(E)-9-(2-iodovinyl)-9H-carbazole: A new coupling reagent for the synthesis of π-conjugated carbazoles

Article abstract of DOI:10.1021/ol200350a

The one-pot synthesis of (E)-9-(2-iodovinyl)-9H-carbazole via sequential ruthenium-catalyzed silylative coupling of N-vinylcarbazole with vinyltrimethylsilane and iododesilylation is reported. Its use as a new building block in the palladium-catalyzed Sonogashira and Suzuki-Miyaura coupling reactions to yield new carbazole-containing (E)-but-1-en-3-ynes and (E,E)-buta-1,3-dienes is demonstrated.

Full text of DOI:10.1021/ol200350a

ORGANIC
LETTERS
2011
Vol. 13, No. 8
1976–1979
(E)-9-(2-Iodovinyl)-9H-carbazole: A New
Coupling Reagent for the Synthesis of
π-Conjugated Carbazoles
ꢀ
Piotr Pawluc,* Adrian Franczyk, Je-drzej Walkowiak, Grzegorz Hreczycho,
Maciej Kubicki, and Bogdan Marciniec
Department of Organometallic Chemistry, Faculty of Chemistry,
Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznan, Poland
piotrpaw@amu.edu.pl
Received February 7, 2011
ABSTRACT
The one-pot synthesis of (E)-9-(2-iodovinyl)-9H-carbazole via sequential ruthenium-catalyzed silylative coupling of N-vinylcarbazole with
vinyltrimethylsilane and iododesilylation is reported. Its use as a new building block in the palladium-catalyzed Sonogashira and
Suzuki-Miyaura coupling reactions to yield new carbazole-containing (E)-but-1-en-3-ynes and (E,E)-buta-1,3-dienes is demonstrated.
Carbazole has attracted considerable interest as a build-
ing block in material science for its known hole-transport-
ing and electroluminescence properties.1 The carbazole
moiety can be easily functionalized or covalently linked
to other molecules to enhance its π-conjugated system.
Several new protocols for the synthesis of N-substituted
π-conjugated carbazoles have been recently reported, based on
the sequential catalytic carbazole N-arylation-Sonogashira
coupling2 or N-arylation-Wittig-Horner reaction;3 how-
ever, the products were limited to those in which the nitrogen
atom was substituted with arylethenyl- or arylethynyl groups.
Recently, carbazole-substituted aromatic enynes with excel-
lent electroluminescence properties have also been prepared
by catalytic dimerization of ethynylphenyl-substituted carba-
zole derivatives.4
On the other hand, N-vinylarylcarbazoles can be ob-
tained via Heck arylation of N-vinylcarbazole;5 however
the main drawback of this reaction is the poor control
of the R- and β-regioselectivity. Alternative approaches to
N-vinylarylcarbazoles based on vinylation of carbazole by
styryl halides6 or sequential silylative coupling-Hiyama
coupling of N-vinylcarbazole have been also reported.7
(1) For recent reviews see: (a) Grimsdale, A. C.; Chan, K. L.; Martin,
R. E.; Jokisz, P. G.; Holmes, A. B. Chem. Rev. 2009, 109, 897. (b) Cheng,
Y.-J.; Yang, S.-H.; Hsu, Ch.-S. Chem. Rev. 2009, 109, 5868. (c) Lo, S.-
Ch; Burn, P. L. Chem. Rev. 2007, 107, 1097.
(2) (a) Adhikari, R. M.; Shah, B. K.; Palayangoda, S. S.; Neckers,
D. C. Langmuir 2009, 25, 2402. (b) Adhikari, R. M.; Mondal, R.; Shah,
B. K.; Neckers, D. C. J. Org. Chem. 2007, 72, 4727. (c) Panthi, K.;
Adhikari, R. M.; Kinstle, T. H. J. Phys. Chem. A 2010, 114, 4550. (d)
Wan, Y.; Yan, L.; Zhao, Z.; Ma, X.; Guo, Q.; Jia, M.; Lu, P.; Ramos-
Ortiz, G.; Maldonado, J. L.; Rodriguez, M.; Xia, A. J. Phys. Chem. B
2010, 114, 11737.
