Rhodium-catalyzed anti selective cross-addition of bis(trimethylsilyl)- acetylene to diarylacetylenes via carbon-silicon bond cleavage

Article abstract of DOI:10.1021/ol8004003

The addition of bis(trimethylsilyl)acetylene to diarylacetylenes proceeds efficiently and selectively in a formal anti fashion in the presence of [Rh(OH)(cod)]₂/bisphosphine and phenol as catalyst and activator, respectively, accompanied by cleavage of one of the C-Si bonds to produce the corresponding (Z)-enynes. The products can further couple with the same or a different diarylacetylene molecule to give rise to (Z,Z)-1,2,5,6-tetraaryl-1,5- hexadien-3-ynes that show relatively strong solid-state fluorescence.

Full text of DOI:10.1021/ol8004003

ORGANIC
LETTERS
2008
Vol. 10, No. 9
1751-1754
Rhodium-Catalyzed Anti Selective
Cross-Addition of Bis(trimethylsilyl)-
acetylene to Diarylacetylenes via
Carbon-Silicon Bond Cleavage
Akinobu Horita, Hayato Tsurugi, Tetsuya Satoh, and Masahiro Miura*
Department of Applied Chemistry, Faculty of Engineering, Osaka UniVersity, Suita,
Osaka 565-0871, Japan
miura@chem.eng.osaka-u.ac.jp
Received February 21, 2008
ABSTRACT
The addition of bis(trimethylsilyl)acetylene to diarylacetylenes proceeds efficiently and selectively in a formal anti fashion in the presence of
[Rh(OH)(cod)]2/bisphosphine and phenol as catalyst and activator, respectively, accompanied by cleavage of one of the C-Si bonds to produce
the corresponding (Z)-enynes. The products can further couple with the same or a different diarylacetylene molecule to give rise to (Z,Z)-
1,2,5,6-tetraaryl-1,5-hexadien-3-ynes that show relatively strong solid-state fluorescence.
Alkyne coupling reactions such as the dimerization of
terminal alkynes via C(sp)-H bond cleavage are of genuine
synthetic utility in preparing π-conjugated enyne com-
pounds.1 However, the selective cross-dimerization of two
different alkynes is, in general, still difficult because the
formation of cross- and homocoupled regio- and stereoiso-
mers is possible. Thus, minute control of the reaction is a
major challenge. One of the rare, leading examples is the
palladium-catalyzed reaction of internal alkynes having an
electron-withdrawing group as acetylene acceptors.2a Such
reactions have been successfully achieved by using other
transition metals.2 Notably, some selective catalytic cross-
addition reactions of terminal silylacetylenes as acetylene
donors to terminal and internal alkynes have recently been
disclosed.3
Meanwhile, the selective cross-addition reactions other
than those involving C-H bond cleavage are known. Thus,
metal or ketone masked terminal alkynes involving stannnyl-
and boryl-acetylenes and propargyl alcohols have been
reported to react via C-M and C-C bond cleavages.4 We
recently reported that in the presence of a rhodium catalyst,
(3) (a) Ru: Katayama, H.; Yari, H.; Tanaka, M.; Ozawa, F. Chem.
Commun. 2005, 4336. (b) Rh: Katagiri, T.; Tsurugi, H.; Funayama, A.;
Satoh, T.; Miura, M. Chem. Lett. 2007, 36, 830. (c) Nishimura, T.; Guo,
X.-X.; Ohnishi, K.; Hayashi, T. AdV. Synth. Catal. 2007, 349, 2669. (d)
Pd: Tsukada, N.; Ninomiya, S.; Aoyama, Y.; Inoue, Y. Org. Lett. 2007, 9,
2919.
(1) (a) Trost, B. M. Angew. Chem., Int. Ed. 1995, 34, 259. (b) Tsuji, J.
Transition Metal Reagents and Catalysts; Wiley: Chichester, U. K., 2000.
(2) (a) Pd: Trost, B. M.; Sorum, M. T.; Chan, C.; Harms, A. E.; Ruhter,
G. J. Am. Chem. Soc. 1997, 119, 698. (b) Lucking, U.; Pfaltz, A. Synlett
2000, 1261. (c) Chen, L.; Li, C.-J. Tetrahedron Lett. 2004, 45, 2771. (d)
Ru: Yi, C. S.; Liu, N. Organometallics 1998, 17, 3158. (e) Rh: Weng, W.;
Guo, C.; C¸ elenligil-C¸ etin, R.; Foxman, B. M.; Ozerov, O. V. Chem.
Commun. 2006, 197. (f) Ito, J.; Kitase, M.; Nishiyama, H. Organometallics
2007, 26, 6412. (g) Ir: Hirabayashi, T.; Sakaguchi, S.; Ishii, Y. AdV. Synth.
Catal. 2005, 347, 872. (h) Ti Akita, M.; Yasuda, H.; Nakamura, A. Bull.
Chem. Soc. Jpn. 1984, 57, 480. (i) U: Wang, J.; Kapon, M.; Berthet, J. C.;
Ephritikhine, M.; Eisen, M. S. Inorg. Chim. Acta 2002, 334, 183.
(4) (a) C-Sn: Shirakawa, E.; Hiyama, T. Bull. Chem. Soc. Jpn. 2002,
75, 1435. (b) C-B: Suginome, M.; Shirakura, M.; Yamamoto, A. J. Am.
Chem. Soc. 2006, 128, 14438. (c) C-C Funayama, A.; Satoh, T.; Miura,
M. J. Am. Chem. Soc. 2005, 127, 15354. (d) Nishimura, T.; Katoh, T.;
Takatsu, K.; Shintani, R.; Hayashi, T. J. Am. Chem. Soc. 2007, 129, 14158.
(e) Shintani, R.; Takatsu, K.; Katoh, T.; Nishimura, T.; Hayashi, T. Angew.
Chem., Int. Ed. 2008, 47, 1447.
10.1021/ol8004003 CCC: $40.75
Published on Web 04/09/2008
2008 American Chemical Society

