Hexapyrrolylbenzene and octapyrrolylnaphthalene

Full text of DOI:10.1021/jo961192c

9
012
J . Org. Chem. 1996, 61, 9012-9015
Hexa p yr r olylben zen e a n d
Octa p yr r olyln a p h th a len e
Sch em e 1. Syn th esis of Hexa p yr r olylben zen e (1)
a n d Octa p yr r olyln a p h th a len e (2)
†
‡
§
|
H. A. M. Biemans, C. Zhang, P. Smith, H. Kooijman,
|
|
,†
W. J . J . Smeets, A. L. Spek, and E. W. Meijer*
Laboratory of Organic Chemistry, Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The
Netherlands, UNIAX Corporation, Santa Barbara,
California 93117, ETH Z u¨ rich, Materials Department,
Institute for Polymers, ETH-Zentrum, Universit a¨ tstrasse 41,
CH-8092 Z u¨ rich, Switzerland, and Bijvoet Center for
Biomolecular Research, Crystal and Structural Chemistry,
Utrecht University, Padualaan 8,
3
584 CH Utrecht, The Netherlands
Received J une 24, 1996
In tr od u ction
Highly symmetrical, propeller-shaped, hexasubstituted
benzenes have received considerable interest due to their
intriguing properties as sterically congested polycyclic
1
aromatic compounds. Recently, octasubstituted naph-
thalenes and decasubstituted anthracenes were studied
as well.2 An interesting one-pot synthesis for some
selected structures is based on the multiple nucleophilic
substitution of perfluoroaromatics with the corresponding
nucleophile. Nitrogen-containing nucleophiles that have
been reacted six times with hexafluorobenzene comprise
performed the reaction of pyrrolylsodium with hexafluo-
robenzene at ambient temperature in DMF (Scheme 1).
Pyrrolylsodium was prepared by the addition of pyrrole
in dry DMF to sodium hydride in dry DMF. To this 1 M
light-brown solution was added dropwise a stoichiometric
amount of hexafluorobenzene in dry DMF. The notice-
able exothermic reaction yielded, after precipitation in
water, hexapyrrolylbenzene 1 as a white solid in 86%
yield. Crystallization from acetonitrile produced large
crystals of 1 from which the crystal structure was
determined (Figure 1). The crystals are orthorhombic,
3
4,5
4
4
-(dimethylamino)pyridine, imidazole, pyrazole, and
4
,6,7
dimethylpyrazole.
In some of these cases an unex-
plained autoacceleration is suggested in the synthesis,
since hexasubstitution seems to be favored over partial
substitution.3
,4
Remarkably, the synthesis of hexa-
pyrrolylbenzene is reported to be unsuccessful when
using pyrrolylsodium and hexafluorobenzene in THF.4
Since hexapyrrolylbenzene 1 and octapyrrolylnaphtha-
lene 2 are expected to be useful as building blocks for
larger π-conjugated systems, we have investigated the
synthesis and properties of these molecules. In this
communication we report on a convenient and general
synthesis toward 1 and 2, present evidence for the
absence of an autoacceleration in the nucleophilic aro-
matic substitution of hexafluorobenzene, and discuss the
crystal structures of 1 and 2.
space group Pna2
1
with 1 molecule per asymmetric unit.
In the crystal the dihedral angles between the least
squares planes through the pyrrole rings and the central
phenyl ring in the “propeller-shaped” molecule range
between 48° and 69°. Comparing these values to those
8
found for other hexasubstituted benzenes (hexaphenyl:
6
6,7
4° to 69°; hexakis(3,5-dimethylpyrazolyl): 85° or be-
tween 51-122° and hexakis(4-(dimethylamino)-1-pyri-
3
dinium): 80°), we conclude that the pyrroles are able to
become more coplanar with the central phenyl ring than
any other substituent known today. The broad range in
which the dihedral angles are found is also somewhat
wider, reflecting the greater conformational freedom in
the hexapyrrolylbenzene. On the other hand, the UV/
vis spectrum of 1 in acetonitrile shows the presence of
some π-stacking between the pyrrole units (Figure 2), as
is concluded from the small shoulder at λ ) 300 nm on
the low-energy side of the main absorption band at λmax
Resu lts a n d Discu ssion
Since it is known that the rate of an SnAr reaction
increases by the use of a more polar aprotic solvent, we
*
To whom correspondence should be addressed
Eindhoven University of Technology.
