Biradical intermediates in the photoisomerization of dibenzodihydropentalenofurans to dibenzosemibullvalenes

Article abstract of DOI:10.1021/jo0101353

The photoisomerization of a few substituted dibenzodihydropentalenofurans to the corresponding dibenzosemibullvalenes is reported. Steady-state photolysis of the dibenzodihydropentalenofurans 3a-d gave the corresponding dibenzosemibullvalenes 2a-d in nearly quantitative yields. The quantum yields of this photoisomerization were found to be in the range 0.17-0.26. Laser flash photolysis studies of the dibenzodihydropentalenofurans 3a-e showed transients, with absorption maxima around 410 nm and decaying by first-order kinetics. The lifetimes were in the range 14-30 μs in degassed benzene at 25 ^°C. These transients were readily quenched (trapped) by molecular oxygen, and the Stern-Voelmer quenching constants were found to be in the range (2.45-3.17) × 10⁹ M^-1 s^-1. As a representative example, the 1,3-biradical intermediate from 3e was trapped by molecular oxygen to give the corresponding endoperoxide 11e. The transients were weakly quenched by triplet/radical quenchers such as 2,2,6,6-tetramethylpiperinyl-1-oxy (TEMPO) and 4-hydroxy-2,2,6,6-tetramethylpiperinyl-1-oxy (HTEMPO), and the quenching constants are found to be in the range (1.09-3.19) × 10⁶ M^-1 s^-1. The decay rates of the transients were found to be temperature dependent and obeyed the Arrhenius equation. For example, the activation energy of the transient from 3a was ～4.5 kcal mol^-1 and the Arrhenius preexponential factor log(A/s^-1) for the decay of the transients was ～7.5. On the basis of our studies, these transients were assigned as the ground-state triplet biradicals, generated by the cleavage of the C-O bond of the starting dibenzodi-hydropentalenofurans.

Full text of DOI:10.1021/jo0101353

3182
J . Org. Chem. 2001, 66, 3182-3187
Bir a d ica l In ter m ed ia tes in th e P h otoisom er iza tion of
Diben zod ih yd r op en ta len ofu r a n s to Diben zosem ibu llva len es1
Meledathu C. Sajimon,2a Danaboyina Ramaiah,*,2a Kakkudiyil G. Thomas,2a,b and
Manapurathu V. George*,2a,b,c
Photochemistry Research Unit, Regional Research Laboratory (CSIR), Trivandrum 695 019, India,
Radiation Laboratory, University of Notre Dame, Indiana 46556, and J awaharlal Nehru Center for
Advanced Scientific Research, Bangalore 560 064, India
mvgeorge@rediffmail.com
Received February 5, 2001
The photoisomerization of a few substituted dibenzodihydropentalenofurans to the corresponding
dibenzosemibullvalenes is reported. Steady-state photolysis of the dibenzodihydropentalenofurans
3a -d gave the corresponding dibenzosemibullvalenes 2a -d in nearly quantitative yields. The
quantum yields of this photoisomerization were found to be in the range 0.17-0.26. Laser flash
photolysis studies of the dibenzodihydropentalenofurans 3a -e showed transients, with absorption
maxima around 410 nm and decaying by first-order kinetics. The lifetimes were in the range 14-
30 µs in degassed benzene at 25 °C. These transients were readily quenched (trapped) by molecular
oxygen, and the Stern-Vo¨lmer quenching constants were found to be in the range (2.45-3.17) ×
109 M-1 s-1. As a representative example, the 1,3-biradical intermediate from 3e was trapped by
molecular oxygen to give the corresponding endoperoxide 11e. The transients were weakly quenched
by triplet/radical quenchers such as 2,2,6,6-tetramethylpiperinyl-1-oxy (TEMPO) and 4-hydroxy-
2,2,6,6-tetramethylpiperinyl-1-oxy (HTEMPO), and the quenching constants are found to be in the
range (1.09-3.19) × 106 M-1 s-1. The decay rates of the transients were found to be temperature
dependent and obeyed the Arrhenius equation. For example, the activation energy of the transient
from 3a was ∼4.5 kcal mol-1 and the Arrhenius preexponential factor log(A/s-1) for the decay of
the transients was ∼7.5. On the basis of our studies, these transients were assigned as the ground-
state triplet biradicals, generated by the cleavage of the C-O bond of the starting dibenzodi-
hydropentalenofurans.
