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Can Polystyrene be Optically Active.

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order of elution from chiral selectors["' as well as unsatisfactory enantiomer separation for particular selectands'"]
may well be due to the temperature dependence of the enantioselectivity, particularly for temperatures close to Ti,,.
It is intriguing to realize that above T,,,, preferential recognition of one enantiomer is not caused by stronger binding
to a non-racemic matrix but is due to the higher degree of
disorder in the resulting chemical interaction.
be able to exhibit optical activity.['.31 By copolymerizing
the mannitol derivative 1 with different comonomers and
subsequent cleavage of the mannitol template, optically
active copolymers (e.g., poly(4-vinylphenylboronic acidco-styrene)) consisting of asymmetric triads of type 2 were
~btained.[~-~]
Received: August 23, 1988 [Z 2938 IE]
German version: Angew. Chem. I01 (1989) 197
[I] a) B. Koppenhoefer, Dissertation, University of Tiibingen 1980; b) B.
Koppenhoefer, E. Bayer, Chromalographia I9 (1984) 123; c) V. Schurig,
Angew. Chem. 96 (1984) 733; Angew. Chem. Inl. Ed. Engl. 23 (1984)
741.
121 V. Schurig, W. Burkle, J. Am. Chem. SOC.104 (1982) 7573.
[3] In principle, a temperature-dependent reversal of the enantioselectivity
may also arise from structural or configurational changes in the nonracemic stationary phase upon heating or from changes of the individual
contributions of compeling mechanisms of chiral recognition.
[4] K. Watabe, R. Charles, E. Gil-Av: 23rd Znt. Symp. Adu. of Chromatogr.
1986 (Chiba, Japan), Abstracts, p. 83; cf., Angew. Chem. 101 (1989) 194;
Angew. Chem. Int. Ed. Engl. 28 (1989) 192.
[S] V. Schurig, A. Ossig, R. Link, J . High Resolut. Chromatogr. Chromatogr.
Commun. I 1 (1988) 89.
[6] a) V. Schurig in J. D. Morrison (Ed.): Asymmetric Synthesis, Vol. Z , Academic Press, New York 1983, p. 59; b) V. Schurig, J . Chromotogr. 441
(1988) 135.
[7] V. Schurig, R. Link in D. Stevenson, I. D. Wilson (Eds.): Chiral Separations. Plenum, London, in press.
[8] The enantioselective selectand-selector system l b and 26 shows some
unusual features which are not fully understood at present. Although
the temperature-dependent reversal of the elution order of l b on Zb is
fully reproducible, - A o . c ( A G o ) = R T . l n ( K D / K c ) almost doubled at,
e.g., 60°C with increasing time of operation of the column, while K,,,.
remained essentially unchanged. Also, an excessive peak-broadening,
reaching a maximum at T,,, has been observed both for the racemic mixture and the single enantiomers.
[9] Recently, in a careful study of the temperature dependence of the separation of l a and l b on Za (1R)in SE-54 polysiloxane in five degree
intervals between 80 and 120°C, T,, values as high as 200°C have been
extrapolated for these Z / E isomers [7].
[lo] V. Schurig, B. Koppenhofer, W. Biirkle, Angew. Chem. 90 (1978) 993;
Angew. Chem. Znt. Ed. Engl. 12 (1978) 937.
[ I 11 Thus, isopropyloxirane is not resolved on 2a at 60°C (T,=) [2]. Besides,
it also shows a temperature dependent reversal of the elution sequence:
V. Schurig, D. Wistuba, unpublished.
Can Polystyrene be Optically Active?**
By Giinter Wuvf* and Pradeep K . Dhal
Dedicated to Professor Kurt Heyns on the occasion of his
80th birthday
The question, as to whether polymers based on vinyl
monomers can exhibit optical activity arising from a chiral
configuration of the main chain (main chain chirality) has
long been a subject of discussion. The conclusion of this
debate, after consideration of all the commonly encountered and well-known structures of both homo- and copolymers, revealed that they are not expected to exhibit any
such characteristics.[*] Careful symmetry considerations
have shown recently that, particularly in the case of copolymers, several chiral structures are possible which should
[*I Prof. Dr. G. Wulff, Dr. P. K. Dhal
Institut fur Organische Chemie und
Makromolekulare Chemie der Universitat
Universitatsstrasse I , D-4000 Diisseldorf (FRG)
[**I On the Chirality of Polyvinyl Compounds, Part 9. This work was supported by the Minister fur Wissenschaft und Forschung des Landes
Nordrhein-Westfalen and the Fonds der Chemischen Industrie. Part 8:
111.
