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Doubly Bridged Prismane and Dewar Benzene Intermediates in an Unusual Photochemical Rearrangement of Tricyclic Phthalic Esters.

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temperature to prevent a phase transition. Further experiments should clarify the situation.
Received: February 13, 1992 [Z5188IE]
German version: Angew. Chem. 1992, 104, 896
CAS Registry numbers:
l a , 103420-86-6; 1 b, 141636-02-4; l c , 141636-05-7; Id, 141636-07-9; le,
112019-20-2; If, 111958-83-9.
[I] J. U. von Schiitz, H. Rieder, H. C. Wolf, H. Meixner. S. Hiinig, Synth.
Me!. 1991. 42, 1761.
[2] Reviews: a) S. Hunig, P. Erk, Adv. Marer. 1991,3, 225-236; b) S. Hiinig,
Pure Appl. Chem. 1990,62, 395-406.
[3] J. S. Miller, A. J. Epstein, Angex.. Chem. 1987,99,332-339; Angew. Chem.
Int. Ed. Engl. 1987, 26, 287-293.
[4] R. Moret, Synrh. Met. 1988, 27, B301bB307.
[5] a) H. Kobayashi, R. Kato, A. Kobayashi, T. Mori, H. Inokuchi, SolidState
Commun. 1988,65,1351-1354; b) A. Kobayashi, T. Mori. H. Inokuchi, R.
Kato, H. Kobayashi, Svnrh. Met. 1988,27, B275-B280; c)S. Kagoshima,
N. Sugimoto, R. Kato. H. Kobayashi, A. Kobayashi, ibid. 1991, 41-43,
1835-1838.
[6] a) A. Kobayashi. R. Kato, H. Kobayashi, T. Mori, H. Inokuchi, SolidState
Commun. 1987, 64, 45-51; b) H. Kobayashi, R. Kato, A. Kobayashi, T.
Mori, H. Inokuchi, Y Nishio, K. Kajita, W. Sasaki, Synrh. M e f . 1988,27,
A289-A297; c) R. Kato, H. Kobayashi, A. Kobayashi, 1 Am. Chem. Soc.
1989, 111, 5224-5232.
(71 a) 0 . Ermer, Adv. Marer. 1991,3,608-611. b) Similar helix structures have
been described for other super-diamond lattices: 0.Ermer, L. Lindenberg,
Chem. Ber. 1990, 123, 1111.
[8] An exact evaluation of these new structural considerations and their implications for the discussion of phase behavior will be published shortly.
[9] For properties induced by collectives of molecules see: F. M. Menger,
Angew,. Chem. 1991,103,1104; Angew,. Chem. Inr. Ed. Engi. 1991,30,1086.
[lo] P. Erk, H. Meixner, T. Metzenthin, S. Hiinig, U. Langohr, J. U. von
Schiitz, H.-P. Werner, H. C. Wolf, R. Burkert, H. W. Helberg, G. Schaumburg, Adv. Mater. 1991. 3, 311-315.
[ l l ] A. Bondi, J. Phvs. Chem. 1964,68,441-451. The special position of l a is
even more apparent by application of these newer van der Waals volumes
(K. Sinzger, Dissrrfation, UniversitLt Wiirzburg, 1992).
[12] S. Tomic, D. Jerome, A. Aumiiller, P. Erk, S . Hiinig, J. U. von Schiitz, J.
Phys. C : Solid State Phys. 1988. 21, L203-L207.
[13] A. Aumiiller, P. Erk, G. Klebe, S. Hiinig, J. U. von Schiitz, H.-P. Werner,
Angew. Chem. 1986, 98, 759-761; Angew. Chem. Int. Ed. Engf. 1986,2S,
740.
[14] L. S. Bartell, K. Kuchitsu, R. J. deNeui, J Chem. Phys. 1961, 35, 12111218.
[lS] S. Fuks, J.-C. Legros, A. Bellemans, Physica 1965, 31, 606-612.
[16] S. C. Greer, L. Meyer, J Chem. Phys. 1970, 52. 468-469.
(171 J. R. Cooper, J. Lukatela, M. Miljak, J. M. Fabre, L. Giral, E. Aharon
Shalom, Solid State Commun. 1978, 25, 949-954.
[18] N. S. Dalal, L. V. Haley, D. J. Northcott, J. M. Park, A. H. Reddoch. J. A.
