Switching a catalyst system from ethene polymerization to ethene trimerization with a hemilabile ancillary ligand

Article abstract of DOI:10.1002/1521-3773(20010702)40:13<2516::AID-ANIE2516>3.0.CO;2-V

A drastic ligand effect was observed in the catalytic ethene conversion by the substituted mono(cyclopentadienyl)titaniumtrichloride/methylalumoxane (MAO) catalysts shown. The catalyst with R = Me produces polyethene, whereas the catalyst with R = Ph selectively trimerizes ethene to 1-hexene. This switch in catalyst performance appears to be the result of a hemilabile behavior of the cyclopentadienyl ligand with the pendant arene group, involving reversible coordination of the arene moiety.

Full text of DOI:10.1002/1521-3773(20010702)40:13<2516::AID-ANIE2516>3.0.CO;2-V

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silica tubes under an argon atmosphere in a glove box. These tubes were
sealed under a 10 Torr atmosphere and then placed in a computer-
[16] D. Röhnert, C. Näther, W. Bensch, Acta Crystallogr. Sect. C 1997, 53,
165 ± 167.
4
controlled furnace. The sample in the first tube was heated to 823 K at
[17] S. Herzog, C. Näther, P. Dürichen, W. Bensch, Z. Anorg. Allg. Chem.
1998, 624, 2021 ± 2024.
1
1.5 Kmin 1, kept at 823 K for three days, very slowly cooled at 4 Kh to
423 K, then cooled to room temperature to afford black hexagonal plates of
the hexagonal modification in about 80% yield. The sample in the second
tube was heated to 873 K at 1.5 Kmin 1, kept at 873 K for two days, when
the furnace was turned off. Black needles of the orthorhombic modification
were obtained in about 70% yield. The reaction mixtures were washed free
of alkali metal chalcogenides with dimethylformamide and then dried with
acetone. Analysis of these crystals with an energy-dispersive X-ray (EDX)-
equipped Hitachi S-4500 scanning electron microscope showed K/Ba/Ti/S
approximately in the ratio 5:1:10:30; the presence of O was observed but
could not be quantified.
[18] W. Bensch, C. Näther, P. Dürichen, Angew. Chem. 1998, 110, 140 ± 142;
Angew. Chem. Int. Ed. 1998, 37, 133 ± 135.
[19] O. Krause, C. Näther, W. Bensch, Acta Crystallogr. Sect. C 1999, 55,
1197 ± 1199.
[20] S. Herzog, C. Näther, W. Bensch, Z. Anorg. Allg. Chem. 1999, 625,
969 ± 974.
[21] W. Bensch, P. Dürichen, C. Näther, Solid State Sci. 1999, 1, 85 ± 108.
[22] F. Q. Huang, J. A. Ibers, Inorg. Chem. 2001, 40, 865 ± 869.
[23] P. Monceau, Electronic Properties of Inorganic Quasi-One-Dimen-
sional Compounds, D. Reidel, Dordrecht, Netherlands, 1985.
Ê
[24] H. Fjellvag, A. Kjekshus, Solid State Commun. 1986, 60, 91 ± 93.
General crystallographic details: Bruker Smart 1000 CCD diffractome-
ter,[30] graphite-monochromatized MoKa radiation (l ˆ 0.71073 Š), Tˆ
153 K. Data were collected by an w scan of 0.38 in groups of 606, 606,
and 606 frames at f settings of 08, 1208, and 2408 for the hexagonal
modification and in groups of 606, 606, 606, and 606 frames at f settings of
08, 908, 1808, and 2708 for the orthorhombic modification. The exposure
times were 15 sframe 1. Intensity data were collected with the program
SMART.[30] Cell refinement and data reduction were carried out with the
use of the program SAINT,[30] face-indexed absorption corrections were
carried out with the program XPREP,[31] and the frame variations were
further corrected with the use of the program SADABS.[30] The structures
were solved with the direct methods program SHELXS and refined with
the least-squares program SHELXL of the SHELXTL-PC suite of
programs.[31] The final refinements included anisotropic displacement
Ê
Ê
[25] S. Furuseth, H. Fjellvag, Acta Chem. Scand. 1991, 45, 694 ± 697.
[26] S. Furuseth, L. Brattas, A. Kjekshus, Acta Chem. Scand. 1973, 27,
2367 ± 2374.
