 129.82 (d × 2), 132.36 (d × 2), 133.58
(s), 134.55 (s), 140.56 (s), 144.30 (s); MS m/z 393 (M+), 238
(M+ Ϫ SO2Tol). Anal. calcd for C18H19NO2SSe: C, 55.10; H, 4.88;
N, 3.57, found: C, 54.63; H, 4.80; N, 3.54%.
Scheme 2 Reagent: CF3SiMe3–Bu4NF–THF.
performed the reaction of 8 with Bu4NF in THF, but only the
acetylene 8 was recovered. In order to ascertain the generality
of this cyclization, we performed the reaction with the non-
seleno-substituted N-pent-4-ynylsulfonamide 10. However, this
reaction did not proceed and gave only the starting amide.12
These results show that the phenylseleno group effectively
12 P. A. Jacobi and J. Guo, Tetrahedron Lett., 1995, 36, 1845.
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