Catalytic versus stoichiometric coupling of alkynes with trimethylvinylsilane mediated by zirconocene

Article abstract of DOI:10.1016/j.tetlet.2009.10.095

Trimethylvinylsilane and disubstituted alkynes underwent coupling reactions in the presence of the lanthanide-originated zirconocene equivalent. Both reactions, stoichiometric and catalytic in zirconium, could be carried out; in the latter case the addition of a stoichiometric amount of AlCl₃ was needed. The catalytic cycle involving bimetallic polarization and a transmetallation step has been proposed.

Full text of DOI:10.1016/j.tetlet.2009.10.095

Tetrahedron Letters 51 (2010) 115–117
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Catalytic versus stoichiometric coupling of alkynes with
trimethylvinylsilane mediated by zirconocene
a
b
Mohamad Soueidan a, Florence Hélion a, , Jean-Louis Namy , Jan Szymoniak
*
a Equipe de Catalyse Moléculaire, CNRS (UMR 8182), ICMMO, Bât 420, Université Paris-Sud, 91405 Orsay, France
b Institut de Chimie Moléculaire de Reims, CNRS (UMR 6229), Université de Reims, 51687 Reims Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Trimethylvinylsilane and disubstituted alkynes underwent coupling reactions in the presence of the lan-
thanide-originated zirconocene equivalent. Both reactions, stoichiometric and catalytic in zirconium,
could be carried out; in the latter case the addition of a stoichiometric amount of AlCl3 was needed.
The catalytic cycle involving bimetallic polarization and a transmetallation step has been proposed.
Ó 2009 Elsevier Ltd. All rights reserved.
Received 22 July 2009
Revised 19 October 2009
Accepted 21 October 2009
Available online 23 October 2009
Keywords:
Lanthanides
Zirconocene
Alkyne coupling
Trimethylvinylsilane
Catalytic reaction
Applications of zirconium in organic synthesis since the 1980s1
have been widely supported by the introduction of a new method
for generating a practical zirconocene (Cp2Zr) equivalent.2,3 Accord-
ing to the described procedure, (1-butene)ZrCp2, the also called
Negishi reagent is formed by treating Cp2ZrCl2 with 2 equiv of n-BuLi
at À78 °C, followed by warming up to room temperature. By apply-
ing this method, Takahashi et al. have described regioselective cross-
coupling reactions of alkynes with vinylsilane.4 By-products arising
from alkyne dimerization have also been observed. To the best of our
knowledge, cross-coupling reactions between alkynes and trim-
ethylvinylsilane in the presence of a catalytic amount of a zirconium
complex have never been reported.
dered La (0.66 mmol) was stirred at room temperature in 3 mL of
THF until a deep red colour appeared. A solution of trimethylvi-
nylsilane (4 mmol) in 2 mL of THF was then added. After 10 min,
alkyne (1 mmol) was added at room temperature. The reaction
was then carried out at room temperature for an additional 12 h.
Finally, hydrolytic workup (HCl 1 M), followed by extraction with
THF
3 Cp ZrCl
3 Cp Zr + 2 LnCl
+ 2 Ln
2
2
2
3
Ln = La, Ce, mischmetall
Scheme 1. Reduction of Cp2ZrCl2 with lanthanides.
Recently we presented a room-temperature-based protocol for
generating of an alkene-free zirconocene equivalent by reducing
Cp2ZrCl2 with lanthanum, cerium or mischmetall5 (Scheme 1).6
Dimerization or cyclotrimerization of alkynes catalytic in zirco-
nocene has next been developed, by using this new protocol and
AlCl3 as additive (Scheme 2).7
In the pursuit of this work, we have studied herein the coupling
of alkynes with vinylsilanes and demonstrated that such reactions
can also be performed with a catalytic amount of lanthanide-orig-
inated zirconocene equivalent.
