High-Temperature Behavior of 1,8-Diethynylanthracene. Benzaceanthrylene, a Transient Intermediate for Cyclopentapyrene

Full text of DOI:10.1021/jo970654z

J . Org. Chem. 1997, 62, 8247-8250
8247
tous 6 is obtained from (CP)-PAHs such as benzo[ghi]-
fluoranthene (C18H10, 7)9b,11 and benzo[c]phenanthrene
(C18H12).10b
High -Tem p er a tu r e Beh a vior of
1,8-Dieth yn yla n th r a cen e.
Ben z[m n o]a cea n th r ylen e, a Tr a n sien t
In ter m ed ia te for Cyclop en ta [cd ]p yr en e
Here we wish to report that FVT of 1,8-bis(1-chloro-
ethenyl)anthracene (2), which at T > 800 °C is quanti-
tatively converted in situ into 1,8-diethynylanthracene
(3),12 gives an unexpected entry to the abundant combus-
tion effluent cyclopenta[cd]pyrene (6). The presence of
10-ethynylaceanthrylene (4), besides 3, in the tempera-
ture range 800-900 °C in combination with the identi-
fication of 6 as primary product at T > 900 °C suggests
that 4 is converted into transient benz[mno]acean-
thrylene (5),13 which immediately rearranges into 6 under
the high-temperature conditions in the gas phase.
FVT precursor 2 was prepared in two steps from
anthracene. Bisacetylation of anthracene gave a 1:1
mixture of 1,5- and 1,8-diacetylanthracene (1); pure 1 was
isolated using preparative column chromatography. Sub-
sequently, 1,8-diacetylanthracene (1) was treated with
PCl5 to give crude 1,8-bis(1-chloroethenyl)anthracene (2),
which was also purified by preparative column chroma-
tography (yield 40% from 1, see the Experimental Sec-
tion).
Aliquots of 2 (0.1 g, 10-2 Torr, sublimation temperature
140 °C and sublimation rate 0.05 g h-1) were transferred
into the unpacked quartz tube (length, 40 cm; diameter,
2 cm; and temperature range, 800-1100 °C) of our FVT
apparatus. The pyrolysate product composition was
determined using 1H NMR, capillary GC-MS, HPLC,
and authentic samples of 3,12 6,1 and 7,11 respectively,
as reference compounds. At 800 °C, 2 is quantitatively
converted into a mixture of 1,8-diethynylanthracene (3)
