Oxovanadium(V)-induced oxidation of alkenylzirconocenes for facile inter- and intramolecular coupling

Article abstract of DOI:10.1016/S0022-328X(98)00978-4

The oxidation reaction of (E)-1-alkenylchlorozirconocenes with an oxovanadium(V) compound at room temperature led to intermolecular homocoupling, giving the corresponding (E,E)-dienes stereoselectively. (E)-1-Alkenyl-1-alkynylzirconocenes underwent the oxovanadium(V)-induced intramolecular cross-coupling of organic substituents on zirconium, leading to the stereoselective formation of the (E)-enynes.

Full text of DOI:10.1016/S0022-328X(98)00978-4

Journal of Organometallic Chemistry 575 (1999) 76–79
Oxovanadium(V)-induced oxidation of alkenylzirconocenes for facile
inter- and intramolecular coupling
Takuji Ishikawa, Akiya Ogawa, Toshikazu Hirao *
Department of Applied Chemistry, Faculty of Engineering, Osaka Uni6ersity, Yamada-oka, Suita, Osaka 565-0871, Japan
Received 11 August 1998; received in revised form 11 September 1998
Abstract
The oxidation reaction of (E)-1-alkenylchlorozirconocenes with an oxovanadium(V) compound at room temperature led to
intermolecular homocoupling, giving the corresponding (E,E)-dienes stereoselectively. (E)-1-Alkenyl-1-alkynylzirconocenes under-
went the oxovanadium(V)-induced intramolecular cross-coupling of organic substituents on zirconium, leading to the stereoselec-
tive formation of the (E)-enynes. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Oxovanadium; Organozirconocene compound; Hydrozirconation; Oxidative coupling
organic synthesis [5,6]. Thus, we investigated the oxida-
tion of alkenylzirconocene derivatives with oxovanadi-
1. Introduction
um(V) compounds.
Organozirconocene derivatives that can be prepared
easily by hydrozirconation of alkynes and alkenes are
useful intermediates in organic syntheses [1]. For exam-
ple, alkenylzirconocenes are utilized widely as precursor
2. Results and discussion
organometallics in the coupling reactions via
Treatment of 1-alkenylchlorozirconocene 1, produced
transmetallation, as represented by the CuCl-induced
by hydrozirconation of 1-alkyne, with an oxovanadi-
coupling to (E,E)-dienes [1–3]. Besides the transforma-
tions via transmetallation, the reductive elimination via
one-electron oxidation is known to occur with the
electron-poor, zero-valent organozirconocenes [4]. The
um(V) compound at room temperature led to inter-
molecular coupling, giving the corresponding
(E,E)-diene 2 stereoselectively (Eq. (1)). Use of two or
three equivalents of VO(OiPr)2Cl raised the yield of 2
choice of metallic oxidants is a key to achieve such an
(Table 1, entries 1–3). The yield of 2 was lowered by
oxidative transformation, and oxovanadium(V) com-
pounds are promising as candidates for this transfor-
mation, because their properties as Lewis acids with
one-electron oxidation capability make oxovanadi-
um(V) compounds fascinating as selective oxidants in
the use of VO(OEt)Cl2, which is a stronger oxidant
than VO(OiPr)2Cl (entries 4–5). This is most probably
due to further oxidation of 2 by VO(OEt)Cl2, and the
reaction at −78°C improved the yield of 2 dramati-
cally (entry 6). Similarly, several 1-alkynes were con-
verted to the expected 1,3-dienes 2 stereoselectively in
good yields under the same conditions as those carried
out in the entry 3 (three equivalents of VO(OiPr)2Cl,
room temperature, entries 7–9).
* Corresponding author. Tel.: +81-6-8797413; fax: +81-6-
8797415.
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(98)00978-4

