PdII-catalyzed Di-o-olefination of carbazoles directed by the protecting N-(2-pyridyl)sulfonyl group

Article abstract of DOI:10.1021/ol400206k

Despite the significance of carbazole in pharmacy and material science, examples of the direct C-H functionalization of this privileged unit are quite rare. The N-(2-pyridyl)sulfonyl group enables the Pd^II-catalyzed ortho-olefination of carbazo

Full text of DOI:10.1021/ol400206k

ORGANIC
LETTERS
PdII-Catalyzed Di‑o‑olefination of
Carbazoles Directed by the Protecting
N‑(2-Pyridyl)sulfonyl Group
XXXX
Vol. XX, No. XX
000–000
ꢀ
ꢀ
ꢀ
Beatriz Urones, Ramon Gomez Arrayas,* and Juan Carlos Carretero*
´
Departamento de Quımica Organica, Facultad de Ciencias, Universidad Autonoma
de Madrid, Cantoblanco, 28049 Madrid, Spain
ꢀ
ꢀ
ramon.gomez@uam.es; juancarlos.carretero@uam.es
Received January 23, 2013
ABSTRACT
Despite the significance of carbazole in pharmacy and material science, examples of the direct CꢀH functionalization of this privileged unit are
quite rare. The N-(2-pyridyl)sulfonyl group enables the PdII-catalyzed ortho-olefination of carbazoles and related systems, acting as both a
directing and readily removable protecting group. This method features ample structural versatility, affording typically the double ortho-
olefination products (at C1 and C8) in satisfactory yields and complete regiocontrol. The application of this procedure to related heterocyclic
systems, such as indoline, is also described.
The unique structural features and biological activities
of carbazole derivatives, including anti-HIV, anticancer,
and antibacterial activities, have led to a great impetus in
the development of carbazole chemistry.1 The properties
imparted by this motif have also found applications
in materials science as optoelectronic or luminescent
materials.2 The most versatile and practical methods for
carbazole synthesis involve the metal-catalyzed cyclization
of either diarylamine derivatives3 (route a, Scheme 1) or
2-aminobiphenyl derivatives4 (pathway b).
(1) For the synthesis and biological activity of carbazoles: (a)
€
Schmidt, A. W.; Reddy, K. R.; Knolker, H.-J. Chem. Rev. 2012, 112,
3193. (b) Roy, J.; Jana, A. K.; Mal, D. Tetrahedron 2012, 68, 6099.
(2) (a) Huang, H.; Wang, Y.; Pan, B.; Yang, X.; Wang, L.; Chen, J.;
Ma, D.; Yang, C. Chem.;Eur. J. 2013, 19, 1828. (b) Gong, W.-L.;
Zhong, F.; Aldred, M. P.; Fu, Q.; Chen, T.; Huang, D.-K.; Shen, Y.;
Qiao, X.-F.; Ma, D.; Zhu, M.-Q. RSC Advances 2012, 2, 10821. (c)
Wang, H.-Y.; Liu, F.; Xie, L.-H.; Tang, C.; Peng, B.; Huang, W.; Wei,
W. J. Phys. Chem. C 2011, 115, 6961. (c) Bubniene, G.; Malinauskas, T.;
Daskeviciene, M.; Jankauskas, V.; Getautis, V. Tetrahedron 2010, 66,
3199 and references therein.
Scheme 1. General Catalytic Methods to Carbazole Derivatives
ꢀ
(3) For recent examples, see: (a) Liegault, B.; Lee, D.; Huestis, M. P.;
Stuart, D. R.; Fagnou, K. J. Org. Chem. 2008, 73, 5022. (b) Watanabe,
T.; Oishi, S.; Fujii, N.; Ohno, H. J. Org. Chem. 2009, 74, 4720. (c)
€
€
Gensch, T.; Ronnefahrt, M.; Czerwonka, R.; Jager, A.; Kataeva, O.;
€
Bauer, I.; Knolker, H.-J. Chem.;Eur. J. 2012, 18, 770.
(4) For selected references, see: (a) Tsang, W. C. P.; Zheng, N.;
Buchwald, S. L. J. Am. Chem. Soc. 2005, 127, 14560. (b) Tsang, W. C. P.;
Munday, R. H.; Brasche, G.; Zheng, N.; Buchwald, S. L. J. Org. Chem.
2008, 73, 7603. (c) Jordan-Hore, J. A.; Johansson, C. C. C.; Gulias, M.;
Beck, E. M.; Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 16184. (d) Youn,
S. W.; Bihn, J. H.; Kim, B. S. Org. Lett. 2011, 13, 3738. For the synthesis
of the carbazole nucleus from anilides and arenes by double CꢀH
activation: (e) Li, B.-J.; Tian, S.-L.; Fang, Z.; Shi, Z.-J. Angew. Chem.,
Int. Ed. 2008, 47, 1115. For metal-free approach: (f) Antonchick, A. P.;
Samanta, R.; Kulikov, K.; Lategahn, J. Angew. Chem., Int. Ed. 2011, 50,
8605. For both Cu-catalyzed and metal-free conditions: (g) Cho, S. H.;
Yoon, J.; Chang, S. J. Am. Chem. Soc. 2011, 133, 5996. See also: Lu, C.;
Markina, N. A.; Larock, R. C. J. Org. Chem. 2012, 77, 11153.
However, despite this significant progress, limitations
still remain with regard to the type of substitution pattern
that can be accessed. For instance, the synthesis of C1/C8-
disubstituted carbazole derivatives5 through these routes is
problematicduetothe stericcongestionnexttothe reactive
(5) For the application of 1,8-disubstituted carbazole ligands in
asymmetric catalysis: Niwa, T.; Nakada, M. J. Am. Chem. Soc. 2012,
134, 13538 and references cited therein.
r
10.1021/ol400206k
XXXX American Chemical Society

