Pd-catalyzed cross-coupling of alkynylsilanols with iodobenzenes

Article abstract of DOI:10.1016/S0040-4039(01)00811-5

Alkynylsilanols were efficiently coupled with iodobenzene derivatives by a Pd catalyst in the presence of TBAF, and the corresponding substituted alkynes could be obtained in good to excellent yields.

Full text of DOI:10.1016/S0040-4039(01)00811-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4833–4835
Pd-catalyzed cross-coupling of alkynylsilanols with iodobenzenes
Sukbok Chang,a,* Soon Ha Yanga and Phil Ho Leeb
aDepartment of Chemistry, Ewha Womans University, Seoul 120-750, South Korea
bDepartment of Chemistry, Kangwon National University, Chuncheon 200-701, South Korea
Received 23 March 2001; revised 23 April 2001; accepted 11 May 2001
Abstract—Alkynylsilanols were efficiently coupled with iodobenzene derivatives by a Pd catalyst in the presence of TBAF, and the
corresponding substituted alkynes could be obtained in good to excellent yields. © 2001 Elsevier Science Ltd. All rights reserved.
Alkynes are one of the most important structural build-
ing blocks in organic synthesis and material science1
and numerous methods are now available for the
preparation of alkyne derivatives; among those, the
Pd-catalyzed acetylenic coupling reaction (Sonogashira
protocol) is most notable.2 Despite recent progress,3
new synthetic routes for the formation of substituted
alkynes are still necessary to achieve milder reaction
conditions and wider substrate scope. Since the pioneer-
ing works initiated by Hiyama4 and Denmark5 the
Pd-catalyzed coupling reactions between alkynylsilanols
or arylsilanols and aryl- or alkenylhalides (or their
equivalents) have been emerging as a powerful new
procedure for the formation of CꢀC bonds. These
coupling reactions are especially notable because the
silicon substrates are non-toxic and reaction conditions
are usually mild.6 During the course of our recent
studies on the practical synthesis of organosilanols,7 we
found for the first time that alkynylsilanols can serve as
facile coupling partners to react with various iodoben-
zene derivatives, which will be described in this
letter.8
With the use of iodobenzene and 1.0 equiv. of
dimethyl(phenylethynyl)silanol as a standard set of cou-
pling partners, various additives were examined for the
coupling reaction (Table 1).
Among the various palladium catalysts tested,
Pd(PPh3)4 was most effective for the coupling reaction
in THF at 60°C. In accord with the previous reports,4,5
tetrabutylammonium fluoride (TBAF) and silver oxide
turned out to be the best activators for the cross-cou-
pling although silver oxide afforded a slightly lower
conversion compared to TBAF (compare entries 4 and
5). Increasing the equivalents of TBAF did not exhibit
Table 1. Effects of various additives on the Pd-catalyzed coupling between dimethyl(phenylethynyl)silanol and iodobenzenea
solvent
Ph I + Ph
SiMe2OH
(1.0 equiv)
+
+
additive
Pd(PPh3)4
(5 mol%)
Ph
Ph
Entry
Additive (equiv.)
Solvent
T (°C)
Time (h)
Conversion (%)b
1
2
3
4
5
6
7
8
None
K2CO3 (1.0)
CsF (1.0)
Ag2O (1.0)
TBAF (1.0)
TBAF (2.0)
TBAF (1.0)
TBAF (1.0)
THF
THF
THF
THF
THF
THF
60
60
60
60
60
60
60
40
12
12
12
2
2
2
B5
B5
B5
78
90
90
Benzene
THF
2
2
62
45
a All reactions were carried out in 0.1 M solutions.
b Conversion was calculated by 1H NMR integration.
* Corresponding author.
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00811-5

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S. Chang et al. / Tetrahedron Letters 42 (2001) 4833–4835
any enhanced effects on the reaction rates and conver-
sion (entry 6). THF was the solvent of choice and other
reaction media were less effective. Conversion of the
coupling was significantly decreased at lower tempera-
tures (entry 8). Other additives, such as K2CO3 or CsF,
were much less effective for the coupling reaction com-
pared to TBAF.
ditions and chemoselective coupling was possible with
3-bromoiodobenzene (entry 5). A silanol bearing steri-
cally bulky substituents on the silicon atom such as B
was less reactive and reaction of the silanol with
iodobenzene afforded the coupled product in moderate
yield (entry 7). Derivatization of the substituents on
alkyne10 had little effect on the reaction efficiency, and
coupling reactions of the aliphatic alkynylsilanols pro-
ceeded with similar rates and yields when compared to
aryl alkynylsilanols. For example, dimethyl(1-
hexynyl)silanol (C) was readily coupled with several
iodobenzene derivatives to generate alkynes bearing
alkyl- or aryl-substituents in high yields (entries 8–10).
In contrast, alkynylsilanols having a halogen group
such as D were less effective in the coupling reactions
and reaction using silanol D with iodobenzenes pro-
ceeded in moderate yields (entries 11 and 12). Conju-
gated enynylsilanol (E) was also competent for the
coupling reaction to afford substituted alkynes in good
yields (entries 13 and 14). Certain advantages of this
new cross-coupling over the related reactions using
Good to excellent yields could be obtained from the
Pd-catalyzed coupling reactions across a wide range of
alkynylsilanols and iodobenzene derivatives using the
established reaction conditions (Table 2).9
Dimethyl(phenylethynyl)silanol (A) reacted with vari-
ous iodobenzenes to afford the corresponding coupled
products in high yields. Electronic variation on the
iodobenzenes exhibited little effect on the reaction rates
and yields (entries 1–5). Reaction of A with 2-iodo-
naphthalene also proceeded with similar efficiency
(entry 6). The coupling reaction of alkynylsilanols with
bromobenzene was negligible under the employed con-
Table 2. Pd-catalyzed coupling of alkynylsilanols with iodobenzene derivativesa
THF
60 oC
Ar
I
+ R1
Si(R2)2OH
(1.5 equiv)
+
Ar
R1
Pd(PPh3)4
(5 mol%)
TBAF
+
(1.0 equiv)
entry
silanol
SiMe2OH
Iodobenzene
Ph
time (h)
yield (%)b
1
2
Ph
(A)
I
2.5
3
97
99
(A)
(A)
I
I
C6H4(p-Me)
C6H4(m-Me)
3
4
2.5
2
95
78
73
(A)
(A)
I
C6H4(p-COMe)
C6H4(m-Br)
I
5
6
2
3
I
(A)
71
52
95
I
C6H4(p-COMe)
(B)
7
8
Ph
Si(i-Pr)2OH
3
2
CH3(CH2)2
SiMe2OH (C)
I
Ph
(C)
(C)
I
I
C6H4(p-Me)
9
3
2
90
72
10
C6H4(p-COMe)
11
12
13
Cl(CH2)3
(D)
SiMe2OH
SiMe2OH
(D)
I
Ph
C6H4(p-Me)
Ph
2
2
2
66
I
62
70
(E)
I
(E)
I
C6H4(p-Me)
1
77
14
a
All reactions were carried out in 0.1 M solutions.
Isolated yields after silica-gel column chromatography.
b

