Kinetic Control in the Palladium-Catalyzed Synthesis of C2-Symmetric Hexabenzotriphenylene. A Conformational Study

Article abstract of DOI:10.1021/ol005916p

(Equation Presented) The hitherto unisolated, thermodynamically unstable C₂-symmetric conformer of hexabenzotriphenylene (1) has been efficiently synthesized by palladium-catalyzed cyclotrimerization of 9,10-didehydrophenanthrene (4). The barriers to conformational interconversion of 1 are examined experimentally and by computational studies.

Full text of DOI:10.1021/ol005916p

ORGANIC
LETTERS
2000
Vol. 2, No. 11
1629-1632
Kinetic Control in the
Palladium-Catalyzed Synthesis of
C -Symmetric Hexabenzotriphenylene. A
2
Conformational Study
Diego Pen˜a, Agust´ın Cobas, Dolores Pe´rez,* Enrique Guitia´n,* and Luis Castedo
Departamento de Qu´ımica Orga´nica, UniVersidad de Santiago de Compostela y
Unidad Asociada al CSIC, 15706 Santiago de Compostela, Spain
qoenrgui@uscmail.usc.es
Received April 8, 2000
ABSTRACT
The hitherto unisolated, thermodynamically unstable C -symmetric conformer of hexabenzotriphenylene (1) has been efficiently synthesized
2
by palladium-catalyzed cyclotrimerization of 9,10-didehydrophenanthrene (4). The barriers to conformational interconversion of 1 are examined
experimentally and by computational studies.
The chemistry of overcrowded PAHs has become a field of
increasing interest in recent years. This kind of molecule
usually displays some grade of distortion from planarity to
minimize steric strain,1 giving rise to interesting structures
such as helicenes,2 twists, or propeller-like geometries.3 The
study of the structure and conformational stability of these
molecules is important not only from the theoretical point
of view but also for their potential applications in catalysis,
material science, or supramolecular chemistry. A particular
subclass of overcrowded PAHs is formed by molecules with
“ideal” D3h symmetry, such as polysubstituted triphenylenes,
which are forced to adopt conformations of lower symmetry
to relieve steric congestion. Studies on the geometries of
these compounds have shown that for most of them C2-
symmetric conformations are preferred to the intuitively more
predictable D3-symmetric ones.4 In general, the experimen-
tally determined geometries are in accordance with the
calculated lowest energy conformations.5,6
An interesting example of a nominally D3h-symmetric PAH
is hexabenzotriphenylene (dibenzo[f,j]phenanthro[9,10-s]-
picene 1), a highly strained molecule whose synthesis has
(1) (a) Herndon, W. C.; Connor, D. A.; Lin, P. Pure Appl. Chem. 1990,
62, 435-444. (b) Herndon, W. C.; Nowack, P. C.; Connor, D. A.; Lin, P.
J. Am. Chem. Soc. 1992, 114, 41-47.
(4) (a) Shibata, K.; Kulkarni, A. A.; Ho, D. M.; Pascal, R. A., Jr. J. Am.
Chem. Soc. 1994, 116, 5983-5984. (b) Shibata, K.; Kulkarni, A. A.; Ho,
D. M.; Pascal, R. A., Jr. J. Org. Chem. 1995, 60, 428-434. (c) Hursthouse,
M. B.; Smith, V. B.; Massey, A. G. J. Fluorine Chem. 1977, 10, 145-155.
(d) Frampton, C. S.; Macnicol, D. D.; Rowan, S. J. J. Mol. Struct. 1997,
405, 169-178.
(2) (a) Martin, R. H. Angew. Chem., Int. Ed. Engl. 1974, 13, 649-660.
(b) Laarhoven, W. H.; Prinsen, W. J. C. In Topics in Current Chemistry,
Vol. 125; Boschke, F. L., Ed.; Springer-Verlag: Berlin, 1984; pp 65-127.
(c) Grimme, S.; Harren, J.; Sobanski, A.; Vo¨gtle, F. Eur. J. Org. Chem.
1998, 1491-1509.
(5) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J.
Am. Chem. Soc. 1985, 107, 3902-3909.
(3) (a) Pascal, R. A., Jr. Pure Appl. Chem. 1993, 65, 105-110. (b) Qiao,
X.; Ho, D. M.; Pascal, R. A., Jr. Angew. Chem., Int. Ed. Engl. 1997, 36,
1531-1532.
(6) Barnett, L.; Ho, D. M.; Baldridge, K. K.; Pascal, R. A., Jr. J. Am.
Chem. Soc. 1999, 121, 727-733.
10.1021/ol005916p CCC: $19.00 © 2000 American Chemical Society
Published on Web 05/02/2000

