Palladium-catalyzed cross-coupling reactions of bromo-substituted group 4 metallocenes

Article abstract of DOI:10.1021/om900353m

Pd-catalyzed Negishi reactions of Zr and Hf complexes bearing η⁵ -4-bromo-2-methylindenyl and η⁵ -2-bromoindenyl ligands with MeZnCl and various aryl- and heteroarylzinc reagents (RZnCl) give in high yields the respective complexes b

Full text of DOI:10.1021/om900353m

3614 Organometallics 2009, 28, 3614–3617
DOI: 10.1021/om900353m
Palladium-Catalyzed Cross-Coupling Reactions of Bromo-Substituted
Group 4 Metallocenes
Alexey N. Ryabov,† Vyatcheslav V. Izmer,† Alexey A. Tzarev,† Dmitry V. Uborsky,†
Andrey F. Asachenko,† Mikhail V. Nikulin,† Jo Ann M. Canich,‡ and
Alexander Z. Voskoboynikov*,†
†Department of Chemistry, Moscow State University, Leninskie Gory, 1/3, Moscow, GSP-1, 119991,
Russian Federation, and ExxonMobil Chemical Company 5200 Bayway Drive, Baytown, Texas 77520
‡
Received May 5, 2009
Summary: Pd-catalyzed Negishi reactions of Zr and Hf
complexes bearing η5-4-bromo-2-methylindenyl and η5-2-bro-
moindenyl ligands with MeZnCl and various aryl- and hetero-
arylzinc reagents (RZnCl) give in high yields the respective
complexes bearing η5-4-R-2-methylindenyl and η5-2-R-inde-
nyl ligands, respectively.
preferably using high-throughput approaches for both catalyst
synthesis and testing. At present, new group 4 metallocene
families are synthesized most often from the respective ligand
families and simple metal derivatives, such as MCln (or their
complexes with donor ligands) and M(NMe2)4, where M = Ti,
Zr, Hf, using transmetalation1,3 and amine elimination reac-
tions,4 respectively. For chiral ansa-zirconocenes, transmetala-
tion using the well-designed dichloride derivatives, such as
One of the most significant areas of growth in the last two
decades has been the development of a new generation of olefin
polymerization catalysts based on group 4 metallocenes.1 These
results form a solid basis for new advances in research and
development of polymerization catalysts targeted mostly at the
design of new highly specific catalytic systems for the preparation
of novel polyolefins with a controlled set of properties, including
morphology, uniform particle size distribution, and physico-
mechanical parameters. It is obvious that reasonable theore-
tical models for catalytic systems involving transition metals are
too complex to allow computations at high levels of theory.2
Therefore, for now and at least for the near future, there is
no alternative to experimental screening of various catalysts,
5
substituted [biphenyl-2,20-diylbis(oxy)]ZrCl2 and particularly
[RN(CH2)3NR]ZrCl2,6 is also widely used. These methodolo-
gies require preliminary ligand synthesis or modification of the
ligand prior to metallocene preparation. Here, we demonstrate
an alternative approach enabling synthesis of novel metallo-
cenes starting from Br-substituted parent metallocene com-
pounds. This synthetic method is based on the Pd-catalyzed
Negishi cross-coupling reaction using group 4 metallocenes
bearing Br-substituted ligands.7 While similar modification of
coordinated ligands in late-transition-metal complexes, parti-
cularly ferrocene derivatives, using catalytic cross-coupling
reactions is well-known,8 similar transformations of early-
transition-metal complexes, which include highly polarized
and reactive metal-ligand bonds,9 have not previously been
described.10
*To whom correspondence should be addressed. E-mail: voskoboy@
org.chem.msu.ru.
(1) (a) Metallocenes: Synthesis, Reactivity, Applications; Togni, A.,
Halterman, R. L., Eds.; Wiley-VCH: Weinheim, Germany, 1998, and
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Catalysts; Baugh, L. S., Canich, J. A. M., Eds.; CRC Press: Boca Raton,
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M. PCT Int. Appl. WO 2,006/065,844, 2006.
