Catalytically active, recyclable zirconocene supported at cross-links within porous polymer disks

Article abstract of DOI:10.1021/ol026094u

(Matrix Presented) Copolymerization of bis(4-vinylbenzylcyclopentadienyl)zirconium dichloride and styrene in a test tube, followed by extraction of the bulk polymer and slicing into "disks" 1 mm thick and 5 mm in diameter, gives a supported zirconocene species that exhibits both stoichiometric and catalytic chemical activity. The disks catalyze reaction of trimethylaluminum and phenylacetylene to give α-methylstyrene with modest turnover numbers and are active over at least six recyclings.

Full text of DOI:10.1021/ol026094u

ORGANIC
LETTERS
2
002
Vol. 4, No. 14
365-2368
Catalytically Active, Recyclable
Zirconocene Supported at Cross-Links
within Porous Polymer Disks
Saphon Hok, Jenny Vassilian, and Neil E. Schore*
2
Department of Chemistry, UniVersity of California, One Shields AVenue,
DaVis, California 95616
neschore@ucdaVis.edu
Received April 28, 2002
ABSTRACT
Copolymerization of bis(4-vinylbenzylcyclopentadienyl)zirconium dichloride and styrene in a test tube, followed by extraction of the bulk
polymer and slicing into “disks” 1 mm thick and 5 mm in diameter, gives a supported zirconocene species that exhibits both stoichiometric
and catalytic chemical activity. The disks catalyze reaction of trimethylaluminum and phenylacetylene to give r-methylstyrene with modest
turnover numbers and are active over at least six recyclings.
We are interested in developing novel methods of tethering
organometallic species of use in organic synthesis to
insoluble polymer supports, with the goal of overcoming
various shortcomings associated with their practical use. In
this context, we sought to construct a robust attachment
between a cross-linked polymer and a zirconocene moiety
that would be active both stoichiometrically and catalytically,
would be readily recyclable for repeated use, and would be
less prone to loss of desired chemical reactivity due to
processes such as hydrolysis or redox-mediated dimerization
than are monomeric analogues.
zirconium derivative with the appropriate vinyl substitution
to permit incorporation into a polystyrene-based system
during the polymerization process itself. This strategy
contrasts with the preparation of previous zirconocene-
bearing polymers, in which a Zr moiety was attached after
polymerization.
Treatment of p-vinylbenzyl chloride with cyclopenta-
dienylsodium in THF containing a catalytic amount of
3
,4
Wang, H.-S.; Su, W.; Wang, R.; Chan, A. S. C. J. Org. Chem. 2000, 65,
2
2
95. (j) Sellner, H.; Karjalainen, J. K.; Seebach, D. Chem. Eur. J. 2001, 7,
873 and refs. therein; (k) Altava, B.; Burguete, M. I.; Garc ´ı a-Verdugo,
The symmetry of the zirconocene system also afforded
us an opportunity to consider the infrequently employed
strategy of situating the functional moieties at the polymer
cross-links themselves.1,2 To this end, we constructed a
E.; Luis, S. V.; Vincent, M. J.; Mayoral, J. A. React. Funct. Polym. 2001,
8, 25 and references therein.
2) (a) Wilson, M. E.; Wilson, J. A.; Kurth, M. J. Macromolecules 1997,
30, 3340. (b) Wilson, M.; Paech, K.; Zhou, W.-J.; Kurth, M. J. J. Org.
Chem. 1998, 63, 5094. (c) Halm, C.; Kurth, M. J. Angew. Chem., Int. Ed.
Engl. 1998, 37, 510.
4
(
(
3) Via covalent bonding through polymer-bound Cp-type ligands. Linked
(
1) Examples of ligands for organometallic species copolymerized as
cross-links: (a) De, B. B.; Lohray, B. B.; Dhal, P. K. Tetrahedron Lett.
993, 34, 2371. (b) De, B. B.; Lohray, B. B.; Sivaram, S.; Dhal, P. K.
to polystyrene: (a) Nishida, H.; Uozumi, T.; Soga, K. Macromol. Rapid
Commun. 1995, 16, 821. (b) Kitagawa, T.; Uozumi, T.; Soga, K.; Takata,
T. Polymer 1997, 38, 615. (c) Hong, S. C.; Terenishi, T.; Soga, K. Polymer
1998, 39, 7153. (d) Hong, S. C.; Ban, H. T.; Kishi, N.; Jin, J.; Uozumi, T.;
Soga, K. Macromol. Chem. Phys. 1998, 199, 1393. (e) Stork, M.; Koch,
M.; Klapper, M.; M u¨ llen, K.; Gregorius, H.; Rief, U. Macromol. Rapid
Commun. 1999, 20, 210. Linked to silica: (f) Iiskola, E. I.; Timonen, S.;
