Synthesis of derivatives of phenanthrene and helicene by improved procedures of photocyclization of stilbenes

Article abstract of DOI:10.1246/bcsj.82.1182

An improved method has been developed for photocyclization of stilbene to construct phenanthrenes and benzo[c]phenanthrenes. This reaction is promoted by iodine while tetrahydrofuran is used as an efficient and inexpensive scavenger of hydroiodic acid produced during the photocyclization sequence. In another process, cyclohexene is used as a reagent for dehydrogenation step in place of THFI₂.

Full text of DOI:10.1246/bcsj.82.1182

1182 Bull. Chem. Soc. Jpn. Vol. 82, No. 9, 1182–1186 (2009)
© 2009 The Chemical Society of Japan
Synthesis of Derivatives of Phenanthrene and Helicene by
Improved Procedures of Photocyclization of Stilbenes
Harish R. Talele, Monik J. Gohil, and Ashutosh V. Bedekar*
Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara 390 002, India
Received February 17, 2009; E-mail: avbedekar@yahoo.co.in
An improved method has been developed for photocyclization of stilbene to construct phenanthrenes and
benzo[c]phenanthrenes. This reaction is promoted by iodine while tetrahydrofuran is used as an efficient and inexpensive
scavenger of hydroiodic acid produced during the photocyclization sequence. In another process, cyclohexene is used as a
reagent for dehydrogenation step in place of THF-I2.
A common method for synthesis of phenanthrene involves
the construction of a central ring by photocyclization of
stilbene.1 This method involves exposure of cis-/trans-stilbene
to UV light, which causes its isomerization to cis form 1, which
undergoes electrocyclic ring formation to produce dihydro-
phenanthrene 2. This intermediate 2 undergoes oxidation in the
presence of iodine to generate the aromatic ring of 3 and HI,
eq 1. This method is used extensively for the preparation of a
large number of polycyclic aromatic compounds,2 helicenes,3
aza-helicenes,4 thia-helicenes,5 complex heterocyclic com-
pounds,6 and molecular devices.7
as phenanthrene (TLC, mp, and NMR). The isolated yield of
3 was excellent and much higher than reportedly obtained
with propylene oxide or with oxygen.1c,3h To the best of our
knowledge THF has not been used as HI scavenger in such
cases, although occasionally used as a solvent. However, there
is a literature reference13 of the combined use of excess of THF
and propylene oxide, where HI should preferentially react with
the more strained epoxide ring than the former. This means that
the THF is actually used only as solvent and may not work as
scavenger of HI in this reported case.13
I2, hν
THF-Toluene
H
I2
hν
ð2Þ
ð1Þ
125 W, 7 h
98%
- 2 HI
H
1
3
1
2
3
There is the possibility of the oxygen from air to act as
oxidant in place of I2 as known from the literature. To test this,
another experiment was run with careful degassing the reaction
mixture by applying vacuum and sonication and subjected to
photoirradiation. However, the same result was obtained
confirming that I2-THF is the actual reagent for the second
step. A separate experiment with I2-toluene but without THF
was very sluggish, establishing the role of THF.
Encouraged by this result a number of stilbene derivatives
were synthesized by Wittig reaction3k and subjected to the
photocyclization under the present, modified conditions,
Table 1. A series of mono- and di-substituted phenanthrenes
were obtained after careful chromatography and showed satis-
factory 1H NMR analysis. It is noteworthy to have succeeded in
photocyclization of nitro-substituted stilbene to the correspond-
ing phenanthrene, which was known to be difficult.1d,1f
As a part of our ongoing research project we are required to
develop an expeditious synthesis of functionalized benzo[c]-
phenanthrene or helicenes. With this aim several ¢-styrylnaph-
thalenes were prepared by Wittig reaction3k and subjected to
the current photocyclization, Table 2. A mixture of cis and
trans isomer was used for the reaction and its ratio had not
much influence on the efficacy of the cyclization.
The oxidative conversion of 2 to 3 is accompanied by
reduction of iodine to produce two moles of hydrogen iodide.
The photolysis is usually carried out in the presence of excess
of propylene oxide or ethylene oxide. The strong acid by-
product, HI, is thus neutralized in the opening of the epoxide
ring of these reagents. Alternatively, the photocyclization can
be carried out in the presence of oxygen8 with good results. It is
reported that when a primary amine is used as the scavenger
of this HI, the cyclization favors an ionic mechanism and
gives dihydro derivative.9 Other bases have been used as
acid scavengers in this type of photocyclization with some
success.4b,10 An analogous reaction of photochemical cycliza-
tion of stilbazole salts within a Nafion membrane micro-reactor
has been reported,11 without using any base nor iodine.
In this article we present the use of tetrahydrofuran as a
neutral, easily available, inexpensive scavenger of hydroiodic
acid produced in this type of photocyclization. The acid
promoted opening of tetrahydrofuran when exposed to in situ
generated HI should give 4-iodobutan-1-ol or 1,4-diiodo-
butane.12 To test this concept a sample of mainly trans-stilbene
was exposed to light from a 125-W high pressure mercury
vapor lamp (hpmv) in a routine immersion well photoreactor
with iodine (1.1 equiv), tetrahydrofuran (20 equiv) in toluene
for 7 h, eq 2. The product formed was separated and identified
Another derivative of stilbene 15 was prepared and subjected
to this photocyclization. The cyclization reaction can take place
Published on the web September 9, 2009; doi:10.1246/bcsj.82.1182

