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An Onium Anion.

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tetrafluoroborate in dichloromethane at - 74 "C. The tetrafluoroborate of cation (3), which was obtained in 47% yield
as greenish yellow needles, decomposed at 204-207 "C (acetonitrile or ethyl acetate)[31,but is very stable under ambient
conditions. Assignment of structure is supported by its 'Hand I3C-NMR spectra shown in Table I.
Due to practical difficulties in the separation, a 5.2:3.2 mixture of (4) and the
was used for the preparation of (6). See
ref. [4].
E Vogel. R. Schubart. W.A. Boll. Angew. Chem. 76. 535 (1964); Angew.
Chem. Int Ed. Engl 3, 510(1964); E. Vogel, H. Gunther. Angew Chem. 79,
429 (1967); Angew. Chem Int. Ed. Engl. 6. 385 (1967); E. Vogel. W A. Boll.
H . Gunther, Tetrahedron Lett. 1965, 609.
K. B. Sharpless, M. A. Umbreit, M. T Nieh, T. C. Flood, J. Am Chem. SOC
94. 6538 (1972).
M . Hatano, A. Tajiri, unpublished results. We wish to thank Professor Hutuno and Dr. Tajiri, Chemical Research Institute of Nan-aqueous Solutions,
Tohoku University. for communicating thelr results to us prior to puhlication.
P. Bischof, J. A. Hashmull. E. Heilbronner, V. Hornung, Helv. Chim. Acta 52.
1745 (1969). E. Haselbach, E Heilbronner, G. Schroder, ibid. 54, 153 (1971).
An Onium Anion
By Douglas Lloyd, Raymond K. Mackie, Glynis Richardson,
and Donald R. Marshall"'
Fig. 1. UV/VIS spectrum of (3) in C H K N (--)
and in CH2C12( ..... ).
The UV/VIS spectrum of (3) (Fig. 1) exhibits a broad and
intense CT absorption in the 300 to 450 nm region which is
solvent dependent, and shows a bathochromic shift in the
less polar dichloromethane. The broad shape of this CT absorption suggests that the band consists of two or more electronic transitions. This is confirmed by the curve resolution
(331 and 387 nm) and by the MCD spectrum of (3) which
shows a positive peak with its maximum at 333 nm and a negative trough at 387 nm in the CT region[*].Since the interaction between the two non-conjugated ethylene n-orbitals
in (3) gives rise to two occupied n-orbitals, the two observed
CT bands at 387 and 333 nm are assigned to the electronic
transitions from the HOMO and the next HOMO of the
two interacting ethylene n-orbitals, to the LUMO of the
tropylium n-orbital, respectively. In fact, the energy
difference of these two CT transitions (0.52 eV) is roughly
equal to the orbital energy gap of bicycloI2.2.2]octadiene,
&(b2(n))-&(al(n))=0.58 eV, measured by PE spectroscopy'9'.
The thermodynamic stability of (3) was evaluated by
means of its pKR+ value, which is found to be 7.0t0.2 in
20% aqueous acetonitrile. It is evident that the stability of the
tropylium ion is markedly diminished in (3) compared with
(2) (PKR + = 8.4,)['] and its dihydro analog (pKR = 8.82)'".
This trend observed on successive introduction of the double
bond in the ethano-bridge system can be attributed to the decrease in the electron-donating inductive effect of the
bridged alkyl groupf2'.
When 6-bromodihydrodiazepinium salts (1) react with alkoxides they may undergo either nucleophilic substitution to
give 6-alkoxy-derivatives (route A) (7), or protodebromination to give the products (8) (route B)['l. It has been suggested that both of these reactions involve initial formation
of the dihydrodiazepine base (2), which can tautomerize to
provide a bisimine species (3) very susceptible to nucleophilic attack[']. The type of product which results seems to be
determined largely by steric factors['].
Received: July 28. 1980 [ Z 684 IE]
German version: Angew. Chem. 93, 195 (1981)
CAS Registry numbers:
(31, 76466-39-2; (4). 56598.81 -3; (5). 291 -70-3; (61, 76466-75-6; (7). 76466-76-7;
(8). 76466-77-8; (9). 76466-78-9
The tautomerization of (2) to (3)involves the loss of a considerable amount of resonance energyfib1,
but there is some
energetic compensation in the loss of vicinal crowding between the 5-, 6- and 7-substituents~l'~.
