Highly Efficient and 1,2-Regioselective Method for the Oligomerization of 1-Hexene Promoted by Zirconium Precatalysts with [OSSO]-Type Bis(phenolate) Ligands

Article abstract of DOI:10.1021/acs.organomet.8b00411

A mixture of zirconium complex 7, which carries a phenyl-substituted [OSSO]-type bis(phenolate) ligand, and dried modified methylaluminoxane (dMMAO) catalyzes the 1,2-regioselective oligomerization of 1-hexene at relatively low catalyst loadings (0.0056 mol %) to produce the corresponding vinylidene-terminated dimer, 5-methyleneundecane (74-80%), and trimer, 7-butyl-5-methylenetridecane (8-11%). The observed turnover frequencies (TOFs) are relatively high (up to 11a€?100 h^-1). When a mixture of 2,6-dimethylphenyl-substituted precatalyst 8 and dMMAO was used, the oligomerization of 1-hexene proceeded effectively to afford predominantly the dimer (87-91%) together with a small amount of the trimer (8-11%) at remarkably high TOFs (up to 6640 h^-1).

Full text of DOI:10.1021/acs.organomet.8b00411

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Highly Efficient and 1,2-Regioselective Method for the
Oligomerization of 1‑Hexene Promoted by Zirconium Precatalysts
with [OSSO]-Type Bis(phenolate) Ligands
Norio Nakata,* Kazuaki Nakamura, and Akihiko Ishii*
Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama
3
38-8570, Japan
*
S Supporting Information
ABSTRACT: A mixture of zirconium complex 7, which carries a
phenyl-substituted [OSSO]-type bis(phenolate) ligand, and dried
modified methylaluminoxane (dMMAO) catalyzes the 1,2-regioselective
oligomerization of 1-hexene at relatively low catalyst loadings (0.0056
mol %) to produce the corresponding vinylidene-terminated dimer, 5-
methyleneundecane (74−80%), and trimer, 7-butyl-5-methylenetride-
cane (8−11%). The observed turnover frequencies (TOFs) are relatively
−
1
high (up to 11 100 h ). When a mixture of 2,6-dimethylphenyl-
substituted precatalyst 8 and dMMAO was used, the oligomerization of
1
-hexene proceeded effectively to afford predominantly the dimer (87−
9
1%) together with a small amount of the trimer (8−11%) at remarkably
−
1
high TOFs (up to 6640 h ).
−
1
INTRODUCTION
highest TOF (1945 h ), and the conversion and dimerization
■
10
selectivity reached 100 and 94%, respectively. Nonmetallo-
cene complexes of zirconium (3) and hafnium (4) supported
by 1,4-dithiabutane-bridged [OSSO]-type bis(phenolate)
ligands, which were developed by Okuda et al., can act as
precatalysts in this reaction. Upon activation with B(C F ) ,
precatalysts 3 and 4 selectively produce vinylidene-terminated
oligo(1-hexene)s (93−95%), albeit at lower TOFs (196 h for
; 3.8 h for 4) compared to those of the 1/MAO system.
Very recently, Abu-Omar et al. presented that B(C F ) -
activated complexes of zirconium (5) and hafnium (6) that
bear [ONNO]-type amine bis(phenolate) ligands catalyze the
oligomerization of 1-hexene to produce exclusively vinylidene-
terminated dimers in up to 97% yield with relatively high
TOFs (430 h for 5; 25 h for 6) at 85 °C. However, these
catalyst systems suffer from low activity even at high
temperature, and their use for the industrial production of
renewable fuels is not feasible.
Recently, our group reported several zirconium complexes
that contain an [OSSO]-type bis(phenolate) ligand based on a
trans-1,2-cyclooctanediyl core. These complexes serve as
precise polymerization catalysts for α-olefins and yield
Over the past two decades, metal-catalyzed α-olefin oligome-
rization has emerged as an important process to generate
1
,2
detergents, synthetic fuels, plasticizers, and lubricants. In this
context, regioselectivity is an important aspect that influences
the composition of the resulting oligomers. For example, the
dimerization of 1-hexene via a metal−hydride intermediate can
produce 10 constitutional and geometric isomers that include
vinylidenes and vinylenes when 1,2-insertion or 2,1-insertion
11
6
5 3
−
1
−
1
3
3
6
5 3
followed by β-H elimination is combined. Of particular
interest is the regioselective oligomerization of 1-hexene, as the
corresponding vinylidene-terminated dimers and trimers such
as 5-methyleneundecane and 7-butyl-5-methylenetridecane
serve as suitable precursors for renewable C8−C22 platforms
−
1
−1
12
4
for jet and diesel fuels. Although extensive research efforts
have been devoted to developing new catalyst systems that
improve the 2,1-regioselectivity in the oligomerization of 1-
hexene for the production of vinylene-terminated oligo(1-
5
hexene)s, only very few catalyst systems have been reported
that insert 1-hexene 1,2-regioselectively to produce vinylidene-
terminated dimers as the major product. In their pioneering
study, Slaugh and Schoenthal discovered that the (η -
6
,7
5
13,14
excellent isotactic poly(α-olefin)s.
