Highly efficient fluorine-promoted intramolecular condensation of benzo[c]phenanthrene: A new prospective on direct fullerene synthesis

Article abstract of DOI:10.1002/ejoc.200900976

Various functional groups have been tested as alternative promoters of the intramolecular condensation of benzo-[c]phenanthrene under flash vacuum pyrolysis conditions. Methyl and fluorine functionalization were found to be promising approaches. Unexpectedly high selectivity was observed in the cyclization of fluorinated benzo[c]phenanthrenes. The mechanism for the condensation reaction and the advantages of fluorine as a promoter for the rational synthesis of fullerenes are discussed. Wiley-VCH Verlag GmbH & Co. KGaA.

Full text of DOI:10.1002/ejoc.200900976

FULL PAPER
DOI: 10.1002/ejoc.200900976
Highly Efficient Fluorine-Promoted Intramolecular Condensation of
Benzo[c]phenanthrene: A New Prospective on Direct Fullerene Synthesis
Konstantin Yu. Amsharov,*[a] Mikhail A. Kabdulov,[a] and Martin Jansen[a]
Keywords: Flash pyrolysis / Cyclization / C-C coupling / Reaction mechanisms / Fullerenes
Various functional groups have been tested as alternative
promoters of the intramolecular condensation of benzo-
[c]phenanthrene under flash vacuum pyrolysis conditions.
Methyl and fluorine functionalization were found to be prom-
ising approaches. Unexpectedly high selectivity was ob-
enes. The mechanism for the condensation reaction and the
advantages of fluorine as a promoter for the rational synthe-
sis of fullerenes are discussed.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
served in the cyclization of fluorinated benzo[c]phenanthr- Germany, 2009)
known promoters) into the large molecules causes serious
Introduction
difficulties during synthesis, separation and purification.
Although such complications can be partially overcome,[9]
FVP of the resulting halogenated large molecules is ham-
pered because the increase in molecular weight significantly
increases the vaporization temperature of the precursor,
causing its considerable decomposition during sublimation.
Moreover, in the FVP experiment the reactive chlorine radi-
cals formed can attack the target molecule and cause vari-
ous side-reactions, which can completely nullify the yield of
the target molecule.[10] The availability of alternative radical
precursors that do not have these disadvantages seems to
be a key prerequisite for successful direct fullerene synthe-
sis. Herein we present the investigation of such alternative
promoters for intramolecular condensation. Fluorine was
found to be an excellent candidate for an efficient intramo-
lecular condensation and seems to possess all the required
properties of an “ideal promoter”.
The discovery of fullerenes has led to appreciable interest
in the synthesis of non-planar “aromatic” structures that
can be regarded as fullerene fragments. Such geodesic poly-
arenes can be obtained by intramolecular CAryl–CAryl con-
densation of polycyclic aromatic hydrocarbons (PAHs),
with all the carbon atoms in the required location, under
flash vacuum pyrolysis (FVP) conditions.[1–5] The presence
of a halogen atom in the initial precursor has been found
to be essential for effective radical generation, which is the
driving force for further cyclization. The same non-haloge-
nated PAH analogues usually do not undergo intramolecu-
lar cyclization or give only trace amounts of the target
products. Chlorine and bromine have been shown to be ef-
fective radical promoters for ring-closure.[1–5] This method-
ology was successfully employed by Scott et al. in the first
rational C60 fullerene synthesis. Pyrolysis of the PAH pre-
cursor, containing all 60 carbon atoms at the programmed
positions needed for fullerene formation and three chlorine
atoms, gave C60 fullerene with a yield of about 0.1–1%.[6]
Despite the relatively low degree of conversion, the high
selectivity makes this approach, in principle, promising for
the synthesis of higher fullerenes including those that do
not form under uncontrollable graphite vaporization. Re-
cently we reported the synthesis of two PAH precursors of
the higher fullerenes C78:4 and C84:20 and demonstrated
their selective conversion to the designed fullerene cages un-
der FVP conditions.[7,8] The yield in both cases was low due
to the lack of effective radical promoters. Unfortunately,
the introduction of chlorine and/or bromine atoms (the best
Results and Discussion
The conversion of benzo[c]phenanthrene (5) to benzo-
[ghi]fluoranthene (6) was chosen as a model reaction to in-
vestigate the new radical promoters. Benzo[c]phenanthrene
is the smallest molecule bearing the so-called fjord region
in which the new bond should be established. Moreover, the
efficiency of chlorine and bromine demonstrated previously
with benzo[c]phenanthrene systems have been proven trans-
ferable to the synthesis of various buckybowl structures.[11]
Earlier studies have shown that the best position for acti-
vation is one of the carbon atoms at which the new bond
should be established. In the system under consideration,
these carbon atoms always belong to the fjord region. Ac-
cording to the condensation mechanism proposed, the radi-
cal formed attacks the adjacent carbon atom with the for-
[a] Max Planck Institute for Solid-State Research,
Heisenbergstrasse 1, 70569, Stuttgart, Germany
Fax: +49-711-6891502
E-mail: k.amsharov@fkf.mpg.de
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.200900976.