196
0 VCH Verlagsgesellschafr mbH, 0-6940 Weinheim, 1989
R
9
R
I
1
0i 4 9
I
R
n
2
3
In this example, the diads having A substituents (4vinylphenyl boronic acid residues incorporated via the
mannitol derivative 1) are decisive for chirality (in this
case, the diad has the (S,S)-configuration).[61The carbon
atoms bearing B substituents d o not represent stereogenic
centers and essentially serve to separate the chiral diads
from one another. Therefore, this spacer can also be composed of a relatively long chain fragment (stereoregular or
atactic) of comonomer units (see 3).1',61 This gives rise to
the question of whether a chain fragment consisting of an
atactic sequence of homopolymer could fulfill the requirements of a spacer for the diads as well. If this were so, it
would be possible to prepare, for the first time, optically
active vinyl homopolymers possessing well-defined main
chain chiral configurations.
In order to address this question, a systematic model
study was carried out for all possible partial chain structures. We analyzed to what extent the asymmetric arrangement at the centers of the diad, which originally had a defined absolute configuration, is retained upon introduction
of intermittent atactic units. The following essential simplifications were introduced:
a) Chain fragments consisting of one, two, o r three monomer units with a statistical distribution of all possible
configurations, were considered to serve as appropriate
bridging groups between the diads in 3.
OS70-0833/89/0202-0196 $ 02.50/0
Angew. Chem. Znl. Ed. Engl. 28 (1989) No. 2
:pseudo-asymmetric). In the arrangements a, b, g, and h,
:he central carbon atom is stereogenic and chirotopic
lasymmetric). However, the structural arrangements in fornulas b and h are mirror images of each other. Consepently, only the central stereogenic centers in the diaster:omeric arrangements a and g (i.e., 25% of all centers) are
:xpected to contribute appreciably to the optical rotation
:same or opposite sign). Similar results are obtained when
:hree or more monomeric units are present in the atactic
?art of the spacer. In contrast, if a single monomeric unit
,onstitutes the separating component, half of the centers of
:he diads should be chirotopic and stereogenic (asymmet3c). This type of chain configuration corresponds to the
:arlier discussed homopolymer type consisting of chiral
hexads 4.",2c.31
From the above analysis, it appears that for chain struc1:ures of type 3 with separating units consisting of atactic
Ihomopolymer fragments, optical activity, albeit to a modI:rate extent, can be expected. The synthesis of this struc1:ure involved copolymerization of the monomer 1 with styI.ene in various molar ratios (see Table I), whereby removal
IIf the 3,4-O-cyclohexylidene-~-mannitol
template yielded
IJptically active copolymers with optical rotations in the
1:ange [a]::,= -28" to -36" (Fig. 2). Quantitative debo1*onation using AgN03/NH3 in acetone['l resulted in the
1rormation of the homopolymer 6a containing distyryl
Two stereocenters on each side of the investigated center were taken into account in determining the asymmetry of the centers in the tactic diads. Moreover, it was
assumed that the arrangement of these four neighboring stereocenters would mainly govern the rotational
contributions arising from the asymmetric center. The
remaining parts of both chains were regarded as being
identical (see Fig. 1).
It was assumed that identical partial structures within
the chain would bring about optical rotations of the
same sign for the entire molecule.
Although the introduction of these simplifications is not
free from problems, in our opinion this approach is adequate for the assessment of possible effects.
Inspection of chain structures in the light of the above
model reveals that, for a separating unit consisting of varying number of monomers, at least 25% of all the centers of
the diads are stereogenic and chirotopic (asymmetric) and
could contribute to the optical rotation. As an illustration,
all possible partial structures for a polymer chain with two
monomeric units in the spacer are shown in Figure 1. By
taking the central carbon atom (one of the centers of the
diad, in the box) as reference, it is evident that in formulas
c and e this center is chirotopic but not stereogenic and
that in formulas d and f it is stereogenic but not chirotopic
R
I
I-.
