Preparation and study of 1,8-di(pyrid-2′-yl)carbazoles

Article abstract of DOI:10.1021/jo801132w

(Figure Presented) A series of three derivatives of 1,8-di(pyrid-2′- yl)carbazole were prepared by Stille-type coupling of 2-(tri-n-butylstannyl) pyridine with the appropriate 1,8-dibromocarbazole. The carbazoles were prepared by appropriate substitution methodologies on the parent carbazole or by palladium-catalyzed cyclization of di-(p-tolyl)amine to provide the carbazole ring system. An X-ray structure of the di-tert-butyl derivative confirmed that both pyridyl groups were oriented for favorable intramolecular H-bonding to the central N-H. Two orientations of the molecule were found in the unit cell and this observation was corroborated by two N-H stretching bands in the solid state IR. Substitution of N-H by N-D led to increased emission intensity through diminished intramolecular deactivation of the excited state. The di-tert-butyl derivative formed a tridentate complex with Pd(II), which showed a red-shifted band attributed to an intraligand charge transfer state.

Full text of DOI:10.1021/jo801132w

Preparation and Study of 1,8-Di(pyrid-2′-yl)carbazoles
Maria S. Mudadu, Ajay N. Singh, and Randolph P. Thummel*
Department of Chemistry, 136 Fleming Building, UniVersity of Houston, Houston, Texas 77204-5003
thummel@uh.edu
ReceiVed August 15, 2007
A series of three derivatives of 1,8-di(pyrid-2′-yl)carbazole were prepared by Stille-type coupling of
2-(tri-n-butylstannyl)pyridine with the appropriate 1,8-dibromocarbazole. The carbazoles were prepared
by appropriate substitution methodologies on the parent carbazole or by palladium-catalyzed cyclization
of di-(p-tolyl)amine to provide the carbazole ring system. An X-ray structure of the di-tert-butyl derivative
confirmed that both pyridyl groups were oriented for favorable intramolecular H-bonding to the central
N-H. Two orientations of the molecule were found in the unit cell and this observation was corroborated
by two N-H stretching bands in the solid state IR. Substitution of N-H by N-D led to increased emission
intensity through diminished intramolecular deactivation of the excited state. The di-tert-butyl derivative
formed a tridentate complex with Pd(II), which showed a red-shifted band attributed to an intraligand
charge transfer state.
Introduction
efficient ESPT to provide the tautomer 2b and almost complete
quenching of the excited state emission4 (Figure 1).
Carbazole represents a simple, symmetrical benzalogue of
indole that can serve to amplify the intramolecular H-bonding
characteristics of 2. To further investigate this interesting system
we have prepared the parent 1,8-di(pyrid-2′-yl)carbazole (3H)
as well as the 3,6-dimethyl (4H) and 3,6-di-tert-butyl (5H)
derivatives. The synthesis, structure, and properties of these
systems will be presented along with some preliminary coor-
dination chemistry.
The juxtaposition of pyridine and indole brings together two
important fundamental aromatic nuclei with interesting comple-
mentary properties. The pyridine molecule is somewhat electron
deficient but is nevertheless a good Lewis base due to the
availability of the nitrogen lone pair electrons. It behaves as an
important ligand and participates readily as an H-bond acceptor.
The indole molecule is comparatively electron rich but only
binds metals in its deprotonated form. It is highly fluorescent
and behaves as a good H-bond donor.
In earlier work we have carefully examined the consequences
of appending a pyridyl group to indole, either through the
2-indole1 or 7-indole position.2 For 2-(pyrid-2′-yl)indole (1) we
found evidence for excited state proton transfer (ESPT) pro-
moted by a cyclic H-bonded alcohol complex to provide an
unstable tautomer with greatly diminished fluorescence.3 On the
other hand, 7-(pyrid-2′-yl)indole (2) exhibits strong intramo-
lecular H-bonding in the ground state, which leads to very
Results and Discussion
Synthesis and Structure. The three carbazoles 3H-5H were
all prepared in the indicated yields by the Stille coupling of the
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Waluk, J. J. Am. Chem. Soc. 1996, 118, 3508–3518. (c) Herbich, J.; Hung, C.;
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Wu, F.; Thummel, R. P.; Waluk, J. Isr. J. Chem. 1999, 39, 309–318. (e) Nosenko,
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Wu, F.; Hardesty, J.; Thummel, R. P. J. Org. Chem. 1998, 63, 4055–4061. (c)
Wu, F.; Chamchoumis, C. M.; Thummel, R. P. Inorg. Chem. 2000, 39, 584–
590.
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10.1021/jo801132w CCC: $40.75
Published on Web 07/31/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 6513–6520 6513

Mudadu et al.
FIGURE 1. Relative orientation of pyridine and indole subunits in 1-5.
SCHEME 1
SCHEME 2
corresponding 1,8-dibromo derivatives with 2-(tri-n-butylstan-
nyl)pyridine.5 Interestingly the dibromo derivatives were all
made by significantly different approaches. To direct bromina-
tion exclusively to the 1,8-positions, one must first incorporate
nitro groups at the 3,6-positions of carbazole (Scheme 1).
