Catalytic dimerization reactions of α-olefins and α,ω-dienes with Cp2ZrCl2/poly(methylalumoxane): Formation of dimers, carbocycles, and oligomers

Full text of DOI:10.1021/ja960227n

J. Am. Chem. Soc. 1996, 118, 4715-4716
Catalytic Dimerization Reactions of r-Olefins and
Scheme 1
4715
2 2
r,ω-Dienes with Cp ZrCl /Poly(methylalumoxane):
Formation of Dimers, Carbocycles, and Oligomers
Jens Christoffers and Robert G. Bergman*
Department of Chemistry, UniVersity of California
Berkeley, California 94720
ReceiVed January 23, 1996
Zirconocene/alumoxane catalysts are known to polymerize
1
R-olefins; chiral zirconocene complexes are especially useful
2
in the stereospecific synthesis of stereoregular polymers or
chiral oligomers.3 Numerous investigations on this topic have
shown that a large alumoxane excess (e.g. Al/Zr ratios of 500:1
up to 10000:1) is most favorable for this polymerization
4
process.
In contrast to this, the use of low Al/Zr ratios has been much
less frequently discussed in the literature.5 In at least one case
this leads to the selective formation of R-olefin dimers instead
MAO (3 days, RT, 1 mol % Zr, Al/Zr ) 4:1) does indeed give
the benzannulated methylenecycloheptane 4, which was sepa-
rated from nonvolatile byproducts by distillation and purified
by column chromatography (SiO2/pentane, 70%) (Scheme 1,
eq 2).
6
of polymers. We wish to report the application of low Al/Zr
ratio zirconocene/poly(methylalumoxane) (MAO) catalysts to
the conversion of dienes to cyclization products and oligomers
with unsaturated intrachain and endgroup functionality.
In our hands, reaction of the 1-alkenes 1a-d with a catalyst
prepared by mixing Cp2ZrCl2 (0.3-1.0 mol %) and a solution
However, a starting material structure that holds the cyclizing
olefinic groups in proximity to one another appears to be
required to achieve cyclization, because all the simple dienes
we have so far investigated lead with high efficiency to linear
oligomers rather than rings. For example, treatment of 1,7-
octadiene (1e) with the same Cp2ZrCl2/MAO mixture gives an
oligomeric material 5 (Scheme 1, eq 3a). The polycondensation
product 5 can be obtained either as lower oligomeric (1 day,
7
of MAO in toluene (ratio Al/Zr ca. 1:1) forms dimeric products
8
-1
2
a-d at modest rates (TON ) ca. 6.1 min at 25 °C after a
brief induction period) without significant portions of higher
oligomers (Scheme 1, eq 1). After 1 day at room temperature
(RT) no starting material 1 is detectable in the reaction mixture,
and the analytically pure dimers 2 can be isolated by fractional
distillation directly from the reaction mixtures in 80-90%
yields.
12
RT, Mw ) 1600, Mw/Mn ) 2.0) or higher oligomeric materials
(
3 days, RT, Mw ) 5400, Mw/Mn ) 2.6). Conversion to
We decided to investigate whether this dimerization catalyst
oligomer is quite selective; to date we have not observed the
formation of higher molecular weight materials, by either
variation of reaction temperature or time, variation of olefin/
Zr/Al ratios, or addition of an inert solvent (toluene).13
9
might cyclize R,ω-dienes, e.g. to form a seven-membered
carbocycle.10 Reaction of o-diallylbenzene 3 with Cp2ZrCl2/
11
(1) (a) Andresen, A.; Cordes, H.-G.; Herwig, J.; Kaminsky, W.; Merck,
A.; Mottweiler, R.; Pein, J.; Sinn, H.; Vollmer, H.-J. Angew. Chem., Int.
