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Direct Transformation of the Second Excited Singlet State of Benzene into Dewar-Benzene.

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However, treating 2-lithiothiophene with quinoline at 20°C
by Giiman's methodf6]leads only to 2-(2-thienyl)quinoline
(2) (38%), indicating steric hindrance of 4-addition by the
peri-hydrogen atom of the quinoline. In the following
reactions of the quinoline system with 2-lithiothiophene
derivatives substitution is also only at the 2-position.
cxpqwa
S
(4)
a-
%QcJUa-0
s
s
'Re
The yield of 2-(2-thienyl)quinoline (2) could be raised to
75% by causing the components to react at 45°C [2 h in
ether/n-hexane (2: I)], the 2-position of the thienyl group
being provided by an analogous synthesis from 2-chloroquinoline instead of quinoline and by NMR spectroscopy.
The lithium compoundc6]obtained therefrom by n-butyllithium [yield 33% on the basis of the yield of alcohol
(m. p. 221 "C) resulting from treatment with benzophenone]
reacts with quinoline to give the trisarene ( 3 ) and with
CuCl, to give the tetrakisarene (4). As shown in the formula
scheme and Table, compound ( 4 ) was accessible in higher
yield by reaction of 2,2'-bithiophene and 2 mol of n-butyllithium with subsequent treatment with quinoline. The
analogous reaction with one mol of the two reagents gave
the trisarene (6) as main product.
Analogous methods afford compounds (8) and ( 9 ) which
are homologs of ( 4 ) and (6) and of interest as starting
materials for protophane syntheses[71.Their preparation
is possible because 1,2-di-(2-thienyl)ethane(7) (an "arenologous diamine", cf. Ref. [2]) is metalated by n-butyl-
Reaction
Treatment [a] of
the metalated [b]
product
Yield
Product
Color
(%)
M. p.
("C)
2 h, 35°C
3 h, 35°C
14
14
189-190
250-251
Yellow
Yellowishred
1 h,
I h,
2 h,
2 h,
37
40 rc1
57
24 [dl
142
189
87
Yellow
Pale yellow
Pale yellow
35°C
35°C
35°C
35°C
[a] Solvent: diethyl ether (+ ca. 10% of n-hexane introduced with nbutyllithium); in the reaction (2) + ( 4 ) a n additional 10% of THF.
[b] Metalation always with n-butyllithium at 0°C (30 min).
[c] 26% of ( 4 ) formed as by-product.
[d] Yield calculated on n-butyllithium which was used in excess to
suppress the formation of (8).
Angew. Chem. internat. Edit. 1 Vof. I0 (1971) / No. 10
lithium in ether/n-hexane (9 :1)only in the thiophene rings
and not on the aliphatic bridge. The yield of (8) is considerably better than that of ( 9 ) since the monolithium
derivative from (7) is in equilibrium with the dilithium
compound and the unmetalated compound.
The Table records the reaction conditions, yields, and
properties of the apparently new compounds prepared,
their structures being proved by elemental analyses, mass
and NMR spectra, and the method of their preparation.
Received: July 5, 1971 [Z 473e I€]
German version: Angew. Chem. 83,799 (1971)
[l] Protophanes and Polyarenes, Part 5.-Part 4: [2]
[2] Th. Kaufmann,Angew. Chem. 83,798 (1971); Angew. Chem. internat. Edit. 10,743 (1971).
[3] H . Wynberg, ?: J . uan Bergen, and R. M. Kelfog, J. Org. Chem. 34,
3175 (1969).
[4] K . Kahmann, H . Sigel, and H.'Erienmryer, Helv. Chim. Acta 47,
1754 (1964).
[S] A . Woltermann and Th. Kaufmann, experiments 1971.
[6] H . Gifman and D.A Shirley, J. Amer. Chem. SOC.71, 1870 (1949).
[7] Th. Kuufmann, G. Beissner, and R. Maibaum, Angew. Chem. 83,
795 (1971); Angew. Chem. internat. Edit. 10,740 (1971).