Ripmeester, D. F. Williams, J. Chem. Phys. 1980, 73, 2515-2517.
[19] a) C:P. Heidmann, K. Andres, D. Schweitzer, Physica 1986, 14338, 357359; b) K. Oshima, H. Uruyama, H. Yamochi, G. Saito, J Phvs. Soc. Jpn.
1988,57, 730-733.
[20] H. Schwenk, E. Hess, K. Andres, F. Wudl, E. Aharon-Shalom, Phys. Lett.
A 1984, 102, 57-60.
1211 The syntheses will be reported elsewhere. All new compounds provided the
expected spectroscopic and analytical data. The extent of deuteration was
determined by 'H NMR spectroscopy (250 MHz) and confirmed by mass
spectrometry. I b (D3: 99.5%); l c (D6: > 99.5%); I d (D6: 99.5%;
D,: 97.3%).
(221 A careful analysis of the IR data of 1d (C. Pecile, personal communication)
conducted parallel to the measurements described here also demonstrated
the phase transition. We thank Prof. Pecile for the results and for providing
us with 100 mg of I d for comparison.
[23] All four-point measurements with Tl and Tt were repeated a number of
times on various samples.
1241 J. U. yon Schiitz, unpublished.
[25] J. U.von Schutz, H.-P. Werner, H. C . Wolf, A. Aumiiller, P. Erk, S . Hiinig,
Proc. 23th Congr. Ampere M a p . Rson. 1986, 158-159.
[26] First results of the X-ray structure analysis on a single crystal (R. Bau, T.
Metzenthin, unpublished results) and on powder (E. Tillmanns, unpublished results: l a : a = 21.607(1), c = 3.8823(6) A, V, = 1812.5 A'; I c :
a = 21.628(2), c = 3.8730(6) A, Vc = 1811.7 A') indicate a minimal shortening of the cell constant c for the deuterated compounds even at room
temperature. Even the pressure-dependent structural investigations of 1 a
point to an increased compressability in the direction of stacking[4].
[27] A. Heidemann, H. Friedrich, E. Giinther, W. HLusler, 2. Phys. B : Condens. Murrer 1989, 76. 335-344.
862
'63 VCH Verlagsgrsellschafi mbH, W-6940 Weinheim, 1992
[28] a) D. Cavagnat. A. Magerl, C. Vettier, 1. S . Anderson, Phys. Rev. Lett.
1985, 54, 193-196; b) D. Cavagnat, A. Magerl, C. Vettier, S. Clough, J.
Phys. C : Solid State Phys. 1986, 19. 6665-6672; c) D. Cavagnat, A.
Magerl, C. Vettier, S. Clough, J. Phys.: Condens. Matter 1989, 1, 1004710 051.
(291 W. Muller-Warmth, R. Schiiler, M. Prager, A. Kollmar, J. Magn. Reson.
1979,34, 83-95.
(301 a) U. Langohr, Disserfation, Universitat Stuttgart, 1991; b) M. Schlingmann, Diplomarbeit, Universitdt Stuttgart, 1989.
[311 a) J. U. von Schiitz, H. C. Wolf, Z. Nufurforsch.A 1972,27,42-50; b) J. U.
von Schiitz, F. Noack, ibid. 1972,27,645-651; c) J. U. von Schiitz, unpublished.
Doubly Bridged Prismane and Dewar Benzene
Intermediates in an Unusual Photochemical
Rearrangement of Tricyclic Phthalic Esters**
By R o y Gleiter* and Bjorn Treptow
Dedicated to Professor Giinther Maier
on the occasion of his 60th birthday
Irradiation of alkylbenzenes in solution at wavelengths
higher than 250 nm usually yields a positional isomerization
of the alkyl groups."] To rationalize this phenomenon, it is
assumed that valence bond isomers of benzene, such as benzvalene, Dewar benzene, and prismane are short-lived intermediates. The involvement of such isomers has been inferred
from isotopic labeling experiments."' Possible ring transposition processes that give rise to apparent 1,2 and 1,3 shifts
are shown schematically in Scheme 1 .
4
x
Scheme 1. Proposed rearrangements that proceed via valence isomers of benzene as intermediates.
