Stereospecific styrene enchainment at a titanium site within a helical ligand framework: Evidence for the formation of homochiral polystyrene

Article abstract of DOI:10.1002/anie.200700783

(Chemical Equation Presented) Tactful: Titanium catalyst precursors in both optically active, enantiomeric forms have been obtained diastereoselectively, and after activation with methylaluminoxane (MAO) they were used for the stereospecific oligomerizati

Full text of DOI:10.1002/anie.200700783

Communications
DOI: 10.1002/anie.200700783
Polymerization Catalysis
Stereospecific Styrene Enchainment at a Titanium Site within a Helical
Ligand Framework: Evidence for the Formation of Homochiral
Polystyrene**
Klaus Beckerle, Ramanujachary Manivannan, Bing Lian, Geert-Jan M. Meppelder,
Gerhard Raabe, Thomas P. Spaniol, Henner Ebeling, Frederic Pelascini, Rolf Mülhaupt, and
Jun Okuda*
The phenomenon of cryptochirality renders isotactic poly(a-
olefin)s such as polypropylene optically inactive, since Cs
symmetry can be assumed for high-molecular-weight poly-
mers formed from prochiral olefins.[1,2] Only for highly
specific sequences of segments within the poly(a-olefin)
chain can optical activity be measured.[3,4] The advent of
chiral, and in rare cases optically active, enantiomerically
pure ansa-zirconocene catalysts with well-defined molecular
structure[5] allowed the observation of the stereochemical
discrimination of the prochiral monomer at a single metal
center during oligomerization.[2,6,7] To the best of our knowl-
edge, it has not yet been determined at what degree of
polymerization the observable optical activity of poly(a-
olefin)s disappears and whether “homochiral” isotactic poly-
(a-olefin)s can be accessed.
mentioned catalyst precursors. By utilizing chain-transfer
methodology in the presence of 1-hexene, we demonstrate
that the insertion of styrene in such postmetallocene cata-
lysts[10] occurs stereospecifically giving optically active iPS
oligomers.
As we reported previously, the stereorigidity of the
titanium catalysts containing a [OSSO]-type ligand depend
rather critically on the presence of a two-carbon backbone
linking the two phenol units and on the presence of bulky
ortho substituents.[11] Therefore, we introduced a trans-1,2-
cyclohexanediyl backbone that connects two 4,6-di-tert-butyl-
phenol (a) or 6-tert-butyl-4-methylphenol (b) groups by a
SCCS link. We followed the reaction sequence delineated in
Scheme 1 and obtained the bis(phenol) rac-1a in three steps.
Starting with cyclohexene oxide, consecutive nucleophilic
We have recently shown that configurationally rigid
bis(phenolato) titanium catalysts derived from linear
[OSSO]-type ligands efficiently polymerize styrene to give
isotactic polystyrene (iPS).[8] iPS was previously prepared
using heterogeneous Ziegler-type catalysts and characterized
as the first crystallizable poly(a-olefin) by Natta et al.[9] We
have now prepared optically active variants of the above-
[*] Dr. K. Beckerle, Dr. R. Manivannan, Dr. B. Lian, G.-J. M. Meppelder,
Dr. T. P. Spaniol, Prof. Dr. J. Okuda
Institute of Inorganic Chemistry
RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
Fax: (+49)241-809-2644
E-mail: jun.okuda@ac.rwth-aachen.de
Prof. Dr. G. Raabe
Institute of Organic Chemistry
RWTH Aachen University
Landoltweg 1, 52056 Aachen (Germany)
Dr. H. Ebeling, Dr. F. Pelascini, Prof. Dr. R. Mülhaupt
Freiburger Materialforschungszentrum
and Institute of Macromolecular Chemistry
Albert-Ludwigs-Universität Freiburg
Stefan-Meier-Strasse 21, 79104 Freiburg (Germany)
[**] This work was supported by the Fonds der Chemischen Industrie
and the Deutsche Forschungsgemeinschaft. R.M. thanks the
Alexander von Humboldt Foundation for a postdoctoral fellowship;
G.-J.M. thanks the Graduiertenkolleg 440 (“Methoden der asym-
metrischen Synthese”) for a doctoral fellowship. We thank Prof. M.