Nitration of carbazole6 with sodium nitrite and nitric acid leads
to a mixture of 1-nitro-, 1,6-dinitro-, and 3,6-dinitrocarbazoles
(7-9) from which the desired 3,6-isomer could be isolated by
chromatography in 34% yield. The bromination of 7 in sulfuric
acid affords a nearly quantitative yield of the dibromo-derivative
10,7 which is then reduced with iron to the diamine in 65%
yield. Finally the amine groups are diazotized in the normal
fashion and removed with hypophosphorous acid to afford 12.8
Preparation of the dimethyl derivative followed a completely
different approach, starting with di(p-tolyl)amine (13), which
was oxidatively cyclized to provide 3,6-dimethylcarbazole (14)
(Scheme 2). Initial attempts involved benzoyl peroxide in
chloroform in the presence of light9 but this reaction results
only in the recovery of the starting material. Rather, the
cyclization of 13 was performed with 1 equiv of palladium(II)
diacetate in refluxing AcOH for 2 h.10 Several attempts to
improve the reaction conditions always provided a 1:1 mixture
of 13 and 14 which are difficult to separate. The incomplete
cyclization of 13 can be attributed to a lack of oxidant in the
mixture when the process is carried out under Ar. Moreover,
recrystallized Pd(II) acetate and freshly distilled AcOH play an
important role in the success of this reaction. When the
cyclization is carried out in the presence of atmospheric oxygen,
carbazole 14 is obtained pure in 53% yield.11 This material was
then brominated to give the dibromodimethyl derivative 15 in
37% yield.12
The tert-butyl analogue of 13 is less accessible so we once
again started with the parent carbazole. Treatment with 4 equiv
of tert-butyl chloride under Friedel-Crafts conditions resulted
in a mixture of di-, tri-, and tetra-tert-butyl-substituted deriva-
tives, with the more reactive 3,6-positions being substituted in
(10) Åkermark, B.; Eberson, L.; Jonsson, E.; Pettersson, E. J. Org. Chem.
1975, 40, 1365–1367.
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Chem.-Eur. J. 1999, 5, 2413–2416. (c) Åkermark, B.; Oslob, J. D.; Heuschert,
U. Tetrahedron Lett. 1995, 36, 1325–1326.
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White, A. J. P.; Williams, D. J. Inorg. Chem. 2003, 42, 3454–3465. (b) Piatek,
P.; Lynch, V. M.; Sessler, J. L. J. Am. Chem. Soc. 2004, 126, 16073–16076. (c)
Coombs, N. D.; Stasch, A.; Cowley, A.; Thompson, A. L.; Aldridge, S. Dalton
Trans. 2008, 332–337. (d) Spitzmesser, S. K.; Gibson, V. C. J. Organomet. Chem.
2003, 673, 95–101. (e) Wentrup, C.; Gaugaz, M. HelV. Chim. Acta 1971, 54,
2108–2111.
(5) Schubert, U. S.; Eschbaumer, C.; Heller, M. Org. Lett. 2000, 2, 3373–
3376.
(6) Grotta, H. M.; Riggle, C. J.; Bearse, A. E. J. Org. Chem. 1964, 29, 2474–
2476.
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T.; Kinoshita, A.; Kawada, H.; Nakada, M. Synlett. 2003, 4, 570–572.
(9) Bhattacharyya, P.; Jash, S. S.; Dey, A. K. J. Chem. Soc., Chem. Commun.
1984, 1668–1669.
6514 J. Org. Chem. Vol. 73, No. 17, 2008

Preparation of 1,8-Di(pyrid-2′-yl)carbazoles
SCHEME 3
TABLE 1. 1H NMR Data for Dipyridylcarbazolesa
TABLE 2. Selected Geometric Parameters from the X-ray
Structure of 5H
Bond Lengths (Å)
molecule A
C6-C7
C11-C12
C16-C17
molecule B
C39-C40
C44-C45
C49-C50
1.481(4)
1.449(3)
1.482(3)
1.481
1,448(3)
1.482(4)
Dihedral Angles (deg)
compd
H2
H3
H4
H3′
H4′
H5′
H6′
molecule A
C5-C6-C7-C8
N1-C6-C7-C25
C15-C16-C17-C18 -7.3(4)
C23-C16-C17-N22 -8.3(3)
molecule B
-10.8(4) N34-C39-C40-C58 18.0(4)
-11.4(4) C38-C39-C40-C41 18.1(4)
3H
4H
5H
20
8.06
7.96
8.23
8.27
7.35
8.00
7.78
8.02
7.92
8.22
8.02
8.05
8.22
7.85
7.81
7.86
7.92
7.30
7.25
7.28
7.26
8.93
8.89
8.90
C48-C49-C50-C51 -23.2(4)
C56-C49-C50-N55 -22.3(3)
10.14
a Recorded in CDCl3 at room temperature.
Hydrogen Bonds (D-H---A)
all three products13 (Scheme 3). These isomers could be
separated efficiently by chromatography on silica gel. Interest-
ingly, the tetra-tert-butyl isomer 16 could be dealkylated at the
1,8-positions to afford additional 17 in 58% yield. Finally,
bromination of this material afforded 19 in 77% yield.14
The final Stille couplings of the dibromo precursors 12, 15,
and 19 with 2-(tri-n-butylstannyl)pyridine were all accomplished
in a similar manner with use of tetrakis(triphenylphosphine)-
palladium(0) as the catalyst. Purification was accomplished by
a combination of chromatography and recrystallization and the
yields of the obtained carbazoles are given in Figure 1.