H. R. Talele et al.
Table 1. Photocyclization of Stilbenes with I2-THF
Bull. Chem. Soc. Jpn. Vol. 82, No. 9 (2009) 1183
Time/h with
125-W hpmv lamp
Isolated yield/%
Mp/°C (Lit)
Entry
1
Stilbene
Phenanthrene
[Reported yield/%]
98
[73]a)
100-01
(98-100)
7
7
3
2
3
4
78
[76]a)
79-81
(79-80)a)
Cl
4
5
Cl
Br
Br
84
[76]a)
80-82
(81-83)a)
20
Br
Br
9
16
40
83
186-188
Cl
Cl
6
7
8
5
6
7
97
[77]b)
194
(194)c)
Br
Br
Br
Br
Br
Br
47
68
100-102
234-235
OMe
OMe
29
Br
NO2
Br
NO2
9
a) Ref. 1d. b) Ref. 2c. c) Ref. 1e.
Table 2. Photocyclization of ¢-Styrylnaphthalenes with I2-THF
Time/h with
125-W hpmv lamp
Isolated yield/%
[Reported yield/%]
Entry
1
Stilbene
Benzo[c]phenanthrene
Mp/°C (Lit)
68-69
13
16
20
22
23
84
67
(67-68)b)
10
2
3
4
5
Cl
Cl
Br
62-64
11
96
[78]a)
87-88
(87-88)c)
Br
12
78-80
(78-79)b)
OMe
OMe
75
83
13
NO2
NO2
150-152
14
a) Ref. 2d. b) Ref. 14a. c) Ref. 14b.