Sch em e 1
In tr od u ction
Several aspects of the phototransformations of dibenzo-
barrelenes are reported in the literature.3 Depending on
the reaction conditions and the substituents, dibenzo-
barrelenes undergo photorearrangement, leading prima-
rily to the corresponding dibenzosemibullvalenes and/or
dibenzocyclooctatetraenes. It has been suggested that the
dibenzocyclooctatetraenes are formed through a singlet-
state pathway, whereas the dibenzosemibullvalenes arise
through triplet state mediated pathways. Recently, we
had reported4a,b the photoisomerization of a few dibenzo-
barrelene derivatives, containing dibenzoylalkene moiety
(1) Contribution No. RRLT-PRU-128 from the Regional Research
Laboratory (CSIR), Trivandrum, India, and NDRL-4246 from Notre
Dame Radiation Laboratory.
(2) (a) Regional Research Laboratory. (b) Radiation Laboratory,
University of Notre Dame. (c) J awaharlal Nehru Center for Advanced
Scientific Research.
(3) (a) Zimmerman, H. E.; Armesto, D. Chem Rev. 1996, 96, 3065-
3112. (b) Zimmerman, H. E. In Organic Photochemistry; Padwa, A.,
Ed.; Marcell Dekker: New York, 1991; Vol. 11, pp 1-36. (c) 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. (d) 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) Sajimon, M. C.; Ramaiah, D.; Muneer, M.; Ajithkumar, E.
S.; Rath, N. P.; George, M. V. J . Org. Chem. 1999, 64, 6347-6352. (b)
Sajimon, M. C.; Ramaiah, D.; Muneer, M.; Rath, N. P.; George, M. V.
J . Photochem. Photobiol. A Chem. 2000, 136, 209-218. (c) Pratapan,
S.; Ashok, K.; Gopidas, K. R.; Rath, N. P.; Das, P. K.; George, M. V. J .
Org. Chem. 1990, 55, 1304-1308.
(Scheme 1). Thus, for example, irradiation of 1a -d led
to the formation of regioselective dibenzosemibullvalenes
2a -d in quantitative yields. However, irradiation of 1e
under similar conditions gave the corresponding dibenzo-
dihydropentalenofuran derivative 3e.4c Interestingly, the
thermal isomerization of the dibenzosemibullvalenes
2a -d gave the corresponding dibenzodihydropentaleno-
furans 3a -d in quantitative yields.4a,b This thermal
isomerization of dibenzosemibullvalenes is analogous to
10.1021/jo0101353 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/23/2001

Photoisomerization of Dibenzodihydropentalenofurans
J . Org. Chem., Vol. 66, No. 9, 2001 3183
the literature.7 The chemistry and kinetic behavior of
photochemically generated biradicals have been areas of
increasing interest in recent years.8 Several classes of
biradical intermediates have been generated from a
variety of precursors.9 For example, the Norrish Type I
and Type II cleavages of ketones,10 photoinduced nitrogen
elimination from azoalkanes,11 and the photocleavage of
strained ring compounds12 have been extensively used
to generate biradicals, and their photochemistry has also
been reviewed in the literature.13 Also, much attention
has been focused recently on understanding the multi-
plicity of these biradicals generated by different meth-
ods.14
Sch em e 2
Although the multiplicity of a biradical is controlled
by the relative spin of the two electrons, which in turn
is found to depend on several factors such as the through-
space interaction of the orbitals containing the unpaired
electrons and the electronic effects of the substituents.14,15
For example, in biradicals if there is an effective through-
space interaction of the orbitals then a large HOMO-
LUMO gap is generated. This large HOMO-LUMO gap
can ultimately lead to a singlet state for the biradical.
Similar singlet-state preferences have also been observed
on substituting highly electronegative atoms between the
radical centers, as in the case of cyclopentane-1,3-diyl
biradicals.14,16 If, on the other hand, the HOMO-LUMO
levels are nearly degenerate, then the resultant biradical
will exist in the triplet state.7b
the vinylcyclopropane-cyclopentene rearrangement, which
is known to occur thermally as well as photochemically.5
In the present study, we report the novel photochemi-
cal isomerization of the dibenzodihydropentalenofurans
3a -e to the corresponding dibenzosemibullvalenes 2a -e
(Scheme 1). Earlier studies6 on the phototransformation
of a few alkyl and phenyl substituted dihydrofuran
derivatives such as 4 and 7 showed the formation of the
corresponding regioisomeric mixtures of cyclopropyl
ketones (5 and 6) and cyclopropyl aldehydes (8 and 9),
respectively (Scheme 2), and these reactions are similar
to the present photoisomerization. The basic difference
between these substituted dihydrofurans and the di-
benzodihydropentalenofurans is that, in the latter case,
the yield of the photoisomerization products were found
to be quantitative and only one isomer is formed due to
the constraints of the polycyclic ring system. The present
study provides a potential example of a photochromic
system, in which both the thermal and the reversible
photoisomerization reactions are very efficient (Scheme
1).
