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Ba44Cu48(CO3)6O87.9 The Structure of УBaCuO2Ф from Simultaneous X-ray and Neutron Powder Diffraction

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case of 12, the highest (Felkin-Ahn) selectivity (7:7a =
96:4, overall yield 83 YO)is obtained with LiBEt,H (superhydride). Selective reduction of 13 to 9 (9:9a = 85:15, overall
yield 84%) can only be achieved with the help of double
asymmetric induction by using the reducing agent (R)-oxazaborolidine developed by Corey et al.[”. ‘’1 In contrast, reduction with (S)-oxazaborolidine gives a 4:96 ratio of 9 : 9 a
Hydroboration does not take place at
(overall yield 89 YO).
either the conjugated o r the isolated double bonds during the
above reaction. O n reducing the 7-OTr protected ketone 13
with L-selectride, a de of 91 % (overall yield 88 YO)
is achieved
to give 7-OTr-9. Thus, with a slightly modified
it is possible to manage with a commercially available reducing agent.
Thus two options exist for the synthesis of compound 1 b.
If we accept the lack of selectivity in the two aldehyde additions and perform the two resulting HPLC separation steps,
an overall yield of 1.3% over 16 steps is obtained, which
corresponds to 2.3 steps per stereogenic unit (five chiral centers and two double bonds). If, on the other hand, we choose
the route via the ketones 12 and 13, 20 steps are required
(overall yield 2%). This corresponds to 2.8 steps per stereogenic unit, but the selectivity of each step is now > 90%!
This sequence can again be reduced to 16 steps if the corresponding Weinreb a m i d e ~ [ are
’ ~ ~employed instead of the
aldehydes 3. Ketones 12 and 13 are then generated directly
in the C-C bond-formation step.
Since the chiral starting material 6 is available in both (R)and (5’)-forms, the two enantiomers of 1 b are also equally
accessible-an indispensable requirement for pharmacological testing. Remarkably, the purity of the synthetic product
is higher than that of the isolated natural product, since
compound 1 b is described as an oil in the literature.[’] The
‘H and 13C NMR spectra are consistent (superimposable)
with those of our synthetic material.[’51The originally assigned absolute and relative configurations of the natural
product[” have thus been confirmed.
Received: April 17. 1993 [Z6019IE]
German version: Angew. Chem. 1993. 105, 1538
[l] a ) Y Kono, J. M. Gardner, K. Kobayashi, Y. Suzuki, S. Tdkeuchi, T.
Skurai. Phvthorhernrstry 1986, 25, 69; b) Y Kono, J. M. Gardner. Y
Suzuki, S. Takeuchi, ibid. 1985,24,2869; c) K. Kohomoto. R. P. Scheffer,
J. 0. Whiteside, Phpoputholog? 1979, 69. 667
[2] H. Danda, M. M. Hansen, C. H. Heathcock, J. Org. Chem. 1990,55.173.
[3] F. W. Lichtenthaler, J. Dinges, Y. Fukuda, Angew. Chem. 1991, 103, 1385;
Angeiv. Chem. In?.Ed. EngI. 1991, 30, 1339.
[4] a) M. Braun, Adv. Carbanion Chem. 1992, 1, 177, b) [2]; c) M. T. Reetz. E.
Rivadeneira, C. Niemeyer, Tetrahedron Left. 1990, 31, 3863; d) E. J.
Corey, S. S. Kim. J. Am. Chem. Sot. 1990.1f2.4976; e) R. 0. Duthaler, P.
Herold, S. Wyler-Helfer. M. Riediker. Helv. Chim. Aciu 1990, 73, 659;
f ) A . G. Myers, K. L. Widdowson, J. An?. Chem. Sot. 1990, 112. 9612;
g) M. Braun. H. Sascha. Angew. Cliem. 1990. f03.1369; Angew. Chem. f n t .
Ed. EngI. 1990, 30. 1318.
(51 Starting from commercially available 6, the primary hydroxyl group is
protected (97%). the ester is reduced to the alcohol with LiAlH, (95%).
and the alcohol is finally oxidized to the aldehyde under Swern reaction
conditions (92%).
161 a) F. Sato, J. Organomet. Chem. 1985, 285. 53; b) F. Sato, Y Kobayashi,
Org. Svnth. 1990, 69. 106.
[7] E. J. Corey. P. L. Fuchs, Tetrahedron Lett. 1972, 3769.