Schmidt, Mainz University, for kindly measuring MALDI TOF mass
spectra.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Scheme 1. Synthesis of the chiral catalyst precursors 2a.
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substitutions by the mercaptophenolate were critical to the
successful synthesis, which made use of anchimeric assistance
in each of the substitution steps. Chiral resolution of rac-1a
was achieved by fractional crystallization and column chro-
matography of the bis((1S)-camphor sulfonate)s, for which
both diastereomers were isolated and their absolute config-
urations determined by X-ray crystallography. After hydro-
lytic removal of the chiral auxiliary, the enantiomerically pure
bis(phenols) (À)-(R,R)-1a and (+)-(S,S)-1a were obtained in
good yield and characterized by their optical rotation and CD
spectra (see the Supporting Information). Complexation of
these ligands at a TiX2 fragment (2: X = Cl, 3: X = OiPr) was
straightforward by reaction of the bis(phenol) with TiX4.
Use of the racemic ligand rac-1a resulted in single crystals
for the dichloro complex 2a. According to X-ray crystallog-
raphy, the chiral backbone led to the formation of a single
configuration (helicity) at the titanium center. Thus, the
racemic ligand derived from rac-1a resulted in the selective
formation of only one enantiomeric pair of the diastereomeric
complex ((L,R,R)-2a and (D,S,S)-2a); the other possible
diastereomers ((D,R,R)-2a and (L,S,S)-2a) were absent
(Figure 1).[12] When we employed the resolved, enantiomer-
ically pure bis(phenol)s (À)-(R,R)-1a and (+)-(S,S)-1a, the
Figure 2. CD spectra of the enantiomerically pure titanium complexes
(À)-(L,R,R)-2a (red line) and (+)-(D,S,S)-2a (black line) in CH2Cl2 at
258C.
termination using 1-hexene to be most practical.[14] By
variation of the styrene/1-hexene ratio, a series of oligostyr-
enes was synthesized using both enantiomers of the precata-
lyst 2a in toluene (Scheme 2, Table 1). A typical pair of
oligomers having Mn of approximately 2000 (runs 5 and 6,
Table 1) showed peaks assignable to chains containing one to
Scheme 2. Synthesis of isotactic polystyrene oligomer.
five 1-hexene units (m = 0–4) attached to a polystyrene
segment (maximum at n ꢀ 30) in the MALDI TOF mass
spectra (see the Supporting Information). H and 13C NMR
1
Figure 1. Structure of the D,S,S enantiomer in the crystal of the
racemic compound 2a (hydrogen atoms omitted; atoms depicted at
the 50% probability level). Selected bond lengths [Š] and angles [8]:
Ti–Cl 2.2639(7), 2.2733(7), Ti–S 2.6106(7), 2.6191(7) Ti–O 1.8639(16),
1.8827(16); Cl-Ti-Cl 108.38(3), O-Ti-O 157.93(7), S-Ti-S 79.91(2).
spectra confirmed the presence of iPS segments terminated
by regio- and stereoirregular oligo(hexene) segments. Homo-
oligomerization of 1-hexene using the same catalyst system
confirmed the formation of oligo(1-hexene) showing the
absence of both regio- and stereoselectivity as well as optical
activity.
corresponding enantiomerically pure titanium complexes
(L,R,R)-2a and (D,S,S)-2a were obtained and completely
The dependence of the specific rotation values on the
molecular weight of the iPS is depicted in Figure 3. It is
evident that for Mn > 5000 no optical activity could be
detected, but below this threshold (corresponding to roughly
45 styrene units with an average of three terminal 1-hexene
characterized.
Specific
rotations
for (L,R,R)-2a
of
[a]2D3
and
=
À277.3 degcm3 gÀ1 dmÀ1
+ 277.3 degcm3 gÀ1 dmÀ1 for (D,S,S)-2a (c = 0.002 gmLÀ1,
CH2Cl2), CD spectra (Figure 2), and a single-crystal structure
determination confirm the chiral structure, in which the
tetradentate ligand is arranged helically around the octahe-
dral titanium center.
units)
specific
rotation
[a]2D3
of
Æ 1.5(1)–
5.9(1) degcm3 gÀ1 dmÀ1 could be measured reproducibly.[15]
Moreover, each enantiomeric titanium catalyst produced
only oligomers with the same sign of optical rotation,
corroborating that enantiomorphic site control is operative.
