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Azo Cope Rearrangements of Nonstabilized Azo Compounds.

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117 (1984) 517: d) S. Hunig. F. Prokschy, ihid. 117 (1984) 534: e) K.
Beck, S. Hunig, ibid. I20 (1987) 477.
[6] See K. Beck, Dissertation. Universitat Wiirzburg 1986.
(71 Species 4 - H a should be described as endo-4-H@,which is completely
transformed within 28 d to ero-4-H@.In contrast to endo-4, exo-4 does
not rearrange to 3 under the same conditions.
[8] 3: 'H-NMR (400 MHz. CDCI,) cf. 14~1."C-NMR (100 MHz, CDCI,):
6 = 18.28, 19.46 (each q, CH,), 31.29 (1, C-7). 38.24 (d, C-7a), 49.92 (d,
C-4a). 51.55 (s. C-S), 85.92. 87.13 (each d, C-l,4), 127.8, 132.2 (each d,
C-5,6).-4: 'H-NMR (400 MHz, CDCI,): 6=0.85 ( s , 3 H, CH,), 0.99 (s,
3H.CH4, 1.41, 1.60(eachd,9-H,,J,,.=8.5 Hz),2.80(bs, IH,4-H),3.39
(d, I H, 3a-H), J L . ~ =1.5 Hz), 4.34 (bs, I H, 7-H), 5.69 (m. I H. 6-H), 6.02
(m, I H, 5-H), 6.29 (5, I H, 2-H). "C-NMR (100 MHz, CDCI,): 6=20.37
(q, CHI). 29.12 (9, CHI), 44.88 (s, C-3). 45.76 (d, C-4), 49.92 (t, C-9).
66.44 (d, C-3a), 75.72 (d, C-7). 131.5, 133.8 (each d, C-5,6), 155.9 (d, C2).
191 Differential thermal analysis gave AH=4.5 kcal/mol (72.1-C) for 3-4
(Dr. H . D . Beckhaus, Universitat Freiburg). Enthalpies of formation determined from force-field calculations (SCRIPT: N. C. Cohen, Terrahedron 37 (1981) 171 I): 3, 58.1 kcal/mol; 4, 62.1 kcal/mol (Dr. H. Burghard, Pharmasynthese, Hoechst AC). We thank the authors for these
[lo] Reviews on cycloaddition with iminium salts: a) M. Lora-Tamayo, R.
Madronero in J. Hamer (Ed.): 1.4-Cycloaddition Reactions. Academic
Press, New York 1967, p. 129f; b) S. M. Weinreb, J. 1. Levin, Heterocycles I2 (1979) 949; c) S. M. Weinreb, R. R. Staib, Tetrahedron 38 (1982)
3087. See also: d) B. Schmied, Dissertation: Universitat Regensburg
1980; e) S. D. Larsen, P. A. Grieco, J . Am. Chem. Soc. 107 (1985)
[I I] The deuterated cycloadducts [D&3 and [D,?I-4 were synthesized from 1
and IDJ-2 according to the same method used for the undeuterated
[I21 The rearrangement was followed by observing the NMR signals for I-H
and 4-H (3) and 3a-H (4).
[I31 See the detailed discussion in M. S . Dewar, S. Olivella, J . J. P. Stewart, J .
Am. Chem. SOC.108 (1986) 5771.
[I41 F.-G. Klarner, B. M. J. Dogan, 0. Ermer, W. von E. Doering, M. P.
Cohen, Angew. Chem. 98 (1986) 109; Angew. Chem. Inf. Ed. Engl. 25
(1986) 108.
1151 R. Huisgen in A. Padwa (Ed.): 1.3-Dipolar Cycloaddition Chemrstry. Wiley-Interscience, New York 1984, p. 48.
[I61 R. Sustmann, Pure Appl. Chem. 40 (1974) 569. See also K. N. Houk in A.
P. Marchand, R. E. Lehr (Eds.): Pericyclic Reactions. Academic Press,
New York 1977, p. 181f.
[I71 The species 5 - H e is an intramolecular charge-transfer complex. After
transfer of an electron, it could also be present as a diradical monocation.
[IS] K. Beck, H. Burghard, G . Fischer, S. Hunig, P. Reinold, Angew. CAem.
99 (1987) 695; Angew. Chem. Int. Ed. Engl. 26 (1987) 672.
