Matrix Isolation of 3,4-Benzocyclodeca-3,7,9-triene-1,5-diyne

Article abstract of DOI:10.1002/(SICI)1099-0690(199805)1998:5<799::AID-EJOC799>3.0.CO;2-G

3,4-Benzocyclodeca-3,7,9-triene-1,5-diyne (3) has been synthesized from two different precursors and characterized by means of matrix-isolation spectroscopy. Energies, structures and IR spectra of the product, the intermediate 9,10-didehydroanthracene (4), and of the transition state of the Bergman cyclization 3 → 4 have been calculated at the B3LYP/6-31G * level of theory.

Full text of DOI:10.1002/(SICI)1099-0690(199805)1998:5<799::AID-EJOC799>3.0.CO;2-G

FULL PAPER
Matrix Isolation of 3,4-Benzocyclodeca-3,7,9-triene-1,5-diyne
Carsten Köttinga, Wolfram Sander*a, Stefan Kammermeierb and Rainer Herges*b
Lehrstuhl für Organische Chemie II der Ruhr-Universitäta,
Universitätsstraße 150, D-44780 Bochum, Germany
Fax: (internat.) ϩ 49(0)234/709-4353
E-mail: sander@neon.orch.ruhr-uni-bochum.de
Institut für Organische Chemie der Universitätb,
Hagenring 30, D-38106 Braunschweig, Germany
Fax: (internat.) ϩ 49(0)531/391-5388
E-mail: R.Herges@tu-bs.de
Received December 18, 1997
Keywords: Bergman cyclization / Matrix isolation / Ab initio calculations / Flash pyrolysis / [10]Annulenes
3,4-Benzocyclodeca-3,7,9-triene-1,5-diyne (3) has been syn-
thesized from two different precursors and characterized by
means of matrix-isolation spectroscopy. Energies, structures
and IR spectra of the product, the intermediate 9,10-didehy-
droanthracene (4), and of the transition state of the Bergman
cyclization 3 Ǟ 4 have been calculated at the B3LYP/6-31G*
level of theory.
The Bergman cyclization of hexa-1,3-diyn-3-ene (1) and Scheme 1
related enediynes to give diradicals such as p-benzyne (2)
has attracted much attention during recent years.[1] The ac-
tivation barrier for the cycloaromatization of 1 was deter-
mined as 28.2 kcal/mol and the reaction enthalpy as 8.5
kcal/mol.[2] For cyclic enediynes, it was shown that the ther-
mal stability of the structure correlates with the distance
between the two triple bonds. The distance between the two
˚
terminal carbon atoms was determined to be 4.12 A in 1,
[1]
˚
but only 3.01 A in the 10-membered ring system 3. As a
consequence, 1 is stable at room temperature, whereas early
attempts by Masamune et al.[3] to synthesize 3 in solution
failed, resulting only in the isolation of products derived
from 9,10-didehydroanthracene (4).
Recently, Schottelius and Chen investigated the photo-
chemistry of 6 by means of trapping experiments in solution
as well as by laser-flash photolysis (LFP).[5] Two transient
species were observed in acetonitrile, a fast transient with a
lifetime of 2.6 µs (λmax ഠ 295 nm), and a slow transient
with a lifetime of 1.0 ms (λmax ഠ 335 nm). Based on kinetic
data, quenching studies, and ab initio calculations, these
species were assigned as 4 and 3, respectively. According to
the ab initio calculations [CAS(6ϫ6)/6-31G* and
CASPT2N/6-31G*], 3 and 4 are close in energy. Since the
LFP experiments revealed that 3 was formed from 4 in an
The matrix isolation and spectroscopic characterization exothermic reaction, and that at room temperature no reac-
of dehydroanthracene 4 was published by Chapman et al. tion of 3 back to 4 was observed within the time scale of
in 1976.[4] UV irradiation (λ > 200 nm) of bis(ketene) 5 in the experiment, it was concluded that 3 lies at least 3Ϫ4
organic glasses produced a reactive intermediate with a kcal/mol below 4. An estimated activation barrier of 12Ϫ13
series of UV/Vis absorptions in the range of 256Ϫ449 nm, kcal/mol for ring closure of 3 accounts for the lifetime being
attributable to 4. Irradiation of 5 in solid argon resulted in of the order of milliseconds.