T. Ishikawa et al. / Journal of Organometallic Chemistry 575 (1999) 76–79
77
(1)
The substitution of the 1-alkenylchlorozirconocene
present some information. Negishi and his coworkers
1 with 1-alkynyllithium at −78°C, followed by treat-
ment with VO(OiPr)2Cl led to a novel cross-coupling
of organic substituents on zirconium, giving the corre-
sponding trans-enyne 4 stereoselectively (Scheme 1,
Table 2). The use of VO(OEt)Cl2, again, decreased
the yield of 4 from the same reason mentioned above
(entries 2–3). Although stoichiometric or excess
amounts of VO(OiPr)2Cl are required for this trans-
formation, this method was applicable to other com-
reported 1,2-migration in the reaction of Cp2ZrCl2
with three equivalents of lithium acetylide to afford
the enyne and diyne upon treatment with HCl(aq.)
and I2, respectively, via the intermediate 8 (Scheme 3)
[7]. We investigated the similar reaction of 1a with
two equivalents of lithium phenylacetylide, which
gave the diene 7 by treatment with HCl(aq.) (Scheme
2). In contrast to this finding, the equimolar reaction
of 1a with 1-octynyllithium gave only styrene and 1-
octyne after treatment with HCl(aq.) (Scheme 1).
These results suggest the involvement of the different
organozirconium intermediates in each reaction. In
the former case using two equivalents of lithium
phenylacetylide, oxidation with VO(OEt)Cl2 was
found to provide the enyne 4b in good yield, proba-
binations of
1
and 1-alkynyllithiums giving
trans-enynes 4 in moderate to good yields under the
similar conditions as indicated in entry 1 (entries 4–
6).
Although the reaction pathway for this coupling
reaction is ambiguous, the following observations
Table 1
Oxidative homocoupling of aIkenylchlorozirconocene 1
a
Entry
1
R
Oxovanadium
Equivalent
Conditions
2
%
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
1b
1c
1d
Ph
Ph
Ph
Ph
Ph
Ph
VO(OiPr)2Cl
VO(OiPr)2Cl
VO(OiPr)2Cl
VO(OEt)Cl2
VO(OEt)Cl2
VO(OEt)Cl2
VO(OiPr)2Cl
VO(OiPr)2Cl
VO(OiPr)2Cl
1
2
3
1
3
3
3
3
3
r.t., 18 h
r.t., 18 h
r.t., 18 h
r.t., 18 h
r.t., 18 h
−78°C, 1 h
r.t., 18 h
r.t., 18 h
r.t., 18 h
2a
2a
2a
2a
2a
2a
2b
2c
2d
34
52
91
26
42
90
88
80
55
n-C6H13
Me3Si
PhCH2OCH2
a Isolated yield.
Scheme 1.

78
T. Ishikawa et al. / Journal of Organometallic Chemistry 575 (1999) 76–79
bly via the intermediate 6 formed by 1,2-migration
(Scheme 2). However, in the latter case, the interme-
diate 3 is assumed to be involved in the equimolar
reaction of 1a with 1-alkynyllithium, allowing to the
oxidative transformation to 4.
The present reactions widen the scope of organozir-
conium compounds in organic synthesis. One-electron
oxidative transformations of organometallics such as
aluminum [8], boron [9], and zirconium provides a
versatile synthetic tool for the carbon–carbon bond
formation via coupling.
on a Yanagimoto Micromelting Point Apparatus. Iso-
lation of products by GPC was carried out on an
LC-08 of the Japan Analytical Industry. All solvents
were dried and distilled. The benzyl ether derivative
1d was prepared according to the standard procedure
[10]. VO(OEt)Cl2 and VO(OiPr)2Cl were obtained eas-
ily from VOCl3 and the corresponding alcohol, and
then distilled [11].
3.1. General procedure for oxidati6e coupling of
alkenylchlorozirconocene 1
To a stirred solution of Cp2ZrHCl (1.0 mmol, 258
mg) in dry dichloromethane (4 ml) at room tempera-
ture under argon, 1-alkyne (1.0 mmol) was added to
generate the 1-alkenylzirconocene 1. After stirring for
1 h at room temperature, VO(OiPr)2Cl (660 mg, 3.0
mmol) was added dropwise to the resulting solution
over 10 min at 0°C. The mixture was stirred for 18 h
at room temperature, and then ether (15 ml) and 1.5
M aqueous HCl (1 ml) were added to the reaction
mixture. After extraction with ether (3×10 ml), the
combined ethereal solution was washed with saturated
NH4C1, saturated NaHCO3, and brine. The organic
3. Experimental
1H-NMR or 13C-NMR spectra were recorded on a
JEOL JNM-GSX-400 spectrometer (400 MHz) and a
Varian MERCURY 300 spectrometer (300 MHz) in
chloroform-d with tetramethylsilane or residual chlo-
roform as an internal standard. Mass spectra were
recorded on a Varian SATURN3 and JEOL JMS-
DX-303. GC analysis was carried out on a SHI-
MADZU GC-8A. Infrared spectra were recorded on
a Perkin-Elmer 1600. Melting points were determined
Table 2
Oxidative cross-coupling of alkenylalkynylzirconocene 3
a
Entry
R
R%
Oxovanadium
Conditions
%
4
2
5
1
2
3
4
5
6
Ph
Ph
Ph
n-C6H13
n-C6H13
Ph
Ph
Ph
VO(OiPr)2Cl
VO(OEt)Cl2
VO(OEt)Cl2
VO(OiPr)2Cl
VO(OiPr)2Cl
VO(OiPr)2Cl
r.t., 18 h
r.t., 18 h
−78°C, 3 h
r.t., 18 h
r.t., 18 h
r.t., 18 h
64
30
63
62
70
57
15
14
14
17
6
28
27
27
25
22
24
n-C6H13
Me3Si
PhCH2OCH2
Ph
10
a Isolated yield.
Scheme 2.
Scheme 3.