and 10-ethynylaceanthrylene (4,14 Scheme 1 and Table
1). Repyrolysis of the pyrolysate at 800 °C supported the
consecutive conversion of 2 into 3 and 4, respectively
[initial composition, 3, 65% and 4, 35% (mass recovery
ca. 100%); and after repyrolysis, 3, 34%; 4, 53%; and 6,
13% (mass recovery 74%)]. Corresponding results were
found upon FVT of 2 at 900 °C [pyrolysate composition,
3, 13%; 4, 40%; and 6, 47% (mass recovery 33%)]. Above
900 °C, 6 was the major low molecular weight product,
Martin Sarobe and Leonardus W. J enneskens*
Debye Institute, Department of Physical Organic Chemistry,
Utrecht University, Padualaan 8,
3584 CH Utrecht, The Netherlands
Received April 11, 1997X
The awareness that the genotoxicity of combustion
exhausts can be accounted for by the presence of a small
number of polycyclic aromatic hydrocarbons (PAHs), of
which cyclopenta-fused derivatives (CP-PAHs), such as
cyclopenta[cd]pyrene (6),1 pose an extraordinary biohaz-
ard,2 has made the elucidation of CP-PAH formation
mechanisms under high-temperature conditions a topical
issue.3 Additional impetus is given by the proposal that
CP-PAHs also may play a role in fullerene build up under
arc and flame conditions.4
The origin of cyclopenta[cd]pyrene (6) during combus-
tion has been rationalized by invoking the intermediacy
of 1-ethynylpyrene, thought to arise from either C2 or
ethyne (C2H2) addition to pyrene.1,5 This proposal was
supported by flash vacuum thermolysis (FVT) experi-
ments using 1-(1-chloroethenyl)pyrene (“masked” 1-eth-
ynylpyrene).6,7 Under FVT conditions 1-(1-chloroethenyl)-
pyrene is quantitatively converted in situ into 1-eth-
ynylpyrene, which subsequently gave 6 at T > 1000 °C
(mass recovery 90%).1 By similar approaches, important
and abundant CP-PAH effluents have been prepared in
reasonable to good yields as well as with good mass
recoveries.8
Recently, other thermal pathways involving either
selective (CP)-PAH isomerizations (“annealing”)9 or PAH
interconversions10 were disclosed. For example, ubiqui-
* Address correspondence to Prof. Dr. L. W. J enneskens, Debye
Institute, Department of Physical Organic Chemistry, Utrecht Uni-
versity, Padualaan 8, 3584 CH Utrecht, The Netherlands. Tel: +31
302533128. Fax: +31 302534533. E-mail: jennesk@chem.ruu.nl.
X Abstract published in Advance ACS Abstracts, October 15, 1997.
(1) Sarobe, M.; Zwikker, J . W.; Snoeijer, J . D.; Wiersum, U. E.;
J enneskens, L. W. J . Chem. Soc., Chem. Commun. 1994, 89 and
references cited.
(2) (a) Howard, J . B.; Longwell, J . P.; Marr, J . A.; Pope, C. J .; Busby,
W. F., J r.; Lafleur, A. L.; Taghizadeh, K. Combustion Flame 1995, 101,
262. (b) For a review, see: J acob, J . Pure Appl. Chem. 1996, 68, 301
and references cited.
(8) (a) Sarobe, M.; Snoeijer, J . D.; J enneskens, L. W.; Slagt, M. Q.;
Zwikker, J . W. Tetrahedron Lett. 1995, 36, 8489. (b) Sarobe, M.; Flink,
S.; J enneskens, L. W.; van Poecke, B. L. A.; Zwikker, J . W. J . Chem.
Soc., Chem. Commun. 1995, 2415. (c) J enneskens, L. W.; Sarobe, M.;
Zwikker, J . W. Pure Appl. Chem. 1996, 68, 219 and references cited.
(d) Scott L. T.; Necula, A. J . Org. Chem. 1996, 61, 386. (e) see also
Rabideau, P. W.; Sygula, A. Acc. Chem. Res 1996, 29, 235 and
references cited. (f) Liu, C. Z.; Rabideau, P. W. Tetrahedron Lett. 1996,
37, 3437.
(9) (a) Scott, L. T.; Roelofs, N. H. J . Am. Chem. Soc. 1987, 109, 5461.
(b) Plater, M. J . Tetrahedron Lett. 1994, 35, 6147. (c) Sarobe, M.;
Snoeijer, J . D.; J enneskens, L. W.; Zwikker, J . W.; Wesseling, J .
Tetrahedron Lett. 1995, 36, 9565. (d) see also Hagen, S.; Nuechter,
U.; Nuechter, M.; Zimmerman, G. Polycyclic Aromatic Compounds
1995, 4, 209. (e) Sarobe, M.; J enneskens, L. W.; Wesseling, J .; Snoeijer,
J . D.; Zwikker, J . W.; Wiersum, U. E. Liebigs Ann./ Recueil. 1997, 1207.
(f) Sarobe, M.; J enneskens, L. W.; Wesseling, J .; Wiersum, U. E. J .
Chem. Soc, Perkin Trans. 2 1997, 703.
(3) (a) Mukherjee, J .; Sarofim, A. F.; Longwell, J . P. Combustion
Flame 1994, 96, 191. (b) Hofmann, J .; Zimmermann, G.; Guthier, K.;
Hebgen, P; Homann, K.-H. Liebigs Ann. 1995, 631.
