Homo-elimination of HF - An efficient approach For intramolecular aryl-aryl coupling

Article abstract of DOI:10.1002/chem.201000374

(Figure Presented) A process of elimination: The throughspace fluorine activation in the benzo[c]phenanthrene cyclisation process has been examined experimentally and confirmed theoretically (see figure). Thermally provoked HF homo-elimination was found to be an efficient approach for selective intramolecular aryl-aryl coupling. The data obtained provide new prospects for the synthesis of non-planar aromatic hydrocarbons and fullerenes employing the flash vacuum pyrolysis technique.

Full text of DOI:10.1002/chem.201000374

DOI: 10.1002/chem.201000374
Homo-elimination of HF—An Efficient Approach for Intramolecular
Aryl–Aryl Coupling
Konstantin Yu. Amsharov,* Mikhail A. Kabdulov, and Martin Jansen[a]
The discovery of fullerenes has given rise to appreciable
interest in the field of synthesis of non-planar “aromatic”
structures, especially in the context of the direct synthesis of
fullerenes.[1–7] The strategy generally followed starts with the
the fullerene precursor necessitates the introduction of a
considerable number of promoter groups. Moreover, the
radical nature of the condensation significantly affects the
selectivity of the process.
synthesis of
a
planar polycyclic aromatic hydrocarbon
In the search for more efficient promoters of ring closure
we have turned our attention to the possibility of using fluo-
rine for this purpose. Recently, we reported on the efficient
intramolecular ring closure in fluorinated benzo[c]phenan-
threnes by HF elimination.[13] Herein, we present experi-
mental and computational evidence that HF elimination is a
synchronous process leading directly to the target molecule
without any intermediates, and therefore, producing no side
products. We confirm that HF elimination is a homo-elimi-
nation process. The data obtained provide new prospects for
rational fullerene synthesis by using fluorine as an activating
group, which seems to possess all the required properties of
an “ideal” promoter of ring closure.
To systematically study the effect of fluorine functionali-
sation on the selectivity of thermally activated aryl–aryl cou-
pling, we have synthesised 1-fluorobenzo[c]phenanthrene
(1) and 2-fluorobenzo[c]phenanthrene (2), and have subject-
ed them to FVP. The products obtained give clear evidence
for selective 1,5 ring closure in the case of 1.[13] Such fluorine
activity in intramolecular condensation is unexpected, since
(PAH) that already contains the carbon framework required
for the formation of the target molecule. Such “unrolled”
molecules can be “rolled up” to form fullerenes under flash
vacuum pyrolysis (FVP) conditions.[8–12] The high tempera-
tures applied at FVP result in intramolecular condensation
of the precursor and provide the energy necessary to build
up the strained fullerene molecule. The presence of halogen
atoms, such as chlorine or bromine, in the initial precursor
has been found to be essential for effective radical genera-
tion, which is the driving force for further aryl–aryl cou-
pling. Although this approach has proven to be quite prolific
for the synthesis of many small non-planar PAHs,[1–5] selec-
tive and high yielding direct synthesis of fullerenes still re-
mains a challenge for organic chemistry. The principal possi-
bility of selective fullerene cage formation through FVP has
[1–5]
[6]
been demonstrated by the examples of C60,
C78 and
C84.[7] However, the rates of conversion to the target mole-
cules have remained disappointingly low. C60 has been ob-
tained with 0.1–1% yield,[3–5] whereas higher fullerenes were
only detected by mass spectrometry.[6,7] The low yields in
converting precursors to the respective fullerene result from
a lack of efficient promoters of intramolecular condensation.
The frequently employed bromine or chlorine functionalisa-
tions reach their limits in the case of large molecules due to
decomposition of such halogenated precursors during subli-
mation. The molar masses will generally be high, since the
À
the C F bond is considered to be one of the most stable
bonds in organic chemistry.[14] In contrast to other halogens,
À
the high stability of the C F bond makes it impossible to
produce a radical by homolytic bond cleavage. The mecha-
nism of the fluorine activation observed in this study is obvi-
ously different and includes synchronous HF elimination.
