Photoisomerization of bridgehead monosubstituted dibenzobarrelenes and interesting thermal isomerization of their photoproducts

Article abstract of DOI:10.1021/jo9905322

Phototransformations of a few bridgehead monosubstituted dibenzobarrelenes, 4a-d, and thermal isomerizations of their primary photoproducts, 7a and 11a-c, are described. Irradiation of 4a in benzene gave a mixture of regioisomeric products, 7a (8b-isomer)

Full text of DOI:10.1021/jo9905322

J . Org. Chem. 1999, 64, 6347-6352
6347
P h otoisom er iza tion of Br id geh ea d Mon osu bstitu ted
Diben zoba r r elen es a n d In ter estin g Th er m a l Isom er iza tion of
Th eir P h otop r od u cts1
Meledathu C. Sajimon,† Danaboyina Ramaiah,*,† Mohammed Muneer,| E. S. Ajithkumar,†
Nigam P. Rath,‡ and Manapurathu V. George*,†,§
Photochemistry Research Unit, Regional Research Laboratory (CSIR), Trivandrum 695 019, India,
Department of Chemistry, University of Missouri, St. Louis, Missouri 63121, and J awaharlal Nehru
Centre for Advanced Scientific Research, Bangalore 560 064, India
Received March 25, 1999
Phototransformations of a few bridgehead monosubstituted dibenzobarrelenes, 4a -d , and thermal
isomerizations of their primary photoproducts, 7a and 11a -c, are described. Irradiation of 4a in
benzene gave a mixture of regioisomeric products, 7a (8b-isomer) and 11a (4b-isomer), in 20 and
70% yields, respectively. In contrast, irradiation of 4b, under identical conditions, yielded only the
corresponding 4b-substituted dibenzosemibullvalene, 11b, in 95% yield. Similarly, irradiation of
the nitro- (4c) and chloro- (4d ) substituted dibenzobarrelenes gave exclusively the corresponding
dibenzosemibullvalenes, 11c (81%) and 11d (81%), respectively. The formation of regioselective
products in these systems has been attributed to the relative stabilities of the diradical intermediates
involved in these transformations. Interestingly, the dibenzosemibullvalene 7a , on refluxing in
xylene, gave the corresponding dibenzopentalenofuran 9a (94%). Thermolysis of 11a and 11c in
o-dichlorobenzene and, likewise, of 11b in xylene, yielded 94-97% of the corresponding dibenzo-
pentalenofurans 12a , 12c, and 12b, respectively. Activation energies for the thermal isomerizations
of 7a to 9a , 11a to 12a , and 11b to 12b have been found to be 21.57, 25.97, and 17.93 kcal/mol,
respectively. The structures of 11a , 11c, 11b, and 12b were established unambiguously through
X-ray crystallographic analysis, whereas the structures of the different photoproducts, 7a , 11a -d ,
12a -c and 9a , have been arrived at on the basis of spectral data and analytical results.
In tr od u ction
bullvalenes arise through triplet state mediated di-π
methane pathway.3,4 Both electronic and steric effects of
the bridgehead substituents play a major role in the
observed regioselectivity of these rearrangements.2-5
In an earlier publication,6 we had reported that the
irradiation of aryl-substituted dibenzobarralenes 1a -f,
gives exclusively the dibenzopentalenofurans 3a -f, de-
rived from the corresponding 8b-substituted semibull-
valenes 2a -f (Scheme 1). The formation of the regio-
selective semibullvalene derivatives 2a -f in these sys-
tems has been rationalized on the basis of a di-π-methane
pathway, in which the aryl group stabilizes the secondary
diradical intermediates through π conjugation, irrespec-
tive of the nature of the substituents present at the para
position of the aryl group. Further, the formation of the
dibenzopentalenofurans 3a -f, as exclusive products, was
attributed to the ease of ring opening of the corresponding
semibullvalene precursors 2a -f.
Several aspects of the phototransformations of diben-
zobarrelenes are reported in the literature.2 Depending
on the reaction conditions, dibenzobarrelenes undergo
photorearrangement, leading primarily to the corre-
sponding dibenzosemibullvalenes or dibenzocyclooctatet-
raenes. It has been established that the cyclooctatet-
raenes are formed under direct irradiation, involving a
singlet state mediated pathway, whereas the semi-
* Corresponding author. Tel.: 91-471-490-392. Fax: 91-471-490-186.
E-mail: mvg@csrrltrd.ren.nic.in.
† Regional Research Laboratory, Trivandrum.
‡ University of Missouri.
§ J awaharalal Nehru Center for Advanced Scientific Research,
Bangalore.
| Present address: Department of Chemistry, Aligarh Muslim
University, Aligarh, India.
(1) Contribution no. RRLT-PRU-108 from the Regional Research
Laboratory (CSIR), Trivandrum, India.
The objective of the present investigation has been to
study the photochemical transformations of some selected
(2) For some recent reviews on the photochemistry of dibenzobar-
relenes and the general di-π-methane rearrangement see: (a) Zim-
merman, H. E. In Rearrangements in Ground and Excited States; de
Mayo, P., Ed.; Academic Press: New York, 1980; Vol. 3, Chapter 16,
pp 131-166. (b) Zimmerman, H. E. In Organic Photochemistry; Padwa,
A, Ed.; Marcel Dekker: New York, 1991; Vol. 11, pp 1-36. (c) Scheffer,
J . R.; Pokkuluri, P. R. In Photochemistry in Organized and Constrained
Media; Ramamurthy, V., Ed.; VCH: New York, 1991; pp 185-246.