[27] J. A. Cody, M. F. Mansuetto, M. A. Pell, S. Chien, J. A. Ibers, J. Alloys
Compd. 1995, 219, 59 ± 62.
[28] Note added in proof: Prof. W. Bensch has called our attention to an
abstract describing the related compound K6TiS18O (R. Tillinski, C.
Näther, W. Bensch, Z. Kristallogr. Suppl. 1999, 16, 66).
[29] D. Fenske, A. Grissinger, M. Loos, J. Magull, Z. Anorg. Allg. Chem.
1991, 598/599, 121 ± 128.
[30] SMART Version 5.054 Data Collection and SAINT-Plus Version
6.02A Data Processing Software for the SMART System, Bruker
Analytical X-Ray Instruments, Inc., Madison, WI, USA, 2000.
[31] G. M. Sheldrick, SHELXTL DOS/Windows/NT Version 5.10, Bruker
Analytical X-Ray Instruments, Inc., Madison, WI, USA, 1997.
parameters and
a secondary extinction correction. Crystal structure
analysis of K4Ba[Ti6OS20]: black hexagonal plate, 0.040 Â 0.078 Â
0.084 mm, hexagonal, P6322, a ˆ 9.3386(4), c ˆ 18.130(1) Š, Vˆ
1369.3(1) Š3, Tˆ 153 K, Z ˆ 2, 1calcd ˆ 3.004 gcm 3, qmax. ˆ 28.888, 11429
reflections measured, 1175 unique, 1064 observed with I > 2s(I), m ˆ
1
52.12 cm
,
min/max transmission ˆ 0.685/0.816, R1 ˆ 0.0374, wR2 ˆ
0.0895. Black needle, 0.096  0.112  0.248 mm, orthorhombic, Fddd, a ˆ
Switching a Catalyst System from Ethene
Polymerization to Ethene Trimerization with a
Hemilabile Ancillary Ligand**
11.106(2), b ˆ 15.338(2), c ˆ 32.265(5) Š, Vˆ 5496.0(13) Š3, Tˆ 153 K,
3
Z ˆ 8, 1calcd ˆ 2.993 gcm
,
qmax. ˆ 28.858, 16008 reflections measured,
1749 unique, 1590 observed with I > 2s(I), m ˆ 51.94 cm 1, min/max trans-
mission ˆ 0.459/0.713, R1 ˆ 0.0336, wR2 ˆ 0.0894.
Patrick J. W. Deckers, Bart Hessen,* and Jan H. Teuben
Further details on the crystal structure investigations may be obtained from
the Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldsha-
fen, Germany (fax: (‡49)7247-808-666; e-mail: crysdata@fiz-karlsruhe.
de), on quoting the depository numbers CSD-411697 for the orthorhombic
modification and CSD-411698 for the hexagonal modification.
The chemistry of transition metal complexes with hemi-
labile ancillary ligands (i.e., with multidentate ligands that
have a mixture of tightly bound and substitutionally labile
functionalities) is enjoying an increasing popularity.[1] These
hemilabile ligands can stabilize reactive metal centers by the
chelate effect, but keep the metal accessible for substrate
molecules by virtue of the substitutionally labile character of
one of the functionalities. Examples of such hemilabile ligands
include phosphine ± ether, cyclopentadienyl ± alkene, and
phosphine ± arene ligands. In some cases, hemilabile ligands
were found to influence the selectivity and stability of
transition metal catalysts.[2] Here we report that the catalyst
system [(h5-C5H4CMe2R)TiCl3]/MAO (MAO ˆ methylalum-
oxane) is transformed from an ethene polymerization catalyst
into an ethene trimerization catalyst, producing 1-hexene, by
simply changing the ligand substituent R from a methyl to a
Received: February 22, 2001 [Z16664]
[1] T. Saito, Adv. Inorg. Chem. 1997, 44, 45 ± 91.
[2] T. Saito in Early Transition Metal Clusters with p-Donor Ligands (Ed.:
M. H. Chisholm), VCH, New York, 1995, pp. 63 ± 164.
[3] J. D. Corbett, Inorg. Chem. 2000, 39, 5178 ± 5191.
[4] J. D. Corbett, E. Garcia, Y.-U. K. Kwon, A. Guloy, Pure Appl. Chem.
