A five-component synthesis of hexasubstituted benzene

Article abstract of DOI:10.1002/1521-3773(20021115)41:22<4291::AID-ANIE4291>3.0.CO;2-D

Seven functional groups are involved in a highly ordered five-component domino process, which leads to a biologically relevant polyheterocycle. In this one-pot transformation, the formation of seven chemical bonds provides a hexasubstituted benzene core (

Full text of DOI:10.1002/1521-3773(20021115)41:22<4291::AID-ANIE4291>3.0.CO;2-D

COMMUNICATIONS
malonate complex 6b. Complex 7 did form the corresponding
2-aryl 1,3-diketone during this thermolysis, but in only 22%
yield.
A Five-Component Synthesis of
Hexasubstituted Benzene**
One would expect that rearrangement of the h2-O,O-bound
complexes to their C-bound tautomers would precede reduc-
tive elimination. Indeed, complexes of 1,3-dicarbonyl anions
that we could not observe as a C-bound isomer did not
undergo reductive elimination in high yield. For example,
arylmalonate anions 2c and 6c, and acetylacetonate 7
produced Pd0 complexes and free malonate or acetylacetone
upon addition of the chelating ligands dppe, dppbz ¼ 1,2-
bis(diphenylphospnanyl)benzene (dppbz), 2,2’-bis(diphenyl-
phosphanyl)-1,1’-binaphthyl (binap), and 1,1’-bis(diphenyl-
phosphanyl)ferrocene (dppf). No complex of a C-bound
anion was detected.
Pierre Janvier, Hugues Bienaymÿ, and Jieping Zhu*
Bis(indolyl)maleimides and indolo[2,3-a]carbazole alka-
loids constitute a rapidly growing family of natural products
with diverse biological activities.[1] Thus, rebeccamycin (1)[2]
and staurosporine (2)[3] are potent topoisomerase I and
MeO
OH
HO
H
O
O
N
N
HN
HO
HN
O
NHMe
OMe
N
In summary, the greater steric hindrance of FcPtBu2 (4),
relative to that of phenylphosphanes, induces reductive
elimination from complexes of typically unreactive ligands
derived from malonate and actylacetonate anions. This steric
effect overrides the stabilizing effect of the h2-O,O-coordina-
tion mode and the electron-withdrawing groups on the central
carbon atom of the malonate anion. Studies on the mechanism
of these new reductive eliminations and the synthesis of
complexes with related stabilized anions are in progress.
O
Me
NH
O
1
2
protein kinase C inhibitors, respectively. Structurally, this
class of compounds is characterized by an hexasubstituted
benzene ring with a fused indolo (forming an indolocarbazole
entity) and a fused lactam or an imide ring with a pendant
sugar moiety. The novelty of the structures combined with
their interesting biological profile have stimulated numerous
synthetic efforts from both academic and industrial research-
ers.[4]
Received: July 22, 2002 [Z19785]
[1] S. Kawaguchi, Coord. Chem. Rev. 1986, 70, 51.
[2] F. A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed.,
Wiley, New York, 1988.
Transition-metal-mediated cyclization of appropriately
functionalized enynes[5] and Diels Alder cycloaddition of
furan[6] are two main strategies used for the synthesis of
hexasubstituted benzenes.[7] Although high level of structural
complexity can be generated from these two key transforma-
tions, the overall efficiency is often counter-balanced by
efforts associated with the synthesis of linear precursors. In
connection with our continued interest in the development of
highly efficient synthesis of druglike polyheterocycles,[8] we
report herein a conceptually new strategy for the synthesis of
hexasubstituted benzenes 3 based on a novel one-pot five-
component domino process.[9,10]
[3] D. F. Shriver, P. W. Atkins, Inorganic Chemistry, 3rd ed., Freeman,
New York, 1999.
[4] J. Tsuji, Acc. Chem. Res. 1969, 2, 144.
[5] B. M. Trost, Tetrahedron 1977, 33, 2615.
[6] A. Pfaltz, M. Lautens, Allylic substitution reactions in Comprehensive
Asymmetric Catalysis I III, Vol. 2 (Eds.: E. N. Jacobsen, A. Pfaltz, H.
Yamamoto), Springer, Berlin, 1999, pp. 833 884.
[7] N. A. Beare, J. F. Hartwig, J. Org. Chem. 2002, 67, 541.
