Catalytic performance of tantalum-η2-alkyne complexes [TaCl3(R1C≡CR2)L2] for alkyne cyclotrimerization

Article abstract of DOI:10.1246/bcsj.77.1009

Structurally characterized tantalum-η²-alkyne complexes [TaCl₃(η²-EtC=CEt)L₂] (1, L₂ = 1,2-dimethoxyethane (DME); 2, L = py) acted as catalysts for the cyclotrimerization of terminal alkynes. The catalytic reaction proceeded at 25 °C within few hours and the trisubstituted benzenes were obtained without the formation of linear oligomers. A new tantalum complex having a terminal alkyne ligand, [TaCl₃(η²-Me₃SiC≡ CH)(dme)l (3), was prepared, and its catalytic performance was also investigated.

Full text of DOI:10.1246/bcsj.77.1009

ꢀ
2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 1009–1011 (2004) 1009
2
Catalytic Performance of Tantalum–ꢀ -Alkyne Complexes
1
2
[
TaCl (R CꢀCR )L ] for Alkyne Cyclotrimerization
3
2
ꢀ
ꢀ
Toshiyuki Oshiki, Hiroshi Nomoto, Kouji Tanaka, and Kazuhiko Takai
Department of Applied Chemistry, Faculty of Engineering, Okayama University, Tsushima, Okayama 700-8530
Received November 5, 2003; E-mail: oshiki@cc.okayama-u.ac.jp
2
2
Structurally characterized tantalum–ꢀ -alkyne complexes [TaCl3(ꢀ -EtCꢁCEt)L2] (1, L2 = 1,2-dimethoxyethane
(
at 25 C within few hours and the trisubstituted benzenes were obtained without the formation of linear oligomers. A
DME); 2, L = py) acted as catalysts for the cyclotrimerization of terminal alkynes. The catalytic reaction proceeded
ꢂ
2
new tantalum complex having a terminal alkyne ligand, [TaCl3(ꢀ -Me3SiCꢁCH)(dme)] (3), was prepared, and its cata-
lytic performance was also investigated.
The transition metal-catalyzed cyclotrimerization of alkynes
characterized; its catalytic behavior toward trimethylsilylacety-
lene is also presented.
is one of the most attractive methods and atom economical
processes for the construction of aromatic rings.1 Recent re-
search in this field has been focused mainly on the regioselec-
–8
9
tive formation of substituted aromatic compounds and the de-
besides
improvements of conventional late transition metal cata-
velopment of new early transition metal catalysts1
0–14
15–17
2
lysts.
An ꢀ -alkyne complex of an early transition metal
is of potential utility as a cyclotrimerization catalyst, because
the cyclotrimerization proceeds via the insertion of an alkyne
into a metal–alkyne bond of the alkyne complex. However,
ð1Þ
2
the isolated ꢀ -alkyne complexes of group 5 metals, except
2
0
for Wigley’s [(DIPP)3Ta(ꢀ -RCꢁCR )] (DIPP = 2,6-diisopro-
18,19
pylphenoxide),
are known to be inert toward the insertion
20,21
of alkynes, which is the first step of cyclotrimerization.
2
In
fact, catalytically active group 5 ꢀ -alkyne complexes have
been prepared in situ in many previous studies.1
0,22–29
In
995, we reported that the stoichiometric cyclotrimerization
1
proceeded when the treatment of terminal alkynes with low-
Results and Discussion
valent tantalum complexes generated in situ (Eq. 1).30 Recent-
ly, we isolated and determined the structures of several tanta-
Tantalum–3-hexyne Complexes 1 and 2 Catalyzed
Cyclotrimerization. The catalytic behavior of the structurally
characterized 3-hexyne complex 131 toward the cyclotrimeriza-
tion of 1-hexyne was investigated. The reaction of 1-hexyne
catalyzed by 1 mol% of 1 proceeded in toluene at 25 C for
18 h to give 1,2,4-tributylbenzene and 1,3,5-tributylbenzene
in almost quantitative yield with 72:28 isomeric ratio
2
lum–ꢀ -internal alkyne complexes with the general formula,
2
0
[
TaCl3(ꢀ -RCꢁCR )(dme)], which were postulated as the in-
ꢂ
31
termediates of cyclotrimerization. In this paper we report
2
on the catalytic behavior of [TaCl3(ꢀ -EtCꢁCEt)(dme)] (1)
2
and [TaCl3(ꢀ -EtCꢁCEt)(py)2] (2) as single-component termi-
(Table 1, run 1). No linear oligomer was formed, and a trace
amount of dibutyldiethylbenzene was detected, which was de-
nal alkyne cyclotrimerization catalysts (Scheme 1). In addition,
a new tantalum complex having a terminal alkyne ligand,
2
rived from 1-hexyne and coordinated 3-hexyne. The [2 + 2 +
,22
[
TaCl3(ꢀ -Me3SiCꢁCH)(dme)] (3), has been synthesized and
2
] cycloaddition is known to be exothermic.4 Actually, upon
the addition of 1-hexyne to a solution of 1 in toluene, the sol-
vent toluene began to boil. The reaction pathway of the cyclo-
5
trimerization can be explained by a ‘common mechanism’ in-
volving the formation of tantalacyclopentadiene and tantalum–
2
ꢀ
tio (A:B = 72:28) of the cyclotrimerization of 1-hexyne is stat-
-terminal alkyne complexes. Therefore, the regioisomeric ra-
istical, which are explained by a random insertion of 1-hexyne
2
into the tantalum–ꢀ -hexyne bond of 1 due to the less bulky
ligand.
At low catalyst concentrations (S/C = 1000), the reaction
Scheme 1.
Published on the web May 6, 2004; DOI 10.1246/bcsj.77.1009

