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Diels-Alder Adducts of Benzene with Arenes and Their [4+2] Cycloreversion.

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Procedure
A solution of freshly sublimed [ S n ( 0 r B ~ ) ~171
] , (1.2 g, 4.53 mmol) in benzene
(25 mL) was added dropwise to a suspension of M(OtBuh M=Sr, Ba
(6 mmol) in benzene ( I 0 mL). The mixture was stirred for 2 h, filtered, and
the filtrate evaporated almost to drmess. The product crystallized out as colorless platelets. Yield: 1.59 g (92%) 4 o r 1.71 g (93%) 5 .
Received: January 14, 1986;
revised: February 4, 1986 [Z 1622 I€]
German version: Angew. Chem. 98 (1986) 367
recording the 'H-NMR spectrum, signals other than that
of benzene are observed, which first build up and then decrease during the course of the reaction. These signals correspond to those expected for o,p'-dibenzene (Fig. I): upon
heating of the sample to room temperature, they disappear
in favor of the benzene signal. At -9O"C, the intermediate
photoproduct reacts with 4-phenyl-1,2,4-triazoledione
(PTAD) to form the adduct 3, the structure of which also
confirms the presence of 2.
CAS Registry numbers
4. 101165-23-5; 5, 101 165-24-6
[I] M. Veith. R. Rosler, Z . Narurjiorsch. B. in press.
[2] M. Veith, R. Rosler. Angew. Chem. 94 (1982) 867; Angew. Chem. Inr. Ed.
Engi. 21 (1982) 858.
131 G.
[4]
Synthesis
A
1968.
E. Coates,
I 1 in
18.analogy
J. A. Heslop,
to a previously
M. E. Redwood,
described method:
D. Ridley,
E. Chablay,
J . Chem. Ann.
SOC.
Chrm. iParir) 8 (1917) 415.
[ 5 ] Synthesis analogous to that of the isopropyl compounds: J. S. Smith 11,
R. T. Doloff, K. S. Mazdiasni, J . Am. Ceram. Soc. 33 (1970) 91.
[6] A. R. West: Solid S f a f e Chemistry. Wiley, Chichester 1984. p. 235.
&-Ph:
&xs.&%&?
1oooc
2
1
3
1
-5OT o r hv
171 M. Veith, F. Tollner, J . Orgonomet. Chem. 246 (1983) 219.
2
0
&
-
4
Diels-Alder Adducts of Benzene with Arenes and
Their [4 + 21 Cycloreversion""
By Achim Bertsch, Wolfram Grimme,* and
Gerd Reinhardt
A continuum of reaction pathways exists for the [4+2]
cycloreversion, from the simultaneous breaking of two single bonds to stepwise bond cleavage. The retrocleavage of
Diels- Alder adducts formed from two arenes should most
closely resemble the ideal synchronous reaction. Both
components lack the two singly occupied p-orbitals necessary to become aromatic, and only simultaneous formation
of these two centers enables the transition state to acquire
part of this stabilization. Having reported earlier the adduct 4 between benzene and naphthalene,"] we now report
the Diels-Alder adducts of benzene both with itself and
with anthracene and naphthacene.
The Diels-Alder trimer 1 of benzene decomposes at
100°C to benzene without evidence for the formation of
the intermediate o,p'-dibenzene 2.i2.81
The retrocleavage of
1 also takes place upon irradiation with a high-pressure
mercury lamp through quartz. If the reaction is carried out
at -90°C in CDzClz in an N M R tube and followed by
CHDCI,
bk
65
I
6~0
5'5
5'0
L'5
6'0
3'5
3'0
2'5
-6
Fig. I . 300-MHz H-NMK spectrum o1 o.p'-dibttnzene 2 in CDICll at
-90°C. The CHDCl? signal has been truncated.
[*] Prof. Dr. W. Grimme, Dipl.-Chem. A. Bertsch, Dr. G. Reinhardt
lnstitut fur Organische Chemie der Universitat
Greinstrasse 4, D-5000 Koln 41 (FRG)
[**I This work was supported by the Fonds der Chemischen tndustrie and
BASF AG, Ludwigshafen.