UNIAX Corp.
†
‡
§
|
ETH Z u¨ rich.
Utrecht University.
1) (a) Pascal, R. A., J r.; McMillen, W. D.; Van Engen, D. J . Am.
Chem. Soc. 1986, 108, 5652. (b) Pascal, R. A., J r.; McMillen, W. D.;
Van Engen, D.; Eason, R. G. J . Am. Chem. Soc. 1987, 109, 4660. (c)
Smyth, N.; Van Engen, D.; Pascal, R. A., J r. J . Org. Chem. 1990, 55,
(
)
254 nm.
In order to investigate the previously suggested au-
3,4
toacceleration effect, an experiment was conducted in
which 1 equimolar amount of hexafluorobenzene was
added to 2 equimolar amounts of pyrrolylsodium (instead
of 6 needed for a stoichiometric and complete reaction).
After the usual workup, the reaction mixture was sub-
jected to column chromatography. The five main prod-
ucts could be isolated, and their structures were eluci-
1
937. (d) Shibata, K.; Kulkarni, A. A.; Ho, D. M.; Pascal, R. A., J r. J .
Am. Chem. Soc. 1994, 116, 5983. (e) Shibata, K.; Kulkarni, A. A.; Ho,
D. M.; Pascal, R. A., J r. J . Org. Chem. 1995, 60, 428. (f) Barbour, R.
H.; Freer, A. A.; MacNicol, D. D. J . Chem. Soc., Chem. Commun. 1983,
62. (g) Downing, G. A.; Frampton, C. S.; Gall, J . H.; MacNicol, D. D.
Angew. Chem., Int. Ed. Engl. 1996, 35, 1547.
2) Qiao, X.; Padula, M. A.; Ho, D. M.; Vogelaar, N. J .; Schutt, C.
E.; Pascal, R. A., J r. J . Am. Chem. Soc. 1996, 118, 741.
3) Weiss, R.; Pomrehn, B.; Hampel, F.; Bauer, W. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1319.
3
(
(
1
13
19
dated using GC/MS, H, C, and F NMR spectroscopy
(
(
(
(
4) Henrie, R. N., II; Yeager, W. H. Hererocycles 1993, 35, 415.
5) Fujii, S.; Maki, Y.; Kimoto, H. J . Fluor. Chem. 1989, 43, 131.
6) Foces-Foces, C.; Llamas-Saiz, A. L.; Claremunt, R. M.; J agerovic,
1
Table 1). The chemical shifts determined with H NMR
are surprisingly consistent throughout the series. When
a pyrrole substituent has two fluorine neighbors, the
N.; J imeno, M. L.; Elguero, J . J . Chem. Soc., Perkin Trans. 2 1995,
359.
7) Foces-Foces, C.; Llamas-Saiz, A. L.; Escol a´ stico, C.; Claramunt,
R. M.; Elguero, J . J . Phys. Org. Chem. 1996, 9, 137.
1
(
(8) Bart, J . C. J . Acta Crystallogr. B. 1968, 24, 1277.
S0022-3263(96)01192-9 CCC: $12.00 © 1996 American Chemical Society

Notes
J . Org. Chem., Vol. 61, No. 25, 1996 9013
F igu r e 1. ORTEP drawing (50% probability level) of hexapy-
rrolylbenzene (1).
F igu r e 3. Octapyrrolylnaphthalene (2). (a) Top view, ORTEP
drawing at 50% probability level with atom labeling scheme.
(b) Side view, PLUTON drawing.
ronaphthalene as the core molecule (Scheme 1). Octa-
pyrrolylnaphthalene (2) was obtained as a yellow solid
in 80% yield by using the conditions as described for the
synthesis of 1. It was crystalized from acetonitrile,
producing large crystals from which the crystal structure
was determined (Figure 3).
F igu r e 2. UV/vis spectra of 1, 2, and phenylpyrrole measured
in acetonitrile.
