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Walk Rearrangement in the 1 2 3 4-Tetra-tert-butylhydroxycyclobutenylium Ion.

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Walk Rearrangement in the
1,2,3,4-Tetra-tert-butylhydroxycyclobutenyliumIon**
By Gunther Maier,* Rolf Emrich, and
Hans-Otto Kalinowski
The oxidative ring-opening of tetra-tert-butyltetrahedrane leads to stable and crystalline isolable homocyclopropenylium salts.['l The ion 1 thus obtained, the structure
of whose hexafluoroantimonate has been confirmed by a n
X-ray structure analysis, shows temperature-dependent
N M R spectra. In the 'H-NMR spectrum, only three singlets are observed at 6= 1.10 (9 H, tBu on C-4), 1.54 (27 H,
tBu on C-1/2/3) and 4.60 (OH) at room temperature. O n
cooling, the signal at 1.54 becomes broader and finally
splits at
I "C into two signals in the intensity ratio 2 : 1
(at - 10°C: 6 = 1.50 and 1.58). The other two singlets remain unchanged.
2c
2a
3.4
3.4
orbital-
+
A
allowed
forbidden
+
4
3
1
h
3
2
1A
IB
5
El
In the room-temperature I3C-NMR spectrum of 1 only
four signals are recorded, at 6=29.3 (CH3, tBu on C-4),
30.9 (CH3, tBu o n C-1/2/3), 41.2 (quaternary, tBu on C-4),
and 101.2 (C-4). All other absorptions (C-1/2/3 and quaternary C of tert-butyl groups attached thereto) are hidden
under these conditions because of poor signal-to-noise ratio as a result of exchange processes. Below +17"C, the
signal for the tert-butyl groups on C-1/2/3 separates into
two signals. In this temperature range. absorptions also
gradually emerge for C- 1/2/3 and the qrlaternary C-atoms
of the tert-butyl groups attached thereto. At -20°C the expected spectrum could finally be observed: 6=29.1 (CH3,
rBu on C-4), 30.6 (CH3, tBu on C-1/3), 31.3 (CH3, rBu o n
C-2), 34.9 (quaternary, tBu on C-2), 38.1 (quaternary, tBu
on C-1/3), 40.9 (quaternary, tBu o n C-4), 101.0 (C-4), 161.5
(C-l/3), 184.9 (C-2).
The reversible changes in the spectra demand a dynamic
process in which the olefinic ring positions in 1 are equilibrated, while C-4 and the substituents attached to it d o
not, however, experience any change. The simplest interpretation of this process is the occurrence of the walk rearrangement 1 A + 1B+ 1C. From the coalescence temperatures (determinable on the signals of the methyl groups in
the 'H- and "C-NMR spectra; a measurement of the coalescence spectrum for C-1/2/3 and the neighboring Catoms was not possible, owing to technical reasons and because of the instability of 1) and the magnitude of the
splitting in the low temperature spectra, an activation barrier of AG+ = 15 kcal/mol could be calculated for the observed rearrangement.
Such an isomerization is known in the case of methylsubstituted homocyclopropenylium salts.['] Still unclear,
but of particular interest,13] is the stereochemistry of this
reaction. According to theoryf4]the rearrangement of the
homocyclopropenylium ion 2 would violate the Woodward-Hoffmann rules. Of the two alternatives-3 and 4
represent the corresponding eclipsed and bisected transition states, respectively-not the orbital symmetry allowed
reaction via 3, but the route via 4 with inversion at the migrating C-atom is to be expected. Accordingly, the substituents A and B would always retain their orientation. This
prerequisite coincides with experiment, for the signals of
C-4 and of the substituents on C-4 in the coalescence spectra remain sharp over the whole temperature range. Their
chemical shifts likewise remain unaffected. The presence
of equal amounts of the endo-exo-isomeric ions and a very
rapid rearrangement into each other can therefore be ruled
out. There remain three possibilities: a) Because of the size
of the rert-butyi groups the "least-motion" process is favored. b) Ring inversion leads very rapidly to an endo,exo
equilibrium that lies totally o n the side of 1 (endo-tert-butyl group). c) An unusually strongly pronounced secondary
orbital interaction[41ensures that the Woodward-Hoffmann
rules no longer hold in this instance. We consider the latter
mentioned explanation not only to be the most attractive
but also the most likely.