Pakkanen, T. T.; H a¨ rkki, O.; Lehmus, P.; Sepp a¨ l a¨ , J. V. Macromolecules
1997, 30, 2853. (g) Schneider, H.; Puchta, G. T.; Kaul, F. A. R.; Raudaschl-
Sieber, G.; Lefebvre, F.; Saggio, G.; Mihalios, D.; Herrmann, W. A.; Basset,
J. M. J. Mol. Catal. A: Chem. 2001, 170, 127.
1
Tetrahedron: Asymmetry 1995, 6, 2105. (c) Minutolo, F.; Pini, D.; Salvadori,
P. Tetrahedron Lett. 1996, 37, 3375. (d) Minutolo, F.; Pini, D.; Petri, A.;
Salvadori, P. Tetrahedron: Asymmetry 1996, 7, 2293. (e) Rheiner, P. B.;
Sellner, H.; Seebach, D. HelV. Chim. Acta 1997, 80, 2027. (f) De, B. B.;
Lohray, B. B.; Sivaram, S.; Dhal, P. K. J. Polym. Sci., Part A: Polym.
Chem. 1997, 35, 1809. (g) Sellner, H.; Seebach, D. Angew. Chem., Int. Ed.
Engl. 1999, 38, 1918. (h) Sellner, H.; Faber, C.; Rheiner, P. B.; Seebach,
D. Chem. Eur. J. 2000, 6, 3692. (i) Yang, X.-W.; Sheng, J.-H.; Da, C.-S.;
1
0.1021/ol026094u CCC: $22.00 © 2002 American Chemical Society
Published on Web 06/11/2002

tetrabutylammonium iodide gave a 59% yield of p-vinyl-
benzylcyclopentadiene as a mixture of double-bond isomers.
Anticipating that this material would be rather prone to
Diels-Alder dimerization, we proceeded immediately to
subject it to deprotonation with NaH, followed immediately
°C for 2-3 days. After cooling, the tube was broken, and
the flexible, light brown polymer rod was sliced into disks
approximately 1 mm in thickness. The disks were extracted
(Soxhlet) with THF for several days, followed by drying
under vacuum at 50 °C, becoming translucent and light
orange in color. Two sets of disks were eventually prepared
for investigation: one (disks A) nominally containing 2.4
mol % Zr as both functional unit and sole cross-linker and
another (disks B) nominally containing 2.3 mol % Zr and
2.7 mol % p-divinylbenzene as additional cross-linker
(Scheme 2).
5
by reaction with ZrCl
4
in THF. The product, bis(p-vinyl-
benzylcyclopentadienyl)zirconium dichloride (1), was char-
1
13
acterized fully spectroscopically (IR, H and C NMR) and
6
by elemental analysis. For solution-phase reactivity com-
parison purposes, the known bis(benzylcyclopentadienyl)
7
analogue (2) was similarly prepared (Scheme 1).
Scheme 1. Preparation of Monomeric Zirconocenes
Scheme 2. Preparation of Polymer Disks A
As expected, attempts to generate polymer beads by
conventional suspension copolymerization of mixtures of 1
and styrene instead gave rise to beads containing hydrolyzed
zirconocene species of the general formula [(ArC
5 4 2
H ) -
8
ZrCl] O. As an alternative, following Sherrington’s strategy,
2
we submitted 1 to bulk copolymerization with styrene under
air- and water-free conditions, both with and without added
p-divinylbenzene. Varying concentrations of 1, styrene, and
divinylbenzene were mixed with dichlorobenzene and warmed
until homogeneous. The solution was transferred to a 5 mm
i.d. Pyrex test tube charged with benzoyl peroxide, heated
to 85 °C, and agitated and purged with Ar bubbling until
polymerization rendered the mixture viscous. Bubbling was
stopped, and polymerization was allowed to continue at 85
(4) Via noncovalent linkages. Impregnation: (a) Soga, K.; Kaminaka,
M. Makromol. Chem., Rapid. Commun. 1992, 13, 221. (b) Soga, K.;
Kaminaka, M. Makromol. Chem. 1993, 194, 1745. Ionic salt: (c) Roscoe,
S. B.; Gong, C.; Fr e´ chet, J. M. J.; Walzer, J. F. J. Polym. Sci. Pt. A: Polym.
Chem. 2000, 38, 2979. (d) Kishi, N.; Ahn, C.-H.; Jin, J.; Uozumi, T.; Sano,
T.; Soga, K. Polymer 2000, 41, 4005. σ-Coordination: (e) Musikabhumma,
K.; Uozumi, T.; Sano, T.; Soga, K. Macromol. Rapid Commun. 2000, 21,
We initially attempted stoichiometric hydrozirconation9
experiments as a test for the qualitative presence of active
10
6
75. (f) Koch, M.; Stork, M.; Klapper, M.; M u¨ llen, K.; Gregorius, H.