[8] Both the intermediate dibromide and the alkyne 8 can be isolated and
characterized. Compound 8 is obtained in 75% yield by both the C,
extension and elimination steps.
[9] W. Stahl, dissertation, Universitit Bonn, 1990.
[lo] In reference [3]. the crystal structure of the 0-protected ACRL toxin I is
described; it has a similar conformation.
[I 11 a) E. J. Corey, Pure Appl. Chrm. 1990,62,1209; b) E. J. Corey, R. B. Bakshi. S. Shibata, C.-P. Chen, V. S. Singh. J. A m . Chem. Sor. 1987, fO0,
7925.
[I21 a) E. J. Corey, J. 0. Link. Te/ruh<,dronLerr. 1989,30,6275; b) D. A. Evans,
1454 0 VCH
vPrlug.~ge.~el.rtlinltmhH, D-69451 Weinheim, 1993
Science 1988, 240, 420; c) D. K . Jones, D. C. Liotta, I. Shinkai, D. J.
Mahre. J. Org. Chem. 1993, 58, 799.
[13] Coupling of 8 with 3 a and detritylation with ZnBr, in step k of Scheme 2
(62 % yield).
1141 S. Nahm, S. M. Weinreb. Tetrahedron Left. 1981, 22. 3815.
[15] All new compounds werecharacterized by their ‘H NMR, ”C NMR, MS,
and IR spectra, as well as by their elemental analyses:
[16] 1 b: Monoclinic, space group P2,. a = 5.196(5). h = 10.000(10), c =
20.395(40) A. B =100.52(19)”; pEdlCd
= 1.168 g ~ m - 2
~=
: 2; STOE fourcircle diffractometer. monochromated Cu,, radiation: R = 0.080, w R =
0.038; S = 2.77. Further details of the crystal structure investigation may
be obtained from the Fachinformationszentrum Karlsruhe. Gesellschaft
for wissenschaftlich-technische Information mbH. D-76344 EggensteinLeopoldshafen (FRG) on quoting the depository number CSD-57341. the
names of the authors. and the journal citation.
Ba4,CU48(C03)6087.9:The Structure of
“BaCuO,” from Simultaneous X-ray and
Neutron Powder Diffraction**
By Miguel A . G. Aranda* and J. Paul Atrfield
“BaCuO,” is often formed as an unwanted impurity in the
synthesis of high-temperature cuprate superconductors. Previous attempts to determine the structure of this disordered
material have been only partially successful, so we have
taken advantage of the different relative X-ray and neutron
scattering factors of Ba, Cu, and 0 and developed the structural model from powder X-ray and neutron diffraction profiles simultaneously. This has enabled us to identify a previously unsuspected carbonate ion within the disordered
region of the structure and to show that the true stoichiometry of “BaCuO,” is Ba,,Cu,,(CO,),O,,
+ x (x = 6.9(3) for
our sample). The mechanism of nonstoichiometry is similar
to that in YBa,Cu,O,+,. Our determination of this structure
(volume of unit cell 6135 A3)demonstrates the power of
simultaneous X-ray and neutron refinements.
Early studies showed that “BaCuO,” is semiconducting
and has a body-centred cubic structure with lattice parameter a zz 18.3 ..&,r’-*I and a small range of nonstoichiometry up
to x = 0.1 2 in “BaCuO, + x .’’I3] The structure was originally
solved by single-crystal X-ray diffra~tion,‘~]
and had subsequently been reexamined by using these data,”] X-ray data
from a new
and powder neutron diffraction profiles for two samples with differing oxygen content^.^'] All of
these studies demonstrate that the structure contains two
unusual clusters, Cu,,O,, cages and Cu,O,, rings, built
from edge-sharing CuO, square planes, which are surrounded by the Ba atoms. However, no satisfactory models for the
disordered region between these entities were found, and so
the exact composition and structure remained uncertain. As
the relative magnitudes of the Ba, Cu, and 0 scattering factors for X-rays c f ( 0 ) = 56,29, and 8 electrons per atom) and
neutrons ( b = 5.25, 7.72, and 5.81 fm) are very different,
they may be used together to identify the atoms present in
the disordered regions of the “BaCuO,” structure. We have
[*] Dr. M. A. G. Aranda. Dr. J. P. Attfield
IRC in Superconductivity, University of Cambridge
Madingley Road, Cambridge CB30HE (UK)
and
Department of Chemistry. University of Cambridge
Lensfield Road. Cambridge CB2 1EW (UK)
Telefax: Int. code (223)336-362
[**I This work was supported by the Spanish Government and the European
Community through a Human Capital and Mobility Fellowship
(M.A.G.A.). We thank Drs. J. Rodriguez-Carvajal (Laboratoire Leon Brillouin. Saclay). A. Hewat (Institut Laue-Langevin Grenoble), and Prof. S.