Azo Cope Rearrangements of Nonstabilized
Azo Compounds**
rangement, resulting in the formation of the expected hydrazone 5.l3]
We now report the first azo Cope rearrangement of the
5 despite the absence of an aryl-group-statype 4 ~ that,
bilized azo group, proceeds without loss of nitrogen and,
depending on the pattern of substitution, affords either the
azo or the hydrazone isomer (partially reversible). In this
reaction, the known charge acceleration of the rearrangement['] is exploited by protonation of the N atom. The
starting point of this reaction sequence is the cycloaddition
of the isopyrazole 6a to the cyclopentadiene 7d, which,
depending on the acid concentration, leads to a mobile
equilibrium between the azo compound Sad and the hydrazone 9ad (as 9ad-H@).It was established that the transformation 8ad-Ha+9ad-H@ proceeds via an intramolecular [3.3] Cope-type rearrangement and not via the appreciably slower retro Diels-Alder reaction.1J1A series of further cycloadducts 8 and 9 that were also capable of undergoing isomerization were correspondingly obtained according to Scheme
The structural dependence of the
rearrangement was determined for five examples (Scheme
6 1 R l X
7 1 Y
[*] Prof. Dr. S. Hunig, Dr. K. Beck, DipLChem. P. Reinold
lnstitut fur Organische Chemie der Universitat
Am Hubland, D-8700 Wiirzburg (FRG)
Dr. H. Burghard [+I, DipLIng. G . Fischer ['I
Hoechst AC, Pharmasynthese
Postfach 800320, D-6230 Frankfurt am Main 80 (FRG)
[+] Force-field calculations
[**I This work was supported by the Fonds der Chemischen Industrie and
BASF AG, Ludwigshafen.
0 VCH Verlagsgesellscliaji mbH. 0-6940 Weinheim. 1987
The numerous hetero-l$dienes for which Cope rearrangements are known include 3,4-diaza- and 2,5-diaza
derivatives. I n the first case, the N-N bond is broken during the rearrangement; in the second case, the azomethine
function is merely shifted."] To the best of our knowledge,
only one 2,3-diaza Cope rearrangement is known, namely,
1 2. This rearrangement, which is expected to form an
azo compound, leads to loss of nitrogen and the formation
of I-butene 3.l2!Recently, the system 4, which is stabilized
by an aryl group, was shown to undergo an azo Cope rear-
By Karin Beck, Harald Burghard, Gabriele Fischer,
Siegjiried Hiinig, * and Petra Reinold
Scheme 1
As shown in Scheme 2, the reversibility of the rearrangement SadT't9ad is completely maintained upon introduction of a methyl group (Sbdzsbd), although the reaction
rate is markedly decreased. The cycloadducts obtained
from isopyrazole 6a and 1,3-cyclohexadiene 7e or isoprene 7f, namely, 8ae or Saf, respectively, undergo only
irreversible isomerization to the dihydropyrazoles 9ae and
9af. In the case of the cycloadducts obtained from
dihydropyridazine 6c and cyclopentadiene 7d, only
rearrangement of 9cd to the azo compound 8cd occurs!
Calculations performed using the SCRIPT force-field
program''' revealed that, for the systems U1 and U4, the
azo compounds are 3.0 kcal/mol and 6.8 kcal/mol, respectively, more stable, whereas, for U2 and U3, the hydrazones are 2.4 kcal/mol and 21.4 kcal/mol, respectively,
lower in energy (Scheme 2). Indeed, treatment with 0.1
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Angew. Chem. Int. Ed. Engl. 26 (1987) No. 7
I Hs
cl min
5 m4n
0 1 TFA
za mtn
.)calculated withoui
methyl groups
lg&=2.1; 9: A,,,,,=245 nrn, I g ~ z 3 . 7 .'H-NMR: 8 : 6=4.8-5.2 (bridgehead protons); 9 : 6=6.6-6.8 (azomethine protons).
i7] SCRIPT: N. C. Cohen, Tetrahedron 37 (1981) 171 I . For the scope of application of force-field calculations as well as the effect of methyl groups,
see E. Osawa, H. Musso, Angew. Chem. 95 (1983) I ; Angew. Chem. Int.
Ed. Engl. 22 (1983) I.
5TFA 35mrn
32d 84"C 10%
Redox Disproportionation of Ge" Compounds:
Synthesis and Structure of I(Me3Si)C(PMez)212GeCI,
and Ip-{(Me3Si)C(PMe2)2]12Cez**
o ~,TFA
-21 4
Scheme 2. Acid-catalyzed diaza Cope rearrangements in CDCI,, at room
temperature if not stated otherwise (the hydrazones 9 are protonated if an
excess of acid is present).
equivalent of TFA results in the smooth isomerization of
9ad, 9bd, and 9cd into the isomeric azo compounds; however, despite the larger difference in energy, 9cd rearranges
significantly more slowly. The influence of structure on the
energy barrier for the isomerization is so large in the case
of U2 and U3 that the calculated differences in energy can
no longer be confirmed. The rearrangements carried out in
the presence of a large excess of acid proceed in the appropriate direction but are not conclusive owing to the appreciably stronger basicity of the hydrazone form. With 0.1
equivalent of TFA, the conversion of 8ae to 9ae asymptotically approaches the limiting value of 10% after 32 d at
84"C, because 9ae-H@is such a poor protonating agent.