It is noteworthy that for the samples of the lowest molecular
weights (runs 1 and 2, Table 1), a decrease in optical rotation
is observed. Although we were unable to produce samples of
lower molecular weight, we ascribe this effect to the lower
stereoselectivity at the earlier stages of insertion, as efficient
enantiofacial discrimination of styrene is effected by the
Following our previous studies on homogeneous styrene
polymerization,[8,11] the enantiomeric pairs of titanium cata-
lyst precursors 2a were activated by methylaluminoxane
(MAO) (to give a configurationally stable alkyl cation with
homotopic sites[11b]) and allowed to polymerize styrene. As
expected, the isotactic polystyrene obtained showed no
optical rotation. After considerable efforts to find a suitable
chain-transfer agent (H2, AlR3, ZnR2)[13] we found chain
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Table 1: Isotactic polystyrene with oligo(1-hexene) terminal groups prepared by using MAO-activated, optically active (À)-(L,R,R)-2a and (+)-(D,S,S)-
2a.
Run[a] [Styrene] [molLÀ1
]
[1-hexene] [molLÀ1
]
Cat.[b]
Yield [mg] Mn [gmolÀ1
]
Mw/Mn
[a]D23[d] [degcm3 gÀ1 dmÀ1
]
[F]2D3 [degcm2 kmolÀ1
]
[c]
[c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0.48
0.48
0.64
0.64
1.6
1.6
1.6
1.6
1.6
1.6
1.6
1.6
3.2
4.5
4.5
4.5
4.5
4.5
4.5
3.4
3.4
2.3
2.3
1.4
1.4
3.2
3.2
(R,R)-2a 494
(S,S)-2a 568
(R,R)-2a 517
(S,S)-2a 377
(R,R)-2a 325
(S,S)-2a 468
(R,R)-2a 153
(S,S)-2a 168
(R,R)-2a 221
(S,S)-2a 235
(R,R)-2a 567
(S,S)-2a 617
(R,R)-2a 517
(S,S)-2a 680
750
790
870
1.24
1.35
1.31
1.25
1.46
1.49
1.46
1.46
1.52
1.58
1.67
1.45
1.68
1.68
À4.9(1)
+4.4(1)
À5.9(1)
+5.6(1)
À4.2(2)
+3.5(2)
À2.9(1)
+2.7(1)
À2.5(1)
+2.2(1)
À1.8(1)
+1.5(1)
Æ0
À37
+35
À45
+61
À81
+65
À78
+73
À94
+74
À80
+67
Æ0
1120
1930
1880
2700
2680
3750
3460
4420
4590
5930
5870
3.2
Æ0
Æ0
[a] Reaction time for runs 1–6: 6 h. [b] Activated by MAO. [c] Determined by GPC using polystyrene standard. [d] c=0.01 gmLÀ1 in CH2Cl2.
that was decanted off, washed twice with 15 mL of pentane, and dried
in vacuo to give rac-2a (0.25 g, 0.37 mmol); yield: 86%. 1H NMR: d =
0.36 (m, 2H, CH (C6H10)), 1.07–1.18 (m, 2H, CH (C6H10)), 1.18 (s,
18H, C(CH3)3), 1.23–1.43 (m, 2H, CH (C6H10)), 1.57 (s, 18H,
C(CH3)3), 1.72–1.85 (m, 2H, CH (C6H10)), 2.43–2.56 (m, 2H, SCH),
4
7.08 (d, 2H, 4JHH = 2.3 Hz, arom. CH (C3)), 7.45 ppm (d, 2H, JHH
=
2.3 Hz, arom. CH (C5)). 13C{1H} NMR: d = 25.10 (CH2 (C6H10)), 29.82
(C(CH3)3), 31.59 (C(CH3)3), 32.41 (CH2 (C6H10)), 34.61 (C(CH3)3),
35.87 (C(CH3)3), 55.59 (SCH), 117.42 (arom. C2), 127.33 (arom. C3),
129.70 (arom. C5), 137.18 (arom. C4), 144.03 (arom. C6), 167.64 ppm
(arom. C1). Analysis (%) calcd for C34H50Cl2O2S2Ti (673.66): C 60.62,
H 7.48; found: C 60.63, H 7.86. Crystals suitable for X-ray analysis
were obtained as toluene solvate by slow evaporation of a toluene
solution at room temperature.[18]
General polymerization procedure: A Schlenk tube was charged
with toluene (calculated for a total volume of 15 mL), 1.2 mL of a
MAOsolution in toluene (10 wt%; Eurecen; used as received), and a
mixture of styrene and 1-hexene (toluene, styrene and 1-hexene were
dried over sodium or calcium hydride and degassed three times
before use; concentrations of the monomers are given in Table 1).
