Micro-flow synthesis and structural analysis of sterically crowded diimine ligands with five aryl rings

Article abstract of DOI:10.3762/bjoc.9.268

Sterically crowded diimine ligands with five aryl rings were prepared in one step in good yields using a micro-flow technique. X-ray crystallographic analysis revealed the detailed structure of the bulky ligands. The nickel complexes prepared from the lig

Full text of DOI:10.3762/bjoc.9.268

Micro-flow synthesis and structural analysis of
sterically crowded diimine ligands
with five aryl rings
Shinichiro Fuse*1, Nobutake Tanabe1, Akio Tannna2, Yohei Konishi2
and Takashi Takahashi1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 2336–2343.
1Department of Applied Chemistry, Tokyo Institute of Technology,
2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8552, Japan and
2Mitsubishi Chemical Croup, Science and Technology Research
Center, Inc., 1000 Kamoshida-cho, Aoba-ku, Yokohama 227-8502,
Japan
doi:10.3762/bjoc.9.268
Received: 17 July 2013
Accepted: 07 October 2013
Published: 01 November 2013
Email:
This article is part of the Thematic Series "Chemistry in flow systems III".
Guest Editor: A. Kirschning
Shinichiro Fuse* - sfuse@apc.titech.ac.jp
* Corresponding author
© 2013 Fuse et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
amidine formation; diimine; flow chemistry; polymerization
Abstract
Sterically crowded diimine ligands with five aryl rings were prepared in one step in good yields using a micro-flow technique.
X-ray crystallographic analysis revealed the detailed structure of the bulky ligands. The nickel complexes prepared from the ligands
exerted high polymerization activity in the ethylene homopolymerization and copolymerization of ethylene with polar monomers.
Introduction
The design of a ligand is a key step in the development of new tion of a Co complex (Figure 1) [22]. Stephan and coworkers
catalysts because the ligand framework influences the reactiv- reported the synthesis of a bulkier chelating ligand 2a, and its
ity of the metal center. That is why sterically crowded and use in the formation of various metal complexes [20,21]. Rojas
neutral-chelating diimine ligands have garnered a great deal of and coworkers reported the synthesis of a series of bulky
attention [1-17]. In recent years, N-aryl 1,3,5-triazapenta-1,4- chelating ligands 2b–g, and detailed their use in the preparation
dienes 1 and 2 have been reported, and they are useful with late of ethylene polymerization catalysts [18,19]. We became
transition metal olefin-polymerization catalysts [18,19], and for curious about the structure and function of 1,2,3,4,5-pentaaryl-
the stabilization and isolation of reactive metal species [20,21]. 1,3,5-triazapenta-1,4-diene ligand 3, which is sterically more
In 1997, Murillo and coworkers reported the synthesis of a hindered, because there are as many as five aryl rings that can
neutral, and bulky chelating ligand 1, and its use in the forma- provide further opportunities to change and tune the steric and
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Beilstein J. Org. Chem. 2013, 9, 2336–2343.
the previous report, 1,2,3,4,5-pentaphenyl-1,3,5-triazapenta-1,4-
diene ligand 3a was prepared by the reaction of N,N′-diphenyl
benzamidine and N-phenylbenzimidochloride in benzene in 13
days [25]. In the present study, we intended to prepare these
ligands from readily available materials in only one step
[18,21].
Results and Discussion
Two equivalents of N-naphthylbenzimidochloride (4) was
reacted with one equivalent of naphthylamine (5) in CH2Cl2 in
the presence of DIEA (N,N-diisopropylethylamine) at room
temperature (Scheme 1). The reaction proceeded smoothly, and
consumption of naphthylamine was confirmed by TLC analysis
within 10 min. After an aqueous workup, the desired product 3b
was obtained in a moderate yield (44%) with the concomitant
generation of naphthylamine (5) and amide 8. Reportedly, 3a
can form an HCl adduct, and the adduct decomposes to the
corresponding amidine and imidochloride [25]. It is conceiv-
able that 3b overreacted with DIEA·HCl to afford 6, or 4 and 7
in the reaction mixture and that 5 and 8 were generated from the
hydrolysis of these compounds under aqueous workup condi-
tions. We decided to use a micro-flow reactor in order to
suppress the overreaction [26,27] because the micro-flow tech-
nique [28-35] enables the precise control of reaction time and
temperature. The micro-flow system was made from simple and
inexpensive laboratory instruments (syringes, syringe pumps,
Figure 1: Sterically crowded, and neutral chelating diimine ligands.
