Synthesis and photochemical transformations of a few olefin-appended 11,12-dibenzoyldibenzobarrelenes

Article abstract of DOI:10.3184/174751915X14267013391940

Several olefin-appended dibenzobarrelenes have been synthesised by Diels-Alder reaction between 9-alkenylanthracenes and dibenzoylacetylene under carefully controlled conditions and their photochemistry was examined. Olefin appendages acted as efficient quenchers of the triplet state of these barrelenes.

Full text of DOI:10.3184/174751915X14267013391940

VOL. 39 APRIL, 199–202
JOURNAL OF CHEMICAL RESEARCH 2015
RESEARCH PAPER 199
Synthesis and photochemical transformations of a few olefin-appended
11,12-dibenzoyldibenzobarrelenes
Ambily Mary Jacoba, Gisha Georgea, Jomon P. Jacoba and Eason M. Mathewb*
aDepartment of Applied Chemistry, Cochin University of Science and Technology, Cochin-682 022, Kerala, India
bResearch and Development Division, Arjuna Natural Extracts Ltd., Aluva-683101, Kerala, India
Several olefin-appended dibenzobarrelenes have been synthesised by Diels–Alder reaction between 9-alkenylanthracenes and
dibenzoylacetylene under carefully controlled conditions and their photochemistry was examined. Olefin appendages acted as efficient
quenchers of the triplet state of these barrelenes.
Keywords: Diels–Alder reaction, dibenzobarrelenes, intramolecular quenching, di-π-methane rearrangement, dibenzocyclooctatetraene,
dibenzosemibullvalene
Photochemistry of barrelene derivatives has attracted
considerable attention.1-11 Their ability to undergo singlet
mediated electrocyclic reactions and triplet mediated di-
π-methane and tri-π-methane rearrangements make them
attractive substrates for mechanistic investigations.2,12-14 Over
the past few decades, several barrelene derivatives were
synthesised and their photochemistry was examined in great
detail. Upon irradiation, barrelenes give the corresponding
cyclooctatetraenes and/or semibullvalene derivatives.15 In most
cases, both singlet mediated cyclooctatetraene generation and
triplet mediated di-π- and tri-π-methane rearranged products
are concurrently generated.15,16 This is due to competing
intersystem crossings under direct irradiation conditions and
partial absorption of light by barrelenes under triplet sensitised
irradiation conditions.
semibullvalenes (or products derived thereof) were formed
in major amounts. In the case of dibenzobarrelene 1, tri-π-
methane rearrangement leading to 1,4-diradical intermediates
2, 3 gave a mixture of dibenzopentalene 4 and cyclooctatetraene
5 derivatives (Scheme 1).13,15
Interestingly, the role of olefinic substituents on barrelene
photochemistry remains unexplored. In this context, we wished
to synthesise a few 11,12-dibenzoyldibenzobarrlenes having
olefin appendages at the 9-position. Olefins are known triplet
quenchers.22,23 Intramolecular quenching is expected to be
more efficient than intermolecular quenching. We reasoned
that efficient intramolecular quenching of the triplet excited
states by the olefin appendage will enable the emergence of
alternative reaction pathways for these barrelenes. We now
describe our efforts to divert the photochemistry of systems
that are known to undergo triplet mediated pathways to other
reaction possibilities.
The nature of substituents plays an important role in
barrelene photochemistry. George et al.15,17-20 have established
that benzoyl groups at 11,12-postions of dibenzobarrelenes
promote intersystem crossing resulting in preferential
generation of triplet mediated products even under direct
irradiation. They examined the photochemistry of several
Results and discussion
Several olefin appended dibenzobarrelenes 8a–c were
synthesised by the Diels–Alder reaction of olefin appended
anthracene derivatives 6a–c with dibenzoylacetylene (7)
(Scheme 2). 9-Alkenylanthracenes 6a–c were synthesised by
employing the Wittig reaction, Grignard reaction and Claisen–
11,12-dibenzoyldibenzobarrelenes having
a
variety of
substituents at the 9- and 10-positions 1.15,21 Depending on the
nature of the 9,10-substituents, either 4b- or 8b-substituted
H5
COC6
H5
COC6
H
C6 5OC
H
C6 5OC
H
C6 5OC
H
C
H
C
3
3
H
3C
ν
h
H
3C
H
3C
H
C
3
1
2
H5
3 COC6
H
3C
H5
COC6
H5
COC6
H
3C
H
H
C
3
C6 5OC
H
H5
3C COC6
5
4
Scheme 1
* Correspondent. E-mail: easonmmathew@gmail.com

200 JOURNAL OF CHEMICAL RESEARCH 2015
Ph
CO
Ph
Ph
CO
Ph
OC
∆
x ene
l
y
R
CO
R
7
a-c
6
a-c
8
= a -
=
H H
C C
R
)
2
- = -
CH CH C6H5
b
)
c -
=
-
H
C
-
H
C
H5
CO C6
)
Scheme 2
Schmidt reaction.Note that introduction of a vinyl substituent at
the bridgehead position of dibenzobarrelenes offers two distinct
possibilities: (1) generation of a tetra-π-methane system capable
of exhibiting interesting photochemistry; and (2) generation of
dibenzobarrelenes where intramolecular triplet quenching is a
distinct possibility.
