Palladium-catalysed cis- and trans-silaboration of terminal alkynes: Complementary access to stereo-defined trisubstituted alkenes

Article abstract of DOI:10.1039/b800181b

Palladium-catalysed cis- and trans-silaboration of terminal alkynes has been developed via the addition of (chlorodimethylsilyl)pinacolborane, followed by a one-pot conversion of the chloro group on the silicon atom to an isopropoxy group. The Royal Society of Chemistry.

Full text of DOI:10.1039/b800181b

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Palladium-catalysed cis- and trans-silaboration of terminal alkynes:
complementary access to stereo-defined trisubstituted alkenesw
Toshimichi Ohmura, Kazuyuki Oshima and Michinori Suginome*
Received (in Cambridge, UK) 8th January 2008, Accepted 1st February 2008
First published as an Advance Article on the web 18th February 2008
DOI: 10.1039/b800181b
Palladium-catalysed cis- and trans-silaboration of terminal
alkynes has been developed via the addition of (chlorodimethyl-
silyl)pinacolborane, followed by a one-pot conversion of the
chloro group on the silicon atom to an isopropoxy group.
synthesis. Herein, we describe the palladium-catalysed sila-
boration of terminal alkynes with 1. Depending on the
reaction conditions of the silaboration, either the cis- or
trans-silaboration products were obtained selectively through
a one-pot conversion into the alkoxysilane derivative.
(Chlorodimethylsilyl)pinacolborane (1) was reacted with
1-octyne (2a) in toluene at room temperature in the presence
of (Z3-C3H5)PdCl(PPh3)13 (1.0 mol%) (Scheme 1). After a
period of 1 h, isopropyl alcohol (1.5 equiv.) and pyridine were
added to the reaction mixture to convert the chloro group on
the silicon atom to an isopropoxy group, giving the chroma-
tographically stable 1-boryl-2-silyloct-1-ene 3a. When the
reaction was carried out with a small excess of alkyne
(1 : 2a = 1.0 : 1.2), the cis-addition product, (Z)-3a, was
isolated in a 91% yield as a single stereoisomer (Z : E =
499 : 1). In sharp contrast, when a small excess of silylborane
(1 : 2a = 1.2 : 1.0) was used under the same reaction
conditions, the trans-addition product, (E)-3a, was isolated
in an 85% yield (Z : E = 11 : 89) after treatment of the
reaction mixture with i-PrOH–pyridine.
There is increasing demand for the development of an efficient
way to synthesise stereodefined organometallic compounds for
stereoselective access to complex organic molecules. Vicinally
dimetallated alkenes are attractive intermediates for the synth-
esis of highly substituted alkenes, via C–C bond forming
reactions using appropriate carbon electrophiles (Fig. 1).1
Regio- and stereoselective synthesis of dimetallic alkenes has
been achieved via the transition metal-catalysed addition of
dimetallic compounds, consisting of silicon, boron, or tin, to
alkynes.2 These reactions generally proceed in a cis fashion to
give cis-1,2-bismetallated alkenes. In contrast, selective synth-
esis of the corresponding trans isomers has been only partly
achieved by hydrometallation of monometallated internal
alkynes, which often suffers from low regio- and stereoselec-
tivity.3–5 Complementary synthesis of cis- and trans-bismetal-
lated alkenes from common starting materials is highly
attractive for the preparation of desirable stereoisomers of
highly substituted alkenes.6
Fig. 1 Vicinally dimetallated alkenes.
Scheme 1 Palladium-catalysed silaboration of 2a with 1.
As a part of our silaboration study,7 we have recently
reported on the palladium-catalysed reaction of alkynes with
silylboranes bearing a heteroatom functional group on the
silicon atom.8 We found that addition of (chlorodimethylsi-
lyl)pinacolborane (1)9 to 1-octyne was much faster than that of
conventional triorganosilylboranes, such as (dimethylphenyl-
silyl)pinacolborane.10,11 The efficiency of the reaction and the
potential synthetic utility of the product, which has an easily
convertible silyl group,12 led us to an investigation aimed at
the application of these products in stereoselective alkene
The reaction process, which was highly influenced by the
stoichiometry of the reagents used, was traced in order to
elucidate the mechanism of the cis- and trans-silaboration
(Scheme 2). In the first step, the palladium-catalysed addition
of 1 to 2a proceeded in a cis fashion to give (Z)-4a selectively,14
regardless of the stoichiometry of the substrate used. The
subsequent reaction of (Z)-4a with isopropyl alcohol also
proceeded stereospecifically to give (Z)-3a, which was also
Department of Synthetic Chemistry and Biological Chemistry,
Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-
ku, Kyoto 615-8510, Japan. E-mail: suginome@sbchem.kyoto-u.ac.jp;
Fax: +81-75-383-2722
w Electronic supplementary information (ESI) available: Experimental
procedures and characterization data for new products. See DOI:
10.1039/b800181b
Scheme 2 Tracing of the reaction.