electrical environments of the ligands [23,24]. However, as far water bath, T-shape mixer, standard tubing and fittings), as
as we could ascertain, this has been reported only once [25]. In shown in Figure 2. The T-shape mixer was made of stainless
this pioneering work, 1,2,3,4,5-pentaphenyl-1,3,5-triazapenta- steel and immersed in a water bath (20 °C). A solution of 4
1,4-diene ligand 3a was prepared, but the report included (0.1 M) in CH2Cl2, a solution of aryl amine 5 or 9 (0.1 M), and
neither the complexation nor a detailed structural study.
DIEA (0.7 M) in CH2Cl2 were introduced using syringe pumps
at the indicated flow rates. The reaction was quenched by
Herein, we report an efficient micro-flow synthesis and struc- pouring the mixture into a saturated aqueous solution of NH4Cl
tural analysis of sterically crowded 1,2,3,4,5-pentaaryl-1,3,5- in CH2Cl2. After an aqueous workup, the products 3b and 3c
triazapenta-1,4-diene ligands 3b and 3c, and their use in the co- were purified by silica gel chromatography. Reaction time was
polymerization of ethylene and polar monomers. According to controlled by changing the flow rates.
Scheme 1: One-step synthesis of the ligand 3b, and a plausible decomposition pathway for 3b to naphthylamine (5) and amide 8.
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Beilstein J. Org. Chem. 2013, 9, 2336–2343.
Figure 2: Micro-flow synthesis of the ligands 3b and 3c.
As expected, the yield of 3b was improved by reducing the ligand 2c where N=C–N–C=N was in a common plane. Interest-
reaction time (Table 1, entries 1–4). The highest yield was ingly, in the case of ligand 3b, the bond lengths of the two
observed for a reaction time of 38 s (Table 1, entry 4). A further amines, C(1)–N(2) and C(2)–N(2) were nearly identical,
shortening of the reaction time resulted in a reduction in the although N=C–N–C=N was not in a common plane.
yield because of substrate recovery (Table 1, entry 5).
Table 1: Micro-flow synthesis of ligands 3b and 3c.
entry flow rate A flow rate B time
Ar–NH2 yielda
[%]
[μL/min]
[μL/min]
[s]
1b
2b
3b
4b
5b
6c
7c
8c
54
27
300
150
75
5
5
5
5
5
9
9
9
65
72
75
84
66
55
69
66
106
214
426
854
214
426
854
53
107
213
427
107
213
427
38
19
50
25
13
aIsolated yield. bReaction tube volume is 400 μL. cReaction tube
volume is 266 μL.
The structure of the ligand 3b was unambiguously determined
by 1H NMR, 13C NMR, IR, HRMS and X-ray crystallographic
analysis [36] (Figure 3). The ORTEP structure of 3b showed
that in the solid state the ligand adopts a non-planar arrange-
ment similar to the previously reported ligands 2a–c, and 2e–g
[18,19,21]. In ligand 3b, N(1) and N(2) nearly occupied a
common plane with C(1) and C(2), while N(3) was twisted out
Figure 3: ORTEP drawing of 3b. Thermal ellipsoids are set at 35%
probability level. Hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and angles [°] are as follows: C(1)–N(1) 1.273(4),
C(1)–N(2) 1.420(4), C(2)–N(2) 1.421(4), C(2)–N(3) 1.278(4),
N(1)–C(1)–N(2) 115.3(3), C(1)–N(2)–C(2) 117.0(3), N(2)–C(2)–N(3)
117.8(3).
of this plane and was nearly perpendicular. The bond lengths The asymmetric ligand 3c was obtained by the coupling of
for imines C(1)–N(1) and C(2)–N(3) were 1.273(4) and N-naphthylbenzimidochloride (4) with aniline (9). The product
1.278(4) Å, respectively, while the bond lengths for amines was obtained in a satisfactory yield (69%) under the conditions
C(1)–N(2) and C(2)–N(2) were 1.420(4) and 1.421(4) Å, res- of entry 7 (25 s), as shown in Table 1. We speculated that the
pectively. Reportedly, the two amine bond lengths were slightly lower yield of 3c compared to 3b came from the insta-
different in the case of ligands, 2a, 2b, 2e, 2f, and 2g for which bility of 3c during purification process. The compound 3c was
N=C–N–C=N was not in a common plane. On the other hand, less stable than 3b. Rojas et al. reported that the regioselec-
the two amine bond lengths were nearly identical in the case of tivity in the nucleophilic addition of an amidine to an
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Beilstein J. Org. Chem. 2013, 9, 2336–2343.