Angew. Chum. 1n1. Ed. Engl. 25 (1986) No. 4
+
The kinetics of the [4 21 cycloreversion of o,p'-dibenzene 2 was determined by flash p h o t ~ l y s i s . A
'~~
solution of
the tribenzene 1 in isooctane (8 x lo-' M) was irradiated
in a quartz cuvette ( I = 10 cm) with a flash from a capacitor
discharge and the transient UV absorption of the dibenzene formed, 2, was recorded at 280 nm. The lifetime of 2
is ca. 0.5 s at room temperature; the kinetic parameters of
the decomposition were derived from nine rate constants
in the temperature range 20-55°C (see Table 2).
In order to synthesize the more stable adducts 5 - 7 , anthracene and naphthacene were allowed to react with p benzoquinone. With anthracene, 5a was the sole product,
whereas with naphthacene a 1 : 1 mixture of the stereoisomers 6a and 7a was obtained. By hydrogenation with
zinc in acetic acid, 5b and a mixture of 6b and 7b were
obtained; these compounds were then converted into the
bistosylhydrazones 5c-7c. Reaction with n-butyllithium in
tetrahydrofuran/n-hexane at 0°C afforded the anthracene
adduct 514]and a 1 : 1 mixture of the isomeric naphthacene
adducts 6 and 7 , respectively.
The adduct 10, involving the outer ring of anthracene,
was obtained from 4a,9,9a,lO-tetrahydroanthracene S.IS1
Cycloaddition of p-benzoquinone afforded the adduct 9a,
which, as described above, was converted into the hydrocarbon 9 via the intermediates 9b and 9c. Before dehydrogenation of the tetralin moiety in 9, the cyclohexadiene
ring must be protected by addition of 4-ethyl-] ,2,4-triazoledione to give adduct 9d. After formation of the naphthalene system in 10d by heating with 2,3-dichloro-5,6-dicyano-p-benzoquinone in tetrachloroethane, the protecting
group is removed by basic hydrolysis and oxidation with
copper(1r) chloride.
The adducts so obtained (Table I) decompose cleanly
into their aromatic components between 0 and 100°C; the
nonseparated isomeric naphthacene adducts 6 and 7 have
the same lifetime. In order to measure the rates of cycloreversion, a degassed solution of the compound in dodecane (lo-' M) was added to a thermostated polarimeter
cuvette (Z= 5 cm); the formation of naphthalene, anthracene, and naphthacene was followed by continuous re-
0 VCH Verlagsgesellschafi mbH. 0-6940 Weinheim. 1986
0570-0833/86/0404-0377 $ 02.50/0
377
cording of their long-wavelength UV absorptions. The Arrhenius parameters of the cycloreversion and the temperature range in which they were determined are given in Table 2.
Table 2. Activation parameters and resonance gain for the [4 + 21 cycloreversion of the adducts of benzene with arenes.
Compd
AT
[“CI
IgA
E.,
[kcal/moll
ACi (77°C) ARE
[kcal/moll
[kcal/molj
20-55
0-14
20-45
56-80
80-95
13.4f0.2
12.ltO.6
12.3t0.2
15.5i0.4
14.9f0.4
16.5f0.2
19.6f0.8
21.8k0.6
29.3k0.7
30.8f0.6
15.7f0.1
20.8f0.1
22.6f0.1
25.3f0.2
28.6f0.2
~
2
4
10
5
6, 7
40.0
30.5
26.4
16.8
11.5
sion of the adducts and the gain in resonance energy, ARE,
calculated according to the SCF-MO method:[@
AG* =33.3+0.9-(0.43+0.03).ARE
a:
b:
0
4;
0
instead o f
N-NHTos
c: Q i
a
N-NHTos
Table I . Yields and some physical data for the compounds 2, 3, 5 , 6, 7 , 9,
and 10. The ‘H-NMR spectra (except for that of 2, see Fig. 1) were recorded
at 90 MHz in CDC13.