Table 1. Reaction of Diphenylacetylene (1a) with
Bis(trimethylsilyl)acetylene (2) to Produce Enyne 3a and
Dienyne 4a
Table 2. Reaction of Diarylacetylenes (1) with
Bis(trimethylsilyl)acetylene (2) to Produce Dienynes 4a
entry
ligand
time (h)
3a, % yieldb (E/Z)
4a, % yieldc
1
2
3
4
dppb
dCypb
dppf
2
2
2
2
2
2
4
4
4
4
67 (10:90)
76 (64)d (4:96)
77 (8:92)
56 (54:46)
73 (3:97)
23 (22:78)
trace
13
10
8
trace
19
15
0
0
0
0
dppp
5e
6f
7g
8h
9i
10j
dCypb
dCypb
dCypb
dCypb
dCypb
dCypb
0
14 (21:79)
2
a Reaction conditions: [1a]/[2]/[Rh]/[ligand]/[PhOH] ) 1:2:0.03:0.03:3
(in mmol), in o-xylene (4 mL) at 130 °C under N2. b GC yield based on the
amount of 1a used. c GC yield based on the half amount of 1a used.
d Isolated yield. e [2] ) 1.2 (mmol). f [PhOH] ) 1 (mmol). g H2O (3 mmol)
in place of PhOH. h Without PhOH. i [Rh(Cl)(cod)]2 in place of
[Rh(OH)(cod)]2. j [Ir(OH)(cod)]2 in place of [Rh(OH)(cod)]2.
the reaction of γ-arylated tert-propargyl alcohols with
bis(trimethylsilyl)acetylene selectively proceeds via cleavage
of the C(sp)-C(sp3) and C(sp)-Si bonds to afford the
corresponding 1-aryl-4-trimethylsilyl-(E)-enynes.5 The reac-
tion is one of the rare examples for alkyne-alkyne coupling
leading to enynes through C-Si bond cleavage, while the
desilylative coupling with aryl and vinyl halides has been
well studied.6 Thus, the enyne products can be readily
converted to π-conjugated fluorescent dihydrofuran deriva-
tives by desilylative Sonogashira coupling and base-promoted
cyclization. In the course of our further study of alkyne
coupling reactions, we have found that diarylacetylenes also
undergo rhodium-catalyzed cross-addition with the bis(si-
lyl)acetylene in a formal anti fashion via cleavage of the
C-Si bond by adding phenol as activator. This is in marked
contrast to the reaction with terminal silylacetylenes under
similar conditions, where syn addition predominates.3b,c The
present products can further couple with the same or a
different diarylacetylene molecule to give rise to (Z,Z)-
1,2,5,6-tetraaryl-1,5-hexadien-3-ynes that show relatively
strong solid-state fluorescence.
a Reaction conditions: [1]/[2]/[Rh]/[dCypb]/[ArOH] ) 1.2:0.5:0.025:
0.025:1.5 (in mmol), in refluxing o-xylene (4 mL) under N2. ArOH ) 3,4,5-
b
3
(MeO) C6H2OH. GC yield based on the amount of 2 used. Value in
c
d
parentheses is GC yield. Reaction with PhOH in place of ArOH. KF
e
(0.1 mmol) was added. KF was added after 2 h and then stirred for 2 h.
activator, respectively, in o-xylene at 130 °C for 2 h. The
expected cross-addition product 3a was formed in 67% yield
with an E/Z ratio of 10:90 together with a small amount of
dienyne 4a (entry 1 in Table 1). The formation of phenoxy-
trimethylsilane as byproduct was also detected by GC-MS.
The yield of 3a and its E/Z ratio were improved up to 76%
and 4:96 by using dCypb [Cy2P(CH2)4PCy2] as ligand in
place of dppb (entry 2). Other bidentate ligands involving
dppf and dppp were not superior to dCypb (entries 3 and 4).
Reducing the amount of 2a to 1.2 mmol little affected the
reaction (entry 5), while the reaction with a reduced amount
of phenol was sluggish (entry 6). It was also confirmed that
the reaction did not take place in the absence of phenol or
with use of water in place of phenol (entries 7 and 8). Related
In an initial attempt, diphenylacetylene (1a) (1 mmol) was
treated with bis(trimethylsilyl)acetylene (2) (2 mmol) in the
presence of [Rh(OH)(cod)]2 (0.015 mmol, 3 mol % Rh), dppb
(0.03 mmol), and phenol (3 mmol) as catalyst, ligand, and
(5) Horita, A.; Tsurugi, H.; Funayama, A.; Satoh, T.; Miura, M. Org.
Lett. 2007, 9, 2231.
(6) (a) Hiyama, T.; Shirakawa, E. Top. Curr. Chem. 2002, 219, 61. (b)
Tsuji, J. Palladium Reagents and Catalysts; Wiley: Chichester, U. K., 2004,
pp 201–229.
1752
Org. Lett., Vol. 10, No. 9, 2008