Scheme 2: Synthesis of the ligand 3c through the coupling of 4 and 9.
imidochloride depends on the employed reaction conditions, – bond lengths for imines C(1)–N(1) and C(2)–N(3) were
in particular, the order of addition and the base selection [37] – 1.269(3) and 1.274(3) Å, respectively, while the bond lengths
and the symmetric ligands were obtained through the coupling for amines C(1)–N(2) and C(2)–N(2) were 1.413(3) and
of N-2,6-di(iPr)-phenylbenzimidochloride with aryl amines in 1.411(3) Å, respectively. The bond lengths for amines
the presence of Et3N in toluene [18,19]. Interestingly, in our C(1)–N(2) and C(2)–N(2) were nearly identical, although
case, only the asymmetric ligand 3c was obtained, although N=C–N–C=N was not in a common plane. These features were
similar reaction conditions were employed (Scheme 2). The similar to that of 3b.
result showed that the nucleophilic addition of amidine 10
occurred from the sterically more hindered nitrogen atom
(path b).
A complexation of the synthesized ligands 3b and 3c with
nickel(II) was performed (Scheme 3) in accordance with a
reported procedure [38]. An equimolar amount of NiBr2(dme)
The structure of the asymmetric ligand 3c was unambiguously (dme = 1,2-dimethoxyethane) and the synthesized ligands were
determined by 1H NMR, IR, HRMS and X-ray crystallographic mixed in CH2Cl2 and stirred for 4 h at room temperature. Free
analysis (Figure 4). The ORTEP structure of 3c showed that in ligands 3b or 3c were not observed by 1H NMR analysis of the
the solid state, the ligand adopted a non-planar arrangement obtained crude mixtures. NMR characterization of the
similar to that of 3b. In the ligand 3c, N(1) and N(2) occupied a complexes 12 and 13 was poor due to the paramagnetic nature
plane that was near that of C(1) and C(2), while N(3) was of the pseudo-tetrahedral nickel centers. In the case of 12, green
twisted out of this plane and was almost perpendicular. The needle-like crystals suitable for X-ray crystallographic analysis
were obtained (Figure 5). The molecular structure confirmed
the formation of a six-membered chelate ring by the ligand in a
N,N binding mode to the nickel dibromide. All the aryl rings
were almost perpendicular to the chelate plane probably due to
the strong steric repulsions among the aryl rings. The tetrahe-
dral geometry around nickel was distorted similar to the previ-
ously reported nickel complexes of 2b, 2f and 2g [18,19]. For
example, Br(1) was almost perpendicular to the plane of the
metal-containing ring. This can be observed in the corres-
ponding angle N(1)–Ni(1)–Br(1) 102.2(3)°, while the angle of
N(1)–Ni(1)–Br(2) was 120.9(3)°. The C(1)–N(1), C(2)–N(3),
Figure 4: ORTEP drawing of 3c. Thermal ellipsoids are set at 35%
probability level. Hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and angles [°] are as follows: C(1)–N(1) 1.269(3),
C(1)–N(2) 1.413(3), C(2)–N(2) 1.411(3), C(2)–N(3) 1.274(3),
N(1)–C(1)–N(2) 116.5(2), C(1)–N(2)–C(2) 119.1(2), N(2)–C(2)–N(3)
117.4(2).
Scheme 3: Complexation of the ligands 3b and 3c with NiBr2(dme).
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Beilstein J. Org. Chem. 2013, 9, 2336–2343.
activity of the catalyst 12 in the copolymerization of ethylene
with 5-norbornen-2-ol (NBO) and ethyl acrylate (EtA) (Table 2,
entries 3 and 4) and propene homopolymerization (Table 2,
entry 5). Although the propene homopolymerization activity of
catalyst 12 was moderate, catalyst 12 exerted a high activity in
the copolymerization of ethylene with both polar monomers.