the formation of CO and 4, showing IR absorptions at 710
The only derivative of 3 that has been described in litera-
and 760 cmϪ1 (Scheme 1).
ture is 3,4-benzocyclodeca-3,7,9-triene-1,5-diyne (10).[6] In
Eur. J. Org. Chem. 1998, 799Ϫ803
WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1998
1434Ϫ193X/98/0505Ϫ0799 $ 17.50ϩ.50/0
799

C. Kötting, W. Sander, S. Kammermeier, R. Herges
FULL PAPER
10, the rigid bridged biphenylene moiety prevents the rings of the anthracene skeleton, but should be large for the
Bergman cyclization and formation of the highly reactive in-plane (B1u) deformation, particularly the vibration at 537
diradical 4, and thus this compound is thermally stable even cmϪ1. Despite this uncertainty, by comparison of the exper-
at room temperature. The aromatic planar all-cis-[10]annu- imental with the calculated spectra, diradical 4 could clearly
lene (11) would require bond angles of 144° at the sp2 car- be ruled out as one of the products trapped in the matrix.
bon atoms, which accounts for the fact that in solution only On the other hand, there is a good agreement between a set
the more stable non-planar conformers were detected by of IR absorptions and the calculated spectrum of enediyne
NMR spectroscopy.[7] We report herein on the matrix iso- 3 (Table 1, Figure 1). Characteristic of the cyclodeca-1,3,7-
lation and spectroscopic identification of 3, synthesized triene-5,9-diyne moiety of 3 are the ring-deformation
from two independent precursors.
modes at 493 (in-plane), 512 (out-of-plane), and 712 cmϪ1
(out-of-plane), which are predicted by theory to within 4,
18 and 2 cmϪ1, respectively.
Table 1. IR-spectroscopic data of compound 3
[b]
[b]
Mode ν˜ [cmϪ1 [a]
]
Irel
(exp.) (calcd.)
ν˜ [cmϪ1 [c]
]
Irel
Description[d]
Sym.
(exp.)
(calcd.)
13
14
18
20
23
34
45
47
56
58
59
60
493
512
20
13
489
530
612
710
745
22
10
13
60
100
7
10-ring δ ip A1
10-ring δ oop B1
Results and Discussion
616
14
ring δ ip
B2
Synthesis and Characterization of 3
712
758
1043
1464
1519
53
100
10 1035
19 1458
54 1516
10-ring δ oop B1
6-ring δ oop B1
Flash-vacuum pyrolysis (FVP) of epoxide 7[8] at 650°C
with subsequent trapping in argon at 10 K resulted in the
complete fragmentation of the starting material (Scheme 2).
Anthraquinone was easily identified in the product mixture
by comparison of the IR spectrum with that of authentic
matrix-isolated material.[9] Strong IR absorptions not at-
tributable to anthraquinone were additionally found at
ring ip
ring ip
ring ip
A1
B2
A1
8
43
94
13
29
41
3040Ϫ3095 br. 3056
10-ring ν CϪH A1
6-ring ν CϪH A1
6-ring ν CϪH B2
6-ring ν CϪH A1
3082
3093
3098
[a]
[b]
Argon at 10 K. Ϫ Relative intensities (based on the strongest
1519, 758, and 712 cmϪ1
Scheme 2
.
[c]
absorption). Ϫ Calculated at the B3LYP/6-31G* level of theory;
IR bands with a relative intensity Irel > 5 are listed; the assignment
[d]
is tentative and is based on band positions and intensities. Ϫ
Approximate description.
Table 2. B3LYP/6-31G*-calculated frequencies of singlet and triplet
4, scaled with 0.963[12][23]
S-4
I [km/mol] Sym.