1990, 62, 103 ± 112.
[5] U. Müller, V. Krug, Angew. Chem. 1988, 100, 277; Angew. Chem. Int.
Ed. Engl. 1988, 27, 293 ± 294.
[6] A. Simon, J. Solid State Chem. 1985, 57, 2 ± 16.
[7] J. D. Corbett, J. Alloys Compd. 1995, 229, 10 ± 23.
[8] F. Rogel, J. Zhang, M. W. Payne, J. D. Corbett in Electron Transfer in
Biology and the Solid State. Inorganic Compounds with Unusual
Properties, Vol. 226 (Eds.: M. K. Johnson, R. B. King, D. M. Kurtz, Jr.,
C. Kutal, M. L. Norton, R. A. Scott), American Chemical Society,
Washington, DC, 1990, pp. 369 ± 402.
[*] Dr. B. Hessen, P. J. W. Deckers, Prof. Dr. J. H. Teuben
Dutch Polymer Institute/Center for Catalytic Olefin Polymerization
Stratingh Institute for Chemistry and Chemical Engineering
University of Groningen
[9] S. A. Sunshine, D. Kang, J. A. Ibers, J. Am. Chem. Soc. 1987, 109,
6202 ± 6204.
[10] S. A. Sunshine, D. Kang, J. A. Ibers, Mater. Res. Soc. Symp. Proc. 1987,
97, 391 ± 396.
[11] D. Kang, J. A. Ibers, Inorg. Chem. 1988, 27, 549 ± 551.
[12] P. M. Keane, J. A. Ibers, Inorg. Chem. 1991, 30, 1327 ± 1329.
[13] J. A. Cody, J. A. Ibers, Inorg. Chem. 1994, 33, 2713 ± 2715.
[14] M. A. Pell, J. A. Ibers, Chem. Mater. 1996, 8, 1386 ± 1390.
[15] M. A. Pell, J. A. Ibers, Chem. Ber. 1997, 130, 1 ± 8.
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
Fax : (‡31)50-3634315
E-mail: hessen@chem.rug.nl
[**] Netherlands Institute for Catalysis Research (NIOK) publication
RUG 01-04-02.
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Table 2. Catalytic ethene conversion with the 1 ± 3/MAO catalyst systems
(toluene solvent, 5 bar ethene, 308C, 15 mmol Ti, Al:Ti ˆ 1000, 30 min run
time).
phenyl group. It appears that the hemilabile behavior of the
cyclopentadienyl ligand with a pendant arene group is
responsible for this drastic switch in catalyst performance.
The catalyst precursors used in this study, [(h5-
C5H4CMe2R)TiCl3] (R ˆ Ph, 1; 3,5-dimethylphenyl, 2; Me,
3), are readily obtained by reaction of 6,6-dimethylfulvene
with the appropriate RLi reagent followed by reaction with
TiCl4, or by reaction of TiCl4 with the corresponding
Me3Si(C5H4CMe2R) reagent.[3, 4] In previous work by us and
others it has been shown that cationic dialkyl derivatives of 1
and 2 can be generated by reaction of the trimethyl complexes
[(h5-C5H4CMe2Ar)TiMe3] with Lewis acids such as B(C6F5)3,
Catalyst (R)
C6 products [g] C10 products [g] PE [g]
Productivity[a]
(wt%)
(wt%)
(wt%)
1 (Ph)
2 (3,5-Me2C6H3)
3 (Me)
20.9 (83)
7.9 (93)
0.5 (17)
3.5 (14)
0.5 (5)
0.1 (4)
0.5 (1.8) 2787
0.1 (1.3) 1053
2.4 (76)
66
[a] In kg C6 product per mole Ti and h.
whereas the absence of a pendant arene group leads to the
predominant formation of PE. This indicates that a) the
pendant arene group is essential to obtaining selective
trimerization, and that b) this catalysis is likely to involve
reversible coordination of the arene moiety to the metal
center. It may be noted that even for the tert-butylcyclopen-
tadienyl system 3/MAO some ethene trimerization is ob-
served. A related observation was made recently by Pellec-
chia et al. in ethene polymerization with [(h5-C5Me5)TiMe2]-
[MeB(C6F5)3] in toluene as catalyst, where the PE obtained
contains a noticeable amount of n-butyl side groups.[5] It was
suggested that the catalyst is partly converted to a species that
trimerizes ethene to 1-hexene, which is then incorporated into
the polymer. Our observations indicate that transient coordi-
nation of the toluene solvent to the metal center may be
instrumental in this,[6] albeit less efficient than the pendant
arene group in the [(h5-C5H4CMe2Ar)TiCl3]/MAO systems as
the interaction is intermolecular rather than intramolecular.