Two motives prompted us to study the mechanism of the
light-induced ring transposition in benzene: 1) We thought
that the tethering of neighboring centers in a benzene derivative might stabilize one ofthe intermediates, and 2) we hoped
by this "rope trick" to generate those doubly bridged Dewar
benzenes and/or prismanes which were not available to
date.[*]
We started our investigations with the doubly bridged Dewar benzene derivatives 1, which could be obtained easily from
cyclic alkynes and dimethyl acetylenedicarboxylates.t2,31 Ir['I
Prof. Dr. R. Gleiter, Dipl.-Chem. B. Treptow
Organisch-chemisches Institut der Universitat
Im Neuenheimer Feld 270, D-W-6900 Heidelberg (FRG)
[**I We are grateful to the Volkswagen-Stiftung, the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the BASF AG (Ludwigshafen) for financial support.
057o-Oa33192107o7-0862B 3.50f ,2510
Angew. Chem. I n f . Ed. Engl. 1992. 31, No. 7
radiation of 1 at wavelengths above 295 nm yielded exclusively the isomeric phthalic ester derivatives 3 (Scheme 2).
No trace of the prismanes 2 could be detected, nor was there
any hint of a photochemical equilibrium between 1 and 3.
Excitation by the unfiltered light from a high-pressure Hg
lamp afforded the isomeric terephthalic esters 4 b 4 d . In the
case of 3a no rearranged product was obtained.
E
E
1
4b- d
t
2
Scheme 2. Rearrangements of 1 on irradiation with light with L > 295 nm. a:
n,m = 4; b: n,m = 5; c : n,m = 6: d: n = 4, m = 5; E = COOMe.
These unexpected results were an impetus for closer investigation. At first we confined our further studies to 3 b because it was the most readily accessiblet2] and gave the
highest yields of 4b. A change of the wavelength to 282 nm
afforded two other compounds, prismane 5 b and Dewar
benzene 6 b. They were unequivocally identified by their
'H NMR, I3C NMR, and UV spectra (see Table 1). They
proved to be direct precursors to 4b, as irradiation converted
them rapidly and quantitatively into 4 b.
Table 1. Selected spectral data of I a, d, 3 a. d, 4b, c, d, 5 b, and 6b, c
l a : ' H NMR (200 MHz, CDCI,): 6 = 3.79 (s, 6H), 2.4-1.0 (m. 16H);
"CNMR (50.32MHz,CDC13): B =163.8, 150.3, 139.1, 57.6. 52.4, 27.0, 25.2,
(Igs) =188 (4.23). 235 sh (3.67):
24.5, 23.3; UV/VIS (acetonitrile): i,,[nm]
correct C,H,O analysis.
ld:'HNMR(300MHz,CDCI,):d= 3.78(~,3H),3.77(~,3H),2.35-2.25(m,
2H), 2.2-1.1 (m, 16H); "C NMR (75.47 MHz, CDCI,): 6 =163.3, 163.25,
150.4, 149.3. 143.5, 141.4, 62.7. 54.5. 51.6, 51.55, 31.1, 28.8. 28.1, 27.7. 27.65.
26.5, 24.4, 24.2, 22.9: high-resolution MS: mi; ( M e- CH,OH) calcd:
284.1413, found: 284.1386
3 a : colorless crystals. m.p. 148-150°C: ' H NMR (200 MHz, CDCI,): 6 =
3.83 ( S , 6H), 2.78 (t, 'J(H,H) = 6.0 Hz, 4H), 2.57 (t, 'J(H,H) = 6.1 Hz, 4H),
1.9-1.6 (m, 8H); 13CNMR (50.32 MHz. CDCI,): 6 =169.9, 139.3, 133.1,
130.1, 52.8, 28.4, 27.7, 23.3, 23.0: UVjVIS (acetonitrile): i.,,,[nm] (lgc) = 213
(4.4), 245 sh (3.45), 287 (2.85): correct C,H.O analysis.
3d: colorless crystals, m.p. 86°C; ' H N M R (300 MHz. CDCI,): 6 = 3.83 (s,
6H), 2.85-2.7 (m, 8H), 1.9-1.5 (m. 10H); I 3 C N M R (75.47MHz. CDCI,):
b =169.8, 169.3, 145.3, 138.4, 136.7, 133.3, 129.4. 129.2, 52.2, 52.3, 31 5, 31.3,
28.6, 27.9, 27.9, 27.2, 26.2, 23.0; UVjVIS (acetonitrile): E.,,,[nm] (Igs) = 216
(4.59), 252 sh (3.79), 288 (3.31); correct C.H.0 analysis.