The lifetime of a ground-state triplet biradical is
governed by the intersystem crossing (ISC) efficiency to
the singlet state. Spin-orbit coupling7a,17 and hyperfine
(7) (a) Salem, L.; Rowland, C. Angew. Chem., Int. Ed. Engl. 1972,
11, 92-111. (b) Borden, W. T. Diradicals. In Encyclopedia of Compu-
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Doubleday, C., J r.; Turro, N. J .; Wang, J . F. Acc. Chem. Res. 1989, 22,
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Chem. Soc. 1996, 118, 4469-4479. (d) Pedersen, S.; Herek, J . L.;
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Another aspect of the present investigation deals with
the study of ground-state triplet biradicals, involved in
these photoisomerization reactions. Biradicals have been
proposed as reaction intermediates in many thermal and
photochemical reactions. Various aspects of both homo-
and heterosymmetric biradicals are well documented in
(14) (a) Zimmerman, H. E.; Armesto, D.; Amezua, M. G.; Gannett,
T. P.; J ohnson, R. P. J . Am. Chem. Soc. 1979, 101, 6367-6383. (b)
Zimmerman, H. E.; Factor, R. E. Tetrahedron 1981, 37, Supplement
1, 125-141. (c) Zimmerman, H. E.; Penn, J . H.; J ohnson, M. R. Proc.
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Acc. Chem. Res. 1982, 10, 312-317.
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H.; Dougherty, D. A. J . Am. Chem. Soc. 1983, 105, 284-290.
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felder, T.; Heubes, M.; Hrovat, D. A.; Kita, F.; Lewis, S. B.; Scheutzow,
D.; Wirz, J . J . Am. Chem. Soc. 1998, 120, 593-594. (b) Abe, M.; Adam,
W.; Nau, W. M. J . Am. Chem. Soc. 1998, 120, 11304-11310. (c) Getty,
S. J .; Hrovat, D. A.; Borden, W. T. J . Am. Chem. Soc. 1994, 116, 1521-
1527.
(17) (a) Langoff, S. R. In Modern Theoretical Chemistry; Schaefer,
H. F., III, Ed.; Plenum: New York, 1977; Vol. 4. (b) Michl, J . J . Am.
Chem. Soc. 1996, 118, 3568-3579. (c) Havlas, Z.; Downing, J . D.; Michl,
J . J . Phys. Chem. A 1998, 102, 5681-5692.
(5) (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 Cyclopropyl Group;
Rappoport, Z., Ed.; Wiley: Chichester, 1985; Part 2, pp 1027-1082.
(d) Nendel, M.; Sperling, D.; Weist, O.; Houk, K. N. J . Org. Chem. 2000,
65, 3259-3268.
(6) (a) McGreer, D. E.; Vinje, M. G.; McDaniel, R. S. Can. J . Chem.
1965, 43, 1417-1418. (b) Wiemann, J .; Thoai, N.; Weisbuch, F.
Tetrahedron Lett. 1965, 2983-2986. (c) Scribe, P.; Monot, M. R.;
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C.; Weimann, J . Tetrahedron Lett. 1970, 4375-4378.

3184 J . Org. Chem., Vol. 66, No. 9, 2001
Sajimon et al.
coupling18 are primarily responsible for the ISC in triplet
biradicals. Also, it has been observed that the lifetimes
of triplet biradicals increase with electron withdrawing
substituents and decrease with electron donating sub-
stituents, when attached to the radical centers.19 The
triplet ground state of a diradical can be readily inferred
by the temperature dependence of its lifetime as well as
the Arrhenius factors, in addition to other spectroscopic
and quenching studies.19 In the present investigation we
have carried out the steady-state photolysis of dibenzodi-
hydropentalenofurans 3a -e, and also have examined the
nature of the transients involved in these rearrange-
ments through nanosecond laser flash photolysis studies.
Resu lts
The dibenzodihydropentalenofuran derivatives 3a -d
were prepared by the thermal isomerization of the
corresponding semibullvalenes 2a -d either under neat
heating or by refluxing in a suitable solvent, as reported
previously (Scheme 1).4a,b However, 3e was prepared by
the steady-state photolysis of the corresponding dibenzo-
barrelene 1e, at ambient temperature.4c All the dibenzo-
dihydropentalenofuran derivatives exhibited character-
istic absorption maxima around 330 nm. Irradiation of
the dibenzodihydropentalenofuran derivatives 3a -d us-
ing a RPR 350 nm light source gave the corresponding
dibenzosemibullvalene derivatives 2a -d , in nearly quan-
titative yields (Scheme 1). The photoisomerization of 3e
to 2e was monitored at low temperature (∼15 °C), using
1H NMR (300 MHz) spectroscopy since 3e was found to
revert back to 2e, at ambient temperature. Interestingly,
the irradiation of 3e at ambient temperature in benzene
saturated with molecular oxygen led to the trapped
endoperoxide 11e in good yield. The absorption maxima
(around 255 nm) of dibenzosemibullvalenes are generally
blue shifted in comparison to the corresponding dibenzo-
dihydropentalenofurans.