Ed. Engl. 1976, 15, 630. (b) Sinn, H.; Kaminsky, W.; Vollmer, H.-J.; Woldt,
R. Angew. Chem., Int. Ed. Engl. 1980, 19, 396.
Under most conditions it is difficult to stop the diene reaction
at the simple dimer stage. However, this can be accomplished
by running the reaction in dilute toluene solution with slow
addition of 1e to the catalyst. Under these conditions 5%
conversion to the dimeric species can be achieved, and the dimer
(2) (a) Ewen, J. A. J. Am. Chem. Soc. 1984, 106, 6355. (b) Kaminsky,
W.; K u¨ lper, K.; Brintzinger, H. H.; Wild, F. R. W. P. Angew. Chem., Int.
Ed. Engl. 1985, 24, 507. (c) Review: Brintzinger, H. H.; Fischer, D.;
M u¨ lhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem., Int. Ed. Engl.
2
e can be isolated in 4% yield; no cyclic or other product is
1
995, 34, 1143.
3) Kaminsky, W.; Ahlers, A.; M o¨ ller-Lindenhof, N. Angew. Chem., Int.
Ed. Engl. 1989, 28, 1216.
4) (a) Kolthammer, B. W. S.; Mangold, D. J.; Gifford, D. R. J. Polym.
Sci. A 1992, 30, 1017. (b) J u¨ ngling, S.; M u¨ lhaupt, R. J. Organomet. Chem.
995, 497, 27. (c) Janiak, C.; Versteeg, U.; Lange, K. C. H.; Weimann, R.;
Hahn, E. J. Organomet. Chem. 1995, 501, 219.
5) Slaugh, L. H.; Schoenthal, G. W. U.S. Patent 4,658,078, 1987; Chem.
Abstr. 1987, 107, P39178s.
6) For examples of other catalytic systems useful for dimerizing alkenes,
1
(
detectable by H NMR (Scheme 1, eq 3b). This suggests that
the dimer is the first-formed product and that it is converted to
oligomer by catalyst-mediated reaction with monomer or other
dimeric/oligomeric fragments.
(
1
Careful examination of the oligomeric products by NMR has
provided information about the overall course of the oligomer-
ization, as well as the reason for the selective conversion to
(
(
1
see: (a) Al-Jarallah, A. M.; Anabtawi, J. A.; Siddiqui, M. A. B.; Aitani, A.
M.; Al-sa’doun, A. W. Catalysis Today 1992, 14, 1. (b) A.-Sa’doun, A.
W. Appl. Catal. A: Gen. 1993, 105, 1. (c) McLain, S. J.; Sancho, J.;
Schrock, R. R. J. Am. Chem. Soc. 1980, 102, 5610. (d) Piers, W. E.;
Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E. Synlett 1990, 74 and references
cited in these papers. We are grateful to Prof. Bercaw for calling our
attention to ref (d).
material of moderate molecular weight. First, H NMR analysis
demonstrates that the oligomeric mixture contains a certain
amount of internal di- or tri-substituted double bonds (ca. 10%
of the integral of the dCH2 protons). This indicates that the
catalyst is capable of alkene isomerization as well as dimer-
ization, presumably via a minor amount of 2,1 insertions.
R-Olefinic endgroups that have been converted to internal
(
7) MAO (solution in toluene) was either purchased from Akzo
Chemicals Inc., Chicago, or prepared following a literature procedure:
Resconi, L.; Boss, S.; Abi, L. Macromolecules 1990, 23, 4489.
(8) 2a: Onopchenko, A.; Cupples, B. L.; Kresge, A. N. Ind. Eng. Chem.
(12) Molecular mass analysis by GPC. Because of their low volatility,
the oligomeric mixtures are difficult to analyze by gas chromatography. In
Prod. Res. DeV. 1983, 22, 182. 2b: Bestmann, H. J.; R o¨ sel, P.; Vostrowsky,
O. Liebigs Ann. Chem. 1979, 1189. 2c: Liu, H. Q.; Deffieux, A.; Sigwalt,
P. Makromol. Chem. 1991, 192, 2111.
+
the EI-MS spectra M signals for species up to m/z 880 (n ) 8) can be
observed.