D-H (Å)
H---A (Å)
DHA (deg)
molecule A
N24-H1-N1
N24-H1-N22
molecule B
N57-H34-N34
N57-H34-N55
0.90(3)
0.90(3)
2.28(3)
2.17(3)
114(2)
120(2)
0.88(3)
0.88(3)
2.26(3)
2.33(3)
120(2)
114(3)
in their arrangement and the torsional twist of the pyridine rings.
This difference seems to be due solely to crystal packing forces.
The tert-butyl groups in both molecules are disordered. An
ORTEP plot of the type A molecule is given in Figure 2 along
with the atomic numbering scheme for both type A and type B
molecules. Table 2 summarizes some pertinent geometric data.
The different arrangements of two molecules of type A and
two molecules of type B are shown in Figure 3. In the center
of the unit cell, there are two molecules of type A, which are
almost flat and parallel to each other. For molecule A, the torsion
angles around the bond connecting the pyridine and carbazole
rings, 11.1° for C(6)-C(7) and 7.8° for C(16)-C(17), are taken
as the average of the two dihedral angles associated with the
inter-ring bond. The two carbazole units are oriented in opposite
directions and the pyridines partially overlap each other in a
staggered manner. The molecules of type B surround the central
type A molecules and are found at the edges of the unit cell.
The type A molecules are paired and each molecule of every
pair overlaps with half of the type B molecule. In particular,
one pyridine lies on the top of the carbazole of the type B
molecule. For the type B molecule, the tert-butyl group of one
of the pair is situated between the pyridine rings of the other
1
The carbazoles were all identified by their characteristic H
NMR spectra and these data are summarized in Table 1. The
H6′ proton exhibits a small coupling constant (JHH ) 3.2-4.2
Hz) and is always the lowest field signal, being strongly
deshielded by the adjacent nitrogen. The remaining protons in
the pyridine ring can be assigned by connectivity to H6′. The
signals for H2 and H4 on the carbazole ring might be easily
confused. We have assigned the more downfield signal to H2,
which may experience greater deshielding due to proximity to
N1′ in conformations having the pyridine nitrogen pointing
outward.
The 3,6-di-tert-butylcarbazole (5H) was further characterized
by an X-ray crystal structure. The unit cell consists of two
independent molecules (types A and B) which differ significantly
(13) Neugebauer, F. A.; Fischer, H. Chem. Ber. 1972, 105, 2686–2693.
(14) Gibson, V. C.; Spitzmesser, S. K.; White, A. J. P.; Williams, D. J. Dalton
Trans. 2003, 2718–2727.
J. Org. Chem. Vol. 73, No. 17, 2008 6515

Mudadu et al.
FIGURE 2. ORTEP plot of the type A molecule of 5H with atom
numbering scheme. Add +33 for numbering of the type B molecule.
FIGURE 4. Preliminary X-ray structure of [Pd(5)Cl] (20).
(1.48 Å) as is the central C-C bond of the carbazole (1.45 Å).
The N-H of the carbazole is H-bonded to both pyridines with
the N-H bond length being 0.88-0.90 Å. The acceptor H---N
bond lengths are not exactly equal but differ by about 0.1 Å
and the H-N---H bond angle is 114° with one pyridine and
120° with the other for both type A and B molecules.
When the carbazole 5H is treated with palladium chloride in
acetonitrile at reflux the complex 20 is formed in 80% yield.
This complex is readily soluble in chloroform and is identified
1
by its H NMR spectrum. Like the free ligand, the complex
shows 6 signals in its aromatic region indicating a symmetrical
structure. The singlets for H2 and H4 along with the signals
for H3′-H5′ are only slightly shifted upon complexation. The
most characteristic signal is H6′ on the two pyridyl rings, which
for the free ligand appears as a doublet at 8.90 ppm with a small
(J ) 4.2 Hz) coupling constant. Complexation with Pd(II) shifts
this signal down to 10.14 ppm due to charge depletion at the
pyridine nitrogens caused by metal binding as well as the
deshielding effect of the proximal chlorine. Two attempts to
solve the crystal structure for this complex were thwarted by
disorder in the crystal lattice. Nevertheless, a reasonable
preliminary structure was obtained and is illustrated in Figure
4. The small binding cavity will not accommodate the metal
with a planar array of the ligand. The Pd is pushed up out of
the plane of the carbazole ring and the chloride is bent even
further out of the plane. Attempts to bind other smaller metals
in this cavity were unsuccessful.
Spectroscopic Properties
The electronic absorption and emission spectra for the
dipyridylcarbazoles 3H-5H and the palladium complex 20 were
recorded in acetonitrile and the data are collected in Table 3.
The absorption profiles for the three carbazoles are nearly
identical with only a slight difference in energy and absorbance.
The dimethyl system gives the lowest energy band at 390 nm,
followed by the di-tert-butyl at 385 nm and the diprotio at 376
nm.