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Zirconium and -Hafnium Compounds. Raston/Ellis Horwood: London,
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pentadienyl ligands of group 4 metal complexes resulting in no mod-
ification of the nearest coordination polyhedron have been described:
e.g., the H/D isotope exchange in η5-cyclopentadienyls,11a Pd/C-
or PtO2-catalyzed hydrogenation of η5-indenyls, giving η5-tetrahy-
droindenyls,11b-11f hydroboration of allyl-, vinyl,-11g,11h and alkynyl-
η5-cyclopentadienyl11i complexes, intramolecular photochemical [2þ2]
cycloaddition of vinyl-η5-cyclopentadienyl complexes,11j-11l catalytic
metathesis of alkenyl-substituted Zr and Hf complexes,11m-11v catalytic
hydrogenation of CdN functionalized zirconocenes,11w and the Man-
nich reaction.11x
1435and references therein.
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619, 280. (c) Izmer, V. V.; Agarkov, A. Y.; Nosova, V. M.; Kuz’mina, L.
G.; Howard, J. A. K.; Beletskaya, I. P.; Voskoboynikov, A. Z. Dalton
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2009 American Chemical Society
pubs.acs.org/Organometallics
Published on Web 06/16/2009

Communication
Organometallics, Vol. 28, No. 13, 2009 3615
The three zirconocenes 1 and rac- and meso-3 and the
hafnium complex 2, bearing the η5-4-bromo-2-methylindenyl
ligand, were selected for this study. Pd-catalyzed arylation of
these metallocenes should result in new metal complexes
having η5-4-aryl-2-methylindenyl ligands. This ligand class is
of particular importance, since η5-4-aryl-2-methylindenyls are
significant structural components of highly active commercial
ansa-zirconocene polymerization catalysts that produce
highly isotactic polypropylene (iPP).1,12,13 One more com-
plex selected for this study bears two η5-2-bromoindenyl
ligands. This zirconocene, 4, can be used as a starting
material in cross-coupling reactions to obtain the Way-
mouth-type complexes (2-arylindenyl)2ZrCl2, which are
more recently believed to be multicentered catalysts that
produce “elastomeric polypropylene” (ePP).12,14
respectively. Finally, complex 4 was obtained in 77% yield
from ZrCl4 and 2 equiv of (2-bromoindenyl)lithium in
CH2Cl2. Metallocenes 1-3 can be kept for a long time at
room temperature, though complex 4 decomposes slowly
under these conditions. Moreover, this compound was found
to be extremely sensitive to moisture. Thus, complexes 1-4
represent two different types of the Br-substituted group 4
metallocenes bearing a halogen atom bonded to either (a) the
Cp ring or (b) the benzene ring fused to the cyclopentadienyl
fragment.
To obtain aryl-/alkyl-substituted metallocenes from 1-4,
we have used Pd-catalyzed cross-coupling reactions with
organozinc reagents (Negishi reaction). This reaction with
aryl halides is known to proceed under mild conditions to
give the desired cross-coupling products in high yields.15 It
should be noted that bromo-substituted zirconocenes and
hafnocenes are very specific substrates for cross-coupling
reactions, as they contain highly polarized and reactive
metal-ligand (Cp0 and Cl) bonds.9 Thus, the mildest condi-
tions should be applied to exclude possible substitution of
ligands in the coordination sphere of zirconium or hafnium,
particularly a transfer of Br from ZnBrCl (formed from
RZnCl during the catalytic reaction) to the transition metal,
as well as the analogous transfer of the organic radical of the
organozinc substrate resulting in undesirable aryl/alkyl
zirconium/hafnium species. It should be noted that in com-
parison to the free Br-substituted ligand, the relatively high
ionicity of Zr--Cp0 and Hf-Cp0 bonds should increase the
electron density on the Br-substituted metal-bound ligand,
which in turn should reduce the reactivity of this substrate in
the Negishi reaction, because oxidative addition of electron-
rich aryl bromides to Pd0 is slower.15 This effect should be of
particular importance for 4, which has the Br substituent in
the Cp fragment; hence, this substrate should be less reactive
than 1-3.