R
H
1
(or R )
H
H
R
1
5% Cp ZrCl
2
2
R
R
H
1
(or H)
1
La, AlCl
3
R
1
= alkyl, aryl
1
rdt = 65-82%
R
1
R
2
Using stoichiometric amounts of Cp2ZrCl2 and La, the coupling
of alkynes and vinylsilane could be achieved. The optimized proce-
dure is as follows: a mixture of Cp2ZrCl2 (1 mmol) and the pow-
10% Cp ZrCl
2
2
R
R
2
1
R
La, AlCl
3
1
R
2
R = R = alkyl, aryl
1
2
rdt = 70-85%
* Corresponding author.
E-mail address: flohelion@icmo.u-psud.fr (F. Hélion).
Scheme 2. Catalytic dimerization and cyclotrimerization of alkynes.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.10.095

116
M. Soueidan et al. / Tetrahedron Letters 51 (2010) 115–117
SiMe
Ph
1) La, THF
15 min, rt
3
Ph
R
R
Ph
R
4) HCl 1 M
3)
Cp ZrCl
2
ZrCp
ZrCp
SiMe
2
2
2
THF, 12h, rt
2)
Me Si
SiMe
3
3
A
SiMe
B
3
3
THF 10 min, rt
1a: 75%
1b: 70%
1c: 71%
R = Ph
R = Me
R = Pr
LaCl
+
3
3) I , 30 min, rt
2
4) HCl 1M
SiMe
3
I
2
SiMe
3
Scheme 3. Stoichiometric coupling of alkynes with trimethylvinylsilane.
Table 1
use of 20 mol % Cp2ZrCl2 (entries 1 and 2), a complete conversion
was observed after a 12-h reaction period, followed by hydrolytic
workup. Under the same conditions, the use of 10 mol % Cp2ZrCl2
resulted in an incomplete conversion even after a prolonged
(48 h) reaction period. However, the decrease of the amount of
THF (entries 3 and 4) led to a complete conversion after 12 h.8 In
all cases, in contrast with the stoichiometric conditions (Scheme 2),
two regioisomers (1 and 3) were isolated in total yields of 61–66%.
It must be noted that in the absence of AlCl3, the reaction did not
take place (with R: Me, n-Pr and Ph).
Catalytic coupling of alkynes with trimethylvinylsilane
1) Cp ZrCl (cat),
2
2
R
Ph
Ph
R
Ph
R
La, AlCl
+
3
+
Me Si
+
3
SiMe
SiMe
2) H O
3
3
3
1
3
Entrya
R
Cp2ZrCl2
THF
Combined isolated yield %
b
(cat) (mol %)
(mL)
of 1 and 3c (1:3)
Under stoichiometric conditions, the appearance of the deep red
colour is indicative of the formation of zirconocene Cp2Zr(II).6,9 Un-
der the catalytic conditions, we did not observe such a colour
change in the absence of AlCl3, possibly due to a slow reduction
rate and a low concentration in zirconium. In contrast, the deep
red colour appeared within a short time (1 min) after the addition
of aluminium chloride. Obviously, this compound plays an essen-
tial role in the generation of Cp2Zr(II) under the catalytic condi-
tions. In both cases, LaCl3 might be involved as a Lewis acid
activator.
1
2
3
4
Me
n-Pr
Me
20
20
10
10
5
5
2
2
1b:3b 64 (55:45)
1c:3c 61 (68:32)
1b:3b 65 (62:38)
1c:3c 66 (63:37)
n-Pr
a
Alkyne: 1 mmol, trimethylvinylsilane 4 mmol, La: 0.66 mmol, AlCl3: 1 mmol,
THF, 12 h, rt.
b
With respect to alkyne.
Determined by 1H NMR.
c
A mechanistic pattern involving bimetallic systems, in which
one metal activates another via a three-centre two-electron bond-
ing (Scheme 4, Eq. 1), analogously to the mode of carbon activation
in the Friedel–Crafts reaction can be proposed10 (Scheme 4, Eq. 2).
In the catalytic reaction, a bimetallic polarization between
Cp2ZrCl2 and AlCl3, leads to the formation of [Cp2ZrCl]+[AlCl4]À.