been shown to be difficult so far.7 Recently Pascal and co-
workers reported the preparation of 1 in 5% yield by FVP
long periods of time (several months), led to the gradual
disappeareance of the original signals and the emergence of
new peaks in the 1H NMR spectrum. Careful revision of the
spectral data showed that the product, as originally isolated
1
from our reaction, presented an H NMR spectrum (Figure
1b) in which each signal was shifted 0.03-0.09 ppm
of phenanthrene-9,10-dicarboxylic anhydride and its struc-
tural characterization as a D3-symmetric molecular propeller.6
Afterward, we reported a more convenient synthesis of 1
based on the palladium-catalyzed cyclotrimerization of
9,10-didehydrophenanthrene (9,10-phenanthryne, 4) (Scheme
1).8,9
1
Figure 1. (a) H NMR spectra (250 MHz) of 1-C2 in CDCl3 at
298 K; (b) 1-D3 in CDCl3 at 298 K (250 MHz); (c) 1-C2 in 3:1
CS2/CD2Cl2 at 200 K (500 MHz); (d) 1-C2 in 3:1 CS2/CD2Cl2 at
298 K (500 MHz).
downfield with respect to the data reported in the literature,11
while the new emerging peaks were coincidental with these
previously reported data (Figure 1a).
Scheme 1
We attributed this unexpected observation to a conforma-
tional change in 1. Taking into account that the compound
obtained by Pascal had D3 symmetry, as determined by an
X-ray analysis, we reasoned that the product obtained by us
should be the thermodinamically less stable and hitherto
unisolated C2-symmetric conformer. The appearance of only
1
four signals in the H NMR spectrum (Figure 1b,d) could
be due to rapid interconversion of the two possible C2
enantiomeric conformers (I and II) at room temperature. Note
that in this case enantiomerization is achieved by slippage
of two of the outer benzo groups (A and B in I and II, Figure
2) and that in this conversion the C2 axis is rotated by 120°.
Subsequent conversions of this type eventually result in
scrambling of the six benzo groups. Satisfyingly, when we
carried out low-temperature NMR experiments, we observed
at 200 K the splitting of each signal into three signals of
equal intensity (Figure 1c), in accordance with the expected
spectrum for a C2-symmetric conformer. This result was
rather surprising since semiempirical and ab initio calcula-
tions had shown a strong preference (by 5-9 kcal/mol) for
the D3 geometry of 1.6 Intrigued by our finding, we decided
to carry out experimental and computational studies on the
conformational stability of 1. Here we report the results of
these studies.
The kinetics for the conversion of C2 to D3 conformations
(II to III) were determined by 1H NMR in 1,1,2,2-
tetrachloroethane-d2, at constant temperatures 55, 60, 65, 71,
and 82 °C. The rate constants k for each temperature were
determined from the first-order plots shown in Figure 3. From
these data, the Arrhenius plot gave the activation energy Ea
of 22.8 kcal/mol, while the activation parameters ∆Hq (22.0
Further work carried out after our preliminary com-
munication revealed two important new observations: (1)
optimization of the procedure followed for the isolation of
1 from the reaction mixture led to an improvement in the
yield from 39% to a remarkable 68%;10 (2) heating of our
compound in solution, or storage at room temperature for
(7) (a) Hacker, N. P.; McOmie, F. W.; Meunier-Piret, J.; Van Meerssche,
M. J. Chem. Soc., Perkin Trans. 1 1982, 19-23. Other authors reported
the preparation of 1, but no characterization was given: (b) Halleux, A. L.
Br. Patent 852,981, 1960. (c) Barton, J. W.; Grinham, A. R. J. J. Chem.
Soc., Perkin Trans. 1 1972, 634-637.
(8) Pen˜a, D.; Pe´rez, D.; Guitia´n, E.; Castedo, L. Org. Lett. 1999, 1, 1555-
1557.
(9) For related Pd-catalyzed cycloadditions of arynes, see: (a) Pen˜a, D.;
Escudero, S.; Pe´rez, D.; Guitia´n, E.; Castedo, L. Angew. Chem., Int. Ed.
1998, 37, 2659-2661. (b) Pen˜a, D.; Pe´rez, D.; Guitia´n, E.; Castedo, L. J.
Am. Chem. Soc. 1999, 121, 5827-5828.
(10) Experimental procedure: To a solution of triflate 3 (106 mg, 0.27
mmol) and Pd2(dba)3‚CHCl3 (14 mg, 0.013 mmol) in CH3CN (5 mL) was
added finely powdered anhydrous CsF (122 mg, 0.8 mmol), and the mixture
was stirred at room temperature for 12 h. The resulting suspension was
filtered under vacuum. The solid obtained was washed with CH3CN (2 ×
1 mL) and Et2O (0.5 mL) and dissolved in CH2Cl2/H2O 9:1 (40 mL) The
organic layer was separated, dried over Na2SO4, filtered through a short
pad of SiO2 (CH2Cl2 as eluant), and concentrated under reduced pressure
to afford 1 (32 mg, 68%) as a yellow solid, only slightly soluble in common
organic solvents: 1H NMR (CDCl3, 500 MHz) δ 8.63 (d, J ) 8.0 Hz, 6H),
8.18 (d, J ) 8.2 Hz, 6H), 7.59 (dt, J ) 7.5, 1.2 Hz, 6H), 7.25 (dt, J ) 7.6,
1.0 Hz, 6H); MS, m/z (%) 528 (M+, 29), 264 (16), 261 (66), 254 (100);
HRMS for C42H24, calcd 528.1878, found 528.1869.
(11) The data that we initially gave for the spectrum in CDCl3, identical
to those previously reported,6,7 were obtained from a sample that had been
heated in solution for extended periods of time in an attempt to obtain a
satisfactory 13C NMR spectrum.
1630
Org. Lett., Vol. 2, No. 11, 2000

