Synthesis of (arylimido)vanadium complexes and their application for oxidative coupling reactions of silyl enol ether derivatives

Article abstract of DOI:10.1039/c0dt00633e

(Arylimido)vanadium(v) complexes, V(NAr)(OEt)Cl₂ [Ar = C ₆H₅, p-BrC₆H₄, p-(MeO)C ₆H₄] or V(NAr)Cl₃ [Ar = o-BrC₆H ₄, o-(MeO)C₆H₄] were synthesized by the reaction of VO(OEt)Cl₂ or VOCl₃, respectively, with the corresponding aryl isocyanates. X-Ray crystal structure determination for V(NAr)(OEt)Cl₂ (Ar = C₆H₅ and p-BrC ₆H₄) elucidates the alkoxido-bridged dimeric building blocks [V(NAr)(OEt)Cl₂]₂, wherein the substituent on the benzene ring was found to affect the character of the imido nitrogen. The ⁵¹V NMR spectroscopic measurements indicated that the electronic state of the vanadium metal centre depends on the ligands connecting to the vanadium and the substituents on the benzene ring. The selective oxidative cross-coupling reaction of silyl ketene acetal with silyl enol ether was achieved using V(Np-BrC₆H₄)Cl₃ or V(No-BrC ₆H₄)Cl₃.

Full text of DOI:10.1039/c0dt00633e

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis of (arylimido)vanadium complexes and their application for
oxidative coupling reactions of silyl enol ether derivatives†
Masafumi Nishina, Toshiyuki Moriuchi* and Toshikazu Hirao*
Received 10th June 2010, Accepted 28th July 2010
DOI: 10.1039/c0dt00633e
(Arylimido)vanadium(V) complexes, V(NAr)(OEt)Cl2 [Ar = C6H5, p-BrC6H4, p-(MeO)C6H4] or
V(NAr)Cl3 [Ar = o-BrC6H4, o-(MeO)C6H4] were synthesized by the reaction of VO(OEt)Cl2 or VOCl3,
respectively, with the corresponding aryl isocyanates. X-Ray crystal structure determination for
V(NAr)(OEt)Cl2 (Ar = C6H5 and p-BrC6H4) elucidates the alkoxido-bridged dimeric building blocks
[V(NAr)(OEt)Cl2]2, wherein the substituent on the benzene ring was found to affect the character of the
imido nitrogen. The 51V NMR spectroscopic measurements indicated that the electronic state of the
vanadium metal centre depends on the ligands connecting to the vanadium and the substituents on the
benzene ring. The selective oxidative cross-coupling reaction of silyl ketene acetal with silyl enol ether
was achieved using V(Np-BrC6H4)Cl3 or V(No-BrC6H4)Cl3.
Table 1 Synthesis and 51V NMR spectra of (arylimido)vanadium(V)
Introduction
complexes 1 and 2
Oxo and imido ligands are recognized to stabilize highly oxidized
transition metals through extensive ligand-to-metal p-donation.1
(Imido)vanadium(V) complexes have attracted much attention2
because of their potential application as catalysts for olefin
polymerization,3 C–H activation4 and related reactions.5 Ligands
connecting to the vanadium metal centre have been reported
to influence the reactivity.3c,3e,3h,3i Compared with oxo ligands,
imido ligands can possess a substituent on the imido nitrogen
so that the steric and electronic characters of the metal centre
are considered to be controlled by the properties of the nitrogen
substituent. In such a sense, the design of (arylimido)vanadium(V)
Complex 1 or 2
R1
R2
Yield/%
51V NMR/ppm
complexes is one of key factors in the development of efficient
reagents. In previous papers, the p-substituent on the benzene
ring of (arylimido)vanadium(V) complexes was demonstrated to
affect the nature of the vanadium metal centre and imido bond
through p-conjugation.6 On the other hand, oxovanadium(V)
alkoxydichloride is allowed to induce the oxidative coupling
reaction of silyl enol ethers.7 We herein report the structural
characterization of (arylimido)vanadium(V) complexes and their
application as a one-electron oxidant in the oxidative coupling
reaction of silyl enol ether derivatives.