Macromolecules 2000, 33, 7713.
5) Anal. Calcd for C14H14: C, 92.26; H, 7.74. Found: C, 92.19; H,
Zr residues in disks A. Using a one-pot procedure, the disks
were combined with Red-Al (Aldrich) and styrene in THF
and allowed to stand for 8 h under Ar. After exposure to
t-BuOOH for an additional 4 h, the disks were extracted,
and the residue after solvent evaporation was examined by
(
1
13
7
.81. H and C NMR indicate the presence of the expected two major
double-bond isomers of the cyclopentadiene ring in a ca. 1.2:1.0 ratio.
6) Anal. Calcd for C28H28Cl2Zr: C, 64.10; H, 4.99; Cl, 13.52; Zr, 17.39.
(
1
Found: C, 63.78; H, 5.05; Cl, 13.62; Zr, 17.90. H NMR (C6D6, 300 MHz)
1
δ: 3.97 (s, 4H), 5.07 (d, J ) 11 Hz, 2H), 5.60 (d, J ) 18 Hz, 2H), 5.63
reversed-phase HPLC and H NMR. The presence of both
(
4
m, 4H), 5.87 (m, 4H), 6.58 (dd, J ) 11, 18 Hz, 2H), 7.03 (d, J ) 8 Hz,
1
- and 2-phenylethanol in the same ratio obtained from
1
H), 7.20 (d, J ) 8 Hz, 4H). H NMR (CDCl3) δ: 3.99 (s, 4H), 5.10 (d,
solution-phase hydrozirconation with either Cp ZrClH or the
2
J ) 11 Hz, 2H), 5.71 (d, J ) 18 Hz, 2H), 6.22 (m, AA′BB′, 8H), 6.68 (dd,
13
J ) 11, 18 Hz, 2H), 7.16 (d, J ) 8 Hz, 4H), 7.34 (d, J ) 8 Hz, 4H).
C
bis(benzyl) analogue was thus confirmed, although the total
amount was too small to allow absolute determination of
yield (Scheme 3).
Given this positive result, we turned to a catalytic system,
alkyne carbometalation based on Negishi’s method.11 In a
typical experiment, 0.5 g of Zr-cross-linked and function-
NMR (C6D6) δ: 36.2, 112.4, 113.4, 117.1, 126.8, 129.4, 133.5, 136.2, 136.9,
13
1
1
40.0. C NMR (CDCl3) δ: 35.7, 112.5, 113.3, 116.8, 126.2, 128.8, 133.4,
35.7, 136.2, 149.2.
(7) (a) Tainturier, G.; Gautheron, B.; Renaut, P.; Etievant, P. C. R. Hebd.
Seances Acad. Sci., Ser. C 1975, 281, 951. (b) Renaut, P.; Tainturier, G.;
Gautheron, B. J. Organomet. Chem. 1978, 148, 35. (c) Dusausoy, Y.; Protas,
J.; Renaut, P.; Gautheron, B.; Tainturier, G. J. Organomet. Chem. 1978,
1
57, 167.
8) Hird, N.; Hughes, I.; Hunter, D.; Morrison, M. G. J. T.; Sherrington,
(
D. C.; Stevenson, L. Tetrahedron 1999, 55, 9575. For a recent application
of grafted polymer disks as acylating agents, see: Tripp, J. A.; Svec, F.;
Fr e´ chet, J. M. J. J. Comb. Chem. 2001, 3, 604.
(9) (a) Hart, D. W.; Schwartz, J. J. Am. Chem. Soc. 1974, 96, 8115. (b)
Schwartz, J.; Labinger, J. A. Angew. Chem. 1976, 88, 402.
(10) Gibson, T. Organometallics 1987, 6, 918.
2366
Org. Lett., Vol. 4, No. 14, 2002

way of comparison, under comparable solution-phase condi-
tions using the analogous soluble catalyst 2, a turnover
number of 6.4 was realized after 2 days of reaction. Thus,
the solution-phase reaction proceeds some 3-10 times faster
than its solid-phase counterpart.
Scheme 3. Hydrozirconation with Disks A
Similar experiments using the more highly cross-linked
disks B displayed reduced reaction rates and correspondingly
lower turnover numbers over the duration of each run (Table
2 2
alized disks were preswelled in 8 mL of CH Cl and then
2
).
treated successively with 10-fold molar excesses of 2 M
trimethylaluminum in hexane and then phenylacetylene.
