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Synthesis Structure and Isomerization of the (E)- and (Z)-Isomers of a Diphosphene.

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5 % of the total protein concentration. It is worth noting
that no similar g < 2 signal could be detected when protein
R2 was reduced by 1,4-dithio-~-threitoI(DTT) at alkaline
in the presence of methyl viologen also
P H . ~ ' ~Dithionite
failed to generate this signal.f7]This indicates that the corresponding species is extremely unstable.
Introduction of air resulted in the instantaneous increase
of the g = 2.00 signal and disappearance of the g < 2 signal.
The regeneration of the tyrosyl radical confirmed that reduced R2 was formed during the anaerobic step.[4asb-7-91
However, radical formation was not total (60 YOyield), probably because the diimide partly mobilized endogenous iron
as suggested by the significant increase of the g N" 4.3 signal.
Instability of Fe" in R2 is well-documented.[81We checked
that protein R2 was not irreversibly denatured by prolonged
incubation with diimide. Actually, diimide-treated R2 could
be fully reactivated through one cycle of iron chelation by
8-hydroxyqauinoline 5-sulfonate and reconstitution in the
presence of Fe" and a~corbate,[~I
as monitoring by EPR
spectroscopy shows.
The new signal resembles those previously reported for the
two mixed-valence Fe"-Fe"' states of hemerythrin (g values
of 1.95, 1.72, 1.57 and 1.96, 1.88, 1.66), methane monoxygenase (g = 1.95, 1.88, 1.78), and purple acid phosphatase
(g = 1.93. 1.72, 1.56).['7 1 3 * 141 Also, a synthetic binuclear
iron complex containing an Fe"-Fe"' cluster has similar
S = 1/2 EPR characteristics arising from the antiferromagnetic interaction of a pair of high spin Fe" and Fe"'
Thus the S = 1/2 EPR spectrum of Figure 1 strongly suggests that a mixed-valence Fe"-Fe"' state was formed during reduction of R2 to reduced R2 by diimide.
Although the g values of the mixed-valance states of binuclear iron proteins fall in the same range, confirming that
the clusters have a similar core structure, the observed differences are nevertheless significant. They reflect distortions at
the Fe" site as well as minor changes of the relative magnitudes of the exchange and zero-field splitting parameters.['6]
It is therefore remarkable that the mixed-valence state of R2
gives an EPR spectrum which closely resembles that of
(semi-metHr), , the mixed-valence state produced by a oneelectron reduction of methemerythrin.['* "1 This suggests
that the two iron atoms are only weakly antiferromagnetically coupled, which in turn suggests that the 0x0 bridge has
been converted into a hydroxo bridge during reduction of
R2.[16-191Moreover, since Fe2+ in (semi-metHr), is sixcoordinate, a similar geometry is likely to occur in the R2
mixed-valence state.
The doubling of the signal could originate from a dipolar
interaction between the paramagnetic Fe"-Fe"' center and
the residual tyrosyl radical, if present in the same polypeptide chain. However, this would also result in the splitting
of the g = 2.00 signal. Since no change in the position or the
width of this signal could be observed, this hypothesis must
be excluded. Therefore, the doubling of the signal is probably related to the dissymmetry of the binuclear iron cluster.
The presence of two structurally nonidentical iron ions[31
may explain the formation of two slightly different mixedvalent di-iron clusters, as found for the semimethemerythrins.[' 7 * *I
As mentioned above, reduced R2 obtained from reduction
of R2 by dithionite exhibits an EPR signal at g 16.Ig1On
the contrary, the Fe"-Fe" center produced during reduction
of R2 by diimide is EPR-silent. This situation is reminiscent
of the one in deoxyhemerythrin, for which the generation of
an analogous low field (g 13) signal depends on the ligand
bound at the free coordination site.[201We therefore tentatively suggest that binding of diimide to the Fe"-Fe" center
1 136 0 VCH VerlagsgesellscJ?ajl mhH. W-6940 Wernheim, 1991
of reduced R2 makes it EPR-silent. Such a binding would
also explain the stabilization of the mixed-valence state. We
are currently investigating the possibility for such coordination chemistry in ribonucleotide reductase.
Received: April 9, 1991 [Z 4560 IE]
German version: Angew. Chem. 103 (1991) 1151
[I] J. Sanders-Loehr in T. M. Loehr (Ed.): Iron Carriers and Iron Proteins,
VCH Publishers, New York 1989, p. 373.