1
2
1
010 Bull. Chem. Soc. Jpn., 77, No. 5 (2004)
[TaCl3(R CꢁCR )L2] for Cyclotrimerization
2
Table 1. Tantalum–ꢀ -Alkyne Complex Catalyzed Cyclotrimerization of Terminal Alkynes (RCꢁCH)
Run
Cat.
R
S/C
Solvent
Time/h
Yield/%
A + B
Isomer ratio
A/B
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
3
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
n-Bu
Ph
Me3Si
HOCH2
n-Bu
Me3Si
100
1000
100
100
100
100
100
100
100
100
100
100
toluene
toluene
hexane
CH2Cl2
CH3CN
DME
18
23
1.5
1.5
1.5
1.5
1.5
20
20
7
99
25
99
88
trace
0
72/28
74/26
76/24
68/32
—
—
—
58/42
52/48
—
THF
0
toluene
hexane
toluene
toluene
hexane
93
99
0
56
99
1
1
1
0
1
2
1.5
20
44/56
52/48
stopped at 25% conversion is probably due to the deactivation
of the catalyst caused by the contamination of a trace amount of
oxygen (run 2). As the solvent, hexane gave the best results, and
the reaction was completed within 1.5 h (>99% conversion),
whereas dichloromethane led to incomplete conversion (run
signals of Me3SiCꢁCH appeared at a remarkable lower field of
ꢁ 15.15 compared to that of the reported Pedersen’s niobium
analogue.26 In the C{ H} NMR spectral data, the two signals
13
1
of alkyne carbon were observed at ꢁ 239.1 and 255.3, which are
2
comparable to those reported for tantalum–ꢀ -alkyne com-
Moreover, the IR absorption band at 1556 cm
plexes.2
1,32,33
ꢃ1
4
). The cyclotrimerization was suppressed by the donor sol-
2
vents, such as acetonitrile, DME, and THF (runs 5–7). In the
case of acetonitrile, pyridine derivatives derived from acetoni-
trile and 1-hexyne were not detected. Phenylacetylene and tri-
methylsilylacetylene were also trimerized with 1 in almost
quantitative yields (runs 8 and 9). Complex 1 did not catalyze
the cyclotrimerization of 2-propargyl alcohol, due to the de-
composition of the catalyst by the hydroxy group (run 10).
An internal alkyne, such as 1-phenylpropyne and 3-hexyne,
did not react with 1 at all. The catalytic reaction using bis(pyr-
idine) catalyst 2 was sluggish (run 11), which is presumably
due to a restraint of coordination of 1-hexyne to the tantalum
center by the strongly coordinated pyridine ligands. Although
the isomer ratio is low, 1,3,5-tributylbenzene was obtained as
a major isomer.
supports the ꢀ -coordination mode of the alkyne fragment.
Thus, the alkyne ligand donates four electrons to the tantalum
center, and the canonical structure of the tantalum–alkyne unit
can be best described as a tantalacyclopropene.3 The inequi-
valent methyl resonances of the coordinated DME ligand
(ꢁ 3.32 and 3.68 in H NMR and ꢁ 62.6 and 68.2 in
C{ H} NMR) indicate that one of the oxygen occupies the
trans position to the alkyne ligand.36 We conclude that the co-
ordination environment around the tantalum of 1c is essentially
the same as those observed for Pedersen’s [NbX3(ꢀ -alkyne)-
(dme)] (X = Cl, Br) and our crystallographically character-
ized [TaCl3(ꢀ -alkyne)(dme)]. The observation of the meth-
ylene protons of the DME as a singlet peak (ꢁ 3.10) accords
4,35
1
1
3
1
2
26
2
31
2
26
with that reported in the case of [NbBr3(ꢀ -EtCꢁCEt)(dme)].
The reaction of 3 with 100 equiv of Me3SiCꢁCH proceeded
smoothly to give tris(trimethylsilyl)benzenes in quantitative
yield without the formation of any organic by-products, such
2
ð2Þ
as linear oligomer (run 12). Thus, the isolated ꢀ -terminal al-
kyne complex 3 also catalyzed the cyclotrimerization, and co-
ordinated Me3SiCꢁCH was completely incorporated into the
cyclotrimers. The same isomer ratio as in the case of 1 (run
9) indicates that the cyclotrimerization by 3 probably proceed-
ed via the same intermediate in 1 catalyzed cyclotrimerization.
2
Synthesis and Characterization of Tantalum–ꢀ -Termi-
nal Alkyne Complex 3 and Its Catalytic Performance. Al-
2
though an tantalum–ꢀ -terminal alkyne intermediate would
2
certainly be formed in the catalytic cycle, isolated ꢀ -terminal
alkyne complexes are still unknown. We chose trimethylsilyl-
Experimental
All manipulations involving air- and moisture-sensitive com-
pounds were carried out using standard Schlenk techniques under
argon. 1,2-Dimethoxyethane (DME), THF, and toluene were pur-
chased from Wako Pure Chemical Industries, Ltd. and stored on
acetylene (Me3SiCꢁCH) as a sterically bulky terminal alkyne,
2
and tried to isolate the tantalum–ꢀ -terminal alkyne complex.
The treatment of the low-valent tantalum with Me3SiCꢁCH
gave 3 in 22% isolated yield as a brown crystalline solid. Com-
2
4
A molecular sieves under argon. Tantalum(V) chloride was pur-
2
plex 3 is the first example of a tantalum complex having an ꢀ -
chased from Nacalai Tesque, Inc. tantalum–ꢀ -alkyne complexes
terminal alkyne ligand. The thermally stable 3 could be stored
at room temperature under argon for one year without decom-
position. Complex 3 is also stable in organic solvents in the ab-
2
2
[
TaCl3(ꢀ -EtCꢁCEt)(dme)] (1) and [TaCl3(ꢀ -EtCꢁCEt)(py)2]
(
2) were prepared according to our previously described proce-
dure.31 1H and 13C{ H} NMR spectra were measured on a JEOL
1
2
1
sence of oxygen and water. The ꢀ -coordination of the alkyne
and coordination environment around the tantalum was con-
firmed by NMR and IR spectroscopies. The terminal hydrogen
JNM-LA400 spectrometer. All H NMR chemical shifts were re-
ported in ppm relative to the protio impurity resonance as follows:
CDCl3, singlet at 7.26 ppm; C6D6, singlet at 7.20 ppm. IR spectra