1aa
1ba
1ca
2ab
2bc
2cb
2da
2ea
MeO
H
Br
MeO
H
Br
H
H
H
H
H
H
MeO
Br
89
82
85
88
73
75
89
85
-4
-136
-146
429
260
263
385
258
H
H
a This work. b Ref. 2c. c Ref. 2d.
Complexes 1 and 2 were characterized by1H NMR spectra. The
51V NMR spectroscopic measurements clarified the substituent
effect on the electronic environment of the vanadium metal centre.
Compared with the 51V chemical shifts of 2, the shifts of 1 were
observed at a higher field with the electron donation from the
ethoxido group (Table 1). The 51V chemical shifts of 1 were
observed at a lower field with an increase in the electron-donating
capability of the substituent on the benzene ring. In the 51V NMR
spectra of the d0 diamagnetic vanadium complexes, the vanadium
metal centre becomes increasingly shielded as the electronegativity
of the imido ligand increases.2c The chemical shifts indicate that the
vanadium metal centre of a (p-methoxyphenylimido)vanadium(V)
complex is more electron-rich than that of non-substituted or
bromo substituted complexes. The p-conjugation between the aryl
group and the imido bond is considered to control the electronic
Results and discussion
The reaction of VO(OEt)Cl2 or VOCl3 with substituted aryl iso-
cyanates in octane at reflux temperature was performed to give the
corresponding (p-substituted phenylimido)vanadium(V) ethoxy-
dichlorides [V(Np-R1C6H4)(OEt)Cl2 (1a: R1 = MeO, 1b: R1 = H,
1c: R1 = Br) and (p- or o-substituted phenylimido)vanadium(V)
trichlorides [V(Np-R1C6H4)Cl3 (2a: R1 = MeO,2c 2b: R1 = H,2d
2c: R1 = Br2c) and V(No-R2H4)Cl3 (2d: R2 = MeO, 2e: R2 = Br)].
Department of Applied Chemistry, Graduate School of Engineering, Osaka
University, Yamada-oka, Suita, Osaka 565-0871, Japan. E-mail: moriuchi@
chem.eng.osaka-u.ac.jp, hirao@chem.eng.osaka-u.ac.jp; Fax: +81 6 6879
7415; Tel: +81 6 6879 7413
† CCDC reference numbers 774799 and 774800. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c0dt00633e
9936 | Dalton Trans., 2010, 39, 9936–9940
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Table 2 Crystallographic data for 1b and 1c
is coordinated in a trigonal-bipyramidal geometry, bridging to
the second vanadium atom via the ethoxido group in an apical
position (Fig. 1b). The V(1)–N(1) distance of 1.6554(14) A and
1b
1c
˚
the nearly linear C(1)–N(1)–V(1) angle of 173.46(13)◦ suggest the
participation of an sp-hybridized character in the imido nitrogen.
The ethoxido-bridged dimeric building blocks were also formed
in the case of 1c, bearing the bromo group on the benzene ring
(Fig. 1d). No notable difference was observed with the V(1)–Cl(1),
V(1)–Cl(2), C(1)–N(1) and V(1)–V(1*) lengths (Table 3). A nearly
linear V(1)–N(1)–C(1) angle was also observed with 1c. However,
as compared with 1b, the imido angle is almost 4◦ bent. This
result indicates that there is less contribution of an sp-hybridized
character from the imido nitrogen of 1c. This is probably due to the
conjugation of the electron-withdrawing group, the bromo group
might weaken the sp-hybridized character. Such a relationship
between the imido bond character and the substituent effect was
also reported with V(NAr)(Oi-Pr)3.6c
Formula
C8H10O2N1Cl2V1
274.02
Orthorhombic
Pbca (no. 61)
11.6418(14)
7.8118(8)
C8H9O1N1Br1Cl2V1
336.92
Monoclinic
P21/n (no. 14)
7.6595(10)
11.7903(14)
13.1806(19)
Formula weight
Crystal system
Space group
˚
a/A
˚
b/A
˚
c/A
24.194(3)
a/◦
b/◦
g /◦
101.426(4)
3
˚
V/A
2200.2(4)
8
1166.7(3)
4
Z
Dc/g cm-3
m(Mo-Ka)/cm-1
T/◦C
1.558
13.465
-100
0.71075
0.0331
0.0781
1.918
47.082
-100
0.71075
0.0484
0.1275
1
1
˚
l(Mo-Ka)/A
a
R1
b
wR2
To evaluate the effect of the aryl substituents and the ligands
binding to the vanadium on the reactivity of the (arylim-
ido)vanadium(V) complexes, the oxidative homo-coupling reac-
tion of silyl ketene acetal 3 was examined, as shown in Scheme 1
and Table 4. By using V(Np-(MeO)C6H4)(Oi-Pr)3,6b the homo-
coupled product 47d was not produced (entry 1). On the other
hand, with the (arylimido)vanadium(V) ethoxydichlorides 1, the
homo-coupled product 4 was obtained. Interestingly, probably
due to the electron-withdrawing properties of the bromo group,
a R1 = RꢀFo| - |Fcꢀ/R|Fo|, b wR2 = [Rw(Fo - Fc2)2/Rw(Fo2)2]1/2
.
