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Diradicals as Intermediates in Chemical Reactions; 2 3-Dimethylene-1 4-cyclohexadiyl.

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Table I . Compounds (2) prepared. IR spectra determined as film on
a Perkin-Elmer Infracord instrument.
CPd.
R
B. p.
rC/torr]
[%I
Yield
IR
[cm-
(2a)
12b)
(2c)
OCH,
OCZHS
N(CHd2
83/12
96/12
56/10-'
96
90
57
1720 [a]
1725 [a]
1720 [a]
La] v(C=N) in a (CF&C=N-
'1
group [lo).
(6.56 g, 20mmol) in anhydrous hexane (40 ml) at 0°C. After
being kept for several hours at room temperature the
mixture is fractionated; yield 8.7 g (96%) of (2 a).
clo[4.2.0]octa-1,5-diene (4)f71 and 3,4-dimethylene-l,5hexadiene (5)[81.
A mixture of products of precisely the same composition
was obtained on gas-phase thermolysis (1lOT) of 5,6dimethylene-2,3-diazbicyclo[2.2.2]oct-2-ene ( 6 ) , which
was obtained (m. p. 40°C) in ca. 8 % yield from dimethyl
1,2-dihydrophthalate through intermediacy of the diol,
ditosylate, and N-phenyltriazolinedione adduct, the lastmentioned intermediate being subjected successively to
catalytic hydrogenation, alkaline hydrolysis, elimination,
and oxidation by CuCI,.
Received: April 2, 1973 [Z 822b IE]
German version: Angew. Chem. 85,542 (1973)
[I] Phosphorus Heterocycles with Pentacoordinated Phosphorus, Part
1: K . Burger, J . Fehn, and E. Moll, Chem. Ber. 104, 1826
2.-Part
( 197I).
[2] D. Kolbah and D. KorunEru in Houben-Weyl-Miiller: Methoden der
Organischen Chemie. Thieme, Stuttgart 1967, 4th Edit., Vol. 10/2, pp.
86ff.
[3] J . R . Bailey and N . H . Moore, J. Amer. Chem. SOC.39, 279 (1917);
J . R . Bailey and A. 7: McPherson, ibid. 39, 1322 (1917).
[4] M . Haring and 7: WagnerJauregg, Helv. Chim. Acta 40, 852 (1957);
7: Wagner-Jauregg and L. Zirngibl, Chimia 22, 436 (1968).
[5] 7: P . Forshaw and A. E. Tipping, J. Chem. SOC.C1971, 2404.
[6] St. Goldschmidf and B. Acksteiner, Liebigs Ann. Chem. 618, 173
(1958).
[7] C . G. Ouerberger and A. K Di Giulio, J. Amer. Chem. SOC. 80,
6562 ( 1958).
[8] N . A. Razumoua, A. A. Petrov, A. Kh. Voznesenskaya, and K . 7:
Nouitskii, Zh. Ohshch. Khim. 36, 244 (1966); Chem. Abstr. 64, 15913g
( 1966).
[9] K . Burger, J . Fehn, and W Thenn, Angew. Chem. 85, 541 (1973);
Angew. Chem. internat. Edit. 12,502 (1973).
An intermediate formation of (3) during decomposition
of (6) can be excluded because, although (3) and (6)
have comparable thermal stability, (3) is not formed on
thermolysis of (6) in either a stationary or a flow system
(detection by VPC).
The formation from either (3) or (6) of exactly the same
product mixture, whose composition is independent of
the reaction temperature and the pressure, indicates a
common intermediate to which we ascribe the structure
of the diradical 2,3-dimethylene-1,4-cyclohexadiyl(2).
Unlike the gas-phase reaction, heating (3) or (6) at 110°C
in dilute cyclohexane solution affords mainly (ca. 65%)
the dimers of ( 2 ) , accompanied by ca. 5 YOof the product
[lo] W J . Chambers, C. W Tullock, and D. D. C o f j a n , J. Amer. Chem.
SOC.84, 2337 (1962); W J . Middleton and C . G . Krespan, 3 . Org. Chem.
30, 1398 ( 1965).
[I I] L. L. Muller and J . Hamer. 1,2-Cycloaddition Reactions. Wiley,
New York 1967, pp. 107ff.
[12] J. Fir1 and K . Burger, unpublished work.
Diradicals as Intermediates in Chemical
Reactions; 2,3-Dimethylene-l,4yclohe~adiyl~~~
By Wolfgang R . Roth and Gerhard Erkerl'l
Tetramethyleneethane ( 1 ) has been repeatedly postulated
as an intermediate in thermal rearrangements[-l],and both
a planar and an orthogonal arrangement have been discussed for its
We have been interested in a
derivative of (I), namely 2,3-dimethylene-l,4-~yclohexadiyl (2), in which the ethano bridge makes a planar or
nearly planar geometry compulsory.