Bruque (University of Malaga) for their help.
+
0570-0833j93/f010-1454 $ 10.00+ .2510
Angew. Chein. Int. Ed. Engl. 1993, 32. No. i0
collected powder X-ray and neutron diffraction data (Fig. 1)
from the same polycrystalline sample to determine the crystal structure and composition of this material (see Experimenpal Procedure).
30
20
Table 1 . Atomic parameter for Ba,,Cu,8(C0,)n08,,,
Atom
Position
Y
Bal
Ba2
Ba3
Cul
Cu2
Cu3
Cu4
C
01
48j
24h
16f
4%
48k
12e
48j
12d
48k
48k
48k
12e
481
24g
48j
481
0.1 507(1)
0.0000
0.364Y( 1)
0.0000
0.1766(1)
0.1766(1)
0.14Y8( 1) 0.3502(1)
0.1261(2)
0.0093(5)
0.0000
0.0000
0.0171 (17)
0.0000
0.0000
0.2500
0.0747(2)
0.0747(2)
0.1457(2)
0.1457(2)
0.2670(2)
0.2670(2)
0.0000
0.0000
0.0Y74(10)
0.0000
0.0949(22) 0.0000
0.2236(7) 0.0000
0.0532(8)
0.2500
02
03
05
06
07
08
09
I
I
I
20
I
I
40
I
I
l
---
60
28 ["I
l
I
l
Table 2. Bond lengths
20
40
11111,111111111 1,1111111 llill 111,11111111111 11,,1,111,11111
60
ze 17
80
-
0.0008(2)
0.0008
0.0008
0.0057(7)
0.0057
0.0057
0.052(7)
0.016(3)
0.005(2)
0.004( 1)
0.005(2)
0.020(4)
0.004(7)
0.04( 1)
0.036(4)
0.004(6)
1.oo
1.oo
1 .oo
1.oo
0.50
1.00
0.25
1.00
1.oo
1.00
1-00
1.00
0.286(14)
0.25
0.50
0.25
maximum (i.e. 25%) occupancy. Variable filling of the 0 6
sites around the Cu4 positions accounts for the range of
oxygen nonstoichiometry, x, and shows local structural
changes similar to those around the "chain" Cu sites in
YBa,Cu,O,+, (0 < y <I).['] When the 0 6 sites are vacant
i401
11,!11,1111'1,1
Occupancy
l
I
I I I I I , ,1,1
0.30Y3( 1)
0.3649(1)
0.1 766( 1)
0.2500
0.1 261(2)
0.2009(3)
0.4313(5)
0.5000
0.1892(3)
0.3443(3)
0.0846(3)
0.3408(7)
0.4461(9)
0.5000
0.4388(7)
0.4468(8)
u,,o[A']
100
80
II
I
1
111111111,1111, 1,1111111 8 \ 1 1 , lllllllilllilll
100
120
Fig. 1. Observed powder diffraction plots (points). calculated (full line), and
9. a) X-ray diffraction (
i
=.
1.54 A);
difference profiles for Ba,,Cu,,(CO,),O,,
b) neutron diffraction, (2. = 1.98 A); positions of reflections marked.
I = intensity (counts, for a) the values are multiplied by 10').
A view of the local geometries in the disordered part of the
Ba,,Cu,,(CO,),O,,
+ x structure is shown in Figure 2. Each
C atom is surrounded by eight 0 sites due to the statistical
disorder of four CO: - orientations. The carbonate group
lies between two Cu,O,, rings and forms a long C u - 0 contact with one of them. The Cu,,O,, clusters are linked by
05-Cu4-07-Cu4-05 bridges (see also Tables 1 and 2) in one
of four possible orientations around [loo], which refine to
Fig. 2. The disordered region of Ba,,Cu,,(CO,),O,,
(Cu small open circles;
0 medium open circles; C filled circle, Ba large hatched circles). All possible Cu
and 0 sites are drawn with bonds showing one particular configuration for the
bridge between two C U , , ~ , ,clusters. and the carbonate group lying between
Iwo CuO, groups from different C U , ~ , rings.