2
properties of the vanadium metal centre. The 51V NMR spectra of
2 indicate that the electron-donating substituent on the benzene
ring is related to the shielding effect of the imido nitrogen.
(p-Tolyl)imidovanadium(V) trichloride has been reported to
be present in chlorido-bridged polymeric building blocks,8 al-
though (arylimido)vanadium(V) triisopropoxide2i,3g,6b and (arylim-
ido)vanadium(V) dialkoxychloride2c form an alkoxido-bridged
packing structure. On the contrary, the assembled structure of
(arylimido)vanadium(V) alkoxydichloride has not been reported,
although these complexes have already been synthesized.2c,3f
To clarify the association properties of (arylimido)vanadium(V)
alkoxydichloride, the single-crystal X-ray structure determina-
tions of 1b and 1c were studied (Table 2).
As observed in the crystal structures of the dialkoxide2c and
trialkoxide complexes,2i,3g,6b V(NC6H5)(OEt)Cl2 (1b) was found
to form dimeric building blocks, in which each vanadium atom
Scheme 1 The homo-coupling reaction of 3, using 1 or 2.
Fig. 1 (a) The crystal structure of 1b, (b) the crystal structure of 1b arising from dimeric building blocks, (c) the crystal structure of 1c and (d) the crystal
structure of 1c arising from dimeric building blocks (hydrogen atoms are omitted for clarity).
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Table 3 Selected bond lengths and angles for 1b and 1c
The redox process of the vanadium species for this coupling
reaction was investigated. After the reaction of entry 7, doublet
peaks of the aromatic ring in H NMR and a broad peak in
1b
1c
1
the 51V NMR disappeared. EPR spectra at room temperature in
acetonitrile solution revealed a resonance at g = 1.990 with the
characteristic octet pattern [Aiso(51V) = 107 ¥ 10-4 cm-1] expected
for the interaction of an unpaired electron of VIV with the 51V
nucleus (I = /2). These findings indicate that the vanadium(V)
species is reduced to vanadium(IV) through a one-electron redox
˚
Bond lengths/A
V(1)–N(1)
V(1)–Cl(1)
V(1)–Cl(2)
1.6554(14)
2.2266(5)
2.2151(5)
1.8187(12)
2.2019(12)
1.381(2)
1.661(3)
2.2249(12)
2.2232(10)
1.814(2)
2.203(2)
1.374(4)
1.896(3)
3.2200(9)
V(1)–O(1)
7
V(1)–O(1a)
C(1)–N(1)
C(4)–Br(1)
process.
V(1)–V(1a)
3.2225(4)
In the homo-coupling reaction of silyl enol ether 5 with 2c
or 2e, which are reactive oxidants towards 3, the homo-coupled
product was not obtained, indicating that 5 is less reactive than 3.