Evidence in favor of this mechanism is provided by the
following facts: (a) the bromine atom of ( l a ) is immune to attack by alkoxide ion (save for conversion into the corre-
[ I ] a) 7: Nukaeuwa, I. Murufa, J. Am. Chem. SOC.99,1996 (1977): b) T Nakaza-
wa, N. Abe, K. Kubo. I . Murata, Tetrahedron Lett. 1979. 4995.
121 T. Nakazawa, Y. Niimoto. K. Kubo, I. Muram, Angew. Chem. 92.566 (1980);
Angew. Chem. Int. Ed. Engl. 19, 545 (1980).
[3] Satisfactory elemental analyses and spectroscopic data were obtained for all
new compounds reported.
I41 T. Nakazuwa, I . Murata, Angew. Chem. 87. 742 (1975); Angew. Chem. Int.
Ed Engl. 14, 711 (1975).
0 Verlag Chemie, GmbH,6940 Weinheim, 1981
Dr. D. Lloyd ['I, Dr. R. K. Mackie. Dr. G. Richardson
Department of Chemistry, Purdie Building
University of St. Andrews
St. Andrews. Fife KY16 9ST (Scotland)
Dr. D. R. Marshall
School of Physical and Molecular Sciences
University College of North Wales
Bangor. Cwynedd LL57 2UW (Wales)
Author to whom correspondence should he addressed.
S 02.50/0
Angew. Chem. Int. Ed. Engl. 20 11981) No. 2
sponding dihydrodiazepine base), presumably because in
(2a) there is minimal crowding between the 5-, 6- and 7-positions in the conjugated form of the base and hence no steric
factor to assist change to the bisimine form["l; (b) N,N'-disubstituted 6-bromodihydrodiazepinium salts such as (13) do
not undergo reactions of types A or B[la].
When the salt (lb) is heated with a molar equivalent of triphenylphosphane in methanol, the product (8b) is formed in
high yield. The p K , for the acid-base equilibrium involving
(lb) is 11.8121.It seems very unlikely, therefore, that in the
presence of triphenylphosphine in methanol there could be
sufficient base form present to sustain the protodebromination reaction proceeding via this base as intermediate. An alternative mechanism, in keeping with the known reactivity
of triphenylphosphane towards brominel'"1, could involve
the following sequence of reactionsf3].
- Protodebromination of (lb) in 1-propanol provided the
other expected products, triphenylphosphane oxide and 1bromopropane, which were isolated and characterized.
- The higher temperatures required for ( l a ) and (13) may
be associated with the diminished crowding between the
5-, 6- and 7-positions in these molecules. making the
C-Br bond slightly stronger and/or raising the energy of
the transition state because of diminished steric assistance. The 6-chloro-analogue of (1b) likewise undergoes
protodechlorination in refluxing I-propanol but not in refluxing methanol. The higher temperature required is
similarly reasonable.
It seemed possible that in an aprotic solvent such an onium anion (9) might gain a proton at the 6-position by transfer from a nitrogen atom.
The N,N-disubstituted dihydrodiazepinium species (13)
could not provide a proton in this way. In accord with this
proposition, (lb) underwent protodebromination when
heated in dry ethyl benzoate but similar treatment of (13)
only produced polymeric material, presumably resulting
from alternative reactions of the intermediate species.
The postulated intermediate species (96) is remarkable in
that it is at the same time an onium ion and a carbanion. Alternative forms which might be considered for this species,
but which would involve substantial distortion of the geometry of the ringf4{are the allene (11) (by analogy with structures considered for cycl~heptatrienylidene[~I)
or the carbene
(12). However contributions from either of these structures
would involve considerable increase in geometric strain in
the molecule, coupled with loss of the delocalization energy
(ca. 20 kcal .mol - ' [Ib1) of the vinamidinium system@].
There is other evidence for the formation of onium anions
in the literature even although they have not been formulated as such.
Bisaminocyclopropenium salts (14) and the vinamidinium
system in (1)@],are electronically similar and reactivity in
keeping with this has been reported[*"]. It has been postulated that the onium ion (15) is formed as an intermediate in
a number of reactions[xb,'l,but it has been described as a carbene (16), which can be another contributing form.
Also the ready decarboxylation of 2-pyridine carboxylic
acids has for some time been attributed to the formation of
the intermediate pyridinium ylid (17)I91. I f the pyridinium
cation is regarded as a delocalized system, this species (17)
provides another example of an onium anion (18).
Received August 6, 1980 [Z 685 IE]
German version: Angew. Chem. 93, 193 (1981)
In the onium ion (9b) the anionic lone-pair is orthogonal
to the delocalized system, which adds further complication
by being an electron-rich cationf'1.