4
4
A
i
i
R
h
Fig. I. Chain fragments consisting of a tactic diad of defined absolute configuration (in circles) with statistical
distribution of all possible configurations in the atactic part. The center under consideration is indicated by a box.
Two neighboring stereocenters on either side are taken into consideration. The separating atactic component consists of two or more monomeric units.
a)AgNO,/NH,
b) l2/NaOH
6b +
F
I, / TI(CF3C00),
Ic
.
i
5
I
i
I
6 q R=H
6b. R= I
7
0 VCH Verlagsgesellschoft mbH. 0-6940 Weinheim, 1989
0570-0833/89/0202-0197 $ 02.50/0
Fig. 2. Preparation of optically active homopolymers
Angew. Chem. Inl. Ed. Engl. 28 (1989) No. 2
197
diads of known absolute configuration in a n otherwise
atactic polystyrene. As can be seen from Table 1, for certain compositions, distinct optical rotations were observed.
Since the boric acid residues could be removed quantitatively, the possibility of any contribution from residual 4(dihydroxybory1)styryl diads to the observed optical activity can be ruled out. Furthermore, the circular dichroic
(CD) spectrum of the polymer 6a (A=217nm,
[ Y ]= - 245 ") does not resemble that typically found for 4(dihydroxybory1)-styryl diads"] (A = 234 nm, [ q =- 2656")
as in the case of 5 , but clearly resembles that of the distyryl diads ( A = 217 nm, [ = - 2395 ") present in the deboronated copolymers of 1 with methacrylonitrile.
Reduction in the optical activity of 6a to one tenth of
the value compared with the copolymer 5 could be explained in the following manner:
m
Approximately 25% of the total (S,S)-diads of 5 only
contribute to the optical activity of 6a.
The diastereomeric arrangements a and g might result
in optical rotations of opposite sign.
The molar rotational contributions of chain segments
bearing comonomer units are likely to be significantly
higher than the corresponding homopolymers (a and
some cases the optical activity of these polymers was attributed to a chiral conformation.
Received: August 29, 1988;
revised: October 4, 1988 [Z 2945 IE]
German version: Angew. Chem. 101 (1989) 198
CAS Registry number:
(1) (styrene) (copolymers), 110590-46-0.
[I] G. Wulff, Angew. Chem. 101 (1989) 22: Angew. Chem. Int. Ed. Engl. 28
(1989) 21.
121 General review articles dealing with optical activity of polymers: a) E.
Selegny (Ed.): Optically Active Polymers, Reidel, Dordrecht 1979: b) M.
Fontanille, A. Guyot (Eds.): Recent Advances in Mechanistic and S.vnfhefic
Aspects of Polymerization, Reidel, Dordrecht 1987, pp. 399-470; c) M.
Farina, Top. Stereochem. 17 (1987) 1.
[3] Reviews: a) G. Wulff, Nachr. Chem. Tech. Lab. 33 (1985) 956: b) in [2b],
p. 399.
[4] G. Wulff, K. Zabrocki, J. Hohn, Angew. Chem. 90 (1978) 567; Angew.
Chem. Inf. Ed. Engl. 17 (1978) 535.
[5] G. Wulff, J. Hohn, Macromolecules I5 (1982) 1255.
[6] G. Wulff, R. Kemmerer, B. Vogt, J. Am. Chem. Soc. 109 (1987) 7449.
171 G. Wulff, P. K. Dhal, Macromolecules 21 (1988) 571.
[S] N. P. Bullen, P. Hodge, F. G. Thorpe, J , Chem. SOC.Perkin Trans. 1 1981,
1863.
[9] G. Wulff, P. K. Dhal, Makromol. Chem. 188 (1987) 2847.
9).
Using an analogous procedure, optically active poly(4iodostyrene) was prepared. 5 was converted first to an optically active poly(styrene-co-4-iodostyrene) 6b by replacing the boric acid residues with iodine.[81 This copolymer
was subsequently subjected to thallium(m) trifluoroacetate
catalyzed iddination['I to produce the desired poly(4-iodostyrene). These homopolymers also showed distinct optical
activity for particular compositions.