HF elimination is a well known decomposition process in
the photolysis of fluoroethylenes.[15–20] Although the photo-
fragmentation would be expected to proceed from an excit-
ed electronic state, it has been shown that dissociation most
likely occurs through the ground state.[15,20] This is supported
by the fact that thermal fragmentation of fluoroethylenes
shows a similar result, and the main decomposition process
is the unimolecular 1,2-HF elimination.[21] Theoretical stud-
ies have revealed that 1,2-HF elimination is the main de-
composition channel, which passes through a four- or three-
À
large number of new C C bonds that need to be formed in
[a] Dr. K. Y. Amsharov, M. A. Kabdulov, Prof. M. Jansen
Department Chemistry III, Institution Max Planck
Institute for Solid State Research
Heisenbergstrasse 1, 70569 Stuttgart (Germany)
Fax : (+49)7116891502
E-mail: k.amsharov@fkf.mpg.de
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000374.
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Chem. Eur. J. 2010, 16, 5868 – 5871

COMMUNICATION
centred transition state. The activation barrier for HF elimi-
nation in fluoroethylenes has been calculated to be as low
as 74 kcalmolÀ1.[22] A similar elimination process has been
identified as the main channel of fluorobenzene decomposi-
tion. The activation barrier in the latter case is higher and
amounts to 92 kcalmolÀ1 (Scheme 1).[23]
benzo[c]phenanthrene molecule, which leads directly to the
target molecule without any intermediates. This process is
supposed to be energetically more favourable than the com-
peting 1,2 elimination.
To corroborate the presented hypothesis, and to prove the
principal possibility of 1,5-HF elimination experimentally,
1,2,3,4-tetrafluorobenzo[c]phenanthrene (8) has been syn-
thesised and subjected to FVP under the same conditions.
The synthetic route to compound 8 is summarised in
Scheme 3. None of the fluorine atoms in 8 has immediate
Scheme 3. The synthetic route to 1,2,3,4-tetrafluorobenzo[c]phenanthrene
8 and its condensation to trifluorobenzoACHTUNRTGNEUNG[ghi]fluoranthene 9. a) PPh3, tolu-
ene, 97% b) 2-naphthaldehyde, KOtBu, EtOH, 87% c) hn, I2, propylene
oxide, cyclohexane, 43% d) FVP, 11008C, 60%.
Scheme 1. 1,2-HF elimination through a four-centred transition state—
the main channel of difluoroethene and fluorobenzene decomposition.
Considering the pyrolysis of 1 and 2, the formation of
benzyne 3 as an intermediate through 1,2-HF elimination
appears to be most probable (Scheme 2). The highly reac-
hydrogen neighbours, and therefore 8 is not able to elimi-
nate HF through conventional 1,2 elimination. Thus, the for-
mation of benzoACTHNUTRGNEUNG[ghi]fluoranthene would unambiguously
confirm that direct 1,5-HF elimination had occurred.
Indeed, FVP of 8 at 9008C resulted in highly selective ben-
zo[c]phenanthrene formation. In particular, no cyclopen-
ta[cd]pyrene derivatives were found in the pyrolysis prod-
ucts, additionally confirming the proposed mechanism. Since
all other channels of HF elimination are blocked in 8, in-
creasing the pyrolysis temperature up to 11008C does not
affect the selectivity substantially. This has made it possible
to achieve a remarkably selective and efficient condensation
of 8 to 9 under FVP conditions with up to 60% yield.
Almost no side products were detected in the pyrolysis
product (see the Supporting Information). This means that
unreacted 8 can be recovered, and in principle condensed to
the target product with close to quantitative yield by repeat-
ed FVP.
Scheme 2. Decomposition of 1- and 2-fluorobenzo[c]phenanthrenes
under FVP conditions through HF elimination.
Quantum chemical calculations show that 1,5 elimination
in the 1-fluorobenzo[c]phenanthrene molecule is possible
through the four-centred transition state. The transition
state and its optimum geometrical parameters are presented
in Figure 1. The activation energy for this process was found
to be only 80.2 kcalmolÀ1. The 1,5-HF elimination is, thus,
tive intermediate 3 could relax by ring closure and result in
the target benzoACHTUNGTRENNUNG[ghi]fluoranthene (4). According to this
mechanism both 1 and 2 should provide comparable activity
in intramolecular condensation. The FVP experiments show
however, that 1 is remarkably more active than 2. For in-
stance, the pyrolysis at 10008C results in 15% of 4 in the
case of 1, whereas pyrolysis of 2 at the same conditions gave
only 1% of the product. Moreover, pyrolysis of 2 preferably
results in the formation of cyclopenta[cd]pyrene (6), where-
as 1 almost exclusively leads to the desired 4. Such a large
difference in the reactivity of 1 and 2 indicates that 3 is not
a relevant intermediate in the condensation path from 1 to
4. The effective cyclisation of 1 can only be understood as a
1,5 elimination of HF from the fjord region of the 1-fluoro-
substantially
favoured
over
the
1,2
elimination
(92 kcalmolÀ1), which is in excellent agreement with our ex-
perimental data.