Its reduction by Ln metal would then be much easier than the
reduction of Cp2ZrCl2. Another mechanistic scenario would involve
2
2
2
2
1
1
1
1
(Eq. 1)
(Eq. 2 )
M Ln
X
X
M Ln
L
M
+
L
M
n
n
C
X
ML
n
ML
C X
+
n
Scheme 4. Metal activations.
a
bimetallic polarization process through zirconocene-LaCl3
assemblies.
The decrease of regioselectivity under the catalytic conditions
ether and chromatography purification, afforded trimethyl-(3-al-
kyl-4-phenyl)-but-3-enyl silanes 1 in good yields (70–75%).8 No
trace of another regioisomer was detected, indicating total double
regioselectivity. Furthermore when iodine was added instead of
the alkyne, 2 was obtained as a sole product.8 This result confirms
which involves the presence of AlCl3 is surprising. To puzzle out
this problem, we performed the following experiment under stoi-
chiometric conditions (see above) and in the presence of AlCl3:
La (0.66 mmol), Cp2ZrCl2 (1 mmol) and AlCl3 (1 mmol) were mixed
together in THF at room temperature, after 10 min vinyl silane
(4 mmol) was added, after another 10 min 1-phenylpropyne
(1 mmol) was added and stirring was maintained for 12 h. After
usual workup, the formation of regioisomers 1b and 3b in good
combined yield (60%) was observed with a 1b:3b ratio very close
(69:31) to that measured under catalytic conditions. Thus, it was
demonstrated that AlCl3 influences the regioselectivity of this
reaction.
A tentative catalytic cycle is depicted in Scheme 5. The reduc-
tion of Cp2ZrCl2 with La in the presence of AlCl3 gives Cp2Zr which
further reacts with trimethylvinylsilane to afford disilylzircona-
cyclopentane A. A reaction with AlCl3 produces the complex C
which undergoes an insertion reaction to give a mixture of alu-
the generation of
(Scheme 3).
a common metallacycle intermediate A
In order to carry out these reactions with a catalytic amount of
Cp2ZrCl2, several conditions were investigated. The influence of
some parameters was studied: molar percentage of Cp2ZrCl2 or of
AlCl3 and concentration.
According to the optimized procedure, the reaction was carried
out by stirring
a mixture of catalytic amount of Cp2ZrCl2
(0.2 mmol; 20 mol %), the powdered La (0.66 mmol) and alumin-
ium chloride (1 mmol) at room temperature in 1 or 3 mL of THF,
until the deep red colour appeared. A solution of vinylsilane
(4 mmol) in 1 or 2 mL of THF was then added. The following proce-
dure and the workup were identical to those reported for the stoi-
chiometric version. The results are presented in Table 1. With the

M. Soueidan et al. / Tetrahedron Letters 51 (2010) 115–117
117
Cl
Cl
Cl
Cp
Cp
AlCl
3
Al
Cl
Zr
Cl
R
Ph
Al
+
-
Ph
+
R
[Cp ZrCl] [AlCl ]
Cp ZrCl
2
2
4
2
+
Me Si
3
Al Cl
SiMe
Cl
La
SiMe
3
3
D
E
LaCl + AlCl
3
3
"
"
Cp Zr
2
[ A-AlCl
3
]
C
Ph
R
SiMe
3
Me Si
2
3
ZrCp
2
AlCl
3
SiMe
3
A
Scheme 5. Mechanistic proposal for catalytic coupling of alkynes with trimethylvinylsilane.
5. Mischmetall is an alloy of Ce, La, Nd and Pr.
6. Denhez, C.; Médégan, S.; Hélion, F.; Namy, J.-L.; Vasse, J.-L.; Szymoniak, J. Org.
Lett. 2006, 8, 2945–2947.
7. Joosten, A.; Soueidan, M.; Denhez, C.; Harakat, D.; Hélion, F.; Namy, J.-L.; Vasse,
J.-L.; Szymoniak, J. Organometallics 2008, 27, 4152–4157.
minacyclopentenes D and E with regeneration of Cp2ZrCl2. The
quenching of D and E should produce 1 and 3, respectively. Addi-
tional experiments are necessary to define the structure of C. How-
ever, it must be considered that AlCl3 is involved in the insertion of
the alkyne into the disilylated metallacycle.