Table 1. Optical activity of co- and homopolymers (in THF). Polymer 5 was
synthesized by radical copolymerization of 1 with styrene in various molar
ratios followed by removal of the template [6]. Further details of the modification of 5 are outlined in the text and Fig. 2. The molecular weights of the
polystyrene samples 6a, as determined by GPC measurements using a polystyrene standard, were found to lie in the range H= 30000-35000 [a].
Experiment
No.
1
2
3
4
5
Mole fraction of 1
inmonomer
in polymer
0.15
0.21
0.24
0.3 1
0.35
0.23
0.32
0.38
0.40
0.47
5
[a]& ["I
6a
-31
-36
-34
-33
-28
-3.5
-3.0
-1.5
-0.5
-1
[a]%I"]
6b
-36
-44
-37
-34
-24
7
-1
-4.0
-4.5
0
-1
~
[a] Representative physical data and elemental analyses for the polymers:
-0.012f0.002" (c=O.32), elemental analpolystyrene 6a (No. 2): amcasured=
ysis: calcd. C 92.26, H 7.74, found C 92.34, H 7.69: 6a (No. 3):
a,,,, = -0.01 1 t0.002" (c=0.35), found C 92.08, H 7.82; poly(4-iodstyrene)
7 (No. 2): a,,,, = -0.015-+0.002" (c=0.38), calcd. C 41.77, H 3.06, found C
41.80, H 2.77; 7 (No. 3): a,,., = -0.015f0.002° (c=0.33), found C 41.88, H
2.70.
A Polyhedral Oligogermane: 4,8-Dibromoocta-tert-butyltetracyclo13.3.0.02~7.03~6]octagermane*
*
By Manfred Weidenbruch*, Fred-Thomas Grimm,
Siegfried Pohl, and Wolfgang S a a k
In spite of much interest in inorganic rings and cages,"]
examples of these types of compound for the heavier elements of the fourth main group are restricted to the few biand polycyclic silicon compounds reported in the last few
years. In addition to derivatives of bicyclot 1.l.O]tetrasilane['] and bicycl0[2.2.O]hexasilane,[~~
the first polycyclic
oligosilanes have recently been reported.l4I Although an
o c t a s i l a c ~ b a n ehas
[ ~ ~also been synthesized, it has yet to be
structurally characterized.
In contrast, bi- and polycyclic oligogermanes have not
previously been reported. In the early literature, the reductive dehalogenation of trichlorophenylgermane was reported to afford a compound of composition (PhGe),,
which, at that time, was believed to be a hexagermabenzene derivative.@' With the benefit of modern knowledge, a
polyhedral germane would appear more likely.l']
We now report the synthesis of a polycyclic oligogermane via a simple route. Treatment of germanium tetrabromide with tert-butyllithium gave a mixture of dibromodi-tert-butylgermane and 1,1,2,2-tetrabromo-l,2-di-tert-b~tyldigermane 118'in 20% yield in each case. Reaction of the
latter with excess naphtha1eneAithium [Eq. (a)] afforded a
4tBuBr,Ge-GeBr2tBu
+ 14C,,H8/Li
--+
1
It has thus become possible, for the first time, to synthesize optically active homopolymers with well-defined
structures based on 1-substituted olefins. The observed optical activity unequivocally originates from the chiral configurations in the main chain. Homopolymers with low optical activity, e.g.,.of the methacrylate type (for a concise
survey see Ref. [l]) reported in the past have always remained ambiguous with respect to their structures and
mechanisms of formation. As the possibility of a configurational chirality in the main chain was not conceivable, in
198
0 VCH Verlagsgesellschafr mbH, 0-6940 Weinheim. 1989
tBu8Ge8Brl
+ 14LiBr + 14CloHs
(a)
2
[*I Prof. Dr. M. Weidenbruch, F.-T. Grimm, Prof. Dr. S . Pohl,
DipLChem. W. Saak
Fachbereich Chemie der Universitat
Carl-von-Ossietzky-Strasse 9-1 1, D-2900 Oldenburg (FRG)
[**I Compounds of Germanium and Tin, Part 4. This work was supported by
the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie. Part 3: M. Weidenbruch, K. Schafers, S . Pohl, W. Saak, K.
Peters, H. G. von Schnering, Z. Anorg. Allg. Chem. (in press).
0570-0833/89/0202-0198 $ 02.50/0
Angew. Chem. Int. Ed. Engl. 28 (1989) No. 2
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