[8] M. Kawatsura, J. F. Hartwig, J. Am. Chem. Soc. 1999, 121, 1473.
[9] J. M. Fox, X. Huang, A. Chieffi, S. L. Buchwald, J. Am. Chem. Soc.
2000, 122, 1360.
[10] L. Djakovitch, K. Kohler, J. Organomet. Chem. 2000, 606, 101.
[11] M. A. Aramendia, V. Borau, C. Jimenez, J. M. Marinas, J. R. Ruiz, F. J.
Urbano, Tetrahedron Lett. 2002, 43, 2847.
The underlying principle of our synthesis is shown in
Scheme 1. A recently developed three-component reaction
based on Ugi chemistry pioneered by Ugi and co-workers[9]
provides the 5-aminooxazole 7,[11] Reaction of the latter with
acyl chloride 8 (X ¼ Cl) should give 5,6-dihydrofuro[2,3-c]-
pyrrol-4-one 10 following a sequence of acylation, intra-
molecular Diels Alder cycloaddition and retro-Diels Alder
cycloreversion.[12] Addition of a second dienophile 11 to the
reaction mixture should initiate an intermolecular Diels
Alder reaction of furan[6,7e] to give, after directed fragmenta-
[12] K. Hiraki, M. Onishi, H. Matsuo, J. Organomet. Chem. 1980, 185, 111.
[13] P. J. Chung, H. Suzuki, Y. Moro-oka, T. Ikawa, Chem. Lett. 1980, 63.
[14] G. R. Newkome, T. Kawato, D. K. Kohli, W. E. Puckett, B. D. Olivier,
G. Chiari, F. R. Fronczek, W. A. Deutsch, J. Am. Chem. Soc. 1981, 103,
3423.
[15] B. F. Straub, F. Rominger, P. Hofmann, Inorg. Chem. Commun. 2000,
3, 358.
[16] J. Ruiz, V. Rodriguez, N. Cutillas, M. Pardo, J. Perez, G. Lopez, P.
Chaloner, P. B. Hitchcock, Organometallics 2001, 20, 1973.
[17] G. R. Newkome, V. K. Gupta, Inorg. Chim. Acta 1982, 65, L165.
[18] G. R. Newkome, V. K. Gupta, H. C. R. Taylor, F. R. Fronczek,
Organometallics 1984, 3, 1549.
[19] T. Kawato, T. Uechi, H. Koyama, H. Kanatomi, Y. Kawanami, Inorg.
Chem. 1984, 23, 764.
[*] Dr. J. Zhu, P. Janvier
[20] H. Kurosawa, K. Ishii, Y. Kawasaki, S. Murai, Organometallics 1989, 8,
1756.
Institut de Chimie des Substances Naturelles, CNRS, 91198 Gif-sur-
Yvette (France)
[21] V. V. Grushin, H. Alper, Organometallics 1993, 12, 1890.
[22] M. S. Driver, J. F. Hartwig, Organometallics 1997, 16, 5706.
[23] G. Mann, C. Incarvito, A. L. Rheingold, J. F. Hartwig, J. Am. Chem.
Soc. 1999, 121, 3224.
Fax : (þ 33)1-69077247
E-mail: zhu@icsn.cnrs-gif.fr
Dr. H. Bienaymÿ
Chrysalon, 11 Avenue Albert Einstein, 69100 Villeurbanne (France)
[**] Financial support from the CNRS and a doctoral fellowship from
Rhodia (PJ) is gratefully acknowledged.
Angew. Chem. Int. Ed. 2002, 41, No. 22
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4122-4291 $ 20.00+.50/0
4291

COMMUNICATIONS
NHR1
O
R6
R3
O
R6
COX
R1N
R2
O
R1NH2 (4)
R2CHO (5)
CN
R2
8
NR4R5
NR4R5
NR4R5
+
N
R3
O
N
R3
7
9
6
O
R6
O
R8
R6
R6
O
R7
NR4R5
R8
R1N
R2
R1N
R2
11
R1N
R2
O
NR4R5
NR4R5
O
R8
R7
12
R7
R3CN
10
3
Scheme 1. Five-component synthesis of hexasubstituted benzene.
tion, the hexasubstituted benzenes 3. The development of
conditions for the three-component synthesis of oxazole that
are compatible with the subsequent two cycloaddition-based
domino processes would enable a five-component synthesis of
hexasubstituted benzene 3. The sequence would allow rapid
and efficient construction of structurally complex molecules
from readily available starting materials.