It is well-known that carbazoles, much like the closely related
indoles, are strongly fluorescent. This fluorescence can often
be quenched through H-bonding with an appropriate acceptor
molecule. In the case of 3H-5H very efficient intramolecular
H-bonding exists and so these systems are only very weakly
fluorescent with quantum yields of about 4.5 × 10-4. A detailed
study of the excited state proton transfer in 5H was recently
reported.15 By exposing 5H to excess D2O we were able to
exchange the N-H proton for deuterium. The resulting stronger
N-D bond should proportionately weaken the D---N hydrogen
FIGURE 3. Top view showing two molecules of 5H: type A (top)
and type B (bottom).
molecule of the pair causing a larger torsional twist, 18.0° and
22.8°, for this arrangement of 5H.
Both type A and B molecules have the pyridine nitrogens
oriented toward the NH of the central carbazole. The bond
lengths for the C-C and C-N bonds in the pyridine and
carbazole rings (1.376-1.414 Å) are in the expected range for
aromatic systems and no significant bond alternation is observed.
The bond connecting the pyridine and carbazole rings is longer
6516 J. Org. Chem. Vol. 73, No. 17, 2008

Preparation of 1,8-Di(pyrid-2′-yl)carbazoles
TABLE 3. Electronic Absorptiona and Emissionb Data for Dipyridylcarbazoles
compd
λmax (nm, log ꢀ)
λem (nm)
Φc
3H
4H
5H
20
205 (4.55), 231 (4.67), 295 (4.46), 316 (4.41), 331 (4.23), 362 (4.04), 376 (4.08)
209 (4.62), 233 (4.77), 297 (4.38), 326 (4.43), 342 (4.26), 376 (4.07), 390 (4.11)
209 (4.55), 233 (4.72), 298 (4.34), 323 (4.38), 337 (4.22), 370 (4.03), 385 (4.06)
231 (4.50), 283 (4.04), 341 (3.79), 410 (3.71), 451 (3.79)
397
414
408
none
4.5 × 10-4
4.5 × 10-4
4.5 × 10-4
b
a Measured in CH3CN (10-5 M) at room temperature. Measured in CH3CN (10-5 M) at room temperature with excitation at absorbance 0.12.
c Measured by comparison with data for 5 from ref 4, using quinine sulfate in 0.1 N H2SO4 as the standard.
FIGURE 6. (a) Top: Electronic absorption spectra of [Pd(tpy)Cl]Cl
(red) and 20 (blue) in CH3CN (5 × 10-5 M). (b) Bottom: Electronic
FIGURE 5. Top: Electronic absorption spectra of 5H (black) and 5D
absorption spectra of 5H and 20 in CH3CN (5 × 10-5 M).
(red) in CH3CN (5 × 10-5 M). Bottom: Emission spectra of 5H (black)
and 5D (red) with excitation at 380 nm. Both taken in CH3CN (5 ×
10-5 M).
state and a more intense luminescence at room temperature. The
Pd complex 20 contains two fused six-membered chelate rings
and thus might be expected to enjoy some of the same
enhancements of its ligand field.
bond. Figure 5 shows both the absorption and emission spectra
for 5H and 5D. While the absorption spectra are nearly
superimposable, the emission spectrum for the deuterated
carbazole is considerably more intense, indicating that the
quenching process is less efficient for this species.
In somewhat related work we and others have examined
polypyridine-type complexes of Pt(II)16 and Ru(II)17 complexes
in which the metal is contained in a six-membered rather than
a five-membered chelate ring. The ligands were designed so as
to maintain complete conjugation of the aromatic system. In
both cases the six-membered chelate ring provided a less strained
coordination environment leading to a more stable ligand field.
This, in turn, resulted in less nonradiative decay of the excited
The absorption spectrum of 20 is shown in Figure 6a
compared with the spectrum of [Pd(tpy)Cl]Cl where tpy ) 2,2′;
6,2′′-terpyridine. While the absorbance of [Pd(tpy)Cl]Cl falls
off sharply beyond 370 nm, the complex 20 shows two broad
bands centered at 410 and 451 nm. Bands in this region are
often assigned to metal-to-ligand charge transfer; however, in
this case, such an assignment is highly unlikely. The PdII/I
potential is too high to make this state easily accessible and
furthermore the anionic ligand would be a very poor charge
acceptor. A more reasonable explanation would involve intrali-
gand charge transfer from the carbazole as donor to the pyridine
as acceptor. This assignment is strengthened by the observation
in Figure 6b that similar absorption bands at 370 and 385 nm
appear for the free ligand 5H and are red-shifted upon
complexation to Pd(II).
(15) Wiosna-Salyga, G.; Dobkowski, J.; Mudadu, M.; Sazanovich, I.;
Thummel, R. P.; Waluk, J. Chem. Phys. Lett. 2006, 423, 288–292.
(16) Hu, Y.-Z.; Wilson, M. H.; Zong, R.; Bonnefous, C.; McMillin, D. R.;
Thummel, R. P. Dalton Trans. 2005, 354–358.
The solid state IR spectra of the complexes were measured
and found to be consistent with the other spectroscopic evidence.
Figure 7 illustrates the IR spectra of 3H-5H as well as that of
the deuterio-derivative 5D. For the parent carbazole 3H the
(17) (a) Abrahamsson, M.; Jager, M.; Osterman, T.; Eriksson, L.; Persson,
P.; Becker, H. C.; Johansson, O.; Hammarstro¨m, L. J. Am. Chem. Soc. 2006,
128, 12616–12617. (b) Abrahamsson, M.; Becker, H. C.; Hammarstro¨m, L.;
Bonnefous, C.; Chamchoumis, C.; Thummel, R. P. Inorg. Chem. 2007, 46,
10354–10364.
