Activator-free oxidative homocoupling of organosilanes catalysed by a palladium-DPPP complex

Article abstract of DOI:10.1039/b303852a

Oxidative homocoupling reaction of aryl- or alkynylsilanes was found to proceed without any added activators using a catalytic amount of a palladium-1,3-bis(diphenylphosphino)propane complex, affording the corresponding biaryls or 1,3-diynes in modest to excellent yields.

Full text of DOI:10.1039/b303852a

Activator-free oxidative homocoupling of organosilanes catalysed by a
palladium–DPPP complex†
Hiroto Yoshida,* Yasuhito Yamaryo, Joji Ohshita and Atsutaka Kunai*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima Univerisity,
Higashi-Hiroshima 739-8527, Japan. E-mail: yhiroto@hiroshima-u.ac.jp, akunai@hiroshima-u.ac.jp;
Fax: +81-824-24-5494
Received (in Cambridge, UK) 7th April 2003, Accepted 2nd May 2003
First published as an Advance Article on the web 21st May 2003
Oxidative homocoupling reaction of aryl- or alkynylsilanes
was found to proceed without any added activators using a
catalytic amount of a palladium–1,3-bis(diphenylphosphi-
no)propane complex, affording the corresponding biaryls or
1,3-diynes in modest to excellent yields.
(PPh3) complex or a ligandless palladium salt in lieu of the Pd–
DPPP complex reduced the yield considerably (Entries 5 and 6).
When the homocoupling reaction was carried out under a
strictly prepared argon atmosphere, 2a was scarcely generated
(Entry 7). In contrast to 1a, trimethyl(phenylethynyl)silane gave
2a only in 18% yield, which implies that introduction of an
electron-withdrawing group (methoxy group) on a silicon atom
is necessary for the smooth reaction (Entry 8).
Palladium-catalysed carbon–carbon bond forming reactions
using organosilanes, such as cross-coupling reaction (Hiyama
coupling),1 have recently received much attention as a promis-
ing alternative to those using organoboranes or organostannanes
owing to their higher chemoselectivities and widespread
availability. However, because of the weakly polarized charac-
ter of a carbon–silicon bond of organosilanes, addition of an
Under the optimized conditions, the homocoupling of various
organosilanes 1 was next examined (Scheme 1). The results are
summarized in Table 2. Similarly to the case of 1a, the
homocoupling of (1-naphthyl)- (1b) or (1-cyclohexenyl)ethy-
nylsilane (1c) readily proceeded to provide the corresponding
1,3-butadiyne (2b or 2c) in moderate yield (Entries 1–3). It
should be noted that the homocoupling was also applicable to an
arylsilane by employing an organosilanol instead of an
organodialkoxysilane as a silicon reagent. In this case, addition
of water improved the product yields significantly.7 Thus, when
dimethyl(phenyl)silanol (1d) was treated in DMSO–H2O, the
homocoupling product, biphenyl (2d), was produced in 70%
yield (Entry 4). Similarly, arylsilanols bearing an electron-
donating group at the para-position (1e–1h) also furnished high
yields of biaryl 2e–2h (Entries 5–8), whereas the reaction of an
electron-deficient arylsilanol (1i) resulted in low yield (Entry
9). The reaction of other silanols bearing a meta-substituted
phenyl (1j–1l), 3,4-(methylenedioxy)phenyl (1m) or 2-naphthyl
group (1n) also took place smoothly, providing the respective
homocoupling products (2j–2n) in high yields (Entries 10–14).
On the other hand, ortho-substituted arylsilanols (1o and 1p) or
a heteroarylsilanol (1q) reacted sluggishly, leading to the
formation of the products (2o–2q) in low to moderate yields
(Entries 15–17). Worthy of note is that the present reaction
conditions allow the employment of a fluoride ion-sensitive
arylsilanol (1r or 1s), whose C–SiMe3 bond remained intact
after the reaction (Entries 18 and 19).
activator such as a fluoride ion,1 a strong base,2a–c a copper(
salt2d or a silver( ) salt2e is often indispensable for the reactions
I)
I
to proceed, and therefore, examples of the successful reactions
free from the added activator are to the best of our knowledge
totally limited to the cross-coupling with allyl carbonates3a or
diene monoxides,3b and the Mizoroki–Heck type reaction with
alkenes.4 Herein we report the palladium–1,3-bis(diphenyl-
phosphino)propane (DPPP) complex-catalysed oxidative hom-
ocoupling5,6 as a novel entry for the activator-free carbon–
carbon bond forming reaction using organosilanes.
First we investigated the reaction of dimethoxy(methyl)(phe-
nylethynyl)silane (1a) in dimethyl sulfoxide (DMSO) at 50 °C
for 12 h without any added activators using a palladium–DPPP
complex under an oxygen atmosphere, and observed that the
homocoupling product, 1,4-diphenylbutadiyne (2a), was pro-
duced in 70% yield (Table 1, Entry 1). The reaction proceeded
as well in DMF to give a 35% yield of 2a (Entry 2), whereas no
trace of 2a was detected in THF or toluene at all (Entries 3 and
4), indicating that a polar solvent plays an important role in this
homocoupling. The use of a palladium–triphenylphosphine
Table 1 Palladium-catalysed homocoupling of (phenylethynyl)silanesa
Entry
Ligand
Solvent
Yield (%)b
1
2
3
DPPP
DPPP
DPPP
DPPP
PPh3
—
DMSO
DMF
THF
toluene
DMSO
DMSO
DMSO
DMSO
70
35
0
0
11
6
4
5c
6
7d
8e
DPPP
DPPP
11
18
a The reaction was carried out in a solvent (1.3 mL) at 50 °C using 1a (0.27
mmol) for 12 h in the presence of Pd(OAc)2 (0.013 mmol) and DPPP (0.020
mmol) under an oxygen atmosphere. b GC yield. c PPh3/Pd = 3. d Under an
argon atmosphere. e Trimethyl(phenylethynyl)silane was used instead of
1a.
† Electronic supplementary information (ESI) available: experimental
procedures and characterization data. See http://www.rsc.org/suppdata/cc/
b3/b303852a/
Scheme 1
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CHEM. COMMUN., 2003, 1510–1511
This journal is © The Royal Society of Chemistry 2003