dibenzocyclooctatetraene is observed only upon prolonged
irradiation. We conclude that all barrelene triplets are quenched
by the olefin appendage leaving the singlet excited state to react.
Since intersystem crossing is efficient here, the singlet excited
state is present only as a minor fraction and hence generation of
the singlet mediated dibenzocyclooctatetrene is at a very slow
rate.
Typically,11,12-dibenzoyldibenzobarrelenesundergoefficient
intersystem crossing leading to triplet-mediated products such
as dibenzosemibullvalenes. In a control experiment, irradiation
of 9-methyl-11,12-dibenzoyldibenzobarrelene (12) under
identical conditions (0.8 mmol in benzene at 300 nm) gave
the corresponding 4b-substituted semibullvalene (13) in near-
quantitative amounts in 30 minutes (Scheme 4).18 This control
experiment revealed that irradiation conditions applied by us
are appropriate for barrelene–semibullvalene rearrangement to
manifest itself in systems where the intramolecular quenching
possibility is nonexistent. Hence intramolecular quenching
and diversion of barrelene photochemistry is facilitated by the
olefin appendages present in them.
Photochemistry of 11,12-dibenzoyldibenzobarrelenes (8a–c)
All irradiation experiments were performed using a Rayonet
photochemical reactor employing 300 nm lamps. Irradiation
of 8a in benzene (8 mM) for 30 minutes resulted in near-
quantitative recovery of unchanged starting material. However,
prolonged irradiation (10h) of 8a in benzene (8 mM) at 300 nm
gave the dibenzocyclooctatetraene 11a as the only photoproduct.
The structural identity of the photoproduct 11a was established
on the basis of spectral and analytical data.
Similar behaviour was exhibited by dibenzobarrelenes 8b
and 8c. Irradiation of 8b and 8c in benzene (8 mM) at 300
nm resulted in the formation of the dibenzocyclooctatetraene
11b and 11c respectively. Here also there was no trace of
dibenzosemibullvalenes, indicating efficient intramolecular
quenching of the triplet excited states of the dibenzobarrelenes
by the olefin appendage at their bridgehead position. The
following mechanism is proposed for the photochemical
outcome (Scheme 3). It may be noted that formation of
Conclusion
We have examined the photochemistry of a few representative
olefin-appended dibenzobarrelenes. Olefin appendages act
1
*
H5
COC6
H5
COC6
H5
COC6
H
C6 5OC
H
H
C6 5OC
C6 5OC
ν
h
enzene
b
R
R
R
a-c
9
a-c
8
I
SC
H
R
H5
3
COC6
*
H5
COC6
H5
H5
COC6
H
C6 5OC
COC6
R
H5
COC6
R
a-c
11
a-c
10
= a -
=
H H
C C
R
)
2
-
= -
CH CH C6H5
b
)
)
-
=
-
-
d
CH CH CO C6H5
Scheme 3

JOURNAL OF CHEMICAL RESEARCH 2015 201
9-Styrylanthracene (6b):25 A mixture of 9-anthraldehyde (4.12 g,
0.02 mol) and benzyltriphenylphosphonium chloride (8.00 g, 0.02
mol) in dichloromethane (35 mL) was stirred vigorously at room
temperature for about 5 min. 50% aqueous solution of NaOH, was
then added from a dropping funnel, at the rate of one drop per 7 s.
The reaction mixture became a clear brown solution, it was stirred for
another 30 min, poured into water and extracted with dichloromethane.
