Cobalt-catalyzed dimerization of α-olefins to give linear α-olefin products

Article abstract of DOI:10.1021/ja056655n

[Cp*P(OMe)₃CoCH₂CH₃]⁺ [BarF]^-, generated by the addition of HBArF to Cp*P(OMe)₃Co(ethene), catalyzes the oligomerization of 1-hexene to give dimers and trimers. When a deficit of the acid is used, linear α-olefin dimers are produced at the expense of trimeric products: e.g., 1-butene, 1-hexene, and 1-octene give 1-octene, 1-dodecene, and 1-hexadecene, respectively. Copyright

Full text of DOI:10.1021/ja056655n

Published on Web 11/18/2005
Cobalt-Catalyzed Dimerization of r-Olefins to Give Linear r-Olefin Products
Richard D. Broene,*,† Maurice Brookhart, William M. Lamanna, and Anthony F. Volpe, Jr.
‡
§
‡,|
Department of Chemistry, Bowdoin College, Brunswick, Maine 04011, Department of Chemistry, UniVersity of
North Carolina, CB 3290, Venable Hall, Chapel Hill, North Carolina 27599-3290, and
3
M Company, 3M Center, St. Paul, Minnesota 55144
Received September 28, 2005; E-mail: rbroene@bowdoin.edu
Linear R-olefins in the C -C20 range have broad utility including
6
use as comonomers in the production of linear low-density
polyethylene and as precursors to detergents, synthetic oils, and
1
plasticizers. These olefins are commercially produced primarily
through oligomerization of ethylene followed by fractionation of
the resulting broad distribution of even-numbered carbon oligo-
1
mers. Catalyst systems have been developed which selectively
2
trimerize ethylene to 1-hexene, and more recently certain chromium
systems have shown selectivity for forming the tetramer, 1-octene.3
An alternate, attractive method for producing specific higher
R-olefins would be the dimerization of low carbon number
R-olefins, for example, conversion of 1-butene to 1-octene or
conversion of 1-hexene to 1-dodecene.
4
Small described the use of cobalt pyridine bis-imine catalysts
that dimerize propene to 1-hexene as a major product through a
cycle involving 1,2 insertion of propene into a metal hydride
followed by 2,1 insertion to yield L
â-Elimination of this species generated 1-hexene together with
-hexene. Use of 1-butene as monomer led to internal olefin dimers
n 3 2 2 2 3
M(CH(CH )CH CH CH CH ).
Figure 1. Catalytic oligomerization of 1-hexene initiated by 1 formed by
protonation of 2 with HBArF. 1,2 and 2,1 refer to the olefin insertion modes.
2
undergoes insertion faster than â-elimination. Insertion of hexene
with high linearity. Indeed, there are no reports of dimerization of
R-olefins other than propene to linear terminal olefin dimers.
Wasserschied reported Ni catalysts which convert 1-butene pre-
dominantly to linear octenes containing internal double bonds.5
Small also reported iron pyridine bis-imine catalysts that convert
into 5 in a 1,2 manner followed by â-elimination produces the major
trimer, C (27 mol %).1
0-12
Dimer A and trimers B and C were
identified by independent synthesis (see Supporting Information).
The formation of the dimer and trimers is slow with turnover
,6
-1
frequencies of about 0.65 h over 12 h. Turnover numbers of about
1
-butene, 1-pentene, and 1-hexene to linear dimers with internal
80 are found after 10 days of reaction.
7
unsaturation. Gibson described similar results for the dimerization
of propene, 1-butene, and 1-hexene using a supported, electron-
poor, cobalt pyridine bis-imine catalyst. We report here cobalt-
Surprisingly, when a deficit of HBArF was used to protonate 2
in the presence of 1-hexene the oligomer distribution was altered;
-dodecene was now formed at the expense of dimer C. For
example, use of 0.82 equiv of HBArF yielded after 3.5 h 80% A,
% B, only 1% C, and the formation of 14% 1-dodecene. After 12
8
1
catalyzed oligomerization of 1-hexene and a mechanistically unique
catalytic process wherein C
4n R-olefins.