J. Org. Chem. Vol. 73, No. 17, 2008 6517

Mudadu et al.
more rapidly indicating a faster exchange of this proton. After
65 min all of 2a has exchanged while approximately 25% of
5H remains as the unexchanged protio species.
Conclusions
A series of three 1,8-di(pyrid-2′-yl)carbazoles was prepared
by Stille coupling of the appropriate dibromocarbazole precur-
sors. These precursors are prepared by three different ap-
proaches. A crystal structure of 5H indicates that the carbazole
NH is H-bonded to both pyridine rings, leading to a syn coplanar
arrangement of the molecule, which crystallizes in two different
forms. The absorption spectra evidence what appears to be an
intraligand charge transfer from the carbazole to the pyridines.
The strong intramolecular H-bond effectively quenches excited
state emission and significantly slows down D-H exchange.
The binding cavity in these dipyridocarbazoles is small but Pd
can be accommodated by out-of-plane twisting of the peripheral
pyridine rings.
FIGURE 7. Solid state IR spectra taken at 25 °C: 3H (red), 4H (blue),
5H (green), 5D (purple), and 20 (black).
Experimental Section
3,6-Dinitrocarbazole (7). A three-necked round-bottomed flask,
equipped with condenser, addition funnel, and stirring bar, was
charged with carbazole (10.03 g, 60.0 mmol) and glacial AcOH
(80 g) under Ar. The mixture was stirred for 1.5 h at 40 °C. Then,
10% HNO3/AcOH (1:1, 16.4 g) was added dropwise and the
mixture was stirred for 2 h at 90-100 °C. The yellow precipitate
was filtered and washed with cold AcOH (50 mL) and then with
water. The solid was warmed in KOH/EtOH (200 mL, 60 g/L) at
50 °C for 30 min and filtered. The insoluble red solid was digested
in 10% HCl at 100 °C for 2 h. The resulting yellow solid was
filtered, washed with water, and dried overnight under vacuum in
the presence of P2O5. Chromatography on alumina, eluting with
CH2Cl2, followed by AcOEt and MeOH, provided 7 (5.25 g, 34%):
mp >300 °C (lit.6 mp 386-387 °C); 1H NMR (DMSO-d6) δ 12.73
(br s, 1H), 9.47 (d, 2H, J ) 2.1 Hz), 8.36 (dd, 2H, J ) 9.3, 2.7
Hz), 7.74 (d, 2H, J ) 9.0 Hz).
FIGURE 8. D2O exchange competition between 2a and 5H as
1
monitored by H NMR. Spectra show the NH resonances for a 1:1
mixture of 2a and 5H. The top spectrum is before the addition of D2O
and the successive spectra are at time intervals of 5, 35, and 65 min.
prominent peak at 3354 cm-1 is associated with the N-H
stretch. In the dimethyl derivative 4H this absorption broadens
and moves to 3375 cm-1 while characteristic C-H bands appear
in the region around 2900 cm-1. For the di-tert-butyl derivative
1,8-Dibromo-3,6-dinitrocarbazole (10). A mixture of 4,7-dini-
trocarbazole (7, 0.23 g, 0.9 mmol), bromine (0.5 mL, 1.5 mmol),
and H2SO4 (6 mL) was warmed for 15 min at 90-100 °C. Then,
additional bromine (0.3 mL, 0.9 mmol) and H2SO4 (6 mL) were
added. The resulting mixture was heated at 100 °C for 2.5 h. After
cooling to room temperature the brown reaction mixture was poured
onto ice and filtered to provide 10 (0.37 g, 99%) as a yellow-green
5H the N-H band splits into two peaks at 3364 and 3405 cm-1
,
which is consistent with the existence of two molecules, type
A and type B, in the solid state structure of this species. For
this molecule the C-H absorption at 2960 cm-1 becomes more
significant. Finally, the deuterated derivative of 5H shows almost
complete disappearance of the two N-H stretching bands and
the appearance of a much weaker single band at an intermediate
frequency of about 3376 cm-1. At the same time a new peak
appears at 2528 cm-1, which is expected for the N-D stretch.
It is possible that upon deuteration the morphology of the solid
state of 5D changes to a single type of molecule rather than the
type A and type B which were observed for 5H. The IR
spectrum of the Pd complex 20 shows the same typical C-H
stretching bands and the clear absence of the N-H stretch.
To compare the H-bonding in 5H with that of 2a, a
competition exchange experiment was carried out involving
these two species. The top spectrum in Figure 8 shows the NH
resonances for an equimolar (0.005 mmol) solution of these two
compounds in CDCl3. Already some interesting features are
evident. For the carbazole 5H, having a bifurcated H-bond, the
NH resonance appears at much lower field due to the stronger
deshielding effect of the “double” H-bond. Notice also that this
peak is sharper, indicating less motion of the proton. Upon the
addition of D2O (0.025 mmol) the NH peak for 2a disappears
7
1
solid: mp >300 °C (lit. mp >320 °C); H NMR (DMSO-d6) δ
12.80 (br s, 1H), 9.54 (d, 2H, J ) 2.7 Hz), 8.56 (d, 2H, J ) 1.8
Hz).