(3) (a) Yang, Z.; Chi, Z.; Yu, T.; Zhang, X.; Chen, M.; Xu, B.; Liu, S.;
Zhang, Y.; Xu, J. J. Mater. Chem. 2009, 19, 5541. (b) Yan, Y.-X.; Tao,
X.-T.; Sun, Y.-H.; Yu, W.-T.; Xu, G.-B.; Wang, Ch.-K.; Zhao, H.-P.;
Yang, J.-X.; Yu, X.-Q.; Zhao, X.; Jing, M.-H. Bull. Chem. Soc. Jpn.
2005, 78, 300.
(4) Liu, Y.; Nishiura, M.; Wang, Y.; Hou, Z. J. Am. Chem. Soc. 2006,
128, 5592.
(5) (a) He, Q.; Sun, Y.; Liu, W.; Xu, S.; Cao, Z.; Cheng, J.; Bai, F.
Tetrahedron Lett. 2007, 48, 4249. (b) He, Q. G.; Cheng, J. G.; Bai, F. L.
Chin. Chem. Lett. 2007, 18, 920. (c) Ma, W.; Wu, Y.; Han, J.; Gu, D.;
Gan, F. Chem. Phys. Lett. 2005, 403, 405.
(6) Lebedev, A. Y.; Izmer, V. V.; Kazyulkin, D. N.; Beletskaya, I. P.;
Voskoboynikov, A. Z. Org. Lett. 2002, 4, 623.
(7) (a) Marciniec, B.; Majchrzak, M.; Prukala, W.; Kubicki, M.;
Chadyniak, D. J. Org. Chem. 2005, 70, 8550. (b) Prukala, W.; Marciniec,
B.; Majchrzak, M.; Kubicki, M. Tetrahedron 2007, 63, 1107.
(8) To the best of our knowledge, only a single example of 4-substituted
(E,E)-buta-1,3-dienylcarbazole: 9-((E,E)-4-nitrobuta-1,3-dien-1-yl)-9H-
carbazole has been reported in the literature: Koike, T.; Hagiwara, M.;
Takeuchi, N.; Tobinaga, S. Heterocycles 1997, 45, 1271.
r
10.1021/ol200350a
Published on Web 03/16/2011
2011 American Chemical Society

Nevertheless, to the best of our knowledge, no examples
of a general method for the synthesis of stereodefined
(E,E)-dienes8 and (E)-enynes9 containing N-substituted
carbazolyl groups has been given so far.
synthesis of (E)-β-aryl vinyl halides from styrenes,12 we
have envisaged that the ruthenium-catalyzed (E)-selective
silylative coupling of N-vinylcarbazole with trimethylvi-
nylsilane followed by N-iodosuccinimide-mediated iodo-
desilylation could be a valuable synthetic method for the
one-pot conversion of N-vinylcarbazole into (E)-9-(2-
iodovinyl)-9H-carbazole. The resulting iodide could be
used as a coupling partner for the stereoselective synthesis
of carbazole-functionalized enynes and dienes.
The silylative coupling reaction of N-vinylcarbazole and
trimethylvinylsilane (1.2 equiv) was conducted follow-
ing the original procedure (RuHCl(CO)(PCy3)2 cata-
lyst (2 mol %), toluene, 24 h, 100 °C, sealed ampule
under Ar atmosphere)7a to give exclusively (E)-9-(2-
trimethylsilylvinyl)-9H-carbazole (GC yield 95%). Treat-
ment of (E)-9-(2-trimethylsilylvinyl)-9H-carbazole with
1.2 equiv of N-iodosuccinimide (NIS) in acetonitrile at
room temperature allowed isolation of stereochemically
pure (E)-9-(2-iodovinyl)-9H-carbazole 1 in 80% isolated
yield (GC yield 98%). To the best of our knowledge, this is
the first iododesilylation process disclosed for β-silylena-
mines.13 Thus, by sequencing the highly (E)-selective sily-
lative coupling of N-vinylcarbazole with a stereospecific
iododesilylation, the stereochemical fidelity of the product
is preserved.