1184 Bull. Chem. Soc. Jpn. Vol. 82, No. 9 (2009)
Synthesis of Derivatives of Phenanthrene
Table 3. Photocyclization of Stilbene and ¢-Styrylnaphtha-
lenes in the Presence of Cyclohexene
at two sites, at “a” or at “b” (eq 3), leading to formation of 16a
or 16b respectively. The cyclization in the present conditions
resulted in the isolation of linear product 16b, established by its
1H NMR. Probably the desired cyclization at “a” site is difficult
due to steric constraints.
Time/h with
125-W hpmv
Benzo[c]-
phenanthrene
Isolated
yield/%
Entry
1
Stilbene
lamp
b
16
23
20
86
63
54
O
O
3
h
ν
O
O
ð3Þ
O
or
2
3
THF-I2
Toluene
22 h
a
O
15
16a
16b
Cl
Br
4
7
Cl
Br
37 %; mp 154-55 °C
Optically pure [5]helicenes was recently used as chiral
source in asymmetric addition of dialkyl zinc to aldehydes.15
This helicenes can be prepared by photolysis of dinaphthyl-
ethene 17 under the present conditions. However, under the
standard conditions of prolonged irradiation, the initially
formed [5]helicene (18), was further converted to benzo[ghi]-
perylene (19), eq 4. This observation of [4n + 2] electrocyclic
ring closure has been previously documented in the literature.16
Br
Br
4
5
17
18
36
35
10
OMe
OMe
I2,
hν
[4n+2]
- 2H
THF-Toluene
13
ð4Þ
43%
preliminary work to a few more examples, Table 3. Further
work in this method is currently underway.
17
18
19
mp 265-70 °C
Lit. 269 °C15
In summary, we have developed two simple and efficient
methods for photocyclization of substituted stilbenes to
phenanthrenes and benzo[c]phenanthrenes using readily avail-
able and cheap chemicals. In the first one the boiling point of
THF is sufficiently high compared to propylene oxide (bp
34 °C) and hence it is convenient to use it in a routine
photoreactor, which gets slightly warm even with water
circulation. In the second method we report novel use of
cyclohexene to assist the second part of the cyclization
sequence. The low cost and easy availability of the reagents
are the main advantages of these methods.
However, under similar conditions it was possible to
synthesize [6]helicene (21) from 20 in moderate yield. The
isolated 21 shows identical melting point17a and NMR to one
reported recently,17c eq 5.
I2, hν
THF-Toluene
ð5Þ
125 W
20 h, 64%
Experimental
20
21
mp 231-2 °C
Lit. 231 °C17a
Preparation of Phenanthrene. Method-1: A solution of
trans-stilbene (0.1 g, 0.55 mmol) and iodine (0.154 g, 0.61 mmol)
in toluene (425 mL) and tetrahydrofuran (0.9 mL, 11.09 mmol, ca.
20 mol equivalent) was irradiated in a standard immersion well
photoreactor with 125-W high pressure mercury vapor lamp for 7 h
or until no starting olefin was seen by TLC. The reaction mixture
was then washed with aqueous sodium thiosulfate, water, brine and
dried over anhydrous sodium sulfate. The concentrated mixture
was purified on silica gel column to afford phenanthrene as white
solid (0.097 g, 98%).
The first step of this photocyclization is the formation of 2
and our present modification deals with the second step, i.e.,
aromatization toward 3. In this part of the conversion we have
envisaged the use of cyclohexene to abstract the hydrogen as
shown in eq 6. To test this concept a sample of trans-stilbene
was exposed to photolysis in the presence of excess of
cyclohexene (20 equiv) in toluene.
Method-2: A solution of trans-stilbene (0.1 g, 0.55 mmol) and
cyclohexene (0.91 g, 1.1 mmol) in toluene (425 mL) was irradiated
in a standard immersion well photoreactor with a 125-W high
pressure mercury vapor lamp for 16 h. The reaction mixture was
dried on anhydrous sodium sulfate, concentrated at reduced
pressure and purified on silica gel column to get phenanthrene as
white solid (0.086 g, 86%).
H
hν
ð6Þ
H
2
1
3
A careful isolation of the product indicated the formation
of phenanthrene in good yield and we have extended this
The molecules listed in Table 1, Table 2 were prepared by
Method-1 and in Table 3 by Method-2.