3,6-Diamino-1,8-dibromocarbazole (11). A mixture of 1,8-
dibromo-3,6-dinitrocarbazole (10, 0.33 g, 0.8 mmol), iron powder
(0.89 g, 16.0 mmol), and conentrated HCl (1.53 g, 16.0 mmol) in
EtOH/H2O (3:1, 100 mL) was mechanically stirred at reflux for
19 h. After cooling, the reaction mixture was filtered and the solvent
evaporated. The aqueous residue was made alkaline with 10%
NaOH and extracted with THF/AcOEt (1:1, 3 × 100 mL). The
combined organic phase was washed with brine (20 mL), dried
(MgSO4), and concentrated to provide 11 as a yellowish solid (0.18
1
g, 65%): mp 238-240 °C dec; H NMR (DMSO-d6) δ 9.79 (s,
1H), 7.03 (s, 2H), 6.94 (s, 2H), 4.95 (br s, 4H); 13C NMR (DMSO-
d6) δ 142.8, 132.0, 125.5, 117.3, 104.1, 103.2; HRMS m/z calcd
for C12H9Br2N3 [M+] 352.9163, found 352.9159.
1,8-Dibromocarbazole (12).8b A solution of water (1.2 mL) and
concentrated HCl (0.5 mL) was heated to reflux. The heat was
removed and 3,6-diamino-1,8-dibromocarbazole (11, 0.15 g, 0.4
mmol) was added. EtOH (1 mL) was added to help dissolution of
the solid. The mixture was refluxed for 45 min. The solution was
allowed to cool to 15 °C in an ice bath and more HCl (0.2 mL)
6518 J. Org. Chem. Vol. 73, No. 17, 2008

Preparation of 1,8-Di(pyrid-2′-yl)carbazoles
was added. When the temperature reached 10-13 °C, a solution
of NaNO2 (0.7 g, 1.0 mL) in H2O (1 mL) was added dropwise.
The yellow mixture was stirred for 30 min at 5-10 °C. Then cold
50% H3PO2 (1.0 mL) was added. The mixture was stoppered loosely
and kept in the refrigerator for 11 h. Then the reaction mixture
was stirred at room temperature for 24 h. The EtOH was evaporated
and the residue was dissolved in CH2Cl2. The two layers were
separated and the aqueous phase was extracted with CH2Cl2 (3 ×
20 mL). The combined organic phase was washed with brine (20
mL), dried (MgSO4), and concentrated to provide 12 as a yellowish
solid (0.09 g, 70%): mp 110-12 °C; 1H NMR (DMSO-d6) δ 11.18
(br s, 1H), 8.18 (d, 2H, J ) 7.8 Hz), 7.66 (d, 2H, J ) 7.8 Hz), 7.17
1,8-Dibromo-3,6-di(tert-butyl)carbazole (19).14 In the manner
described for 15, a mixture of 3,6-di(tert-butyl)carbazole (18, 0.81 g,
2.9 mmol), bromine (0.96 g, 6.0 mmol), and glacial AcOH (128
mL) gave a crude product, which after chromatography on silica
gel, eluting with CH2Cl2, provided 19 (0.98 g, 77%) as a white
solid: mp 174-175 °C (lit.14 mp 171-172 °C); 1H NMR (CDCl3)
δ 8.14 (br s, 1H), 7.97 (d, 2H, J ) 1.2 Hz), 7.64 (d, 2H, J ) 1.8
Hz), 1.43 (s, 18H).
1,8-Di(pyrid-2′-yl)carbazole (3H). A two-necked round-bottomed
flask, equipped with a magnetic stir bar, condenser, and rubber
septum, was charged with 1,8-dibromocarbazole (12, 72.0 mg, 0.23
mmol), 2-(tri-n-butylstannyl)pyridine (85%, 238.0 mg, 0.55 mmol),
tetrakis(triphenylphosphine)palladium(0) (5% mol, 23.1 mg, 0.02
mmol), and dry toluene (2 mL) was refluxed under Ar for 20 h.
After cooling to room temperature, water (1 mL) was added and
the mixture was concentrated under reduced pressure. Chromatog-
raphy on silica gel, eluting with CH2Cl2 provided 3 (53.0 mg, 27%)
as a white solid, which was recrystallized from CH2Cl2/hexane:
1
(t, 2H, J ) 7.8 Hz); H NMR (CDCl3) δ 8.27 (br s, 1H), 7.92 (d,
2H, J ) 8.1 Hz), 7.57 (d, 2H, J ) 7.83 Hz); 13C NMR (CDCl3)
137.6, 128.68, 124.7, 121.1, 119.7, 104.4; HRMS m/z calcd for
C12H7Br2N [M+] 322.8945, found 322.8940.
3,6-Dimethylcarbazole (14). A mixture of di-p-tolyamine (0.39
g, 2.0 mmol), palladium(II) diacetate (0.45 g, 2.0 mmol), and glacial
AcOH (40 mL) was refluxed for 1.5 h in the presence of
atmospheric oxygen.11 After cooling to room temperature, the
mixture was concentrated. The red residue was dissolved in CH2Cl2
and filtered through Celite. The organic solution was washed with
brine (20 mL), dried (MgSO4), and concentrated. Chromatography
on alumina, eluting with CH2Cl2/petroleum ether provided 14 (0.19
g, 53%) as a white solid: mp 215-217 °C (lit.12e mp 219-220
°C); 1H NMR (acetone-d6) δ 10.08 (br s, 1H), 7.86 (d, 2H, J ) 0.9
Hz), 7.36 (d, 2H, J ) 8.1 Hz), 7.19 (d, 2H, J ) 8.4 Hz), 2.48 (s,
6H).