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Table 1 Palladium-catalysed cis- and trans-silaboration of terminal
alkynes
Table 2 Isomerization of (Z)-3a to (E)-3a
Entry
Pd
1
i-PrOH
Pyridine
(Z)-3a : (E)-3aa,b
1
2
3
4
5
6
7
+
ꢁ
+
+
ꢁ
+
+
+
ꢁ
+
+
+
+
ꢁ
13 : 87
499 : 1
499 : 1
499 : 1
8 : 92
499 : 1
499 : 1
Excess alkyne Excess Si–B
Product % yield (Z : E) % yield (Z : E)
Entry R (Alkyne)
+
+
+
+
+
92 (11 : 89)e
87 (8 : 92)
91 (8 : 92)
82 (9 : 91)
82 (7 : 93)
84 (11 : 89)
81 (7 : 93)
80 (62 : 38)
91 (67 : 33)
76 (90 : 10)f
+
+
1
2
3
n-C6H13 (2a)
n-C4H9 (2b)
n-C8H17 (2c)
3a
3b
3c
93 (499 : 1)
93 (499 : 1)
91 (499 : 1)
90 (499 : 1)
81 (499 : 1)
94 (499 : 1)
87 (499 : 1)
87 (499 : 1)
90 (499 : 1)
87 (499 : 1)
+
+
+
c
ꢁ
+
ꢁ
c
ꢁ
4
5
TBSO(CH2)2 (2d) 3d
TBSO(CH2)3 (2e) 3e
a
b
Determined by GC analysis. Small amounts of structure-undefined
c
products were formed for all reactions investigated. Me3SiCl was
used instead of 1.
6
7
8
Cl(CH2)3 (2f)
NC(CH2)3 (2g)
Ph (2h)
3f
3g
3h
9
10
cyclo-C6H11 (2i) 3i
t-Bu (2j) 3j
groups were tolerated (entries 4–7). However, the trans-sila-
boration protocol was not applicable to the sterically demand-
ing alkynes, such as 2h, 2i, and 2j (entries 8–10). These results
clearly indicate that the sterically less-hindered alkynes are
suitable for the trans-silaboration.
a
(Z3-C3H5)PdCl(PPh3) (1.0 mol%), 1 (0.40 mmol), and 2 (0.48 mmol)
in toluene (0.2 mL) were stirred at room temperature for 1–3 h.
Pyridine (0.72 mmol) and i-PrOH (0.60 mmol) were added and the
b
mixture was stirred at room temperature for 1 h. (Z3-C3H5)-
PdCl(PPh3) (2.0 mol%), 1 (0.48 mmol), and 2 (0.40 mmol) in toluene
(0.2 mL) were stirred at room temperature for 1–3 h. Pyridine (0.72
mmol) and i-PrOH (0.60 mmol) were added and the mixture was
The isomerization step was examined separately from the
silaboration step using isolated (Z)-3a (Table 2). In the pre-
sence of (Z3-C3H5)PdCl(PPh3) (2.0 mol%), 1 (20 mol%),
i-PrOH (25 mol%), and pyridine (30 mol%), geometrically
pure (Z)-3a was isomerized to (E)-3a to the same extent as in
the one-pot reaction (Z : E = 13 : 87) after heating to 50 1C for
a period of 24 h (entry 1). On the other hand, no isomerization
took place in the absence of either the palladium complex, 1,
or i-PrOH (entries 2–4), whereas pyridine was not essential for
driving the isomerization (entry 5). Two additional experi-
ments were carried out to elucidate the role of the chloro-
substituted silylborane (entries 6 and 7). In the presence of
c
d
stirred at 50 1C for 24 h. Isolated yield. Determined by GC analysis
of the crude mixture. Carried out with 1 (2.4 mmol) and 2
e
f
1
(2.0 mmol). Determined by H NMR analysis.
not dependent on the reaction conditions. However, it was
found that (Z)-3a underwent slow isomerization to (E)-3a only
when an excess of 1 was used in the first step. The ratio of
(Z)-3a : (E)-3a became constant (ca. 1 : 9) after a period of 96 h
at room temperature.15
Various terminal alkynes were subjected to the cis- and
trans-silaboration protocols (Table 1). To obtain cis-addition
products, the silaboration was performed with excess alkyne
(1 : 2 = 1.0 : 1.2) in the presence of (Z3-C3H5)PdCl(PPh3)
(1.0 mol%), followed by treatment with i-PrOH at room
temperature for a period of 1 h. The reaction of various
aliphatic alkynes 2a–g proceeded efficiently at room tempera-
ture to give the corresponding (Z)-3a–g in yields of 81–94%
with perfect Z selectivities (entries 1–7). Functional groups
such as silyloxy, chloro, and cyano groups were tolerated
under these reaction conditions (entries 4–7). Phenylacetylene
(2h), cyclohexylacetylene (2i), and 3,3-dimethylbut-1-yne (2j)
also successfully underwent the cis-silaboration to produce
(Z)-3h–j in yields of 87–90% with the Z/E ratios of 499 : 1
(entries 8–10).