results and the desire to further explore reactions catalyzed by
OSSO]-type complexes, we would like to report herein the
high performance of dried-modified methylaluminoxane-
Motivated by these
C H ) ZrCl (1)/Me Al system with an Al/Zr ratio of 4:1
5
5 2
2
3
realizes the dimerization of 1-hexene under complete 1,2-
[
regioselectivity at turnover frequencies (TOFs) of 189−960
−
1
8
h
to afford 5-methyleneundecane in up to 95% yield. Some
15
activated (dMMAO) zirconium precatalysts 7 and 8, which
substituted zirconocenes and ansa-type complexes also
oligomerize 1-hexene in the presence of a minimal excess of
4d,e,9
MAO to yield 5-methyleneundecane as the main product.
Among these, 1,3-siloxo-bridged zirconocene 2 showed the
Received: June 14, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00411
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Chart 1. Previously Reported Precatalysts for the 1,2-
Regioselective Oligomerization of 1-Hexene
integration ratio of vinylidene peaks suggested an oligomeric
distribution of 78% of dimer, 9% of trimer, and 13% of other
oligomers. The GC-MS analysis of the obtained oligomers
revealed the following distribution: 76% of dimer, 11% of
trimer, 6% of tetramer, 4% of pentamer, and 3% of hexamer
(
Figure 1). This oligomeric distribution was in relatively good
1
3
agreement with that calculated based on the C NMR data.
When the oligomerization temperature was increased to 40 °C,
the TOF and conversion values increased to 7560 h− and
1
4
1
2%, respectively (Table 1, run 2). The neat oligomerization of
-hexene with 7/dMMAO was successful at 25 or 40 °C, and
the dimer was obtained as the main product (74−78%) under
complete 1,2-regioselectivity, and increased TOF (7510 h at
−
1
possess [OSSO]-type ancillary ligands with ortho, para-
diphenylphenol or ortho-{2,6-dimethylphenyl (Dmp)}-para-
methylphenol substituents, respectively, in the 1,2-regioselec-
tive oligomerization of 1-hexene using relatively low catalyst
−1
2
5 °C; 8580 h at 40 °C) and conversion values (42% at 25
°
C; 48% at 40 °C) were observed (Table 1, runs 3 and 4).
However, a full conversion of 1-hexene was not achieved, not
even upon increasing the catalyst loading (0.056 mol %) and/
or extending the reaction time (24 h) (Table 1, run 5). Thus, it
seems that the reaction does not reach completion due to the
deactivation of the Zr−H-active species during the oligome-
1
6
loadings.
RESULTS AND DISCUSSION
■
3
The oligomerization of 1-hexene catalyzed by dMMAO-
activated zirconium complexes 7 and 8 was conducted under
varying reaction conditions (Scheme 1), and the results are
rization. To investigate the effect of different loadings of the
cocatalyst, we subsequently screened varying amounts of
dMMAO in the oligomerization of 1-hexene (Table 1, runs 6−
8
). Reducing the amount of dMMAO to 100, 150, and 200
Scheme 1. Oligomerization of 1-Hexene Using Zirconium
Precatalysts 7 and 8
equiv led to the formation of vinylidene-terminated oligomers
that contain high contents of dimers (74−80%) under
significant improvement of the conversion and TOF. Especially
noteworthy is the use of 150 equiv of dMMAO, which afforded
−
1
the highest TOF (11 100 h ) in this study (Table 1, run 7).
The neat oligomerization of 1-hexene (3.0 g, 35.6 mmol)
using precatalyst 8 (0.002 mmol, 0.0056 mol %) and different
loadings of dMMAO (150−300 equiv) at 25 °C for 1 h also
efficiently furnished vinylidene-terminated oligomers in 11−
20% conversion (Table 1, runs 9−12). In contrast to the
aforementioned 7/dMMAO system, reducing the amount of
dMMAO affects the TOF of this oligomerization. Thus, when
precatalyst 8 was activated with 250 equiv of dMMAO, the
oligomerization of 1-hexene showed the highest TOF (3,530
−
1
h ) for the 8/dMMAO system (Table 1, run 11). Despite the
sterically more congested environment at the zirconium center
−
1
of 8, the corresponding TOFs (1,950−3,530 h ) are still
higher than those of the previously reported precatalysts 1−
8
−12
summarized in Table 1. Upon activation with 250 equiv of
dMMAO (0.50 mmol), phenyl-substituted precatalyst 7 (0.002
mmol, 0.0056 mol %) promoted the oligomerization of 1-
hexene (3.0 g, 35.6 mmol) in toluene to yield 1.00 g
6.