(d) de Lucchi, O.; Adam, W. In Comprehensive Organic Synthesis; Trost,
B. M., Paquette, L. A., Eds.; Pergamon Press: Oxford, 1991; Vol. 5,
pp 193-214. (e) Chen, J .; Scheffer, J . R.; Trotter, J . Tetrahedron 1992,
48, 3251-3274. (f) Zimmerman, H. E.; Sulzbach, H. M.; Tollefson, M.
B. J . Am. Chem. Soc. 1993, 115, 6548-6556. (g) Scheffer, J . R.; Yang,
J . In CRC Handbook of Organic Photochemistry and Photobiology;
Horspool, W. M., Song, P. S., Eds.; CRC Press: Boca Raton, FL, 1995;
Chapter 16, pp 204-221.
(4) (a) Zimmerman, H. E.; Grunewald, G. L. J . Am. Chem. Soc. 1966,
88, 183-184. (b) Zimmerman, H. E.; Binkley, R. W.; Givens, R. S.;
Sherwin, M. A. J . Am. Chem. Soc. 1967, 89, 3932-3933. (c) Pokkuluri,
P. R.; Scheffer, J . R.; Trotter, J . J . Am. Chem. Soc. 1990, 112, 3675-
3676. (d) Chen, J .; Pokkuluri, P. R.; Scheffer, J . R.; Trotter, J . J .
Photochem. Photobiol., A 1991, 7, 21-26. (e) For some of the similarly
substituted dicarbomethoxy esters, see; Iwamura, M.; Tukada, H.;
Iwamra, H. Tetrahedron Lett. 1980, 21, 4865-4868.
(5) (a) Kumar, C. V.; Murthy, B. A. R. C.; Lahiri, S.; Chackachery,
E.; Scaiano, J . C.; George, M. V. J . Org. Chem. 1984, 49, 4923-4929
(b) Kumar, S. A.; Ashokan, C. V.; Das, S.; Wilbur, J . A.; Rath, N. P.;
George, M. V. J . Photochem. Photobiol., A 1993, 71, 27-31. (c)
Ramaiah, D.; Kumar, S. A.; Ashokan, C. V.; Mathew, T.; Das, S.; Rath,
N. P.; George, M. V. J . Org. Chem. 1996, 61, 5468-5473.
(3) (a) Ciganek, E. J . Am. Chem. Soc. 1966, 88, 2882-2883. (b)
Rabideau, P. W.; Hamilton, J . B.; Friedmann, L. J . Am. Chem. Soc.
1968, 90, 4465-4466.
(6) Pratapan, S.; Ashok, K.; Gopidas, K. R.; Rath, N. P.; Das, P. K.;
George, M. V. J . Org. Chem. 1990, 55, 1304-1308.
10.1021/jo9905322 CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/30/1999

6348 J . Org. Chem., Vol. 64, No. 17, 1999
Sajimon et al.
Sch em e 1
dibenzoylalkene moieties were prepared through the
reaction of the appropriate anthracenes with dibenzoyl-
acetylene (DBA) by neat heating or under refluxing
conditions using a suitable solvent or in the presence of
a Lewis acid catalyst such as aluminum chloride. All the
dibenzobarrelenes 4a -d have been fully characterized
by analytical results and spectral data. For example, the
1H NMR spectrum of 4a showed a singlet at δ 5.34 that
is due to the bridgehead proton, and the 13C NMR
spectrum showed characteristic signals at δ 54.17 and
70.04, assigned to the bridgehead sp3 carbons. In addi-
tion, the latter showed two peaks at δ 194.32 and 194.41
which are due to the benzoyl carbonyl groups and a peak
at δ 203.71 which was assigned to the carbonyl of the
acetyl group.
Photolysis of 4a in benzene afforded a mixture of the
regioisomeric products 7a and 11a in 20 and 70% yields,
respectively (Scheme 2). Yields reported herein are the
isolated yields of products, after purification through
recrystallization. In contrast, irradiation of 4b in ben-
zene, gave 11b (95%) as the exclusive product. Similarly,
irradiation of 4c and 4d , under identical conditions,
yielded 81% of 11c and 81% of 11d , respectively, as the
only isolable products in each case. To ascertain the exact
composition of the various regioisomeric mixtures formed
in these photoreactions, we have carried out the photoly-
sis at ambient temperature in both CDCl3 and C6D6 and
monitored the progress of the reaction in each case by
1H NMR. Irradiation of 4a in CDCl3 for 1.6 h resulted in
a mixture of 7a (21%) and 11a (72%), along with 7% of
examples with a view to examining the role of both steric
and electronic effects of the bridgehead substituents on
the regioselectivity of these photorearrangements. Fur-
ther, it was of interest to investigate the thermal isomer-
ization of the primary photoproducts 7a and 11a -c,
formed in these reactions. The substrates that we have
examined in the present study include 9-acetyl-11,12-
dibenzoyl-9,10-dihydro-9,10-ethenoanthracene (4a), 9-tert-
butyl-11,12-dibenzoyl-9,10-dihydro-9,10-ethenoanthra-
cene (4b ), 11,12-dibenzoyl-9,10-dihydro-9-nitro-9,10-
ethenoanthracene (4c), and 9-chloro-11,12-dibenzoyl-
9,10-dihydro-9,10-ethenoanthracene (4d ). The photoprod-
ucts that we have subjected to thermal isomerization
include 8b-acetyl-8c,8d-dibenzoyl-4b,8b,8c,8d-tetrahy-
drodibenzo[a,f]cyclopropa[c,d]pentalene (7a ), 4b-acetyl-
8c,8d-dibenzoyl-4b,8b,8c,8d-tetrahydrodibenzo[a,f]cyclo-
propa[c,d]pentalene (11a ), 4b-tert-butyl-8c,8d-dibenzoyl-
4b,8b,8c,8d-tetrahydrodibenzo[a,f]cyclopropa[c,d]pen-
talene (11b), and 8c,8d-dibenzoyl-4b-nitro-4b,8b,8c,8d-
tetrahydrodibenzo[a,f]cyclopropa[c,d]pentalene (11c).