The crystals are tetragonal, space group P4
1 1
2 2 and
the unit cell contains four molecules. Each molecule lies
upon a crystallographic two-fold rotation axis. As ex-
pected, the dihedral angles between the least squares
planes through the pyrrole rings and the acene in 2 range
between 59° and 78°. Again, the pyrroles can obviously
be more coplanar with the central naphthalene unit as
signals of the two different pyrrole protons are centered
around 6.96 and 6.43 ppm. If a pyrrole unit is neighbored
by one fluorine atom and another pyrrole unit, its
chemical shifts are approximately 6.52 and 6.26 ppm, and
if the pyrrole unit is sandwiched between two other
pyrrole rings the chemical shifts are 6.18 and 6.08 ppm,
respectively. The products found reflect the expected
ortho-para activation, while indeed a preferred formation
of the highly substituted compounds is found. That
preference for 1 is even more pronounced, when hexa-
fluorobenzene was added very slowly to pyrrolylsodium;
the product consisted of >90% hexapyrrolylbenzene (as
2
compared to octaphenylnaphthalene (68° to 69°). As in
2
the case of octaphenylnaphthalene, the naphthalene unit
is twisted and the pyrroles neighboring the central
C(21)-C(22) bond are displaced above and below the
plane. The twist of the naphthalene core, expressed as
the angle between the two symmetry-related C(6)-C(11)
bonds, amounts to 21.84(12)°. NMR and UV/vis spec-
troscopy of 2 in solution clearly shows the presence of
1
determined by H-NMR). Apparently, these experimen-
tal results are indicative of the autoacceleration effect
as suggested by other authors.3
,4
stacking of the pyrrole units in 2 (λshoulder ) 340 nm, λmax
88 nm, Figure 2). The effect is even more pronounced
here than in the case of 1.
)
However, when the
2
addition sequence is reversed by the slow addition of 2
equimolar amounts of pyrrolylsodium to 1 equimolar
amount of hexafluorobenzene, the product consists of
Con clu sion
>
90% of 1,4-dipyrrolyl-2,3,5,6-tetrafluorobenzene. This
result clearly contradicts autoacceleration, where, ir-
respective of the addition sequence, a preferred formation
of hexapyrrolylbenzene would have to take place. An
instantaneous exothermic reaction between pyrrolylso-
dium and hexafluorobenzene, however, accounts for all
the results found, and we are prompted to conclude that
the proposed autoaccelerated synthesis is incorrect.
The reactivity of pyrrolylsodium toward other fluori-
nated aromatics was tested by the reaction with octafluo-
We have demonstrated the great ease with which
multiple-pyrrolyl aromatic compounds can be synthe-
sized. Due to their sterically congested structure, they
may be regarded as building blocks for larger π-conju-
gated systems aiming at star-shaped molecules with new
electrooptical properties.9
(9) Keegstra, M. A.; De Feyter, S.; De Schryver, F. C.; M u¨ llen, K.
Angew. Chem., Int. Ed. Engl. 1996, 35, 774.

9
014 J . Org. Chem., Vol. 61, No. 25, 1996
Notes
Ta ble 1. Selected Da ta of Ma in Rea ction P r od u cts Obta in ed a fter Ad d ition of 1 Equ im ola r Am ou n t of
Hexa flu or oben zen e to 2 Equ im ola r Am ou n ts of P yr r olylsod iu m in DMF
a
As determined by GC. b The values between brackets indicate the relative intensities of the signals. c Hexafluorobenzene was used as
d
external reference (-162 ppm). The P stands for a pyrrole unit.
Exp er im en ta l Section
diffractometer on rotating anode. Data were corrected for Lp
effects and for a linear decay of 1% of the reference reflections,
but not for absorption. The structure was solved by automated
Gen er a l In for m a tion . 1H, 13C, and 19F NMR spectra were
recorded on a 400 MHz spectrometer operating at 400.13, 100.62,
and 376.48 MHz, respectively. Analytical gas chromatography
was done using a CPSIL5CB column (length 25 m, diameter 0.32
mm, film 0.12 µm).
Chemical reagents and solvents were purchased from com-
mercial sources and used as received with the exception of DMF
which was dried over barium oxide.