4 b : colorless needles, m.p. 149'C; ' H N M R (300 MHz, CDCI,): 6 = 3.87 (s,
6H). 2.7-2.55 (m, XH), 1.9-1.7 (m. 4H). 1.7-1.5 (m, 8 H ) ; " C N M R
(75.47 MHz, CDCI,): 6 =271.1, 137.2, 134.4, 51.9, 32.3, 31.8, 27.2; UVjVIS
(acetonitrile): %,,,[nm] (Igs) = 208 (4.27), 230 sh (3.70). 282 (3.06); high-resolucalcd: 330.1831, found: 330.1826: correct C.H.0 analysis.
tion MS: m / z (Ma)
4c: colorless needles, m.p. 169 "C; 'H N M R (200 MHz, CDCI,). 6 = 3.88 (s,
6H), 2.7-2.6 (m, 8H), 1.8-1.6 (m, XH), 1.45-1.25 (m, 8 H ) ; " C N M R
(50.32MH2, CDCI,): 6 =173.1, 135.5, 135.3, 51.8. 31.0, 29.2, 26.1; UVjVIS
(CH,CI,): E.,,,[nm] ( l g ~=
) 206 (4.62). 230 sh (3.91), 280 (3.19): high-resolution
MS: m/r ( M e ) calcd: 358.2144. found: 358.2150.
4d: colorlesscrystals, m.p. =118-119"C; ' H N M R (200MHz. CDCI,): 6 =
3.88 (s, 6H). 2.7-2.55 (m, XH), 1.85-1.5 (m, 10H); "C NMR (50.32 MHz,
CDCI,): 6 =170.8, 137.2, 134.8, 131.0, 52.0. 32.2, 31.8, 27.4. 26.7. 22.4; UVi
VIS (acetonitrile): L,,,[nm] (Ige) = 206 (4.59), 228 sh (3.94). 282 (3.29): correct
C,H.O analysis.
5 b : ' H N M R (300 MHz, CDCI,): 6 = 3.68 (s. 6H). 2.5-2.35 (m. 2H). 2.252.1 (m. 2H), 2.0-1.1 (m, 16H); I3C N M R (75.47 MHz, CDCI,): 6 =170.5,
56.0, 54.5, 50.8.48.4, 32.3,28.6,27.9,24.5,24.2; UVjVIS (n-pentane): i,,,[nm]
(lgs) = 203 (3.85). 270 sh (2.85); high-resolution MS: mi; ( M a ) calcd:
330.1831, found: 330.1827.
6b: ' H N M R (300 MHz, CDCI,): 6 = 3.71 (s, 6H), 2.9-2.7 (m, 2H), 2.5-2.35
(m, 2H), 2.1-1.4 (m. 16H); "C NMR (75.47 MHz. CDCI,): 6 =173.9. 164.2.
134.6, 59.3, 50.8, 31.2, 30.9, 27.9, 27.3, 27.2: UVjVIS (n-pentane): L,,,[nm]
(lge) = 218 (4.17), 270 sh (3.47); high-resolution MS: m / z (Me)calcd:
330.1831, found: 330.1819
6c: 'H N M R (300 MHz, CDCI,): 6 = 3.71 (s. 6H). 3.0-2.8 (m, 2H). 2.4-2.15
(m,4H), 2.15-1.3 (m, 18H); "C NMR (75.47 MHz, CDCI,): 6 = 174.2, 164.1.
136.8, 60.3, 50.7, 30.4. 27.3, 26.6. 25.9, 24.6, 24.2; high-resolution MS: m / z
( M a ) calcd: 358.2144, found: 358.2169
30-d
hv
70-d
9b-d
8b-d
For the mechanism of the complete photochemical rearrangement we assume that the benzvalene derivative 7 b,
[5]metacyclophane 8b, and Dewar benzene 9 b serve as additional intermediates. We account for the almost complete
conversion of 3 b into 4 b through this complex series of
rearrangements as follows: As a result of the short 1,3
bridge, 8 b is extremely bent thus generating considerable
strain,r41which under the given conditions can be relieved
either by the back reaction or in several steps via the prismane 5 b to 4b. The back reaction is usually invoked to
explain the transformation of [5]metacyclophanes to 1,2bridged arenes, although benzvalene intermediates have not
yet been observed directly.r51In our particular case, 8 b can
also follow the photochemically allowed sequence in
Scheme 3 to yield the relaxed tereuhthalic ester derivative
4b. It is noteworthy that the geometry of both Dewar ben-
$$L)&LJ(cw
E
I
(H2C)m
( C W m
E
E
(CH2)m
5b-d
6b-d
4b-d
Scheme 3. Sequence of reactions for the rearrangements of 3 b-d to 4 b-d.