Complexes 1 and 2 were obtained in 84 and 76% yields,
respectively, via transmetalation between the lithium salt of
4-/7-bromo-2-methylindene and Cp*MCl3 (M = Zr, Hf) in
toluene. rac- and meso-3 were synthesized from the Et3Sn
derivative of bis(4-bromo-2-methylindenyl)dimethylsilane
and ZrCl4 in toluene and then isolated in 21 and 29% yields,
(11) (a) Larsonneur, A.-M.; Choukroun, R.; Jaud, J. Organometallics
1993, 12, 3216. (b) Wild, F. R. W. P.; Zsolnai, L.; Huttner, G.;
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Chem. 1987, 328, 87. (d) Bandy, J. A.; Green, M. L. H.; Gardiner, I. M.;
Prout, K. J. Chem. Soc., Dalton Trans. 1991, 2207. (e) Rheingold, A. L.;
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1869. (f) Hollis, T. K.; Rheingold, A. L.; Robinson, N. P.; Whelan, J.;
Bosnich, B. Organometallics 1992, 11, 2812. (g) Erker, G.; Nolfe, R.;
In the first series of experiments, we found that the
16
palladium complex Pd(PtBu3)2 catalyzes cross-coupling
reactions of metallocenes 1 and 2 with MeZnCl and various
aryl- and heteroarylzinc reagents (Table 1). On the evidence
of NMR spectroscopy, these reactions proceed even at room
temperature to form the target zirconium and hafnium
complexes involving 4-methyl-, 4-aryl-, and 4-heteroaryl-2-
methylindenyl fragments in high or almost quantitative
yield. No exchange of Cl ligands at Zr (or Hf) by Br was
observed under the conditions studied. Though these reac-
tions were performed on the 100-400 mg scale, in most
cases, analytically pure products were isolated in good
to high yield after treatment of the reaction mixtures by
MeSiCl3 (to remove an excess of organozinc reagent) and the
following crystallization of the crude materials from com-
mon organic solvents (see the Supporting Information for
experimental details). Treatment of the crude reaction mix-
tures by MeSiCl3 was not applied during isolation of 25-28,
which have reactive functional groups. It should be noted
that the synthesis of the zirconocenes bearing highly reactive
cyano and carboxylic groups demonstrates a very important
advantage of this chemistry, as the synthetic protocols
for such promising metallocene targets have not been de-
€
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669. (r) Horn, C. R.; Martın-Alvarez, J. M.; Gladysz, J. A. Organome-
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tallics 2002, 21, 5386and references therein. (s) Kuwabara, J.; Takeuchi,
D.; Osakada, K. Organometallics 2005, 24, 2705. (t) Kuwabara, J.;
Takeuchi, D.; Osakada, K. Chem. Commun. 2006, 3815. (u) Ogasawara,
M.; Watanabe, S.; Fan, L.; Nakajima, K.; Takahashi, T. Organometal-
lics 2006, 25, 5201and references therein. (v) Buchowicz, W.; Jerzykie-
wicz, L. B.; Krasiska, A.; Losi, S.; Pietrzykowski, A.; Zanello, P.
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Organometallics 2006, 25, 5076. (w) Axenov, K. V.; Kehr, G.; Frohlich,
R.; Erker, G. J. Am. Chem. Soc. 2009, 131, 3454. (x) Knuppel, S.; Wang,
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C.; Kehr, G.; Frohlich, R.; Erker, G. J. Organomet. Chem. 2005, 690,
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therein.
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(13) Spalek, W.; Kuber, F.; Winter, A.; Rohrmann, J.; Bachmann, B.;
(15) (a) Negishi, E.; Liu, F. In Metal-Catalyzed Cross-Coupling
Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH: Weinheim,
Germany, 1998; pp 1-47. (b) Negishi, E. In Organozinc Reagents. A
Practical Approach; Knochel, P.; Jones, P., Eds.; Oxford University Press:
Oxford, U.K., 1999; pp 213-243, and references therein .
Antberg, M.; Dolle, V.; Paulus, E. F. Organometallics 1994, 13, 954.
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(b) Hauptman, E.; Waymouth, R. M.; Ziller, J. M. J. Am. Chem. Soc.
