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Generation of the Tetraphenylcyclobutadiene Radical Cation by Ionizing Gas-Phase Decarbonylation.

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pentane solution at 2 "C to [2-I3C]-n-butane without giving
isobutane. Under the reaction conditions, however, the
protonated methylcyclopropane intermediate (or transition
state) is quenched by the hydride-donor n-pentane before
rearranging to an incipient primary cation leading to isobutane.
I
Received: August 13, 1981;
revised: October 21, 1982 [ Z 409 1E]
German version: 4ngew. Chem. 95 (1983) 51
3
The cation 1 (60% I3C enriched at the cationic center)
was prepared from the correspondingly labeled tert-butyl
chloride in FSO3H/SbF5/SOZC1F solution at -78 "C. A
sample was sealed in a 10 mm NMR tube and heated to
70°C in an oil bath. The I3C-NMR spectrum of the ion
was recorded intermittently at room temperature (Fig. 1).
13
C-scrambling is negligible at room temperature, but is essentially complete in 20 h at 70 "C. Even at 100 "C the rate
of scrambling is too slow to be observed by NMR linebroadening, and hence it was not possible to determine
any activation parameters for the process. A lower limit of
E A= 30 k~al/mol[~"]
can be estimated for the scrambling
process, which should correspond to the energy difference
between the tert-butyl and isobutyl cations. A similar lower
limit of 28 kcal/mol was estimated by Saunders and HagenlZblfrom the absence of hydrogen scrambling in the hexadeuterio-tert-butyl cation.
Fig. 1. 'H-decoupled I3C-NMR spectrum (20 MHz) of 1 in FS0,H/SbF5/
S02CIFat room temperature. A: immediately after preparation, B: after 2.5 h
at 70 "C, C: after 9.5 h at 70 "C.
[I] a) M. Saunders, P. Vogel, E. L. Hagen, J. Rosenfield, Acc. Chem. Res. 6
(1973) 53; b) M. Saunders, J. Chandrasekhar, P. von R. Schleyer in P. De
Mayo: Rearrangements in Ground and Excited States, Vol. I , Academic
Press, New York 1980, Chapter 1.
[2] a) M. Saunders, J. Rosenfield, J . Am. Chem. SOC.91 (1969) 7756; b) M.
Saunders, E. L. Hagen, J . Am. Chem. SOC.90 (1968) 2436.
[3] a) D. M. Brouwer, E. L. Mackor, Proc. Chem. SOC.1964. 147; b) D. M.
Brouwer, Red. Trav. Chim. Pays-Bas 87 (1968) 210.
[4] a) G. J. Karabatsos, F. M. Vane, J. Am. Chem. Soe. 85 (1963) 729; b) We
believe that the scrambling occurs through a monomolecular pathway
since no hydrogen-deuterium exchange was observed in deuterated superacids. No deprotonation of the tert-butyl cation to isobutylene leading
to a dimer or oligomer occurs under the reaction conditions.
[5] D. M. Brouwer, Trav. Chim. Pays-Bas Red. 8 7 (1968) 1435.
Generation of the
TetraphenylcyclobutadieneRadical Cation by
Ionizing Gas-Phase Decarbonylation* *
By Wolfgang Blum, Harry Kurreck, Wilhelm J. Richter,
Helmut Schwarz*, and Helga n i e s
Dedicated to Professor Hans- Werner Wanzlick on the
occasion of his 65th birthday
Oxidation of tetra-tert-butylcyclobutadiene and -tetrahedrane with A1Cl3/CHZCIZand t-C,HF, respectively, leads
to formation of radical cations""], whose ESR and collisional activation spectra are identical. MNDO analysis'lal
of the valence isomerization, tetrahedrane + cyclobutadiene indicates for the sterically non-hindered tetramethyl
system that the tetramethyltetrahedrane radical cation possesses no local minimum on the potential energy hypersurface; it spontaneously isomerizes to the tetramethylcyclobutadiene radical cation["]. From this it was concluded
that a flattening to the cyclobutadiene system also occurs
with the radical cation of the sterically more restricted tetra-tert-butyltetrahedrane.