A n g w . Chem. Int. Ed. Engl. 1992, 31. No. 7
0 VCH
9 b and prismane
boat conformation[610f
5 b is already inherent
8 b. Theindriving
the assumed
force
zene
unsymmetric
in this series is the irreversibility of prismane formation
(9 b + 5 b). The interaction of the carbomethoxy groups with
the cyclopropane portion of the prismane in 5 b stabilizes the
distal bonds,['] thus allowing fragmentation only to 6 b,
Verlugsgesell.whuft mhH, W-6940 Weinheim. 1992
0570-0833/92/0707-0863 $3.50+ ,2510
863
which in turn isomerizes immediately to 4b. This was corroborated by the fact that irradiation of 5b did not give rise
to any of its precursors but afforded exclusively 4 b. A similar
argument can be considered for the decomposition of 7 b.
The C-C bond linking the carbomethoxy groups in the bicyclobutane moiety is destabilized for the same reason as
above (in this case the interaction is with two vicinal groups),
so cleavage of this bond leading to [S]metacyclophane 8 b is
kinetically favored over the back reaction, which affords the
thermodynamically more stable 3 b.
The postulated intermediates 7 b, 8 b, and 9 b have not yet
been detected. We attribute this both to their low equilibrium
concentrations and their increased reactivity. As already
mentioned, 3a did not undergo this rearrangement. Even
with extended irradiation times and shorter wavelengths
(254 nm) only a slow polymerization was observed, probably
because the potential [4]metacyclophane 9a does not exist
under the given conditions. All efforts to isolate a
[4]metacyclophane derivative have failed so far because of its
enormous strain energy.''. 91 In our case we assume that the
path leading to 4 a is interrupted at the second step
(7a 8a). Further evidence for benzvalene 7d and [6]metacyclophane 8 c was obtained from the fact that 3c and 3d did
afford their respective terephthalic ester isomers 4 c and 4d.
However, under identical conditions as those used for 4b,['']
the yields of both 4 c (30% after 12 h) and 4d (24% after
12 h) were considerably smaller. Because the equilibrium
concentration was lower, 5c could only be identified in the
reaction mixture by the typical 6 value for the protons of the
methoxy groups in the 'H NMR spectrum. In the case of 4,
none of the intermediates could be fully characterized for the
same reason. The reduced rate in both cases can be explained
--f
since the path via 7d' would lead to a highly strained
[4]metacyclophane 8 d'. A steric factor (tethering of neighboring centers) is necessary, because irradiation of unsubstituted phthalic ester does not yield terephthalic ester. In conclusion, a tuned interplay of steric and electronic effects is
responsible for the creation of doubly bridged prismanes
with C, symmetry from tricyclic phthalic ester derivatives.
Experiments in which the ester groups are substituted by
different functions are currently in progress to substantiate
the electronic argument.
Received: January 13, 1992 [Z 5119 IE]
German version: Angen,. Chem. 1992, 104, 879
CAS Registry numbers:
l a , 141634-78-8; Ib, 130434-04-7, l c , 130434-06-9; Id, 141634-79-9; 3a.
51037-17-3; 3b, 130434-08-1; 3c, 130434-09-2; 3d, 141634-80-2; 4b, 14163481-3; 4c, 141634-82-4;4d, 141663-42-5; 5b, 141663-43-6;5c, 141663-44.7; 5d,
141663-45-8; 6b, 141634-83-5; 6c, 141634-84-6; 6d, 141634-85-7.
[l] Reviews: D . Bryce-Smith, A. Gilbert in Rearrangements in Ground and
E~witedStates, (Ed.:P. de Mayo), Academic Press, New York, 1980,Vol. 3
p. 349; P. A. Wender, T. W von Geldern in Photochemisiry in Organic
S-ynthesis (Ed.: J. D . Coyle), The Royal Society of Chemistry, 1986, p. 226.