(4) (a) Chang, T-M.; Naim, A.; Ahmed, S. N.; Goodloe, G.; Shevlin,
P. B. J . Am. Chem. Soc. 1992, 114, 7603. (b) Pope, C. J .; Marr, J . A.;
Howard, J . B. J . Phys. Chem. 1993, 97, 11001. (c) Osterodt, J .; Zett,
A.; Vo¨gtle, F. Tetrahedron 1996, 52, 4949. (d) Crowley, C.; Taylor, R.;
Kroto, H. W.; Walton, D. R. M.; Cheng, P.-C.; Scott, L. T. Synth. Metals
1996, 77, 17 and references cited. (e) see also Lafleur, A. L.; Howard,
J . B.; Taghizadeh, K.; Plummer, E. F.; Scott, L. T.; Necula, A; Swallow,
K. C. J . Phys. Chem. 1996, 100, 17421.
(5) (a) Bockhorn, H.; Fetting, F.; Wenz, H. W. Ber. Bunsen-Ges. Phys.
Chem. 1983, 87, 1067. (b) Lafleur, A. L.; Gagel, J . J .; Longwell, J . P.;
Monchamp, P. A. Energy Fuels 1988, 2, 709. (c) Baum, T.; Lo¨ffler, S.;
Lo¨ffler, P.; Weilmu¨nster, P.; Homann, K.-H. Ber. Bunsen-Ges. Phys.
Chem. 1992, 96, 841. (d) Lafleur, A. L.; Howard, J . B.; Marr, J . A.;
Yadev, T. J . Phys. Chem. 1993, 97, 13539.
(10) (a) Scott, L. T. Pure Appl. Chem. 1996, 68, 291 and references
cited. (b) Sarobe, M.; J enneskens, L. W.; Wiersum, U. E. Tetrahedron
Lett. 1996, 37, 1121. (c) Scott, L. T.; Bratcher, M. S.; Hagen, S. J . Am.
Chem. Soc. 1996, 118, 8743. (d) Clayton, M. D.; Rabideau, P. W.
Tetrahedron Lett. 1997, 38, 741.
(11) Studt, P.; Win, T. Liebigs Ann. Chem. 1983, 519.
(12) Katz, H. E. J . Org. Chem. 1989, 54, 2179.
(6) Ethynyl-PAH can undergo either decomposition or oligomeriza-
tion upon heating, viz. upon sublimation from the sample flask into
the hot zone of the FVT apparatus.8b,9e,f
(7) Under FVT conditions, appropriately substituted ethynyl-PAHs
give CP-PAHs by an ethynyl-ethylidene carbene rearrangement
followed by carbene C-H insertion. For a review, see: Brown, R. F.
C.; Eastwood, F. W. Synlett 1993, 9 and references cited.
(13) For an earlier theoretical description of benz[mno]aceanthrylene
(5), see: Dias, J . R. J . Chem. Inf. Comput. Sci. 1992, 32, 2.
(14) A 1H NMR chemical shift difference (∆δ 1.42 ppm) is found for
the two cyclopentene protons of 4 as a consequence of deshielding of
H(1) by the ethynyl substituent at C(10) (see the Experimental
Section); Hesse, M.; Meier, H.; Zeeh, B. Spektroscopische Methoden in
der Organischen Chemie; Thieme Verlag: Stuttgart, 1991.
S0022-3263(97)00654-3 CCC: $14.00 © 1997 American Chemical Society

8248 J . Org. Chem., Vol. 62, No. 23, 1997
Notes
Ta ble 1. P yr olysa te P r od u ct Com p osition Obta in ed
u p on F VT of 2a
Sch em e 2
mass
T/°C
recovery/%
3/%
4/%
6/%
7/%
800
100
74b
33c
32c
31c
65
35
34b
13
53b
40
13b
47
>99
95
900
1000
1100
<1
5
a
1H NMR integral ratios, capillary GC, and HPLC gave
identical results. bMass recovery and pyrolysate product composi-
tion after repyrolysis at 800 C. c Carbonization occurs (see text).
o
Sch em e 1
whereas at T > 1000 °C minor amounts of benzo[ghi]-
fluoranthene (7)11 were also identified (Scheme 1 and
Table 1). The presence of 7, besides 6, indicates that the
latter rearranges presumably by a ring-contraction/ring-
expansion process (1,2-C/1,2-H shift).8c,9b It is noteworthy
that at T g 900 °C the mass recoveries decrease due to
carbonization (T ) 800 °C, ca. 100% and T > 900 °C, ca.