This difference in the reactivity was applied to the selective cross-
coupling reaction of silyl enol ether 5 and silyl ketene acetal 3 to
give 1,4-keto ester 67c (Scheme 2). Treatment of 3 with an equal
amount of 5 in the presence of 2c or 2e led to the predominant
formation of the cross-coupled keto ester 6, with a small amount
of the homo-coupled by-product 4 (Table 5, entries 1 and 2). The
yield using 2e was 15% lower than that using 2c. It should be
noted that the reaction is more selective with 200 mol% of 5 and
the formation of the by-product 4 was completely suppressed by
using 2c.
Bond angles/◦
C(1)–N(1)–V(1)
N(1)–V(1)–Cl(1)
N(1)–V(1)–Cl(2)
N(1)–V(1)–O(1)
N(1)–V(1)–O(1a)
O(1)–V(1)–Cl(1)
O(1)a–V(1)–Cl(1)
O(1)–V(1)–Cl(2)
O(1)a–V(1)–Cl(2)
O(1)–V(1)–O(1)a
Cl(1)a–V(1)–Cl(2)
V(1)–O(1)–V(1a)
173.46(13)
96.33(5)
97.29(5)
99.91(6)
173.66(6)
118.12(4)
86.00(3)
121.21(4)
87.00(3)
78.84(4)
115.12(2)
106.16(5)
169.5(2)
98.89(12)
95.17(10)
100.95(13)
173.90(14)
115.05(8)
86.33(7)
122.55(9)
85.25(7)
73.84(10)
116.21(4)
106.16(11)
a 1b; -x + 1, -y + 1, -z + 1, 1c; -x + 1, -y, -z.
In summary, (arylimido)vanadium(V) ethoxydichloride
V(NAr)(OEt)Cl2 [Ar
= C6H5 (1b), p-BrC6H4 (1c)] was
Table 4 The homo-coupling reaction of 3, using 1 or 2
demonstrated to form dimeric building blocks, wherein the
substituent on the benzene ring was found to affect the character
of the imido nitrogen. Ligands connecting to the vanadium metal
centre and the substituents on the benzene ring were elucidated
to affect the electronic properties of the vanadium nuclei and
control the reactivity. These findings permitted the selective
oxidative coupling reaction of silyl ketene acetal and silyl enol
ether to provide a useful synthetic protocol. The application of
(arylimido)vanadium(V) complexes to other oxidation reactions
is currently under way.
Entry
Complex
Yield of 4/%a
1
2
3
4
5
6
7
8
9
V(Np-(MeO)C6H4)(Oi-Pr)3
No reaction
8
6
18
63
73
75
65
79
1a
1b
1c
2a
2b
2c
2d
2e
a Determined by 1H NMR using durene as an internal standard.
the use of 1c resulted in an approximately 10% higher yield than
that when using 1a or 1b (entries 2–4). From the above-mentioned
insufficient reactivity of 1, (arylimido)vanadium(V) trichloride 2
was employed. The yield of 4 was raised to 63% by use of V(Np-
(MeO)C6H4)Cl3 (2a). The oxidative homo-coupling reaction of
3 was found to depend largely on the ligands connecting to
the vanadium. Then, the position and electronic properties of
the substituent on the benzene ring were changed to study the
substituent effect. As shown in entries 5 and 8 or entries 7 and 9,
the position of the substituent was not found to affect the reactivity
of the complex. On the other hand, as shown in the reactivity of
1, the reaction was dependent on the electronic character of the
substituent. Use of (bromophenylimido)vanadium(V) trichlorides
2c and 2e resulted in higher yields than those resulting from the use
of non-substituted or methoxido-substituted complexes (entries
5–9). The reactivity of the (arylimido)vanadium(V) complexes
was found to be changed dramatically by the ligands binding
to the vanadium metal centre and tuned finely by the electronic
properties of the substituent on the benzene ring. These findings
are consistent with the above-mentioned chemical shifts of 51V
NMR (Table 1).
Scheme 2 The cross-coupling reaction of 3 with 5, using 2c or 2e.
Table 5 The cross-coupling reaction of 3 with 5, using 2c or 2e
Yield/%a
Entry
Complex 2
Amount of 5 used/mol%
6
4
1
2
3
4
2c
2e
2c
2e
100
100
200
200
74
59
84
80
17
18
0
trace
a Determined by 1H NMR using durene as an internal standard.
