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Optically Active Polyvinyl Compounds with Chirality in the Main Chain.

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Directed Synthesis of [Mo3SI3 ] 2 - , an Isolated Cluster
Containing Sulfur Atoms in Three Different States of
Bonding
By Achim Miiller, Sabyasachi Sarkar, Ram Gopal Bhattacharyya, Siedried Pohl, and Mechtild Dartmann"]
The directed synthesis of cluster-compounds is a current
problem in transition metal chemistry"'. For the subgroup
VI metals (Mo and W) with d" configuration, cluster types
are expected to be dumb-bell shaped (in the case where n= I),
triangular ( n = 2), tetrahedral (n = 3) and octahedral (n= 4) (here
n also indicates the number of metal-metal bonds originating
from each metal atom). Recently, the first binary species with
n = l , uiz [ M O ~ ( S , ) ~ ] ~has
- , been isolated in the form of
its ammonium salt[']. We now report the synthesis of the
homologous cluster with n = 2 from a solution containing
Mo". Hitherto, the transition metal sulfur clusters, as model
compounds of bioinorganic interest, could only be obtained
by unusual reactions from unusual reactants and often only
in low
The dark-red, crystalline compound (NH4)2[Mo3S(S2)6].
1 / 2 H 2 0 (1) is formed in aqueous solutions of molybdenum(1v) ions by treatment with polysulfide ions, and has
been characterized by elemental analysis, DTA-, TG- and
magnetic measurements, as well as by ESCA-, VIS-, IR- and
Raman spectra, and a single-crystal structure analysis[4! The
salt is diamagnetic and the main IR absorption bands of
the anion occur at 545 (m), 51 1 [m; v(SS)], 459 [w; v(Mo3S)],
386 (w), 362 (w), 342 (m), 330 cm-' [s; v(MoS)] (assignments[51
by the use of 92Mo- and '"Mo-isotopes). The compound
is extraordinarily stable and is not even decomposed by hydrochloric acid. It is almost insoluble in water and is only
moderately soluble in dimethylformamide.
Like [ M O ; ( S ~ ) ~ ] ~ - [the
' ~ new cluster
also
contains both terminal and bridging disulfide groups (Fig.
l), and the equivalent bonds in both the anions have roughly
v
the same length[']. As expected, the Mo-Mo distances (2.67 A)
are somewhat shorter in ( I ) and the coordination number
of molybdenum is again 9[41.
The Si- ligands favor high coordination numbers and are
suitable for stabilizing metal clusters, since an optimum shielding of the metal atoms is achieved through the high coordination numbers arising due to the small MSz-angle. From the
I'[
Angew. Chem. lnt. Ed. Engl. 17 ( 1 9 7 8 ) N o . 7
Experimental
30ml of saturated ammonium polysulfide is added to a
reddish brown solution of 2.Og of (NH4)6M07024.4HZ0and
1.5g of NHzOH.HCl in 30ml water. The resulting solution
is warmed to 40-50°C for 1 h, then cooled to room temperature, filtered, and the filtrate heated at ca. 90°C for 3-4 h.
Dark-red crystals of (1) then separate. The crystals are washed
with (NH4)&, H 2 0 , ethanol, CS2 and ether; yield: 0.5g.
( I ) can also be obtained directly by adding M O C ~ ~ ( C ~ H ~ N ) ~
to a polysulfide solution (the high tendency of formation
is comparable with that of (NH4)2[Mo2(S~)6].2H~0).
Received: April 25, 1978 [Z 994 IE]
German version: Angew. Chem. 90, 564 (1978)
CAS Registry numbers:
( N H ~ ) ~ [ M o ~ S ( S ~67031-31-6;
)~].
(NH4)6M07024, 12027-67-7; (NH4)2Sr,
9080- 17-5
[ I ] R . B. King, Prog. Inorg. Chem. I S , 287 (1972).
[2] A. Miiller, W - 0 , N o k , B . Krebs, Angew. Chem. 90, 286 (1978); Angew.
Chem. Int. Ed. Engl. 17, 279 (1978).
[3] H . Vuhrmkamp, Angew. Chem. 87, 363 (1975); Angew. Chem. Int. Ed.
Engl. 14, 322 (1975).
[4] Details of the X-ray structure analysis will be published later. The water
content follows from elemental analysis and T G but could not be finally
confirmed by structure analysis.