The above example can be compared with the BersonWillcott rearrangement of norcaradienes, which, according
to investigations by Klarner et al.,[51likewise proceed contrary to the requirements of the rules of conservation of orbital symmetry with inversion at the migrating C-atom.
[*] Prof. Dr. G. Maier, Dip1.-Chem. R. Emrich, Dr. H.-0. Kalinowski
Received: December 28, 1984;
supplemented: January 31, 1985 [Z 1125 1E]
German version: Angew. Chem. 97 (1985) 427
lnstitut fur Organische Chemie der Universitat
Heinrich-Buff-Ring 58, D-6300 Giessen (FRG)
[**I
Small Rings, Part 56. This work was supported by the Deutsche Forschungsgerneinschaft and the Fonds der Chemischen Industrk-Part
55: G. Maier, M. Hoppe, K. Lanz, H. P. Reisenauer, Tetrahedron Lett.
25 (1984) 5645.
Anyen Chrrn Int. Ed. Engl. 24 (1985) No. 5
2
1c
[I] G. Maier, R. Emrich, K.-D. Malsch, K.-A. Schneider, M. Nixdorf, H. Irngartinger, Chem. Ber., in press.
0 VCH Verlugsgesellschuft mbH, 0-6940 Weinheim. 1985
0570-0833/85/0505-0429 S 02.50/0
429
(21 V. A. Koptyug, 1. A. Shleider, 1. S. Isaev. J . Org. Chem. USSR 7 (1971)
864: 1. A. Shleider, I. S. Isaev, V. A. Koptyug, 2nd. 8 (1972) 1357.
[3] a) T. S. Sorensen, A. Rauk in A. P. Marchand, R. E. Lehr: Pericvclic Reacfrons. Vol. 2, Academic Press, London 1977, p. 52-55; b) M. Saunders, J.
Chandrasekhar, P. von R. Schleyer in P. de Mayo: Rearrangements in
Ground and Excited States. Vol. I . Academic Press, London 1980, p. 4447; c) F.-G. Klarner, Top. Stereochem. I5 (1984) I.
[4] a) ab initio: W. A. Hehre, A. J. P. Devaquet, J. Am. Chem. Soc. 98 (1976)
4370; [bid. 96 (1974) 3644: b) MINDOIZ: K. Morio, S. Masamune, Chem.
Lett. 1974, 1251.
[S] F. G. Klarner, Angen. Chem. 86 (1974) 270; Angew. Chern. Int. Ed. Engl.
13 (1974) 268; F. G. Klarner, S. Yaslak, M. Wette, Chem. Ber. 112 (1979)
1168.
complexes 6[9h1with tetracarbonylchromium, -molybdenum, and -tungsten and with ruthenium(i1) fragments. Table 1 lists these data and some redox potentials; in Table 2,
the tris(a-diimine)ruthenium(lr) complexes of 2,2'-bipyridine and of the bidiazine series are compared.
MIt,
6b: MIt,
6c : MI,
6a:
d6-Metal Complexes of 4,4'-Bipyrimidine,
an Ambident Ligand with High x-Acceptor Ability**
By Sylvia Ernst and Wolfgang Kaim*
Owing to the importance of ligands of the 2,2'-bipyridine type 1 in inorganic and analytical chemistry""] and especially in the photochemistry of coordination compounds"'] there have been numerous attempts at purposefully modifying this system. Besides the substitution at the
heterocyclic ringsr2a1and the enlargement of the a-diimine
7c-system,12h1
the incorporation of further nitrogen centers
in 1 has recently also attracted interest ;I3-'] coordination
compounds of 3,3'-bipyridazine 2,I3l 2,2'-bipyrazine 4,141
and 2,2'-bipyrimidine 5I5lhave been described in connection
with the use of corresponding ruthenium(r1) complexes as
photocatalysts.["]
=
Cr(C0)4
=
Mo(CO)~
6e
= W(CO),
6 d : M 4 = [Ru( 1)$
@
Table 1. Long-wavelength absorption maxima A(MLCT) [nm] [a] and redox
potentials E [ V vs SCE] [b] of d6-metal complexes of 4,4'-bipyrimidine.