These photoisomerization reactions were monitored by
both UV and 1H NMR spectroscopy and also by thin-layer
chromatography, thereby excluding the possibility of
other photoproducts in these cases. The spectral profile,
as a function of irradiation time, for the isomerizaton of
3a to 2a , for example, is shown in Figure 1. Figure 1
reveals an isosbestic point around 280 nm, indicating the
presence of only two species. The inset of Figure 1 shows
the absorption spectrum of the photoproduct, dibenzo-
semibullvalene 2a, indicating that the absorption maxima
of the photoproduct and the starting material are well
separated. The quantum yields for the photoisomeriza-
tion of 3a -d to the corresponding semibullvalenes 2a -d
were found to be in the range of 0.17-0.26 (Table 1).
Nanosecond laser flash photolysis studies revealed that
the transient absorption spectra of the dibenzodihydro-
pentalenofurans 3a -e in benzene show absorption
maxima around 410 nm. Figure 2 shows the transient
absorption spectra of 3a , as a representative example.
The intrinsic lifetimes of the transients in benzene at 25
°C were found to be in the range 14-30 µs and were
sensitive to the presence of oxygen. The Stern-Vo¨lmer
oxygen quenching constants for the biradicals derived
F igu r e 1. Irradiation profile of 3a , recorded at each 30 s
interval (irradiated with 350 nm monochromatic light source).
The absorption at 335 nm (characteristic of 3a ) decreases with
increase in irradiation time. Inset shows the UV absorption
spectrum of 2a .
from 3a -e were in the range 2.45-3.17 × 109 M-1 s-1
(Table 1). Both TEMPO (kq ) 1.21 × 106 M-1 s-1 in a
representative case, 3a ) and HTEMPO weakly quenched
the transients, and the rate constants were found to be
in the range 1.09-3.19 × 106 M-1 s-1
.
The decay rates of the transients showed significant
temperature dependence. The temperature effect on
decay rate of the transient from 3a was studied in
degassed toluene in the range of 30-70 °C (measured at
each 10 °C increase). The decay rate was found to
increase with temperature in an Arrhenius manner, and
the Arrhenius parameters were calculated by linear
regression method using standard equations. The activa-
tion energy for the transient from 3a was found to be
nearly 4.35 ( 0.4 kcal mol-1 and the Arrhenius preex-
ponential factor (log A/s-1) was 7.34 ( 0.3. To examine
the effect of the initial concentration on decay rate, the
rates were monitored for solutions of different optical
densities. It was found that the decay rate is independent
of the initial concentration of the dibenzodihydropenta-
lenofuran. An attempt was made to record the phos-
phorescence spectrum of 3a , but it did not show any
significant phosphorescence at 77 K in toluene-methyl-
cyclohexane glass.20
We assign the phototransients of dibenzodihydro-
pentalenofurans (3a -e) as triplet biradicals on the basis
of the following observations: (a) The biradical interme-
diate from 3e was trapped by molecular oxygen to give
the corresponding endoperoxide derivative 11e. (b) The
transients were quenched (trapped) by oxygen with rate
constants in the range 2.45-3.17 × 109 M-1 s-1. (c) Both
TEMPO and HTEMPO quenched the transients at higher
concentrations with rate constants in the range 1.09-
3.19 × 106 M-1 s-1. (d) The decay rates of these transients
are independent of the initial concentration of the di-
benzodihydrodpentalenofurans. (e) Decay rate shows
significant temperature dependence and the Arrhenius
preexponential factor (log A/s-1) is relatively small (7.5,
as in the case of 3a , for example).
(18) (a) De Kanter, F. J .; Kaptein, R. J . Am. Chem. Soc. 1982, 104,
4759-4766. (b) Turro, N. J .; Kraulter, B. Acc. Chem. Res. 1980, 13,
369-377.
(19) Kita, F.; Adam, W.; J ordan, P.; Nau, W. M.; Wirz, J . J . Am.
Chem. Soc. 1999, 121, 9265-9275.
(20) The phosphorescence measurements were carried out to observe
the excited triplet state. For photophysical aspects of similar diradical
precursors, see: Adam, W.; Fragale, G.; Klapstein, D.; Nau, W. M.;
Wirz, J . J . Am. Chem. Soc. 1995, 117, 12578-12592.