[S] Because of thecoupling in the ring system thedesignation of the vibrations
is only approximate.
[6] The central unit has already been found in [(C5H5)MoSI4 (cf. [3]).
Optically Active Polyvinyl Compounds with Chirality
in the Main Chain[**]
Fig. I . Structure of the cluster [ M o 3 S l 3 I 2 - in crystals of ( I ) [schematic].
[*] Prof. Dr. A. Miiller, Dr. S. Sarkar ['I, Dr. R. G. Bhattacharyya
Dr. S. Pohl, and M. Dartmann
Fakultat fur Chemie der Universitat
Universitatsstrasse, D-4800 Bielefeld 1 (Germany)
Alexander von Humboldt Foundation fellow.
above mentioned scheme the existence of the homologous
cluster [ M O Y ' S ~ ( S ~ ) ~could
] ~ - be predictedL6! Obviously the
stability of these compounds is also connected with the occurrence of only tricoordinated sulfur in the central units
( M O ( S ~ ) ~ M O }{[M
~~
o ~, S ( S Z ) ~(in
}
(1 )) and probably
{Mo4S4}L6l(for the low reactivity cf. [31).
[+I,
By Giinter W u g Karl Zabrocki, and Johann Hohn"]
We recently described a method for the preparation of
synthetic polymers having cavities containing functional
groups in a definite steric arrangement[" 'I. Functionally substituted polymerizable vinyl compounds were bound to a
suitable chiral template molecule and then copolymerized
under stringent crosslinking conditions. After removal of the
template from the polymer the latter contains microcavities,
whose shape is largely determined by the shape of the template,
and within which the functional groups are in a fixed stereochemistry corresponding to the structure of the template. Such
polymers can be used for the resolution of racemates['].
In order to clarify whether the asymmetry of the microcavities is due only to an asymmetric crosslinking of the polymer,
or also to chiral regions in the linear chains, we used for
our investigations the previously described monomer ( I ) [ I . 3l
with D-mannitOl as template and copolymerized it by radical
initiation together with methyl methacrylate (molar ratio
16:84) in benzene, but this time without the addition of a
further crosslinking agent.
[*] Prof. Dr. G. Wulff, Dr. K. Zabrocki, Dipl.-Chem. J. Hohn
Institut fur Organische Chemie und Biochemie der Universitlt
Gerhard-Domagk-Str. 1, D-5300 Bonn 1 (Germany)
[**I Chirality of Polyvinyl Compounds, Part I . This work was supported
by the Fonds der Chemischen Industrie.
535
m
121, isotactic. meso form
131, asymmetric head -to- head linkage
After removal of the crosslinking template D-mannitol, we
now obtained a soluble copolymer of p-vinylphenylboronic
acid and methyl methacrylate with a monomer ratio of 1 : 2
= 120000, membrane osmomet(average molecular weight
ric). The copolymer had an optical activity of [w]& = - 29.4”
(a= -0.123 f0.002”, acetone/water 9 : 1, c =0.42) [monomer
(I): [x]:g5= +270 (chloroform)].
In order to prove that incorporation or incomplete removal
of D-mannitol was not responsible for the observed optical
activity, the experiment was repeated with 14C-labeled Dmannitol. We found that only 0.2 % of the total D-mannitol
was still present in the polymer, thus ruling out the possibility
of it having any notable influence on the optical rotation.
Moreover, addition of D-mannitol to polymer solutions was
found to lead to a change in the rotation to positive values
until precipitation of the polymer due to progressive crosslinking by the mannitol.
From these findings we conclude that our copolymer of
1-substituted olefins possesses chirality localized in the main
chain by asymmetric induced polymerization. Optically active
polymers or copolymers of I-substituted olefins were previously unknown-excepting those having chirality centers
in the side-chain~[~I.
According to the literatureL4.51 optical
activity is not expected to occur in polyvinyl compounds,
since in each step of the polymerization of a I-substituted
olefin a chiral center is formed from a prochiral center but
in atactic polymers, with irregularly, in syndiotactic with regularly alternating R- and S-configuration. Chains of this type
could indeed be chiral, but the optical activity is intramolecularly compensated and therefore is not measurable. Isotactic
polymers (2)[61 could begin with an R- or with an S-configuration with formation of two enantiomorphic chains. If, however,
the difference in the end groups can be neglected (R-CHI
g-R), as is always permissible in the case of longer chain
lengths, the molecule possesses a symmetry plane (meso form)
and does not show any optical activityr4].