The design of differently functionalized building blocks
and development of reaction sequences that would allow
other key structural units are of intense ongoing synthetic
interest. In this regard, stereodefined functionalized alke-
nyl halides suitable for cross-coupling reactions have been
among the most successfully exploited for olefin motif
elaboration. (E)-9-(2-Iodovinyl)-9H-carbazole would of-
fer a synthetically versatile variant of N-vinylcarbazole
which could be used as a coupling partner in palladium-
catalyzed cross-coupling reactions allowing regio- and
stereoselective construction of complex (E)-9-(but-1-en-
3-yn-1-yl)-9H-carbazoles and (E,E)-9-(buta-1,3-dien-1-yl)-
9H-carbazoles.
In this communication we report a one-pot synthesis of
(E)-9-(2-iodovinyl)-9H-carbazole 1 from N-vinyl-carba-
zole by silylative coupling/iododesilylation sequence and
its application as a new reagent in the palladium-catalyzed
Sonogashira and Suzuki-Miyaura cross-coupling reac-
tions (Scheme 1).
During the course of our experiments, we have found that
molecular iodine (1 equiv) in CH2Cl2 could also be employed
for the iododesilylation of (E)-9-(2-trimethylsilylvinyl)-9H-
carbazole at room temperature. However, it seemed to be less
effective than NIS and gave the product with moderate yield
and selectivity (GC yield 60%, E/Z= 8/2). In contrast, when
iodine monochloride ICl (1 equiv) in CH2Cl2 was applied, no
iodovinylcarbazole product was detected and decomposition
of (E)-9-(2-trimethylsilylvinyl)-9H-carbazole to N-vinylcar-
bazole and vinyl iodide was observed.
Scheme 1. New Synthetic Strategy to Highly π-Conjugated
N-Substituted Carbazoles
After several attempts we found that iododesilylation of
(E)-9-(2-trimethylsilylvinyl)-9H-carbazole in the presence
of NIS occurred efficiently also when the 3:1 mixture of
acetonitrile and toluene was employed as the solvent with-
out affecting either the reaction yield and stereoselectivity.
This result prompted us to attempt the iododesilyla-
tion step in one pot with silylative coupling without
further purification of the (E)-9-(2-trimethylsilylvinyl)-
9H-carbazole intermediate. In a typical procedure,
N-vinylcarbazole, trimethylvinylsilane (1:1.2 ratio), and
RuHCl(CO)(PCy3)2 catalyst (2 mol %) were dissolved in
dry toluene (0.5 M concentration) and heated under an Ar
atmosphere in a Schlenk bomb flask fitted with a plug
valve at 110 °C for 24 h. Next, after cooling the reaction
mixture to room temperature, a 3-fold excess of acetoni-
trile and 1.2 equiv of solid N-iodosuccinimide were added.
Treatment of the silylative coupling product with NIS
caused iododesilylation in a stereospecific manner, giving
In the past two decades, we have developed the silylative
coupling of olefins with vinyl-substituted organosilicon
compounds occurring in the presence of complexes con-
taining initially or generating in situ M-H and M-Si
bonds.10 The silylative coupling, in combination with
subsequent desilylation reactions such as Hiyama cross-
coupling and halodesilylation, appears to be a valuable
step to provide highly conjugated π-electron compounds
such as stilbenes, aryl-substituted polyenes, or styryl
halides.11
As we have previously reported, the silylative coupling
of N-vinylcarbazole with vinylsilanes catalyzed by a ruthe-
nium-hydride complex occurred stereoselectively to give
(E)-9-(2-silylvinyl)-9H-carbazoles in high yields.7 On the
basis of our recent results on the highly stereoselective
(9) To the best of our knowledge, only a single example of carbazole-
containing (E)-but-1-en-3-yne: (E)-9-(but-1-en-3-ynyl)-9H-carbazole
has been reported: Wang, B. Chin. J. Org. Chem. 2005, 1, 81.
(10) For recent reviews, see: (a) Marciniec, B. Acc. Chem. Res. 2007,
40, 943. (b) Marciniec, B. Coord. Chem. Rev. 2005, 249, 2374.
ꢀ
(12) Pawluc, P.; Hreczycho, G.; Szudkowska, J.; Kubicki, M.; Mar-
ciniec, B. Org. Lett. 2009, 11, 3390.