30%); a thin carbon lining is deposited on the wall of the
quartz tube inside the furnace.15
No evidence for the rearrangement of 4 into 10-
ethynylacephenanthrylene, which is also a potential FVT
precursor of 6 by ring-contraction/ring-expansion, was
found. This is in line with FVT experiments using either
9-ethynylanthracene or aceanthrylene in the temperature
range 800-1100 °C. At T > 1000 °C, both precursors
gave, besides aceanthrylene, acephenanthrylene in only
small amounts (ca. 5%).9e These results further cor-
roborate that, at least under FVT conditions, cyclopenta-
fusion occurs more readily than ring-contraction/ring-
expansion.8a-c,9e-f
To rationalize the formation of 6 from 4, benz[mno]-
aceanthrylene (5)13 is invoked as transient intermediate.
In going from 4 to 5, the ethynyl substituent at C(10)
has to undergo ethyne-ethylidene carbene equilibration
followed by carbene C-H insertion (Scheme 1).15 Ap-
parently, 5 is susceptible to rearrangement by ring-
contraction/ring-expansion9a,16 under the high-tempera-
ture conditions in the gas phase.
This interpretation is corroborated by AM117 calcula-
tions (Scheme 2). Benz[mno]aceanthrylene (5) is pre-
dicted to be less stable than either 6 or 7 [∆Hf °(5) ) 129.3
kcal mol-1, ∆Hf °(6) ) 109.4 kcal mol-1, and ∆Hf °(7) )
116.6 kcal mol-1]. In addition, the activation enthalpy
(∆H‡) for C-H carbene insertion in going from 9 to 5
[∆H‡(9f5) ) 12.7 kcal mol-1] is smaller than that in
going from 8 to 4 [∆H‡(8f4) ) 15.6 kcal mol-1].18 These
results give credence to our proposal that the consecutive
conversion of 3 into 4 and 5 will be viable under FVT
conditions. The fleeting existence of 5 is rationalized by
the AM1 ∆H‡ values connecting 5 to 9, 10, and 11,
respectively [∆H‡(5f9) ) 106.0 kcal mol-1, ∆H‡(5f10, 1,2-
(15) Six-membered ring formation is generally less favorable than
cyclopenta-fusion: Scott, L. T.; Hashemi, M. M.; Meyer, D. T.; Warren,
H. B. J . Am. Chem. Soc. 1991, 113, 7082.
(16) See also Mertz, K. M., J r.; Scott, L. T. J . Chem. Soc., Chem.
Commun. 1993, 412 and references cited.
(17) (a) Dewar, M. J . S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J . J .
P. J . Am. Chem. Soc. 1985, 107, 3902. (b) Stewart, J . J . P. QCPE No.
504, Bloomington, IN, 1990.
(18) For the C-H carbene insertion of 1-ethylideneanthracene to
9e
aceanthrylene, ∆H‡(AM1) ) 14.1 kcal mol-1
.