Hexa p yr r olylben zen e (1). A 0.43 g (17.9 mmol) amount of
NaH was stirred in 15 mL of DMF using a 25 mL two-necked
flask fitted with a calcium chloride tube. Through a dropping
funnel was added 1.20 g (17.9 mmol) of pyrrole. After the
liberation of H gas had ceased, the reaction mixture was stirred
2
for an additional 30 min. Then a solution of 0.5 g (2.69 mmol)
of hexafluorobenzene in 5 mL of DMF was added dropwise in
5 min, and the reaction mixture was stirred for an additional
h. It was poured into 100 mL of ice-water, and the white
2
direct methods (SHELXS86). Refinement on F was carried out
by full-matrix least-squares techniques (SHELXS-93); no obser-
vance criterium was applied during refinement. Refinement
converged at a final wR2 value of 0.1370, R1 ) 0.0635 (for 1653
reflections with Fo > 4σ(F )) S ) 1.040, for 349 parameters.
o
Hydrogen atoms were included in the refinement on calculated
positions riding on their carrier atoms. A final difference Fourier
-
3
showed no residual density outside -0.25 and 0.27 e Å .
P a r tia l Su bstitu tion of Hexa flu or oben zen e by P yr r ole.
A 0.125 g (5.2 mmol) amount of NaH was stirred in 5 mL of
DMF using a 25 mL two-necked flask fitted with a calcium
chloride tube. Through a dropping funnel was added 0.35 g (5.2
mmol) of pyrrole. After the liberation of H
reaction mixture was stirred for an additional 30 min. Then
.49 g (2.63 mmol) of hexafluorobenzene in 5 mL of DMF was
added in one batch and the mixture was stirred for an additional
2
gas had ceased, the
1
2
0
precipitate was filtered and dried overnight in air yielding 1.08
g (86%). Crystallization from acetonitrile yielded large colorless
crystals from which the crystal structure was determined.
C H N , M ) 468.57; elemental analysis: C 76.94 (76.90), N
7.83 (17.94), H 6.05 (5.16); GC/MS (m/z) 468; H NMR (CDCl
2
h. It was poured into 100 mL ice-water, and the white
precipitate was filtered and dried overnight in air yielding 0.36
g. The mixture was subjected to column chromatography using
30 24 6
2 2
silica and CH Cl /hexanes as eluent (from a 1/3 ratio to 1/1 ratio).
1
1
3
)
Five products were isolated and characterized:
δ 6.17 (12H, dd, J ) 2.2 Hz, J ) 2.2 Hz), 6.07 (12H, dd, J ) 2.1
1
3
1,4-Di(N-pyrrolyl)tetrafluorobenzene: GC-yield: 2%; GC/MS
Hz, J ) 2.1 Hz); C NMR (CDCl
3
) δ 134.3, 120.9, 110.6; mp >
(
1
m/z) 280; 1H NMR (CDCl
3
) δ 6.43, 6.96; C NMR (CDCl ) δ
13
3
3
×
50 °C; crystal data of 1: colorless, block-shaped crystal (0.35
10.9, 118.7, 122.3, 142.0; 19F NMR (CDCl
1,2,4-Tri(N-pyrrolyl)trifluorobenzene: GC-yield: 3%; GC/MS
) δ -148.
0.35 × 0.30 mm), orthorhombic, space group Pna2
1
(no. 33)
3
with a ) 10.2570(12), b ) 19.797(2), c ) 11.811(2) Å, V )
3
-3
1
2
)
<
398.3(5) Å , Z ) 4, D
x
) 1.298 g cm , F(000) ) 984, µ(Mo KR)
(m/z) 326; H NMR (CDCl
3
) δ 6.26 (4H), 6.43 (2H), 6.50 (4H),
-
1
13
0.8 cm , 2914 reflections measured, 2675 independent, (1.03°
6.97 (2H); C NMR (CDCl ) δ 110.8, 110.8, 110.9, 119.2, 121.6,
3
1
9
θ < 27.49°, ω scan, T ) 150 K, Mo KR radiation, graphite
monochromator, λ ) 0.71073 Å) on an Enraf-Nonius CAD4 Turbo
3
121.8, 122.4, 125.4, 133.7, 142.5, 144.6, 147.7; F NMR (CDCl )
δ -134, -143, -148.