1
mp 215-217 °C; H NMR (DMSO-d6) δ 13.54 (br s, 1H), 9.01
(d, 2H, J ) 3.6 Hz), 8.30 (t, 4H, J ) 7.5 Hz), 8.19 (d, 2H, J ) 7.2
Hz), 7.99 (td, 2H, J ) 5.7, 1.8 Hz), 7.45 (dd, 2H, J ) 6.9, 4.8 Hz),
1
7.36 (d, 1H, J ) 7.8 Hz); H NMR (CDCl3) δ 13.54 (br s, 1H),
8.93 (dd, 2H, J ) 4.2, 1.5 Hz), 8.22 (d, 2H, J ) 7.8 Hz), 8.06 (d,
2H, J ) 8.4 Hz), 8.00 (d, 2H, J ) 7.8 Hz), 7.85 (td, 2H, J ) 7.85,
1.8 Hz), 7.35 (t, 2H, J ) 7.8 Hz), 7.30 (dd, 1H, J ) 7.5, 5.1 Hz);
13C NMR (CDCl3) δ 157.4, 148.7, 138.4, 136.9, 124.2, 123.5, 121.6,
121.4, 120.6, 120.4, 118.9; HRMS m/z calcd for C22H15N3 [M +
H] 322.1344, found 322.1345.
1,8-Dibromo-3,6-dimethylcarbazole (15).12 To a warm (90 °C)
solution of 3,6-dimethylcarbazole (14, 0.40 g, 2.2 mmol) in glacial
AcOH (87 mL) was added a solution of bromine (0.73 g, 4.6 mmol)
in AcOH (6 mL) in one portion. The mixture was stirred at 90 °C
under Ar for 3.5 h. After cooling to room temperature, the reaction
mixture was concentrated and dried. The solid obtained was washed
with hexane, filtered, and dried to provide 15 as a white solid (0.28
1,8-Di(pyrid-2′-yl)-3,6-dimethylcarbazole (4H). In the manner
described for 3H, a mixture of 1,8-dibromo-3,6-dimethylcarbazole
(15, 0.53 g, 1.5 mmol), 2-(tri-n-butylstannyl)pyridine (85%, 1.56 g,
3.6 mmol), and tetrakis(triphenylphosphine) palladium(0) (92.0 mg,
0.08 mmol) in dry toluene (12 mL) was refluxed under Ar. After
24 h, more 2-(tri-n-butylstannyl)pyridine (0.22 g, 0.5 mmol) was
added and the reaction mixture was refluxed for a further 17 h.
After cooling to room temperature, water (1 mL) was added and
the mixture was concentrated under reduced pressure. Chromatog-
raphy on alumina, eluting with CH2Cl2/hexane (1:1) provided a
solid, which was washed with Et2O, filtered, and dried to provide
1
g, 37%): mp 192-193 °C; H NMR (acetone-d6) δ 10.16 (br s,
1H), 8.56 (s, 2H), 7.60 (s, 2H), 2.52 (s, 6H). This material was
used in the subsequent Stille coupling step without further
purification.
1
4 as a yellow solid (0.09 g, 17%): mp 242-243 °C; H NMR
1,3,6,8-Tetra(tert-butyl)carbazole (16), 3,6-Di(tert-butyl)carba-
zole (17), and 1,3,6-Tri(tert-butyl)carbazole (18).13 To a suspension
of carbazole (2.90 g, 17.4 mmol) and freshly distilled 2-chloro-2-
methylpropane (9.65 g, 104.2 mmol) was added AlCl3 (2.32 g, 17.4
mmol) during 1 h. Concentrated HCl (50 mL) was added and the
mixture was stirred at room temperature for 24 h. Then, ice-water
was added. The mixture was extracted with Et2O (3 × 50 mL).
The aqueous phase was concentrated to a smaller volume and
extracted with Et2O (3 × 40 mL). The combined organic phase
was washed with brine (20 mL), dried (MgSO4), and concentrated.
Chromatography on silica gel, eluting with benzene/hexane (1:1)
provided 16 (2.09 g, 30%), 17 (1.15 g, 24%), and 18 (1.04 g, 18%)
(CDCl3) δ 12.98 (br s, 1H), 8.89 (d, 2H, J ) 3.9 Hz), 8.03 (d, 2H,
J ) 8.1 Hz), 7.96 (s, 2H), 7.81 (t, 2H, J ) 7.5 Hz), 7.78 (s, 2H),
7.25 (t, 2H, J ) 6.0 Hz), 2.62 (s, 6H); 13C NMR (acetone-d6) δ
158.4, 150.0, 137.9, 137.8, 128.7, 125.5, 125.1, 122.6, 122.5, 121.3,
120.9, 21.7; HRMS m/z calcd for C24H19N3 [M + H] 350.1657,
found 350.1656.