On the other hand, for polymers with 1,2-substituted segments, which are obtainable e.g. from cyclic olefins such as
maleic anhydride, the occurrence of chirality in the main
chain is to be expected[’”], and numerous optically active
polymers have already thus been obtained by asymmetric
ind~ction‘~].
We should like to discuss here how also optically active
polyvinyl or polyvinylidene compounds with chirality in the
main chain can occur, and thus offer an explanation for the
optical activity observed by us.
One possibility would be by head-to-head polymerization
of the monomers (3), in which 1,2-disubstituted chiral segments can be formed, as in the case of 1,2-substituted monomers. However, in the case of the monomers used by us such a
manner of polymerization can be ruled out.
Chirality in the main chain would also be possible if the
substituents near the center of the chain in an isotactic polymer
chain change to the other side of the main chain ( 4 ) with
respect to the Fischer projection. Since each part of the chain
.a,,
536
1Cl inverse - biblock -isotactic ,chiral
”
n
”
R
-
4
k
I
R
Triad,lI
I
Triad Ill
151, asymmetric copolymerization
has in itself a strong isotactic structure, we propose for this
type of arrangement the designation “inverse-biblock isotactic”. It should be pointed out that in an “inverse-biblock
isotactic” chain the configuration of all asymmetric C atoms
remains the same, whereas in a strictly isotactic chain it changes
in the middle from R to S. With identical chain ends ( 4 )
has only one twofold symmetry axis and is therefore chiral
as a whole.
While the concept “inverse-biblock isotactic” applies for
both uni- as well as co-polymers, there should also be a
type of chirality of polymer chains which can occur only
in copolymers. ( 5 ) represents an example of several possibilities. In this copolymer the three different triads I, I1 and
I11 are asymmetric.
It must be decided whether in our case the optical activity
is due to an “inverse-biblock isotactic” structure or to an
asymmetric copolymerization. Moreover it must be remembered that optical activity can already occur if such structures
are only partially realized in the chains.
In the case of “inverse-biblock isotactic” structures, a unipolymer must also be optically active, so we tried to synthesize
optically active poly(p-vinylphenylboronic acid). (I) was polymerized and the mannitol subsequently removed; or (I) and
p-vinylphenylboronic acid glycol ester were copolymerized
Fig. I. Specific optical rotatlon of copolymers of meth) I methacrylate and ( I )
after removal of D-mannitol. Equiv. ”/, ( I ) ( I equiv. ( I ) = ‘h mol ( I )) in the
polymerization mixture and rotation of various wavelengths are given.
Angew. Chem. I n t .
Ed. Engl. 1 7 (1978) No. 7
and the mannitol and glycol removed. The polymers obtained,
however, showed no significant optical activity. Further indications were given by the copolymerizations of methyl methacrylate with varying amounts of ( 1 ) . The specific rotation of
these copolymers after removal of the mannitol is strongly
dependent upon the molar ratio of the comonomers (cf. Fig.
I), the maximum negative rotation being reached at 16 mob%
( 1 ) (36 equiv.% boronic acid groups). This means that a
definite composition of the copolymerization mixture corresponds to the highest asymmetric induction. We also obtained
copolymers with negative rotation after removal of D-mannitOl
on using styrene as comonomer of ( 1 ) .
These results suggest the likelihood of an asymmetric copolymerization which can be explained in terms of the spatial
structure of ( 1 ) : A methyl methacrylate molecule could be
introduced stereo- and enantiospecifically in the chain between
each two p-vinylphenylboronate moieties in the 1,2- and 5,6positions of the mannitol, with asymmetrical triads being
formed as in Formula ( 5 ) , Triad I.
Received: December 16, 1977 [Z 962a IE]
Revised: March 3, 1978
German version: Angew. Chem. 90, 567 (1978)
Publication delayed at authors' request
[11 G . Wulf/. A Sor/irrii. K . Zohrocki, Tetrahedron Lett. 1973, 4329.
[2] G. Wurlf; Nachr. Chem. Tech. Lab. 25, 239 (1977); G. Wulf, W Vesper,
R. Grohe-Einsler, A. Sarhan, Makromol. Chem. 178,2799 (1977); G. Wuw,
R . Grobe-Einsler, W Vesper, A . Sarhan, ibid. 178, 2817 (1977).