[2] P. Reichard, Annu. Rev. Biochem. 57 (1988) 349.
[3] P. Nordlund, H. Eklund, B. M. Sjoberg, Nature (London) 345 (1990) 593.
[4] a) M. Fontecave, R. Eliasson, P. Reichard, J. Biol. Chem. 264 (1989) 9164;
b) M. Fontecave, C. Gerez, M. Atta, A. Jeunet, Biochem. Biophys. Kes.
Commun. 168 (1990) 659.
[5] L. Petersson, A. Graslund, A. Ehrenberg, B. M. Sjoberg, P. Reichard, J.
Biol. Chem. 255 (1980) 6706.
[6] Another weak signal is observed at g = 4.3 indicating the presence of
adventitiously hound iron unavoidable in standard preparations of the
[7] M. Sahlin, A. Graslund, L. Petersson, A. Ehrenberg, B. M. Sjoherg, Biochemistry 28 (1989) 2618.
[8] M. Fontecave, C. Gerez, D. Mansuy, P. Reichard, J. Biol. Chem. 265
(1990) 10919.
[9] J. B. Lynch, C. Juarez-Garcia, E. Munck, L. Que, J. Biol. Chem. 264 (1989)
[lo] Protein R2 was prepared from an overproducing strain of E. coli: B. M.
Sjoberg, S . Hahne, M. Karlsson, H. Jornwall, M. Goransson, B. E. Uhlin,
J. Biol. Chem. 26f (1986) 5658.
[ll] a) J. Thiele, Justus Liebigs Ann. Chem. 271 (1892) 127; b) W. G. Hanstein,
J. B. Lett, C. E. McKenna, T. G. Traylor, Proc. Nail. Acad. Sci. U S A . 58
(1967) 1314.
[12] The reaction was carried out as described for diimide but 1,4-~-ditbiothreit01 (10-20mM) was used instead.
[13] L. Que, Jr., R. C. Scarrow: Metal Clusters in Proteins 372 (ACS Symp.
Ser. (1988) 152).
[14]M. P. Woodland, D. S . Patil, R. Cammack, H. Dalton, Biochim. Biophys.
Acta. 873 (1986) 237.
(151 J. R. Hartman, R. L. Rardin, P. Chandhuri, K. Pohl, K. Wieghardt, B.
Nuber, J. Weiss, G. C. Papaefthymiou, R. B. Frankel, S. J. Lippard, J. Am.
Chem. Soc. 109 (1987) 7387.
[16] P. Bertrand, B. Guigliarelli, C. More, Nouv. J. Chim. 15 (1991), in press.
[17] L. L. Pearce. D. M. Kurtz, Jr., Y M. Xia, P. G. Debrunner, J. Am. Chem.
Soc. 109 (1987) 7286.
[18] J. M. Me Cormick, E. I. Solomon, J. Am. Chem. Soc. 112 (1990) 2005.
[19] M. J. Maroney, D. M. Kurtz, Jr., J. M. Nocek, L. L. Pearce, L. Que,J. Am.
Chem. Soe. 108 (1986) 6871.
[20] R. C. Reem, E. I. Solomon, J. Am. Chem. Soc. 109 (1987) 1216.
Synthesis, Structure, and Isomerization
of the (E)- and (2)-Isomers of a Diphosphene""
By Edgar Niecke,* Oliver Altmeyer, and Martin Nieger
Dedicated to Professor Leopold Horner
on the occasion of his 80th brithday
Aryl-substituted (E)-diazenes (E = N) I can be photochemically converted into the (Z)-form 11, which, in turn,
can be reconverted thermally into the (E)-form.['' Since the
isomerization I1 + I generally proceeds with release of energy, aryl-substituted diazenes have found lasting interest as
storage systems for solar energy.l2I
[*I Prof. Dr. E. Niecke, Dr. 0. Altmeyer, Dr. M. Nieger
Anorganisch-chemisches Institut der Universitat
Gerhard-Domagk-Strasse 1, W-5300 Bonn (FRG)
[**I This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Cbemischen 1ndustrie.-Partly based on a lecture delivered
at the International Conference on Phosphorus Chemistry, Tallinn, USSR,
September, 1989: E. Niecke, 0. Altmeyer, D. Barion, R. Detsch, C.