Notes
J . Org. Chem., Vol. 62, No. 23, 1997 8249
H) ) 89.8 kcal mol-1, and ∆H‡(5f11, 1,2-C) ) 84.6 kcal
mol-1]. These theoretical results suggest that the con-
version of 5 to either 10 [∆H‡(5f10, 1,2-H)] or 11
[∆H‡(5f11, 1,2-C)] is considerably more favorable than
the back-conversion of 5 to 9 [∆H‡(5f9), retro-carbene
C-H insertion, Scheme 2]. In addition, the AM1 ∆H‡
values for the rearrangement of 5 to 6 by either a
consecutive 1,2-C/1,2-H shift process or vice versa
[1,2-C/1,2-H, ∆H‡(5f11, 1,2-C) ) 84.6 kcal mol-1 and
1,8-Bis(1-ch lor oeth en yl)a n th r a cen e (2). A mixture of 1
(1.10 g, 4.2 mmol) and PCl5 (2.19 g, 10.5 mmol) in CH2Cl2 (100
mL) was heated to reflux for 5 h. After cooling to room
temperature, water (100 mL) was added. After standard workup
and purification by preparative column chromatography (silica,
eluent chloroform), 2 was isolated as a yellow solid. Yield of 2:
0.51 g (1.7 mmol, 40%). Mp: 108-109 °C. 1H NMR: δ 9.10 (s,
1H), 8.48 (s, 1H), 8.02 (d, 3J (H,H) ) 8.6, 2H), 7.57 (d, 3J (H,H) )
3
3
7.2 Hz, 2H), 7.45 (dd, J (H,H) ) 7.2 Hz, J (H,H) ) 8.6 Hz, 2H),
2
2
5.94 (d, J (H,H) ) 1.1 Hz, 2H), 5.69 (d, J (H,H) ) 1.1 Hz, 2H).
13C NMR: δ 138.5, 137.1, 131.2, 129.7, 128.6, 127.4, 126.7, 124.9,
122.8, and 118.0 ppm. GC/MS: m/z (%) 298 (50) [M•+ with
isotope pattern], 263 (40) [(M - Cl)+ with isotope pattern].
Elemental analysis calcd for C18H12Cl2: C, 72.26; H, 4.04.
Found: C, 72.08; H, 4.07.
∆H‡(11f6, 1,2-H)
) ; 1,2-H/1,2-C,
11.2 kcal mol-1
∆H‡(5f10, 1,2-H) ) 89.8 kcal mol-1 and ∆H‡(10f6, 1,2-
C) ) 23.9 kcal mol-1] indicate that the 1,2-C/1,2-H shift
process is preferred over the 1,2-H/1,2-C shift process,
i.e. 5 presumably rearranges to 6 via carbene 11. Fur-
thermore in line with our experimental observations, the
AM1 ∆H‡ values for the rearrangement of 5 to 6 are
smaller than those for the conversion of 6 into 7
[1,2-C/1,2-H, ∆H‡(6f13, 1,2-C) ) 105.7 kcal mol-1 and
1
1,8-Dieth yn yla n th r a cen e (3) H NMR: δ 9.44 (s, 1H), 8.46
(s, 1H), 8.03 (d, 3J (H,H) ) 8.6 Hz, 2H), 7.79 (d, 3J (H,H) ) 6.9
Hz, 2H), 7.45 (dd, 3J (H,H) ) 8.6 Hz, 3J (H,H) ) 6.9 Hz, 2H), 3.62
(s, 2H). 13C NMR: δ 131.6, 131.5, 131.4, 129.5, 127.5, 125.0,
123.8, 120.4, 82.6 and 81.7 ppm. GC/MS: m/z (%) 226 (100)
[M•+]. The spectrocopic results are in agreement with available
literature data.12
∆H‡(13f7, 1,2-H)
) ; 1,2-H/1,2-C,
12.8 kcal mol-1
∆H‡(6f12, 1,2-H) ) 87.0 kcal mol-1 and ∆H‡(12f7, 1,2-
C) ) 37.0 kcal mol-1]. This is in agreement with our
temperature conversion data; 7 is obtained at the expense
of 6 at T > 1000 °C (Table 1).8c,9b,19
10-Eth yn yla cea n th r ylen e (4).14 Compound 4 was isolated
from the 800 °C pyrolysate (50 mg) by column chromatography
(silica, eluent n-hexane). Mp: 82-84 °C. 1H NMR: δ 8.51 (d,
3J (H,H) ) 5.3 Hz, 1H), 8.48 (s, 1H), 8.11 (d, 3J (H,H) ) 8.7 Hz,
3
3
1H), 7.96 (d, J (H,H) ) 8.4 Hz, 1H), 7.86 (d, J (H,H) ) 7.0 Hz,
In summary, under FVT conditions, in situ generated
1,8-diethynylanthracene (3) is consecutively converted
into 10-ethynylaceanthrylene (4) and benz[mno]acean-
thrylene (5). Under the high-temperature conditions in
the gas phase, 5 instantaneously rearranges into cyclo-
penta[cd]pyrene (6).1,2 Hence, a novel entry to the
abundant combustion effluent 6 from a previously un-
expected C18H10 PAH precursor, viz. 3, involving consecu-
tive carbene C-H insertion reactions followed by isomer-
ization (“annealing”) processes has been disclosed. These
results contribute to the understanding of the formation
processes responsible for the ubiquitous formation of a
specific (CP)-PAH, such as 6, during combustion.