Interestingly, none of the carbon atoms involved changes
its hybridisation during the reaction, which is atypical for an
elimination process. The formal increase in unsaturation, a
necessary prerequisite of any elimination process, is realised
through the formation of a new ring in the system. The p
system of benzo[c]phenanthrene is not involved in the con-
densation process and the elimination process can thus be
considered as a four-centred homo-elimination. To estimate
Chem. Eur. J. 2010, 16, 5868 – 5871
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K. Yu. Amsharov et al.
In summary, the HF homo-elimination has proven to be
an efficient approach for intramolecular aryl–aryl coupling,
for example, in closing five membered rings in PAHs. The
through-space fluorine activation mechanism has been iden-
tified experimentally and confirmed by quantum chemical
calculations. The activation energies found are qualitatively
in good accordance with FVP experimental data. Taking
into consideration that fluorine can promote the desired
ring closure only if hydrogen is placed in a neighbouring po-
sition in space in the precursor structure, it seems to be pos-
sible to fully control the direction of the condensation. Ac-
cording to our results highly efficient intramolecular con-
densations of appropriately functionalised PAH to non-
planar PAHs by homo-HF elimination can be achieved. Use
of fluorine as an activating group solves the problem of se-
lectivity in FVP and should provide an effective conversion
of the respective planar PAH precursors to the desired ful-
Figure 1. Transition-state geometry for HF homo-elimination predicted
with the DFT B3LYP/6-311G(d,p) method. On the right, only those
AHCTUNGTRENNUNG
AHCTUNGTRENNGlUN erene cage.
bonds broken or formed during the reaction are shown. Bond lengths are
given in ꢃ, angles in degrees. The geometry of the benzo[c]phenanthrene
moiety in the transition state is characterised by sufficient deviation from
planarity. The four-centred transition state is highly distorted from square
geometry, and the H atom is placed virtually on the C–F axis.
Experimental Section
The synthesis of 1- and 2- fluorobenzo[c]phenanthrenes and FVP details
were reported elsewhere.[13] The full synthesis routes for new compounds
7, 8 and 9 are given in the Supporting Information. DFT was employed
to characterise the ground electronic potential energy surface for the HF
elimination reactions. The DFT calculations were carried out with the
the influence of the p system on the condensation process,
the model reaction between benzene and fluorobenzene to
form biphenyl has been investigated theoretically. Indeed, a
low energetic path to biphenyl with a similarly low activa-
tion energy barrier of 84.2 kcalmolÀ1 was also found for this
system. This confirms our conclusion that HF elimination in
the system considered is actually a homo reaction. Ordinari-
ly, such a process cannot possibly occur due to entropy rea-
sons, but it becomes viable in the case of benzo[c]phenan-
threne, in which the benzene rings are already oriented in
B3LYP hybrid functional and the 6–311GACTHNUTRGNEUNG
(d,p) standard basis set,[26]
which previously had been applied successfully for the investigation of
HF elimination in fluorobenzene.[23] Frequency calculations were then
performed to characterise stationary points as minima or saddle points
and to evaluate zero point vibrational energies. The transition states were
found by following the minimum energy path from the reactant to the ex-
pected product. The activation energies are given including zero point
energy corrections. All the calculations were carried out by using the
GAUSSIAN 03 program package.[27]
À
an appropriate way in space. In other words, Csp2 H and
À
Csp2 F groups can interact directly through space, whereas
the rest of the molecule is not involved in the process, but is
just interconnecting the reacting atoms. This interaction fi-
À
nally results in formation of a new Csp2 Csp2 bond and elimi-
nation of HF.
Acknowledgements
We thank Prof. D. Gudat for the measurement of 19F NMR and Dr. U.
Wedig for helpful discussions.
The geometries of the transition states for benzene–fluo-
robenzene condensation and for the cyclisation in 1 were
found to be similar, except for a higher deviation from pla-
narity in the case of the biphenyl system. The angular orien-
tation of benzene rings with a 1138 angle was found to be
optimal for homo HF elimination (see the Supporting Infor-
mation). Such an orientation of benzene rings in benzo[c]-
phenanthrene requires partial perturbation of the aromatic
system, but according to our calculations the deviation from
planarity does not contribute significantly to the energy. As
an important consequence, the cyclisation occurs through a
transition state with an already curved p system, which is re-
quired for geodesic PAHs and fullerenes. Regarding the
condensation of fullerene precursors in which the “benzene
rings” are already oriented in an appropriate angular
manner, such a bent geometry seems to be suitable for con-
densation and the activation barrier for HF elimination is
expected to be low.