Conclusion
In summary, sterically crowded diimine ligands 3b and 3c were
prepared in one step in good yields using a micro-flow tech-
nique. One of the advantages of using microreactors is the ease
of scale-up. It should be possible to scale-up our developed
process by either continuous running or by increasing the
number of the microreactors. X-ray crystallographic analysis
revealed the detailed structure of ligands 3b and 3c. Interest-
ingly, 3c retained an asymmetric structure which ran contrary to
a previous report. Unexpectedly, both bond lengths of the two
amines C(1)–N(2) and C(2)–N(2) in both ligands were nearly
identical, although N=C–N–C=N was not in a common plane.
The complexation of 3b and 3c with nickel afforded 12 and 13.
X-ray crystallographic analysis of 12 revealed that all the aryl
rings are nearly perpendicular to the chelate plane. The nickel
complex 12 exerted a high polymerization activity in both
ethylene homopolymerization and the copolymerization of
Figure 5: ORTEP drawing of 12. Thermal ellipsoids are set at 35%
probability level. Hydrogen atoms and cocrystallized CH2Cl2 are
omitted for clarity. Selected bond lengths [Å] and angles [°] are as
follows: C(1)–N(1) 1.277(11), C(1)–N(2) 1.393(11), C(2)–N(2)
1.396(11), C(2)–N(3) 1.305(11), Ni(1)–N(1) 1.947(8), Ni(1)–N(3)
1.943(8), N(1)–C(1)–N(2) 122.8(8), C(1)–N(2)–C(2) 126.2(7),
N(2)–C(2)–N(3) 121.2(8), C(2)–N(3)–Ni(1) 127.5(6), N(1)–Ni(1)–N(3)
89.3(3), C(1)–N(1)–Ni(1), 127.7(6), N(1)–Ni(1)–Br(1) 102.2(3),
N(1)–Ni(1)–Br(2) 120.9(3).
C(1)–N(2) and C(2)–N(2) bond lengths were 1.277(11) Å, ethylene with polar monomers. The synthesized ligands 3b and
1.305(11) Å, 1.393(11) Å and 1.396(11) Å respectively, indi- 3c retained as many as five aryl rings, which offered another
cating the double and single bond character around the imines opportunity to change the steric and electric environment. The
and amine nitrogen, respectively.
developed process should be valuable for the preparation of
various 1,2,3,4,5-pentaaryl-1,3,5-triazapenta-1,4-diene ligands
A series of olefin polymerizations were briefly tested as shown and for the creation of novel and useful catalysts.
in Table 2. Both catalysts 12 and 13 exerted high activity for the
Experimental
ethylene homopolymerization (Table 2, entries 1 and 2). As far
as we could ascertain, neither propene homopolymerization nor General
copolymerization of ethylene with polar monomers by using the NMR spectra were recorded on a JEOL Model ECP-400 (400
catalysts derived from N-aryl-1,3,5-triazapenta-1,4-dienes 1–3 MHz for 1H, 100 MHz for 13C) instrument in the indicated
have been reported [18,19]. Thus, we tested the polymerization solvent. Chemical shifts are reported in units of parts per
Table 2: Evaluation of polymerization activities of nickel complexes 12 and 13.a
entry
catalystb
monomerc
temperature, time
[°C, h]
Vp
[kg/mol·h]
1
2
3
4
5
12
13
12
12
12
ethylene
ethylene
60, 0.5
60, 0.5
60, 1
140
110
25
35
5
ethylene/NBO
ethylene/EtA
propene
60, 1
60, 1
aEntries 1–4 were carried out in a 2 L autoclave reactor in 1 L toluene in the presence of 500 equiv of modified methyl aluminoxane (MMAO)[39] as
cocatalyst and 400 psig of ethylene. Entry 5 was carried out in a 2 L autoclave reactor in 750 mL propene in the presence of 500 equiv of MMAO as
the cocatalyst. bEntries 1, 4 and 5 were carried out using 0.11 mmol of the catalyst. Entries 2 and 3 were carried out using 0.10 and 0.09 mmol of the
catalysts, respectively. cEntries 3 and 4 were carried out using 500 equiv of NBO or EtA.