We now report on an independent method of generating
the previously unknown tetraphenylcyclobutadiene radical
cation 2. In the decarbonylation of 2,3,4,5-tetraphenyl-2,4cyclopentadienone 1 ( M @ O=45% rel. int.) by electron impact ionization (70 ev>, a signal is, inter alia, observed at
m / z 356 (35%). The collisional activation mass spectrurnl2]
using MS/MS conditions'31 with ions of m / z 356, massselected by a triple quadruple instrument, only contains a
single signal at m / z 178, which can be assigned to the diphenylacetylene radical cation 4.
[*I Prof. Dr. H. Schwarz, H. T h i s
The most likely mechanism14b1for the scrambling process is rearrangement via the primary isobutyl cation 2 involving the delocalized protonated methylcyclopropane intermediate (or transition state) 3 to the sec-butyl cation 4.
The sec-butyl cation scrambles all hydrogens and carbons
and reverts back to the tert-butyl cation through the well
documented pathway!']. Furthermore, it was shown by
Brouwer[" that [l-'3C]-n-butane isomerized in HF/SbF,/
Angew. Chem. Int. Ed. Engl. 22 (1983) No. I
Institut fur Organische Chernie der Technischen Universitat
Strasse des 17. Juni 135, D-1000 Berlin 12 (Germany)
Prof. Dr. H. Kurreck
Institut fur Organische Chemie der Freien Universitat
Takustrasse 3, D-1000 Berlin 33 (Germany)
Dr. W. J. Richter, W. Blum
Ciba-Geigy AG, CH-4002 Basel (Switzerland)
[**I This work was supported by the Fonds der Chemischen Industrie and
the Deutsche Forschungsgemeinschaft. We thank Dr. W. Kieslich for
supplying a sample of l a .
0 Verlag Chemie GmbH, 6940 Weinheim, 1983
0570-0833/83/0101-005l $ 02.50/0
51
!i'
2a
Ph
3a
Scheme 1.
= I3C.
When the experiment was carried out using the [2,4I3Cz]labeled isotopomer l a the following results were obtained :
as expected, m / z 356 is quantitatively shifted to m / z 358,
and its MWMS spectrum contains only a single signal at
m / z 179 ( e4a) after isotopic correction[41. The following
conclusion can be drawn from this: Decarbonylation of 1
by electron impact ionization leads to an intermediate,
which must be a derivative of ionized cyclobutadiene 2.
Generation of a tetraphenyltetrahedrane radical cation 3,
whose existence was postulated previouslyf5I, is completely
inconsistent with the data obtained for l a . If the tetrahedrane intermediate 3a were really involved o r a carbon
scrambling of the C, moiety in the [M-CO]@' ion obtained from l a were to occur, then the distribution pattern
shown in Scheme 1 should be observed in the formation of
the ionized diphenylacetylene isotopomers (4, 4a, and
4b). This does not occur. The results are only compatible
with route a and certainly exclude route b and isomerization c.
example, exclusively 2a was obtained in 74-76% yield
(Table 2[,') from l a in acetonitrile or dimethylformamide
(DMF), whereas electrochemical reduction of l a in methanol gave dimethyl 3,4-diphenyladipate 3a as the sole productf3]in 75% yield.