(
9) A cyclization reaction of R,ω-dienes using Cp2ZrMe2-catalysis and
(13) We see no detectable amounts of cyclic products, which contrasts
with observations made in the reactions of dienes with conventional
Ziegler-Natta systems, cf.: (a) Ruiz de Ballesteros, O.; Venditto, V.;
Auriemma, F.; Guerra, G.; Resconi, L.; Waymouth, R. M.; Mogstad, A.-L.
Macromolecules 1995, 28, 2383. (b) Mitani, M.; Oouchi, K.; Hayakawa,
M.; Yamada, T.; Mukaiyama, T. Chem. Lett. 1995, 905, as well as with
Scandium catalysts (ref 6d).
a stoichiometric amount of Al2Me6 has been reported, see: Shaughnessy,
K. H.; Waymouth, R. M. J. Am. Chem. Soc. 1995, 117, 5873.
(
10) There are only very few examples of a transition-metal catalyzed
seven-membered ring formation. Review: Dyker, G. Angew. Chem., Int.
Ed. Engl. 1995, 34, 2223.
(
11) Echavarren, A. M.; Stille, J. K. J. Am. Chem. Soc. 1988, 110, 1557.
S0002-7863(96)00227-2 CCC: $12.00 © 1996 American Chemical Society

4
716 J. Am. Chem. Soc., Vol. 118, No. 19, 1996
Communications to the Editor
Scheme 2
Scheme 3
using this analysis and an iteration algorithm results in i ) 0.25,
j ) 0.50, and k ) 0.25. This prediction is in excellent agreement
with the values obtained by careful integration of the 13C{ H}
NMR spectrum, although the latter gives an experimental error
of about 5% due to the signal-to-noise ratio and imperfect
spectral resolution.
1
alkenes are unreactive toward further dimerization reaction, and
this might be the molecular weight limiting process.
Concerning the overall structure of the oligomers, it appears
that the material formed from 1,7-octadiene contains internal
vinylidene units separated by either aliphatic tetramethylene,
hexamethylene, or octamethylene moieties.14 Evidence for this
is provided by the presence of four different types of resonances
for the olefinic carbon atoms in the 13C{ H} NMR spectrum
due to four possible different environments for the vinylidene
units,15 which is consistent with the constitution of 5 illustrated
in Scheme 1. There are three sets of resonances for the aliphatic
carbon atoms, each consisting of three lines: set 1: R-CH2,
R′-CH2, R′′-CH2; set 2, â-CH2, â′-CH2, â′′-CH2; and set 3,
γ-CH2, γ′-CH2, and δ-CH2. For stoichiometric reasons, j equals
i + k.
The formation of this distribution of oligomer isomers can
be understood by considering an overall dimerization process
in which, formally, one of the two C-C double bonds is retained
and the other one is converted into a C-C single bond. We
assume that the initial step in the formation of oligomer 5 is
the dimerization of 1,7-octadiene 1e. The dimeric product 2e
has two R-olefinic ends, which are reactive in further dimer-
ization steps. However, the two terminal double bonds in 2e
are not equivalent; one of the terminal bonds is attached to an
aliphatic tetramethylene unit, and the other one has a hexa-
methylene moiety (abbreviations t-4-CH2 and t-6-CH2 in Scheme
A final question concerns the detailed mechanism by which
the overall vinyl dimerization reaction illustrated in Schemes 1
and 2 actually occurs. We suggest the sequence of well-known
elementary steps shown in Scheme 3. We proposed that the
catalytically active species in this process is a hydridozirconium
1
16
complex (A). Insertion of the 1-alkene 1 into the Zr-H bond
gives the Zr-alkyl complex B, which can either â-eliminate to
re-form the starting materials or insert into another equivalent
of 1 to form C. â-Elimination of 2 from C regenerates the
hydrido complex A. We believe that the formation of only
dimeric product might be caused by the presence of a chlorine
rather than an alkyl ligand coordinated to the active Zr catalyst;17
the presence of Cl apparently makes â-elimination occur more
rapidly than further insertion. In support of this hypothesis,
we observe that, when treated with 1-alkenes, the systems Cp2-
1
8
Zr(Me)Cl /MAO and Cp2ZrMe2/LiCl/MAO also give only
dimeric product, whereas Cp2ZrMe2/MAO and Cp2ZrCl2/(large
excess MAO) form higher oligomeric products. In the latter
cases one expects that all the chloro ligands at the polymerizing
Zr centers have been displaced by methyl groups; in the former
cases substantial numbers of Cl ligands may be retained.