The organic layer was separated, washed with water twice and dried
over anhydrous MgSO4. Solvent was removed under reduced pressure
and the pasty residue obtained was recrystallised from 1-propanol, to
obtain shining yellow plates of 6b. Yield 67%; m.p. 131 °C; IR vmax
(KBr) 3024 cm-1 (=CH), 1621 cm-1 (C=C); UV lmax (CH3CN) 270 (e
19,800), 300 (e 7,900), 350 (e 1,100), 390 (e 60,000); 1H NMR (CDCI3)
d 6.95 (1H, d, JAX = 16.56 Hz, vinylic), 7.25–8.41 (14H, m, aromatic),
7.92 (1H, d, JAX = 16.56 Hz, vinylic); 13C NMR (CDCI3) d 124.87,
125.12, 125.41, 125.98, 126.43, 126.56, 127.96, 128.65, 128.78, 129.71,
131.48, 132.72, 137.26. Anal. calcd for C22H16: C, 94.25; H, 5.75; found:
H5
COC6
H5
COC6
H5
COC6
8b
H
C6 5OC
H
C
3
ν
h
4b
enzene
b
H
C
3
94
%
12
13
Scheme 4
as efficient triplet quenchers resulting in slow but exclusive
generation of singlet mediated products.
Experimental
All melting points are uncorrected and were determined on a
Neolab melting point apparatus. All reactions and chromatographic
separations were monitored by TLC. Aluminium sheets coated with
silica (Merck) were used for TLC. Visualisation was achieved by
exposure to iodine vapour or UV radiation. Column chromatography
was carried out with slurry-packed silica (Qualigens 60–120 mesh).
All steady state irradiations were carried out using a Rayonet
Photochemical Reactor (RPR). Solvents for photolysis were purified
and distilled before use. Absorption spectra were recorded using a
Shimadzu 160A spectrometer and IR spectra were recorded using a
ABB Bomem (MB series) FTIR spectrophotometer respectively.
C, 94.47; H, 5.42%.
trans-3-(9-Anthryl)-1-phenylprop-2-en-1-one (6c):
26
A mixture
of 9-anthraldehyde (4.12 g, 0.02 mol), acetophenone (2.44 g, 0.02
mol) and potassium hydroxide pellets (1.80 g, 0.03 mol) in methanol
(30 mL) was stirred at 60 °C for 48h and later kept in refrigerator for
48h. The solid product that separated out was filtered and purified by
recrystallisation from a mixture (1:2) of hexane and dichloromethane
to give 6c. Yield 60%; m.p. 120 °C; IR vmax (KBr) 1653 (C=O) cm-1;
UV lmax (CH3CN) 250 (e 27,400), 330 (e 6,400), 380 (e 39,800), 420
1
The H and 13C NMR spectra were recorded at 300 and 75 MHz on
1
(e 81,700); H NMR (CDC13) d 7.21 (1H, d, JAX., = 16.2 Hz, vinylic),
7.50–8.50 (14H, m, aromatic), 8.81 (1H, d, JAX = 16.2 Hz, vinylic);
13C NMR (CDCI3) d 125.29, 125.41, 126.42, 128.41, 128.73, 128.90,
129.64, 130.17, 131.08, 131.32, 133.05, 137.92, 141.88, 189.68. Anal.
calcd for C23H16O; C, 89.58; H, 5.23; found: C, 89.20; H, 5.04%.
a Bruker FTNMR spectrometer with tetramethylsilane (TMS) as
internal standard. Chemical shifts are reported in parts per million
(ppm) downfield of TMS. FAB mass spectra were recorded on
a JEOL SX 102/DA-6000 using argon/xenon as the FAB gas, at
Central Drug Research Institute (CDRI), Lucknow. Elemental
analysis was performed on Elementar Systeme (Vario ELIII) at the
Sophisticated Test and Instrumentation Centre (STIC), Kochi.
Synthesis of 9-alkenyldibenzobarrelenes (8a–c); general procedure
A sample of 9-alkenylanthracenes (6a–c, 5 mmol) was dissolved
in the minimum quantity of dry xylene and DBA (7) (11 mmol) was
added. The reaction mixture was refluxed for 12h and progress of the
reaction was monitored through TLC. Solvent was removed under
reduced pressure and the residue obtained was purified by column
chromatography. Elution with 1:1 hexane-dichloromethane mixture
yielded the corresponding barrelenes.
Synthesis of 9-olefin appended anthracenes (6a–c); general
procedure
24
9-Vinylanthracene (6a):
(a) Wittig reaction: A solution of
9-anthraldehyde (4.12 g, 0.02 mol) in dry THF (80 mL), was added
to the stirred mixture of methyltriphenylphosphonium iodide (24.24
g, 0.06 mol) in dry THF (120 mL) under nitrogen atmosphere. The
reaction mixture was stirred at room temperature for 5h. An aqueous
solution of NaOH (50%), was then added from a dropping funnel.