RI HI
Aryl-C=C-COOC
R
1
C,H5-C H-CH,-C
I
Novel One-Step Synthesis of
Oxocyclopentanecarboxylates by
Electrochemical Reduction of Cinnamic Acid Esters
By Ikuzo Nishiguchi* and Tsuneaki Hirashima
The electrochemical reduction of a$-unsaturated esters
in aqueous solution usually gives the corresponding hydrodimers, i.e. adipic acid derivatives, as the main products['C.21.We have now found that oxocyclopentanecarboxylates 2 are formed in moderate to good yields by electrochemical reduction of cinnamate derivatives 1 . A remarkable solvent effect was observed in this dimerization. For
[*] Dr. 1. Nishiguchi, Dr. T. Hirashima
Osaka Municipal Technical Research Institute
1-6-50 Morinomiya, Jyoto-ku, Osaka 536 (Japan)
52
0 Verlag Chemie GrnbH, 6940 Weinheim, 1983
COOCH,
2
I
OOCH,
C6H5-C H-CH,-COOC
H,
3a
4
Table 2. Electrochemical reduction of l a - l h to Za-2h [a].
Received: September 21, 1982 [Z 153 IE]
German version: Angew. Chem. 95 (1983) 59
[I] a) H. Bock, B. Roth, G. Maier, Angew. Chem. 92 (1980) 213; Angew.
Chem. In/. Ed. Engl. 19 (1980) 209; b) R. Wolfschiitz, H. Schwarz, unpublished results; c) For the photo induced generation of the tetramethylcyciobutadiene radical cation see: Q. B. Broxterman, H. Hogeveen, D. M.
Kok, Tetrahedron Lett. 22 (1981) 173.
[Z] K. Levsen, H. Schwarz., Angew. Chern. 88 (1976) 589; Angew. Chem. In/.
Ed. Engl. I5 (1976) 509.
131 H. Schwarz, Nachr. Chern. Tech. Lab. 29 (1981) 687.
[4] A weak signal at m / z 180 (6%) can be attributed to incomplete ["C2]-labeling of l a . 38% of 1 is present as the ['3C,]-isotopomer, and the natural
["CI-abundance in the [M-COIQQ ion of this species gives rise to the
formation of m / z 180. A signal at m / z 178 is not observed in the MS/MS
spectrum of m / z 358.
IS] M. M. Bursey, T. A. Elwood, Org. Mass Specrrom. I (1968) 531.
- Wbo
+Ze
H,
Yield [Yo] [b]
M . p . ["C]
76
67
63
78
62
52
64
75
126-127
83-87
75-80
131-133
101-104
121-123
115-116
134-135
[a] Cell, which is fitted with a stirring device, consists of a ceramic diaphragm, Cu cathode, Pt anode, D M F as solvent, tetraethylammonium p-toluenesulfonate as supporting electrolyte, at 10- 15 "C, cathode potential
- 1.9 to -2.2 V us. SCE, 4.0 F/mol. [b] Isolated yield. [c] 2-Pyridyl.
Information on the reaction pathway was obtained by
electrochemically reducing l h and 3a (molar ratio 1 : 1)
under similar conditions as in Table 2. As a result, exclusively the dimeric cyclization product 2h was formed in
74% yield, and 3a was recovered quantitatively: 2a was
not detected. This result clearly shows that the reaction
proceeds via a dimeric anionic intermediate such as 4ls1,
which yields the cyclization product 2 or the simple hydrodimer 3 depending upon the proton-donating ability of
the solvent: this also excludes the possibility of Dieckmann condensation of 3 to 2 under the reaction conditions.
Received: March 9, 1981 [Z 406 IE]
revised: October 22, 1982
German version: Angew. Chern. 95 (1983) 61
The complete manuscript of this communication appears in:
Angew. Chem. Suppl. 1983. 70-74
[I] c) N. L. Weinberg: Technique of Electroorganic Synthesis, Part 11, Wiley,
New York 1974, pp. 87-136, and references cited therein.
[2] M. M. Baizer, J. M. Anderson, J . Electrochem. Soc. I l l (1964) 223.
0570-0833/83/0101-0052 .$ 02.50/0
Angew. Chem. I n / . Ed. Engl. 22 (1983) No. I
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generation, ionizing, gas, radical, cation, tetraphenylcyclobutadien, phase, decarbonylation
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