Direct Transformationof the Second Excited
Singlet State of Benzene into Dewar-Benzene
By D.Bryce-Smith, A. Gilbert, and D.A . Robinson"]
We have found that the S, state of benzene (but not the S,
state) can isomerize directly into Dewar-benzene (bicyclo[2.2.0]hexa-2,5-diene) by a symmetry allowed process
which appears to provide the first example of a non-dissociative photochemical reaction from an upper excited
singlet state.
Irradiation of liquid benzene under nitrogen at 254 nm
populates the S, state ('B,u) and thence the T, state ( 3 B , ~ ) ,
and gives the benzene isomers fulvene['I and benzvaleneF2],
but no trace of Dewar-benzene. On the other hand, this
latter isomer is among those formed by irradiation of
liquid benzene over the 160-210 nm range, a process
which directly populates both the S, (lB,u) and S, ('E,u)
states of benzenef3].The interesting question arises whether
Dewar-benzene is being formed by a hitherto unknown
type of direct isomerization of the S, and/or S, states: such
transformations would be symmetry allowed, as wouId
that of the T , state, but would not be thus allowed from
the S, or So statesc4].It was therefore desirable to identify
more precisely the state of benzene which transforms into
Dewar-benzene.
['I
Prof. D. Bryce-Smith, Dr. A. Gilbert, and Dr. D. A. Robinson
Department of Chemistry, University of Reading
Whiteknights Park, Reading R G 6 2 AD (England)
745
Use of a microwave-generated discharge in argon/iodine[']
as radiation source (h206 nm) has made possible the
irradiation of liquid benzene specifically in the So + S,
band (A,,, 203 nm) without direct population of the S,
state. Such irradiation of liquid benzene under standardized
conditions (20°C, 90 min, 2-ml samples) led typically to
concentrations of Dewar-benzene, fulvene, and benzvalene
of 3013, 5 0 1 5 , and 150F10ppm, respectively (GLC
retention times, UV-spectrum of fulvene, half-lives of
Dewar-benzene and benzvalene). The amounts and proportions of these isomers were the same from both airsaturated and argon-saturated benzene'"'. Thus the s,
state of benzene is clearly implicated at some stage in the
formation of Dewar-benzene, but these results on their
own do not exclude the intermediacy of So, S,, or triplet
states produced from the S, state by internal conversion.
The intermediacy of the So and S, states in the formation
of Dewar-benzene i s rendered improbable by the absence
of this isomer from the products of prolonged irradiations
(e.g. 48 h) of liquid benzene in the So S, band, both at
254 nm and over the whole band. One could only reconcile
the intermediacy of So or S, states with this evidence by
the rather strained postulation of the need for some critically high vibrational levels inaccessible by direct irradiation in the So+ S, band, and, as noted above, the transformations would be symmetry forbidden. The idea of
vibrationally excited intermediates also seems at variance
with the observed need for a liquid- rather than a gasphase',].
--f
The possible intermediacy of triplet states of benzene in
the formation of Dewar-benzene was tested by comparison
of the rate of formation of Dewar-benzene from pure
benzene with that from dilute solutions of benzene in ciscyclooctene and cyclooctane. cis-Cyclooctene acts as an
energy acceptor from T, benzene, giving the trans-isomer[61,
and other olefins are well known to behave similarly[71.In
fact, irradiation of 10% solutions of benzene in cis-cyclooctene at 206 nm as above surprisingly increased the rates
of formation of Dewar-benzene, fulvene, and benzvalene
approximately 20-fold in comparison with the rates from
pure liquid benzene under comparable conditions, and a
similar degree of insensitivity to dissolved air was observed[**''. A similar increase (22-fold) was found with the use
of cyclooctane as medium, so the dilution effect is physical
rather than chemical in nature. The greatly increased rate
of formation of Dewar-benzene in the presence of the olefin
rules out any appreciable contribution from triplet benzene
as an intermediate. Thus the evidence leaves S, benzene as
the only reasonable direct precursor of Dewar-benzene.