Gartner, J. Hein, M. Nieger, F. Reichert, Phosphorus Sulfur Silicon Kelat.
Hem. 49/50 (1990) 321.
S 3.50 f .25/0
Angew. Chem. Int. Ed. Engl. 30 (1991) No. 9
In contrast, very little has been reported about (E/Z)-isomerizations of the homologous diphosphenes (E = P);r31an
isomeric pair, from which the two forms could be isolated is
thus far unknown.[41We report here the synthesis and structure of such an isomeric pair, the isomeric interconversion,
and a secondary reaction of the (2)-isomer.
The hydrazinodihalogenophosphanes,1 a and 1 b, which
are accessible from phosphorus(m) chloride or bromide respectively and lithium tris(trimethylsilyl)hydrazide, react
with 2,4,6-tri-tert-butylphenyl(trimethylsilyl)phosphide to
give the thermostable diphosphanes 2 a and 2 b, respectively.
Above - l O T , these eliminate halosilane and form the P = P
double-bond systems 3 a and 3 b, respectively (Scheme 1).
Remarkable about this reaction is that, in the case of the
chloro derivative 2a, the (E)-isomer 3 a is formed selectively
from the educts precipitating only in one diasteromeric form,
whereas in the case of the bromo derivative 2 b, the Z-isomer
3 b is formed selectively.
- LiX
their intensities should belong to the symmetry-forbidden
n+-n*-transition (&,,,[nm] = 318 (3a), 319(3b)) andn2-n*transition of the NPP bond system (~,,,[nm] = 355 (3a), 365
(3 b)), respectively.
The diphosphenes, which are storable almost indefinitely
in the solid state, equilibrate in solution with formation of
the isomeric form, whereby the establishment of equilibrium,
with 3a:3b = 11 :6 (293 K), indicates a similar thermodynamic stability for both isomers (AAG 1 kcalmol-').
Spectroscopic analysis of the reversible transformation
3 a e 3 b (ko,,,,,, = 5.5(5) x IO-'s-';
k,,,,,,,, = 6.7(5) x
lo-' s-') affords the free activation enthalpies
AG&,,,,,, = 25.5(5) and AGG,,,,,, = 25.4(5) kcalmol-I.
These are significantly lower than the calculated rotational
barrier for the parent compound HP=PH (34.0 kcal
mol- [51), which possibly can be attributed to the stabilization of the radical transition state.16]The free activation enthalpy for the (Z/E)-isomerization, however, is markedly
higher than that for the diphosphene aryl-P=P-aryl
(AGG,3)z,E = 20.3 kcalmol-I,
aryl = 2,4,6-tB~,C,H,~~~),
whose (Z)-form could therefore not be isolated.
Figure 1 shows the structure of the diphosphenes 3a, b in
the crystal.['] As in the already previously characterized aryl-
la, b
€i!' e)A
, ryl
CI) R(X)P-l':ir,)Aryl
(X = Br)
A - SIMe,X
- StMe,X
Scheme 1. l a , 2a: X = CI; l b , 2b: X
aryl = 2,4,6-fBu3C,H,.
Br; R
If it is assumed that the thermally-induced cleavage of
halosilane from the diphosphanes 2a, b takes place via a
syn-elimination and the rotameric equilibrium is controlled
by steric effects, then the elimination must proceed from
different diasteromeric pairs of enantiomers. On the basis of
the very similar 31P-NMRdata of the educts (6 = 181, - 41,
Jp,p= 293 Hz (2a); 182, - 42, Jp,p= 275 Hz (2b)) the (E)isomer 3 a must be formed from the S,R-(R,S)-form with
trans arrangement of the aryl and hydrazino substituents
(gl-rotamer), whereas the (2)-isomer 3b must be formed
from the S,S-(R,R)-form with trans arrangement of the aryl
and bromine substituents (g2-rotamer).