The only family of catalysts known thus far to give highly
selective ethene trimerization is based on chromium (mix-
tures of Cr salts with alkylaluminum compounds and added
ligands, especially imidazoles, or coordinating solvents such as
1,2-dimethoxyethane).[7] Very recently, selective trimerization
of higher alkenes with triazacyclohexane ± chromium species
was also reported.[8] The catalysis in these systems is proposed
to proceed via metallacyclic intermediates, chromacyclopen-
tanes and chromacycloheptanes, where b-H abstraction from
the latter, followed by reductive elimination, leads to
1-hexene formation and a low-valent Cr species. The latter
can then couple two ethene molecules to regenerate the
chromacyclopentane.[9, 10] It is possible that a similar cycle is
operative in the present titanium system. Several catalytic
C-C coupling reactions have been reported in neutral Ti
systems that involve the TiIV/TiII couple (e.g., in the bis-
(aryloxy)titanium compounds).[11] The question is then
how the system converts from the cationic species
‡
and that in these cationic species [(C5H4CMe2Ar)TiMe2] the
arene moiety is h6-coordinated to the metal center. For Arˆ
3,5-Me2C6H3 this yields a relatively stable 16-electron com-
plex that does not polymerize propene at ambient temper-
ature and pressure.[4] For Ar ˆ C6H5 the coordination is
noticeably weaker, and a modest activity in propene polymer-
ization was reported.[3]
We have now observed that activation of the trichloride 1
with 1000 equiv of the cocatalyst MAO in toluene under
ethene pressure generates a catalyst that converts ethene
predominantly to 1-hexene. Results of catalytic ethene
conversion experiments under various conditions are listed
in Table 1. The catalyst produces at ambient temperatures
olefin trimerization products with high selectivity (95 ±
98 wt% overall). These products consist of two fractions: C6
products (>99% 1-hexene) and C10 products (cotrimers of
Table 1. Catalytic ethene conversion with the 1/MAO catalyst system (toluene
solvent, 15 mmol Ti, Al:Ti ˆ 1000, 30 min run time).
p(ethene)
[bar]
T
[8C]
C6 products [g]
(wt%)
C10 products [g]
(wt%)
PE [g]
(wt%)
Produc-
tivity[a]
2
5
30
30
30
50
80
8.0 (87)
20.9 (83)
47.2 (86)
12.4 (83)
3.3 (76)
1.0 (11)
3.5 (14)
5.1 (9)
1.6 (11)
0.2 (4)
0.2 (1.6)
0.5 (1.8)
1.4 (2.6)
0.7 (4.6)
0.8 (19)
1066
2787
6292
1653
440
10
5
5
[a] In kg C6 product per mole Ti and h.
ethene and 1-hexene). The C10 fraction consists mainly
(>75%) of 5-methyl-non-1-ene. In addition, some 1-octene
(1 wt%) and polyethene (PE, 1 ± 3 wt%) is produced. The
rate of production of 1-hexene increases with increasing
ethene pressure, the C6 productivity being about 550 ± 600 kg
per mole Ti, h, and bar over a range of 2 ± 10 bar in ethene
pressure. The thermal stability of the catalyst is modest, and
increasing the reaction temperature decreases the overall
catalyst productivity due to catalyst degradation (as seen from
the decrease in the ethene uptake rate over the run period)
and increases the relative amount of PE produced (see
Table 1).
The importance of the pendant arene group on the ancillary
ligand for 1-hexene production can be seen by comparing the
performance of the three catalyst systems 1 ± 3/MAO (Ta-
ble 2). Making the pendant arene group more electron-rich by
adding methyl substituents significantly diminishes the cata-
lyst activity (but the selectivity for trimerization is retained),
‡
[(C5H4CMe2Ar)TiMe2] (generated by the reaction of the
corresponding trichloride with MAO) to a cationic metal-
lacyclopentane. A proposal for this process is shown in
Scheme 1.
