Platinum chloride/Xphos-catalyzed regioselective hydrosilylation of functionalized terminal arylalkynes

Article abstract of DOI:10.1016/j.tetlet.2008.02.060

Totally regioselective hydrosilylation of functionalized terminal arylalkynes was achieved using PtCl₂ associated with the air-stable and bulky Xphos ligand with various silanes. Regardless of the electronic nature of the substituents on the ar

Full text of DOI:10.1016/j.tetlet.2008.02.060

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 2429–2431
Platinum chloride/Xphos-catalyzed regioselective
hydrosilylation of functionalized terminal arylalkynes
*
ˆ
Abdallah Hamze, Olivier Provot, Jean-Daniel Brion, Mouad Alami
Univ Paris-Sud, CNRS, BioCIS, UMR 8076, Laboratoire de Chimie The´rapeutique, Faculte´ de Pharmacie,
ˆ
rue J.B. Cle´ment, Chatenay-Malabry F-92296, France
Received 26 November 2007; revised 7 February 2008; accepted 11 February 2008
Available online 14 February 2008
Abstract
Totally regioselective hydrosilylation of functionalized terminal arylalkynes was achieved using PtCl2 associated with the air-stable
and bulky Xphos ligand with various silanes. Regardless of the electronic nature of the substituents on the aromatic ring, a single
b-(E)-vinylsilane was obtained in excellent yields.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Hydrosilylation; Alkynes; Regioselectivity; Platinum; Phosphane ligands; Xphos
Functionalized b-(E)-styrylsilanes 2, which have
emerged as powerful intermediates in organic synthesis,1
can in principle be accessed by metal-catalyzed hydrosilyla-
tion of terminal arylalkynes.2 Although the platinum cata-
lyzed hydrosilylation of alkynes is well documented,
however, the reaction with functionalized terminal aryl-
alkynes to provide b-(E)-styrylsilanes has received scant
attention.3 Recently, significant progress with non-substi-
tuted phenylacetylene in terms of regioselectivity has been
made for the b-(E)-styrylsilane formation4 using the pre-
formed [Pt(CH2@CHSiMe2)2O] in conjunction with air-
sensitive, pyrophoric, and difficult-to-handle P(tBu)3. The
remaining challenge is to obtain high b-(E)-selectivity from
functionalized arylalkynes without compromising reagent
stability and practicality. Herein, we report that PtCl2/
Xphos provides an efficient catalyst system for the hydro-
silylation of a wide variety of functionalized phenylacetyl-
enes 1 with various silanes.
arylalkynes.5 Unfortunately, in the case of terminal
alkynes, the regioselectivity of the H–Si bond addition
was found to be weak.6 Therefore, we anticipated that
the tuning of platinum complex catalysts would affect the
regioselectivity of H–Si bond addition. The hydrosilylation
regioselectivity of 1a with HSiEt3 was studied under several
reaction conditions (platinum catalysts, ligands, and sol-
vents) according to Scheme 1. The best results in term of
yield and selectivity were achieved when commercially
available PtCl2 (5 mol %) and Xphos ligand (10 mol %)
were used in THF. Accordingly, 2a was exclusively formed
R
R
[Si]-H
α
β
+
PtCl2/Xphos,
THF, 60 ºC
R
[Si]
α-isomer
[Si]
β-(E)-isomer
1
2
3
a: R = H
Previously, we reported that platinum oxide proved to
be a versatile catalyst for the hydrosilylation of internal
P
Xphos
=
i-Pr
i-Pr
*
Corresponding author. Tel.: +33 01 46 83 58 87; fax: +33 01 46 83 58
i-Pr
28.
Scheme 1.
E-mail address: mouad.alami@u-psud.fr (M. Alami).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.02.060