,
[A] and selected angles ["I
Disroncrs
Bal-01 x 2
2.940(5)
Bdl-02 X 2
2.745(4)
Bal-03x 2
2.744(4)
2.819(3)
Bal-O5x 1
Bal-06 x 0.3 2.69(2)
B a l - 0 8 x 0.5 2.72(1)
Bal-09 x 0.5 3.253(7)
Ba2-02x2
2.863(6)
2.970(6)
Ba2-03 x 2
Ba2-06x 1.1 2.751(8)
Angles
08-C-08 x 1 99.1(4)
Ba2-07 x
Ba2-08 X
Ba2-08 x
Ba2-09 x
Ba3-01 x
Ba3-02 x
Ba3-03 x
Cul-02x
Cul-03 x
Cul-09 x
for Ba,,Cu,,(CO,),O,,
0.5 2.58(1)
1
2.9?(1)
2
3.161(8)
1
2.761(3)
3
2.647(6)
3
3.174(6)
3
2.884(5)
2
1.914(4)
2
1.960(4)
0.3 2.50(2)
08-C-09 x 2
Cu2-01 x
CU2-01 X
Cu3-01 x
Cu3-05x
(34-05 x
014-07 x
Cu4-06 x
C-08 x
C-09 x
2
2
4
1
1
1
1.2
2
1
+.
1.911(7)
2.141(8)
1.947(6)
2.56(1)
1.69(2)
1.89(4)
1.83(2)
1.22(1)
1.38(2)
130.5(2) Cul-09-C x 1 180
(x = 0), the Cu4 sites contain C u + in a typical two-coordi-
nate environment and the other Cu sites have an average
formal charge of 1.90. This is comparable to the charge
distribution in YBa,Cu,O,. The occupation of two 0 6 sites
around one Cu4 site in each bridge (x = 6) gives a distorted
square planar coordination (right hand side of the bridge in
Fig. 2) with short C u - 0 bonds, consistent with oxidation to
C u 3 + . The mean oxidation state for the Cu atoms in the
clusters is now +2.12, showing that they act as charge reservoirs during the oxidation of the Cu4 sites, as is found in
YBa,Cu,O,+,. Filling a third 0 6 site giving x = 9 is sterically possible and results in an unusual trigonal environment
around C u (left hand side of bridge in Fig. 2). Our refined
value of x = 6.9(3) does not represent a significant filling of
this site.
As Ba,,Cu,,(CO,),O,,
+ x is stoichiometric in carbonate,
and all previous structure determinations found an atom at
the C site, we conclude that carbonate ions are required to
stabilize this structure. Evidence that the analogous nickel
compound, cubic "BaNiO, ," contains carbonate ions has
very recently been reported,[g]although the proposed composition, Ba,,Ni,,(CO,),O,,
, is slightly different. The compound "BaCuO,,, ,''['I
which decomposes into cubic
"BaCuO,+," above 800"C,['0."1 is probably an oxocarbonate as well. The formation of Ba,,Cu,,(CO,),O,,
+
as an equilibrium phase at 850°C in air highlights the
ability of Ba and related elements to form very stable oxocarbonates. This has recently led to the development of superconducting phases such as (Ba,., ,Sro,,,)CuI. (CO,),, 02,,
( T , = 40 K)II2]and T10,,Pbo,,Sr4Cu,(C0,)0,, a 70 K superconductor containing layers of carbonate g r o u p ~ .31~ ’Rigorously excluding carbon should prevent the formation of
“BaCuO,” during the synthesis of superconducting cuprates
and may also enable new, superconducting, infinite-layer
type materials (Ba,Sr), _,CuO, to be prepared with small or
zero strontium c0ntents.1~~1
Our determination of the complex disorder within this
large unit cell (61 35
demonstrates the power of simultaneous refinements. The combined information from powder
X-ray and neutron patterns of moderate quality has proved
more effective than previous individual single-crystal X-ray
data sets.
w3)
Experimental Procedure
“BaCuO,” was synthesized by heating an intimate, equimolar (Ba:Cu = 1:1)
mixture of BaCO, (Aldrich. 99.98%) and CuO (Aldrich, 99.99 O h ) at 850 “C
in air for 24 h. The sample was reground and reheated at the same temperature
for a further 24 h.