Photoisomerization of Dibenzodihydropentalenofurans
J . Org. Chem., Vol. 66, No. 9, 2001 3185
Ta ble 1. Lifetim es a n d Oxygen Qu en ch in g Con sta n ts of th e Tr a n sien ts fr om th e Diben zod ih yd r op en ta len ofu r a n s 3a -e
quenching
constant,c
k(O2),
quenching
constant,d
triplet birdical
intermediatea
k(HTEMPO),
quantum
compd
λ
max, nm
lifetime (µs)b
109 M-1 s-1
106 M-1 s-1
yielde (φ)
3a
3b
3c
3d
3e
410
410
410
420
410
28.4 ( 1.3
14.9 ( 0.60
24.5 ( 0.1.2
30.7 ( 1.5
11.7 ( 0.70
2.45 ( 0.03
3.04 ( 0.14
2.96 ( 0.10
3.17 ( 0.12
2.89 ( 0.13
1.31 ( 0.03
2.96 ( 0.19
3.19 ( 0.26
1.40 ( 0.12
1.09 ( 0.13
0.26
0.17
0.20
0.21
a
The triplet biradicals were generated from the corresponding pentalenofurans by flash photolysis with a Nd:YAG 355 nm laser.
Mean value and standard error of samples of about 20-50 individual measurements in degassed benzene at 25 °C. c Determined by
monitoring the decay rates in degassed, air-saturated, and oxygen-saturated benzene solutions and calculated using linear regression
b
d
method. Determined by adding known volumes of 0.5 M HTEMPO solution in benzene. TEMPO was also found to quench with similar
rate constants. e Quantum yields for photoisomerization of the dibenzodihydropentalenofurans, determined by using azobenzene
actinometry.35
Sch em e 3
F igu r e 2. Transient absorption spectra of 3a , in degassed
benzene at 25 °C. Inset shows the decay monitored at 430 nm.
yields were not quantitative.23 A probable pathway for
Discu ssion
the photoisomerization of dibenzodihydropentalenofurans
3a -e to the corresponding dibenzosemibullvalenes 2a -e
is shown in Scheme 3, which involves the cleavage of the
C-O bond, resulting in the formation of biradical inter-
mediates 10a -e. This mechanism is analogous to the
photoisomerization of furan derivatives to the corre-
sponding cyclopropene-3-carboxaldehydes.24 For example,
in the photoisomerization of furan, it has been suggested
that the primary photochemical reaction step involves
the cleavage of the C-O bond to generate a 1,5-biradical.
This view was reasonably substantiated by theoretical
calculations.25 In this isomerization, the ground-state σ,π
diradical, which is strongly stabilized by the pentadiene-
type resonance, is postulated as the precursor for the
ring-closed cyclopropene-3-carboxaldehyde.
The dibenzodihydropentalenofurans under study (3a -
e) can be viewed as aryl ketones, and hence, most likely
the triplet excited states of these are involved in the
photocleavage, to give triplet biradicals 10a -e. Further
transformation (ISC) of the triplet biradicals to the
corresponding carbon centered singlet 1,3-biradicals would
lead to the corresponding dibenzosemibullvalenes. Since
no transients corresponding to the triplet excited states
of the dibenzodihydropentalenofurans could be detected
The isomerization of dibenzosemibullvalenes 2a -d to
the corresponding dibenzodihydropentalenofurans 3a -d
takes place under thermal conditions and is analogous
to the reported thermal ring-enlargement of vinylcyclo-
propanes to the corresponding cyclopentenes.5 The isomer-
ization of vinylcyclopropanes to cyclopentenes is also
reported to proceed through a singlet-state-mediated
photochemical pathway.21 We have observed that the
dibenzosemibullvalenes 2a -e are photochemically stable
under our reaction conditions but thermally unstable.4
The thermal isomerization of dibenzosemibullvalenes can
proceed through an adiabatic pathway involving the
ground state potential energy surface and similar ther-
mally reversible photoisomerizations are reported in the
literature.22 The thermodynamic parameters of these
isomerizations are in support of a concerted sigmatropic
rearrangement, involving a cyclic transition state.4a,b,5
The driving force for the thermal isomerization can be
the release of the strain energy associated with the fused
cyclopropane ring system in the dibenzosemibullvalenes.
Interestingly, the dibenzodihydropentalenofurans 3a-e
are thermally stable but photochemically isomerizable to
the corresponding dibenzosemibullvalenes (2a -e). It is
interesting to compare the photochemistry of the present
system to that of 2-pyrazoline, where the photoisomer-
ization was observed to be reversible even though the
(23) Engel. P. S.; Bodager, G. A. J . Org. Chem. 1986, 51, 4792-
4796.
(24) (a) Srinivasan, R. Pure. Appl. Chem. 1968, 16, 65-74. (b)
Chaquin, D.; Furth, B.; Kossanyi, J . C. R. Acad. Sci. Paris 1974, C279,
359. (c) Kobayashi, Y.; Kumadaki, I.; Oshawa, A.; Sekine, Y. Tetrahe-
dron Lett. 1974, 2841-2844. (d) Chambers, R. D.; Lindley, A. A.;
Fielding, H. C. J . Fluorine Chem. 1978, 12, 337.