The reaction mixture was stirred for another 30 min, poured into
water and extracted with dichloromethane. The organic layer was
separated, washed with water and dried over anhydrous MgSO4.
Solvent was removed under reduced pressure and the product
mixture was separated by column chromatography and purified by
recrystallisation from a mixture of hexane and dichloromethane (3:2)
to give 6a in pure form.
(b) Grignard reaction: A solution of anthrone (15.0 g, 0.08
mol) dissolved in dry THF (200 mL), was added to a mixture of
vinylmagnesium bromide (11.80 g, 0.09 mol) in dry THF (80 mL),
maintained at 55 oC. The mixture was refluxed for 4h, cooled
and then hydrolysed with 4N HCl (50mL). The organic layer was
extracted with benzene (70 mL). The extract was washed with water
and dried with anhydrous Na2SO4. The filtrate was added at room
temperature to P2O5 (10 g) in anhydrous benzene (250 mL) with
stirring over a 40 h period. Subsequently the filtrate was evaporated
to dryness, giving a crude solid, which was purified by column
chromatography over silica gel using hexane as eluent to give 6a in
pure form. Yield 70%; m.p. 65 °C; IR vmax (KBr) 3075 cm-1 (=CH2),
3043 cm-1 (=CH), 1625 cm-1 (C=C); UV lmax (CH3CN) 250 (e
73,000), 330 (e 2,500), 385 (e 4,900); 1H NMR (CDCI3) d 5.66 (1H,
dd, Jtrans = 17.7 Hz, Jgem = 2.1 Hz, vinylic), 6.04 (1H, dd, Jtrans = 11.4
Hz, Jgem = 2.1 Hz, vinylic), 7.46-8.40 (l0H, m, aromatic and vinylic).
Anal. calcd for C16H12:C, 94.08; H, 5.92; found: C, 94.3l; H, 5.72%.
8a: Yield 93%; m.p. 152 oC; IR nmax (KBr) 1651 (C=O, ketone) cm-1;
UV lmax (CH3CN) 256 (e 12,080), 300 (e 2,320); 1H NMR (CDCl3) d
5.48 (1H, d, Jtrans =18.2 Hz, vinylic), 5.54 (1H, s, methine), 5.95 (1H, d,
Jcis =11.5 Hz, vinylic), 6.57–6.65 (1H, m, vinylic), 7.10–7.58 (18H, m,
aromatic); 13C NMR (CDCl3) d 53.25, 59.51, 123.29, 124.03, 124.13,
125.13, 125.65, 128.28, 128.49, 128.64, 128.83, 129.17, 130.13, 133.02,
133.42, 137.01, 137.57, 145.11, 145.33, 151.38, 159.00, 193.94, 195.99;
FAB-MS m/z 439 (M++1), 333 (M+-COPh) and other peaks. Anal.
calcd for C32H22O2: C, 87.65; H, 5.06; found: C, 87.79; H, 5.35%.
8b: Yield 91%; m.p. 190 oC; IR nmax (KBr) 1662 (C=O, ketone) cm-1;
UV lmax (acetonitrile) 216 (e 27,040), 256 (e 15,200), 300 (e 2,310),
350 nm (e 470); 1H NMR (CDCl3) d 5.58 (1H, s, methine), 6.83 (1H, d,
JAX =17 Hz, vinylic), 6.91 (1H, d, JAX =17 Hz, vinylic), 7.06–7.59 (23H,
m, aromatic); 13C NMR (CDCl3) d 53.21, 58.92, 121.65, 123.35, 124.02,
125.11, 125.63, 126.38, 128.10, 128.23, 128.41, 128.63, 128.76, 129.15,
129.79, 132.98, 133.31, 136.68, 136.95, 137.56, 138.54, 145.02, 145.72,
151.57, 159.09, 193.92, 196.02. Anal. calcd for C38H26O2: C, 88.69; H,
5.09; found: C, 89.10; H, 5.31%.
