Oligomerization and simultaneous cyclization of ethylene to methylenecyclopentane catalyzed by zirconocene complexes

Article abstract of DOI:10.1016/S0022-328X(99)00749-4

The oligomerization of ethylene catalyzed by Cp₂ZrL₂ (L=Cl, Me, OC₆H₄-Me-p) with ethylaluminoxane or Et₃Al as cocatalyst (Al/Zr=100, 150°C, P(C₂H₄)=1.4 MPa) afforded not only common chain oligomers but also a cyclic oligomer, methylenecyclopentane. The selectivity of methylenecyclopentane reached 39% under optimal conditions. The addition of C₅H₅N to the catalytic system of Cp₂ZrCl₂/Et₃Al was capable of further improving the selectivity of methylenecyclopentane to 43%.

Full text of DOI:10.1016/S0022-328X(99)00749-4

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 599 (2000) 143–146
Oligomerization and simultaneous cyclization of ethylene to
methylenecyclopentane catalyzed by zirconocene complexes
Mei Wang *, Yumei Shen, Mingxing Qian, Rui Li, Ren He
Department of Chemistry, School of Chemical Engineering, Open Laboratory of Comprehensi6e Utilization of Carbon Resources,
Zhongshan Road 158-46, Dalian Uni6ersity of Technology, Dalian 116012, PR China
Received 7 October 1999; received in revised form 29 November 1999; accepted 4 December 1999
Abstract
The oligomerization of ethylene catalyzed by Cp2ZrL2 (L=Cl, Me, OC6H4-Me-p) with ethylaluminoxane or Et3Al as cocatalyst
(Al/Zr=100, 150°C, P(C2H4)=1.4 MPa) afforded not only common chain oligomers but also a cyclic oligomer, methylene-
cyclopentane. The selectivity of methylenecyclopentane reached 39% under optimal conditions. The addition of C5H5N to the
catalytic system of Cp2ZrCl2/Et3Al was capable of further improving the selectivity of methylenecyclopentane to 43%. © 2000
Elsevier Science S.A. All rights reserved.
Keywords: Ethylene oligomerization; Zirconocene complexes; Cyclization; Methylenecyclopentane
by aging and reaction temperature, the Al/Zr ratio and
the hydrolysis extent of Et3Al, reached 39% under
1. Introduction
optimal conditions. The addition of C5H5N as a third
component to the catalytic system of Cp2ZrCl2/Et3Al
was capable of further improving the selectivity of
MCP to 43%. As far as we know, this is the first
Zirconocene/aluminoxane catalysts have attracted in-
tense interest due to their catalytic activity and special
stereoselectivity in the polymerization of a-olefins. Re-
cently, the development of homogeneous catalytic
example of catalytic formation of a cyclic product
chemistry of zirconocenes has been extended to the field
direct from ethylene oligomerization. The oligomeriza-
of fine chemical synthesis, e.g. selective oligomerization
tion and simultaneous cyclization of ethylene occurring
of a-olefins, enantioselective reaction of monosubsti-
in one-pot really provides a potential convenient access
tuted alkenes and monocyclization of a,v-dienes, by
using low Al/Zr ratios or combining zirconocenes with
other cocatalysts [1]. In contrast to methylaluminoxane
to five-membered carbocyclic compounds, which are
very useful building blocks for organic synthesis [3].
(MAO), the use of other higher alkylaluminoxanes as
cocatalysts has been much less frequently reported in
the literature [2].
In our attempts to tune Cp2ZrL2 (L=Cl, Me,
2. Results and discussion
OC6H4Me-p)/ethylaluminoxane (EAO) to be a catalytic
system for the oligomerization of ethylene to low-car-
bon linear a-olefins, we found that in addition to
common oligomers, the oligomerization of ethylene cat-
alyzed by Cp2ZrL2 with EAO or Et3Al as cocatalyst
also afforded an unexpected cyclic product, methylene-
cyclopentane (MCP). The selectivity of MCP, affected
Pretreatment of Cp2ZrCl2 (0.025 mmol) with EAO
(Al/Zr=100) in anhydrous toluene (20°C, 30 min),
followed by exposure to ethylene (1.4 MPa, 150°C, 2 h)
gave not only chain oligomers in 72% yield, but also an
unusual monocyclic product, methylenecyclopentane in
28% yield based on GC analysis. Intrigued by this
surprising lead, we examined the oligomerization of
ethylene with other zirconocene complexes as procata-
lysts under identical conditions of temperature, pressure
and Al/Zr ratio. Some representative results are sum-
* Corresponding author. Fax: +86-411-2633041.
E-mail address: sunsuwangmei@hotmail.com (M. Wang)
0022-328X/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00749-4