T. Ishikawa et al. / Journal of Organometallic Chemistry 575 (1999) 76–79
79
layer was dried over MgSO4, and concentrated.
Purification by chromatography on a silica-gel column
eluting with hexane–chloroform gave the 1,3-diene 2
as shown in Table 1.
(s, 2H), 4.14 (dt, J=5.5 Hz, J=1.8 Hz, 2H).
13C-NMR (100 MHz, CDCl3) l 139.4, 138.0, 131.5,
128.4, 128.3, 128.2, 127.7, 127.7, 123.2, 111.7, 90.0,
87.4, 72.3, 69.8. Anal. Calc. for C18H16O: C, 87.06;
H, 6.49. Found: C, 87.12; H, 6.70.
2d: Rf=0.20 (hexane–chloroform v/v 1:1). IR
(NaCl) 3028, 2852, 1495, 1453, 1359, 1099, 991, 736,
1
697 cm−1. H-NMR (300 MHz, CDCl3) l 7.38–7.26
(m, 10H), 6.38–6.24 (m, 2H), 5.92–5.74 (m, 2H), 4.53
(s, 4H), 4.08 (d, J=5.7 Hz, 4H). 13C-NMR (75 MHz,
CDCl3) l 138.1, 131.9, 129.8, 128.3, 127.6, 127.5,
72.1, 70.2. Anal. Calc. for C20H22O2: C, 81.60; H,
7.53. Found: C, 81.38; H, 7.27.
Acknowledgements
The use of the facilities of the Analytical Center,
Faculty of Engineering, Osaka University is
acknowledged. This work was partly supported by a
Grant-in-Aid for Scientific Research on Priority Areas
from the Ministry of Education, Science, and Culture,
Japan.
3.2. General procedure for oxidati6e coupling of
alkenylalkynylzirconocene 3
To a solution of Cp2ZrHCl (1.0 mmol, 258 mg) in
dry dichloromethane (4 ml), 1-alkyne (1.0 mmol) was
added at room temperature under argon. After
stirring for 1 h at room temperature, a solution of
1-alkynyllithium (1.0 mmol) in dry ether (3 ml) was
added via cannula to the thus obtained
1-alkenylzirconocene at −78°C. The solution was
warmed to room temperature and stirred for an
additional 1 h. The reaction mixture was added via
cannula to a solution of VO(OiPr)2Cl (660 mg, 3.0
mmol) in dichloromethane (4 ml) at 0°C. After
stirring for another 18 h at room temperature, and
then ether (15 ml) and 1.5 M aqueous HCl (1 ml)
were added to the reaction mixture. After extraction
with ether (3×10 ml), the combined ethereal solution
was washed with saturated NH4C1, saturated
NaHCO3, and brine. The organic layer was dried
over MgSO4, and concentrated. The crude product
was purified by a silica-gel column chromatography
eluting with hexane–chloroform or GPC to give the
trans-enyne 4.
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4d: Rf=0.25 (hexane). IR (NaCl) 2956, 1568, 1489,
1
1249, 974, 840 cm−1. H-NMR (300 MHz, CDCl3) l
7.47–7.42 (m, 2H), 7.35–7.28 (m, 3H), 6.55 (d,
J=19.2 Hz, 1H), 6.18 (d, J=19.2 Hz, 1H), 0.19 (s,
9H). 13C-NMR (100 MHz, CDCl3) l 145.8, 132.7,
131.6, 128.3, 128.2, 123.3, 89.8, 89.6, −1.7. Anal.
Calc. for C13H16Si: C, 77.93; H, 8.05. Found: C,
77.62; H, 8.45.
4e: Rf=0.30 (hexane–chloroform v/v 2:1). IR
(NaCl) 3030, 2846, 1489, 1360, 1115, 953, 756, 690
1
cm−1. H-NMR (300 MHz, CDCl3) l 7.48–7.42 (m,
2H), 7.40–7.28 (m, 8H), 6.33 (dt, J=16.2 Hz, J=5.5
Hz, 1H), 6.02 (dt, J=16.2 Hz, J=1.8 Hz, 1H), 4.57
.