Oligomerization of 1-hexene catalyzed by [Cp*CoP(OMe)
BArF- (1) [generated in situ by addition of 1 equiv of HBArF
n
H2n R-olefins can be converted, in part,
5
to C2nH
h similar ratios of A, B, C, and dodecene were observed of which
about 25% had isomerized to internal olefins.13 Runs were carried
out varying the HBArF between 40 and 85 mol %.14 As expected
higher turnover frequencies were observed with higher HBArF
loadings. The turnover frequency was proportional to the concentra-
tion of 1-hexene, and productivity doubled with a doubling of the
catalyst concentration. When a 4:1 ratio of isolated agostic ethyl
Et]+
3
-
+
[
(
HBArF ) (3,5-(CF
3
)
2
C
6
H
3
)
4
B
(Et
2 2 3
O) H ] to Cp*CoP(OMe) -
9
ethene) (2)] was carried out in 1,2-difluorobenzene solutions (40
vol % 1-hexene) at 25 °C under argon. One dimer and two trimers
along with minor amounts of tetramers were formed; the structures
of the dimer and trimers are shown in Figure 1. After one turnover
and loss of the ethyl group, the catalytic cycle can be described as
illustrated with the hydride as chain carrier. Dimer A (66 mol %
based on GC analysis using an internal standard) is the result of
+
-
3 2 3
salt [Cp*CoP(OMe) CH CH ] [BArF] and neutral ethylene com-
plex 2 was employed in catalysis, the same ratio of A/B/dodecene
was observed as when 80 mol % HBArF was used. Similar
reactivity was seen in the dimerization of 1-octene to 1-hexadecene
and 1-butene to 1-octene.
A mechanism consistent with formation of 1-dodecene at the
expense of trimer C is shown in Figure 2. Using less than 1 equiv
of HBArF results in formation of a mixture of agostic hexyl
complex 6 and neutral 1-hexene complex 7 after one turnover of
1
,2 insertion of 1-hexene into the Co-hexyl species 3 to give 4.
â-Elimination of 4 provides dimer A. A second 1,2 insertion of
-hexene into 4 followed by â-elimination produces the minor
1
trimer B (7 mol %). When 3 undergoes 2,1 insertion with 1-hexene,
metal migration down the chain via â-elimination/readdition
produces the terminal dodecyl agostic species 5 which apparently
2
the mixture of neutral and cationic C starting complexes 1 and 2.
†
Bowdoin College.
Following 2,1 insertion of 1-hexene into 6 and isomerization to
the agostic dodecyl complex 5 (see Figure 1), proton exchange
between 5 and the neutral 1-hexene complex 7 yields the agostic
‡
University of North Carolina.
§
3M Corporation.
|
Currently at Symyx Technologies.
1
7194 J. AM. CHEM. SOC. 2005, 127, 17194-17195
9
10.1021/ja056655n CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
a potentially valuable process, the catalyst reported here is not
practical due to competitive formation of branched dimers and slow
turnover frequencies. However, these studies suggest generally how
chain-walking, proton transfer, and olefin exchange can be coupled
to achieve this unique dimerization process. More efficient catalysts
based on this scheme are being sought.
Acknowledgment. We thank the NSF (Grant No. CHE-
0107810) for financial support of this work. R.D.B. acknowledges
the NSF for a research opportunity award and Bowdoin College
for a Porter Fellowship for advanced study.
Figure 2. Formation of 1-dodecene via intermolecular agostic H transfer.
Scheme 1. Intermolecular Exchange of Agostic Hydrogen
Supporting Information Available: Full experimental procedures
for the dimerization and oligomerization reactions and for the synthesis
of reference compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
References
(
1) Vogt, D. In Applied Homogeneous Catalysis with Organometallic
Compounds; Cornils, B., Herrmann, W., Eds.; VCH: New York, 1996;
Vol. 1, pp 245-257.
(
2) For Cr catalysts: (a) Carter, A.; Cohen, S. A.; Cooley, N. A.; Murphy,
A.; Scutt, J.; Wass, D. F. Chem. Commun. 2002, 858. (b) McGuinness,
D. S.; Wasserscheid, P.; Morgan, D. H.; Dixon, J. T. Organometallics
2005, 24, 552. (c) For Ta catalysis: Andes, C.; Harkins, S. B.; Murtuza,
S.; Oyler, K.; Sen, A. J. Am. Chem. Soc. 2001, 123, 7423. (d) For Ti
catalysis: Deckers, P. J. W.; Hessen, B.; Teuben, J. H. Angew. Chem.,
Int. Ed. 2001, 40, 2516.