(21) Cooke, R. S. Chem. Commun. 1970, 454-455.
(22) Quinkert, G. Angew. Chem., Int. Ed. Engl. 1972, 12, 1072-
1087 and references therein.
(25) Salem, L. J . Am. Chem. Soc. 1974, 96, 3486-3501.

3186 J . Org. Chem., Vol. 66, No. 9, 2001
Sajimon et al.
in the phosphorescence studies, it is assumed that a facile
cleavage of the C-O bond occurs in the excited state to
give the ground-state triplet biradicals.
quenching rates are similar to those measured through
different techniques such as photoacoustic calorimetry
28
by Herman and Goodman (5.3 × 109 M-1 s-1
)
and
Toscano and co-workers29 using time-resolved infrared
techniques (>2 × 109 M-1 s-1). The quenching of tran-
sients by TEMPO and HTEMPO, needs special mention.
TEMPO and HTEMPO are known to quench both excited-
state triplets and also biradical intermediates.30 For
example, quenching rates of the excited-state triplets
from dibenzobarrelenes31 by TEMPO and HTEMPO are
in the range (0.24-0.43) × 109 M-1 s-1 and similar
quenching rates were also observed for ground-state
biradicals.32 However, there are also cases where similar
nitroxyl radicals quench the biradicals at much lower
rates (1.7 × 107 M-1 s-1).9,33 In the case of the triplet
biradicals derived from dibenzodihydropentalenofurans
(3a -e), the quenching rate constants by TEMPO and
HTEMPO were found to be considerably slower (1.09-
3.19 × 106 M-1 s-1). The lack of effective quenching of
the diradical intermediate 10a -e by these nitroxyl
radicals could be attributed to steric hindrance near the
radical sites.
While there are several reports for the photochemical
ring cleavage of arylcyclopropane derivatives,26 the di-
benzosemibullvalenes, 2a -d under our irradiation condi-
tions did not yield any photoproducts.27 To probe the
involvement of any phototransients in these cases, we
have carried out some preliminary studies of a few
dibenzosemibullvalenes (2c,d ), through nanosecond laser
flash photolysis technique. These compounds, on laser
excitation (λexc ) 337.1 nm) showed transients, with
absorption maxima around 340 nm, which were not
quenched by oxygen (see the Supporting Information).
These transients are tentatively assigned as the corre-
sponding singlet 1,3-biradicals,8b,16a,b generated by the
cleavage of the cyclopropyl σ-bond. The fate of the singlet
biradical could be the coupling reaction to give back the
semibullvalene derivatives, as we have not isolated any
photoproduct under steady-state irradiation conditions
of these cases.
The intrinsic lifetimes of the triplet biradicals gener-
ated from the dibenzodihydropentalenfurans (3a -e) are
in the range 14-30 µs. Adam and co-workers recently
reported19 that the lifetimes of ground-state triplet bi-
radicals are considerably enhanced by substitution of
electron-withdrawing groups at the radical center, whereas
the electron-donating substituents decreased their life-
times. On the basis of this suggestion, it is of interest to
compare the lifetimes of the triplet biradicals derived
from the disubstituted dibenzodihydropentalenofurans
3c, 3d , and 3e. For example, 3c and 3d , where acetyl
and benzoyl groups, respectively, are attached to the
radical center, exhibited lifetimes of 24.5 and 30.7 µs,
respectively. On the other hand, for the diradical inter-
mediate derived from 3e, in which the radical center is
attached to the electron donating p-methoxyphenyl group,
the lifetime was found to be 11.7 µs, which is considerably
lower in comparison to 3c and 3d . However, the ap-
preciable difference in the lifetimes of the transients
derived from monosubstiuted dibenzodihydropentaleno-
furans 3a and 3b are rather difficult to rationalize. One
possible explanation could be in terms of the electronic
effects of the substituents. For example, in the case of
3b, the electron withdrawing acetyl group can increase
the HOMO-LUMO gap via the through-bond interaction
and thereby resulting in a singlet preference for the
biradical intermediate and consequent decrease in triplet
lifetime (14.9 µs), in contrast to the biradical derived from
3a (28.4 µs).
The strong temperature dependence34,19of the decay of
the biradicals and a relatively small value of around 7.5
(characteristic of a spin forbidden process) for Arrhenius
preexponential factor (log A/s-1) obtained in the case of
3a is in the range reported for ground-state triplet 1,3-
biradicals (6-10).19 The preexponential factor for the
decay of singlet biradicals, on the other hand, falls in the
range 10-14.15a Therefore, the results of our quenching
and kinetic studies are in support of the assignment of
the transients as ground-state triplet biradicals.