The reaction of the resulting aminofuran with N-phenyl-
maleimide (11a) in toluene was next examined. When this
reaction was performed at 708C in toluene, we were able to
isolate the unstable oxa-bridged intermediate 12a as a
COOMe
Whereas the cycloaddition of oxazole with acetylene is a
well-established method for furan synthesis,[12] the corre-
sponding reaction of 5-aminooxazole was, to the best of our
knowledge, unknown. To test the feasibility of the overall
process, the reaction between purified 5-aminooxazole 7a and
acyl chloride 8a[13] was first examined (Scheme 2). The
O
O
MeOOC
N
N
N
O
O
O
N
O
N
N
O
13(90%)
12a (47%)
NHC4H9
COCl
O
N
C6H13
N
O
mixture of two diastereomers in 47% yield. However, on
heating the solution in toluene at reflux, hexasubstituted
benzene was produced directly. In view of the similar reaction
conditions leading to the production of 10a and 3a, we
reasoned that it might be possible to combine these two
domino processes. In the event, heating a solution of oxazole
7a and acyl chloride 8a in the presence of triethylamine for
12 h, followed by addition of N-phenylmaleimide 11a, pro-
vided 3a in over 90% overall yield (Scheme 2).
Encouraged by the efficiency of the two consecutive
domino processes, we set out to explore conditions that
would enable its combination with the three-component
synthesis of 5-aminooxazole 7. Initial results regarding the
quest for this one-pot five-component domino process with
lithium bromide gave unsatisfactory results.[14] On the other
hand, a stoichiometric amount of ammonium chloride[15] and a
catalytic amount of camphorsulfonic acid (CSA, 10%) were
able to promote the five-component domino process, which
led to the hexasubstituted benzene 3a in 40 and 52% yields,
respectively (Scheme 3, X ¼ Cl). Subsequent experiments
indicated that the use of pentafluorophenyl ester 8b (X ¼
OC6F5) instead of the acyl chloride 8a (X ¼ Cl) provided an
improved yield of 3a (67%). Significantly, in this operation-
ally simple five-component domino process, at least seven
reactive functionalities participated in the chemical trans-
formation that led to the concomitant creation of seven
+
Bn
7a
8a
O
toluene
Et3N, reflux
C4H9
+
BnCN
N
N
O
O
N
C6H13
10a
O
O
O
O
N
Ph
11a
N
O
N
O
3a
Scheme 2. From 5-aminooxazole to hexasubstituted benzene.
reaction proceeded smoothly to provide the 5,6-dihydro-
furo[2,3-c]pyrrol-4-one 10a (> 95% yield). A triple domino
sequence involving acylation/intramolecular Diels Alder cy-
cloaddition/retro Diels Alder cycloaddition could explain the
reaction outcome. On the other hand, the reaction of 7a with
dimethylacetylenedicarboxylate (DMAD) provided the Mi-
chael adduct 13; no Diels Alder cycloaddition took place.
Attempts to promote the cycloaddition between 13 and
DMAD led only to the recovery of starting materials or to
degradation under forcing conditions.
ꢀ
ꢀ
chemical bonds (two C N and five C C bonds) and a
biologically relevant polyheterocycle.
4292
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4122-4292 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 22

COMMUNICATIONS
O
CN
C4H9NH2 (4a)
N
O
O
O
O
O
N
C6H13CHO (5a)
N
6
N
N
MeO
MeO
O
O
toluene, additive
then
N
N
O
O
COX
8
Et3N
then
3a (67%)
3b (32%)
O
N
O
Ph
11a
COOMe
O
O
O
O
N
N
O
O
N
N
N
N
O
O
N
N
O
O
O
N
O
F
3a
3c (54%)
3d (45%)
Additive
Equiv.
X
Yield of 3a
OMe
NH4Cl
CSA
CSA
1
0.1
0.1
Cl
Cl
OC6F5
40%
52%
67%
COOMe
O
O
O
O
N
Scheme 3. One-pot five-component synthesis of polyheterocycles with an
hexasubstituted benzene core under various conditions.
N
N
O
N
O
O
O
The use of three different amines, three aldehydes, one
isocyanoacetamide, two pentafluorophenyl 3-arylprop-2-
ynoates, and three dienophiles as starting materials allowed
the synthesis of the polyheterocycles shown in Scheme 4. The
potential and general applicability of this five-component
domino process are readily seen from these selected exam-
ples.