[3] The configuration and conformation proposed for compound ( 1 ) is
supported by the results of an X-ray analysis of D-mannitoltris(1,2;3,4;5,6-phenylboronate):A. Gupta, A. Kirfel, G . Will, G. Wulji
Acta Crystallogr. 8 3 3 , 637 (1977).
[4] Review: R . C . Schulz, E. Kaiser, Adv. Polym. Sci. 4, 236 (1965); M.
Farina, G. Bressan, Stereochem. Macromol. 3, 181 (1968).
[5] a) C . L. Arcus, J. Chem. SOC. 1955, 2801; b) G. Natta, P. Pino, G.
Muzzanti, Gazz. Chim. Ital. 87, 528 (1957); c) C . Schuerch, J. Polym.
Sci. 40, 533 (1 959).
161 Theoligomerchains in formulas ( 2 ) - ( 5 ) serve for schematic representations of high polymers.
Directed Cooperativityand Site Separation of Mercapto
Groups in Synthetic Polymers"]
By Giinter Wulffand Ilona Schulze"]
With the aid of chiral template molecules it has been possible
to prepare polymers having chiral cavities which contain functional groups in definite stereochemical arrangement[*]. The
ability of such polymers to separate racemates might depend
upon the asymmetric structure of the cavities13] and the
exactness of orientation of the functional groups. In order
to acquire more detailed information about the influence of
the stereochemistry of the functional groups on the specificity
we sought a model system which is sensitive only to the
distance between the functional groups.
For this purpose bis(p-vinylbenzyl) disulfide ( 1 )[41 was copolymerized under various conditions with styrene and divinylbenzene. O n polymerization the two vinyl groups of the disulfide ( 1 ) ought to be incorporated at various positions of
a growing chain or in various chains ("discontinuate words"
arrangement['"]). If the disulfide bridges are reductively transformed into mercapto groups then they should remain in
a definite related position to each other in a strongly crosslinked polymer (cf. Fig. 1).
Since mercapto groups can be selectively re-oxidized to
di~ulfides[~],
retention of this spatial relationship can be quanti[*I Prof. Dr. G. Wulff, Dr. I. Schulze
Institut fir Organische Chemie und Biochemie der Universitat
Gerhad-Domagk-Str. 1, D-5300 Bonn 1 (Germany)
Angew. C h m . I n t . Ed. Engl. 17 (1978) No. 7
tatively determined on the basis of the re-oxidized portion
(degree of re-oxidation).
Fig. 1. SH-groups in definite stereochemical arrangement by polymerization
of ( 1 ) .
For an exact assessment, however, comparison is necessary
with a polymer which contains the mercapto groups in the
same concentration but in a random distribution (Fig. 2).
Polymers of this type were obtained from p-vinylbenzyl thioacetate (2)[61. On complete site separation of the mercapto
groups no reoxidation of these polymers should occur.
Table 1 lists the composition of the monomeric mixtures
used for the preparation of the polymers. Using the two
monomers a total of three series of polymers were prepared
having as far as possible comparable polymeric properties
and containing the same amounts of sulfur. A complete comparability is not to be expected owing to the different polymerization properties of a mono- and a difunctional monomer.
Table 1. Percentage composition of the monomer mixture used for preparation
of the polymers [a].
Monomers
Styrene
Divinylbenzene (techn.)
Polymers
B2
A1
A2
B1
75.8
72.5
-
-
79.8
73.3
~
2.8
C1
C2
7.2
7.2
90.5
89.9
[a] In each case the quoted monomer mixtures (in wt-%) were polymerized
in the presence ofthe same volume ofacetonitrile/toluene (2: 1) with azobis(is0butyronitrile) for 7 d at 50°C.
Polymers B 1, B2, C 1 and C 2 were macroporous
Table 2. Properties ,of the polymers.
A1
% SH-Groups [a]
97
Degree ofreoxidation [b] 99
A2
B1
97
69
68
98
Polymers
82
96
33
C1
C2
95
95
96
<5
[a] Percent sulfur present as -SH after treatment with diborane. SH-groups
determined by titration with AgN03 in isopropanol [7]. Total sulfur content
determined by elemental analysis.
[b] Percent SH-groups that could be reoxidized with I2 in methanol.
537
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