Similar results were observed for oligomerizations
carried out at 40 °C, which produced oligo(1-hexene)s in
excellent 1,2-regioselectivity with slightly increased TOFs
−
1
13
(3,700 h ) (Table 1, run 13). The C NMR and GC-MS
analyses revealed that the thus obtained oligomers possess
three different chain lengths (cf. SI), i.e., there is a strong
preference for the formation of the dimer (85−91%) relative to
the trimer and tetramer. Interestingly, the oligomerization of 1-
hexene can be carried out even with a low catalyst loading
(0.0019 mol %) under retention of a remarkably high TOF
(
2
conversion = 33%) of a product of low volatility after 1 h at
5 °C (Table 1, run 1). The corresponding TOF (5950 h ) is
−
1
approximately 6 times higher than that of Et Al-activated
3
zirconocene 1 (0.063 mol %, conversion = 58.6%, 40 °C),
which was reported by Slaugh and Schoenthal (TOF < 960
−
1
8
h ). The microstructure of the produced oligomers was
1
13
1
−1
determined by H and C NMR spectroscopy. In the H
NMR spectrum (see Figure S1 in the Supporting Information),
a sharp signal due to the vinylidene-terminated dimer was
observed at 4.68 ppm. Other much smaller signals appeared at
(6,640 h ) (Table 1, run 14). The resulting oligomers exhibit
an exceptional selectivity toward vinylidene termini (>99%)
and show a dimer distribution (89%) similar to that in run 12,
indicating that the catalyst loading does not influence the
regioselectivity and the formation ratio of the dimer.
4
.68 (overlapping with the aforementioned signal) and 4.73
ppm, which are consistent with those of isolated 7-butyl-5-
The resulting dimer-enriched oligo(1-hexene)s represent a
promising potential precursor for the alternative jet diesel fuel
5-methylundecane. Hydrogenation of the oligo(1-hexene)s
obtained from run 14 in Table 1 (dimer, 89%; trimer, 8%;
tetramer, 3%) with a catalytic amount of 5% Pd/C (1 mol %)
12,17
methylenetridecane.
In sharp contrast, signals correspond-
ing to vinylene-terminated oligomers were not observed,
indicating that the oligomerization proceeded under excellent
1
,2-regioselectivity (>99%). In the 1 C{ H} NMR spectrum
3
1
(
see Figure S2 in the Supporting Information), the relative
in MeOH under an atmosphere of H led to the quantitative
2
B
DOI: 10.1021/acs.organomet.8b00411
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 1. Oligomerization of 1-Hexene Using Zirconium Precatalysts 7 and 8 Activated with dMMAO
f
oligomer distribution (%)
run
cat.
Al/Zr
temp (°C)
conv. (%)
TOF (h−1)
vinylidene selectivity (%)
e
dimer
trimer
others
b
1
7
7
7
7
7
7
7
7
8
8
8
8
8
8
250
250
250
250
250
100
150
200
150
200
250
300
250
250
25
40
25
40
25
25
25
25
25
25
25
25
40
25
33
42
42
48
77
61
62
42
14
11
20
11
21
13
5950
7560
7510
8580
57
10900
11100
8460
2570
2020
3530
1950
3700
6640
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
76
76
78
74
80
80
79
79
91
89
87
87
88
89
10
11
9
11
10
9
8
8
8
9
14
13
13
15
10
11
13
12
b
2
3
4
5
c
6
7
8
9
g
1
2
3
2
g
1
1
1
1
1
0
1
2
3
4
g
10
11
10
8
g
g
2
d
g
3
a
b
c
Conditions: 0.002 mmol 7 or 8; 1-hexene (3.0 g, 35.6 mmol); reaction time = 1 h. Solvent: toluene (5 mL). Conditions: 0.02 mmol 7; 1-hexene
d
e
1
(
3.0 g, 35.6 mmol); reaction time = 24 h. Conditions: 0.002 mmol 8; 1-hexene (9.0 g, 107 mmol); reaction time = 1 h. Determined by H NMR
spectroscopy in CDCl . Determined by C NMR spectroscopy in CDCl . Only the tetramer was formed.
f
13
g
3
3
Scheme 3. Oligomerization of 1-Octene Using Zirconium
Precatalyst 8 and dMMAO
18
cane) are consistent with literature values. The TOF (2880
−
1
h ) is comparable to that in run 12 for the oligomerization of
1
-hexene and remarkably higher than those of arene-
18,19
substituted cyclopentadienyl zirconium complexes
dimeric lanthanide hydrides.
and
20
Figure 1. GC-MS chart for oligo(1-hexene)s in CDCl obtained using
3
7
/dMMAO at 25 °C (Table 1, run 1).