ane (8a) was detected by GC-MS spectrum of the reaction
mixture along with 10% yield of 2a (eqn. 1). The smooth
homocoupling observed only in a polar solvent can be rationally
explained by assuming that the solvent would coordinate to a
Lewis acidic silicon atom of 1, induced by an electron-
withdrawing oxygen atom, to form a pentacoordinate species,
which may be subject to the transmetalation (Cycle A)11 or the
oxidative addition (Cycle B).
In conclusion, we have developed the palladium–DPPP
complex-catalysed activator-free oxidative homocoupling reac-
tion of organosilanes. Further studies on details of the
mechanism as well as application of the present catalytic system
to other activator-free carbon–carbon bond forming reactions of
organosilanes are in progress.
(1)
Scheme 2 depicts two plausible catalytic cycles of the
homocoupling reaction. Cycle A includes the formation of
palladium(II) peroxide complex 4 resulting from the reaction of
palladium(0) complex 3 with molecular oxygen as reported
previously.8 Subsequently, 2 mol of organic moieties in 1 would
transmetalate to a palladium(II) complex to provide dio-
rganopalladium complex 6 via complex 5,9 followed by
reductive elimination of homocoupling product 2 with regen-
erating 3. Alternatively, complex 5 is formed through insertion
of oxygen into a Pd–Si bond of complex 9, which is generated
by oxidative addition of 1 to 3 (Cycle B).10 At present, no
definitive evidence is available that determines the reaction
pathway. The ultimate fate of a silyl moiety has been identified
to be disiloxane 8 through intermediary silyl peroxide 7, as
verified by the homocoupling of dimethyl(phenyl)(phenylethy-
nyl)silane (1t), where 1,1,3,3-tetramethyl-1,3-diphenyldisilox-
Notes and references
1 Reviews: (a) T. Hiyama, in Metal-Catalyzed Cross-Coupling Reactions,
ed. F. Diederich and P. J. Stang, Wiley-VCH, Weinheim, 1998, chap.
10; (b) T. Hiyama and Y. Hatanaka, Pure Appl. Chem., 1994, 66, 1471;
(c) Y. Hatanaka and T. Hiyama, Synlett, 1991, 845.
2 (a) K. Gouda, E. Hagiwara, Y. Hatanaka and T. Hiyama, J. Org. Chem.,
1996, 61, 7233; (b) E. Hagiwara, K. Gouda, Y. Hatanaka and T.
Hiyama, Tetrahedron Lett., 1997, 38, 439; (c) S. E. Denmark and R. F.
Sweis, J. Am. Chem. Soc., 2001, 123, 6439; (d) Y. Nishihara, K.
Ikegashira, K. Hirabayashi, J.-i. Ando, A. Mori and T. Hiyama, J. Org.
Chem., 2000, 65, 1780; (e) K. Hirabayashi, A. Mori, J. Kawashima, M.
Suguro, Y. Nishihara and T. Hiyama, J. Org. Chem., 2000, 65, 5342.
3 In these cases, an alkoxide ion, generated in situ from the allylic
electrophiles, acts as the activator. (a) H. Matsuhashi, Y. Hatanaka, M.
Kuroboshi and T. Hiyama, Tetrahedron Lett., 1995, 36, 1539; (b) H.
Matsuhashi, S. Asai, K. Hirabayashi, Y. Hatanaka, A. Mori and T.
Hiyama, Bull. Chem. Soc. Jpn., 1997, 70, 1943.
4 Although this reaction proceeds smoothly without an aid of an activator
in the presence of a stoichiometric amount of Pd(OAc)2, addition of