S. Chang et al. / Tetrahedron Letters 42 (2001) 4833–4835
2000, 2, 2935.
4835
trimethylsilyl (TMS) acetylenes (Sila–Sonogashira cou-
pling) can be clearly found. For example, this method
does not require expensive tris(diethylamino)sulfonium
trimethyldifluorosilicate (TASF)8a,b or CuCl8c as an
additive. In addition, the presented procedure provides
coupled alkyne products with higher yields in shorter
reaction times when compared to those obtained for the
TMS variant, in general.
4. (a) Hatanaka, Y.; Hiyama, T. Synlett 1991, 845; (b)
Hirabayashi, K.; Mori, A.; Kawashima, J.; Suguro, M.;
Nishihara, Y.; Hiyama, T. J. Org. Chem. 2000, 65, 5342.
5. (a) Denmark, S. E.; Wehrli, D. Org. Lett. 2000, 2, 565;
(b) Denmark, S. E.; Neuville, L. Org. Lett. 2000, 2, 3221.
6. For a recent example on the use of silanols in the Heck
type reactions, see: Hirabayashi, K.; Ando, J.-i.;
Kawashima, J.; Nishihara, Y.; Mori, A.; Hiyama, T. Bull.
Chem. Soc. Jpn. 2000, 73, 1409.
In conclusion, we have demonstrated that various types
of alkynylsilanols could be efficiently coupled with
iodobenzenes with the use of a Pd/TBAF system to
afford the corresponding substituted alkyne derivatives
in good to excellent yields. High efficiency of the cou-
pling reaction combined with practical preparation of
the alkynylsilanols would make this procedure an
appealing alternative to the existing methods for the
generation of substituted alkynes.11
7. Lee, M.; Ko, S.; Chang, S. J. Am. Chem. Soc. 2000, 122,
12011.
8. There are precedent examples of Pd-catalyzed coupling
reactions of alkynyl(trimethyl)silanes: (a) Hatanaka, Y.;
Hiyama, T. J. Org. Chem. 1988, 53, 918; (b) Hatanaka,
Y.; Matsui, K.; Hiyama, T. Tetrahedron Lett. 1989, 30,
2403; (c) Nishihara, Y.; Ikegashira, K.; Mori, A.;
Hiyama, T. Chem. Lett. 1997, 1233.
9. Representative experimental procedure: To a stirred solu-
tion of iodobenzene (40.8 mg, 0.20 mmol) in THF (2.0
mL) was added dimethyl(phenylethynyl)silanol (55.4 mg,
0.3 mmol) followed by Pd(PPh3)4 (12.0 mg, 5 mol%) and
tetrabutylammonium fluoride (1.0 M in THF, 0.20 mL).
The reaction mixture was stirred at 60°C for 2.5 h. After
removal of the solvent under the reduced pressure, the
residue was purified by flash column chromatography on
silica gel (hexane) to afford diphenylacetylene in 94%
yield.
Acknowledgements
This work was supported by the Center for Molecular
Design and Synthesis (CMDS) at KAIST.
References
10. All alkynylsilanols used in this study were prepared in
almost quantitative yields according to the procedure
reported from this laboratory (Ref. 7).
1. (a) Patai, S. The Chemistry of the CarbonꢀCarbon Triple
Bond; Wiley: New York, 1978; (b) Schwab, P. F. H.;
Levin, M. D.; Michl, J. Chem. Rev. 1999, 99, 1863.
11. Although a recent report by the Mori group (Ref. 3)
showed that the coupling could be carried out without
Cu-cocatalyst or amines, the reaction times were gener-
ally longer and the product yields were lower than those
of the present procedure.
2. (a) Brandsma, L. Preparative Acetylenic Chemistry;
Elsevier: New York, 1988; (b) Siemsen, P.; Livingston, R.
C.; Diederich, F. Angew. Chem., Int. Ed. 2000, 39, 2632.
3. For example, see: Mori, A.; Kawashima, J.; Shimada, T.;
Sugura, M.; Hirabayashi, K.; Nishihara, Y. Org. Lett.
.