The anhydride of bicyclo[2.2.0] hex-5-ene-2,3-dicarboxylic
acid[5]was converted into 2,3-dimethylenebicyclo[2.2.0]hexane (3) by successive catalytic hydrogenation, reduction to the diol, conversion into the ditosylate, and elimination by potassium tert-butoxide/DMS0I6I. In the gas phase
at 110°C (3) rearranges to a mixture (67:33) of bicy[*] Prof. Dr. W. R. Roth and DipLChem. G. Erker
Lehrstuhl fur Organische Chemie 1 der Universitat
4630 Bochum, Universitatsstrasse 150 (Germany)
Angew. Chem. internat. Edit.
/ Vol. I 2 (1973) / N o . 6
mixture ( 4 ) + ( 5 ) (67: 33) and ca. 30 % of polymer. The
two main products of the dimer fraction, amounting
together to about 80%, were isolated by gas chromatography; on the basis of their spectroscopic properties they
are regarded as tricycle[ 10.4.0.04.9]hexadeca-4,8,12,16tetraene (7) and 15-methylenetricyclo[9.3.I .03.*]pentadeca-3,7,1I-triene (8)19].
The same dimers, free from the rearrangement products
(4) and ( 5 ) , were formed on photolysis (Philips HPK
125, Pyrex filter)of (6) (0.1 % solution in pentane), together
with polymers.
On irradiation of a 1 M solution of (6) in hexafluorobenzene at -180°C in an EPR spectrometer a progressively
broadening singlet is overlaid by a triplet spectrum that
can be ascribed to (2): A six-line spectrum (D=O.O204
cm- ; E =0.00159 cm- I ) appears centered at 3300 gauss
and the (Am=2) transition with lower intensity at 1650
gauss["].
This triplet species observed in the EPR spectrum seems
to be responsible for dimer formation. Thus, if either (3)
or ( 6 ) , dissolved in hexachlorobutadiene, is thermolyzed
in the preheated probe of an NMR instrument, intense
emission signals corresponding to the absorption signals
of the dimers are observed. The same nuclear polarized
spectra are recorded on photolysis of ( 6 ) , in solution
in hexachlorobutadiene, if this is carried out in the NMR
apparatus[' I].
503
A necessary condition for the occurrence of the CIDNP
signals is that at least one reactant in the dimer formation
shall be in a triplet state112J.In view of the EPR experiments
the intermediate (2) can fill this role. The second reactant
can be either a similar triplet molecule or a molecule
of starting material. Indications that the dimerization
occurs preferentially between two triplet molecules are
provided by the facts that the CIDNP signals appear only
in emissionfl'J and further that on thermolysis or photolysis of ( 3 ) or ( 6 ) , respectively, the triplet diradical cannot
be trapped by 1,3-cyclohexadiene or 1,2-dimethylenecyclohexane.
That (2) reacts differently according to whether it is produced in the gas phase or in solution may be an expression
of a difference in spin multiplicities. In the gas phase
the conversion into the triplet state occurs only slowly
and cannot compete with the rate of formation of ( 4 )
and ( 5 ) ; but in solution the intersystem crossing probability is very much greater and we observe the reactions
of the triplet.
Received: March 5, 1973 [Z 818a IE]
German version: Angew. Chem. 85.510 (1973)
[I] We thank Prof. W Grimmr for informing us of his independent
work on this problem [2].
[2] W Grimme and H. J . Rorhrr, Angew. Chem. 85.512 (1973); Angew.
Chem. internat. Edit. 12. 505 (1973).
[3] R. Hoffnrunn, J. Chem. SOC.8 1 9 7 0 , 1675.
[4] J . J . Gujrwski and C. N. Shih, J. Amer. Chem. SOC.94, 1675 (1972).
[5] E. E. can Tamelm, S. P . Pappas, and K . L. Kirk, J. Amer. Chem.
SOC.93, 6092 (1971).
[6] N. L. Badd and C:S. Chany, J. Amer. Chem. SOC.94, 7593. 7594
( 1972).
[7] The spectroscopic properties of all the new compounds are in accord
with the structures here assigned.
[8] L. Skurtrbol and S. Solomon, J. Amer. Chem. SOC.87, 4506 (1965).
[9] Thespectroscopic propertiesof(7)and (8)differ from those reported
by EuuU and Chang [6] for the Diels-Alder adducts of (31.
[lo] We thank Prof. D. Schulte-Frohlinde, Mulhe~m/Ruhr,for recording
this spectrum.
[ I l l We thank Prof. L . J . Oostrrhofand Dr. R. Kuptain, Leiden, for
carrying out this experiment and for help in interpreting its result.