This mixture was stirred for 20 min at 408C, followed by addition of
0.5 mL of a 0.25 mm stock solution of the enantiomerically pure
complex (À)-(R,R)-2a or (+)-(S,S)-2a (17 mg, 25 mmol) in 10 mL of
toluene. The reaction mixture was stirred at 408C for 2 h, the reaction
quenched by addition of 0.5 mL of isopropyl alcohol, and the polymer
precipitated from 100 mL of acidified methanol. The product was
redissolved in chloroform and precipitated again from acidified
methanol. This procedure was repeated after filtration of the
chloroform solution through a layer of silica. The oligomers were
dried in vacuo for several hours.
Figure 3. Dependence of the specific rotation [a]2D3 and molar rotation
[F]2D3 on the number-averaged molecular weight of isotactic oligostyr-
*
ene terminated by 1-hexene. Specific rotation ( ) and molar rotation
~
*
( ) of samples prepared with the complex (À)-(L,R,R)-2a; ( ) optical
~
rotation and molar rotation ( ) of samples prepared with the complex
(+)-(D,S,S)-2a.
interplay of the growing chiral chain and the stereorigid
ligand sphere. There appears to be an agreement of the molar
rotation value [F]D23 of approximately À70 degcm2 kmolÀ1
(CHCl3) for oligomers prepared using (L,R,R)-2a with the
[F]3D0 value of À85.7 degcm2 kmolÀ1 (CHCl3) reported for the
model compound (À)-(2R,4R)-2,4-diphenylpentane.[16,17]
In conclusion, we have demonstrated that configuration-
ally stable, enantiomerically pure titanium postmetallocene
catalysts are capable of producing isotactic oligostyrenes that
show measurable optical activity up to a degree of polymer-
ization of 45. We are currently investigating the absolute
stereoselectivity of the insertion of the prochiral monomer
into the titanium–alkyl bond within the helical coordination
sphere.[17]
Received: February 21, 2007
Keywords: asymmetric catalysis · chirality · polymerization ·
.
tacticity · titanium
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Experimental Section
rac-2a: Neat titanium tetrachloride (0.08 g, 0.43 mmol) was added
dropwise to a solution of the racemic bis(phenol) rac-1a (0.24 g,
0.43 mmol) in 30 mL of pentane at À108C; the mixture was warmed
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[15] Catalyst or ligand residues do not show any optical rotation
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[17] A molecular model based on a so-called octant scheme by Pino
et al.[2] suggests that at a L,R,R titanium site, styrene inserts via
the si (pro-R) face with secondary regiochemistry, resulting in a
chain of R configurations; likewise styrene inserts at a D,S,S
titanium site via the re (pro-S) face forming in a chain of S
configurations.
[18] X-ray crystal structural data for complex rac-2a:
C75H108Cl4O4S4Ti2, Mr = 1439.45, T= 110 K, crystal dimensions
0.3 ” 0.3 ” 0.2 mm3, monoclinic, P21/c (no. 14), a = 9.9475(9), b =
30.095(3), c = 26.477(2) Š, a = g = 908, b = 100.385(2)8, Z = 4,
U = 7796.6(12) Š3, 1calcd = 1.226 gcmÀ3, m = 0.494 mmÀ1, 114914
collected, 21765 unique (Rint = 0.0619), final R1 = 0.0618, wR2
[I > 2s(I)] = 0.1178, residual electron density extremes were
0.536 and À0.379 eŠÀ3. CCDC-637376 contains the supplemen-
tary crystallographic data for this paper. These data can be
obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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