144
M. Wang et al. / Journal of Organometallic Chemistry 599 (2000) 143–146
Both aging and reaction temperature prove to be
important factors in optimizing the selectivity for MCP
(Table 2). For example, the selectivity of MCP in-
creased from 3 to 31% as the reaction temperature rose
from 120 to 165°C, while Cp2ZrCl2/EAO was pre-
treated at 60°C. The selectivity of MCP also depends
on the Al/Zr ratio. Under the identical reaction condi-
tions, when the Al/Zr ratio was 25:1, the oligomeriza-
tion of ethylene gave 1-hexene in 55% yield, and the
selectivity of MCP was only 3%. With an increase of
the Al/Zr ratio from 40:1 to 100:1 the selectivity of
MCP grew from 8 to 27%. Further enhancement of the
Al/Zr ratio to 200:1 did not show an evident effect on
the selectivity of the cyclic oligomer.
marized in Table 1. In the case of Cp2ZrL2/EAO, MCP
was obtained in 17–28% yield. The replacement of
EAO with Et3Al gave even higher selectivity to MCP
(39%). In contrast, when the hydrolysis extent of Et3Al
changed from 1 to 2 (H2O/Et3Al), the selectivity to
MCP dropped to 12%. The reaction of ethylene cata-
lyzed by Ind2ZrCl2 with EAO as cocatalyst mainly gave
polymers, and with Et3Al gave only 8% of MCP. If
zirconocene complexes were replaced by simple zirco-
nium complexes without h5-cyclopentadienyl ligands,
such as ZrCl4, ZrCl2(OR)2 and ZrCl2(acac)2, with either
EAO, Et3Al or Et2AlCl as cocatalyst, chain olefins were
produced selectively [4]. Chain oligomers are also the
exclusive products for the oligomerization of ethylene
catalyzed by the previously reported non-zirconocene
compounds, ZrR4, Zr(OR)4 and Zr(OCOR)4, in combi-
nation with alkylaluminum chloride [5]. These contrast-
ing results distinguish catalysis with metallocenes from
that with non-metallocenes, implying that zirconocene
complexes may perform a different behavior from the
simple zirconium catalysts in the oligomerization of
ethylene and that the pseudo-sandwich structure of
zirconocene complexes plays an important role in the
formation of the cyclic oligomer.
To confirm the structure of the cyclic product, we
collected the reaction mixture and distilled it carefully.
A fraction containing 56% of the cyclic oligomer and
44% of toluene was obtained and the sample was
1
characterized by GC–IR, GC–MS, H- and 13C-NMR
spectra. With the exception of the signals of toluene,
the data obtained were identical to the structure of
methylenecyclopentane.
The formation of a significant portion of MCP in
ethylene oligomerization catalyzed by Cp2ZrL2/EAO
prompted us to investigate the reaction of 1,5-hexadi-
Table 1
Activities and product distributions for selected catalytic systems a
Catalytic system
Catalytic activity
MCP (%) d
C4ꢀ10 chain
olefins (%) d
]C12 chain
(kg mol−1 (Zr) h−1
)
olefins (%) d
Cp2ZrMe2/EAO
21
37
66
65
31
57
17
17
28
39
43
8
42
36
35
39
33
62
41
47
37
22
24
30
Cp2Zr(OC6H4-p-Me)2/EAO
Cp2ZrCl2/EAO b
Cp2ZrCl2/Et3Al b
Cp2ZrCl2/Et3Al/C5H5N b,c
Ind2ZrCl2/Et3Al b
a Reaction conditions: Zr=0.05 mmol; Al/Zr=100; aging temperature 100°C (30 min); reaction temperature 150°C (2 h); P(C2H4)=1.4 MPa;
toluene 30 ml.
b Zr=0.025 mmol; aging temperature 20°C.
c With third component: Al/Zr/C5H5N=100:1:1.
d Determined by GC analysis (SE-30, 30 m×0.25 mm) with internal standard (n-heptane).
Table 2
Activities and product distributions for selected reaction conditions a
Aging
temperature (°C)
Reaction
temperature (°C) (mol/mol)
EAO/Zr
Catalytic activity
MCP (%) c
C4ꢀ10 chain
olefins (%) c
]C12 chain
(kg mol−1 (Zr) h−1
)
olefins (%)c
60 b
60 b
150
20
20
20
120
165
150
150
150
150
100:1
100:1
100:1
100:1
40:1
21
28
43
39
36
14
3
31
15
27
8
27
43
49
34
48
64
70
26
36
39
38
33
25:1
3
a Other reaction conditions are the same as those in Table 1.
b With third component: Al/Zr/Ph2PCH2COONa=100:1:1.
c Determined by GC analysis.