On the other hand, to obtain the trans-silaboration pro-
ducts, the silaboration step was carried out with excess silyl-
borane (1 : 2 = 1.2 : 1.0) in the presence of 2.0 mol% of
(Z3-C3H5)PdCl(PPh3), followed by treatment with i-PrOH at
50 1C for a period of 24 h. The reaction of the primary alkyl-
substituted alkynes 2a–g successfully gave (E)-3a–g in 81–92%
yields with high E-selectivities (Z : E = 11 : 89–7 : 93, entries
1–7). Functional groups such as silyloxy, chloro, and cyano
Scheme 3 A synthetic application to stereoselective preparation of all
isomers of 1,2-diaryloct-1-ene.
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trimethylsilyl chloride instead of the silylborane, no isomer-
ization proceeded in either the presence or absence of pyridine.
These results clearly suggest that the isomerization is pro-
moted by a catalytically active palladium species that is
formed with 1 and i-PrOH.16
3 Hydrometallation of R0–CRC–M (M = SiR3, BR2, and SnR3)
tends to give gem-dimetallated alkenes. For examples, see: (a) J. J.
Eisch and M. W. Foxton, J. Org. Chem., 1971, 36, 3520; (b) K.
Uchida, K. Utimoto and H. Nozaki, J. Org. Chem., 1976, 41, 2941;
(c) G. Zweifel and S. J. Backlund, J. Am. Chem. Soc., 1977, 99, 3184;
(d) A. Hassner and J. A. Soderquist, J. Organomet. Chem., 1977, 131,
C1; (e) T. N. Mitchell and A. Amamria, J. Organomet. Chem., 1983,
252, 47; (f) B. H. Lipshutz, R. Kell and J. C. Barton, Tetrahedron
Lett., 1992, 33, 5861; (g) L. Deloux, E. Skrzypczak-Jankun, B. V.
Cheesman and M. Srebnik, J. Am. Chem. Soc., 1994, 116, 10302.
4 For hydrometallation of 1-silyl-1-alkynes yielding trans-1,2-bismetal-
lated 1-alkenes, see: (a) P. F. Hudrlik, R. H. Schwartz and J. C. Hogan,
J. Org. Chem., 1979, 44, 155; (b) J. A. Soderquist, J. C. Colberg and L.
D. Valle, J. Am. Chem. Soc., 1989, 111, 4873; (c) H. X. Zhang, F. Guibe
and G. Balavoine, J. Org. Chem., 1990, 55, 1857; (d) H. Urabe, T.
Hamada and F. Sato, J. Am. Chem. Soc., 1999, 121, 2931.
5 Stereoselective catalytic trans-bismetallation has been achieved
only for ethyne (C2H2): (a) B. L. Chenard and C. M. Van Zyl, J.
Org. Chem., 1986, 51, 3561; (b) T. Hayashi, H. Yamashita, T.
Sakakura, Y. Uchimaru and M. Tanaka, Chem. Lett., 1991, 245.
For trans-bisstannylation of 1,3-diynes with a stannylcopper
reagent, see ref. 1b.
6 (a) T. N. Mitchell, A. Amamria, H. Killing and D. Rutschow, J.
Organomet. Chem., 1986, 304, 257; (b) E. Piers and R. T. Skerlj, J.
Chem. Soc., Chem. Commun., 1986, 626.
7 For recent examples, see: (a) T. Ohmura and M. Suginome, Org.
Lett., 2006, 8, 2503; (b) T. Ohmura, H. Furukawa and M.
Suginome, J. Am. Chem. Soc., 2006, 128, 13366; (c) T. Ohmura,
H. Taniguchi and M. Suginome, J. Am. Chem. Soc., 2006, 128,
13682; (d) T. Ohmura, H. Taniguchi, Y. Kondo and M. Suginome,
J. Am. Chem. Soc., 2007, 129, 3518.
8 T. Ohmura, K. Masuda and M. Suginome, J. Am. Chem. Soc.,
2008, 130, 1526.
9 T. Ohmura, K. Masuda, H. Furukawa and M. Suginome, Orga-
nometallics, 2007, 26, 1291.