3
3
1H), 7.80 (d, J (H,H) ) 6.6 Hz, 1H), 7.59 (dd, J (H,H) ) 8.4 Hz,
3J (H,H) ) 6.6 Hz, 1H), 7.38 (dd, 3J (H,H) ) 8.7 Hz, 3J (H,H) )
7.0 Hz, 1H), 7.09 (d, 3J (H,H) ) 5.3 Hz, 1H), 3.58 (s, 1H). GC/
MS: m/z (%) 226 (100) [M•+]. HRMS calcd for C18H10 226.0783,
found 226.0780. It is noteworthy that upon standing at room
temperature solutions of 4 appear to undergo dimerization (1H
NMR: characteristic multiplet at δ 4.22 ppm).21 Consequently,
the 13C NMR spectrum of 4 had to be obtained from 13C NMR
spectrum of the 900 °C pyrolysate (22 mg) by comparison with
the 13C NMR data of pure 3 and 6 (See Table 1 and Supporting
Information). 13C NMR δ 139.9, 135.5, 134.5, 132.2, 131.3, 130.8,
129.5, 128.9, 127.8, 127.7, 126.7, 126.3, 126.1, 126.0, 123.7, 118.7,
84.9 and 82.0 ppm.
3
Cyclop en ta [cd ]p yr en e (6). 1H NMR: δ 8.43 (d, J (H,H) )
7.7 Hz, 1H), 8.40 (s, 1H), 8.30 (d, 3J (H,H) ) 7.6 Hz, 1H) 8.11
(m, 3H), 8.03 (m, 2H), 7.43 (d, 3J (H,H) ) 5.1 Hz, 1H), 7.25 (d,
3J (H,H) ) 5.3 Hz, 1H). 13C NMR: δ 138.9, 135.3, 133.3, 131.7,
130.6, 130.3, 130.0, 128.3, 127.5, 127.1, 126.7, 126.6, 126.4, 126.2,
124.1, 122.4, 122.0 and 120.5 ppm. The spectrocopic results are
in agreement with available literature data.1
Exp er im en ta l Section
Gen er a l P r oced u r es. All reactions were carried out under
a N2 atmosphere. Column chromatography was performed on
Merck Kiesegel 60 silica (230-400 ASTM). Melting points are
uncorrected. 1H (300.13 MHz) and 13C (75.47 MHz) NMR
spectra were recorded in CDCl3 with TMS as internal standard.
Ca u tion : Many polycyclic aromatic hydrocarbons are poten-
tial mutagens and carcinogens. Hence, they should be handled
with care.