T. Oshiki et al.
Bull. Chem. Soc. Jpn., 77, No. 5 (2004) 1011
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´
´
were recorded using Nicolet PROTEGE 460-T. The melting point
was measured in a sealed tube on a Yanaco MP-S3 apparatus and
was uncorrected.
Preparation of [TaCl3(Me3SiCꢀCH)(dme)] (3). Toluene (12
mL) was added to TaCl5 (594 mg, 1.66 mmol) in a 80 mL Schlenk
tube, and then DME (12 mL) was slowly added to the resulting yel-
low suspension. Zn powder (163 mg, 2.49 mmol) was added to the
mixture in one portion at room temperature. After stirring the mix-
ture at room temperature for 60 min, low-valent tantalum was pre-
pared in almost quantitative yield. Trimethylsilylacetylene (235
ꢂ
mL, 1.66 mmol) was added to the suspension and stirred at 25 C
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brown powder was extracted with toluene (15 mL). Hexane (20
mL) was added as a layer to the solution, and placed in a ꢃ20
ꢂ
C freezer. Upon standing overnight, brown crystals were deposit-
ed. Removal of the supernatant by a syringe afforded 174 mg (0.37
ꢂ
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ꢃ1
(
(
ꢂ
CꢁC), 309 (ꢂTa{Cl) cm . 1H NMR (C6D6) ꢁ 0.61 (s, 9H,
CH3)3Si–), 3.10 (s, 4H, –OCH2CH2O–), 3.32 (s, 3H, –OCH3),
1
3
1
3
ꢁ
.68 (s, 3H, –OCH3), 15.15 (s, 1H, HCꢁ). C{ H} NMR (C6D6)
0.3 ((CH3)3Si–), 62.6 (–OCH3), 68.2 (–OCH3), 70.9
(
(
–OCH2CH2O–), 75.4 (–OCH2CH2O–), 239.1 (ꢁCSi), 255.3
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26 J. B. Hartung and S. F. Pedersen, Organometallics, 9, 1414
(1990).
HCꢁ).
Catalytic Cyclotrimerization of Terminal Alkynes. A typi-
cal procedure is described for the trimerization of 1-hexyne cata-
lyzed by 1. To a yellow solution of 1 (34.8 mg, 7.57 mmol) in tol-
uene (5 mL) was added 1-hexyne (0.87 mL, 7.57 mmol), which
ꢂ
was stirred at 25 C for 18 h. After the reaction, the reaction mix-
ture expose to the air and a small amount of silica gel was added.
The mixture was passed through a short column on silica gel (hex-
ane as an eluent) to remove any inorganic products, and all vola-
tiles were removed in vacuo to give tributylbenzenes (620 mg,
2
.52 mmol). The isomer ratio of the products was analyzed by
gas chromatography (Quadrex 007 OV-17 (50 m ꢄ 0.32 mm))
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