[I21 G.L. Closs, J. Amer. Chem. SOC.93, 1546 (1971).
Mechanism of AIlenic Dimerization
By Wolfgang R . Roth, Margerita Heiber, and
Gerhard Erkerl'l
Tetramethyleneethane ( I ) has been repeatedly postulated
as an intermediate in allenic dimerizationf'I. We have
sought proof of this hypothesis in the case of 1,2,6,7-octatetraene (2), which if it reacts analogously should give rise
to the diradical2,3-dimethylene- 1,4-cycIohexadiyl(3). The
behavior of (3) in the gas phase and in solution has
already been described['?
According to Skattebd and Solomon131,(2) rearranges
at 310°C to 2,3-dimethylene-1,5-hexadiene
( 4 ) . We have
now observed that thermolysis of (2) in the gas phase
at 120°C leads, not only to ( 4 ) , but also to bicyclo[4.2.0]octa-1,5-diene ( 5 ) and 1,2-divinyl-l-cyclobutene
(6)l4]. The rearrangement is strictly first order and is
characterized by the activation parameters E, = 24.8 k0.2
kcal mol-* and A = 0 . 9 x 101os-l.
(2)
-
+
+
3
The relative concentrations of the rearrangement products
are independent of the reaction temperature (86150"C)
but not of the pressure. Whereas ( 4 ) is formed almost
exclusively below 10-2 torr, yet at ca. 1 torr product ratios
of ca. 60:25: 15 are observed for the isomers ( 4 ) , ( 5 ) ,
and (6) and are shifted in favor of ( 5 ) by increasing
pressures, so that in solution (infinitely high pressure) the
ratios 40:60:0 are attained.
This pressure-dependence can be understood if the rearrangement of (2) is formulated as proceeding via a vibrationally excited intermediate to which we assign the structure (3)"'. If the intermediate is able to get rid of its excess
energy sufficiently rapidly by collision (high pressure),
then we find that the ratio for (4) :( 5 ) is very close to that
expected from (3) which is 33:6712]. As the pressure
is lowered, (3) transfers increasing amounts of its excess
energy to its rearrangement products ( 4 ) and ( 5 ) , which,
insofar as they are not deactivated by collision, can either
regenerate (3) or afford divinylcyclobutene ( 6 ) .
As shown in independent experiments, (6) rearranges at
170°C to a mixture of ( 4 ) and ( 5 ) , and at 250°C ( 5 )
passes irreversibly into ( 4 ) . The formation of (6) from
( 4 ) and/or ( 5 ) thus appears t o contradict thermodynamics,
but it becomes intelligible if one remembers that equilibrium conditions d o not exist in this middle pressure
range. At very low pressures, which provide a sufficient
lifetime for the excited molecule to equilibrate, we observe
the expected, almost exclusive formation of the thermodynamically most stable product ( 4 ) .
Thermolysis of (2) in solution (cyclohexane, 120"C)
affords, along with ( 4 ) and ( 5 ) , a ca. 30% yield of the
dimerization products of the triplet diradical (3)['1. If
this reaction is carried out in the NMR spectrometer,
CIDNP signals of the dimers appearing in emission can
be recorded. That the ratio of triplet to singlet reactionL21
is distinctly less than for 2,3-dimethylenebicyclo[2.2.0]hexane or 5,6-dimethylene-2,3-diazabicyclo[2.2.2]oct-2-ene
can be due to the vibrationally excited state of ( 3 ) ,through
which the lifetime of the diradical is shortened and the
chance of transition to the triplet is lessened.
Received. March 5 , 1973 [Z 818b IE]
German version: Angew. Chem. 85, 511 (1973)
r]Prof. Dr. W. R. Roth, Dipl.-Chem. M. Heiber, and
Dip[.-Chem. G. Erker
Lehrstuhl fur Organische Chemie der Universitat
4630 Bochum, Universitatsstrasse 150 (Germany)
504
[ I ] J . E. Baldwin and R . H . Fleming, Fortschr. Chem. Forsch. 15,
28 1 ( 1970).
[2] W R . Roth and G. Erkrr. Angew. Chem. 85. 510 (1973); Angew.
Chem. internat. Edit. 12, 503 (1973).
[3j L. Skarrubd and S . Solomon, J. Amer. Chern. SOC.87, 4506
( 1965).
[4] The spectroscopic properties of all new compounds are in accord
with the assigned structure.
[5] The postulated excess energy of (3) can be estimated at ca.
25 kcal mol-' by consideration of the changes in bond energy (replacement of two x- by one u-bond: 50 kcal mol- '), the gain in resonance
Angew. Chem. internat. Edit. / Vol. 12 ( 1 9 7 3 ) / No. 6
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