9938 | Dalton Trans., 2010, 39, 9936–9940
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A representative procedure for the oxidative homo-coupling
reaction of 3
Experimental
General procedures and instrumentation
The silyl ketene acetal 3 (20.2 mg, 0.10 mmol) in dichloromethane
(500 mL) was added dropwise to a suspension of the (arylim-
ido)vanadium(V) complex (0.10 mmol) in dichloromethane
(500 mL) at◦ -85 ◦C. The suspension was stirred at -85 ◦C for
3 h, at -15 C for 3 h and then the solution was stirred at room
temperature for 1 h. 5% aqueous Na2S2O3 (0.5 mL) and ether
(3 mL) were added to the reaction mixture, which was extracted
with ether (6 mL ¥ 3). The combined ethereal solution was washed
with saturated aqueous NaHCO3 (1.5 mL) and saturated aqueous
NaCl (1.5 mL), dried over MgSO4 and concentrated in vacuo. The
yield was determined by using durene (6.7 mg, 0.05 mmol) as an
internal standard.
All manipulations were carried out under an atmosphere of
nitrogen in a drybox or using standard Schlenk techniques. All
reagents and solvents were purchased from commercial sources
and were further purified by the standard methods, if necessary.
The solvents employed were dried by refluxing in the presence
of appropriate drying reagents, distilled under an atmosphere of
argon, de-gassed and stored in a drybox. The complexes V(Np-
(MeO)C6H4)Cl3 (2a),2b V(NC6H5)Cl3 (2b)2d and V(Np-BrC6H4)Cl3
(2c),2b the silyl ketene acetal 39 and the silyl enol ether 510 were
obtained as reported. 1H NMR spectra were recorded on a JEOL
JNM-ECP 400 (400 MHz) or aVarian MERCURY300(300 MHz)
spectrometer. The chemical shifts were referenced to the residual
resonances of deuterated solvents. 51V NMR spectra were obtained
with a JEOL JNM-ECP 400 (105 MHz) spectrometer with VOCl3
as an external standard.
A representative procedure for the oxidative cross-coupling reaction
of 3 with 5
The silyl ketene acetal 3 (20.2 mg, 0.10 mmol) dissolved in
dichloromethane (0.5 mL) was added dropwise to a suspension
of the (arylimido)vanadium(V) complex (0.20 mmol) and the silyl
enol ether 5 (19.2 mg, 0.10 mmol) in dichloromethane (1.5 mL)
at -85 ◦C. The suspension was stirred at -85 ◦C for 3 h and at
-15 ◦C for 3 h. 5% aqueous Na2S2O3 (1 mL) and ether (6 mL)
were added to the reaction mixture, which was extracted with
ether (6 mL ¥ 3). The combined ethereal solution was washed
with saturated aqueous NaHCO3 (1.5 mL) and saturated aqueous
NaCl (1.5 mL), dried over MgSO4 and concentrated in vacuo. The
yield was determined by using durene (13.4 mg, 0.1 mmol) as an
internal standard.
Synthesis of (arylimido)vanadium(V) ethoxydichloride
[V(Np-(MeO)C6H4)(OEt)Cl2 (1a), V(NC6H5)(OEt)Cl2 (1b),
V(Np-BrC6H4)(OEt)Cl2 (1c)]
A mixture of the isocyanate (3.0 mmol) and oxovanadium(V)
ethoxydichloride (3.0 mmol) was stirred in octane (30 mL) under
an atmosphere of nitrogen at reflux temperature for 16 h. Solvent
was evaporated under reduced pressure and the thus-obtained
compound was washed with hexane. Saturated hexane solution
was chilled at -30 ◦C to give the desired crystal.
1
1a: Isolated yield 89%, H NMR (300 MHz, CD2Cl2) d 7.50
(d, J = 7.8 Hz, 2H), 6.81 (d, J = 7.8 Hz, 2H), 5.56 (br, 2H),
3.86 (s, 3H), 1.65 (br, 3H), 51V NMR (105 MHz, CD2Cl2)
-4 ppm.