1,8-Di(pyrid-2′-yl)-3,6-di(tert-butyl)carbazole (5H). In the manner
described for 3H, a mixture of 1,8-dibromo-3,6-di(tert-butyl)car-
bazole (19, 0.51 g, 1.2 mmol), 2-(tri-n-butylstannyl)pyridine (85%,
1.66 g, 3.8 mmol), and tetrakis(triphenylphosphine)palladium(0)
(69.3 mg, 0.06 mmol) in dry toluene (4 mL) provided a crude
material. Chromatography on alumina, eluting with AcOEt/hexane
(1:4) provided impure 5. A second chromatography on alumina,
eluting with hexane, then CH2Cl2/hexane (1:1) provided 5 (0.38 g,
74%) as a white solid, which was recrystallized from CH2Cl2/
as white solids. 16: mp 189 °C (lit.13 mp 191-192 °C); H NMR
1
(CDCl3) δ 8.14 (br s, 1H), 7.95 (d, 2H, J ) 1.5 Hz), 7.46 (d, 2H,
J ) 2.4 Hz), 1.60 (s, 18H), 1.46 (s, 18H). 17, mp 212 °C (lit.13 mp
228-229 °C); 1H NMR (CDCl3) δ 8.08 (d, 2H, J ) 1.2 Hz), 7.46
(dd, 2H, J ) 8.7, 2.4 Hz), 7.34 (d, 2H, J ) 8.7 Hz), 1.44 (s, 18H).
18, mp 134-136 °C (lit.13 mp 136-137 °C); 1H NMR (CDCl3) δ
8.07 (d, 1H, J ) 1.8 Hz), 7.96 (d, 1H, J ) 2.1 Hz), 7.47 (dd, 1H,
J ) 8.4, 1.8 Hz), 7.42 (d, 1H, J ) 1.5 Hz), 7.37 (d, 1H, J ) 8.4
Hz), 1.57 (s, 9H), 1.46 (s, 9H), 1.45 (s, 9H).
1
hexane: mp 255-256 °C; H NMR (CDCl3) δ 12.88 (br s, 1H),
8.90 (dd, 2H, J ) 3.9, 0.8 Hz), 8.23 (d, 2H, J ) 1.2 Hz), 8.06 (d,
2H, J ) 7.8 Hz), 8.02 (d, 2H, J ) 1.5 Hz), 7.83 (td, 2H, J ) 8.1,
2.1 Hz), 7.25 (m, 2H) 1.54 (s, 18H); 13C NMR (CDCl3) δ 158.3,
149.1, 141.6, 137.3, 136.7, 124.3, 121.3, 121.2, 120.4, 118.0, 35.0,
32.4 (one carbon hidden). Anal. Calcd for C30H31N3: C, 83.14; H,
7.16; N, 9.70. Found: C, 83.05; H, 7.33; N, 9.67.
3,6-Di(tert-butyl)carbazole (17).13 A mixture of 16 (1.53 g, 3.9
mmol) and concentrated H2SO4 (30 mL) was stirred at room
temperature for 24 h. Ice water (200 mL) was added, and the
mixture was extracted with AcOEt (5 × 50 mL). The combined
organic phase was washed with water (20 mL) and brine (20 mL),
dried (MgSO4), and concentrated. Chromatography on silica gel,
eluting with benzene/hexane (1:1) provided 17 (0.63 g, 58%), which
was identical with the material obtained from the direct alkylation
of carbazole.
[(5)PdCl] (20). A mixture of 5H (50 mg, 0.109 mmol) and
PdCl4K2 (40 mg, 0.109 mmol) in MeCN (10 mL) was refluxed
under Ar for 24 h. The solvent was evaporated under reduced
pressure. Chromatography on silica gel, eluting with acetone/hexane
1
(2:1) gave 20 as a brown solid (0.050 g, 80%): H NMR (CDCl3)
δ 10.14 (d, 2H, J ) 6.0 Hz), 8.27 (s, 2H), 8.23 (d, 2H, J ) 8.7
Hz), 7.95-7.90 (t, 2H, J ) 7.8 Hz), 7.92 (s, 2H), 7.28-7.24 (t,
J. Org. Chem. Vol. 73, No. 17, 2008 6519

Mudadu et al.
2H, J ) 8.4 Hz), 1.62 (s, 18H); 13C NMR (CDCl3) δ 155.9, 152.1,
141.7, 139.9, 138.1, 126.4, 122.2, 122.0, 121.3, 120.5, 119.8, 34.9,
32.1; HRMS m/z calcd for C30H30N3ClPd [M - Cl]+ 538.1474,
found 538.1485.
Supporting Information Available: General Experimental
Methods; X-ray analysis of 5H, Table S1 giving data
collection and processing parameters for 5H, 1H and 13C
1
NMR spectra for 3H-5H, 11, 12, and 20, H NMR for 7,
10, and 14-19, and X-ray crystallographic data (CIF files)
for 5H. This material is available free of charge via the
Internet at http://pubs.acs.org.
Acknowledgement. We thank the Robert A. Welch Founda-
tion (E-621) and the National Science Foundation (CHE-
0352617) for financial support. We would also like to thank
Dr. James Korp for assistance with the X-ray determinations
and Professor Sebastiano Campagna for helpful discussions.
JO801132W
6520 J. Org. Chem. Vol. 73, No. 17, 2008