6a
6b
6c
6d Id1
630 (3.46)
580 (3.72)
608 (3.67)
522 (3.56)
503 (sh)
427 (3.76)
460 (2.89)
6f
0.64
[cl
[cl
I.45
-0.80
- 0.7 I
-0.69
-0.58
- 1.56
Icl
- 0.66
[cl
- 1.45
- 1.40
- 1.19
[a] Measurements in tetrahydrofuran (THF), except in the case of 6d
(acetonitrile). [b] Measurements in dimethylformamide (DMF)/O. I M
Bu4N@CI07,cyclovoltammetry at a glassy carbon electrode; potentials in
acetonitrile are ca. 0.1 V more negative. [c] Irreversible steps. [d] Bis(hexafluorophosphate).
1
2
Table 2. Long-wavelength absorption maxima L(MLCT) [nm] and redox potentials E [V vs. SCE] of tris(2,2'-bipyridine)- and tris(bidiazine)ruthenium(ii)
complexes [a].
3
4
Complex
5
We have now carried out Hiickel MO calculations for all
symmetrical bidiazines with a-diimine structure, including
the remaining isomer 4,4'-bipyrimidine 3.I6l Such calculations surprisingly revealed that the hitherto neglected'']
complex ligand 3 is the most n-electron deficient compound: F::&=
-0.583 (2), -0.460 (3), -0.515 (4), and
-0.518 ( 5 ) .
In accordance with the specific properties of the pyrazine system,['] 2,2'-bipyrazine 4 was considered to be a ligand with an especially low-lying n* level;L3,41
however, the
n-acceptor activity expected to operate towards d 6 metal
centers is compensated by the strongly reduced CF basicity
of this 1,4-diazine ligand.14'-d1
Since pyrimidines are stronger bases than pyrazines, we
prepared 3 according to the method of Effenberger,['"]
found the pK,, to be 1.50 (pK,, (4)=0.45[4'1), and were in
fact able to establish a distinct red shift of the metal-to-ligand charge transfer (MLCT) absorptions for the chelate
€2:
d(MLCT) (Igt)
[Ru(l),]" [3]
[Ru(2),lZo [3]
452 (4.15)
444 (4.06)
410 (4.08)
[Ru(3)Jz0
496 (4.28)
461 (sh)
[ R U ( ~ ) ?[4b]
] ~ ~443 (4.18)
[Ru(5),]" [4d] 454 (3.93)
418 (3.91)
€2:
E::Y"d
1.27
1.58
-1.34 - 1.53 - 1.78 -2.18
- 1.00 - 1.25 - 1.54 - 1.84
1.75
-0.57
-0.71
-0.90
1.95
1.69
-0.71
-0.91
-0.89
- 1.16 - 1.76
- 1.28 - 1.99
- 1.08
- 1.38
[b]
[a] Measurements on bis(hexafluorophosphates) in acetonitrile. [b] Irreversible reduction [4d].
Intermediates could be isolated during the formation of
complexes with the ambident, potentially bridging ( N ' ,
N") o r chelate-forming (N3, N3') ligand 3. Thus, the primary
coordination
of
photolytically
generated
THF.W(C0)' initially takes place at the peripheral nitrogen centers N ' and N'I9'I before a conversion into the
chelate complex occurs [Reaction (l)].
Consistent with the HMO calculations, both the ligand 3
as well as its complexes exhibit markedly more positive reduction potentials than corresponding coordination com-
[*] Priv.-Doz. Dr. W. Kaim, Dip1:Chem.
S. Ernst
lnstitut fur Anorganische Chemie der Universitat
Niederurseler Hang, D-6000 Frankfurt am Main 50 (FRG)
[**I
This work was supported by the Deutsche Forschungsgemeinschaft, the
Fonds der Chemischen Industrie, the Hermann-Willkomm-Stiftung, the
Flughafen Frankfurt/Main AG, BASF AG, Degussa AG, and by the
Karl-Winnacker-Stiftung of Hoechst AG.
430
0 VCH Verlagsgesellschaf, mhH, 0-6940 Weinheim, 1985
N-
A
r uc
6f
OS70-0833/8S/OS05-0430$ 02..50/0
Angew. Chem. In[. Ed. Engl. 24 i198S) No. S
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ion, rearrangements, tetra, tert, butylhydroxycyclobutenylium, walk
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