H. R. Talele et al.
Bull. Chem. Soc. Jpn. Vol. 82, No. 9 (2009) 1185
Spectral Data of Selected Molecules. 3-Bromo-6-chloro-
phenanthrene (6): 1H NMR (400 MHz, CDCl3): ¤ 7.56-7.58 (dd,
J = 8.5, 2.1 Hz, 1H), 7.67-7.73 (m, 3H), 7.75-7.77 (d, J = 8.5 Hz,
1H), 7.81-7.83 (d, J = 8.5 Hz, 1H), 8.54 (d, J = 1.96 Hz, 1H),
8.71 (d, J = 1.64 Hz, 1H). IR (KBr): ¯ 3051, 2923, 1588, 1497,
7.90-7.95 (m, 4H), 7.96-8.01 (m, 4H). IR (KBr): ¯ 3042, 1601,
¹1
1500, 1468, 1434, 844, 828, 799, 753 cm
.
We are grateful to the Department of Science and Technol-
ogy, New Delhi for the financial support (SR/SI/OC-42/
2006), Dr. P.P. Wadgaonkar (National Chemical Laboratory,
Pune) and Mr. S.P. Sahoo (Sun Pharmaceuticals Pvt Ltd.,
Vadodara) for the NMR analysis.
1429, 1409, 1109, 1088, 1074, 1021, 836, 771, 740, 696, 587,
¹1
520 cm
.
3-Bromo-6-methoxyphenanthrene (8): 1H NMR (400 MHz,
CDCl3): ¤ 4.03 (s, 3H), 7.54-7.56 (d, J = 8.8 Hz, 1H), 7.65-7.74
(m, 3H), 7.72-7.74 (d, J = 9.2 Hz, 1H), 7.78-7.81 (d, J = 8.8 Hz,
1H), 7.91 (d, J = 2.4 Hz, 1H), 8.71 (d, J = 1.5 Hz, 1H). IR (KBr):
References
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1073, 1028, 866, 843, 777, 592, 553, 537 cm
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¹1
1497, 1437, 1342, 1075, 1028, 845, 736, 588, 509 cm
2-Chlorobenzo[c]phenanthrene (11):
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1H NMR (400 MHz,
2
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¹1
838, 779, 747 cm
.
3
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1H), 7.71-7.74 (d, J = 8.4 Hz, 1H), 7.74-7.76 (d, J = 7.2 Hz, 1H),
7.80-7.82 (d, J = 8.4 Hz, 1H), 7.82-7.85 (d, J = 8.5 Hz, 1H),
7.87-7.90 (d, J = 8.6 Hz, 1H), 7.91-7.93 (d, J = 8.6 Hz, 1H),
8.02-8.04 (d, J = 7.9 Hz, 1H), 9.03-9.05 (d, J = 8.5 Hz, 1H), 9.48
(s, 1H). IR (KBr): ¯ 3044, 1600, 1588, 1485, 1440, 1109, 1082,
¹1
1039, 839, 781, 746, 599, 570, 529 cm
.
2-Methoxybenzo[c]phenanthrene (13): 1H NMR (400 MHz,
CDCl3): ¤ 4.01 (s, 3H), 7.27-7.30 (dd, J = 8.8, 2.5 Hz, 1H), 7.59-
7.62 (m, 1H), 7.64-7.68 (m, 1H), 7.68-7.70 (d, J = 8.4 Hz, 1H),
7.79-7.88 (m, 3H), 7.92-7.94 (d, J = 7.9 Hz, 1H), 8.01 (dd,
J = 7.8, 1.4 Hz, 1H), 8.58-8.59 (d, J = 2.4 Hz, 1H), 9.18-9.20 (d,
J = 8.5 Hz, 1H). IR (KBr): ¯ 3001, 2951, 2925, 1608, 1505, 1449,
¹1
1239, 1219, 1102, 1045, 881, 836, 749 cm
.
2-Nitrobenzo[c]phenanthrene (14):
1H NMR (400 MHz,
CDCl3): ¤ 7.68-7.72 (m, 1H), 7.77-7.81 (m, 1H), 7.81-7.84 (d,
J = 8.5 Hz, 1H), 7.91-7.93 (d, J = 8.5 Hz, 1H), 7.96-7.98 (d,
J = 7.8 Hz, 1H), 7.98-8.00 (d, J = 8.5 Hz, 1H), 8.04-8.06 (dd,
J = 8.7, 2.4 Hz, 1H), 8.06-8.09 (d, J = 8.9 Hz, 1H), 8.35-8.38 (dd,
J = 8.8, 2.2 Hz, 1H), 8.95-8.97 (d, J = 8.5 Hz, 1H), 9.99-10.0 (d,
J = 2.0 Hz, 1H). IR (KBr): ¯ 3052, 3006, 2924, 1601, 1530, 1510,
¹1
1378, 1334, 845, 828, 800, 759, 736 cm
.
4
Aza-helicenes: a) H. A. Staab, M. Diehm, C. Krieger,
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1H NMR
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overlapping m, 2H), 7.69-7.82 (four overlapping d, 4H), 7.97-
7.99 (dd, J = 7.8, 1.5 Hz, 1H), 8.51 (s, 1H), 9.01-9.04 (d,
J = 8.4 Hz, 1H). IR (KBr): ¯ 3049, 2905, 1602, 1494, 1463, 1200,
¹1
1172, 1119, 1071, 1036, 857, 833, 796, 761 cm
Benzo[ghi]perylene (19):
1H NMR (400 MHz, CDCl3): ¤
7.97-8.05 (d, J = 7.8 Hz, 2H), 8.05-8.09 (d, J = 8.8 Hz, 2H),
8.11-8.15 (d, J = 8.9 Hz, 2H), 8.16-8.21 (dd, J = 7.7, 0.8 Hz, 2H),
8.34 (s, 2H), 8.98-9.02 (dd, J = 7.8, 0.8 Hz, 2H).
[6]Helicene (21): 1H NMR (400 MHz, CDCl3): ¤ 6.65-6.69
(ddd, J = 8.2, 1.0, 1.0 Hz, 2H), 7.19-7.23 (dd, J = 7.7, 7.2 Hz,
2H), 7.57-7.59 (d, J = 8.6 Hz, 2H), 7.83-7.88 (d, J = 8.0 Hz, 2H),
5
Thia-helicenes: a) S. Maiorana, A. Papagni, E. Licandro, R.
Annunziata, P. Paravidino, D. Perdicchia, C. Giannini, M. Bencini,

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10 Authors in Ref. 6b have studied photocyclization on
tosylethenes with bases such as DBU, Et3N, and CaCO3 during
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cyclization with elimination of -Ts as a leaving group.
11 X.-H. Li, L.-Z. Wu, L.-P. Zhang, C.-H. Tung, Org. Lett.