8. Materials and methods. All reactions were performed under an atmosphere of
argon using standard Schlenk techniques. Prior to use tetrahydrofuran was
distilled under argon from sodium benzophenone ketyl. 1H NMR spectra were
recorded in CDCl3 on a Brucker 250 DPX or 360 AVANCE. Chemical shifts are
reported in delta (d) units, expressed in parts per million (ppm). 13C NMR
spectra were recorded in CDCl3 on a Brucker 250 DPX. Chemical shifts are
reported in delta (d) units, expressed in parts per million (ppm). Coupling
constants are expressed in hertz (Hz). High-resolution mass spectra (HRMS)
were obtained with a MAT-95-S Finnigan. GC–MS were obtained with a DSQ-
thermo electron instrument. (Z)-(3,4-Diphenyl-but-3-enyl)-trimethyl-silane
(1a): Purification: eluent pentane/EtOAc (95:5). Yellow oil. Yield (75%). 1H
NMR (CDCl3): d 0.04 (s, 9H), 0.71 (m, 2H), 2.51 (m, 2H), 6.48 (s, 1H), 6.90–7.40
(m, 10H). 13C NMR (CDCl3): d 145.8, 141.5, 137.6, 128.9, 128.6, 128.4, 127.8,
126.8, 126.0, 124.8, 34.8, 15.1, À1.7. HRMS: [M+] calcd for C19H24Si 280.1642;
found, 280.1632. (E)-Trimethyl-(3-methyl-4-phenyl-but-3-enyl)-silane (1b):
Purification: eluent pentane/EtOAc (95:5). Yellow oil. Yield (70%). 1H NMR
(CDCl3): d 0.05 (s, 9H), 0.80 (m, 2H), 1.90 (s, 3H), 2.12 (m, 2H), 6.34 (s, 1H), 7.30
(m, 5H). 13C NMR (CDCl3): d 141.6, 138.8, 128.8, 128.0, 125.7, 123.4, 34.9, 17.6,
15.0, À1.7. HRMS: [M+] calcd for C14H22Si 218.1485; found, 218.1485. (E)-
Trimethyl-(4-phenyl-propylbut-3-enyl)-silane (1c): Purification: eluent pentane/
EtOAc (99:1). Yellow oil. Yield (71%). 1H NMR (CDCl3): d 0.04 (s, 9H), 0.75 (m,
2H), 0.92 (t, J = 7.3, 3H), 1.50 (m, 2H), 2.20 (m, 4H), 6.30 (s, 1H), 7.27 (m, 5H).
13C NMR (CDCl3): d 145.9, 138.8, 128.6, 128.0, 125.7, 123.7, 32.5, 31.3, 21.5,
15.1, 14.3, À1.7. HRMS: [M+] calcd for C16H26Si 246.1798; found, 246.1792. 1-
Iodo-1,4-bis-trimethylsilanyl-butane (2). Yellow oil. 1H NMR (CDCl3): d 0.04 (S,
9H); 0.71 (m, 2H); 1.50 (m, 2H), 1,91 (m, 2H), 3.18 (m, 1H). GC–MS (EI, 70 eV)
m/z (%): 329 (17), 328 (100), 313 (11), 285 (14), 273 (34). (E)-Trimethyl-(3-
methyl-4-phenyl-but-3-enyl)-silane (1b) and (Z)-trimethyl-(4-methyl-3-phenyl-
but-3-enyl) silane (3b): Purification: eluent pentane/EtOAc (95:5), Yellow oil.