X-Ray structure analysis
1
1b: Isolated yield 89%, H NMR (300 MHz, CD2Cl2) d 7.26–
All measurements for 1b and 1c were made on a Rigaku RAXIS-
RAPID Imaging Plate diffractometer with graphite monochro-
mated Mo-Ka radiation. The structures of 1b and 1c† were solved
by direct methods and expanded using Fourier techniques. The
non-hydrogen atoms were refined anisotropically. The H atoms
were placed in idealized positions and allowed to ride with the
C atoms to which each was bonded. Crystallographic details are
summarized in Table 2. Selected bond lengths and bond angles are
listed in Table 3.
7.46 (multi, 5H), 5.57 (q, J = 6.9 Hz, 2H), 1.62 (t, J = 6.9 Hz, 3H),
51V NMR (105 MHz, CD2Cl2) -136 ppm.
1c: Isolated yield 85%, 1H NMR (300 MHz, CD2Cl2) d 7.50 (d,
J = 8.4 Hz, 2H), 7.29 (d, J = 8.4 Hz, 2H), 5.53 (q, J = 6.9 Hz,
2H), 1.58 (t, J = 6.9 Hz, 3H), 51V NMR (105 MHz, CD2Cl2)
-146 ppm.
Synthesis of (o-substituted phenylimido)vanadium(V) trichloride
[V(No-(MeO)C6H4)Cl3 (2d), V(No-BrC6H4)Cl3 (2e)]
EPR measurement
A mixture of the isocyanate (3.0 mmol) and oxovanadium(V)
trichloride (3.0 mmol) was stirred in octane (30 mL) under an
atmosphere of nitrogen at reflux temperature for 16 h. Solvent
was evaporated under reduced pressure and the thus-obtained
compound was washed with hexane. The desired compound was
collected by filtration and dried in vacuo.
The silyl ketene acetal 3 (20.2 mg, 0.10 mmol) in dichloromethane
(500 mL) was added dropwise to a suspension of the imidovana-
dium(V) complex 2c (32.7 mg, 0.10 mmol) in dichloromethane
(500 mL) at -85 ◦C. The suspension was stirred at -85 ◦C for 3 h,
◦
at -15 C for 3 h and at room temperature for 1 h and then was
1
2d: Isolated yield 89%, H NMR (300 MHz, CD2Cl2) d 7.53
taken into a drybox. The suspension was concentratedin vacuo and
was washed with hexane. The thus-obtained dark compound was
re-precipitated with ether at -30 ◦C and dissolved in acetonitrile
(10-3 M). EPR spectrum was recorded on a JEOL JES-RE1XE
spectrometer under irradiation of a high-pressure mercury lamp
(USH-1005D) by focusing at the sample cell in the EPR cavity
at 298 K. The magnitude of modulation was chosen to optimize
the resolution and signal-to-noise (S/N) ratio of the observed
(d, J = 8.1 Hz, 1H), 7.28 (dd, J = 8.1, 7.5 Hz 1H), 7.00 (d, J =
7.8 Hz, 1H), 6.92 (dd, J = 7.8, 7.5 Hz, 1H), 4.03 (s, 3H), 51V NMR
(105 MHz, CD2Cl2) 385 ppm.
1
2e: NMR yield 85%, H NMR (300 MHz, CD2Cl2) d 7.74 (d,
J = 7.8 Hz, 1H), 6.64 (d, J = 7.5 Hz, 1H), 7.43 (dd, J = 7.8, 7.5 Hz,
1H), 7.20 (dd, J = 7.8, 7.8 Hz, 1H), 51V NMR (105 MHz, CD2Cl2)
258 ppm
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g value was calibrated using a Mn2+ marker.
Acknowledgements
M.N. acknowledges a JSPS fellowship for young scientists and
expresses special thanks for the Global COE (center of excel-
lence) program ‘Global Education and Research Center for Bio-
Environmental Chemistry’ of Osaka University. We thank Dr
T. Suenobu and Prof. S. Fukuzumi at Osaka University for the
measurement of the EPR spectra. Thanks are due to the use of
the 51V NMR from the Analytical Center, Graduate School of
Engineering, Osaka University and the donation of VO(Oi-Pr)3
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9940 | Dalton Trans., 2010, 39, 9936–9940
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