Con clu sion s
In the present study, we have examined the photo-
isomerization of a few dibenzodihydropentalenofurans to
the corresponding semibullvalenes, addressing the mecha-
nistic aspects. Previously, we had reported the reverse
thermal isomerization of the dibenzosemibullvalenes.4a,b
The photoisomerization of dibenzodihydropentaleno-
furans was found to proceed through biradical intermedi-
ates generated by the cleavage of the C-O bond. Based
on quenching and kinetic studies the transients were
assigned as ground-state triplet biradicals. The lifetimes
of the biradicals were found to be influenced by substit-
uents, attached to the radical centers. In a representative
case, the biradical intermediate trapped with molecular
oxygen to give the corresponding endoperoxide.
Exp er im en ta l Section
The UV spectra were recorded on a Shimadzu UV-3010 PC
NIR scanning UV spectrophotometer. The irradiation profile
in Figure 1 was recorded with a solution of n-decane (∼5 ×
Facile oxygen quenching and the isolation of the
endoperoxide derivative 11e from the irradiation of 3e
in the presence of oxygen strongly support the interme-
diacy of the triplet biradical species in the transformation
of the dibenzodihydropentalenofurans (3a -e) to the
corresponding semibullvalenes (2a -e). The observed
oxygen quenching rate constants ((2.45-3.17) × 109 M-1
s-1) are in good agreement with the reported values for
triplet cyclopentane-1,3-diyl biradicals.19 Further, these
(28) Herman, M. S.; Goodman, J . L. J . Am. Chem. Soc. 1988, 110,
2681-2683.
(29) Showalter, B. M.; Bentz, T. C.; Ryzhkov, L. R.; Hadad, C. M.;
Toscano, J . P. J . Phys. Org. Chem. 2000, 18, 309-312.
(30) (a) Watkins, A. R. Chem. Phys. Lett. 1974, 29, 526-528. (b)
Scaiano, J . C. Chem. Phys. Lett. 1981, 79, 441-443. (c) Encinas, M.
V.; Scaiano, J . C. J . Photochem. 1979, 11, 241-247. (d) Barton, D. H.
R.; Charpiot, B.; Ingold, K. U, J ohnston, L. J .; Motherwell, W. B.;
Scaiano, J . C.; Slanforth, S. J . Am. Chem. Soc. 1985, 107, 3607-3611.
(31) Ramaiah, D.; Kumar, S. A.; Asokan, C. V.; Mathew, T.; Das,
S.; Rath, N. P.; George, M. V. J . Org. Chem. 1996, 61, 5468-5473.
(32) Scaiano, J . C.; McGimpsey, W. G.; Leigh, W. J .; J akobs. S. J .
Org. Chem. 1987, 52, 4540-4544
(26) Ichinose, N.; Mizuno, K.; Otsuji, Y.; Caldwell, R. A.; Helms, A.
H. J . Org. Chem. 1998, 63, 3176-3184 and references therein.
(27) Wavelength-dependent steady-state irradiation of the diben-
zosemibullvalenes was carried out using 254, 300, and 350 nm light
source (RPR). No formation of the corresponding dibenzodihydropen-
talenofuran derivatives was observed; only unchanged starting materi-
als are isolated in quantitative yields.
(33) Small, R. D., J r.; Scaiano, J . C. Macromolecules 1978, 11, 840-
841.
(34) Buchwalter, S. L.; Closs, G. L. J . Am. Chem. Soc. 1979, 101,
4688-4694.

Photoisomerization of Dibenzodihydropentalenofurans
J . Org. Chem., Vol. 66, No. 9, 2001 3187
10-4 M). The irradiation was carried out using Oriel optical
bench, by using a 360 nm band-pass filter, in a quartz cell (6
× 6 mm) at a distance of 30 cm from the light source. The
quantum yield measurements were carried out using azoben-
zene as the actinometer.35
Gen er a l P r oced u r e for th e Stea d y-Sta te P h otolysis of
Diben zod ih yd r op en ta len ofu r a n s. A degassed solution of
3a -d in benzene (100 mL) was irradiated in a Rayonet
photochemical reactor (RPR 350 nm) at room temperature. The
solvent was removed under vacuum, and the product thus
obtained was recrystallized from a mixture (4:1) of dichloro-
methane and acetonitrile.
3a : irradiation of 3a for 1 h gave a 94% of 2a , mp 213-214
°C (mixture mp).
3b: irradiation of 3b for 20 min gave a 96% of 2b, mp 182-
183° (mixture mp).
3c: irradiation of 3c for 40 min to give 97% of 2c, mp 181-
182 °C (mixture mp).
3d : irradiation of 3d for 40 min gave a 97% of 2d , mp 192-
193 °C (mixture mp).