In both isomers 3a, b the phosphorus atom in the p-position to the hydrazino substituents is strongly shielded :
(6(31P) = 311(PC) vs 481(PN) (3a); 190(PC) vs 358(PN)
(3b)), which is consistent with the structure in the crystal (see
below), and which can be interpreted in terms of the formation of a 4e-3c-NPP n-system. The relative shift differences
between the ( E ) - and (Z)-form, A6(31P),are of comparable
magnitude for correspondingly substituted phosphorus
atoms (AS = 123(PN), 121(PC)) and compare with those of
the (E/Z) isomeric pair aryl-P=P-aryl (A6 = 124, Aryl =
The UV-VIS spectrum shows two absorption maxima above 300 nm, outside of the range of the
n-n * transitions of the aryl substituent, which on the basis of
Angew. Chem. I n f . Ed. Engl. 30 (1991) No. 9
Fig. 1. Structure of the (Qdiphosphene 3a in the crystal (top) and structure of
the (Z)-diphosphene 3 b in the crystal (bottom). Important bond lengths [pm]
and angles ["I of 3a and 3 b (in square brackets): P-P 203.7(2) [202.7(3)], P-N
170.0(3) [168.6(7)], P-C 186.4(4) [187.8(9)]. N-N 147.4(4) [150.6(8)]; P-P-N
106.1(1) [126.3(3)], P-P-C 97.6(1) [121.4(3)], P-N-N 112.5(2) [107.5(5)].P-N-Si
129.3(2) [136.4(4)], N-N-Si 118.1(2) [114.4(5)]; N-P-P-C 179.4(2) [1.5(6)].
substituted diphosphene~,[~]
the aryl substituent is almost
orthogonal to the central n-bond system (3a: 79; 3b: 89").
The planar coordinated N atoms of the hydrazino substituents and the P atoms of the double bond system, on the
other hand, lie in a plane, which enables formation of a 4e, 3c
n-system. This is accompanied by a larger valence angle at P1
compared to P2, both in the (E)-form (106 vs 98") as well as
in the (Z)-form (126 vs 121"). In the latter, the P-P distance
is also shorter (202.7(2) vs 203.7(3) pm in the (E)-form),
Verlugsgesellschafi mbH. W-6940 Weinheim, 1991
OS70-0833/91/0909-ll37$3.50+ .2S/0
which can be explained in terms of the considerably greater
angle at the phosphorus atoms and the associated greater
s-part in the phosphorus bonds. This interpretation would
also be consistent with the larger P-P coupling constants for
the (Z)-isomer (554 compared to 516 Hz for the @)-form).
A comparison of the structural data of 3 a with those of the
correspondingly substituted iminophosphane 4[’01 ((E/Z)(E)-(Me,Si),N(Me,Si)N
- P = N - 2,4,6-tBu3C,H,
isomers in the iminophosphane system have thus far not
been observed because of the energetically favored nitrogen
inversion[”’) shows that the stereoelectronic influence of the
silylated hydrazino substituents is comparable in both bond
Similarly to the well-known isolable E-diphosphenes,[”’
3b also slowly transforms in solution at room temperature[l3linto the two symmetrically substituted diphosphenes
6 and 7,which can be detected in the reaction solution after
a few days.[141It is assumed that their formation proceeds via
the kinetically highly labile cyclotetraphosphane 5, a head/
head dimer of 3b as intermediate.[”]
1 a [l bj: A solution of PCI, [PBr,] (22 mmol) in ether (50 mL) was treated at
- 40°C with an equimolar amount of Li[N,(SiMe,),] dissolved in 100 mL of
ether. The reaction mixture was warmed to room temperature, stirred for 12 h,
and then the solvent was removed by evaporation. The residue was taken up in
n-pentane, LiCl[LiBr]removed by filtration, and the crude product obtained on
evaporation of the filtrate to dryness was recrystallized from a little n-pentane.
Yield 94%, m.p. 115-116°C [90%, 85-SST]. ”P NMR (C,D,): 6 = 165
[168]; ‘H NMR (C,D,): 6 = 0.23 [0.30] (s, NSiMe,) 0.40 [0.37] (s, N(SiMe,),).
3a[3b]:Asolutionofla[lb](5.0 mmol)inether(20mL) wascooledto -78°C
and treated dropwise with an equimolar amount of LiP(SiMe,)Mes* (Mes * =
2,4,6-fBu3C,H,). The mixture, containing the NMR spectroscopically characterized phosphane 2a [2b] was warmed to room temperature and stirred for 2 h.