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Intramolecular Condensation of Benzo[c]phenanthrene
mation of the required bond. Alternatively, the promoter separation gave the benzo-stilbene 4 as a cis/trans isomeric
can be introduced into one of the neighbouring positions. mixture. No additional separation was performed because
After homolytic cleavage of the corresponding C–R bond a the isomers interconvert photochemically in the next step.
1,2-hydrogen shift takes place, which leads to the displace- Benzo[c]phenanthrenes 5 were synthesized from the benzo-
ment of the radical position.[12] Although such activation is stilbenes by using the Katz modification of the Mallory
less effective, it often helps to avoid synthetic difficulties photocyclization reaction.[17]
associated with the introduction of the promoter group into
the sterically hampered region.
Considering the synthetic routes to fullerene-related
PAHs, two approaches have been proven to be prolific. The
first is based on the aldol trimerization of cyclic ketones.
Although this reaction was discovered more than 100 years
ago, it only recently became a powerful method for the syn-
thesis of C3 symmetrical PAHs as a result of the work of
Scott and co-workers.[13,14] This approach works very well
in many cases, but still has some limitations in the synthesis
of big molecules such as the precursors of higher fullerenes.
This is mostly because of the poor solubility of intermediate
products.[9,14] The reaction often stops after dimerization
and the target sample usually contains a considerable
amount of insoluble side-products.[16] An alternative route,
which avoids the solubility disadvantages, has been devel-
oped by Echavarren and co-workers. It is based on the ad-
dition of an appropriate electrophilic reagent to the truxene
trianion and the subsequent Pd-catalyzed cyclization, which
gives the desired product in moderate yield and high pu-
rity.[15,16] Unfortunately this method has serious disadvan-
tages. Namely the last step, the Pd-catalyzed cyclization, ex-
cludes the option of introducing chlorine or bromine into
the structure. Thus, neither of these approaches provides a
way to design the required fullerene precursor molecules
using chlorine or bromine as radical promoter. To solve this
problem we have examined alternative radical promoters
that seem to be promising whilst taking the above-men-
Figure 1. Synthetic route to benzo[c]phenanthrenes 5a–5g (5h was
synthesized by the debromination of 5g).
tioned synthetic approaches into consideration. Although
in pristine benzo[c]phenanthrene 5, both R3 and R1 can be
Pyrolysis experiments were carried out in an improved
used for activation, the R1 position was chosen preferen- FVP furnace that allowed simultaneous control of the pres-
tially because of its more convenient synthetic access. sure and the flow of the carrier gas in the system. A mov-
All benzo[c]phenanthrene derivatives 5 were synthesized able oven made it possible to regulate the sublimation tem-
by using the modified two-pot synthetic strategy[12] (Fig- perature and tune the sublimation speed during the FVP
ure 1). Benzyl bromides 2 were obtained by free-radical bro- experiments. A special constriction of the injector decreases
mination of the corresponding methylbenzenes 1 with N- the probability of contact between sample molecules and
bromosuccinimide (NBS) in the presence of dibenzoyl per- the walls of the heated quartz tube (Figure 2 and the Sup-
oxide (PBO) in CCl4 (or commercially available bromides porting Information).
were used). The corresponding benzyl bromide and tri-
Benzo[c]phenanthrene (5d) and 2-chlorobenzo[c]phenan-
phenylphosphane were heated at reflux in toluene to form threne (5a) were selected as reference compounds. Pyrolysis
the phosphonium salt 3, which was filtered and used in the of 5a showed that effective condensation takes place under
next step without additional purification. Wittig reactions FVP conditions and the efficiency of the condensation in-
were carried out in anhydrous THF using n-butyllithium as creases with increasing temperature of pyrolysis and
the base (ethanol and potassium tert-butoxide were used for decreasing pressure in the system. At the same time the
the synthesis of the fluoro derivatives). Chromatographic selectivity for benzo[ghi]fluoranthene (6) formation de-
Figure 2. The main products of FVP of the benzo[c]phenanthrene derivatives.