A variety of evidence is available in support of this mechanism, and, consequently, of the postulate of this onium anion
- If perdeuteriomethanol is used as solvent a 6-deuteriodihydrodiazepinium salt is formed.
- The cation (la), which will not undergo protodebromination with alkoxided'"', is protodebrominated by triphenylphosphane, although a higher boiling solvent, l-pentanol, is required to promote the reaction.
- The salt (13), which cannot form a dihydrodiazepine
base, also undergoes protodebromination when heated
with triphenylphosphane in I-pentanol.
Angew. Chem. Inr. Ed. Engl. 20 (1981) No. 2
[I] a) A . M. Gorringe. D. Lloyd. F. I. Wasson, D. R. Marshall, P. A. Duffid. J.
Chem. SOC.C 1969, 1449; h) D.Lloyd. D. R. Marshall, Chem. Ind. (London)
1972. 335; C) D Lloyd, H. McNab, D. R. Marshall. J. Chem. SOC. Perkin
Trans. 11975, 1260.
[2] R P. Bell, D.R. Marshall. J . Chem. SOC. 1964. 2195.
131 a) See J. 1. G. Cadogan. R. K. Mackie, Chem. SOC.Reviews 3, 87 (1974); b) cf.
A. J. Burn, J. I . G. Cadogan, 3. Chem. SOC.1963, 5788.
[4) For the normal geometry see G. Ferguson, W. C. Marsh, D. Lloyd. D. R. Marshall, 1. Chem. SOC.Perkin Trans. I1 1980, 74.
[ S ] Infer alia W. M. Jones. L. Ennis. J. Am. Chem. SOC.89. 3069 (1967); R. L.
Tyler, W. M . Jones, Y. Ohm, J. R. Sadin. ibid. 96. 3765 (1974).
I61 D. Lloyd. H . McNab. Angew. Chem. 88,496 (1976); Angew. Chem. Int Ed
Engl. 15. 459 ( 1 976).
171 D. Lloyd, R. K. Mackie. H . McNab, K. S. Tucker. D. R. Marshall. Tetrahedron 32, 2339 (1976).
181 a) R. Weis, C. Priesner, Angew. Chem. 90. 484 (1978); Angew. Chem. Int.
Ed. Engl. 17.445 (1978); R. Werss, H. WOK Chem. Ber. 113, 1746 (1980). b)
0 Verlag Chemie, GmbH, 6940 Weinheim, 1981
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R. Weiss, C. friesner, H. WOK Angew. Chem. 91. 505 (1979). Angew. Chem
h . Ed. End. 18,472 (1979). C) R. WerSS, M . Herid, H. Wolf,Angew. Chem.
91. 506 (1979); Angew. Chem. Int. Ed. Engl. 18,473 (1979).
[9] a) H. Quasf, E. Franke!feld, Angew. Chem. 77, 680 (1965); Angew. Chem.
Int. Ed. Engl. 4, 691 (1965); K . W.Ram, R. K. Howe, W. G. Phillips, J. Am
Chem. SOC.91.61 15 (1969); H . Quasf,E. Schmirt, Justus Liehigs Ann. Chem.
732, 43 (1970). h) A similar species has also been postulated in reactions of
pyridine-N-oxides: R. A. Abramovitch, M . Saha, E. M . Smith, R. T. Cours, J .
Am. Chem. SOC.89, 1537 (1967).
the quenching agent (3) was chosen such that the lifetime of
the ( 3 s ) state was reduced to about 0.5 ps. The decay then
pseudo first-order kinetics, and hence the Observed
decay rate of (3S), kob,, could be equated to the rate of the
quenching process, k,, [(3)].
The Electronic Triplet State
of a Peralkylated Cyclobutadiene'"]
By Jakob Wirz, Adolf Krebs, Hermann SchmaIstieg
and Herbert AngIiker[']
In accordance with the results of ab initio calculations, recent experiments have established that the ground state of
cyclobutadiene (1) is a singlet (So) and that its geometry is
probably rectangular[']. In contrast, the energy level and
geometry of the triplet state (T,) have not yet been determined. All efforts to detect (31) existing in thermal equilibrium with (1) have been unsuccessful. Even the use of derivative (Z), stabilized by sterically demanding substituents,
produced no ESR signal in solution or solid state studies at
temperatures up to +100°C~21.
It is therefore probable that
the triplet excitation energy of (2) exceeds 40 kJ/mol.