(13) No report in the literature has been found on the halodesilyla-
tion of β-nitrogen-substituted vinylsilanes except the iododesilylation of
β-silylvinyltoluenesulfonamides: Timbart, L.; Cintrat, J. Ch. Chem.;
Eur. J. 2002, 8, 1637.
ꢀ
(11) Pawluc, P.; Prukala, W.; Marciniec, B. Eur. J. Org. Chem. 2010,
2, 219.
Org. Lett., Vol. 13, No. 8, 2011
1977

(E)-9-(2-iodovinyl)-9H-carbazole 1 in high geometrical
purity (E/Z = 97/3) within 2 h (Scheme 2). Column
chromatography of the resulting product (silica gel, eluent:
hexane/ethyl acetate 50:2) afforded pure compound 1 in a
76% overall isolated yield.
usefulness of 1, we investigated palladium-catalyzed
Sonogashira15 and Suzuki-Miyaura16 coupling reactions.
The cross-coupling reactions of 1 with terminal alkynes
were carried out under oxygen-free Sonogashira reaction
conditions, which have been reported previously for
haloenamides.17 Sonogashira coupling of 1 with selected
alkynes (Table 1) proceeded smoothly at room tempera-
ture in the presence of a Pd(PPh3)4/CuI catalytic system to
give corresponding (E)-9-(but-1-en-3-yn-1-yl)-9H-carba-
zoles 2a-h in good yields (75-90%). As terminal alkynes
wereused in excess, formation ofa byproduct, symmetrical
1,3-diynes, was observed (10-20%) in the reaction mix-
ture; however, they could be separated by column chro-
matography. In effort to explore the scope of the method,
we have screened the coupling of 1 with alkyl-, aryl-, and
silyl-substituted acetylenes (Table 1).
Scheme 2. One-Pot Synthesis of Key Precursor: (E)-9-(2-
Iodovinyl)-9H-carbazole 1
The E-configuration of the carbon-carbon double
bond in 1 was determined on the basis of the 1H and 13
C
Table 1. Sonogashira Coupling of (E)-9-(2-Iodovinyl)-9H-car-
bazole 1 with Terminal Alkynes
NMR spectra.14 Moreover, compound 1 proved to be a
solid and yielded a crystal amenable to X-ray structure
determination (Figure 1). The non-hydrogen substituents
of double bonds are in a trans disposition, as can be seen
from the values of the torsion angle N12-C14-C15-
I16- 176.5(3)°.14
compound
R
E/Za
yield (%)b
2a
2b
2c
2d
2e
2f
Ph
9/1
99/1
98/2
98/2
98/2
99/1
99/1
99/1
-
76
90
84
85
80
75
88
80
0
4-PhC6H4
4-t-BuC6H4
SiMe3
n-C6H13
C6H11
2g
2h
-
Ph2(OSiMe3)C
t-Bu
OEt
a E/Z ratio determined by 1H NMR. b Yield of isolated products.
Unfortunately, no reaction took place with ethynyl ethyl
ether. All the reactions were stereospecific, and in most
cases, the (E)-double bond geometry was strongly favored,
with an approximately 99:1-99:2 E/Z ratio as measured
by 1H NMR and GC-MS (Table 1).
Compounds 2a-h were isolated and characterized
spectroscopically.14 The E-configuration of the double
bond of (E)-9-(4-phenylbut-1-en-3-yn-1-yl)-9H-carbazole
2a and (E)-9-(5,5-diphenyl-5-(trimethylsiloxy)pent-1-en-
3-yn-1-yl)-9H-carbazole 2g was further confirmed by
single-crystal X-ray diffraction analysis.14 We have also
found that Suzuki-Miyaura coupling of 1 with 4,4,5,
5-tetramethyl-2-vinyl-1,3,2-dioxaborolane (2 equiv) in
the presence of the Pd(PPh3)4 catalyst (10 mol %) under
biphasic conditions (toluene (0.1 M), aqueous K2CO3
(3 equiv, 2 M), and ethanol (15 equiv)) gives (E)-9-(buta-
1,3-dien-1-yl)-9H-carbazole 3a in 65% isolated yield upon
24 h at 50 °C (Table 2). The reaction was highly stereo-
specific giving the product with retention of configuration
(E/Z = 94:6); however, a side decomposition product,
9H-carbazole, was also isolated in 10% yield. The optimal
conditions established for the reaction of compound 1 with
vinylborane were applied to selected (E)-aryl vinyl boronic
acids providing good yields (66-72% measured by GC) of
Figure 1. Perspective view of the molecule of 1. Elipsoids are
drawn at 50% probability level, hydrogen atoms are shown as
spheres of arbitrary radii.