2,’H-NMR:6=6.32(q,ZH),6.27(q,2H),5.43(m,4H),3.63(sext,2H),2.58
( s , 2 H)
3, M.p.= 156°C (dec.); ‘H-NMR: S=7.39 (m, 5H), 6.52 (q, 2H), 6.07 (9,
2H), 5.90 (4, 2H), 4.80 (m, 2H), 3.52 (m, 2H), 2.58 (m, 2H)
5, Yield 8% from 5 a ; m.p.=ISO”C (dec.); ‘H-NMR: 6=7.5-7.0 (m, 8H),
5.38 ( s , 4H), 4.21 ( s , 2H), 3.06 (s, 2 H )
6,7,Yield4%from 6 a , 7 a ; ‘H-NMR:6=7.8(m+2~,4H),7.5-7.0(m,6H),
5.38 ( m + s , 4H), 4.29 (s, 2H), 3.13 (s, 2 H )
9,Yield 15%from 9 a ; m.p.=154”C, ‘H-NMR:6=7.1 (m,4H),6.42(q,2H),
5.43 (AA‘BB‘, 4H), 2.95 ( s , 2H), 2.5 (m. 6H), 2.2 (m. 2H)
10, Yield 32% from 9 ; m.p.= 156°C (dec.); ‘H-NMR: 6=7.64 (s, 2H), 7.63
(AA‘BB‘,4H),6.70(q,2H),5.63(~,4H),4.10(m,ZH),2.92(~,2H)
The following linear relationship exists between the observed free energies of activation AG’ for the cyclorever-
The correlation coefficient is r = 0.995 and the confidence
level 99%. Thus, the transition state for the cycloreversion
of the adducts is uniformly reduced in energy by 43% of
the amount of resonance energy to be gained, AG+ extending over a range of 13 kcal/mol. Structural parameters
apparently have no influence on AGC in this series of
closely related adducts. The linear relationship found is
only in accordance with a synchronous process, since the
breaking of only one bond in the rate-determining step
would effect a gain of resonance energy in the intermediate diradicals over a range of only 2 k~al/mol.[’~
Received: December 5, 1985 [Z 1569 IE]
German version: Angew. Chem. 98 (1986) 361
[ I ] W. Grimme, H:G. Koser, Angew. Chem. 92 (1980) 307; Angew. Chem
I n / . Ed. Engl. 19 (1980) 307.
[2] W. Grimme, G. Reinhardt, Angew. Chem. 95 (1983) 636; Angew Chem.
Inr Ed. Engl. 22 (1983) 617.
131 We thank Prof. Dr. J. Wirz. Universitat Basel, for the kind invitation to
carry out these measurements in his laboratories.
[4] The synthesis described here: R. Waldraff, Duserturion, Universitat Koln
1983; 5 was obtained for the first time by N. C. Yang, M. J. Chen, P.
Chen, K. T. Mak, J . Am. Chem. SOC.104 (1982) 853: see also ibid. I06
(1984) 7310.
[ 5 ] K. Takeda, J. Horibe, H. Minato, J. Chem. SOC. Perkin Truns. 1 1973.
22 12.
161 M. J. S. Dewar, C. d e Llano, J. Am. Chem. SOC.91 (1969) 789.
[7] W. C. Herndon, J. Org. Chem. 46 (1981) 2119.
[8) Note added in proof: o,p’-Dibenzene 2 has meanwhile been synthesized
independently by another photochemical route and characterized by its
UV spectrum (R. Braun, M. Kummer, H.-D. Martin, M. B. Rubin, Angew
Chem. 97 (1985) 1054; Angew. Chem. l n t . Ed. Engl. 24 (1985) 1059). The
half-life at 40°C reported is ca. 5 ms, which is close to that of 10.0 ms
measured by us.
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A large number of books on writing good English are
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Ed. Engl. 25 (1986) No. 4
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