Oxygen Tr a p p in g Exp er im en ts. A solution of 3e (200 mg,
0.38 mmol) in benzene (300 mL, dry) was saturated with
oxygen and irradiated using a Hanovia 450 W lamp for 1.5 h,
with continuous oxygen purging. The irradiated mixture, after
removal of the solvent under reduced pressure, was chromato-
graphed over silica gel by using chromatotron. Elution with a
mixture (1:9) of ethyl acetate and hexane gave 90 mg (45%) of
the starting dibenzodihydropentalenofuran derivative 3e, mp
168-169 °C (mixture mp), followed by 60 mg (29%) of the
endoperoxide 11e, mp 251-252 °C: IR νmax (KBr) 3075, 1695
cm-1; UV λmax (CH3OH) 257 nm (ꢀ 32, 400); 1H NMR (C6D6) δ
3.18 (3H, s), 5.17 (1 H, s), 6.59-8.31 (22H, m); 13C NMR (C6D6)
δ 54.62, 59.60, 84.38, 88.93, 105.01, 113.88, 121.14, 124.52,
125.69, 127.46, 127.70, 127.87, 128.02, 128.34, 128.75, 128.98,
130.28, 130.41, 131.23, 131.43, 132.87, 133.29, 133.54, 135.65,
141.32, 147.96, 150.87, 159.67, 191.69, 195.80; exact mol wt
calcd for C37H26O5 (M + H)+ 551.1858, found 551.1849 (high-
resolution mass spectrometry).
ing the bridgehead and/or the alkyl protons. However, in the
case of 3e, the irradiation was carried out at ∼15 °C and the
spectrum was recorded immediately.
La ser F la sh P h otolysis Stu d ies. Nanosecond laser flash
photolysis studies were performed using a Laser Photonics
PRA/model UV-24 nitrogen laser system (337.1 nm, 2 ns pulse
width, 2-4 mJ /pulse) and/or by the third harmonic laser pulse
from a Nd:YAG laser GCR-12 series, Quanta Ray (355 nm, 10
ns pulse width, ∼70 mJ /pulse). A kinetic absorption spectrom-
eter (LKS-20 Applied Photophysics) has been used to detect
the changes in optical density, after the laser excitation. A
typical experiment consisted of several replicate shots per
single measurement. The intrinsic lifetimes were measured
in benzene, at 25 °C, after purging with argon for 30 min.
Oxygen quenching constants were determined by monitoring
the decay rates of the respective substrates in air and oxygen
saturated benzene solutions. Quenching studies using HTEM-
PO were carried out by following the decay rates, after the
addition of calculated amounts of a 0.5 M solution of the
quencher in benzene. The transient absorption spectra were
recorded in degassed benzene, using a flowcell. The optical
density of the dihydrodibezopentalenofurans solutions at 355
nm was in the range 0.5-1.5.
The kinetics of the triplet decay were studied over the
temperature range of 30-70 °C, and the desired temperature
was achieved by circulating water through the cell jacket. The
temperature inside the cell was calibrated using a digital
thermometer. The activation energy and Arrhenius collision
factor were evaluated using linear regression method by
applying standard equations.
Ack n ow led gm en t. We would like to thank Prof. W.
Adam and Prof. R. W. Fessenden for their helpful
suggestions. We express our gratitude for financial
support of this work to the Office of Basic Energy
Science of the U.S. Department of Energy; the Council
of Scientific and Industrial Research and the Depart-
ment of Science and Technology, Government of India;
the Volkswagen Foundation, Germany; and the J awa-
harlal Nehru Center for Advanced Scientific Research,
Bangalore.
Anal. Calcd for C37H26O5: C, 80.73; H, 4.72. Found: C, 80.34;
H, 4.92.
The experiment was also monitored by 1H NMR spectros-
copy in C6D6, by irradiation using an Oriel optical bench under
identical conditions.
Su p p or tin g In for m a tion Ava ila ble: Transient absorp-
tion spectra of 3b-e, Stern-Vo¨lmer oxygen quenching plots
(3a -e), and HTEMPO quenching plots (3a -e), 1H, 13C, DEPT-
135 NMR spectra of 11e, Arrhenius plot for the decay of the
transient from 3a , and transient decay and transient absorp-
tion spectra of 2c. This material is available free of charge
via the Internet at http://pubs.acs.org.
1H NMR Mon itor in g of th e P h otor ea ction s of 3a -d . A
solution of 3a -d in C6D6 (10 mg in 0.5 mL) in an NMR tube
was degassed for 10 min and irradiated using an RPR 350 nm
1
light source for 20 min, and its H NMR spectrum (300 MHz)
was recorded. The photoconversion was monitored by observ-
(35) Gauglitz, G. J . Photochem. 1976, 5, 41-47.
J O0101353