1
the unchanged starting material 4a , as analyzed by H
NMR, following the bridgehead and acetyl protons. Thus,
the 1H NMR spectrum of the irradiated (45 min) solution
of 4a in CDCl3 showed the acetyl and bridgehead protons
of 7a as singlets at δ 2.55 and 4.94, whereas, in 11a , they
appeared as singlets at δ 1.92 and 4.64. When the
irradiation of 4a was continued for another 30 min, the
product mixture consisted of only 7a and 11a in the ratio
of 1:3.5, as analyzed by the integration of their bridge-
head and acetyl protons. In contrast, irradiation of 4b
in C6D6 for 45 min showed the formation of only 9c (75%),
along with the unchanged starting material 4b (25%),
Resu lts
1. Stea d y-Sta te P h otolysis a n d P r od u ct Id en tifi-
ca tion . The starting dibenzobarrelenes containing 1,2-
Sch em e 2

Bridgehead Monosubstituted Dibenzobarrelenes
J . Org. Chem., Vol. 64, No. 17, 1999 6349
as analyzed by 1H NMR, following the bridgehead
protons. Similarly, the irradiation of 4c and 4d resulted
in the formation of the corresponding regioselective
1
products, 11c and 11d , respectively, as evidenced by H
NMR.
The structures of the dibenzosemibullvalenes 7a and
11a -d were arrived at on the basis of analytical data
and spectral evidence. The 1H NMR spectrum of 7a
showed a singlet at δ 4.94, which is characteristic of the
bridgehead 4b proton. The bridgehead 8b protons of the
regioisomers 11a -d were observed as singlets in the
range of δ 4.64-4.78. The 13C NMR spectra of 7a and
11a -d , showed the characteristic 4 peaks, in each case
in the region of δ 47.46-103.67, assigned to the sp3
carbons of the pentalene moiety. In addition, two peaks
corresponding to benzoyl carbonyl groups appeared, in
each case in the region of δ 191.33-196.85. The struc-
tures of the photoproducts 11a , 11c, and 11d were
further confirmed through X-ray crystal structure deter-
mination.7
F igu r e 1. Kinetic plot for the thermal isomerization of 11b
to 12b at 120 °C. Inset shows the increase in absorbance of
12b at 335 nm as a function of time: (a) 0; (b) 5; (c) 10; (j) 45
min.
2. Th er m a l Isom er iza t ion of Dib en zosem ib u ll-
va len es to Diben zo-p en ta len ofu r a n s. The primary
photoproducts, dibensemibullvalenes 7a and 11a -c were
found to be thermally unstable and to undergo interesting
isomerization to the corresponding dibenzopentalenofu-
ran derivatives. For example, refluxing of a solution of
7a in xylene for 30 min resulted in the formation of 9a
in a 92% yield, whereas 11a when refluxed in o-dichlo-
robenzene for 20 min, gave of 12a in 94%. Similarly, the
thermolysis of 11b in xylene and 11c in o-dichloroben-
zene, resulted in the formation of 12b (97%) and 12c
(94%), respectively. The structures of the products 12a -c
and 9a were arrived at on the basis of analytical results
Ta ble 1. Kin etic a n d Th er m od yn a m ic Da ta for th e
Th er m a l Isom er iza tion of 7a , 11a , a n d 11ba
thermal
temp rate constant
(K)
Ea
∆Sq
(eu)
isomerizationb
k (10-3 s)
(kcal mol-1
)
7a to 9a
383
393
403
428
438
448
393
398
403
1.10
2.21
4.49
1.27
2.51
4.46
1.80
2.40
3.19
21.57 ( 0.7
25.97 ( 0.6
17.93 ( 0.1
-18.50
-13.86
-27.93
11a to 12a
11b to 12b
1
and spectral evidence. The H NMR spectra of 12a , 12b,
and 12c showed the bridgehead protons at δ 6.40, 6.57,
and 6.53, respectively. In addition, they showed only one
peak, in each case, in the region of δ 199.05-197.20,
assigned to the benzoyl carbonyl group, clearly demon-
strating the participation of one of the benzoyl carbonyls
in the furan ring formation. The structure of 12b was
further confirmed unambiguously through X-ray crystal-
a
Average of more than two experiments. b The thermal isomer-
ization monitoring wavelengths (λmax) are 325 nm for 7a to 9a ,
320 nm for 11a to 12a , and 335 nm for 11b to 12b.
thermal isomerization of 11b to 12b at 120 °C, and the
inset shows the increase in absorbance of 12b at 335 nm
as a function of time. The kinetic parameters for the
isomerization of 7a to 9a , 11a to 12a , and 11b to 12b
are presented in Table 1.
1
lographic analysis.7 The H NMR spectrum of 9a , on the
other hand, showed the acetyl and bridgehead 4b protons
as singlets at δ 2.14 and 5.39, respectively. The 13C NMR
spectrum showed two peaks at δ 198.03 and 203.83,
assigned to the carbonyls of benzoyl and acetyl groups,
respectively.