We report here that we have generated and observed the
triplet state of the isolable, alkyl-substituted cyclobutadiene
(3)131 using the technique of flash photolysis (Nd laser at 353
or 530 nm, pulse duration 20 ns, kinetic measuring configuration). The direct excitation of (3) produced no short or long
term changes in its absorption spectrum in the range 250 to
820 nm. This was anticipated, since we had found earlierl4"]
that electronically excited [4n]annulenes undergo very rapid
radiationless decay, so that fluorescence emission or intersystem crossing to the TI state is effectively prevented. We
therefore determined the bimolecular rate constants k,, for
the quenching of several excited triplet sensitizers ( 3 S )by (3)
in degassed benzene (25 "C). For this purpose we used polycyclic arenes which have characteristic triplet absorption
bandsIsaland the triplet energies of which are
The decay kinetics of the ( 3 S )state in the absence of (3) in
a time range > 10 ps was dominated by (3S)-(3s) annihilation. For this reason the concentration (0.001 to 0.1 M ) [ ~ I of
Priv.-Doz. Dr. J . W i n ['I, Dr. H. Angliker
Physikalisch-chemisches Institut der Universitat
Klingelbergstrasse 80. CH-4056 Basel (Switzerland)
Prof. Dr. A. Krebs, Dipl.-Chem. H. Schmalstieg
Institut f i r Organische Chemie und Biochemie der Universitat,
Martin-Luther-King-Platz 6, D-2000Hamburg 13 (Germany)
Part 4 of "Electronic structure and photophysical properties of planar, conjugated hydrocarbons with a 4n-membered ring". Part 5 of "Isolahle cyclobutadienes". This work was supported by the Swiss National Science Foundation (Project Nr. 2.21 1-0.79). the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie. Parts 2 and 3 [4]. Part 4 [3a], resp.
0 Verlag Chemie, GmbH. 6940 Weinheim, 1981
6) 1 kJ/moll
Fig. 1. Logarithmic plot of the rate constants k,, for triplet energy transfer from
several sensitizers (S) to the cyclobutadiene derivative (3). against the triplet energy ET of the sensitizers (S). For the sake of clarity, only the carbon framework
of the benzene rings is shown.
Figure 1 shows k,, plotted logarithmically against the triplet energy ET of the sensitizers (S). The hypothesis that the
quenching of (3s)
by (3) can be assigned to a triplet energy
transfer process is decisive for the interpretation of the results. Since the sensitizers ([(S)]FZ
to lo-' M) were not
consumed, even upon prolonged irradiation, the process of
quenching must regenerate the starting materials. A reversible electron transfer (uia a triplet exciplex) is improbable on
energetic grounds. Furthermore, all solutions immediately
returned to their original absorbance in the visible region after the absorption of (3S)was quenched; if ion-pair intermediatesofthe type '(St ...37) or 3 ( S : ...3 + )h a v i n g ~ 2 2 0 n s
had been involved, they would have produced a strong transient absorption in the visible region. On the basis of semiempirical calculations on cyclobutadiene
we anticipated that (33) would exhibit a relatively weak, broad
absorption band in the near UV.
In most cases, measurements could not be made in this spectral region due to the strong background absorption of the
parent solutions. A suitable sensitizer, by means of which the
UV region became accessible, proved to be 2,3-dimethyl-I,4naphthoquinone (4).Its absorption maximum lay close to
the laser wavelength of 353 nm and because (4)had a very
high rate of quenching, k,, = 7 x lo9 M - ' s - ', a concentration
of [(3)]=0.01 M was sufficient to quench ( 3 4 ) within ca. 20
ns. Under these conditions we were able to observe a shortlived product ( T = 240 ns, 1st order decay) of the quenching
process which exhibited a weak absorption band increasing
gently in intensity from 400 to 300 nm. The spectrum of this
sequential product corresponds qualitatively to that expected
for (33), but is not consistent with the assignment
[ '1 Author to whom correspondence sould he sent.
7 0
A section through the energy surfaces of So (3) and TI (3)
is shown in Figure 2; the diagram displays the consistent re] , qualitasults of numerous ab initio calculations on ( l ) [ ' and
tively takes account of the asymmetric steric interaction arising from the a-methyl groups of (3).The equilibrium geometries shown for So (3) and T I (3) differ considerably; accordingly the vertical excitation energy E;! is much larger than
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Angew. Chem. Inf. Ed. Engl. 20 (1981) No. 2
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anion, onium
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