CONCLUSION
■
In summary, we have established a highly regioselective
method for the oligomerization of 1-hexene employing
zirconium(IV) complexes 7 and 8 supported by aryl-
substituted [OSSO]-type bis(phenolate) ligands, which are
based on the trans-cyclooctanediyl platform. Upon activation
with dMMAO, these precatalysts actively oligomerize 1-hexene
at low catalyst loadings (0.0019−0.0056 mol %) with excellent
vinylidene selectivity. The TOF values can be tuned by
changing the aryl substituent at the ortho position of the
phenolate moiety in the [OSSO]-type ligand and the number
of dMMAO equivalents employed. Notably, the highest TOF
formation of a viscous oil of saturated hydrocarbons, for which
1
3
the C NMR analysis revealed 5-methylundecane as the major
product (Scheme 2), which could be isolated in 81% yield by
vacuum distillation.
Scheme 2. Hydrogenation of Oligo(1-hexene)s
−1
(
11 100 h ) in this study was observed for phenyl-substituted
precatalyst 7. For the dominant formation of the correspond-
ing dimer, 5-methyleneundecane, in up to 91% yield with
−
1
Compound 8 (0.002 mmol, 0.0075 mol %) and dMMAO
250 equiv) were also applied to the oligomerization of 1-
remarkably high TOFs (1950−6640 h ), the use of Dmp-
substituted precatalyst 8 was crucial. These studies on well-
defined [OSSO]-type complexes may help to improve
production processes for renewable jet fuels from bio-derived
precursors.
(
octene under similar conditions (neat, 25 °C, 1 h), which
produced predominantly vinylidene-terminated oligo(1-
1
octene)s (>99%), as evident from H NMR data (Scheme
1
3
3
). The C NMR spectrum of these oligo(1-octene)s showed
the formation of the dimer as the major product (91%),
together with minor fractions of the trimer (7%) and tetramer
EXPERIMENTAL SECTION
■
General. All manipulations of air- and/or moisture-sensitive
compounds were performed either using standard Schlenk line
techniques or in a Japan E300 glovebox under an inert atmosphere of
(
2%) (see Figure S9 in the Supporting Information). The
obtained NMR data for the dimer (i.e., 7-methylenepentade-
C
DOI: 10.1021/acs.organomet.8b00411
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
argon. Toluene, 1-hexene, and 1-octene were dried and degassed over
ACKNOWLEDGMENTS
■
2
a potassium mirror by a freeze−thaw cycle prior to use. Other
1
13
1
This work was supported by JSPS KAKENHI Grants
5410035 and 17K05771 (to N.N.).
chemicals were used as received. H (400 MHz) and C{ H} (101
MHz) NMR spectra were obtained with a Bruker AVANCE-400
spectrometer using CDCl or C D as a solvent at room temperature.
3
6
6
GC-MS charts were obtained with a Bruker SCION SQ 456-GC/MS
system equipped with an InertCap Pure-WAX 30 m column.
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2 2
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NMR and GC-MS charts of the obtained oligomers
(PDF)
AUTHOR INFORMATION
■
*
*
(5) For examples on the 2,1-regioselective oligomerization of 1-
Corresponding Authors
E-mail: nakata@chem.saitama-u.ac.jp.
E-mail: ishiiaki@chem.saitama-u.ac.jp.
hexene using group IV metal complexes, see: (a) Lian, B.; Beckerle,
K.; Spaniol, T. P.; Okuda, J. Group 4 Metal Complexes That Contain
a Thioether-Functionalized Phenolato Ligand: Synthesis, Structure,
and 1-Hexene Polymerization. Eur. J. Inorg. Chem. 2009, 2009, 311−
ORCID
316. (b) Xu, T.; Liu, J.; Wu, G.; Lu, X. Highly Active Ethylene
Norio Nakata: 0000-0003-4120-7924
Akihiko Ishii: 0000-0003-4638-1294
Notes
Polymerization and Regioselective 1-Hexene Oligomerization Using
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The authors declare no competing financial interest.
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DOI: 10.1021/acs.organomet.8b00411
Organometallics XXXX, XXX, XXX−XXX