The oligomers formed in these reactions are unique because
they contain terminal methylene units along the chain, which
could provide a potential method for introducing novel func-
tional groups at these positions. Further studies underway in
our laboratory are directed at obtaining a better understanding
of the mechanism of the oligomerization reaction and at
extending this type of transformation to other dimerization
catalysts and to a wider range of dienes and multiply unsaturated
compounds.
1
, eq 3b). The terminal R-olefinic groups in dimer 2e can react
with another equivalent of 2e or with 1,7-octadiene 1e. In the
latter case there are four different products possible (Scheme
2
). Upon reaction of a tetramethylene terminus (t-4-CH2) with
1
e either an internal tetramethylene unit (i-4-CH2) and a
hexamethylene terminus (t-6-CH2) can be formed (eq 4a) or an
internal hexamethylene unit (i-6-CH2) and a t-4-CH2 terminus
are generated (eq 4b). On the other hand, the reaction of a
t-6-CH2 unit with 1e also has two possibilities. One is the
formation of i-6-CH2 and t-6-CH2 (eq 5a), and the second is
the formation of i-8-CH2 and t-4-CH2 (eq 5b).
Acknowledgment. We acknowledge the National Science Founda-
tion (Grant No. CHE-9113261) for generous financial support of this
work and the Alexander von Humboldt-Stiftung for a fellowship for
J.C. We are grateful to Prof. T. D. Tilley and S. Mao for assistance
with GPC measurements and to Prof. Tilley, Prof. J. E. Bercaw
Following this logic further, as the isomers shown in Scheme
continue to oligomerize, reactions of t-4-CH2 and t-6-CH2
(
(
California Institute of Technology), and Drs. Rick Kemp and Bob Job
Union Carbide Corp.) for helpful discussions.
2
with themselves or each other give six possible combinations.
Here only internal aliphatic units are formed, either i-4-CH2,
i-6-CH2, or i-8-CH2; no terminal t-4-CH2 or t-6-CH2 units are
generated (see supporting information for details). This makes
it clear that there are three different internal aliphatic methylene
chains possible: i-4-CH2, i-6-CH2, and i-8-CH2. Terminal
Supporting Information Available: Experimental details, char-
acterization data for all products, and an outline of the structure analysis
of 5 (10 pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the journal,
can be ordered from the ACS, and can be downloaded from the Internet;
see any current masthead page for ordering information and Internet
access instructions.
R-olefinic endgroups are always t-4-CH2 or t-6-CH2.
A
computer simulation of the formation and statistical distribution
of tetramethylene, hexamethylene, and octamethylene units
JA960227N
(
14) There is only one precedent for a polymeric compound of similar
(16) We assume that the reaction is initiated by an insertion reaction
between a Zr-CH3 complex and a CdC double bond, followed by
â-elimination to give the catalytically active Zr-H species.
(17) The reaction between Cp2ZrCl2 and MAO in 1:1 ratio, without
alkene present, has been reported before, see: Cam, D.; Giannini, U.
Makromol. Chem. 1992, 193, 1049.
constitution: (a) Yang, X.; Jia, L.; Marks, T. J. J. Am. Chem. Soc. 1993,
1
15, 3392. (b) Jia, L.; Yang, X.; Yang, S.; Marks, T. J. J. Am. Chem. Soc.
996, 118, 1547.
1
(
15) The four possible different environments for a vinylidene unit are
as follows: (CH2)4(CdCH2)(CH2)6, (CH2)6(CdCH2)(CH2)6, (CH2)4(CdCH2)-
CH2)8, (CH2)6(CdCH2)(CH2)8.
(
(18) Jordan, R. F. J. Organomet. Chem. 1985, 294, 321.