metal complexes, [Rh(Cl)(cod)]2 and [Ir(OH)(cod)]2, as
catalysts were not effective for this reaction (entries 9 and
10).
Scheme 2
We next examined the selective synthesis of the 2:1-
coupled dienyne 4a. The reaction of 1a (1.2 mmol) with 2
(0.5 mmol) using [Rh(OH)(cod)]2 and dCypb in refluxing
o-xylene for 24 h afforded 4a in 58% isolated yield (entry 1
in Table 2) It is noted that 4a had exclusively Z,Z-
configuration. The use of an electron-rich phenolic com-
pound, 3,4,5-(MeO)3C6H2OH, in place of phenol itself was
found to more effectively promote the reaction (entry 2).
The use of KF (20 mol %) together with 3,4,5-(MeO)3-
C6H2OH further accelerated the reaction, and it was com-
pleted within 6 h to give 4a in 69% yield (entry 3). Bis(4-
methoxyphenyl)acetylene (1b) and bis(2-thienyl)acetylene
(1d) were also reacted with 2 to give the corresponding
dienynes 4b and 4d with good yields (entries 4 and 7).
Addition of KF, after the first 1:1 coupling was completed
by heating for 2 h, was found to effectively promote the
second reaction (entries 5 and 8). In contrast, the reaction
of bis(4-chlorophenyl)acetylene (1c) was sluggish even using
KF. As depicted in Scheme 1, however, the sequential
reactions of 1b with 2 and the product (Z)-3b with 1c gave
crossed-dieneyne 5 with a good yield.
trimethylsilane gives silylalkynylrhodium B.8 The successive
insertion of 1 to the rhodium-carbon bond affords vinyl-
rhodium C. (Z)-Enyne 3 is formed by protonolysis after the
geometrical isomerization of C to D, probably via a
zwitterionic form.9 It should be noted that the reaction of
1a with 2 in the presence of D2O was found to give 3a
deuterated at the ortho position of the 2-phenyl group (63%)
as well as the expected 1-vinyl position (16%) (Scheme 3).
Scheme 1
Scheme 3
This suggests that arylrhodium E that is formed by the 1,4-
migration of rhodium from the vinyl position to the ortho
position participates in the termination step.10 The deuterium
distribution indicates that the termination takes place mainly
in E rather than D, which may enhance the anti selectivity.
It is also worth noting that in our previous work the rhodium-
catalyzed reaction of 1a with tert-butyldimethylsilylacetylene
was observed to occur selectively in a syn manner, where
the termination by protonolysis with acetylenic hydrogen is
The dienynes 4 and 5 that consist of a π-conjugated six-
carbon scaffold may exhibit fluorescent properties.7 Interest-
ingly, 4a-c and 5 were found to show relatively strong
fluorescence in their solid state in a range of 458-506 nm,
and are relatively more luminous than or comparable to
tris(8-hydroxyquinolino)aluminum (Alq3), which is a typical
green emitter, by a factor range of 3.7-1: 4a 2.2 (458 nm),
4b 3.7 (466 nm), 4c 1.2 (491 nm), 5 1.0 (506 nm) (see also
Figure S1 in the Supporting Information). The fluorescent
efficiency of 4b is especially remarkable. However, they
showed only weak fluorescence in solution.
(7) Only an isomer of 1,2,5,6-tetraphenyl-1,5-hexadien-3-yne is known.
It seems to be the same with 4a. Israelshvilli, S.; Edlitz-Pfeffermann, J.
J. Am. Chem. Soc. 1952, 74, 5780.
(8) (a) Fangou, K.; Lautens, M. Chem. ReV. 2003, 103, 169. (b) Mu¨ller,
J.; Tschampel, M.; Pickardt, J. J. Organomet. Chem. 1988, 355, 513. (c)
Yamane, M.; Uera, K.; Narasaka, K. Bull. Chem. Soc. Jpn. 2005, 78, 477.
(9) (a) Ojima, I.; Clos, N.; Donovan, R. J.; Ingallina, P. Organometallics
1990, 9, 3127. (b) Jun, C. H.; Crabtree, R. H. J. Organomet. Chem. 1993,
447, 177.
A plausible mechanism for the formation of enyne 3 is
illustrated in Scheme 2, in which neutral ligands are omitted.
At the initial stage, the reaction of phenol with a hydroxy-
rhodium(I) species gives phenoxyrhodium A and the trans-
metalation between A and 2 with liberation of phenoxy-
(10) (a) Oguma, K.; Miura, M.; Satoh, T.; Nomura, M. J. Am. Chem.
Soc. 2000, 122, 10464. (b) Hayashi, T.; Inoue, K.; Taniguchi, N.; Ogasawara,
M. J. Am. Chem. Soc. 2001, 123, 9918.
Org. Lett., Vol. 10, No. 9, 2008
1753