T-4
ν˜ [cmϪ1
] I [km/mol]
Sym. ν˜ [cmϪ1
]
B3u
B1u
B3u
B2u
B1u
B3u
B3u
B1u
B3u
B2u
B1u
B2u
B1u
B2u
B1u
B2u
B1u
B2u
B1u
B2u
88.5
227.8
2.2
6.2
4.8
2.7
B3u
B1u
B3u
B3u
B2u
B1u
B3u
B2u
B1u
B1u
B1u
B2u
B2u
B1u
B2u
B1u
B2u
B1u
B2u
83.0
218.3
2.0
2.5
378.0
376.0
3.7
459.4
483.6
1.6
518.0
137.8
8.9
587.8
4.5
571.3
630.2
1.7
744.2
103.1
62.4
1.9
734.5
102.7
6.4
770.8
991.6
923.2
1120.9
1211.0
1276.7
1319.8
1428.0
1439.5
1522.4
1616.5
3075.8
3086.4
3093.9
3.0
948.4
2.6
3.8
1066.0
1072.7
1242.6
1317.7
1454.9
1584.0
1618.6
3086.1
3102.1
3105.3
60.5
5.5
0.8
8.1
2.1
1.8
0.9
2.8
1.0
3.8
The IR spectra of 3 and 4 were calculated at the B3LYP/
6-31G* level of theory (Tables 1 and 2).[10][11] According to
these calculations, singlet and triplet 4 should exhibit IR
absorptions distinct from those of 3. However, since the
level of theory used in these calculations (B3LYP/6-31G*)
is not adequate for describing a singlet diradical, the com-
puted vibrational energies of singlet 4 are probably some-
8.4
5.7
9.0
2.7
25.3
18.8
33.5
49.8
68.0
In order to further corroborate the characterization of
what less reliable than the values for 3. The insufficient con- 3, 9,10-diiodoanthracene (9) was pyrolysed under similar
sideration of correlation should have little effect on stretch- conditions as an independent precursor. FVP of 9 at 650°C
ing and bending vibrations mainly localized at the terminal produced the same set of IR absorptions as assigned to 3
800
Eur. J. Org. Chem. 1998, 799Ϫ803

Matrix Isolation of 3,4-Benzocyclodeca-3,7,9-triene-1,5-diyne
FULL PAPER
Figure 1. (a) IR spectrum of the products of a flash-vacuum pyrolysis of 7 (Ar, 10 K); (b) IR spectrum of 8 (Ar, 10 K); (c) calculated
spectrum of 3 (B3LYP/6-31G*; scaled)
(Figure 2). However, at this temperature the decomposition since 9 proved to be stable even towards prolonged UV ir-
of 9 is still incomplete, and thus the yield of 3 is lower radiation. This is explained by the rapid in-cage recombina-
than that with 7 as the precursor (Scheme 2). Attempts to tion of the radical pair formed on photolysis of matrix-iso-
synthesize 3 or 4 by irradiation of matrix-isolated 9 failed, lated 9.
Figure 2. (a) IR spectrum of the products of a flash-vacuum pyrolysis of 9 (Ar, 10 K); (b) IR spectrum of 9 (Ar, 10 K); (c) calculated
spectrum of 3 (B3LYP/6-31G*; scaled)
Eur. J. Org. Chem. 1998, 799Ϫ803
801

C. Kötting, W. Sander, S. Kammermeier, R. Herges
FULL PAPER
Calculation of the 4 Ǟ 3 Rearrangement
The computed activation barrier of 1 Ǟ 2 is in good
agreement with the experimental value (error: 2.4 kcal/mol).
Both the enediyne and the transition structure are almost
pure closed-shell systems and are well described by DFT.[14]
However, as expected, the error in the reaction enthalpy is
large (12.4 kcal/mol) due to the high diradical character of
2.[15] In a first approximation, we assume that the error is
of similar magnitude in 2 and 4, since the π systems of the
annelated benzene rings in 4 are orthogonal with respect to
the in-plane “radical” orbitals. On the basis of the above
data we estimate enediyne 3 to be 6Ϫ8 kcal/mol more stable
than diradical 4. The theoretically determined activation
enthalpy of only 10Ϫ12 kcal/mol and the exothermic reac-
tion enthalpy for the retro-Bergman cyclization 4 Ǟ 3 are
consistent with the exclusive formation of 3 as the product
of the FVP experiments (Figure 4).