3
Ben zo[gh i]flu or a n th en e (7). 1H NMR: δ 8.14 (d, J (H,H)
3
) 7.0 Hz, 2H), 7.95 (m, 6H), 7.70 (t, J (H,H) ) 7.0 Hz, 2H). 13C
NMR: δ 137.4, 133.2, 128.3, 127.7, 126.7, 126.6, 126.4, 125.0
and 123.4 ppm. The spectrocopic results are in agreement with
available literature data.11
Gen er a l F la sh Va cu u m Th er m olysis P r oced u r e. A com-
mercial Thermolyne 21100 tube furnace containing an unpacked
quartz tube (length 40 cm and diameter 2.5 cm) was used in all
FVT experiments. The temperature conversion data was de-
termined by evaporating aliquots (0.05 g h-1) of 2 into the quartz
tube at a pressure of 10-2 Torr (Table 1). The product composi-
1,8-d ia cetyla n th r a cen e (1).20 To a cooled suspension (0 °C,
ice bath) of fresh AlCl3 (5.29 g, 40 mmol) in CH2Cl2 (30 mL) was
added 2.94 g (38 mmol) acetyl chloride. After stirring until a
clear solution was obtained, powdered anthracene (2.67 g, 15
mmol) was added in small portions at room temperature. After
stirring overnight, the reaction mixture was cooled to 0 °C (ice
bath) and hydrolyzed with 0.5 M HCl [30 mL (0 °C, ice bath)].
After standard work up a mixture of 1,5-di- and 1,8-diacetylan-
thracene (1) (ratio 1:1) was obtained (yield 3.2 g, 12 mmol, 80%).
The 1,8-diacetyl isomer was isolated and purified using prepara-
tive column chromatography (silica, eluent chloroform). Yield
of 1: 1.12 g (4 mmol, 27%). Mp: 174-176 °C (lit.20 mp: 178
1
tion of the pyrolysates was determined using H NMR integra-
tion ratios, capillary GC, and HPLC.
AM1 Ca lcu la tion s. AM1 geometry optimization (MOPAC
6.0) was executed without imposing symmetry constraints until
GNORM ) 0.5.17 Transition states (TS) were located using a
reaction coordinate and, subsequently, refined using the Eigen-
vector Following routine (keyword TS) until GNORM ) 0.5. All
minima and transition states were characterized by a Hessian
calculation (keywords Force and Large); either none or only one
imaginary vibration, respectively, was found. ∆Hf ° and ∆H‡
values are reported in kcal mol-1 (1 cal ) 4.184 J , Scheme 2).
3
°C). 1H NMR: δ 10.17 (s, 1H), 8.43 (s, 1H), 8.10 (d, J (H,H) )
3
3
8.5 Hz, 2H), 7.94 (d, J (H,H) ) 6.9 Hz, 2H), 7.48 (dd, J (H,H) )
6.9 Hz, 3J (H,H) ) 8.5 Hz, 2H), 2.84 (s, 6H) ppm. 13C NMR: δ
201.6, 136.5, 133.0, 131.7, 129.2, 128.8, 127.5, 124.5, 124.4 and
30.0 ppm. GC/MS: m/z (%) 262 (55) [M•+], 247 (100) [M - CH3+],
219 (80) [M - C2H3O+].
Ack n ow led gm en t . Financial support from the
Basque Government [Beca de Formacion de Investiga-
(19) For the rearrangement of 6 to 7, AM1 calculations suggest that
the 1,2-H/1,2C route is preferred (see also Scheme 2). It is noteworthy
that AM1 overestimates the activation enthalpy ∆H‡ for 1,2-H
shifts.8c,17
(20) Bassilios, H. F.; Shawky, M.; Salem, A. Y. Recl. Trav. Chim.
Pays-Bas 1963, 82, 298.
(21) For the photodimerization of a related CP-PAH, viz. acean-
thrylene, see: Plummer, B. F.; Singleton, S. F. Tetrahedron Lett. 1987,
28, 4801 and references cited.

8250 J . Org. Chem., Vol. 62, No. 23, 1997
Notes
related transition states (TS) presented in Scheme 2 (64
pages). This material is contained in libraries on microfiche,
immediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
dores (M.S.)] and the assistence of A.C. van der Kerk-
van Hoof (HRMS) are gratefully acknowledged.
Su p p or tin g In for m a tion Ava ila ble: 1H and 13C NMR
spectra of compounds 1-4, 6, 7 and the 900 °C pyrolysate of
2, as well as AM1 Archive files for all minima (3-13) and
J O970654Z