8c: Yield 86%; m.p. 120 oC; IR nmax (KBr) 1650, 1665 (C=O, ketone)
1
cm-1; UV lmax (CH3CN) 254 (e 12,330), 300 (e 3,900); H NMR
(CDCl3) d 5.58 (1H, s, methine), 7.13–7.77 (24H, m, aromatic and
vinylic), 7.89 (1H, d, JAX = 16.5 Hz, vinylic); 13C NMR (CDCl3) d 53.21,
58.90, 122.81, 124.20, 125.37, 125.93, 128.31, 128.65, 128.78, 129.12
133.02, 133.15, 133.65, 136.74, 137.26, 139.36, 144.58, 144.74, 151.67,
157.65, 189.62, 193.73, 195.57; FAB-MS m/z 543 (M++1), 437 (M+-
COPh) and other peaks. Anal. calcd for C39H26O3: C, 86.32; H, 4.83;
found: C, 86.54; H, 5.11%.

202 JOURNAL OF CHEMICAL RESEARCH 2015
Photochemical reactions of dibenzobarrelenes 8a–c
References
A degassed solution of barrelenes (8a–c, 0.8 mmol) in benzene (100
mL) was irradiated using RPR 300 nm lamp for 10h. Progress of the
reaction was monitored by TLC. Removal of the solvent under reduced
pressure gave a residual solid, which was chromatographed over silica
gel. Elution with a mixture (1:1) of dichloromethane and hexane
gave unchanged starting materials (21 to 33%). Further elution with a
mixture (3:2) of dichloromethane and hexane gave the corresponding
dibenzocyclooctatetraene products 11a–c (58–65%).
11a: Yield 65%; m.p. 108 oC; IR nmax (KBr) 1659 (C=O, ketone) cm-1;
1H NMR (CDCl3) d 4.96 (1H, d, JAX = 16.9 Hz, vinylic), 5.26 (1H, d,
JAX = 10.7 Hz, vinylic), 6.66–6.73 (1H, m, vinylic), 7.12–8.11 (18H, m,
aromatic), 7.64 (1H, s, vinylic); 13C NMR (CDCl3) d 121.21, 126.56,
127.39, 127.44, 127.83, 127.99, 128.24, 128.35, 128.67, 128.88, 129.00,
129.27, 129.70, 129.91, 130.02, 130.62, 130.75, 130.92, 131.26, 132.24,
132.49, 132.75, 133.78, 135.27, 135.69, 136.26, 136.51, 136.77, 137.87,
140.78, 141.03, 141.24, 144.47, 196.12, 197.66; FAB-MS m/z 439
(M++1), 333 (M+-COPh) and other peaks. Anal. calcd for C32H22O2: C,
87.65; H, 5.06; found: C, 87.42; H, 4.88%.
1
2
3
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5
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7
8
9
10 S. J. Cristol, R. L. Kaufman, S. M. Opitz, W. Szalecki, and T. H. Bindel, J.
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11b: Yield 62%; m.p. 182 oC; IR nmax (KBr) 1667 (C=O, ketone), 1655
cm-1; 1H NMR (CDCl3) d 6.21 (1H, d, JAX = 16.0 Hz, vinylic), 7.10–8.12
(19H, m, aromatic and vinylic), 7.67 (1H, s, vinylic) ; FAB-MS m/z 453
(M++1), 347 (M+-COPh) and other peaks. Anal. calcd for C38H26O2: C,
88.69; H, 5.09; found: C, 89.01; H, 5.35%.
11c: Yield 58%; m.p. 170 oC; IR nmax (KBr) 1664 (C=O, ketone) cm-1;
1H NMR (CDCl3) 7.12–7.96 (26H, m, aromatic and vinylic); 13C NMR
(CDCl3) d121.89, 123.64, 124.76, 125.54, 125.87, 126.08, 127.38, 127.68,
127.76, 128.11, 128.25, 128.35, 128.59, 128.80,128.89, 129.13, 129.15,
129.77, 129.83, 130.14, 130.47, 131.13, 131.86, 132.23, 132.39, 133.56,
136.11, 136.35, 136.64, 137.83, 140.64, 142.88, 143.39, 148.07, 195.29,
196.80; FAB-MS m/z 543 (M++1), 437 (M+-COPh) and other peaks.
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A degassed solution of barrelene 12 (0.8 mmol) in benzene (100 mL)
was irradiated using RPR 300 nm lamp for 30 minutes. Progress of
the reaction was monitored by TLC. Removal of the solvent under
reduced pressure gave a residual solid, which was recrystallised
from a mixture (1:1) of dichloromethane and ethanol to give the
corresponding dibenzosemibullvalene 13 (94%). Characterisation data
for compounds 12 and 13 are available in the literature.18
Received 16 January 2015; accepted 27 February 2015
Paper 1503148 doi: 10.3184/174751915X14267013391940
Published online: 13 April 2015
26 B. Dong, M. Wang and C. Xu, Luminescence, 2013, 28, 628.

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