Halosilane cleavage and a deepening in color accompanied formation of the
diphosphene 3 a [3b]. After removal of the volatile components, the residue was
taken up in n-pentane and filtered to remove LiCI[LiBr]. The crude product
obtained on evaporation was recrystallized from a little n-pentane. Yield: 2.1 g
(79%), m.p. 158-160°C [l.OOg (40%), 110-112”C]. “P{’H}NMR (C,D,):
6 = 481, 311 (J(P,P) = 554 Hz) [358, 190 (J(P,P) = 516 Hz)]; ‘H NMR
(C,D,):6 =7.43[7.4](t,J(H,P) = 1.24Hz,[<0.2 Hz],2H(aryl), 1.55[1.74](d,
.J < 0.2 HZ [1.4 Hz], 18H, 0-IBu), 1.36 [1.30] (s, 9H, p - t B ~ )0.37
[0.25] (t. 9 H ,
J(H,P) = 0.55 Hz [0.4 Hz], PNSiMe,), 0.29 [0.06] (d, 18H, J(H,P) = 0.2 Hz
[<0.2 Hz], ”@Me,),).-MS
(70eV). 3 a : m / z 554 (Me, 13%), 497
( M e - fBu, 2), 309 (Me - Mes*, 14). 278 ((SiMe,),N,P@, 48). 276 (Mes*Pe,
5), 73 (SiMey, 100); 3b: m j z : 554 (Me, 15%), 497 (Ma - tBu, 4). 309
(Me - Mes,* 25), 276 (Mes* Pe, 3), 263 ((SiMe,),N,Pa - Me, 51), 245
(Mes,* 2.5), 205 ((SiMe,),NNP@, 41), 160 ((SiMe,),Na, l), 73 (SiMey, loo),
57 (fBu@,81).
Received: April 11, 1991 [Z4565 IE]
German version: Angew. Chem. 103 (1991) 1158
CAS Registry numbers:
l a , 111437-98-0: l b , 135193-25-8; 2a, 135193-22-5; Zb, 135193-27-0; 3a,
135193-23-6;3b, 135193-28-1;4, 135193-24-7;6,83466-54-0; 7,135193-26-9;
Li[N,(SiMe,),], 22846-03-3; LiP(SiMe,)(2,4,6-tBuC,H2), 91443-42-4.
[I] G . S. Hartley, Nature (London) 140 (1937) 281 ; E. R. Talaty, J. C . Fargo,
Chem. Commun. 1967, 65.
[2] D. P. Fisher, V. Piermattie, J. C. Dabrowiak, J. Am. Chem. SOC.99 (1977)
284; A. W. Adamson, A. Vogler, H. Kunkely, R. Wachter, ibid. 100 (1978)
[3] A.-M. Caminade, M. Verrier, C. Ades, N. Palilous, M. Koenig, J. Chem.
Soc. Chem. Commun. 1984,875; M. Yoshifuji, T. Sato, N. Inamoto, Chem.
Left. 1988. 1735.
1 1 38
Q VCH Verlagsgesellschafi mbH. W-6940 Weinheim, 1991
[4] Stable
isomers of metal complexed diphosphenes have already
been reported: M. Yoshifuji, T. Hashida, N. Inamoto, K. Hirotsu, T.
Horiuchi, T. Higuchi, K. Ito, S. Nagase, Angew. Chem. 97 (1985) 230;
Angew. Chem. Int. Ed. Engl. 24 (1985) 211.
[5] “Bonding Properties of Low Coordinated Phosphorus Compounds”,
W. W. Schoeller in M . Regitz, 0.J. Scherer (Ed.): Mulliple Bonds and LON’
Coordination in Phosphorus Chemistry, Thieme, Stuttgart 1990, p. 5.
[6] In contrast to diazenes, the (Z/E)-isomerization in diphosphenes proceeds
via a rotation of the double bond.
[7j X-ray structure analysis of 3a (3bj (C,,H,,N,P,Si, [C2,H5,N2P2Si3j)yellow crystals, crystal dimensions 0.4 x 0.5 x 0.6 [0.2 x 0.8 x 0.81 mm; M, =
554.9 [554.9]; space group P 2 J n [P2,/n] (No. 14), a = 990.9(2) [934.5(3)],
b = 3551.9(9) [1592.8(8)], c = 1014.3(2) [2456.7(8)] pm, fl = 97.52(2)”
[100.16(3)”], V = 3.539 13.6001nm3, 2 = 4 [4], ecaIcd= 1.04 [1.02] gem-,,
~(Mo,,) = 0.24 [0.23] mm-’ ; 6210 [5021] symmetry independent reflections (28,,, = 50” [SWj),3813 [2778] reflections with IF( > 40(F) used for
the structure solution (direct methods) and refinement (307 [307] parameters), non-hydrogen atoms anisotropic, H atoms refined with a “riding”
gP,g =
model; R = 0.062 [0.100] (R,= 0.059 [0.088], c 1
0.0003 [O.O].The relatively poor R value for the compound 3 b is attribited
to the unfavorable crystal shape (platelets) and the poor diffraction by the
crystals investigated. Therefore, in the case of 3b, an empirical absorption
correction was carried out with the program DIFABS [S]. See [lob].