Cu(OAc)2 and LiOAc is essential for the successful catalytic reaction:
(a) K. Hirabayashi, J.-i. Ando, J. Kawashima, Y. Nishihara, A. Mori and
T. Hiyama, Bull. Chem. Soc. Jpn., 2000, 73, 1409; (b) K. Hirabayashi,
Y. Nishihara, A. Mori and T. Hiyama, Tetrahedron Lett., 1998, 39,
7893.
5 Homocoupling of organosilanes using a stoichiometric amount of CuCl:
(a) J. Yoshida, K. Tamao, T. Kakui and M. Kumada, Tetrahedron Lett.,
1979, 1141; (b) K. Ikegashira, Y. Nishihara, K. Hirabayashi, A. Mori
and T. Hiyama, Chem. Commun., 1997, 1039; (c) Y. Nishihara, K.
Ikegashira, F. Toriyama, A. Mori and T. Hiyama, Bull. Chem. Soc. Jpn.,
2000, 73, 985; (d) Using CuI catalyst and a fluoride ion: S.-K. Kang, T.-
H. Kim and S.-J. Pyun, J. Chem. Soc., Perkin Trans. 1, 1997, 797; (e)
Using PdCl2(PEt3)2/CuI catalyst and a fluoride ion: S. Yamaguchi, S.
Ohno and K. Tamao, Synlett, 1997, 1199; (f) Using PdCl2 catalyst and
CuCl2/LiCl: F. Babudri, A. R. Cicciomessere, G. M. Farinola, V.
Fiandanese, G. Marchese, R. Musio, F. Naso and O. Sciacovelli, J. Org.
Chem., 1997, 62, 3291.
Table 2 Palladium–DPPP-catalysed homocoupling of organosilanesa
Entry
Organosilane
Time (h)
Yield (%)b
Product
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
1a
1b
1c
1d
1e
1f
1g
1h
1i
1j
1k
1l
1m
1n
1o
1p
1q
1r
1s
12
12
12
24
12
24
24
24
24
18
24
24
30
24
24
24
48
12
24
69
57
50
70
94
90
86
70
11
80
77
78
85
86
30
55
33
87
80
2a
2b
2c
2d
2e
2f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
2q
2r
2s
a The reaction was carried out in DMSO at 70 °C (alkynylsilane) or DMSO/
H2O at 100 °C (arylsilane). See ESI† for details. b Isolated yield based on
1.
6 We have already reported the palladium–DPPP-catalysed base-free
oxidative homocoupling of arylboronic esters: H. Yoshida, Y. Yamaryo,
J. Ohshita and A. Kunai, Tetrahedron Lett., 2003, 44, 1541.
7 In the absence of water, the reaction of 1d gave only a 40% yield of 2d
along with disiloxane, (PhMe2Si)2O, that cannot participate in the
homocoupling. Thus we consider that the addition of water would
inhibit the dehydrative condensation of silanols into disiloxanes.
8 (a) J. E. Lyons, in Oxygen Complexes and Oxygen Activation by
Transition Metals, ed. A. E. Martell and D. T. Sawyer, Plenum Press,
New York, 1988, pp. 233–251; (b) S. S. Stahl, J. L. Thorman, R. C.
Nelson and M. A. Kozee, J. Am. Chem. Soc., 2001, 123, 7188; (c) B. A.
Steinhoff, S. R. Fix and S. S. Stahl, J. Am. Chem. Soc., 2002, 124,
766.
9 Transmetalation of an organosilanol to palladium(II) acetate without an
aid of an activator has been proposed as a key step in the Mizoroki–Heck
type reaction, see ref. 4.
10 A referee kindly suggested the possibility of this catalytic cycle.
11 The coordination of a polar solvent to a silicon atom was also considered
to be necessary for the smooth transmetalation of organosilanes to a
Scheme 2 Plausible catalytic cycles of the homocoupling.
copper(I) salt, see ref. 2d and 5c.
CHEM. COMMUN., 2003, 1510–1511
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