Structure determination: X-ray powder diffraction data were recorded for
20 = 6-1 17”on a Siemens D501 diffractometer by using monochromated Cu,,
radiation, counting for 20 s per 0.03” step. Neutron powder diffraction data
were collected on instrument DlA at the Laboratoire Leon Brillouin, Saclay,
France, by using a wavelength of 1.9841(2)8, for 26, = 5- 132.5’ in 0.05”steps.
Rietveld analysis [I 51 with the GSAS package [16] enabled the structural model
to he fitted to either or both data sets. No impurity phases were observed.
Starting coordinates were those reported for ”BaCuO,,,,” from powder neutron refinement [7], but this model gave a poor fit to our neutron data. Examination of the structure revealed an anomalous atom at (1/4,0,1/2), which has
been reported in all previous studies as either an 0 atom [4, 5, 71 or a partially
occupied Cu site [6]. We refined the scattering factor of this atom by using the
X-ray and neutron data separately, giving ./[O) zz 9 electrons per atom and
h = 7 fm, respectively. These values clearly rule out partial occupation by Ba or
Cu. although they are consistent with 0. However, the long distances ( > 3.2 8,)
from this site to the nearest cations make oxygen occupation chemically unrealistic. We therefore considered possible cationic impurity atoms: an obvious
candidate was carbon in view of the scattering factors YfO) = 6 electrons per
atom, h = 6.65 fm)and the ability of barium to form thermally stable carbonates.
+
Carbon refined to full occupancy at (1/4,0,1/2)in both the X-ray and neutron
refinements, and a neutron difference Fourier map revealed two partially occupied oxygen positions (08 and 09) approximately 1.3 8, from this site. These
positions define four possible orientations of a C0:- group, pivotally disordered around the C atom. Including the carbonate group improved the profile
fits and further simultaneous X-ray and neutron refinements enabled us to
resolve other disordered features more clearly than in previous determinations.
Disorder in the Cu2 position, which has been modeled by refining this atom off
(OJJ),
reflects a slight disorder in the orientation of the near-spherical Cu,,O,,
cages around the symmetry axes. The final model (space group fm3m.
u = 18.3069(2)A. Z = 2) shown in Table 1 has composition Ba,,Cu,,(C0,)608,+.r (.x = 6.9(3)) and gives good fits (Fig 1) to both X-ray
(RwF= 0.040, R, = 0.066, R factors are defined in refs. 1151 and [16]) and
neutron ( R w p= 0.081. R, = 0.057) profiles. This model gives satisfactory bond
lengths and angles for all atoms (Table 2) and bond valence calculations [17]
give total valence of 4.1 for C and values between 1.8 and 2.3 for all the Ba. Cu.
and 0 sites. except Cu2 and 0 6 where low valences of 1.6 indicate further slight
disorder. Further details of the crystal structure investigation may he obtained
from Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlichtechnische Information mbH. D-76344 Eggenstein-Leopoldshafen (FRG) on
quoting the depository number CSD-57708, the names of the authors and the
journal citation.
IR Spectroscopy: The presence of carbonate groups was corroborated by the
IR spectrum (Perkin-Elmer 1710 FT spectrometer, Csl disk) of “BaCuO,”
which showed typical carbonate absorptions at 1410 and 865 cm-‘ ; those for
BaCO, are at 1440 and 875 cm-’.
Received: April 30, 1993 [Z 6047 IE]
German version: Angew. Chem. 1993. 105. 1511
[l] M. Arjomand, D.J. Machin. J. Chem. Soc. Dulton Truws. 1975, 10611066.
[2] H. N . Migeon, F. Jeannot. M. Zanne. J. Aubry, Rev. Chim. Miner. 1976.
13. 440-445.
1456 0 VCH
Veriugsgcs~~lischuft
mhH, 0-69421 Wemheim. /993
[3]
H.N. Migeon, M. Zanne, E Jeannot. C. Gleitzer, Rev. Chim. Miner. 1977,
14, 498-502.
[4] R. Kipka, H. K. Muller-Buschbaum, Z. Nufurforsch. B 1977,32,121- 123.
151 W. Gutau. H. K. Muller-Buschbaum. .lLess Common Met. 1989, /52,
L11-Ll3.
[6] E. F. Paulus, G. Miehe, H. Fuess, I. Yehia, U. Lochner, J. Solid Sfute
Chem. 1991. 90,17-26.
I71 M. T. Weller, D. R. Lines, J. Chem. Soc. Chem. Commun. 1989,484-485.