3) Bollmann, A. et al. J. Am. Chem. Soc. 2004, 126, 14712.
4) Small, B. L. Organometallics 2003, 22, 3178.
hexyl complex 6 and neutral 1-dodecene complex 9. Exchange of
coordinated 1-dodecene in 9 by 1-hexene yields free 1-dodecene
and neutral 1-hexene complex 7, which reenters the catalytic cycle.
Two experiments provide evidence that this bimolecular process
is a viable pathway for this transformation. Isolated cationic ethyl
complex 10 was allowed to react with 1-hexene for 12 h at which
time GC analysis showed a turnover frequency for 1-dodecene of
(
(
(
5) Ellis, B.; Keim, W.; Wasserscheid, P. Chem. Commun. 1999, 337.
(6) Chauvin, Y.; Olivier, H. In Applied Homogeneous Catalysis with
Organometallic Compounds; Cornils, B., Herrmann, W. A., Eds.; VCH:
New York, 1996; Vol. 1, pp 258-268.
(
7) Small, B. L.; Marcucci, A. J. Organometallics 2001, 20, 5738.
(8) Tellmann, K. P.; Gibson, V. C.; White, A. J. P.; Williams, D. J.
-
1
Organometallics 2005, 24, 280.
0
.015 h . At that time, the neutral complex 2 (ratio 10/2 ) 4/1)
(
9) Brookhart, M.; Lincoln, D. M.; Volpe, A. F.; Schmidt, G. F. Organome-
tallics 1989, 8, 1212.
was added, and another aliquot was taken after a further 12 h
-
1
showing a TOF of 0.05 h . As a comparison, the TOF for
(
10) Preliminary accounts of this work have appeared in (a) Brookhart, M.;
Volpe, A. F.; DeSimone, J. M.; Lamanna, W. M. Polym. Prepr. (Am.
Chem. Soc., DiV. Polym. Chem.) 1991, 32, 461. (b) Volpe, A. F., Jr. Olefin
Polymerization Catalysts Based on Highly Electrophilic Co(III) Com-
plexes: Mechanistic Studies and Catalyst Design. Ph.D. Thesis, University
of North Carolina Chapel Hill, 1991. The structure of dimer C was reported
incorrectly in (a).
-
1
formation of dimer A decreased from 0.76 to 0.64 h over the
same period of time. Addition of 1-hexene to a solution of the
neutral ethene complex in the absence of either HBArF or 10 gave
no oligomerization. This clearly demonstrates that the addition of
neutral 2 to 10 increases the rate of formation of 1-dodecene. In
the second experiment, ethyl agostic complex 10 and propene
(11) Replacement of P(OMe) with PMe results in a >20:1 ratio of the dimer
3
3
A formed through 1,2 insertion relative to trimer C. Presumably the larger
3 3
size of PMe relative to P(OMe) directs the insertion through the 1,2
complex 11 were mixed in C
over time by H NMR spectroscopy (Scheme 1). Within 50 min,
signals for the propyl agostic complex 12 and the neutral ethene
6
D
5
Cl, and the reaction was monitored
mode. Use of ligands less sterically demanding than P(OMe)3 should
improve the regioselectivity of the insertion.
1
9
(12) Different batches of catalyst provide slightly different ratios of A/B/C.
For instance, use of the same catalyst batch for the oligomerization as is
used for the reaction shown in Figure 2 gives a ratio of 81:6:13.
13) The isomerization of 1-dodecene is assumed to be metal catalyzed, but
we have not yet identified the active species. No free HBArF was observed
when a deficit was used to promote the reaction.
complex 2 were observed. This shows that the transfer of an agostic
(
H to a neutral species is possible. Over longer periods of time,
+
-
3 2
Cp*CoH(P(OMe) ) BArF formed and ultimately became the
major species in solution.
In summary, this work demonstrates an unprecedented process,
conversion of R-olefins to linear R-olefin dimers. Although this is
(14) Use of greater than 1 equiv of HBArF gave olefin isomerization to internal
olefin products presumably through an acid-catalyzed process.
JA056655N
J. AM. CHEM. SOC.
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VOL. 127, NO. 49, 2005 17195