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K. Y. Amsharov, M. A. Kabdulov, M. Jansen
FULL PAPER
creases due to the conversion of 6 to the thermodynamically fjord region. Moreover, the subsequent cleavage of the Me
more stable cyclopenta[cd]pyrene (8), in agreement with lit- group could provide the activation for the next cyclization,
erature data.[11,18,19] Thus, the pyrolysis of 5a at 1100 °C which makes it a rather interesting candidate for the cycliza-
and a pressure of 2.5 mbar with a N2 flow of 12 mL/min tion of fullerene precursors in which a large number of new
gave 8 in 50% yield, whereas 6 was formed in only 20% cycles should be created.
yield. (Here and elsewhere in the text molar percentages are
given.) The optimal conditions for the condensation of 5a
to 6 were found to be T = 1000 °C, 2 mbar pressure and N2
flow of 8 mL/min. In this case 6 was obtained in almost
80% yield, whereas unactivated benzo[c]phenanthrene (5d)
gave only 3% of 6 under the same conditions. The efficiency
of the alternative promoters was investigated by compari-
son with the efficiency of 5a and 5d under the conditions
required for the sublimation of large fullerene precursor
molecules (pressure 0.5 mbar, temperature 1100 °C). The re-
sults of the FVP experiments are summarized in Table 1.
Table 1. The effect of promoters on the condensation of benzo[c]-
phenanthrenes 5. Pyrolysis at 1100 °C (p = 0.1 mbar). The yields
are given as a molar ratio to benzo[ghi]fluoranthene (6). Bond en-
ergy (Ph–R) according ref.[20,21]
R1
Ph–R bond
energy[b]
5
6
7
8
9
Figure 3. HPLC profile of the products of the FVP of 5b (FVP
conditions: T = 1000 °C, p = 0.1 mbar).
[kcal/mol]
5a
5b
5c
5d
5e
5f
Cl
97.1
4.56
6.69
16.27
22.5
13.02
8.36
1
1
1
1
1
1
0.82
1.79
0.80
–
2.07
0.07
1.7
0
CH3 103.5 (89.7)[b]
0.94 (0.6)[a]
CF3
H
Ph
F
110.5
112.9
118
1.53 (0.2)[a]
The deviation from the correlation between C–R bond
energy and efficiency of cyclization in the case of 5e is prob-
ably caused by the lower dissociation energy of the C–H
bond in the fjord region, which stimulates ring-closure
without cleavage of the phenyl group. Indeed, a significant
amount of phenylbenzo[ghi]fluoranthene (9e) was registered
in the pyrolysis products. Nevertheless, a rather low degree
of conversion and the formation of many side-products li-
1.1
–
1.25 (0.7)[a]
1.19 (0.1)[a]
127.2
[a] Concentrations were estimated on the assumption of equal sorp-
tion with benzo[ghi]fluoranthene. [b] The weakest bond in 5b
(PhCH2–H).
As can be seen from Table 1, the efficiency of the ring- mit the use of a Ph group as a cyclization promoter. Pyroly-
closure is in good correlation with the C–R bond dissoci- sis of 5c containing a CF3 group does not show good acti-
ation energy, a measure of the effective homolytic cleavage vation either and gave a complex mixture of the products.
of the corresponding C–R bond. However, the phenyl and
Unexpectedly, fluorine was found to be highly efficient in
fluorine derivatives do not fit the correlation. According to the activation. (The results of FVP of fluorinated benzo[c]-
the data obtained, the methyl group can be considered as a phenanthrenes are summarized in Table 2.) Although some
prospective promoter. The mechanism for the cyclization of activation effect of fluorine in 1,3-difluorobenzo[c]phenan-
5b is definitely different to the “classic” mechanism. The threne has been described previously,[11] the activity in the
homolytic cleavage of the ArCH2–H bond to give a stable case of 2-fluorobenzo[c]phenanthrene (5f) is highly unex-
benzyl radical needs only 89.7 kcal/mol.[20] The radical pected. The C–F bond is considered a very stable bond in
formed stimulates the ring-closure without loss of the organic chemistry[22] and accordingly the proposed mecha-
methyl group. Thus, low-temperature pyrolysis results nism[12] should strongly inhibit its condensation. Because
mostly in the formation of methylbenzo[ghi]fluoranthene our experiments have shown high fluorine efficiency in ring-
9b. Increasing the temperature results in the subsequent closure it is clear that the mechanism of condensation is
cleavage of the methyl group and formation of a benzo[ghi]- different to that proposed for other halogens. The reason
fluoranthene radical which finally yields 6. Interestingly, why fluorine activates the condensation reaction and also
significant transformation of 6 and 9 into the correspond- the mechanism of the process have remained widely unex-
ing cyclopenta[cd]pyrenes takes place even at 1000 °C (Fig- plored.