3. Kin etics of Th er m a l Isom er iza tion of 7a to 9a ,
11a to 12a , a n d 11b to 12b. The kinetic studies were
carried out in either n-decane or n-tridecane by following
the change in absorbance as a function of time at
wavelengths characteristic of the appropriate end prod-
ucts. In the case of 7a for example, the isomerization was
carried out in n-decane at 110 °C by following the
increase in absorbance at 325 nm (characteristic of 9a ).
The Arrhenius parameters were evaluated by measuring
the rates at 110, 120, and 130 °C.
Discu ssion
The formation of the dibenzosemibullvalenes 7a and
11a -d in the phototransformations of 4a -d can be
rationalized in terms of the pathways shown in Scheme
2. Path a involves a [9a,12] benzo-vinyl bridging leading
to the diradical intermediates 5a and 6a , which can
subsequently be transformed to the regioisomeric 8b-
substituted product 7a . Path b involves a [4a,11] benzo-
vinyl bridging leading to the diradical intermediates
8a -d which can undergo further transformations to give
rise to the 4b-substituted products 11a -d . It is interest-
ing to note that the acetyl-substituted dibenzobarrelene
4a gives rise to both 4b-isomer 7a and 8b-isomer 11a , in
the ratio of 1:3.5, whereas the other dibenzobarrelenes
4b-d gave exclusively the 4b-isomers 11b-d .
The observed regioselectivity in these reactions could
be directly correlated to the relative stabilization of the
1,3-diradical intermediates 5a and 8a -d , formed ini-
tially, and also the subsequently formed diradical inter-
mediates 6a and 10a -d . It has been observed earlier8
that strongly electron-withdrawing substituents with
Similarly, the thermal isomerization of 11a to 12a was
carried out in n-tridecane by following the increase in
absorbance at 320 nm (characteristic of 12a ), whereas
the isomerization of 11b to 12b was carried out by
monitoring the absorbance change at 335 nm (charac-
teristic of 12b). Figure 1 shows the kinetic plot for the
(7) Full details of the crystal and molecular structures will be
published separately.

6350 J . Org. Chem., Vol. 64, No. 17, 1999
Sajimon et al.
minimal steric crowding favor Path a, leading to diradical
intermediates analogous to 5a and 6a and ultimately to
the 8b-substituted product. Similarly, aryl substituents
at the bridgehead position of the starting dibenzobar-
relenes favor the formation of the 8b-aryl-substituted
dibenzosemibullvalenes.8 In contrast, alkyl and methoxy
substituents at the bridgehead position favor Path b,
leading to 4b-substituted dibenzosemibullvalenes.9
The fact that 11a is formed as the major product in
the reaction of 4a and that both 11b and 11d are formed
as exclusive products from 4b and 4d , respectively, would
suggest that the directing effect of these substituents is
similar to those of alkyl and methoxy substituents. The
case of the nitro-substituted derivative 4c deserves
special mention. In this case, the reaction also proceeds
through Path b, leading to the product 11c. This could
be due to the effective hyperconjugative stabilization10-12
of the radical intermediate 6c by the nitro group, thereby
decreasing the probability of this radical undergoing
further transformations. The net effect of this could result
in a reaction that originates only from the isomeric
radical intermediate 10c, leading to the formation of 11c
as observed.
isomer 11a (Table 1). The activation energy for the
isomerization of 7a is lower than that of 11a . This could
be due to steric and electronic effects of the acetyl group
on the cyclopropane ring. It is equally important to note
that a very low activation barrier was observed for the
isomerization of 11b to 12b, when compared with those
of 7a and 11a . Although the tert-butyl substituent in 11b
is away from the cyclopropane ring, the observed low
activation barrier could be due to the steric interactions
of the tert-butyl group with the adjacent benzoyl group
of the cyclopropane ring. Both molecular modeling studies
and strain energy calculations12b of these systems are in
support of this explanation.
Exp er im en ta l Section
The equipment and procedure for melting point determi-
nation and spectral recordings are described in earlier
publications.5c,17 All steady-state irradiation experiments were
carried out in a Srinivasan-Griffin-Rayonet Photochemical
Reactor (RPR 300 nm) or by using a Pyrex filtered light from
a Hanovia 450 W medium-pressure mercury lamp. Solvents
for photolysis experiments were purified and distilled before
use. The petroleum ether used was the fraction with a bp of
60-80 °C.
The thermal isomerizations of the dibenzosemibull-
valenes 7a and 11a-c to the corresponding dibenzopental-
enofurans are analogues to the rearrangement of vinyl-
cyclopropanes to cyclopentenes, reported in the litera-
ture.13 These isomerizations could proceed through a
concerted pathway, involving a [1,3] sigmatropic shift or
through 1,3-diradical intermediates.14 The activation
energy13b for the rearrangement of vinylcyclopropane to
cyclopentene is 45 kcal mol-1. It is known that increased
carbon substitution, extended conjugation, and the pres-
ence of heteroatoms all lower the activation energy for
the reorganization toward cyclopentene.13-15 Our kinetic
studies reveal that the isomerizations of 7a and 11a ,b
follow first-order kinetics, as expected. The activation
energy for the thermal isomerization of 7a to 9a , for
example, was found to be 21.57 kcal mol-1, whereas the
change in entropy was -18 eu, which is typical of
sigmatropic rearrangements involving cyclic transition
states.16
Sta r tin g Ma ter ia ls. Dibenzoylacetylene (DBA),18 mp 110-
111 °C; 9-acetylanthracene,19 mp 75-76 °C 9-tert-butylan-
thracene,20 mp 104-105 °C; and 9-nitroanthracene,21 mp 145-
146 °C were prepared by reported procedures. 9-Chloroanthra-
cene, mp 100-101 °C, from Aldrich was used as received.