The question whether Dewar-benzene can also arise from
S, ('E,u) benzene was investigated by the use of an oxygen
lamp emitting over the range ca. 160-220nm'31, and a
filter of 0 . 9 % ~aqueous LiCl to isolate the So+ S, band
when necessary. It was found that Dewar-benzene can also
arise from population of the S, state: the results are consistent with direct isomerization of this state or of some
state (e.g. S , benzene) derived from it.
states to the S , state (which is normally unity for most
organic molecules that have been studied in dilute solution)
is significantly less than unity in the cases of benzene and
some methylbenzenes[']. The present results suggest that
part at least of the discrepancy can be attributed to a tendency for 1,4-bonding in the S, state of benzene.
Received: June 14,1971 [Z 474a IE]
German version: Angew. Chem. 83,803 (1971)
[l] J. M. Blair and D. Bryce-Smith, Proc. Chem. SOC.1957,287; H . J.
F. Angus, J . M . Blair, and D. Bryce-Smith, J. Chem. SOC.1960,2003.
[Z] K. E. Wilzbach, J . S. Ritscher, and L. Kaplan, J. Amer. Chem. SOC.
89, 1031 (1967); L. Kaplan and K . E . Wilzbach, ibid. 90, 3291 (1968).
[3] H . R . Ward and J . S . Wishnok, J. Amer. Chem. SOC.90,1085, 5353
(1968).
[4] D. Bryce-Smith and H . C. Longuet-Higgins, Chem. Commun. 1966,
593; I . Huller, J. Chem. Phys. 47, 1117 (1967); D. Bryce-Smith, Pure
Appl. Chem. 16,47 (1968); R. Hofmann and R. B. Woodward, Accounts
Chem. Res. 1, 17 (1968).
[ 5 ] P. Harteck, R. R. Reeces, and B. A. Thompson, 2. Naturforsch. 19a,
2 (1964).
[6] B. H. Orger, Ph. D. Thesis, University of Reading 1969; cf. J. S.
Swenton, J. Org. Chem. 34, 3217 (1969).
[?I P. J. Kropp and H . J . Krauss, J. Amer. Chem. SOC.89,5199 (1967),
and references therein.
[8] D. Bryce-Smith, Chem. Comm. 1969, 806.
[9] C. L. Braun, S. Kato, and S. Lipsky, J. Chem. Phys. 39, 1645 (1963).
Inhibition of Bond Exchange in Cyclooctatetraenes
By D. Bryce-Smith, A. Gilbert, and J . Grzonka"]
The [4n]annulenes so far studied exhibit (a) bond-alternation, and (b) bond-exchange" - Previous workers have
shown that the rate of bond-exchange in the cyclooctatetraene ring tends to be decreased by mono- and 1,Z-disubstitution, and that in the latter case the equilibrium
favors the less overcrowded tautomer
We now
report NMR and chemical evidence which, taken together,
suggests that certain 1,2-disubstituted cyclooctatetraenes
exist exclusively in form A.
Dimethyl cyclooctatetraene-l,2-dicarboxylate( I ) shows
an NMR spectrum (Table) which remains unchanged over
the temperature range -40 to +20O0C, apart from minor
R'
A
€3
It is interesting that Bruurr, Kato, and Lipsky found that
the internal conversion efficiency from upper electronic
p*]The effect of the atmosphere on the process was examined for
samples irradiated for 10 min since with the argon-saturated solutions
significant amounts of an opaque polymer formed on the cell windows
when the irradiation time was greater than ca. 30 min.
[***I The same mixture of cyclooctene-benzene 1,3-adducts was formed as is produced by irradiation at 254 nm, a result which suggests that
higher excited states of benzene were not directly involved in the intermolecular cycloaddition process : cf. ref. [8].
746
f 4)
[*] Prof. D. Bryce-Smith, Dr. A. Gilbert, and J. Grzonka
Department of Chemistry, University of Reading
Whiteknights Park, Reading R G 6 2 A D (England)
Angew. Chem. internal. Edit. / Vol. I0 (1971) N o . 10
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