ure 3), although such a transformation usually needs a tem-
The condensation of 1-fluorobenzo[c]phenanthrene (5h)
perature of more than 1150 °C.[18] The formation of 7 in gave even more exciting results. Namely, a remarkable de-
only trace amounts confirms that no direct C–Me bond- gree of cyclization with a high selectivity, uncharacteristic
cleavage takes place in low-temperature FVP. In summary, for pyrolysis, was observed in the low-temperature experi-
the Me group promotes rather selective ring-closure, al- ment (Figure 4). Although only 10% of 5h was converted
though it is not directly involved in the sterically hampered into 6, the selectivity of this process was found to be more
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Intramolecular Condensation of Benzo[c]phenanthrene
Table 2. Relative yields of the pyrolysis products (mol-%).
The mechanism for the condensation of 5a is the same
as for its brominated analogue, which has been investigated
previously.[4,13] In the first step, the homolytic cleavage of
the weakest bond (C–Cl) takes place. The radical 11 can be
quenched by a hydrogen radical with the formation of
benzo[c]phenanthrene (7) or rearrange to 12 or 14. As has
also been demonstrated previously, β-scission of the phenyl
radical (82.2 kcal/mol) is less favourable than a 1,2-hydro-
gen shift (58.4 kcal/mol) and radical 11 converts rather se-
lectively to radical 12.[13] Subsequent ring-closure gives 13,
which can be quenched by a hydrogen radical with the for-
mation of the targeted 6. Increasing the pyrolysis tempera-
ture results in reduced selectivity for the transformation of
11 into 12. As a result, both 12 and 14 can be formed from
11 with the same probability. Whereas 11 leads to the for-
mation of benzo[ghi]fluoranthene (6), the intermediate 14
rearranges to carbene 16, cyclization of which results in cy-
clopenta[cd]pyrene (8). As can be seen from Figure 6 the
temperature-dependent 6/8 ratio is in a good agreement
with the proposed mechanism. Thus, at low temperature the
ratio of 6/8 in the pyrolysis products of 5a is about 10,
whereas at high temperature it approaches 1.
Com-
T [°C]
Yield [%]
pound
5
7
6
8
5h
5f
800
850
900
1000
1100
92.6
89.5
85.3
81.8
59.6
0
0
0
0
0
7.6
10
13
14.8
28.4
0.2
0.5
1.6
3.4
11.8
900
1000
1100
100
96.7
79.3
0
0.05
0.7
0
1.3
9.4
0
1.6
11.3
than 97%. This means that unreacted 5h can be recovered
and, in principle, 5h can be condensed to the targeted prod-
uct with close to quantitative yield by repeated FVP.
In the case of methylarenes the cleavage of the ArCH2–
H bond (89.7 kcal/mol) is favoured over Ar–CH3 cleavage
(103.5 kcal/mol). Thus, the pyrolysis of 5b results preferen-
tially in the formation of the benzylic radical 17 which
transforms into 18 and subsequent ring-closure gives 19.
The radical 19 can rearrange to 20, which finally yields
benzo[ghi]fluoranthene 9b. Although the low-temperature
pyrolysis results mostly in the formation of 9b, a remarkable
amount of methylcyclopenta[cd]pyrene 10b was observed in
the product mixture. The most likely route to 10b is by the
rearrangement of radical 19. Indeed, increasing the pyroly-
sis temperature to 1000 °C shows the formation of a con-
siderable amount of cyclopenta[cd]pyrene (8; Figure 6).
This can be a result of cleavage of the CH3 group in 9b and
subsequent rearrangement of the benzo[ghi]fluoranthene
radical 23 to 24. This also confirms that the benzo[ghi]fluo-
ranthene radical 19 could rather easily be transformed into
the corresponding cyclopenta[cd]pyrene 10b. Pyrolysis at
1100 °C is not selective and results in a complex product
mixture. The formation of benzo[c]phenanthrene (7) under
Figure 4. HPLC profile of the FVP of 5h (FVP conditions: T =
850 °C, p = 0.1 mbar).