‡
The initially generated species [(C5H4CMe2Ph)TiMe2] will
undergo multiple ethene insertions into the Ti Me bonds
(where during ethene capture the arene is likely to be
noncoordinating) to produce bis(n-alkyl) compounds
‡
[(C5H4CMe2Ph)Ti(CH2CH2R)2] . These are in equilibrium
(through b-H elimination) with hydride ± olefin species.
Normally, dissociation of the olefin is energetically unfavor-
able unless it is driven out by another incoming ligand (e.g.,
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Scheme 1. Proposed reaction pathway for catalyst transformation and catalytic ethene trimerization in the system studied.
was passed over columns of Al2O3 (Fluka), BASF R3 ± 11 supported Cu
ethene, which will then rapidly insert into the M H bond,
resulting in the normal chain transfer process for catalytic
ethene polymerization).[12] In the system with the pendant
arene group it is possible that the alkene is driven out by the
incoming arene, which will lead to a 16-electron hydride ± alkyl
oxygen scavenger, and molecular sieves (4 Š) under nitrogen atmosphere
prior to use. Ethene (AGA polymer grade) was passed over BASF R3-11
supported Cu oxygen scavenger and molecular sieves (4 Š). Cyclooctane
(used as internal standard) was distilled from sodium prior to use. Product
analyses were performed by GC on a HP 6890 instrument equipped with a
HP-1 dimethylpolysiloxane column (19095 Z-123) and by GC/MS using a
HP 5973 mass-selective detector attached to a HP 6890 GC instrument.
‡
species, [(h5,h6-C5H4CMe2Ph)Ti(H)(CH2CH2R)] . Reductive
elimination then releases CH3CH2R and forms a TiII inter-
mediate which can capture two ethene molecules to give a
cationic titanacyclopentane. This species can then perform the
catalytic ethene trimerization (Scheme 1), following the path-
way described above for the Cr-based catalysts.
We are presently investigating the validity of this proposal.
Although we did observe that the independently generated
ionic species [(h5,h6-C5H4CMe2Ar)TiMe2][MeB(C6F5)3] reacts
with ethene in [D8]toluene to generate 1-hexene (NMR tube
experiment), we have as yet not been successful in observing
and isolating the proposed cationic titanacyclopentane inter-
mediate. Nevertheless, we feel that the present proposal is
reasonable given the observations made thus far.
In conclusion, we have identified a catalyst system, [(h5-
C5H4CMe2R)TiCl3]/MAO, that, by attachment of a pendant
arene group to the cyclopentadienyl ancillary ligand (R ˆ
aryl), can be transformed from an ethene polymerization
catalyst to a selective ethene trimerization catalyst. Although
ethene oligomerizations that produce 1-hexene in excess of
the amount expected from Schultz ± Flory product distribu-
tions, and ethene polymerization catalysts that produce
branched polyethenes are known (e.g., for certain Ziegler-
type catalyst systems),[13] to our knowledge the present system
is the first highly selective ethene trimerization catalyst that is
not based on chromium. The principle that a hemilabile ligand
functionality can divert the metal species from one catalytic
cycle into another may be applicable to other types of
catalysis.
General procedure for the catalytic ethene conversions: A stainless-steel
1-L autoclave (Medimex), fully temperature- and pressure-controlled and
equipped with solvent and catalyst injection systems, was preheated in
vacuo for 45 min at 1008C. The reactor was cooled to the desired
temperature, charged with 200 mL of toluene, and pressurized with ethene.
After equilibrating for 15 min, the appropriate amount of MAO/toluene
was injected, together with 25 mL of toluene. Subsequently, a mixture of
2.50 mL of cyclooctane (internal standard) and 1.0 mL of a 15 mm stock
solution of the titanium halide complex in toluene was injected, together
with 25 mL of toluene, to start the reaction. During the reaction the ethene
pressure was kept constant to within 0.1 bar of the initial pressure by
replenishing flow. The run was ended by adding an aliquot of ethanol, and
the reactor was vented. Remaining residual MAO was destroyed by adding
further ethanol, and samples of the reaction mixture were taken to analyze
and quantify the soluble components by GC and GC/MS. The polymer was
stirred for 1 h in acidified ethanol, repeatedly rinsed with ethanol on a glass
frit, and dried in vacuo at 708C overnight.