An isomerization of 8af to 9af with 0.1 equivalent of TFA
cannot be detected even after 12 d at 95°C. The reaction of
6a with isoprene 7f establishes that the regioselective rearrangement driven by an excess of TFA proceeds
intramolecularly. In three competing reactions (monitored
by CC), 8af, 9af, and the isomer 9'af are simultaneously
formed. Therefore, 9af, which is formed exclusively ac3TFA. CHCl3
By Hans H . Karsch,* Brigitte Deubelly, Jurgen Riede.
and Gerhard Muller
Low-valent, monomeric germanium compounds are
known; to the best of our knowledge, however, there are
no reports concerning spontaneous redox reactions of germ a n i u m ( ~ compounds
under normal conditions. We have
already shown that the stabilization of phosphane complexes of main-group elements can be extended to lowvalent elements (e.g., Ge", Sn", Pb")[ll if anionic phosphane ligands, such as phosphinomethanides 1, are employed.
[R2P-CXY]0 1, X, Y = H , PMe2, SiMe?; R=alkyl, aryl
We show here, by way of germanium as example, that
such ligands can also be used to initiate redox reactions of
low-valent element-phosphane complexes, leading to the
formation of novel products having element-element
SnC12 smoothly reacts with Li[(Me2P)2CX] ( l a ,
X = PMe2; lb , X = SiMel) to give Sn"-phosphane complexes having pseudo trigonal-bipyramidal configurations.12]An analogous reaction can also be carried out with
GeCI2.dioxane [Eq. (a)]; the germanium complexes 2 , corGeC12.dioxane
+ 2 Li[(PMe2)2CX]
Ge[(PMe2)2CX]2 (a)
la, b
Za, b
responding to the tin complexes, are formed. The colorless
crystals131of 2a,b are stable indefinitely at room temperature. However, when the phosphinomethanide l b is used
in less than the required stoichiometric amount, a redox
process occurs, which presumably proceeds via the intermediate [(Me3Si)C(PMe2)2]GeC1. For optimized stoichiometry, the reaction can be formulated as Equation (b).["'
9'ai ( 4 9 % )
cording to U3, is not formed by cycloreversion, since 9'af
is absent. Accordingly, all isomerizations presumably involve a one- o r two-step, intramolecular [3.3] rearrangement.141
Received: March 2, 1987 [ Z 2119 IE]
German version: Angew. Chem. 99 (1987) 695
[I] Review: R. P. Lutz, Chem. Rev. 84 (1984) 206; H. Heimgartner, H.-J.
Hansen, H. Schmid in H. Bohme, H. Viehe (Eds.): fminium Salrs in Organic Chemistry. Part 11. Wiley, New York 1979, p. 655.
[2] R. V. Stevens, E. E. McEntire, W. E. Barnett, E. Wenkert, J . Chem. Soc.
Chem. Commun. 1973. 662.
[3] T. Mitsuhashi, J . Am. Chem. Sor. 108 (1986) 2400.
[4] K. Beck, S. Hiinig, Angew. Chem. 99 (1987) 694; Angew. Chem. Int. Ed.
Engl 26 (1987) 670.
[5] Cf K. Beck, Dissertation and P. Reinold, Diplomarbeit. Universitat WURburg 1986.
161 All new compounds were characterized by elemental analyses, UV spectra (hexane), 'H-and "C-NMR spectra (400 and 90 MHz, respectively,
CDCI,), and I R and mass spectra. Typical signals: UV: 8 : Am.,, =380 nm,
Angew Chem. I n t . Ed. Engl. 26 (1987) No. 7
3 Ge"CI:. dioxane
[(Me,Si)C(PMe2)2]zGe:4 b
The oxidation product 3b can also be synthesized from
GeCI, and l b [Eq. (c)], which confirms its formulation as a
Ge'" complex.[51 Complex 3b is the first example of a
trans-octahedral structure (Fig. 1)L6'having 4 P and 2 CI
donors at a germanium center.['I
[*] Priv.-Doz. Dr. H. H. Karsch, DipLChem. B. Deubelly, J . Riede,
Dr. G. Miiller
Anorganisch-chemisches lnstitut der Technischen Universitat Munchen
Lichtenbergstrasse 4, D-8046 Garching (FRG)
I**] Complexes with Phosphinomethane and methanide Ligands, Part 12.Part 1 1 : H. H. Karsch, A. Appelt, B. Deubelly, G. Miiller, J . Chem. Soc.
Chem. Commun.. in press.
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rearrangements, azo, nonstabilized, compounds, cope
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