The potential offered by functionalization with fluorine these conditions shows that direct Ar–CH3 bond-cleavage
for fullerene precursor closing is clearly promising. First, takes place in 5b as well.
fluorine does not increase the molecular weight of the pre-
The phenyl group also promotes intramolecular cycliza-
cursor drastically, which is an important prerequisite for tion. According to quantum chemical calculations the pres-
facile sublimation. The C–F bond is stable to sublimation, ence of Ph remarkably decreases the C–H bond dissociation
which should exclude decomposition of the precursor mate- energy in the fjord region. Thus, the formation of 21 from
rial during vaporization. Secondly, because of its small size 5e needs only 106.8 kcal/mol instead of the 112.9 kcal/mol
fluorine can be introduced into the sterically hampered typical of homolytic CAr–H bond-cleavage. The radical in-
fjord region of the precursor molecule rather easily. Finally, termediate 21 stimulates ring formation (22) and finally
the high resistance of the C–F bond to many organic reac- yields 9e.
tions provides opportunities for introducing the required
Because of the high stability of the C–F bond, fluori-
number of fluorine atoms at the appropriate positions.
nated benzo[c]phenanthrene 5f is not able to produce radi-
As stated above, the mechanism for ring-closure in the cal 11 by C–F bond cleavage. Our experiments have shown
benzo[c]phenanthrenes is different for different functional that rather effective elimination of the HF takes place al-
groups. The mechanism of condensation for 5a, 5b, 5e, 5f though not through the intermediate 11 as no benzo[c]-
and 5h are summarized in Figure 5.
phenanthrene (7) was found in the pyrolysis products. Of
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K. Y. Amsharov, M. A. Kabdulov, M. Jansen
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Figure 5. Proposed mechanism for the condensation of 5a, 5b, 5e, 5f and 5h under FVP conditions.
mation of 5f to 14 needs 78.6 kcal/mol and 89.6 kcal/mol
to form 15. Interestingly, according to the mechanism pro-
posed, the pyrolysis of 5f should result in the selective for-
mation of cyclopenta[cd]pyrene as the more favourable 14
leads directly to 8. Indeed, low-temperature FVP experi-
ments show remarkable selectivity in cyclopenta[cd]pyrene
formation, which approaches 1 only at high temperatures
(Figure 6). More interesting results were obtained by pyrol-
ysis of 5h. In this case intermediate 14 appears to be the
only reasonable product of a concerted elimination of HF.
Such a process should result in the highly selective forma-
tion of 8. However, the opposite effect was observed experi-
mentally. Even high-temperature pyrolysis resulted in a high
preference for benzo[ghi]fluoranthene formation (the ratio
of 6/8 at 1000 °C pyrolysis exceeds 4). Low-temperature py-
rolysis is selective with a ratio of 6/8 of almost 50, which is
unusually high for a pyrolysis experiment (Figure 6). Be-
cause no other side-products were detected in the product
mixture, the selectivity of the condensation process is better
than 97%. Such an effect can be explained by a 1,5-elimi-
nation of HF with the formation of 6 with no intermediate
products. Although such an elimination step is indeed sig-
Figure 6. Influence of functional groups in benzo[c]phenanthrenes
on the temperature-dependent ratio of 6/8.
the possible ways to eliminate HF from 5f, mechanisms via
the benzynes 14 and 15 are the only possible candidates.
According to quantum chemical calculations the transfor-
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Intramolecular Condensation of Benzo[c]phenanthrene
catalytic amount of dibenzoyl peroxide (PBO; 5–10 mg) were dis-
solved in 150 mL) CCl4 and heated at reflux until the reaction had
gone to completion, as determined by TLC analysis (2–6 h). The
reaction mixture was then allowed to cool, washed twice with
water, dried with MgSO4 and concentrated in vacuo. The crude
product was purified chromatographically on silica gel using petro-
leum ether (PE) as the eluent.
nificantly more favoured thermodynamically (6.0 kcal/mol
instead of 73.7 kcal/mol for the generation of 14), it is diffi-
cult to compare these two processes. According to the ex-
perimental data the activation barrier for the reaction of 5h
to 6 is lower than for the reaction of 5h to 14, but still not
low enough to warrant high selectivity and high conversion
simultaneously. Although it appears desirable to corrobo-
rate the mechanism for fluorine-promoted condensation
suggested here by additional investigations, high selectivity
and moderate conversion together with other attractive
properties of fluorine makes it a very promising promoter
for the condensation of PAHs to fullerenes.
Synthesis of Phosphonium Salts: The corresponding benzyl bromide
(100 mmol) and triphenylphosphane (110 mmol) were dissolved in
toluene (70 mL) and the mixture was heated at reflux with vigorous
stirring for 12 h. During this time a white crystalline solid precipi-
tated from solution. The reaction mixture was cooled to room tem-
perature and then filtered. The solid was washed with toluene and
hexane to give a total of 85–95% yield of the phosphonium salt as
a white powder. The resulting salt was used in the next step without
additional purification.