Received: February 27, 2001 [Z16694]
[1] For a recent review on transition metal complexes with hemilabile
ligands, see: C. S. Slone, D. A. Weinberger, C. A. Mirkin, Prog. Inorg.
Chem. 1999, 48, 233 ± 350.
[2] See for example: a) E. Lindner, B. Keppeler, H. A. Mayer, K.
Gierling, R. Fawzi, M. Steinmann, J. Organomet. Chem. 1996, 526,
175 ± 184; b) S. Mecking, W. Keim, Organometallics 1996, 15, 2650 ±
2656; c) M. Nandi, J. Jin, T. V. RajanBabu, J. Am. Chem. Soc. 1999,
121, 9899 ± 9900, and references therein.
[3] J. Sassmannshausen, A. K. Powell, C. E. Anson, S. Wocadlo, M.
Bochmann, J. Organomet. Chem. 1999, 592, 84 ± 94.
[4] P. J. W. Deckers, A. J. van der Linden, A. Meetsma, B. Hessen, Eur. J.
Inorg. Chem. 2000, 929 ± 932.
[5] C. Pellecchia, D. Pappalardo, G.-J. Gruter, Macromolecules 1999, 32,
4491 ± 4493.
[6] D. J. Gillis, M.-J. Tudoret, M. C. Baird, J. Am. Chem. Soc. 1993, 115,
2543 ± 2545.
[7] a) J. R. Briggs (Union Carbide), US-A 4668838, 1987 [Chem. Abstr.
1987, 107, 178604]; b) W. K. Reagan, B. K. Conroy (Phillips Petro-
leum), EP 416304, 1990 [Chem. Abstr. 1992, 116, 60189]; c) M.
Tamura, K. Uchida, K. Iwanaga, Y. Ito (Sumitomo Chemical), EP
Experimental Section
All manipulations of catalyst and cocatalyst solutions were carried out
under purified nitrogen atmosphere using standard glove-box and Schlenk
techniques. The compounds 1 ± 3 were prepared according to literature
procedures.[3, 4]
A
toluene solution of MAO (26 wt%, Akzo Nobel
Chemicals) was used as received. Toluene (Aldrich, anhydrous, 99.8%)
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699648, 1995 [Chem. Abstr. 1996, 124, 344431]; d) Y. Araki, H.
Nakamura, Y. Nanba, T. Okano (Mitsubishi Chemical), US-A
5856612, 1997 [Chem. Abstr. 1997, 127, 161493].
immobilized catalysts to asymmetric cyclopropanation and
aziridination led to modest enantioselectivity, which could be
related to unfavorable interactions between the metal com-
plex and the support.[10, 11] Better results were recently
reported by Evans et al. for a hetero Diels ± Alder reaction
with CuII ± box in the presence of Florisil, although the use of a
poor solvent for the metal complex was required for effective
recovery of the catalyst.[12]
To develop a versatile, reusable, insoluble box ligand
equivalent to the homogeneous systems 1, the preparation
of bis-oxazoline covalently bound to an inert polystyrene
matrix was designed. The chiral monomer 2a was synthesized
by the route depicted in Scheme 1,[13] together with the soluble
model 2b of the supported ligand.
[8] R. D. Köhn, M. Haufe, G. Kociok-Köhn, S. Grimm, P. Wasserscheid,
W. Keim, Angew. Chem. 2000, 112, 4519 ± 4521; Angew. Chem. Int. Ed.
2000, 39, 4337 ± 4339.
[9] J. R. Briggs, J. Chem. Soc. Chem. Commun. 1989, 674 ± 675.
[10] R. Emrich, O. Heinemann, P. W. Jolly, C. Krüger, G. P. J. Verhovnik,
Organometallics 1997, 16, 1511 ± 1513.
[11] a) M. G. Thorn, J. E. Hill, S. A. Waratuke, E. S. Johnson, P. E.
Fanwick, I. P. Rothwell, J. Am. Chem. Soc. 1997, 119, 8630 ± 8641;
b) S. A. Waratuke, M. G. Thorn, P. E. Fanwick, A. P. Rothwell, I. P.
Rothwell, J. Am. Chem. Soc. 1999, 121, 9111 ± 9119.