Species with considerable diradical character such as 4
are notoriously difficult to describe using single-reference
ab initio and DFT methods. Unfortunately, adequate con-
sideration of correlation in systems as large as 4 is prohibi-
tive. We used the B3LYP hybrid functional[10][11] and a 6-
31G* basis set for our theoretical treatment of the Bergman
system 4 Ǟ 3, because this method is known to perform
well in predicting vibrational frequencies of closed-shell
structures[12] (see above), and because it is superior to most
ab initio perturbation methods (e.g. MP2) in treating corre-
lation. Errors in calculated kinetic and particularly thermo-
dynamic energies of the system 4 Ǟ 3 due to inadequate
consideration of correlation are estimated by comparing
theoretical (B3LYP/6-31G*) and experimental data of the
parent system 2 Ǟ 1. In the latter system, activation and
reaction enthalpies were experimentally determined within
error limits of only 0.5Ϫ1.0 kcal/mol (∆H° ϭ 28.2 ± 0.5
kcal/mol, ∆H° ϭ 8.5 ± 1.0 kcal/mol[2][13][14]). Table 3 gives
the relative energies of 1, [1 Ǟ 2]°, 2, 3, [3 Ǟ 4]° and 4,
while Figure 3 depicts the structures of 3, [3 Ǟ 4]° and 4.
Stationary points were characterized by normal-coordinate
analysis and the B3LYP energies include zero-point contri-
butions.
Figure 4. Calculated (B3LYP/6-31G*) corrected relative energies of
the Bergman cyclization 3 Ǟ 4
Table 3. Calculated relative energies of the Bergman cyclizations
1 Ǟ 2 and 3 Ǟ 4
1
TS [1 Ǟ 2]° S-2
T-2
Erel (B3LYP/6-31G* ϩ ZPE) 0.0
30.6
28.2
2.4
20.9 7.8
Based on the estimated activation barrier of 18 kcal/mol
and an activation entropy of Ϫ3.7 cal molϪ1 KϪ1 (B3LYP/
6-31G*) for the Bergman cyclization leading back to the
aromatic diradical 4, the half-life of 3 should be of the order
of a few seconds at 20°C. Thus, in accordance with the LFP
experiments,[5] enediyne 3 is predicted to be unstable at
room temperature.
Erel (experimental)[2][13]
0.0
0.0
8.5
Ϫ
Ϫ
error
12.4
3
TS [3 Ǟ 4]° S-4
T-4
Erel (B3LYP/6-31G* ϩ ZPE) 0.0
corrected relative energies 0.0
20.3
ca. 18
19.8 7.9
6Ϫ8
Ϫ
This also explains the failure of the synthesis of 3 at-
tempted by Masamune.[3] Under various reaction con-
ditions anthracene was obtained as the principal product.
The formation of 9,10-dideuterated anthracene in deuter-
ated solvents clearly indicated the ring-closure to 4, which
was subsequently trapped by the solvent. In our hands, pre-
parative-scale pyrolysis of 7 and work-up at room tempera-
ture did not produce 3 either, but rather anthracene and
polymeric material.
Figure 3. Calculated (B3LYP/6-31G*) structures of 3, [3 Ǟ 4]° and
4
This work was financially supported by the Deutsche Forschungs-
gemeinschaft and the Fonds der Chemischen Industrie.
Experimental Section
Matrix Spectroscopy: Matrix-isolation experiments were per-
formed by standard techniques with an APD CSW-202 Displex
closed-cycle helium cryostat. Matrices were produced by co-depo-
sition of a large excess of argon (Messer-Griesheim, 99.9999%, ap-
proximately 0.15 mmol/min) and the trapped species onto a cold
CsI window. To obtain optically clear matrices, the cold window
was maintained at 30 K during deposition, and the matrix was
subsequently cooled to 10 K. Ϫ Pyrolyses were carried out in a
802
Eur. J. Org. Chem. 1998, 799Ϫ803

Matrix Isolation of 3,4-Benzocyclodeca-3,7,9-triene-1,5-diyne
FULL PAPER
[1]
[2]
[3]
[4]
[5]
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IR bands of anthraquinone (Ar, 10 K): ν˜ ϭ 625 (7), 702 (33),
812 (8), 908 (3), 933 (4), 939 (33), 1159 (2), 1170 (6), 1207 (3),
1266 (2), 1289 (96), 1306 (43), 1316 (46), 1332 (56), 1477 (3),
1597 (28), 1681 (100), 1707 (1), 1725 (2), 1969 (1), 3079 (4) cmϪ1
(rel. intensity).