Reactions were allowed to proceed at room temperature for
4
-7 days, followed by decantation of the now reddish-orange
solutions from the disks, washing of the disks with additional
CH Cl , and workup of the combined filtrates (Scheme 4).
Table 2. Carbometalation of Phenylacetylene Using Disks B in
CH Cl
2 2
2
2
cycle no.
run time (days)
turnover no.
1
6
6
7
7
4
4
1.70
2.03
2.20
1.03
0.69
0.57
2
Scheme 4. Catalytic Carbometalation with Disks A or B
3
4
5a
6
a
Six months elapsed time between runs 5 and 6.
The disks were collected, dried in vacuo, and reused up to
six times.
Although yields were generally determined by NMR, we
did carry out a careful gravimetric yield determination with
one fresh polymer batch. A total mass balance of 88% was
obtained, including 17% of R-methylstyrene, corresponding
to a turnover number of 3.3 over a 4 day reaction time. Thus
there is only minimal material loss to handling or other
processes. Turnover numbers, based upon the ratio of
Finally, a series of experiments was run using a fluorous
solvent, perfluorohexane, in conjuction with CH Cl , based
2
2
upon reports that fluorous solvents can under certain
circumstances enhance catalyst activity and accelerate polymer-
12
supported reactions. We found little effect on the results
of these processes (Table 3).
1
R-methylstyrene product to recovered phenylacetylene ( H
NMR), ranged from 0.5 to 6. Best overall results were
obtained with disks A (Table 1): turnover after 6 days rose
Table 3. Carbometalation of Phenylacetylene Using Disks A in
6 2 2
1:1 n-C F14/CH Cl
cycle no.
run time (days)
turnover no.
Table 1. Carbometalation of Phenylacetylene Using Disks A in
CH Cl
2 2
1
2
3
4
4
4
1.86
1.04
1.20
cycle no.
run time (days)
turnover no.
1
2
3
4
6
6
6
7
7
4
4
2.63
6.05
3.19
1.63
1.59
2.43
1.36
From these results, one can conclude that the highest
reactivity is found in the polymer with the least cross-linking
and that reactivity increases in cycles immediately following
the first. The latter may result from the presence of any of
several complexes involving the polymer-bound zirconocene
5
6a
7
3
and the excess AlMe used in the first cycle, which may
reduce or eliminate an induction period. A detailed analyti-
cal investigation of the species present throughout the active
a
Six months elapsed time between runs 6 and 7.
11
“
life” of such disks is underway to address this issue.
In summary, we have developed the first cross-link-bound
from 2.6 in the first run to 6.0 in the second and then dropped
and leveled off through run 5. At this point, these disks were
stored dry under an inert atmosphere for 6 months and then
reused successfully for two more 4-day runs. During the
course of these recyclings, the disks displayed fragility,
cracking upon repeated swelling and deswelling and in some
cases fracturing to somewhat smaller pieces, which nonethe-
less were still large enough to cause no difficulty in use or
handling (decantation, filtration, or other manipulation). By
zirconocene-functionalized polymer system via what we
(
11) (a) Negishi, E.; Okukado, N. Tetrahedron Lett. 1978, 27, 2357. (b)
Murray, K. S.; Newman, P. J.; Taylor, D. J. Am. Chem. Soc. 1978, 100,
252. (c) Negishi, E.; Yoshida, T. J. Am. Chem.. Soc. 1981, 103, 4985. (d)
2
Negishi, E.; Matsushita, H. Org. Synth. 1984, 62, 31. (e) Negishi, E.; Van
Horn, D. E.; Yoshida, T. J. Am. Chem. Soc. 1985, 107, 6639.
(12) (a) Vinson, S. L.; Gagn e´ , M. R. Chem. Commun. 2001, 1130. (b)
Morphy, J. R.; Rankovic, Z.; York, M. Tetrahedron Lett. 2001, 42, 7509.
Org. Lett., Vol. 4, No. 14, 2002
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believe is the first example of zirconocene incorporation as
a polymerization monomer. We have also demonstrated that
bulk polymerization of an otherwise sensitive organometallic
can give rise to polymer-bound species exhibiting both
catalytic activity and recyclability, provided that the polym-
erization process is suitably controlled. We are currently
looking at ways to make more physically robust disks and
to expand the range of organometallic species that can be
bound within them. These efforts will be described in due
course.
Acknowledgment. We thank the National Science Foun-
dation (CHE-0078191) for research support. Funding sources
for the NMR instrumentation at UC Davis include the NSF
CRIF program (CHE-9808183), the UC Davis Office of
Research, and the Office of the Dean, Division of Mathemat-
ics and Physical Sciences.
Supporting Information Available: Detailed descrip-
tions of experimental procedures. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL026094U
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