Synthetic application of the silaboration products was
demonstrated by the stereoselective synthesis of 1,2-diaryl-
oct-1-ene via a Suzuki–Miyaura coupling followed by a Hiya-
ma coupling (Scheme 3). The Suzuki–Miyaura coupling of
(Z)-3a with 4-iodotoluene (Ar1–I) in the presence of a Pd–
S-PHOS catalyst17 gave alkenylsilane (Z)-5 in an 80% yield.
The subsequent Hiyama coupling of (Z)-5 with 4-fluoroiodo-
benzene (Ar2–I) under phosphine-free conditions18 afforded
(Z)-2-(4-fluorophenyl)-1-(4-methylphenyl)oct-1-ene [(Z)-6] in
a 66% yield. On the other hand, the Suzuki–Miyaura coupling
of (Z)-3a with Ar2–I followed by Hiyama coupling with Ar1–I
gave (Z)-8, the regioisomer of (Z)-6, in a comparable yield.
A complementary two-step transformation was applied to
(E)-3a, producing (E)-6 and (E)-8 in reasonable yields. These
reactions demonstrate that all four isomers of 1,2-diarylalk-1-
enes are accessible from a single set of reactants, i.e., 1-octyne,
p-tolyl iodide, and p-fluorophenyl iodide, with the use of
common reagents for each step. The methods of synthesis
are only differentiated by the reagent ratio in the silaboration
step and the order of the aryl iodides.
In conclusion, we have established palladium-catalysed cis-
and trans-silaboration of terminal alkynes, which can be
controlled by the stoichiometry of the silylborane and alkyne.
The products, (Z)- and (E)-1-boryl-2-silylalk-1-enes, serve as
useful synthetic builiding blocks for the stereoselective synth-
esis of stilbene derivatives.
10 (a) M. Suginome, H. Nakamura and Y. Ito, Chem. Commun., 1996,
2777; (b) S.-y. Onozawa, Y. Hatanaka and M. Tanaka, Chem.
Commun., 1997, 1229; (c) M. Suginome, T. Matsuda, H. Naka-
mura and Y. Ito, Tetrahedron, 1999, 55, 8787; (d) J. C. A. Da Silva,
M. Birot, J.-P. Pillot and M. Petraud, J. Organomet. Chem., 2002,
646, 179; (e) M. Suginome, H. Noguchi, T. Hasui and M.
Murakami, Bull. Chem. Soc. Jpn., 2005, 78, 323.
11 For a related study on the catalytic silaboration of alkynes, see: (a)
M. Suginome, T. Matsuda and Y. Ito, Organometallics, 1998, 17,
5233; (b) T. Segawa, Y. Asano and F. Ozawa, Organometallics,
2002, 21, 5879.
This work is supported by a Grant-in-Aid for Scientific
Research on Priority Areas (No. 19028027, ‘‘Chemistry of
Concerto Catalysis’’) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
12 (a) T. Hiyama, in Metal-Catalyzed Cross-Coupling Reactions, ed.
F. Diedrich and P. J. Stang, Wiley-VCH, Weinheim, 1998, ch.10;
(b) T. Hiyama and E. Shirakawa, Top. Curr. Chem., 2002, 219, 61.
13 P. Kisanga and R. A. Widenhoefer, J. Am. Chem. Soc., 2000, 122,
10017.
14 A possible mechanism of cis-silaboration involves oxidative addi-
tion of the Si–B bond to Pd(0), followed by regioselective cis-
insertion of the C–C triple bond into the Pd–B bond and subse-
quent reductive elimination of the (Z)-adduct with regeneration of
Pd(0). See ref. 10c.
15 For a related study on the Z/E isomerization under the catalytic
bismetallation conditions, see: (a) H. Watanabe, M. Kobayashi, K.
Higuchi and Y. Nagai, J. Organomet. Chem., 1980, 186, 51; (b) H.
Matsumoto, I. Matsubara, T. Kato, K. Shono, H. Watanabe and
Y. Nagai, J. Organomet. Chem., 1980, 199, 43; see also refs. 5b and
10c.
16 For a general description of alkene isomerization, see (a) F. J. McQuil-
lin, D. G. Parker and G. R. Stephenson, in Transition Metal Organo-
metallics for Organic Synthesis, Cambridge University Press,
Cambridge, 1991, ch. 2, p. 27; (b) G. W. Parshall and S. D. Ittel, in
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(c) R. H. Crabtree, in The Organometallic Chemistry of the Transition
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17 T. E. Barder, S. D. Walker, J. R. Martinelli and S. L. Buchwald, J.
Am. Chem. Soc., 2005, 127, 4685.
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