Conclusions
Synthesis of Benzo-stilbenes: The whole procedure was carried out
in a round-bottomed flask under argon. The phosphonium salt
(34 mmol) was dissolved in anhydrous THF (100 mL). The mixture
was cooled to –78 °C and stirred while n-butyllithium (38 mmol)
was added dropwise over 10–20 min. The orange-red solution ob-
tained was allowed to warm to room temperature and stirred for
an additional 45 min. At this point, 2-naphthaldehyde (34.1 mmol)
was added as a solution in anhydrous THF. After stirring for 12 h
the reaction mixture turned light-yellow. THF was removed and
the resulting material was passed through a short silica gel plug
using DCM as the eluent. A white solid of the title compound as
a mixture of cis/trans isomers was obtained in a yield of 65–80%.
The resulting stilbene was used in the next step without additional
purification.
Intramolecular condensation creating new rings is the
key step in the direct synthesis of fullerenes by the folding
of PAHs of an appropriate composition and constitution.
By using benzo[c]phenanthrene as a model system we have
investigated its intramolecular cyclization under FVP con-
ditions with different promoters introduced into the pristine
PAH. The methyl group and fluorine were found to be pro-
spective promoters for intramolecular condensation under
FVP conditions. The methyl group activates the cyclization
without being introduced into the sterically hampered fjord
region and can provide double-radical-promoted cycliza-
tion. Such a process would be particularly beneficial for ful-
lerene synthesis in which a high number of new cycles need
to be created. We observed an unexpectedly high selectivity
in the intramolecular condensation promoted by fluorine
by FVP. The high resistance of the C–F bond to many or-
ganic reagents and its small size and low molecular weight
make it a perfect candidate for the synthesis of effective
fullerene precursors. The fluorine-promoted condensation
mechanism suggested seems to allow full control of the pro-
cess. Thus, if the fluorine atom has only one neighbouring
hydrogen atom in the precursor structure the elimination
will be directed towards the desired product. Such an ap-
proach may fully solve the problem of effective activation
in the direct synthesis of fullerenes by FVP. The corre-
sponding investigations are in progress.
Synthesis of Benzo[c]phenanthrenes: The corresponding benzo-stil-
bene (10 mmol) as a cis/trans mixture was dissolved in cyclohexane
(350 mL). The resulting solution was placed in a 500-W water-co-
oled quartz photochemical reactor and iodine (11 mmol) was then
added. Argon was bubbled through the stirred solution for 15–
20 min before an excess of propylene oxide (10 mL) was added.
After irradiation for 10–30 h the colour of iodine had disappeared.
The mixture was washed with aqueous Na2S2O3 to remove residual
traces of iodine, concentrated in vacuo and then purified by flash
chromatography on silica gel. A mixture of light petroleum (PE)
and dichloromethane (DCM; 1:1) was used as eluent. The targeted
compound was obtained with 60–95% yield as a white solid.
(E/Z)-2-[2-(2-Bromo-5-fluorophenyl)ethenyl]naphthalene (4g): 2-
Bromo-5-fluorobenzylphosphonium bromide (9.4 g, 17.7 mmol),
tBuOK (1.99 g, 17.7 mmol) and 2-naphthaldehyde (2.77 g,
17.7 mmol) were dissolved in ethanol (200 mL). The mixture was
heated at reflux overnight and neutralized by the addition of 1 
HCl. The product was concentrated by evaporation under reduced
pressure, diluted with water and extracted with DCM. The DCM
solution was dried with MgSO4 and filtered through silica gel (PE/
DCM, 1:1). After evaporation the title compound was obtained as
a white solid (4 g, 70%).
Experimental Section
Mass spectra were recorded with a Shimadzu AXIMA resonance
spectrometer. All NMR spectra were measured at 20 °C. Rf values
were determined on TLC-PET sheets coated with silica gel and a
fluorescent indicator (254 nm; layer thickness 0.25 mm, medium
pore diameter 60 Å; Fluka). Elemental analyses were performed
with an Elementar VARIO EL elemental analyser. HPLC analyses
2-Chlorobenzo[c]phenanthrene (5a): White solid (yield 70%). Rf =
1
0.38 (hexane). M.p. 61.4–61.8 °C. H NMR (CDCl3, 300 MHz): δ
were carried out by using
a Cosmosil Buckyprep column
= 7.48 (dd, J1 = 8.57, J2 = 2.01 Hz, 1 H), 7.52–7.68 (m, 2 H), 7.69–
7.88 (m, 5 H), 7.93 (dd, J1 = 7.87, J2 = 1.36 Hz, 1 H), 8.96 (d, J =
8.43 Hz, 1 H), 9.03 (d, J = 1.82 Hz, 1 H) ppm. 13C NMR (CDCl3,
75 MHz): δ = 126.12, 126.41, 126.64, 126.67, 126.94, 127.05,
127.18, 127.36, 128.14, 128.69, 129.91, 130.11, 131.15, 131.48,
131.73, 132.35, 133.52 ppm. MS (LDI-TOF): calcd. for C18H11Cl
262.0549m/z ; found 262.06 [M]+. C18H11Cl1 (262.74): calcd. C
82.29, H 4.22; found C 81.9, H 4.2.