2430
A. Hamze et al. / Tetrahedron Letters 49 (2008) 2429–2431
Table 1
PtCl2/Xphos-catalyzed hydrosilylation of terminal alkynes 17
Entry
1
Alkyne 1
HSiR3
a:ba Ratio
Vinylsilanesb
2
Yieldc (%)
98
1a
1b
1c
1d
1e
1f
HSiEt3
0:100
2a
SiEt3
MeO
Me
2
3
HSiEt3
0:100d
0:100
0:100
0:100
0:100
2:98
2b
2c
2d
2e
2f
91
91
86
80
94
65
83
67
68e
98
75
83f
MeO
Me
SiEt3
HSiEt3
SiEt3
MeO2C
NC
4
HSiEt3
MeO2C
NC
SiEt3
5
HSiEt3
SiEt3
SiEt3
Br
6
HSiEt3
Br
MeO
Me
7
1b
1c
1d
1d
1g
1g
1h
HSi(OEt)3
HSi(OEt)3
HSi(OEt)3
HSiMe2OEt
HSiEt3
2g
2h
2i
MeO
Me
Si(OEt)3
8
2:98
Si(OEt)3
MeO2C
MeO2C
9
30:70
3:97
MeO2C
MeO2C
Si(OEt)3
10
11
12
2j
SiMe2OEt
Me
Me
0:100
0:100
19:81
2k
2l
SiEt3
Me
Me
HSi(OEt)3
HSiEt3
Si(OEt)3
CO2Me
SiEt3
13
2m
CO2Me
a
Determined by 1H NMR and GC.
b
c
d
e
f
All of the reported compounds exhibited spectral data in agreement with the assigned structures.
Isolated yield.
A 18:82 a:b mixture was obtained in the absence of Xphos ligand.
Reaction was performed at room temperature.
Isolated yield of the vinylsilane a:b mixture after column chromatography.
and the analysis of the crude reaction mixture by 1H NMR
spectroscopy and GC revealed no trace of either 3a or the
b-(Z)-vinylsilane demonstrating that the H–Si bond addi-
tion proceeded exclusively in a syn fashion. To the best
of our knowledge, this is the first example of terminal
arylalkyne hydrosilylation being catalyzed by PtCl2 cata-
lyst associated with stable and commercially available
monodentate Xphos ligand.
Next, we used the PtCl2/Xphos catalyst system for eval-
uating the scope of this hydrosilylation with a range of