P r ep a r a tion of 4a . To a mixture of 9-acetylanthracene
(2.20 g, 10 mmol) and aluminum chloride (1.35 g, 10 mmol) in
dichloromethane (40 mL, dry) was added DBA (2.34 g, 10
mmol) in small amounts at 0 °C. The reaction mixture was
stirred for 1 h and poured over crushed ice, acidified with
hydrochloric acid (6.5 N, 500 mL), and extracted with dichlo-
romethane. Removal of the solvent gave a solid residue, which
was chromatographed over silica gel. Elution of the column
with a mixture (1:9) of ethyl acetate and hexane gave 900 mg
(45%) of the unchanged anthracene derivative, mp 75-76 °C
(mixture mp). Further elution with a mixture (1:4) of ethyl
acetate and hexane gave 1.80 g (40%) of 4a , mp 231-232 °C,
after recrystallization from a mixture (1:1) of dichloromethane
and methanol: IR υmax (KBr) 1727, 1664 cm-1; UV λmax (CH3-
1
CN) 211 nm (ꢀ 35 900); H NMR (CDCl3) δ 2.41 (3H, s), 5.34
(1H, s), 6.99-7.68 (18H, m); 13C NMR (CDCl3) δ 32.00, 54.17,
70.04, 123.78, 123.97, 125.57, 125.91, 128.27, 128.34, 128.68,
128.79, 133.11, 133.28, 136.66, 137.19, 142.91, 144.64, 151.24,
154.64, 194.32, 194.41, 203.71; mass spectrum m/e (relative
intensity) 454 (M+, 1), 105 (100). Anal. Calcd for C32H22O3: C,
84.56; H, 4.88. Found: C, 84.90; H, 4.89.
P r ep a r a t ion of 4b . A mixture of 9-tert-butylanthracene
(468 mg, 2 mmol) and DBA (468 mg, 2 mmol) in toluene (60
mL, dry) was refluxed for 6 h. The solvent was removed under
reduced pressure to give a solid residue, which was chromato-
graphed over silica gel. Elution of the column with a mixture
It is interesting to compare the energetics of the
thermal isomerization of the 8b-isomer 7a and the 4b-
(8) (a) Murthy, B. A. R. C.; Pratapan, S.; Kumar, C. V.; Das, P. K.;
George, M. V. J . Org. Chem. 1985, 50, 2533-2538. (b) Kumar, S. A.;
Ramaiah, D.; Eldho, N. V.; Das, S.; Rath, N. P.; George, M. V. J .
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5893.
(10) For a review on steric effects in free radical chemistry, see
Ru¨chardt, C. Top. Curr. Chem. 1980, 88, 1-32.
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6701.
(12) (a) We thank one of the reviewers for suggesting the hyper-
conjugative effect of the nitro group. (b) Our calculations using AM1
semiempirical molecular orbital method employing simplified model
systems analogous to 1,3-diradical intermediate 6a revealed the
localization of electron potential on the nitro group.
(13) (a) Wenkert, E. Acc. Chem. Res. 1980, 13, 27-31. (b) Hudlicky,
T.; Reed, J . W. In Comprehensive Organic Synthesis; Trost, B. M.;
Paquette, L. A., Eds.; Pergamon Press: Oxford, 1991; Vol. 5, pp 899-
970. (c) Carpenter, B. K. In The Chemistry of the Cyclopropyl Group,
Part 2; Rappoport, Z., Ed.; Wiley: New York, 1987; p 1027.
(14) (a) Griller, D.; Ingold, K. U. Acc. Chem. Res. 1980, 13, 317-
323. (b) Gajewski, J . J .; Olson, L. P.; Willcott, M. R. J . Am. Chem.
Soc. 1996, 118, 299-306. (c) Baldwin, J . E.; Villarica, K. A.; Freedberg,
D. I.; Anet, F. A. L. J . Am. Chem. Soc. 1994, 116, 10845-10846.
(15) (a) Marino, J . P.; Laborde, E. J . Org. Chem. 1987, 52, 1-10.
(b) Reissig, H.-U. Topp. Curr Chem. 1988, 144, 73.

Bridgehead Monosubstituted Dibenzobarrelenes
J . Org. Chem., Vol. 64, No. 17, 1999 6351
(1:9) of ethyl acetate and hexane gave 170 mg (35%) of the
unchanged anthracene derivative, mp 104-105 °C (mixture
mp). Further elution with a mixture (1:4) of ethyl acetate and
hexane yielded 540 mg (57%) of 4b, mp 238-239 ïC, after
recrystallization from a mixture (1:4) of dichloromethane and
methanol: IR υmax (KBr) 1681 cm-1; UV λmax (CH3CN) 248 nm
recrystallization from a mixture (1:4) of dichloromethane and
methanol: IR υmax (KBr) 1628 cm-1; UV λmax (CH3CN) 281 nm
(ꢀ 27 980); 1H NMR (CDCl3) δ 1.52 (9H, s), 4.65 (1H, s), 6.53-
7.77 (18H, m); 13C NMR (C6D6) δ 30.75, 37.17, 47.94, 61.51,
77.13, 81.19, 123.17, 127.70, 128.34, 130.22, 136.69, 138.42,
151.53, 152.24, 196.19, 196.85; mass spectrum m/e (relative
intensity): 468 (M+, 1), 105 (100). Anal. Calcd for C34H28O2:
C, 87.15; H, 6.02. Found: C, 86.98; H, 6.08.