(4.6ϫ250 mm, methanol, UV detection). Chromatographic purifi-
cations were carried out with flash grade silica gel Kieselgel 60
(0.06–0.2 mm; Roth). Quantum chemical calculations were per-
formed by the DFT method with B3LYP/6-31G using the
Gaussian03 software package.[23]
Synthesis of Benzyl Bromides: The relevant methylbenzene
(100 mmol) and N-bromosuccinimide (NBS; 100 mmol) with a
Eur. J. Org. Chem. 2009, 6328–6335
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6333

K. Y. Amsharov, M. A. Kabdulov, M. Jansen
FULL PAPER
2-Methylbenzo[c]phenanthrene (5b): White solid (yield 57%). Rf =
0.31 (hexane). M.p. 75.8–76.2 °C. H NMR (CDCl3, 300 MHz): δ
(silica gel, PE). White solid (yield 66%). Rf = 0.25 (hexane). M.p.
85.0–85.4 °C. 1H NMR (CDCl3, 300 MHz): δ = 7.31 (td, J1 = 2.42,
1
= 2.57 (s, 3 H), 7.34 (d, J = 8.13 Hz, 1 H), 7.48–7.86 (m, 7 H), 7.93
(dd, J1 = 6.5, J2 = 1.42 Hz, 1 H), 8.86 (s, 1 H), 9.01 (d, J = 10.6 Hz,
1 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 22.24, 125.65, 125.90,
125.97, 126.88, 126.95, 127.23, 127.26, 127.39, 127.65, 127.82,
128.37, 128.5, 130.42, 130.49, 131.14, 131.58, 133.45, 135.83 ppm.
MS (LDI-TOF): calcd. for C19H14 [M]+ 242.1096; found: 242.11.
C19H14 (242.32): calcd. C 94.18, H 5.82; found C 94.1, H 5.9.
J2 = 8.32 Hz, 1 H), 7.49–8.09 (m, 8 H), 8.73 (dd, J1 = 2.1, J2 =
12.37 Hz, 1 H), 8.99 (d, J = 8.34 Hz, 1 H) ppm. 13C NMR (CDCl3,
75 MHz): δ = 112.46 (d, J = 23.4 Hz), 115.16 (d, J = 24.23 Hz),
126.01, 126.19 (d, J = 2.42 Hz), 126.51, 126.66, 126.72, 126.77,
127.08 (d, J = 1.06 Hz), 127.11, 128.09, 128.70, 130.29, 130.6 (d, J
= 9.13 Hz), 131.37 (d, J = 8.83 Hz), 131.53, 133.4, 161.45 (d, J =
244.15 Hz) ppm. MS (LDI-TOF): calcd. for C18H11F [M]+
246.0845; found 246.06. C18H11F (246.28): calcd. C 87.78, H 4.50;
found C 87.3, H 4.4.
2-Trifluoromethylbenzo[c]phenanthrene (5c): White solid (yield
73%). Rf = 0.40 (hexane). M.p. 109.6–110.0 °C. 1H NMR (CDCl3,
300 MHz): δ = 7.55–7.75 (m, 4 H), 7.82–7.9 (m, 3 H), 7.93–8.08
(m, 2 H), 8.92 (d, J = 8.48 Hz, 1 H), 9.33 (s, 1 H) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 121.66 (q, J = 3.38 Hz), 125.38 (q, J =
4.5 Hz), 126.41, 126.60, 126.84, 126.88, 127.42, 128.39, 128.79,
129.11, 129.33, 129.48, 129.94, 131.46, 133.7 ppm. MS (LDI-TOF):
calcd. for C19H11F3 [M]+ 296.0813; found 296.10. C19H11F3
(296.29): calcd. C 77.02, H 3.74; found C 76.1, H 4.0.
Supporting Information (see also the footnote on the first page of
this article): NMR, UV/Vis spectra and HPLC profiles of new com-
pounds, FVP apparatus, energies of C–H bond cleavage and ener-
gies of HF elimination from various benzo[c]phenanthrenes.
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2-Phenylbenzo[c]phenanthrene (5e): White solid (yield 55%). Rf =
1
0.18 (hexane). M.p. 131.5–131.9 °C. H and 13C NMR spectra are
in good agreement with the data reported previously.[24] MS (LDI-
TOF): calcd. for C24H16 [M]+ 304.1252; found 304.13. C24H16
(304.39): calcd. C 94.70, H 5.30; found C 94.5, H 5.2.