M. Wang et al. / Journal of Organometallic Chemistry 599 (2000) 143–146
145
elimination to give 1,5-hexadiene and hydridozir-
conocene (VII). It is rational that the free 1,5-hexadiene
thus formed in the reaction is prone to quickly re-form
the intermediate (V) by coordination and insertion to
the hydridozirconocene (VII). Therefore, after reaction
and hydrolysis, a very small amount of 1,5-hexadiene
was present in the reaction mixture. The experimental
results supposed that ethylzirconocene species formed
by the reaction of Cp2ZrCl2 and EAO render the
oligomerization of ethylene via a different pathway
from that catalyzed by Cp2ZrCl2 and MAO. The differ-
ent reactivities between the ethyl- and methylzir-
conocene species may be caused by the b-Me in the
ethyl group, which readily undergoes b-H elimination.
More experimental supports are needed to further clar-
ify the proposed Scheme 1.
Scheme 1. Proposed catalytic cycle for oligomerization and cycliza-
tion of ethylene
ene in the presence of the same catalyst. Zirconium-cat-
alyzed monocyclization of 1,5-hexadiene to five-mem-
bered cyclic compounds has been reported by
3. Conclusions
Waymouth using
Cp2*ZrMe2/AlMe3/B(C6F5)3, Bazan using [C5H5BPh]2-
ZrCl2/MAO and Walther using [(Siam)ZrCl3]2 (Siam=
a
three-component catalyst of
In conclusion, we have found a catalytic cyclization
reaction concurring with ethylene oligomerization cata-
lyzed by a zirconocene/EAO or /Et3Al system. Al-
though the selectivity to methylenecyclopentane is not
very high, the moderate yield already shows a potential
of this unique cyclization reaction as a convenient way
to prepare substituted cyclopentanes, which can be used
as intermediates for the production of some fine chemi-
cals, and to offer feed stock for the production of
cycloolefin copolymers with interesting properties. Our
further studies underway are directed at optimizing the
catalytic system and reaction conditions to improve the
selectivity of methylenecyclopentane and at acquiring a
better understanding of the mechanism of the novel
cyclization reaction concurring with ethylene
oligomerization.
N,N%-bis(trimethylsilyl)benzamidinate)/MAO
and
[(Ph3SiO)2ZrMe2(bpy)](tol)2/MAO as catalysts [6]. In
the latter two cases, methylenecyclopentane was selec-
tively produced in the reactions, while under compara-
ble conditions the use of Cp2ZrCl2 afforded 19% of
MCP and 14% of methylcyclopentene and methyl-
cyclopentane [6c]. Our studies showed that the mono-
cyclization of 1,5-hexadiene could be catalyzed by
Cp2ZrCl2/EAO at 150°C, yielding 58% of MCP and
21% of methylcyclopentene, along with a small amount
of dimer and trimer of 1,5-hexadiene. In comparison
with the Cp2ZrCl2/MAO catalyst, the catalytic system
of Cp2ZrCl2/EAO has a preference for monocyclic
products to chain oligomers.
We assume that in the oligomerization of ethylene
two types of active species [7], the neutral diethylzir-
conocene intermediate (II) and the monoethylzir-
conocene cation (I) [8], are formed competitively. The
former leads to MCP and the latter results in normal
chain oligomers. The intermediacy of metallacyclopen-
tane (III), previously postulated for zirconocene-cata-
lyzed dimerization and carbometalation of a-olefins
and alkynes [9–11], and for chromium-catalyzed
trimerization of ethylene [12], might be involved in the
construction of the cyclic oligomer. A very small
amount of 1,5-hexadiene in the oligomeric mixture has
been identified by GC and GC–MS. This result can be
interpreted in terms of the mechanism shown in Scheme
1 based on well-known fundamental processes and our
experimental results. The intermediate (V) could un-
dergo two competitive reactions: (i) intramolecular in-
4. Experimental
4.1. General methods
All reactions and operations were carried out under a
dry, oxygen-free dinitrogen atmosphere with standard
Schlenk techniques. Ethylene was purified by passage
through a column of molecular sieve (4 A). Triethyl
aluminum was carefully distilled in vacuo and then
,
partially
hydrolyzed
by
the
reaction
with
Al2(SO4)3·18H2O in toluene at 0ꢀ5°C. Toluene was
distilled from sodium/benzophenone ketyl prior to use.
Zirconium tetrachloride was purchased from Merck–
Schuchardt. Other commercially available reagents,
C5H5N, HOC6H4Me-p and Al2(SO4)3·18H2O were used
without further purification. Complexes Cp2ZrCl2 [13],
Cp2ZrMe2 [14] and Cp2Zr(OC6H4-Me-p)2 [15] were pre-
pared according to literature methods.
sertion of the end double bond to afford
a
(cyclopentylmethyl)zirconocene (VI) [6a], and (ii) b-