[12] P. Margl, L. Deng, T. Ziegler, J. Am. Chem. Soc. 1999, 121, 154 ± 162,
and references therein.
[13] See for example: S. Murtuza, S. B. Harkins, G. S. Long, A. Sen, J. Am.
Chem. Soc. 2000, 122, 1867 ± 1872, and references therein.
OTs
Ar
O
COOEt
COOEt
COCl
COCl
Ar
O
a – c
d
An Insoluble Polymer-Bound Bis-Oxazoline
Copper(ii) Complex: A Highly Efficient
Heterogeneous Catalyst for the
O
Ar
O
Ar
Enantioselective Mukaiyama Aldol Reaction**
O
O
O
O
e
Simonetta Orlandi, Alessandro Mandoli, Dario Pini,
and Piero Salvadori*
NH HN
OH HO
tBu
N
N
tBu
tBu
tBu
In recent years asymmetric catalytic procedures have been
emerging as a major option for the preparation of chiral
compounds in enantiomerically enriched form, especially
when the catalyst system consists of readily available, cheap
and nontoxic components.[1] However, the recovery of the
catalyst and its reuse is often mandatory for large-scale
applications. At present, one of the most promising solutions
for this problem seems to be the anchoring of effective soluble
systems on an insoluble matrix, ideally without any reduction
of catalytic performances with respect to the homogeneous
phase.[2, 3] While this strategy is well established in reduction
and oxidation processes,[4, 5] few examples have been reported
for C C bond-forming reactions.[6]
2a Ar = 4-CH2CHC6H4
2b Ar = Ph
Scheme 1. a) NaH, THF, reflux, 24 h, 65 ± 95%; b) 18m KOH, reflux, 3 h,
82 ± 89%; c) (COCl)2, cat. DMF, CH2Cl2, RT, 18 h; d) (S)-tBuCH(NH2)-
CH2OH (2 equiv), Et3N, CH2Cl2, RT, 35 min; e) TsCl, Et3N, cat. DMAP,
CH2Cl2, RT, 27 h, 50 ± 55% over two steps. Ts ˆ p-toluenesulfonyl,
DMAP ˆ 4-dimethylaminopyridine.
The AIBN-initiated copolymerization of 2a with styrene
and divinylbenzene (DVB), in the presence of toluene as a
porogen agent,[14] followed by exhaustive extraction with THF
and CH2Cl2, afforded 3 (Scheme 2). It was characterized by
IR spectroscopy (nÄC ˆ 1655 cm ),[15] and its content of
1
ˆ
N
In our continuing effort to develop heterogeneously
catalyzed asymmetric Mukaiyama aldol reactions,[7] we be-
came interested in bis-oxazoline (box) ligands 1.[8] In spite of
the versatility of this class of chiral auxil-
1
chiral ligand (0.31 mmolg ) was evaluated by nitrogen
elemental analysis.
0.18 mmolg was determined by equilibrating 3 with an
A
maximum metal uptake of
1
excess of Cu(OTf)2 in THF.
iaries, few supported copper box complexes
O
O
have been reported to date.[9] They rely on
ion exchange to anchor them to an organic
or inorganic host. The application of these
N
N
R
R
1
O
[*] Prof. P. Salvadori, M. Sc. S. Orlandi, Dr. A. Mandoli, Prof. D. Pini
Á
Dipartimento di Chimica e Chimica Industriale, Universita di Pisa
a
2a
Centro di Studio del C.N.R. per le Macromolecole Stereordinate ed
Otticamente Attive
O
O
Via Risorgimento 35, 56126 Pisa (Italy)
Fax : (‡39)050-918-409
N
N
tBu
tBu
E-mail: psalva@dcci.unipi.it
3
Á
[**] This work was supported by MURST, Universita di Pisa (National
Project ªStereoselezione in Sintesi Organica, Metodologie ed Appli-
cazioniº), and Consiglio Nazionale delle Ricerche (CNR).
Scheme 2. a) Styrene, DVB (2a:styrene:DVB ˆ 7:42:51), AIBN, toluene
(60% v/v), 808C, 18 h. AIBN ˆ azobisisobutyronitrile.
Angew. Chem. Int. Ed. 2001, 40, No. 13
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
1433-7851/01/4013-2519 $ 17.50+.50/0
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