[6] L. T. Scott, M. M. Boorum, B. J. McMahon, S. Hagen, J.
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2-Fluorobenzo[c]phenanthrene (5f): White solid (yield 66%). Rf =
0.33 (hexane). M.p. 63.7–64.1 °C. H NMR (CDCl3, 300 MHz): δ
1
= 7.31 (td, J1 = 2.42, J2 = 8.32 Hz, 1 H), 7.49–8.09 (m, 8 H), 8.73
(dd, J1 = 2.1, J2 = 12.37 Hz, 1 H), 8.99 (d, J = 8.34 Hz, 1 H) ppm.
13C NMR (CDCl3, 75 MHz): δ = 112.46 (d, J = 23.4 Hz), 115.16
(d, J = 24.23 Hz), 126.01, 126.19 (d, J = 2.42 Hz), 126.51, 126.66,
126.72, 126.77, 127.08 (d, J = 1.06 Hz), 127.11, 128.09, 128.70,
130.29, 130.6 (d, J = 9.13 Hz), 131.37 (d, J = 8.83 Hz), 131.53,
133.4, 161.45 (d, J = 244.15 Hz) ppm. MS (LDI-TOF): calcd. for
C18H11F [M]+ 246.0845; found 246.04. C18H11F (246.28): calcd. C
87.78, H 4.50; found C 86.1, H 4.5.
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[14] M. M. Boorum, L. T. Scott in Modern Arene Chemistry (Eds.:
D. Astruc), Wiley-VCH, Weinheim, 2002, pp. 20–31, and refer-
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4-Bromo-1-fluorobenzo[c]phenanthrene (5g): White solid (yield
1
50%). Rf = 0.34 (hexane). H NMR (CDCl3, 300 MHz): δ = 7.12–
7.21 (m, 1 H), 7.49–7.59 (m, 2 H), 7.69–7.77 (m, 1 H), 7.78–7.86
(m, 2 H), 7.87–7.95 (m, 2 H), 8.08–8.28 (m, 2 H) ppm. 13C NMR
(CDCl3, 75 MHz): δ = 112.87 (d, J = 25.74 Hz), 117.42 (d, J =
3.47 Hz), 120.3 (d, J = 13.67 Hz), 124.44 (d, J = 3.25 Hz), 125.17
(d, J = 2.94 Hz), 125.56, 125.93 (d, J = 2.19 Hz), 126.29, 127.47,
129.09, 129.22, 129.57 (d, J = 16.9 Hz), 129.83 (d, J = 3.4 Hz),
130.21 (d, J = 8.99 Hz), 131.82, 132.93, 133.35 (d, J = 4.45 Hz),
158.85 (d, J = 254.41 Hz) ppm. Crystal data: monoclinic; space
group P21/c, a = 7.2750(8), b = 11.6037(12), c = 15.4111(16) Å, β
= 91.147(2)°, V = 1300.7(2) Å3, Z = 4, 2θmax = 54.26°, –9ϽhϽ9,
–14ϽkϽ14, –19ϽlϽ19, λ = 0.71073 Å, T = 137(2) K, final R =
0.0369 (Rw = 0.1030).
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[21] J. W. Coomber, E. Whittle, Trans. Faraday Soc. 1967, 63, 2656–
CCDC-743279 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2667.
[22] D. O’Hagan, Chem. Soc. Rev. 2008, 37, 308–319.
[23] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
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Vreven, G. Scalmani, K. N. Kudin, S. S. Iyengar, J. Tomasi, V.
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Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
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1-Fluorobenzo[c]phenanthrene (5h): 4-Bromo-1-fluorobenzo[c]phen-
anthrene (1 g, 3.1 mmol) was dissolved in anhydrous THF (50 mL).
All procedures were carried out under argon. The mixture was
cooled to – 78 °C and stirred while n-butyllithium (3.1 mmol) was
added dropwise. The reaction was quenched after 3 min by the ad-
dition of cooled acetic acid (3 mL). The reaction mixture was
warmed to room temperature. Afterwards H2O was added and the
product was extracted with DCM, dried with Na2SO4 and the sol-
vents evaporated. The compound was purified by chromatography
6334
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2009, 6328–6335

Intramolecular Condensation of Benzo[c]phenanthrene
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boul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Lia-
shenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T.
Keith, M. A. Al-Laham, C. Y. Peng, A. Nanayakkara, M.
Challacompe, W. Chen, M. W. Wong, J. A. Pople, Gaussian03,
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Received: August 27, 2009
Published Online: November 4, 2009
Eur. J. Org. Chem. 2009, 6328–6335
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
6335