quartz tube placed 3 cm in front of the cold window. In order to
obtain short contact times, a heating zone of only 3 cm in length
was used. A large excess of argon and the precursor were passed
through the pyrolysis zone and the products were directly trapped
on top of the cold window at 10 K. Ϫ Infrared spectra were re-
corded using a Bruker IFS-66 FT-IR spectrometer with a standard
resolution of 1 cmϪ1 in the range 400Ϫ4000 cmϪ1. Irradiations
were carried out by means of Osram HBO 500 W/2 mercury high-
pressure arc lamps in Oriel housings equipped with quartz optics
or a Gräntzel mercury low-pressure lamp (254 nm). IR radiation
from the arc lamp was absorbed by a 10-cm path of water. Schott
cut-off filters were used (50% transmission at the wavelength speci-
fied) in combination with dichroic mirrors.
[7]
[8]
[9]
Tetradehydrodiepoxydianthracene 7: A solution of 191.1 mg
(1.136 mmol) of m-chloroperbenzoic acid in 100 ml of dichloro-
methane was added dropwise to a solution of 200 mg (0.568 mmol)
of tetradehydrodianthracene[21] in 200 ml of dichloromethane. The
mixture was stirred for 30 min at room temp. and then the solvent
was evaporated. The yellowish residue was washed with dichloro-
methane and recrystallized from toluene. Yield: 161.4 g (74%), m.p.
[10]
[11]
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[13]
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6401Ϫ6412.
1
312Ϫ314°C. Ϫ H NMR (400 MHz, CDCl3): δ ϭ 7.13 (dd, J ϭ
[14]
[15]
High level ab initio calculations[16][17][18][19] are in good agree-
ment with experimental data (∆H° ϭ 28.5 kcal/mol, ∆H° ϭ 8.0
kcal/mol).[16]
3.1, 3.1 Hz, 8 H, CH, arom.), 6.91 (dd, J ϭ 3.1, 3.1 Hz, 8 H, CH,
arom.). Ϫ 13C NMR (100.6 MHz, CDCl3): δ ϭ 139.9 (C, arom.),
126.4 (CH, arom.), 122 (CH, arom.), 70.2 (CϪO). Ϫ MS (70 eV);
m/z (%): 385 (24), 384 [Mϩ] (76), 176 (100). Ϫ C28H16O2 (384.13):
calcd. C 95.42, H 4.58; found C 95.31, H 4.51. Ϫ IR (Ar, 10 K):
ν˜ ϭ 456 (6), 483 (8), 603 (4), 626 (19), 654 (71), 704 (75), 727 (3),
743 (5), 753 (25), 784 (100), 865 (19), 905 (11), 913 (24), 935 (31),
950 (6), 1032 (4), 1139 (5), 1160 (3), 1217 (2), 1338 (14), 1348 (9),
1364 (12), 1369 (8), 1375 (7), 1425 (5), 1448 (27), 1452 (15), 1462
(20), 1467 (6), 1952 (5), 3064 (12), 3081 (6) cmϪ1 (rel. intensity).
According to a CASSCF(8,8) calculation, the weightings of the
two configurations of the bonding and antibonding combi-
nation of the two in-plane (“radical”) π orbitals are 0.53 and
0.32. This is close to a pure diradical system in which both
configurations would have an equal contribution.[20]
E. Kraka, D. Cremer, J. Am. Chem. Soc. 1994, 116, 4929Ϫ4936.
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[16]
[17]
[18]
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[19]
[20]
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9,10-Diiodoanthracene (9): Diiodide 9 was synthesized according
to a procedure described by Duerr, Chung and Czarnik.[22] Ϫ IR
(Ar, 10 K): ν˜ ϭ 572 (11), 606 (12), 667 (9), 750 (60), 754 (15), 900
(8), 913 (14), 916 (100), 924 (8), 1031 (25), 1165 (10), 1167 (20),
1255 (80), 1261 (8), 1266 (10), 1301 (31), 1439 (6), 1443 (38), 1449
(12), 1525 (9), 1529 (15), 1612 (5), 3044 (2), 3056 (3), 3079 (4), 3095
(8) cmϪ1 (rel. intensity).
[21]
[22]
[23]
A. P. Scott, L. Radom, J. Phys. Chem. 1996, 100, 16502Ϫ16513.
[97398]
Eur. J. Org. Chem. 1998, 799Ϫ803
803