Generation of low-valent alkoxy niobium from Nb(OEt)5 and Grignard reagents and their use as catalysts in the cyclotrimerization of isocyanates

Article abstract of DOI:10.1016/j.jorganchem.2013.05.035

A new highly active low-valent alkoxyniobium species has been developed following the reaction of Nb(OEt)5 with Grignard reagent such as i-PrMgCl or EtMgCl, with the material behaving as an efficient catalyst for cyclotrimerization reaction of isocyanates. In this reagent system, the existence of a lowvalent niobium species was confirmed by the formation of the niobiumealkyne complex prepared from the Nb(OEt)5/Grignard reagent system and an alkyne. Furthermore, the hydrolysis and diallylation reactions of the niobiumealkyne complex provided further confirmation of its existence, with corresponding (Z)-alkene and diallylated products being isolated in good yields.

Full text of DOI:10.1016/j.jorganchem.2013.05.035

Journal of Organometallic Chemistry 741-742 (2013) 109e113
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Generation of low-valent alkoxy niobium from Nb(OEt) and Grignard
5
reagents and their use as catalysts in the cyclotrimerization of
isocyanates
*
Makoto Ozaki, Yasushi Obora , Yusuke Tada, Yasutaka Ishii
Department of Chemistry and Materials Engineering, Faculty of Chemistry, Materials and Bioengineering, Kansai University, Suita, Osaka 564-8680, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 April 2013
Received in revised form
A new highly active low-valent alkoxyniobium species has been developed following the reaction of
Nb(OEt) with Grignard reagent such as i-PrMgCl or EtMgCl, with the material behaving as an efficient
5
catalyst for cyclotrimerization reaction of isocyanates. In this reagent system, the existence of a low-
valent niobium species was confirmed by the formation of the niobiumealkyne complex prepared
21 May 2013
Accepted 25 May 2013
5
from the Nb(OEt) /Grignard reagent system and an alkyne. Furthermore, the hydrolysis and diallylation
reactions of the niobiumealkyne complex provided further confirmation of its existence, with corre-
sponding (Z)-alkene and diallylated products being isolated in good yields.
Ó 2013 Elsevier B.V. All rights reserved.
Keywords:
Low-valent niobium
Alkoxyniobium
Catalytic reaction
Cyclotrimerization
Isocyanate
1. Introduction
aryl iodides [5g], In this particular cross-coupling reaction, the use
3
of a low-valent niobiumealkyne (i.e. NbCl ealkyne) was found to
Low-valent transition metals have recently been the subject of
considerable levels of interest as reagents for use in organic syn-
thesis. Low-valent early transition metals in particular, such as Ti
be inefficient as a reagent for the purposes of the desired reaction,
and the cross-coupling reaction was found to proceed with a
greater degree of efficiency following the addition of 3 equiv of i-
(
II) [1], Zr (II) [2], Ta(III) [3], and Nb(III) [4] have been reported to
PrOLi to the NbCl
3
ealkyne complex, which led to the formation of a
behave as reducing reagents in the reactions of unsaturated com-
pounds such as alkynes with electrophiles. Furthermore, low-
valent niobium complexes, such as NbCl (DME) have been used
3
as thermally stable early-low-valent metal reagents for a variety of
other transformations as well as catalysis.
Nb(i-OPr) ealkyne complex as a coupling partner. This result
3
indicated that a low-valent niobium complex bearing an alkoxy
substituent could be used as an efficient alternative to the standard
catalysts and reagents used in this particular field of organic syn-
thesis. The development of an efficient method for the preparation
of low-valent alkoxyniobium compounds from the corresponding
and readily available penta-alkoxy niobium compounds would
therefore be highly desirable for further research toward the used
of low-valent metals in organic synthesis. It is noteworthy that the
penta-alkoxy niobium compounds are widely used as precursor
materials for preparation of thin films using the metal-organic
chemical vapor deposition (MOCVD) technique [6]. In addition,
several examples of low-valent aryloxyniobium complexes such as
aryloxyniobiumeolefin [7a,b], aryloxyniobiumediene [7c], and
aryloxyniobiumeimine [7d] are known.
We recently reported several low-valent niobium complex
mediated reactions, including the NbCl
of 1H-indenes from aliphatic ketones and aryl-substituted alkynes
5a], and the NbCl
(DME)-catalyzed [2 þ 2 þ 2] cycloaddition re-
actions of terminal alkyne compounds to a variety of different
olefinic compounds, including simple alkenes [5b], -dienes [5c],
and internal alkynes [5d]. In the latter case, the active niobium
3
(DME)-mediated synthesis
[
3
a,u
species generated from the NbCl
cycloaddition reactions [5e].
5
/HeSi system catalyzed the
We have also reported the development of the nickel-catalyzed
cross-coupling reaction of niobium(III)ealkyne complexes with
Isocyanurates are a group of versatile compounds that can be used
to enhance the physical properties of a wide variety of polyurethanes
and coating materials [8]. Polymers bearing isocyanurate groups
show increased levels of thermal resistance, flame retardation, and
chemical resistance, as well as film-forming characteristics [9]. For
*
Corresponding author. Tel.: þ81 6 6368 0876; fax: þ81 6 6339 4026.
E-mail address: obora@kansai-u.ac.jp (Y. Obora).
0
022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.05.035