[S] N. Walker, D. Stuart, Acfa Crysfallogr.A 3 9 (1983) 158.
[S] “Diphosphenes” M. Yoshifuji in M. Regitz, 0. J. Scherer (Eds.): Muftipfe
Bonds and Low Coordination in Phosphorus Chemisfry,Thieme, Stuttgart
1990, p. 321.
[lo] a) Important bond lengths [pm] and angles [“I of4: P-N 167.2(4),155.9(4),
N-N 148.6(5), N-C 144.5(5); P-N-N 111.0(3), P-N-C 115.3(3), N-P-N
107.3(2); N-P-N-C - 177.7(3). X-ray structure analysis of 4:
C,,H,,N,PSi,, yellow crystals, crystal dimensions 0.25 x 0.5 x 0.5 mm;
M, = 538.0; space group P2Jn (No. 14), a = 991.3(3), b = 3462.0(19),
c = 1010.7(4)pm,
p = 99.33(3)”, V = 3.423 nm3, Z = 4, ecrlsd=
1.04 gem-,, p(MoKo)= 0.20 mm-’; 4472 symmetry independent reflections (28,,, = 45”), 2792 reflections with F > 4n(F) used for the structure
solution (direct methods) and refinement (307 parameters), non-hydrogen
atoms anisotropic, H atoms refined with “riding” model; R = 0.058
(R,= 0.055, w-’= ’ ( F ) + gF2, g = 0.0005). b) Further details of the
crystal investigation are available on request from the Fachinformationszentrum Karlsruhe, Gesellschaft fur wissenschaftlich-technische Infomation mbH, W-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the
depository number CSD-55424, the names of the authors, and the journal
[Ill E. Niecke, D. Gudat, Angew. Chem. 103(1991)251; Angew. Chem. fnr. Ed.
Engl. 30(1991) 217.
[I21 E. Niecke, B. Kramer, M. Nieger, Angew. Chem. 101 (1989) 217; Angew.
Chem. Int. Ed. Engl. 28 (1989) 215.
[I31 The decomposition is accelerated by the action of light.
[14] The compounds 6 and 7 were identified ,lP-NMR spectroscopically by
addition of authentic samples, aryl-P=P-aryl (6 = 492 [15]) and RNP=P-NR (6 = 325 [16]), respectively.
[I51 M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu, T. Higuchi, J. Am. Chem.
Soc. 104 (1982) 6161.
[I61 0. Altmeyer, Disserfafion,Universitat Bonn 1990.
~ ’ (+n
Methylating Reductive Dimerization of
Aromatic Carbonyl Compounds, a Novel
Organometallic Reaction **
By Thomas Kauffmann,* Jan Jordan, and Karl-Uwe Voss
The reductive dimerization of carbonyl compounds with
titanium“] or tungsten reagents[’. 31 according to Equation (a) is already a well-established reaction in organic synthesis. We have now found a related reaction, according to
[*] Prof. Dr. T. Kauffmann, Dr. J. Jordan, Dipl.-Chem. K.-U. Voss
Organisch-chemisches Institut der Universitat
Corrensstrasse 40, W-4400 Munster (FRG)
[**I Transition Metal Activated Organic Compounds, Part 35. This work was
supported by the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie. - Part 34. T. Kauffmann, A. Hulsdiinker, D.
Menges, H. Nienaber, L. Reithmeier, S . Robbe, D. Scherler, J. Schrickel,
D. Wingbermiihle, Tetrahedron L e f t . 31 (1990) 1553.
Angew. Chem. I n f . Ed. Engl. 30 (199f) No. 9
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structure, synthesis, isomerization, isomers, diphosphene
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