Notes
,2,4,5-Tetra(N-pyrrolyl)difluorobenzene: GC-yield: 36%; GC/
J . Org. Chem., Vol. 61, No. 25, 1996 9015
1
1.254 g cm-3, F(000) ) 1360, µ(Mo KR) ) 0.8 cm-1, 15226
reflections measured, 4791 independent, Rint ) 0.0925, (1.1° <
θ < 29.99°, T ) 100K) on an Enraf-Nonius FAST area detector
1
13
MS (m/z) 373; H NMR (CDCl
CDCl ) δ 110.8, 121.6, 125.4, 148.4; F NMR (CDCl
,2,3,4,5-Penta(N-pyrrolyl)fluorobenzene: GC-yield: 9%; GC/
3
) δ 6.26 (8H), 6.53 (8H); C NMR
19
(
3
3
) δ -133.
3
1
on rotating anode. A disordered solvent area (V ) 243 Å , 11.7
1
MS (m/z) 421; H NMR (CDCl
6
1
3
) δ 6.08 (6H), 6.18 (6H), 6.26 (4H),
electrons, probably partially occupied by water) was treated with
1
3
10
.52 (4H); C NMR (CDCl
21.7, 126.4, 126.5, 133.7, 151.1; F NMR (CDCl
: GC yield 50%; other data are given above.
Octapyr r olyln aph th alen e (2). A 0.71 g (29.6 mmol) amount
3
) δ 110.5, 110.6, 110.8, 120.9, 121.1,
the SQUEEZE/BYPASS procedure. Where relevant, data are
1
9
3
) δ -127.
given without disordered solvent contribution. Refinement
1
converged at a final wR2 value of 0.1432, R1 ) 0.0570 (for 4144
reflections with Fo > 4σ(F )) S ) 1.123, for 227 parameters. A
o
of NaH was stirred in 25 mL of DMF using a 50 mL two-necked
flask fitted with a calcium chloride tube. Through a dropping
funnel was added 2.00 g (29.8 mmol) of pyrrole. After the
final difference Fourier showed no residual density outside -0.29
-
3
and 0.36 e Å . Atomic coordinates, bond lengths and angles,
and thermal parameters of both structure determinations have
been deposited at the Cambridge Crystallographic Data Centre.
2
liberation of H gas had ceased, the reaction mixture was stirred
for an additional 30 min. Then a solution of 0.6 g (2.21 mmol)
of hexafluorobenzene in 10 mL of DMF was added dropwise in
Ack n ow led gm en t. This work was supported in part
H.A.M.B., W.J .J .S., A.L.S.) by the Netherlands Foun-
1
5 min, and the reaction mixture was stirred overnight. It was
(
poured into 100 mL ice-water, and the yellow precipitate was
filtered and dried overnight in air yielding 1.15 g (80%).
Crystallization from acetonitrile yielded large yellow crystals
dation of Chemical Research (SON) with financial aid
from the Netherlands Organisation for Scientific Re-
search (NWO). X-ray data of compound 2 were kindly
collected by A. M. M. Schreurs. Part of this work was
conducted at the Materials Department of the Univer-
sity of California at Santa Barbara.
from which the crystal structure was determined. C42
32 8
H N , M
)
648.78; elemental analysis: C 76.35 (77.76), N 17.20 (17.27),
1
H 5.14 (4.97); GC/MS (m/z) 648; H NMR (CDCl
3
) δ 6.19 (8H,
) δ 135.9, 133.5,
27.7, 124.3, 121.0, 110.0, 109.8; mp > 350 °C; crystal data of 2
unless stated otherwise, the same procedure has been followed
1
3
m), 5.98 (16H, m), 5.82 (8H, m); C NMR (CDCl
3
1
(
J O961192C
as for compound 1): yellow, block-shaped crystal (0.4 × 0.3 ×
0
1
.2 mm), tetragonal, space group P4
1.5659(11), c ) 25.680(9) Å, V ) 3435.2(13) Å , Z ) 4, D
1 1
2 2 (no. 92) with a )
3
x
)
(10) van der Sluis, P.; Spek, A. L. Acta Crystallogr. 1990, A46, 194.