3e (63%)
3f (51%)
O
O
N
N
O
In conclusion, we have developed a novel five-component
domino process for the synthesis of highly functionalized
polyheterocycles from simple and readily accessible starting
materials. The overall process leads to the creation of seven
chemical bonds and delivers five elements of diversity into the
compact polyheterocycle, thus providing a large increase in
molecular complexity. This constitutes a rare example in
which more than four reagents were assembled together to
provide a biologically relevant scaffold.[16] The operational
simplicity and good chemical yield made these novel hetero-
cycle syntheses highly attractive in diversity-oriented parallel
synthesis.[17]
O
O
3g (32%)
Scheme 4. Five-component synthesis of polyheterocycles with an hexasub-
stituted benzene core: selected structures.
reaction mixture was stirred for an additional 15 min at 1108C. After being
cooled to room temperature, the reaction mixture was diluted with water
and extracted with EtOAc. The combined organic extracts were washed
with brine, dried (Na2SO4), and evaporated under reduced pressure. The
crude reaction mixture was purified by preparative TLC (silica gel, eluent:
AcOEt/heptane ¼ 1:2) to give the corresponding hexasubstituted benzene
3a (46.6 mg, 67%) as a slight yellow oil. IR: n˜ ¼ 3011, 2963, 2931, 2862,
1764, 1714, 1684, 1600, 1502, 1438, 1383, 1234 cmꢀ1 1H NMR (CDCl3,
;
Experimental Section
250 MHz): d ¼ 7.58 7.28 (m, 10H), 5.01 (t, J ¼ 3.5 Hz, 1H), 3.95 (ddd, J ¼
6.8, 9.2, 13.8 Hz, 1H), 3.61 (m, 4H), 3.00 (ddd, J ¼ 5.0, 8.9, 13.8 Hz, 1H),
2.95 (m, 4H), 2.63 (m, 1H), 2.09 (m, 1H), 1.62 (m, 2H), 1.33 (sextet, J ¼
7.2 Hz, 2H), 1.19 (m, 6H), 1.02 (m, 1H), 0.92 (t, J ¼ 7.2 Hz, 3H), 0.84 (t, J ¼
6.6 Hz, 3H), 0.69 ppm (m, 1H); 13C NMR (CDCl3, 75 MHz): d ¼ 165.9,
165.8, 165.3, 149.1, 143.6, 137.4, 137.1, 134.6, 131.5, 130.1, 129.7, 129.1, 129.0,
128.2, 128.1, 127.7, 127.6, 126.7, 126.0, 67.0, 57.0, 52.0, 39.7, 31.4, 29.9, 28.9,
28.5, 22.4, 21.8, 20.2, 13.9, 13.7 ppm; MS (ES, positive mode): m/z [MþH]þ:
580.0
Typical procedure: Heptanal 5a (20.0 mL, 0.144 mmol) was added to a
solution of butylamine (4a, 15.2 mL, 0.156 mmol) in dry toluene (1.0 mL).
After the mixture was stirred at room temperature for 30 min, isocyanide 6
(29.0 mg, 0.120 mmol) and camphorsulfonic acid (3.0 mg, 0.012 mmol,
0.1 equiv) were added successively. The reaction mixture was stirred at
608C until the disappearance of isonitrile. The reaction mixture was cooled
to 08C. Et3N (83.6 mL, 0.60 mmol) was added, followed by a solution of
pentafluorophenyl ester 8b (64.0 mg, 0.2 mmol) in toluene (1.0 mL).
Stirring was continued at room temperature for 30 min and at 1108C for
12 h. N-Phenylmaleimide (23.0 mg, 0.132 mmol) was then added. The
Received: July 15, 2002 [Z19740]
Angew. Chem. Int. Ed. 2002, 41, No. 22
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4122-4293 $ 20.00+.50/0
4293

COMMUNICATIONS
[1] For reviews, see: a) G. W. Gribble, S. J. Berthel in Studies in Natural
Products Chemistry, Vol. 12 (Ed.: Atta-Ur-Rahman), Elsevier, New
York, 1993, pp. 365 409; b) J. Bergman, T. Janosik, N. Wahlstrˆm in
Advances in Heterocyclic Chemistry, Vol. 80 (Ed.: A. R. Katritzky),
Academic Press, New York, 2001, pp. 1 71.