Total yield [64%, (1b:3b/55:45)] Table 1, entry 1; [65%, (1b:3b/62:38)] Table 1,
entry 3. Regioisomer (1b): 1H NMR (CDCl3): d 0.04 (s, 9H), 0.80 (m, 2H), 1.90 (s,
3H), 2.12 (m, 2H), 6.34 (s, 1H), 7.30 (m, 5H). Regioisomer (3b): 1H NMR (CDCl3):
d 0.01 (s, 9H), 0.56 (m, 2H), 1.57 (d, J = 7, 3H), 2.33 (m, 2H), 5.56 (q, 1H, J = 7),
7.30 (m, 5H). (E)-Trimethyl-(4-phenyl-3-propyl-but-3-enyl)-silane (1c) and (Z)-
trimethyl-(3-phenyl-hept-3-enyl)-silane (3c). Purification: eluent pentane/EtOAc
(95:5), Yellow oil. Total yield [61%, (1c:3c/68:32)] Table 1, entry 3; [66%,
(1c:3c/63:37)] Table 1, entry 4. Regioisomer (1c): 1H NMR (CDCl3): d 0.05 (s,
9H), 0.75 (m, 2H), 0.92 (t, J = 7.3, 3H), 1.50 (m, 2H), 2.20 (m, 4H), 6.30 (s, 1H),
7.27 (m, 5H). Regioisomer (3c): 1H NMR (CDCl3): d 0.00 (s, 9H), 0.57 (m, 2H),
0.74 (t, J = 8, 3H), 1.35 (m, 2H), 1.90 (m, 2H), 2.20 (m, 2H), 5.48 (t, J = 8, 1H).
9. Soueidan, M.; Hélion, F.; Namy, J.-L.; Szymoniak, J. Organometallics 2008, 27,
2474.
The proposed mechanism makes it possible to rationalize the
difference of regioselectivity observed between the stoichiometric
and catalytic conditions. In the stoichiometric reaction (Scheme 3),
the displacement step involves disilylzirconacyclopentane inter-
mediate A, which in turn reacts with alkyne to form zirconacycl-
opentene B. In the catalytic reaction, the intermediate C is
involved in the displacement of the ligand to form a mixture of alu-
minacyclopentenes D and E.
In conclusion, we have carried out cross-coupling reactions be-
tween alkynes and trimethylvinylsilane, by using zirconocene
(Cp2Zr) generated by the reduction of Cp2ZrCl2 with lanthanide
metals. The stoichiometric conditions gave coupling products in
good yields and with total regioselectivity. The reaction using a
catalytic amount of Cp2ZrCl2 and AlCl3 as an additive was also suc-
cessful. However, a mixture of regioisomers was formed under
these conditions. On this basis, a plausible catalytic cycle has been
proposed. It is worth noting that this work is a new example of the
use of Cp2Zr as a catalyst.
Acknowledgements
We thank the CNRS and the Ministère de l’Enseignement Supé-
rieur et de la Recherche for their financial support.
References and notes
1. (a) For reviews, see: Titanium and Zirconium in Organic Synthesis; Marrek, I., Ed.;
Wiley-VCH: Weinheim, 2002; (b) Negishi, E. Dalton Trans. 2005, 827–848; (c)
New Aspects of Zirconium Containing Organic Compounds in Topics in
Organometallic Chemistry; Marek, I., Ed.2005; Springer: Berlin; Vol. 10 .
2 Negishi, E.; Cederbaum, F. E.; Takahashi, T. Tetrahedron Lett. 1986, 27, 2829–2832.
3. Earlier works employed Na or Mg as reductants typically in the presence of
alkynes or bipyridine, see for example: (a) Watt, G. W.; Drummond, F. O., Jr. J.
Am. Chem. Soc. 1970, 92, 826–828; (b) Wailes, P. C.; Weigold, H. J. Organomet.
Chem. 1971, 28, 91–95.
10. For a review of the Friedel–Crafts reactions, see: Olah, G. A.; Krishnamurti, R.;
Prakash, G. K. S. Friedel–Crafts alkylations. In Comprehensive Organic Synthesis;
Trost, B. M., Fleming, I., Eds.; Pergamon Press: Oxford, 1991; Vol. 3, p 293.
4. Takahashi, T.; Xi, Z.; Rousset, C. J.; Suzuki, . N. Chem. Lett. 1993, 6, 1001–1004.