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Beilstein J. Org. Chem. 2013, 9, 2336–2343.
million (ppm) relative to the signal (0.00 ppm) for internal silica gel (0% to 10% Et2O in hexane with 1% Et3N) to give
tetramethylsilane for solutions in CDCl3 (7.26 ppm for 1H, 77.0 1,2,3,4,5-pentaaryl-1,3,5-triazapenta-1,4-diene 3b or 3c.
ppm for 13C). Multiplicities are reported by using the following
abbreviations: s, singlet; d, doublet; t, triplet; q, quartet; m, 1,3,5-Trinaphthyl-2,4-diphenyl-triazapenta-1,4-diene (3b):
multiplet; br, broad; and, J, coupling constants in Hertz. IR 1H NMR (400 MHz, CDCl3) δ 8.22 (brd, J = 5.8 Hz, 1H), 8.15
spectra were recorded on a Perkin-Elmer Spectrum One FTIR (brs, 1H), 7.89 (brd, J = 5.4 Hz, 1H), 7.74–7.90 (m, 2H), 7.73
spectrometer. HRMS (ESI–TOF) was measured with a Waters (brd, J = 6.8 Hz, 2H), 7.58 (brd, J = 7.8 Hz, 1H), 7.10–7.58 (m,
LCT PremierTM XE. All reactions were monitored by thin-layer 20H), 7.07 (t, J = 7.8 Hz, 1H), 7.39 (brd, J = 7.8, 1H), 6.64 (brs,
chromatography carried out on 0.25 mm E. Merck silica gel 1H); 13C NMR (67.8 MHz, CDCl3) δ 156.2, 145.6, 136.1,
plates (60F-254) with UV light, visualized by ceric sulfate solu- 135.2, 134.7, 134.0, 130.1, 129.5, 128.8, 128.3, 128.2, 127.2,
tion. Flash column chromatography was performed on Silica 126.3, 125.9, 125.8, 125.6, 125.2, 125.0, 123.6, 123.5, 123.0,
Gel 60 N, purchased from Kanto Chemical Co. The T-shape 122.5, 121.6, 118.0, 115.7; FTIR (neat) 3055, 1624, 1572,
mixer (Flom Co. Ltd., #9513, 19 mm × 28 mm × 8 mm, diam- 1529, 1495, 1393, 791, 771 cm−1; HRMS (ESI–TOF, m/z): [M
eter 0.6 mm) was made of stainless steel and had a T-shape + H]+ calcd. for C44H32N3, 602.2596; found, 602.2613.
channel. The reaction tube (diameter 0.5 mm) was made of
Teflon®. A Harvard Pump 11 Plus Single Syringe (HARVARD 1,3-Dinaphthyl-2,4,5-triphenyl-triazapenta-1,4-diene (3c):
apparatus), a KDS 100 syringe pump, and a KDS 200 syringe 1H NMR (400 MHz, CDCl3) δ 8.29 (d, J = 6.4 Hz, 2H), 8.25 (d,
pump (KD Scientific) were used to inject compounds into the J = 8.3 Hz, 2H), 7.78 (brd, J = 7.8 Hz, 1H), 7.73 (d, J = 7.8 Hz,
T-shape mixers. The workup process included quenching of the 2H), 7.61 (d, J = 7.8 Hz, 1H), 7.36–7.60 (m, 9H), 7.40–7.20 (m,
reactions, liquid–liquid extraction, washing and drying, and was 2H), 7.16 (t, J = 7.8 Hz, 1H), 7.05–6.60 (m, 7H), 6.54 (brs, 1H),
performed using a Zodiac CCX-1200 (Tokyo Rikakikai Co., 5.71 (brd, J = 6.8, 1H); FTIR (neat) 3056, 1631, 1595, 1524,
Ltd.). Chromatographic separation was performed using a 1497, 1438, 1323, 775, 698 cm−1; HRMS (ESI–TOF, m/z): [M
Purif®-α2 (Shoko Scientific Co., Ltd.).
+ H]+ calcd. for C40H30N3, 552.2440; found, 552.2435.