[2] P. Moreau, F. Anizon, M. Sancelme, M. Prudhomme, D. Sevõre, J. F.
Riou, J. F. Goossens, J. P. Hÿnichart, C. Bailly, E. Labourier, J. Tazzi,
D. Fabbro, T. Meyer, A. M. Aubertin, J. Med. Chem. 1999, 42, 1816
1822.
Formation and Characterization of the First
Monoalumoxane, LAlO¥B(C6F5)3**
Dante Neculai, Herbert W. Roesky,*
Ana Mirela Neculai, Jˆrg Magull, Bernhard Walfort,
and Dietmar Stalke
Dedicated to Professor Heribert Offermanns
on the occasion of his 65th birthday
[3] P. Schupp, C. Eder, P. Porksch, V. Wray, B. Schneider, M. Herderich, V.
Paul, J. Nat. Prod. 1999, 62, 959 962.
[4] For a review, see: U. Pindur, Y. S. Kim, F. Mehrabani, Curr. Med.
Chem. 1999, 6, 29 69.
[5] a) K. P. C. Vollhardt, Angew. Chem. 1984, 96, 525; Angew. Chem. Int.
Ed. Engl. 1984, 23, 539 556; b) A. J. Fletcher, S. D. R. Christie, J.
Chem. Soc. Perkin Trans. 1 2000, 1657 1668; c) S. Saito, Y. Yama-
moto, Chem. Rev. 2000, 100, 2901 2916.
[6] a) H. N. C. Wong, T. K. Ng, T. Y. Wong, Heterocycles 1983, 20, 1815
1840; b) H. N. C. Wong, T. K. Ng, T. Y. Wong, Y. D Xing, Heterocycles
1984, 22, 875 890; c) C. O. Kappe, S. S. Murphree, A. Padwa,
Tetrahedron 1997, 53, 14179 14233; d) P. Vogel, J. Cossy, J. Plumet,
O. Arjona, Tetrahedron 1999, 55, 13521 13642.
[7] Recent selected examples: metal-catalyzed: a) B. Witulski, T. Stengel,
Angew. Chem. 1999, 111, 2521 2524; Angew. Chem. Int. Ed. 1999, 38,
2426 2430; b) D. Suzuki, H. Urabe, F. Sato, J. Am. Chem. Soc. 2001,
123, 7925 7926; c) T. Sugihara, A. Wakabayashi, Y. Nagai, H. Takao,
H. Imagawa, M. Nishizawa, Chem. Commun. 2002, 576 577; d) M.
Petit, G. Chouraqui, P. Phansavath, C. Aubert, M. Malacria, Org. Lett.
2002, 4, 1027 1029; cycloaddition: e) A. Padwa, J. P. Snyder, E. A.
Curtis, S. M. Sheehan, K. J. Worsencroft, C. O. Kappe, J. Am. Chem.
Soc. 2000, 122, 8155 8167.
It has been shown that alumoxanes of the general formula
(RAlO)n for n > 1 can be obtained by the controlled reaction
of organoaluminum compounds with either water or water
contained in hydrated salts or (Me2SiO)3.[1] Although the
simplest member of the series, namely (RAlO), was predicted
to be obtainable based on the analogy with aluminum
imides,[2] its formation and characterization has remained
ꢀ
elusive, presumably because it implies the presence of an Al
O double bond, which is likely to be very unstable even
though p interactions between Al and O atoms have been
invoked by several groups.[3] However, compounds with such
bonds may be either sterically (by using bulky ligands bonded
to the aluminum) or electronically (by using Lewis acids)
stabilized. Our approach for the stabilization was to use
H2O¥B(C6F5)3, which has been shown to act as a strong
Br˘nsted acid,[4] and whose ability to protonate M R bonds
ꢀ
has been verified.[5] Furthermore if a monoalumoxane is
formed, B(C6F5)3 may hinder the aggregation owing to its
strong Lewis acid character.[6]
[8] a) J. Drews, Science 2000, 287, 1960 1964; b) W. Wess, M. Urmann, B.
Sickenberger, Angew. Chem. 2001, 113, 3443 3453; Angew. Chem.
Int. Ed. 2001, 40, 3341 3350.