[Sl D. W. Murphy, S. A. Sunshine, P. K. Gallagher, H. M. O’Bryan, R. J.
Cava. B. Batlogg, R. B. van Dover. L. F. Schneemeyer, S. M. Zahurak
Chemislrj, of High-Temperature Superconductors (Eds.: D. L. Nelson,
M. S. Whittingham. T. F. Goerge) (ACS Symp. Ser. 1987.351. 181-191).
[9] R. GottSchdll. R. Schollhorn. Solid Sture Ionks 1993. 59.93-98.
[lo] M. Machida, K . Yasuoka. K. Eguchi. H. Arai, J. SolidState Chem. 1991,
91, 176--179.
[ l l ] S. Petricek. N. Bukovec, P. Bukovec, J. S o i i d S m e Chem. 1992, 99.58-62.
[12] K. Kinoshita, T. Yamada, Nature 1992, 357, 313-315.
1131 M Huve. C. Michel. A. Maignan, M. Hervieu, C. Martin, B. Raveau,
P l z ~ ~ i cCu (Amsterrlam) 1993, 205, 214-224.
[14] M. Takano. M. Azuma. Z. Hiroi, Y. Bando. Y. Takeda, Physica C(Amsferdum) 1991, 176.441-444.
1151 H. M. Rietveld, J. Appl. Crystullogr. 1969, 2, 65-71.
[I61 A. C.Larson, R. B. Von Dreele, Los Alamos Laboratory Rep. No, LAUR-86-748, 1987.
[17] N. E.Brese. M. OKeefe. Actu Crystullogr. Secr. B 1991. 47. 192-197.
Synthesis and Reactivity of Hexafluoro-1,Z
diisocyanocyclobutanechromiumComplexes**
By Dieter Lentz,* Frank Nowak, Dagrnar Preugschat.
and Markus Wasgindt
Di- and triisocyanides have recently acquired considerable
significance, owing to their interesting coordination chemistry,“’ in particular their ability to function as polydentate
ligands. Fluorinated cycloalkyl isocyanides and fluorinated
diisocyanides are hitherto unknown. The chemistry of perfluorinated isocyanides, inaccessible through conventional
isocyanide syntheses,[’] was until recently restricted to trifluoromethyl isocyanider3I and pentafluorophenyl isocyanide.141 The more thoroughly investigated trifluoromethyl isocyanide is particularly noteworthy due to its
unusual ligand proper tie^'^] and reactivity.16] As shown by
Fehlhammer et al., pentacarbonyl(cyano)chromate undergoes radical-type alkylation,[’] giving rise to pentacarbonylchromium complexes of functionalized isocyanides.
We recently succeeded in synthesizing the first fluorinated
alkenyl isocyanide F,C=CF-NC 1 by the formation of the
corresponding [Cr(CO),] complex, followed by flash vacuum pyrolysis.[81We now report the synthesis and preliminary
reactions of the thus far unique complex-stabilized, fluorinated diisocyanide.
If pentacarbonyl(trifluoroviny1 isocyanidejchromium 2 is
heated to 70 “C under atmospheric pressure, cleavage of the
isocyanide ligand does not take place; instead a [2 + 21 cycloaddition reaction occurs. [2 21 Cycloaddition reactions
are thermally forbidden according to the Woodward- Hoffmann rules,rg1and d o not normally occur. Fluorinated alkenes, on the other hand, display a strong tendency to undergo
[ 2 + 21 cycloadditions,I’O1from which four possible isomeric
+
[*] Priv.-Doz. Dr. D. Lentz. F. Nowak, Dr. D. Preugschat, M. Wasgindt
Institut fur Anorganische und Analytische Chemie der Freien UniversitHt
Fabeckstrasse 34-36. D-14 195 Berlin (FRG)
Telefax: Int. code + (30)838-2424
[**I This work was supported by the Fonds der Chemischen Industrie, the
Deutsche Forschungsgememschaft, and the Graduiertenkolleg “Synthese
und Strukturuntersuchung niedermolekularer Verbindungen” of the Freie
UniversitHt and Technische Universitiit, Berlin. We thank Hoechst AG for
a gift of 1.2-dichloro-l,1,2-trifluoroethane.
$ 1 0 .0 0 i .25/0
057o-0~33/93j1oJ0-1454
Angew. CXem. f n f . Ed. Engl. 1993, 32, No. I0
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