À
Keywords: C C coupling · cyclization · flash pyrolysis ·
polycycles · reaction mechanisms
[1] M. M. Boorum, Y. V. Vasilꢁev, T. Drewello, L. T. Scott, Science 2001,
294, 828–831.
[2] B. Gꢂmez-Lor, C. Koper, R. H. Fokkens, E. J. Vlietstra, T. J. Cleij,
L. W. Jenneskens, N. M. M. Nibbering, A. M. Echavarren, Chem.
Commun. 2002, 4, 370–371.
[3] L. T. Scott, M. M. Boorum, B. J. McMahon, S. Hagen, J. Mack, J.
Blank, H. Wegner, A. de Meijere, Science 2002, 295, 1500–1503.
[4] K. Y. Amsharov, M. Jansen, Z. Naturforsch. B 2007, 62, 1497–1508.
[5] K. Y. Amsharov, K. Simeonov, M. Jansen, Carbon 2007, 45, 337–
343.
[6] K. Y. Amsharov, M. Jansen, J. Org. Chem. 2008, 73, 2931–2934.
[7] K. Y. Amsharov, M. Jansen, Chem. Commun. 2009, 2691–2693.
[8] G. Mehta, H. S. P. Rao, Tetrahedron 1998, 54, 13325–13370.
5870
www.chemeurj.org
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 5868 – 5871

Homo-elimination of HF for Intramolecular Aryl–Aryl Coupling
COMMUNICATION
[9] M. J. Plater, M. Praveen, D. M. Schmidt, Fullerene Sci. Technol.
1997, 5, 781–800.
[10] L. T. Scott, Pure Appl. Chem. 1996, 68, 291–300.
[11] L. T. Scott, Angew. Chem. 2004, 116, 5102–5116; Angew. Chem. Int.
Ed. 2004, 43, 4994–5007.
[12] V. M. Tsefrikas, L. T. Scott, Chem. Rev. 2006, 106, 4868–4884.
[13] K. Y. Amsharov, M. Kabdulov, M. Jansen, Eur. J. Org. Chem. 2009,
6328–6335.
[14] D. OꢁHagan, Chem. Soc. Rev. 2008, 37, 308–319.
[15] B. A. Balko, J. Zhang, Y. T. Lee, J. Phys. Chem. A 1997, 101, 6611–
6618.
[16] M. J. Berry, G. C. Pimentel, J. Chem. Phys. 1969, 51, 2274.
[17] D. J. Donaldson, D. G. Watson, J. J. Sloan, Chem. Phys. 1982, 68, 95–
107.
[18] J. Gonzꢄlez-Vꢄzquez, A. Fernandez-Ramos, E. Martinez-Nunez,
S. A. Vazquez, J. Phys. Chem. A 2003, 107, 1389–1397.
[19] J. Gonzꢄlez-Vꢄzquez, E. Martinez-Nunez, A. Fernandez-Ramos,
S. A. Vazquez, J. Phys. Chem. A 2003, 107, 1398–1404.
[20] J. J. Lin, S. M. Wu, D. W. Hwang, Y. T. Lee, X. Yang, J. Chem. Phys.
1998, 109, 10838–10846.
[21] J. M. Simmie, E. Tschuiko, J. Phys. Chem. 1970, 74, 4075.
[22] E. Martinez-NuÇez, S. A. Vazquez, Struct. Chem. 2001, 12, 95–100.
[23] C. Y. Wu, Y. J. Wu, Y. P. Lee, J. Chem. Phys. 2004, 121, 8792–8799.
[24] M. Sarobe, L. W. Jenneskens, U. E. Wiersum, Tetrahedron Lett.
1996, 37, 1121–1122.
[25] The mixture of cis/trans isomers can be used in photocyclisation
without separation, since both isomers interconvert under UV irra-
diation.
[26] A. D. J. Becke, Chem. Phys. 1993, 169–178, 5648–5652.
[27] Gaussian 03, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
ery, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian, Inc., Wall-
ingford CT, 2004.
Received: February 11, 2010
Published online: April 13, 2010
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