[9] For recent reviews on multicomponent reactions, see: a) G. H. Posner,
Chem. Rev. 1986, 86, 831 844; b) L. Weber, K. Illgen, M. Almstetter,
Synlett 1999, 366 374; c) A. Domling, I. Ugi, Angew. Chem. 2000,
112, 3300 3344; Angew. Chem. Int. Ed. 2000, 39, 3168 3210; d) H.
Bienaymÿ, C. Hulme, G. Oddon, P. Schmitt, Chem. Eur. J. 2000, 6,
3321 3329.
[10] For recent reviews on domino process, see: a) L. F. Tietze, Chem. Rev.
1996, 96, 115 136; b) M. H. Filippini, J. Rodriguez, Chem. Rev. 1999,
99, 27 76; c) R. A. Bunce, Tetrahedron 1995, 51, 13103 13160;
d) L. F. Tietze, F. Haunert in Stimulating concepts in Chemistry (Eds.:
M. Shibasaki, J. F. Stoddart, F. Vˆgtle), 2000, pp. 39 64.
[11] X. Sun, P. Janvier, G. Zhao, H. Bienaymÿ, J. Zhu, Org. Lett. 2001, 3,
877 880.
Indeed, the reaction of LAlMe2 (where L is a monoanionic
b-diketiminato ligand, Scheme 1)[7] and H2O¥B(C6F5)3 in
toluene gave LAlO¥B(C6F5)3 (1), which was filtered off at
room temperature and crystallized from dichloromethane
(ꢀ268C). In contrast, when the same reagents were allowed to
react in THF at 558C for 2 h, after the solvent had been
removed, an oily product was formed which crystallized as an
isomer of 1, formulated as LAl(C6F5)OB(C6F5)2 (2;
Scheme 1).
[12] a) For a review on the intramolecular Diels Alder reaction of oxazole
and acetylene, see: P. A. Jacobi in Advances in Heterocyclic Natural
Product Synthesis, Vol. 2 (Ed.: W. H. Pearson), 1992, pp. 251 298; for
a recent application in natural product synthesis, see: b) B. Liu, A.
Padwa, Tetrahedron Lett. 1999, 40, 1645 1648; c) P. A. Jacobi, K. Lee,
J. Am. Chem. Soc. 2000, 122, 4295 4303; d) L. A. Paquette, I.
Efremov, J. Am. Chem. Soc. 2001, 123, 4492 4501.
[13] 3-Arylpropiolic acid was synthesized from the corresponding cinna-
mate according to a literature procedure: K. J. Schofield, J. C. E.
Simpson, J. Chem. Soc. 1945, 512 520.
[14] E. Gonzalez-Zamora, A. Fayol, M. Bois-Choussy, A. Chiaroni, J. Zhu,
Chem. Commun. 2001, 1684 1685.
Scheme 1. Synthesis of compounds 1 and 2.
[15] a) P. Cristau, J.-P. Vors, J. Zhu, Org. Lett. 2001, 3, 4079 4082; b) P.
Janvier, X. Sun, H. Bienaymÿ, J. Zhu, J. Am. Chem. Soc. 2002, 124,
2560 2567.
[16] For a five-component synthesis of a-acylaminocarbonamide, see: a) I.
Ugi, C. Steinbr¸ckner, Chem. Ber. 1961, 94, 2802 2814; for a seven-
component reaction, see: b) A. Dˆmling, I. Ugi, Angew. Chem. 1993,
105, 634 635; Angew. Chem. Int. Ed. Engl. 1993, 32, 563 564.
[17] a) S. L. Schreiber, Science 2000, 287, 1964 1969; b) P. Arya, D. T. H.
Chou, M. G. Baek, Angew. Chem. 2001, 113, 351 358; Angew. Chem.
Int. Ed. 2001, 40, 339 346.
[*] Prof. Dr. H. W. Roesky, D. Neculai, A. M. Neculai, Prof. Dr. J. Magull
Institut f¸r Anorganische Chemie
Universit‰t Gˆttingen
Tammannstrasse 4, 37077 Gˆttingen (Germany)
Fax : (þ 49)551-393-373
E-mail: hroesky@gwdg.de
Dr. B. Walfort, Prof. Dr. D. Stalke
Institut f¸r Anorganische Chemie
Universit‰t W¸rzburg
Am Hubland, 97074 W¸rzburg (Germany)
[**] This work was supported by the Deutsche Forschungsgemeinschaft.
4294
¹ 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0044-8249/02/4122-4294 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2002, 41, No. 22