P h otolysis of 4c. A solution of 4c (457 mg, 1 mmol) in
benzene (400 mL) was irradiated (RPR, 300 nm) for 2 h. The
solvent was removed under vacuum, and the residual solid
thus obtained was chromatographed over silica gel. Elution
with a mixture (1:4) of ethyl acetate and petroleum ether gave
370 mg (81%) of 11c, mp 211-212 °C, after recrystallization
from a mixture (1:4) of dichloromethane and methanol: IR υmax
(KBr) 1664 cm-1; UV λmax (CH3CN) 274 nm (ꢀ 20 800); 1H NMR
(CDCl3) δ 4.78 (1H, s), 6.85-7.92 (18H, m); 13C NMR (CDCl3)
δ 47.46, 60.82, 76.61, 103.67, 121.00, 121.15, 126.37, 126.82,
128.07, 128.40, 128.58, 128.67, 129.27, 129.68, 130.10, 133.00,
133.29, 133.38, 136.34, 136.43, 145.05, 146.03, 191.23, 193.71;
mass spectrum m/e (relative intensity) 457 (M+, 3), 105 (100).
Anal. Calcd for C30H19NO4: C, 78.76; H, 4.19; N, 3.06. Found:
C, 78.97; H, 4.27; N, 2.87.
1
(ꢀ 28 700); H NMR (CDCl3) δ 1.71 (6H, s), 1.96 (3H, s), 5.32
(1H, s), 7.07-8.89 (18H, m); 13C NMR (CDCl3) δ 32.54, 34.19,
54.19, 70.63, 124.05, 124.06, 124.82, 125.49, 127.12, 127.93,
128.10, 128.54, 129.38, 132.94, 136.80, 138.41, 144.45, 147.48,
152.88, 156.88, 194.94, 196.73; mass spectrum m/e (relative
intensity) 468 (M+, 3), 105 (100). Anal. Calcd for C34H28O2: C,
87.15; H, 6.02. Found: C, 87.18; H, 5.99.
P r ep a r a tion of 4c. An equimolar mixture of 9-nitroan-
thracene (2.23 g, 10 mmol) and DBA (2.34 g, 10 mmol) under
neat heating at 140-150 ïC for 5 h gave a product mixture,
which was chromatographed over silica gel. Elution of the
column with a mixture (1:9) of ethyl acetate and hexane gave
970 mg (44%) of the unchanged anthracene derivative, mp
145-146 °C (mixture mp). Further elution with a mixture (1:
4) of ethyl acetate and hexane gave 1.83 g (40%) of 4c, mp
180-181 ïC, after recrystallization from a mixture (1:4) of
dichloromethane and methanol: IR υmax (KBr) 1678 cm-1; UV
λ
max (CH3CN) 288 nm (ꢀ 22 000); 1H NMR (CDCl3) δ 5.48 (1H,
P h otolysis of 4d . A solution of 4d (580 mg, 1.3 mmol) in
benzene (300 mL) was irradiated (Hanovia 450 W) for 1 h.
Removal of the solvent under vacuum gave a residual solid,
which was chromatographed over silica gel. Elution with a
mixture (1:9) of ethyl acetate and petroleum ether gave 470
mg (81%) of 11d , mp 181-182 °C, after recrystallization from
a mixture (1:4) of dichloromethane and methanol: IR υmax
(KBr) 1681 cm-1; UV λmax (CH3CN) 253 nm (ꢀ 28 800); 1H NMR
(CDCl3) δ 4.70 (1H, s), 6.75-7.89 (18H, m); 13C NMR (CDCl3)
δ 48.78, 61.04, 78.39, 79.36, 120.91, 128.26, 128.32, 129.46,
130.08, 136.98, 137.26, 149.64, 150.58, 191.93, 195.06; mass
spectrum m/e (relative intensity) 446 (M+, 1), 105 (100). Anal.
Calcd for C30H19ClO2: C, 80.62; H, 4.29. Found: C, 80.62; H,
4.36.
s), 7.08-7.74 (18H, m); 13C NMR (CDCl3) δ 53.23, 98.48,
121.97, 126.23, 128.29, 128.45, 128.82, 133.41, 140.07, 142.11,
148.34, 151.35, 191.91, 193.16; mass spectrum m/e (relative
intensity) 457 (M+, 20), 105 (100). Anal. Calcd for C30H19NO4:
C, 78.76; H, 4.19; N, 3.06. Found: C, 78.99; H, 4.19; N, 2.70.
P r ep a r a tion of 4d . A mixture of 9-chloroanthracene (1.20
g, 5.6 mmol) and DBA (1.30 g, 5.6 mmol) in toluene (50 mL,
dry) was refluxed for 13 h. The solvent was removed under
reduced pressure, and the solid thus obtained was chromato-
graphed over silica gel. Elution with a mixture (1:9) of ethyl
acetate and hexane gave 310 mg (26%) of the unchanged
anthracene derivative, mp 100-101 °C (mixture mp). Further
elution with a mixture (1:4) of ethyl acetate and hexane gave
ï
1.7 g (68%) of 4d , mp 179-180 C, after recrystallization from
1H NMR Mon itor in g of th e P h otor ea ction s of 4a -d . A
solution of 4a in CDCl3 (10 mg in 0.5 mL) in an NMR tube
was irradiated with an RPR 300-nm light source for 1 h and
a mixture (1:4) of dichloromethane and methanol: IR υmax
(KBr) 1661 cm-1; UV λmax (CH3CN) 255 nm (ꢀ 33 000); 1H NMR
(CDCl3) δ 5.52 (1H, s), 7.10-7.69 (18H, m); 13C NMR (CDCl3)
δ 52.07, 73.29, 122.25, 123.44, 128.22, 128.41, 129.14, 133.14,
142.69, 143.28, 149.06, 156.02, 192.88, 193.01; mass spectrum
m/e (relative intensity) 446 (M+, 3), 212 (100), 105 (88). Anal.