146
M. Wang et al. / Journal of Organometallic Chemistry 599 (2000) 143–146
The following spectrometers were used to detect the
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4.2. Ethylene oligomerization
Ethylene oligomerization experiments using the vari-
ous catalyst precursors were carried out by the follow-
ing general procedure. A 75 ml autoclave, equipped
with a magnetic stirring bar, was dried in vacuo at
200°C for 30 min, cooled to room temperature and
filled with pre-purified dinitrogen. A toluene solution of
EAO, in
a
calculated molar ratio of 100:1
(EAO:precatalyst), was added to a Schlenk bottle,
which was charged with 0.025 or 0.05 mmol of precata-
lyst and 15 ml of toluene. All suspended substances
were dissolved after being stirred for a few minutes.
The solution was transferred into the autoclave and the
Schlenk bottle was washed three times with 5 ml of
toluene. After 30 min of pre-reaction, 1.4 MPa of
ethylene was introduced to the autoclave and the sys-
tem was heated to the reaction temperature for 2 h.
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ice-water for 30 min. The autoclave was vented and
weighed. The weight of products was calculated by the
weight difference of the autoclave before and after
reaction. The catalytic system was quenched by addi-
tion of 1 ml of NaOH/EtOH saturated solution. The
product distribution was determined by GC analysis of
the obtained solution with n-heptane as internal
standard.
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We gratefully acknowledge the Chinese National
Natural Science Foundation (Grant no. 29773008) and
the Open Laboratory of Comprehensive Utilization of
Carbon Resources, the Chinese Ministry of Education,
for financial support of this research.
.