This approach to (E)-9-(2-iodovinyl)-9H-carbazole 1
provides a useful synthesis of various β-substituted vinyl-
carbazoles via elaboration of the resulting iodide function-
ality into other substituents by using palladium-catalyzed
cross-coupling processes. Thus, to demonstrate the synthetic
(14) For details see Supporting Information.
(15) For recent reviews, see: (a) Sonogashira, K. In Metal-Catalyzed
Cross-Coupling Reactions; Stang, P. J.; Diederich, F., Eds.; Wiley-VCH:
Weinheim, 1998; pp 203-229. (b) Viciu, M. S.; Nolan, S. P. In Modern
Arylation Methods; Ackermann, L., Ed.; Wiley-VCH: Weinheim, 2009;
Chapter 6, pp 183-220. (c) Chinchilla, R.; Najera, C. Chem. Rev. 2007,
107, 874.
(16) (a) Suzuki, A. In Handbook of Organopalladium Chemistry for
Organic Synthesis; Negishi, E.-I., Ed.; Wiley-Interscience: New York, 2002;
Vol. 1; pp 249-262. (b) Beller, M. Adv. Synth. Catal. 2009, 351, 3027.
(17) Mulder, J. A.; Kurtz, K. C. M.; Hsung, R. P.; Coverdale, H.;
Frederick, M. O.; Shen, L.; Zificsak, C. A. Org. Lett. 2003, 5, 1547.
1978
Org. Lett., Vol. 13, No. 8, 2011

In conclusion, the proposed one-pot ruthenium catalyzed
silylative coupling of N-vinylcarbazole with trimethylvinyl-
silane followed by iododesilylation provides the facile
entry to (E)-9-(2-iodovinyl)-9H-carbazole, which is a versa-
tile building block shown to undergo subsequent palladium-
catalyzed Sonogashira and Suzuki-Miyaura coupling reac-
tions en route to carbazole-containing (E)-but-1-en-3-ynes
and (E,E)-buta-1,3-dienes.
Table 2. Synthesis of (E)-9-(Buta-1,3-dien-1-yl)-9H-carbazoles
compound
R
isomeric purity (%)a
yield (%)b
3a
3b
3c
3d
Hc
Ph
94 (E)
80 (65)
66 (50)
70
80 (E,E)d
84 (E,E)e
85 (E,E)f
4-MeC6H4
3-FC6H4
72
a Determined by GCMS. b Measured by GC (isolated yields in
parentheses). c 2 equiv of 4,4,5,5-tetramethyl-2-vinyl-1,3,2-dioxaboro-
lane and 10 mol % of Pd(PPh3)4 were used. d (E,E)/(E,Z)/(Z,Z) =
80:16:4. e (E,E)/(E,Z)/(Z,Z) = 84:10:6. f (E,E)/(E,Z)/(Z,Z) = 85:10:5.
Acknowledgment. Financial support from the Ministry
of Science and Higher Education (Poland), Grant N N204
148540 is gratefully acknowledged.
the desired (E,E)-9-(buta-1,3-dien-1-yl)-9H-carbazoles
(3b-d, Table 2). Thus, the coupling reactions were
performed in a toluene/ethanol mixture using Pd(PPh3)4
(5 mol %) as a catalyst, in the presence of a 2 M aqueous
solution of K2CO3 (3 equiv) at 50 °C.
Supporting Information Available. Experimantal pro-
cedures, NMR spectra, and compound characterization
data as well as X-ray analysis details. This material is free
of charge via the Internet at http://pubs.acs.org.
Org. Lett., Vol. 13, No. 8, 2011
1979