110
M. Ozaki et al. / Journal of Organometallic Chemistry 741-742 (2013) 109e113
example, triaryl isocyanurates are often used as activators for the
polymerization and postpolymerization of 3-caprolactam in the
production of nylons with high melt viscosities [10]. Triallyl iso-
cyanurate has been used in the preparation of flame-retardant
laminating materials for electrical devices as well as in the prepara-
tion of copolymer resins that are water-resistant, transparent, and
impact-resistant [11].
Table 1
Optimization of the Nb-catalyzed cyclotrimerization reaction of n-hexyl isocyanate
a
(1a).
Traditionally, isocyanurates have been synthesized via the
cyclotrimerization of the corresponding isocyanates. A number of
reports have been published concerning this particular method of
construction using a variety of different Lewis-basic catalysts,
including N-heterocyclic carbenes [12], carbamate anions [13],
amines [14], fluoride anions [15], p-toluenesulfinate [16], cyanates
Entry
Catalyst precursor
Reducing agent
Conv. (%)
Yield of 2a (%)b
1
2
3
4
5
6
7
8
9
1
1
1
1
1
Nb(OEt)
Nb(OEt)
None
5
5
EtMgCl
None
> 99
77
> 99
> 99
< 1
> 99
10
87
> 99
> 99
> 99
> 99
> 99
> 99
89(83)
Trace
21
[17], alkoxyalkenes [18], and phosphines [19]. Several other reports
have also been published concerning the transition metal catalyzed
cyclotrimerization reactions of isocyanates, including the use of
copper and nickel halides [20], palladium (0) [21], organozinc ha-
lides [22], and zirconium compounds [23] as the catalytic species.
EtMgCl
EtMgCl
EtMgCl
EtMgCl
None
Nb(OPh)
NbCl
Ta(OEt)
NbCl (DME)
NbCl (DME)
5
63.
5
Trace
36
5
c
3
n.d.
LiOEtd
i-PrMgCl
n-BuLi
Trace
65
92
88
89
5
Herein, we wish to disclose the development of an Nb(OEt) /
3
EtMgCl and i-PrMgCl reagent system for the generation of a low-
valent alkoxyniobium species, and the subsequent evaluation of
its high level of activity in the catalytic cyclotrimerization of
isocyanates.
Nb(OEt)
Nb(OEt)
Nb(OEt)
Nb(OEt)
Nb(OEt)
Nb(OEt)
5
5
5
5
5
5
0
1
2
3
4
LiAlH
4
e
f
EtMgCl
EtMgCl
EtMgCl
46
73
g
2
. Results and discussion
a
ꢀ
Conditions: 1a (2 mmol) was stirred with the catalyst in solvent (2 mL) at 60 C
for 2 h.
b
To begin with, we conducted the cyclotrimerization reaction of
an isocyanate in the presence a Nb(OEt) /Grignard reagent catalytic
5
Determined by GC. The number in the parentheses indicates the isolated yield.
Ethyl hexylcarbamate was detected in <5% yield by GC as by-product.
c
Not detected by GC.
LiOEt (0.6 mmol) was used.
Solvent was Toluene (2 mL).
Reaction was conducted at room temperature.
system. To develop a deeper understanding of the reaction system,
and ultimately to optimize the reaction conditions, n-hexyl isocy-
anate (1a) was selected as a model substrate for the transformation.
The results of our initial investigation are shown in Table 1. When
the cyclotrimerization reaction of the isocyanate was performed in
d
e
f
g
5
Reactionwasperformed inthe presenceof Nb(OEt) (5 mol%)and EtMgCl(5mol%).
the presence of Nb(OEt)
5
(10 mol%) and EtMgCl (10 mol%) in THF
ꢀ
(
2 mL) at 60 C for 2 h under an atmosphere of Ar, trihexyl iso-
cyanurate (2a) was produced in 83% yield (Table 1, entry 1). The
yield of the product was considerably reduced when either the
niobium(V) ethoxide or Grignard reagent was removed (Table 1,
entries 2 and 3), demonstrating the importance of both materials to
the success of the reaction.
We then proceeded to investigate a variety of different catalysts.
5 5 5
When Nb(OPh) , NbCl and Ta(OEt) (Table 1, entries 4, 5, and 6)
were used as the catalysts, the desired product 2a was obtained in
low yields. It is noteworthy that the conventional low-valent
Isocyanates bearing alkyl substituents such as the n-hexyl (1a)
and benzyl (1b) groups reacted well to afford the corresponding
cyclotrimerization compounds in 83% (2a) and 62% (2b) yields,
respectively (Table 2, entries 1 and 2). Aromatic isocyanates were
also well tolerated as substrates under the optimized reaction
conditions. For example, when phenyl isocyanate (1c) was used as
the substrate, triphenyl isocyanurate (2c) was obtained in good
yield (Table 2, entry 3). When other aromatic isocyanate bearing
methyl and fluoro substituents at the para-position of their phenyl
ring were subjected to the optimized conditions, they gave the
corresponding isocyanurates in moderate yields (Table 2, entries 4
and 5). Unfortunately, however, when t-butyl isocyanate (1f) was
used as the substrate, the corresponding isocyanurate (2f) was not
detected by gas chromatographic (GC) analysis of the reaction
mixture (Table 2, entry 6).
3
niobium complex NbCl (DME), which generally exhibits good
levels of catalytic activity in related transformations, did not show
any catalytic activity toward the present reaction (Table 1, entries
7
e8).
The introduction of a variety of different additives was also
investigated, with EtMgCl being identified as the most effective.
Several other reducing agents, however, such as i-PrMgCl, n-BuLi,
Urabe and Sato et al. [1j,k] previously reported that the use of
and LiAlH
4
, provided similar levels of activity (Table 1, entry 1 and
Ti(OiPr)
4
in conjunction with i-PrMgCl led to the formation of the
entries 9e11).
corresponding low-valent alkoxytitanium species. Based on these
results, we hypothesized that the low-valent niobium species
generated from the Nb(OEt) /Grignard reagent system was the
5
catalytically active species in this reaction. To obtain some experi-
mental evidence to support the existence of a low-valent niobium
species as the active catalytic species in this cyclotrimerization
reaction of isocyanates, we conducted a complexation reaction with
an alkyne and carried out a subsequent experiment to verify the
formation of the hypothesized low-valent metal species in this
system (Scheme 1).
Different solvents were also investigated for the reaction, with
THF provided comparable yield of 2a to toluene (Table 1, entries 1
and 12). This reaction was also found to provide the desired product
at ambient temperature, albeit in relatively low yield (Table 1, entry
13). A reduction in the amount of the catalyst present in the reaction
from 10 to 5 mol % was found to be well tolerated, with the product
2
a being isolated in a reduced yield of 73% (Table 1, entry 14).
With the optimized conditions in hand, we proceeded to
investigate the substrate scope of the reaction. The results are
shown in Table 2. Under the optimized reaction conditions, a va-
riety of different isocyanates (1) were converted to the corre-
sponding cyclotrimerization products in moderate to high yields.
The hydrolysis of 4-octyne (3) (0.7 mmol) was conducted in the
presence of Nb(OEt)
(2 mL) at 60 C for 2 h, and subsequently quenched via the addition
5
(2.5 mmol) and i-PrMgCl (2.5 mmol) in THF
ꢀ