Calcd for C34H28O2: C, 80.62; H, 4.29. Found: C, 80.78; H,
4.29.
1
1
40 min, and its H NMR spectrum was recorded. The H NMR
spectrum revealed that the product mixture consisted of 21%
of 7a and 72% of 11a , along with 7% of the unchanged starting
material 4a . When the irradiation was continued for another
further 30 min, the ratio of the products 7a and 11a was found
to be 1:3.5.
P h otolysis of 4a . A solution of 4a (400 mg, 0.88 mmol) in
benzene (300 mL) was irradiated (Hanovia 450 W) for 40 min.
Removal of the solvent under vacuum gave a residual solid,
which was chromatographed over silica gel. Elution of the
column with a mixture (1:4) of ethyl acetate and petroleum
ether gave (280 mg, 70%) 11a , mp 213-214 ïC, after recrys-
In a separate experiment, the irradiation of the dibenzobar-
relene 4b in C6D6, for 45 min, under identical conditions,
resulted in the formation of 11b (75%), along with the
1
unchanged starting material 4b (25%), as analyzed by the H
NMR. Similarly, the irradiation of 4c in C6D6 for 8 h gave 50%
of 9c, along with 50% of the unchanged starting material,
whereas the irradiation of 4d in CDCl3 for 75 min, under
identical conditions, resulted in the formation of 11d (63%),
along with some unchanged starting material 4d (37%).
Th er m a l Isom er iza tion of 7a . A solution of 7a (100 mg,
0.22 mmol) in dry toluene (30 mL) was refluxed for 30 min.
The solvent was removed under vacuum, and the residual solid
was recrystallized from a mixture (1:4) of dichloromethane and
methanol to give 94 mg (94%) of 9a , mp 237-238 °C: IR υmax
(KBr) 1724, 1661 cm-1; UV λmax (CH3CN) 328 nm (ꢀ 34 500);
1H NMR (CDCl3) δ 2.14 (3H, s), 5.39 (1H, s), 7.04-7.92 (18H,
m); 13C NMR (CDCl3) δ 27.78, 55.52, 83.26, 102.75, 121.36,
124.02, 124.79, 125.84, 127.85, 128.14, 128.21, 128.27, 128.38,
128.51, 128.70, 128.81, 129.84, 130.22, 130.74, 132.45, 137.88,
141.08, 142.37, 149.41, 152.23, 198.03, 203.83. Anal. Calcd for
tallization from
a mixture (1:4) of dichloromethane and
methanol: IR υmax (KBr) 1657 cm-1; UV λmax (CH3CN) 202 nm
(ꢀ 31 700); 1H NMR (CDCl3) δ 1.92 (3H, s), 4.64 (1H, s), 6.75-
7.85 (18H, m); 13C NMR (CDCl3) δ 28.67, 47.82, 61.80, 75.87,
77.77, 121.89, 128.51, 128.57, 128.91, 130.02, 134.86, 148.04,
149.28, 194.21, 195.11, 202.34; mass spectrum m/e (relative
intensity) 454 (M+, 3), 105 (100). Anal. Calcd for C32H22O3: C,
84.56; H, 4.88. Found: C, 84.34; H, 4.87.
Further elution of the column gave 80 mg (20%) of 7a , mp
154-155 °C, after recrystallization from a mixture (1:4) of
dichloromethane and methanol: IR υmax (KBr) 1721, 1654
cm-1; UV λmax (CH3CN) 209 nm (ꢀ 32 000); 1H NMR (CDCl3) δ
2.55 (3H, s), 4.94 (1H, s), 6.82-8.05 (18H, m); 13C NMR (CDCl3)
δ 30.92, 53.41, 59.29, 67.62, 67.87, 121.95, 128.48, 128.68,
129.11, 129.71, 136.27, 150.49, 151, 70, 192.99, 195.20, 202.51.
Anal. Calcd for C32H22O3: C, 84.56; H, 4.88. Found: C, 84.37;
H, 4.91.
C
32H22O3: C, 84.56; H, 4.88. Found: C, 84.82; H, 4.92.
Th er m a l Isom er iza tion of 11a . A solution of 11a (150 mg,
0.33 mmol) in dry o-dichlorobenzene (40 mL) was refluxed
under an argon atmosphere for 20 min. The solvent was
removed under vacuum, and the residual solid thus obtained
was recrystallized from a mixture (1:4) of dichloromethane and
methanol to give 140 mg (94%) of 12a , mp 185-186 °C: IR
υmax (KBr) 1721, 1665 cm-1; UV λmax (CH3CN) 325 nm (ꢀ
P h otolysis of 4b. A solution of 4b (468 mg, 1 mmol) in
benzene (400 mL) was irradiated (RPR, 300 nm) for 50 min.
The solvent was removed under vacuum, and the residual solid
obtained was chromatographed over silica gel. Elution of the
column with a mixture (1:4) of ethyl acetate and petroleum
ether gave 445 mg (95%) of 11b, mp 182-183 °C, after
1
34 200); H NMR (CDCl3) δ 1.83 (3H, s), 6.40 (1H, s), 7.07-

6352 J . Org. Chem., Vol. 64, No. 17, 1999
Sajimon et al.