[2] R. Gleiter, B. Treptow, Angew. Chem. 1990,102, 1452; Angew>.Chem. fnt.
Ed. Engl. 1990, 29, 1427.
[3] H. Hogeveen, D. M. Kok in The Chemistry of' Triple-Bonded Functional
Groups, Suppl. C , Part 2 (Eds.:S. Patai, Z. Rappoport), Wiley, Chichester,
1983, Chap. 23 and references cited therein.
[4] J. W. van Straten, W. H. de Wolf, F. Bickelhaupt, Tetrahedron Lett. 1977,
4667: L. W. Jenneskens, F. J. J. de Kanter, L. A. M. Turkenburg, H. .I.R.
de Boer, W. H. de Wolf, F. Bickelhaupt, Tetrahedron 1984, 40, 4401.
[5] L. W. Jenneskens. H. J. R. de Boer, W. H. de Wolf, F. Bickelhaupt, J. Am.
Chem. Sac. 1990, f12, 8941.
[6] For an X-ray structure of a [5]metacyclophane see: L. W. Jenneskens, J. C.
[7]
[8]
- 4d
191
(101
7d
1111
8d
Klamer, H. J. R. de Boer, W. H . de Wolf, F. Bickelhaupt, C. H. Stam,
Angew. Chem. 1984, 96, 236; Angew. Chem. Int. Ed. Engl. 1984,23, 238.
H. Irngartinger,D . Kallfass, E. Litterst, R. Gleiter, Arta Crystallogr. 1987,
C 43, 266; R. Hoffmann, Tetrahedron Lett. 1970, 2907; H. Giinther, ibid.
1970, 5173.
L. A. M . Turkenburg, J. W. van Straten, W. H. de Wolf, F. Bickelhaupt, J.
Am. Chem. SOC.1980, 102, 3256; G. B. M. Kostermans, M. Hogenbirk,
L. A. M. Turkenburg, W H. de Wolf, F. Bickelhaupt, ibid. 1987,109,2855.
G. B. M. Kostermans, P. van Dansik, W. H. de Wolf, F. Bickelhaupt, J.
Am. Chem. SOC.1987, 109, 7887; J. Org. Chem. 1988,53,4531.
All irradiations were carried out in a 250 mL vessel at 10°C under argon
with a Phillips HPK 125 W lamp. Concentrations: 0.003 molL-' in diethyl ether.
S. Hirano, H. Hara. T. Hiyama, S. Fujita, H. Nozaki, Tetrahedron 1975,
31, 2219.
3d
8
Template-Controlled Organization of a Fluoride
Surface-An Analogue of a Crown Ether in the
Reaction of [{(q5-C,Me,)TiF,),] with Sodium
Fluoride**
E
it-
E
By Herbert W Roesky,* Mansoreh Sotoodeh,
and Mathias Noltemeyer
8d'
7d'
Dedicated to Professor Klaus Weissermel
on the occasion of his 70th birthday
Scheme 4
by the postulated intermediates. Firstly, [6]metacyclophane
8c is distinctly more stable'"] than the lower homologue 8 b,
and the strain releasing steps (8c 5 c 4 4c) are thus less
forceful. Secondly, it is noteworthy that the conversion of 3 d
to 4 d occurred at about half the rate of that of 3 b to 4b (52 YO
after 12 h). A plausible explanation is given in Scheme 4.
Whereas in the case of 3 b two identical benzvalene intermediates afford 8 b, only the path via 7d is possible for 3d,
--f
864
8
Y C H Verlu~sgesellschaf'lmbH, W-6940 Weinheim, 1992
So far in studies concerning molecular recognition, host
molecules were applied whose receptor surface comprises
almost exclusively oxygen, sulfur, nitrogen, and/or phospho[*I
[**I
Prof. Dr. H. W. Roesky, Dipl.-Chem. M. Sotoodeh, Dr. M. Noltemeyer
Institut fur Anorganische Chemie der Universitat
Tammannstrasse 4, D-W-3400 Gottingen (FRG)
This work was supported by the Deutsche Forschungsgemeinschaft, the
Volkswagen-Stiftung, and the Fonds der Chemischen Industrie.
OS70-0833/92/1)707-0864$3.50+.25/0
Angen. Chem. Int. Ed. Engl. 1992. 31, No. 7
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