Experimental details
General procedure for the preparation of nickel
General procedure for the preparation of imidochlorides: complexes 12 and 13
The mixture of N-(1-naphthyl)benzamide and SOCl2 (2 mL/ The following manipulations were performed under an inert
mmol amide) was stirred at 65 °C for 4 h. The reaction mixture atmosphere using standard glove box techniques. A solution of
was concentrated in vacuo. The residue was used for the next the prepared ligand 3b or 3c (1 equiv) and NiBr2(dme) (1
reaction without further purification.
equiv) in dry CH2Cl2 (50 mL/mmol) was stirred for 4 h at room
temperature. The obtained crude mixture was used for the poly-
General procedure for micro-flow synthesis of 1,2,3,4,5- merization without purification.
pentaaryl-1,3,5-triazapenta-1,4-dienes 3b and 3c: A T-shape
mixer and reaction tube were immersed in a water bath (20 °C). Procedure for the preparation of nickel complex 12
Syringe pumps and a mixer were connected using a Teflon® crystals
tube (diameter 0.25 mm). Imidochloride 4 was dried azeotropi- The following manipulations were performed under an inert
cally with toluene. Aryl amine 5 or 9 (0.1 M), DIEA (0.7 M) atmosphere using standard glove box techniques. A solution of
and imidochloride 4 (0.1 M) were dissolved in CH2Cl2 under an the prepared ligand 3b (66 mg, 0.11 mmol) in 40 mL of dry
argon atmosphere and were stored in syringes. Each solution CH2Cl2 was slowly added to NiBr2(dme) (34 mg, 0.11 mmol).
was introduced to a T-shape mixer using the syringe pump. The The resultant mixture was stirred for 1 h at room temperature.
mixed solution went through the reaction tube, and the resul- Then, the stirring was stopped and the mixture was allowed to
tant solution was poured into vigorously stirred saturated stand overnight at room temperature. Green colored needle-like
aqueous NH4Cl (1.5 mL) in CH2Cl2 (2 mL). After being stirred crystals of 12, suitable for X-ray analysis were obtained.
for several minutes, Et2O (30 mL) was added to the reaction
mixture under vigorous stirring for 30 s. The aqueous layer was Procedure for the homopolymerization of ethylene
separated and added to saturated aqueous NaHCO3 (2 mL) after To a 2 L autoclave reactor, 1,000 mL of dry toluene and
being vigorously stirred for 30 s. The aqueous layer was sep- MMAO (500 equiv, 6.5 wt % in toluene) were added. The
arated and brine (2 mL) was added followed by vigorous stir- resultant mixture was heated to 60 °C, then the crude nickel
ring for 30 s. After removing the aqueous layer, the organic complex (1 equiv) was injected under an ethylene pressure of
layers were dried over Na2SO4 (10 g) and concentrated in 400 psig, which was fed continuously at that pressure over the
vacuo. The residue was purified by column chromatography on course of the reaction. After being stirred for 0.5 h, ethanol was
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Beilstein J. Org. Chem. 2013, 9, 2336–2343.
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The resultant mixture was collected, and concentrated in vacuo
to afford a crude polymer. Polymerization activities were calcu-
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polar monomers
To a 2 L autoclave reactor, 1,000 mL of dry toluene and
comonomers (500 equiv) along with MMAO (500 equiv,
6.5 wt % in toluene) were added. The resultant mixture was
heated to 60 °C, then the crude nickel complex (1 equiv) was
injected under an ethylene pressure of 400 psig, which was fed
continuously at that pressure over the course of the reaction.
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To a 2 L autoclave reactor, MMAO (500 equiv, 6.5 wt % in
toluene) and 750 mL of propene were added. Then the crude
nickel complex (1 equiv) was injected with nitrogen. The resul-
tant mixture was heated to 60 °C. After being stirred for 1 h,
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was vented. The resultant mixture was collected, and concen-
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Acknowledgements
The authors thank Dr. Hidehiro Uekusa, Tokyo Institute of
Technology for X-ray crystallographic analysis of intermedi-
ates and Dr. Shigekazu Ito, Tokyo Institute of Technology for
fruitful discussion, and Global COE Program “Education and
Research Center for Emergence of New Molecular Chemistry,”
MEXT, Japan, for financial support.
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