relatively fast. Thus, participation of desilylated monosily-
lacetylene in the present reaction is considered to be, if any,
only little. The electron-rich phenol in combination with KF
seems to effectively enhance the reaction step of 3 with the
corresponding aryloxyrhodium A in the second coupling.
was low. 1-Phenyl-2-trimethylsilylacetylene was far less
reactive and only a small amount of the coupled product
(less than 5%) was detected by GC-MS. These results
suggest that relatively bulky and electron-rich silylacetylenes
are suitable for the present reaction. The reaction of 4-octyne
as acetylene acceptor, however, did not take place. We also
examined the coupling of boryl(silyl)acetylene 10 with 1a.
In this case, 3a was obtained as the single major product.
This is attributable to higher reactivity of the boryl function.4b
In summary, we have developed a rhodium-catalyzed
stereoselective cross-addition of bis(trimethylsilyl)acetylene
to diarylacetylenes to selectively produce the corresponding
(Z)-enynes. This represents a rare example of alkyne-alkyne
coupling via C(sp)-Si bond cleavage. The absence of
terminal acetylene hydrogen is a crucial factor for the high
Z-selectivity. This reaction also enables us to construct a
stereodefined and highly fluorescent π-conjugated dienyne
system.
Scheme 4
Acknowledgment. This work was supported by a Grant-
in-Aid from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan. We thank Ms. Y. Miyaji
for the measurement of NMR spectra (Osaka University).
The formation of 4 and 5 indicates that other internal
silylacetylenes may react under the present conditions. It was
found that 3,3-dimethyl-1-trimethylsilyl-1-butyne (6) ef-
ficiently couples with 1a to afford enyne 7 in 89% yield with
an E/Z ratio of 3:97. The reaction of 1-trimethylsilyl-1-
hexyne (8) also gave the expected enyne 9, albeit the yield
Supporting Information Available: Standard experimen-
tal procedure and characterization data for new compounds.
This material is available free of charge via the Internet at
http://www.pubs.acs.org.
OL8004003
1754
Org. Lett., Vol. 10, No. 9, 2008