1995, 117, 11586. (c) Lin, S.; Waymouth, R. M. Chem. Rev. 2002, 35, 765
and references therein.
(16) Dai, C.; Fu, G. C. J. Am. Chem. Soc. 2001, 123, 2719.

3616 Organometallics, Vol. 28, No. 13, 2009
Table 1. Products of the Pd-Catalyzed Cross-Coupling Reactions of 1 and 2 with RZnCla,b
Ryabov et al.
a Reactions were carried out using 2 mol % of Pd(PtBu3)2 and a 1/1.50 ratio of 1 or 2 to organozinc reagent in THF at room temperature for 4 h, unless
otherwise stated. The subsequent treatment of the reaction mixtures with MeSiCl3 was used to decompose the excess organozinc reagent and isolate the
product (except for compounds 26-28). b The isolated yield of analytically pure metallocene is shown; the total yield (in parentheses) was determined by
NMR spectroscopy. In all cases the studied conversion of 1 or 2 was >99%. c The reaction was carried out for 15 h.
scribed previously in the literature.17 Their possible uses
could range from supported catalysis to organic synthesis
and material chemistry. Synthesis of 25-28 was achieved
using the functionalized organozinc substrates obtained
from the respective Grignard reagents recently developed
by Knochel et al.18 Alternatively, the functionalized orga-
nozinc compounds obtained via direct insertion of zinc into
the aryl-halogen bond according to the Rieke protocol19 or
Knochel method20 work as well. The isolated zirconocenes
were unambiguously characterized by NMR spectroscopy,
and additionally, 26, bearing a 4-EtOC(O)C6H4 fragment,
was studied by X-ray crystal structure analysis (Figure 1).
The screening of phosphine ligands confirmed that Pd
(PtBu3)2 and a mixture of Pd(dba)2 with 2 equiv of PtBu3
are the most active catalytic systems for the Negishi reac-
tion of 1 with 2-p-tolylzinc chloride (Table 2). On the
evidence of NMR spectroscopy, Pd(PtBu3)2 works effec-
tively at 0.4 mol % concentration of Pd to give the
respective coupling product in 73% yield (91% conversion
of 1) at room temperature for 2 h. On the other hand, the
catalyst based on di-tert-butyl[2-(9-phenanthryl)phenyl]
phosphine (entry 6) was found to possess moderate activ-
ity. Interestingly, other catalysts with bulky o-biphenyl-
phosphine ligands or Xantphos were altogether less active
in the coupling reaction studied. Surprisingly, whereas for
Pd(PtBu3)2 we observed no exchange of the Cl ligand of Zr
for Br, the catalysts based on Pd(dba)2 gave mixtures of the
respective zirconium dichloride and chloride-bromide
complexes (in a ratio of 1 to 0.05-0.3 depending on the
phosphine used).
These positive results laid a basis for the following study of
more practical targets, i.e. rac-3 and complex 4, which, after
being arylated, should form promising precatalysts for iPP
and ePP synthesis, respectively. First, rac-3 and 2.6 equiv of
2-p-tolylzinc chloride were shown to give the desired disub-
stituted cross-coupling product rac-34 in almost quantitative
yield in the presence of 2 mol % of Pd(PtBu3)2 in THF for
4 h at room temperature (Scheme 1). Similarly, meso-3
gave pure meso-30 under these conditions. Analytically pure
rac- and meso-34 were isolated in 85 and 79% yields after
treatment of the reaction mixtures with MeSiCl3. Therefore,
no epimerization of both starting ansa complexes and the
respective chiral products was observed under the conditions
employed. It should be noted that possible epimerization of
the racemic complexes in the presence of zinc salts21 could be
the only impediment to a high-yield synthesis of rac-34 using
the cross-coupling protocol.
(17) The only known approach to Zr complexes bearing a carbo-
xylic group bonded to a η5-Cp0 ligand via a -CH2CHdCH- or
-CH2CH2CHdCH- linker was found to be Ru-catalyzed metathesis
of zirconocenes including vinyl functions.11s,11t
(18) Knochel, P.; Krasovskiy, A.; Sapountzis, I. In Handbook of
Functionalized Organometallics: Applications in Synthesis; Knochel, P.,
Ed.; Wiley-VCH: Weinheim, Germany, 2005; Vol. 1, pp 109-172, and
references therein .