Figure 2. Schematic drawings of the C2 and D3 conformations and the transition states of hexabenzotriphenylene (1).
kcal/mol), ∆Sq (-12.3 cal/(mol K)), and ∆Gq (26.2 kcal/
mol) could be calculated by application of the Eyring
equation. This moderately high barrier to conversion reflects
the relative kinetic stability that we observed for the C2
conformations at room temperature or below. The barrier to
the much faster interconversion of the C2 enantiomers was
computational methods. The ground states and the transition
states for the interconversion processes were located at the
semiempirical AM113 level using the SPARTAN program.14
The nature of any stationary structure was evaluated by
way of vibrational analyses, with such structures identified
as minima or transition states by the presence of zero or one
imaginary vibrational frequencies, respectively. The geom-
etries initially obtained were fully optimized at the ab initio
Hartree-Fock level using the basis sets 3-21G and 6-31G-
(d) of the GAUSSIAN program.15 These fully optimized
geometries for the C2 (II) and D3 (III) conformations, and
for the transition states proposed for the racemization C2-
C2 (TS-I) and for the conversion C2-D3 (TS-II), are
represented in Figure 2, together with schematic drawings
which illustrate these geometries by showing the relative
disposition of the six outer benzene rings of the molecule.
The structures of the transition states TS-I and TS-II
resemble somehow the geometries of the transition states
proposed for the racemization of [n]helicenes (n ) 5-8),16,17
1
estimated by dynamic NMR.12 We registered the H NMR
spectrum of 1-C2 in CS2/CD2Cl2 (3:1) at different temper-
atures between 25 and -73 °C (200 K) and found that the
spectrum displays coalescence of the signals marked with
an asterisk in Figure 1c, at Tc ) 247 ( 2 K, with ∆ν ) 102
Hz (at 500 MHz). From these observed values, the rate
constant kc (k247 ) 226 s-1) and the free energy of activation
∆Gq (11.7 kcal/mol) for the racemization were determined.
The barriers to interconversion of the conformers of
hexabenzotriphenylene (1) were also estimated by using
(12) Kessler, H. Angew. Chem., Int. Ed. Engl. 1970, 9, 219-235.
(13) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. P. J.
Am. Chem. Soc. 1985, 107, 3902-3909.
(14) MacSpartan Plus; Wavefunction, Inc.; 18401 Von Karman Ave.,
#370, Irvine, CA 92715.
(15) Gaussian 94, Revision E.2. Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.;
Cioslowski, J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng,
C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E.
S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A.
Gaussian, Inc., Pittsburgh, PA, 1995.
Figure 3. Rates of conversion of 1-C2 to 1-D3 in 1,1,2,2-
tetrachloroethane-d2 at constant temperatures 55, 60, 65, 71, and
82 °C.
(16) Lindner, H. J. Tetrahedron 1975, 31, 281-284.
(17) (a) Janke, R. H.; Haufe, G.; Wu¨rthwein, E.-U.; Borkent, J. H. J.
Am. Chem. Soc. 1996, 118, 6031-6035. (b) Grimme, S.; Peyerimhoff, S.
D. Chem. Phys. 1996, 204, 411-417.
Org. Lett., Vol. 2, No. 11, 2000
1631