M. Ozaki et al. / Journal of Organometallic Chemistry 741-742 (2013) 109e113
111
Table 2
a
Nb-catalyzed cyclotrimerization reactions of a variety of different isocyanates.
[Nb]
(OEt)n
Nb
n
n
R
N
Pr
Pr
THF (2 mL)
Nb(OEt) (10 mol%)
5
O
R
O
R
o
EtMgCl (10 mol%)
60 C, 2 h
3
n
n
R
N C O
Pr
Pr
N
N
0.7 mmol
THF (2 mL)
o
A
6
0 C, 2 h
O
2
1
Br (2.8 mmol) 5
n
n
Pr
Pr
Entry
1
ReNCO
nHexeN]C]O
Product (2)
yield (%)b
6
1a
83
[Nb]
Yield
71%
41%
n.d.
i
Nb(OEt) (2.5 mmol) / PrMgCl (2.5 mmol)
5
N C O
Nb(OEt) (2.5 mmol) / EtMgCl (2.5 mmol)
2
1b
62
5
NbCl (DME) (2.5 mmol)
3
Scheme 2. Allylation of 4-octyne.
3
4
5
1c
71
63
N C O
use of EtMgCl which showed a higher level of catalytic activity in
the cyclotrimerization of isocyanates, gave lower yield of 4.
Although a detailed reason and experimental evidence of this
outcome is not confirmed, this might due to the formation of un-
stable niobium species such as niobium hydride during the reaction
course [24].
1d
N C O
1e
1f
50
F
N C O
6
tBueN]C]O
n.d.c
For the generation of the low-valent alkoxytitanium compounds
a
ꢀ
described above, the addition of 2 equiv of a reducing agent such as
Conditions: 1a (2 mmol) was stirred with the catalyst in solvent (2 mL) at 60
C
i
for 2 h.
a Grignard reagent to Ti(O Pr)
4
was required to allow the reaction to
b
c
Isolated yields.
Not detected by GC.
proceed [1j,k]. Interestingly, the current method only required the
addition of 1 equiv of Grignard reagent to react with the Nb(OEt)
5
to maintain the low-valent metal species. In addition, to evaluate
the reactivity of the niobiumealkyne complex, a reaction was
conducted between the alkyne and a suitable electrophile in the
2
of water to provide (Z)-4-octene (or D -4-octene) (4) in 91% yield by
GC analysis. This result indicated that the alkoxyniobiumealkyne
complex A was formed in this reaction from the low-valent
niobium species, which was derived itself from the reaction of
presence of the Nb(OEt)
The allylation reaction of 4-octyne (3) with allyl bromide pro-
ceed smoothly in the presence of the Nb(OEt) /Grignard reagent to
afford the diallyl product 5 in 71% yield. The use of EtMgCl afforded
5 in 41% yield. In contrast, the allylation compound was not ob-
5
/Grignard reagent system (Scheme 2).
5
13
Nb(OEt)
5
with i-PrMgCl. In addition, the C NMR spectrum of the
low-valent niobiumealkyne complex A revealed the presence of
alkyne carbon peaks assignable to the Nbealkyne (4-octyne)
compound at 216.9 ppm (Fig. S1 in the Supplementary materials),
tained when NbCl
result indicated that the Nbealkyne species generated from the
reaction of Nb(OEt) and the Grignard reagent with the alkyne
showed a higher level of activity than the conventional Nbealkyne
species generated from the reaction of NbCl (DME) with an alkyne.
3
(DME) was used under these conditions. This
13
which was comparable to the C NMR peak reported for the related
alkyne carbon of the NbCl ealkyne (4-octyne) complex at
50.9 ppm (Fig. S2 in Supplementary materials) [5f]. In contrast, the
5
3
2
3
n
n
Pr
Pr
3
(OEt)n
Nb
0
.7 mmol
Nb(OEt)5
.5 mmol
+
RMgCl
THF (2 mL)
n
n
Pr
Pr
2
2.5 mmol
o
6
0 C, 2 h
A
(
D) H
H (D)
H O (D O)
2
2
n
n
Pr
Pr
(> 98%D incorporated)
4
R
(GC yield)
91%
i
Pr
Et
55%
Scheme 1. Hydrolysis of 4-octyne.