The Negishi reaction of 4 bearing two η5-2-bromoindenyl
ligands was found to proceed more slowly than similar
reactions of 1-3. On the evidence of NMR spectroscopy, 4
(19) Rieke, R. D. Aldrichim. Acta 2000, 33, 52and references therein.
(20) Sase, S.; Jaric, M.; Metzger, A.; Malakhov, V.; Knochel, P.
J. Org. Chem. 2008, 73, 7380and references therein.
(21) Lin, R. W. U.S. Patent 5,965,759, 1999.

Communication
Organometallics, Vol. 28, No. 13, 2009 3617
Scheme 1. Negishi Reaction of rac-3 with p-Tolylzinc Chloride
Scheme 2. Negishi Reaction of 4 with p-Tolylzinc Chloride
after 3 h at 65 °C (Scheme 2). The loss of product is likely due
to the instability of 4 in solution. For instance, this complex
has a ca. 15 h half-life in C6D6 solution at 20 °C, and our
attempts to carry out the synthesis of 35 at this temperature
failed, since 4 decomposed more quickly than it reacted to
form the target aryl-substituted product. It should be noted
that no formation of the unsymmetrical complex (2-bromoin-
denyl)(2-p-tolylindenyl)ZrCl2 was detectedby NMRspectros-
copy in all cases studied. Also, we did not observe a transfer of
the p-tolyl or Br fragment to the Zr atom. Finally, the
analytically pure 35 was isolated in 40% yield after treatment
of the reaction mixture with MeSiCl3 and the following
crystallization of the product from toluene-hexanes.
Figure 1. Molecular structure of 26, including 50% thermal
ellipsoids and the atom-labeling scheme.
Table 2. Influence of the Catalyst Structure on the Cross-Cou-
pling Reaction of 1 with p-Tolylzinc Chloridea
In conclusion, we have demonstrated a new methodology
for the synthesis of aryl-/alkyl-substituted group 4 metallo-
cenes, which are known to be components of promising
olefin polymerization catalysts. Whereas usually the desired
metallocenes are synthesized by starting from well-designed
cyclopentadienyl-type ligands and group 4 metal precursors,
the above-described cross-coupling approach is more prac-
tical and enables easy access to a large library of precatalysts
from a single bromo-substituted parent metallocene. This
useful technique can be applied both for the synthesis of
target complexes including metallocenes bearing highly re-
active functionalities, such as cyano and carboxylic groups,
and for high-throughput development of new promising
olefin polymerization catalysts. The study of other halo-
substituted group 4 metal complexes, their cross-coupling
chemistry, and, particularly, the respective functionalized
derivatives is underway.
a Reactions were carried out using 1 mol % of Pd catalyst and a 1/1.50
ratio of 1 to p-tolylzinc chloride in THF at room temperature for 1 h. The
subsequenttreatment with MeSiCl3 was applied to decompose the excess
organozinc reagent. b Yield of (4-p-tolyl-2-methylindenyl)Cp*ZrCl2 on
the evidence of NMR spectroscopy. c Conversion of 1 on the evidence of
NMR spectroscopy. d Xantphos = 9,9-dimethyl-4,5-bis-(diphenylpho-
sphino)xanthene.
Acknowledgment. Financial support from ExxonMobil
Chemical Co. is gratefully acknowledged. We also thank
Prof. Irina P. Beletskaya.
Supporting Information Available: Text, tables, figures, and
CIF files giving synthetic procedures, characterization data for
new compounds, and details of the X-ray crystal structure
determinations of 2 and 26. This material is available free of
charge via the Internet at http://pubs.acs.org.
and 2.6 equiv of p-tolylzinc chloride gave the desired Way-
mouth-type complex 35 in 67% yield (with almost quantita-
tive conversion of 4) in the presence of 4 mol % of Pd(PtBu3)2