with two outer benzo groups oriented face-to-face. The
transition state for the racemization C2-C2 (TS-I) presents
Cs symmetry, while the transition state for the conversion
C2-D3 (TS-II) shows a highly distorted structure with C1
symmetry. This greater distortion accounts for the experi-
mentally observed (and theoretically predictable) higher
barrier to interconversion of C2 to D3 conformation than to
racemization of C2 conformers.18
large deviations of the ab initio Hartree-Fock barriers could
be attributed to the neglect of electron correlation effects,
which are expected to be important for the conformational
changes that proceed through highly distorted transition
states.
Single-point calculations at B3LYP19 and BLYP20 levels
with several basis sets using RHF/6-31G(d) optimized
geometries were also performed. The density functional
methods (DFT), which include to some extent electron
correlation effects, gave also barriers in good agreement with
the experimental value for the racemization of the C2
conformers and only 2.5-3.5 kcal/mol above the experi-
mental barrier for the C2-D3 process. The BLYP method
gave slightly better values than the popular hybrid B3LYP
method.21
The results of the computations for the interconversion
barriers are given in Table 1. All calculations showed that
Table 1. Experimental and Calculated Barriers to
Conformational Interconversions in Hexabenzotriphenylene
(kcal/mol)a
In conclusion, we have shown that the palladium-catalyzed
cyclotrimerization of 9,10-didehydrophenanthrene is an ef-
ficient, high-yielding method for the synthesis of hexaben-
zotriphenylene (1) and that the structure of the product
obtained by this mild procedure has C2 symmetry, in contrast
with previous results by other authors who isolated the
thermodynamically more stable D3 conformer. Our studies
on the conformational stability of 1 have demonstrated that
the racemization of the C2-symmetric conformers is a rapid
process, while the conversion to 1-D3 has a much higher
barrier, which makes the C2 conformers relatively persistent
at room temperature or below. The computational calcula-
tions performed, especially the semiempirical AM1 and DFT
methods, are in good agreement with the experimental results
and also with reports on mechanistically related processes
such as the racemization of helicenes.
C2-C2
C2-D3
∆Hq
method
∆Hq
∆Gq
experimental
AM1
RHF/3-21G
RHF/6-31G(d)
B3LYP/6-31G(d)
B3LYP/6-311G(d,p)
BLYP/6-31G(d)
BLYP/6-311G(d,p)
11.7b
11.5b
22.0
22.2c
31.2
30.2
26.5
26.7
25.5
24.5
10.6b
14.2
12.5
10.6
11.4
10.4
10.1
a Differences in zero-point energies and thermal corrections were
estimated at AM1 level to be smaller than 1 kcal/mol and are not included
in ab initio and DFT calculations. b Determined for T ) 247 K. c Determined
for T ) 341 K.
the barrier to interconversion of C2 to D3 is substantially
higher than the one to C2-C2 conversion. However, the
quantitative results varied notably depending on the theoreti-
cal level used. Semiempirical AM1 calculated energies match
the experimental results fairly well while, as expected, the
ab initio Hartree-Fock calculations considerably overesti-
mate the barriers of both processes, particularly for the C2-
D3 transformation, and the performance is not improved
significantly by increasing the size of the basis set. An
analogous trend has previously been reported for studies on
the racemization of [5]helicene,17 which demonstrates the
usefulness of AM1 calculations for this kind of system. The
Acknowledgment. Financial support from the DGES
(PB96-0967) is gratefully acknowledged. We also thank the
Spanish Ministry of Education and Culture for the award of
a fellowship to D. Pen˜a.
Supporting Information Available: Variable tempera-
ture 1H NMR spectra of 1, details of the kinetic experiments,
and full geometry and energy information. This material is
available free of charge via the Internet at http://pubs.acs.org.
OL005916P
(19) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
(20) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. ReV. B 1998, 37, 785-789.
(21) For the racemization of [5]helicenes, the BLYP method has also
been shown to be superior than the hybrid B3LYP (ref 17).
(18) This proposal parallels in every respect the analysis of the
conformational interconversion of perchlorotriphenylene (ref 4b), except
for the symmetry of the C2-D3 transition state.
1632
Org. Lett., Vol. 2, No. 11, 2000