8.02 (18H, m); 13C NMR (CDCl3) δ 27.85, 52.85, 85.95, 87.80,
120.83, 124.94, 126.42, 126.52, 127.89, 128.07, 128.22, 128.43,
128.53, 128.58, 129.24, 129.85, 130.27, 130.52, 132.95, 134.46,
136.75, 141.48, 141.91, 147.76, 152.25, 199.05, 203.93. Anal.
Calcd for C32H22O3: C, 84.56; H, 4.88. Found: C, 84.30; H,
4.90.
activation parameters Ea and ∆Sq were evaluated using
standard equations (Table 1).
A degassed solution of 7a in n-decane (2 × 10-5 M) was
heated with stirring at 110 °C, and aliquots were removed after
each 5-min time interval to record the UV spectrum. The
formation of the product was observed by monitoring the
absorption change at 325 nm, which is characteristic of the
dihydropentalenofuran derivative 9a . Similarly, the experi-
ment was repeated at 120 and 130 °C, to calculate the
corresponding rate constants. The thermal isomerization of
11a (5 × 10-5 M) in n-tridecane was followed by a monitoring
of the absorption change at 320 nm (characteristic absorption
of 12a ) and a repetition of the experiment at three different
temperatures, namely, 155, 165, and 175 °C. The thermal
isomerization of 11b (5.3 × 10-5 M) in n-decane was similarly
carried out at three different temperatures (120, 125, and 130
°C), and the progress of the reaction was monitored at 335
nm (characteristic absorption of 12b). The activation param-
eters for these thermal isomerizations are summarized in
Table 1.
Th er m a l Isom er iza tion of 11b. A solution of 11b (100 mg,
0.21 mmol) in dry xylene (30 mL) was refluxed under an argon
atmosphere for 20 min. The solvent was removed under
vacuum, and the residual solid was recrystallized from a
mixture (1:4) of dichloromethane and methanol to give 97 mg
(97%) of 12b, mp 212-213 °C: IR υmax (KBr) 1627 cm-1; UV
λ
max (CH3CN) 335 nm (ꢀ 28,400); 1H NMR (CDCl3) δ 1.22 (9H,
s), 6.57 (1H, s), 7.04-7.92 (18H, m); 13C NMR (CDCl3) δ 28.79,
36.76, 84.10, 90.67, 121.67, 127.35, 127.98, 128.19, 128.69,
139.98, 140.84, 146.25, 150.47, 152.31, 203.15. Anal. Calcd for
C
34H28O2: C, 87.15; H, 6.02. Found: C, 87.11; H, 5.77.
Th er m a l Isom er iza tion of 11c. The thermal isomerization
of 11c was achieved by refluxing a solution of 11c (100 mg,
0.23 mmol) in o-dichlorobenzene (30 mL) for 20 min. The
solvent was removed under vacuum to give a residual solid,
which was recrystallized from a mixture (1:1) of chloroform
and hexane to give 94 mg (94%) of 12c, mp 285-286 °C: IR
X-r a y Str u ctu r e Deter m in a tion of 11a , 11c, 11d , a n d
12b . Good-quality crystals of 11a , 11c, 11d , and 12b were
mounted on glass fibers in random orientations and subjected
to X-ray crystallographic analysis, employing a Siemens R3
automated four circle diffractometer. Data reduction and
structure solution were achieved by SHELXTL-PLUS (VMS)
structure solution package.22 Full details on the crystal and
molecular structures will be published elsewhere.7
1
υmax (KBr) 1664 cm-1; UV λmax (CH3CN) 315 nm (22 300); H
NMR (CDCl3) δ 6.53 (1H, s), 7.26-8.34 (18H, m); 13C NMR
(CDCl3) δ 64.83, 85.14, 104.77, 121.72, 124.14, 125.46, 126.55,
126.97, 127.19, 127.95, 128.16, 128.44, 128.67, 128.81, 129.88,
128.79, 130.65, 131.27, 132.79, 136.99, 138.87, 142.92, 152.46,
160.07, 197.20. Anal. Calcd for C30H19NO4: C, 78.76; H, 4.19;
N, 3.06. Found: C, 78.87; H, 4.27; N, 2.68.
Ack n ow led gm en t. We (M.C.S., D.R., M.M., A.K.,
and M.V.G.) thank the Council of Scientific and Indus-
trial Research, Department of Science and Technology,
Government of India; the Regional Research Laboratory,
Trivandrum; the Volkswagen Foundation, Germany
(D.R.); the University of Missouri-St. Louis (N.P.R.); and
J awarharlal Nehru Centre for Advanced Scientific
Research, Bangalore (M.V.G.) for financial support of
this work.
Kin etic Mea su r em en ts. The kinetics of thermal isomer-
ization of 7a to 9a , 11a to 12a , and 11b to 12b was studied
by UV spectroscopy, using n-decane or n-tridecane as solvent.
The required temperature was maintained using an oil bath,
and aliquots of the reaction mixture were removed at desired
time intervals. The reaction, in each case, was quenched by
dipping it in ice-salt mixture. In all cases, ln[A0/(A0 - A)] was
plotted against time to give a straight line, and the rate
constant, k, was determined from the slope. The experiment
was repeated for at least three different temperatures to
determine the corresponding rate constants. The values of ln
k were plotted against 1/T (K) to give a straight line, and the
Su p p or tin g In for m a tion Ava ila ble: X-ray ORTEP dia-
grams of 11a , 11c, and 12b copies of 1H and 13C NMR spectra
of 7a , 9a , 11a -d , and 12a -c; kinetic plots of thermal
isomerzations of 7a , 11a , and 11b. This material is available
free of charge via the Internet at http://pubs.acs.org.
(22) Sheldrick, G. M. Siemens Analytical X-ray Division, Madison,
WI, 1991.
J O9905322