112
M. Ozaki et al. / Journal of Organometallic Chemistry 741-742 (2013) 109e113
3
. Conclusions
the Kansai University Research Grants (Grant-in Aid for Encour-
agement of Scientists, 2012), and METI.
5
In conclusion, we have developed a Nb(OEt) /Grignard reagent
system as a new highly active low-valent alkoxyniobium species.
This material behaved as an efficient catalyst for the cyclo-
trimerization of a variety of different isocyanates as well as a good
reagent for the allylation reaction of alkynes.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.05.035.
4
. Experimental section
References
4
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Nb(OEt) (64 mg, 0.2 mmol) in THF (2 mL), and the resulting
5
mixture was stirred at room temperature for 0.5 h under an at-
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4.3. Hydrolysis of 4-octyne
(
1
[
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5
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(
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to a mixture of Nb(OEt) (800 mg, 2.5 mmol) and THF (2 mL), and the
5
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[
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6
2
0
C under an atmosphere of Ar. Allyl bromide (5) (339 mg,
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mixture was then quenched via the addition of a 10% (v/v) HCl solu-
tion. Thesolventwasthenremovedinvacuotogivethecrudeproduct,
which was purified by silica gel column chromatography using n-
hexane as the eluent to give compound 6 (95 mg) in 71% yield.
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