A. Hamze et al. / Tetrahedron Letters 49 (2008) 2429–2431
2431
2. For a review, see: (a) Trost, B. M.; Ball, Z. T. Synthesis 2005, 853–887;
functionalized terminal alkynes (Table 1). para-Substituted
arylalkynes 1b–f were cleanly hydrosilylated with Et3SiH in
the presence of the PtCl2/Xphos couple to their corres-
ponding b-(E)-adducts with excellent yields whatever is
the nature (electron donating or electron withdrawing
group) of the substituent (entries 2–6). Replacement of
Et3SiH by (EtO)3SiH resulted in similar yields and b-(E)-
selectivities (entries 7 and 8) except in the case of arylalkyne
1d with a para electron withdrawing group (entry 9). For-
tunately, we were pleased to observe that the replacement
of (EtO)3SiH by HSiMe2OEt led to b-(E) vinylsilane 2j
with an excellent regioselectivity (entry 10).
With the ortho-substituted alkyne 1g, again a total b-
regiocontrol was observed with either Et3SiH or (EtO)3SiH
(entries 11 and 12). This result clearly demonstrated that
the regioselectivity of the H–Si bond addition is governed
by steric effects induced by Xphos ligand rather than
ortho-directing effect (ODE) as we previously reported.5,6,8
To support this explanation, the hydrosilylation of ortho
methoxyphenylacetylene was conducted without Xphos
and produced a 38:62 ratio of a:b regioisomers. With
ortho-methoxycarbonyl phenylacetylene 1h, the PtCl2-
catalyzed hydrosilylation was less selective and led to a
regioisomeric mixture with a preference for the b-isomer
(a:b = 19:81, entry 13) indicating that ODE,5 which is
opposed to steric effects, rebalances the isomeric distri-
bution, thus increasing the amounts of a-adduct.
´
(b) De Bo, G.; Berthon-Gelloz, G.; Tenant, B.; Marko, I. Organo-
metallics 2006, 25, 1881–1890; (c) Chauhan, M.; Hauck, B.; Keller, L.;
Boudjouk, P. J. Organomet. Chem. 2002, 645, 1–13; (d) Wu, W.; Li, C.
J. Chem. Commun. 2003, 1668–1669; (e) Wang, F.; Neckers, D. C. J.
Organomet. Chem. 2003, 665, 1–6; (f) Poyatos, M.; Maisse-Franßcois,
A.; Bellemin-Laponnaz, S.; Gade, L. H. Organometallics 2006, 25,
2634–2641; (g) Na, Y.; Chang, S. Org. Lett. 2000, 2, 1887–1889; (h)
Trost, B.; Ball, Z. J. Am. Chem. Soc. 2001, 123, 12726–12727; (i) Faller,
J.; D’Alliessi, D. Organometallics 2002, 21, 1743–1746.
3. For recent Rh-catalysis, see: (a) Takeuchi, R.; Nitta, S.; Watanabe, D.
J. Org. Chem. 1995, 60, 3045–3051; (b) Sato, A.; Kinoshita, H.;
Shinokubo, H.; Oshima, K. Org. Lett. 2004, 6, 2217–2220; (c) Zeng, J.
Y.; Hsieh, M. H.; Lee, H. M. J. Organomet. Chem. 2005, 690, 5662–
5671; (d) Mori, A.; Takahisa, E.; Yamamura, Y.; Kato, T.; Mudalige,
A. P.; Kajiro, H.; Hirabayashi, K.; Nishihara, Y.; Hiyama, T.
Organometallics 2004, 23, 1755–1765; For recent Ir-catalysis, see: (e)
Miyake, Y.; Isomura, E.; Iyoda, M. Chem. Lett. 2006, 35, 836–837. For
recent Co-catalysis, see: (f) Tojo, S.; Isobe, M. Tetrahedron Lett. 2005,
46, 381–384.
4. (a) Denmark, S. E.; Wang, Z. Org. Lett. 2001, 3, 1073–1076; (b) Itami,
K.; Mitsudo, K.; Nishino, A.; Yoshida, J.-I. J. Org. Chem. 2002, 67,
2645–2652; (c) Aneetha, H.; Wu, W.; Verkade, J. G. Organometallics
2005, 24, 2590–2596.
5. Hamze, A.; Provot, O.; Alami, M.; Brion, J.-D. Org. Lett. 2005, 7,
5625–5628.
6. Hamze, A.; Provot, O.; Brion, J.-D.; Alami, M. Synthesis 2007, 2025–
2036.
7. Typical procedure: Under nitrogen atmosphere, PtCl2 (0.05 mmol) and
Xphos (0.1 mmol) in THF (0.5 mL) were heated at 60 °C for 15 min.
Then, terminal alkyne (1 mmol) and triethylsilane or triethoxysilane
(1.5 mmol) were successively added via a syringe, and the mixture was
stirred at 60 °C for 1 h. After evaporation of the solvent, the residue
was purified by column chromatography to yield b-(E)-vinylsilane 2.
Vinylsilane 2a: Yield: colorless oil, 91%. TLC: Rf 0.5 (Et2O/cyclo-
hexane, 5/95, SiO2). IR (neat, cmÀ1): 2952, 2909, 2874, 2835, 1606,
1570, 1508, 1463, 1441, 1416, 1378, 1332, 1303, 1294, 1250, 1171, 1106,
1037, 1014, 986, 843, 789, 749, 717. 1H NMR (300 MHz, CDCl3): d
0.57 (q, 6H, J = 7.8 Hz), 0.90 (t, 9H, J = 7.8 Hz), 3.72 (s, 3H), 6.17 (d,
1H, J = 19.3 Hz), 6.70–6.82 (m, 3H), 7.30 (d, 2H, J = 8.7 Hz). 13C
NMR (75 MHz, CDCl3): d 3.7 (3CH2), 7.6 (3CH3), 55.4 (OCH3), 114.0
(2CH), 123.1 (CH), 127.6 (2CH), 131.7 (C), 144.3 (CH), 159.6 (C). MS
(ESI): 248 (M+). Anal. Calcd for C15H24OSi (248.44): C, 72.52; H,
9.74. Found: C, 72.48; H, 9.82.
In conclusion, we have established that PtCl2/Xphos is
an efficient catalyst system for the hydrosilylation of func-
tionalized terminal alkynes with various silanes. This quite
simple procedure is characterized by functional group com-
patibility and a good generality. Additionally, our results
demonstrated that commercially available and air-stable
Xphos ligand associated to the PtCl2 catalyst constitutes
an attractive catalytic system for the univocal synthesis of
b-(E)-vinylsilanes from terminal arylalkynes and should
find many applications in organic synthesis.
Vinylsiloxane 2g: Yield: yellow oil, 65%; ratio a:b (2/98 of isomers).
TLC: Rf 0.50 (Et2O/cyclohexane, 30/70, SiO2). IR (neat, cmÀ1): 3288,
2974, 2891, 2883, 1606, 1572, 1508, 1465, 1442, 1418, 1390, 1293, 1250,
Acknowledgment
1
1169, 1099, 1071, 1032, 994, 956, 832, 797, 776, 748, 709, 685, 640. H
NMR (300 MHz, CDCl3): d 1.20 (t, 9H, J = 7.0 Hz), 3.72 (s, 3H), 3.80
(q, 6H, J = 7.0 Hz), 5.92 (d, 1H, J = 19.5 Hz), 6.80 (d, 2H, J = 8.3 Hz),
7.08 (d, 1H, J = 19.5 Hz), 7.34 (d, 2H, J = 8.3 Hz). 13C NMR
(75 MHz, CDCl3): d 18.3 (3CH3), 55.3 (3CH2), 58.7 (OCH3), 113.9
(2CH), 114.7 (CH), 128.3 (2CH), 130.6 (C), 133.7 (CH), 160.3 (C). MS
(ESI): 319 (M+Na)+. Anal. Calcd for C15H24O4Si (219.40): C, 60.78;
H, 8.16. Found: C, 60.65; H, 8.15.
The CNRS is gratefully acknowledged for financial sup-
port of this research.
References and notes
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