site, especially in route a. Because of the higher nucleo-
philic character of the C3 and C6 positions of the carba-
zole, the electrophilic substitution at the C3 and/or C6
positions constitutes an efficient alternative approach for
the direct functionalization of the carbazole skeleton.6 As a
consequence of this electronic distribution, substitution at
the less activated C1 and C8 of the carbazole typically
requires prior protection of the 3,6-positions.2aꢀc,5
In stark contrast to the tremendous progress made with
related nitrogen aromatic systems,7 methods for the cata-
lytic direct CꢀH functionalization of the carbazole core
are quite rare, and only recently have the first successful
examples appeared. Chu, Wu and co-workers disclosedthe
PdII-catalyzed direct ortho-arylation of carbazoles bearing
a N-(pyridin-2-yl) directing group with potassium aryl-
trifluoroborates.8 Patureau et al. have reported the direct
dehydrogenative C1ꢀN carbazolation of NH-carbazoles by
the cooperative action of Ru and Cu catalysts.9 Within this
context, new methods for the regiocontrolled direct CꢀH
functionalization, providing orthogonal selectivities to
those currently available, as well as other types of function-
alization, are of great interest in carbazole synthesis.
We have recently reported the PdII-catalyzed direct
olefination of N-alkyl N-(2-pyridyl)sulfonyl anilines and
arylalkylamines.10 On the basis of the excellent structural
flexibility displayed by thismethod, we hypothesized thatit
could be also extended to the CꢀH functionalization of
carbazole derivatives. To our delight, this was indeed the
case, and herein we disclose a reliable protocol for the PdII-
catalyzed ortho-olefination of N-(2-pyridyl)sulfonyl carba-
zoles and structurally related heterocyclic systems. To the
best of our knowledge, our work constitutes the first
example of a Pd-catalyzed direct ortho-olefination of the
carbazole nucleus.11
Table 1. Optimization of the N-Directing/Protecting Group
entry
PG (substrate)
H (1)
[ox]
[Fþ]c
[Fþ]c
[Fþ]c
[Fþ]c
conv (%)a yield [6/7, (%)]b
d
€
(6) For instance, see: (a) Dierschke, F.; Grimsdale, A. C.; Mullen, K.
1
2
3
4
5
6
7
8
9
ꢀ
ꢀ
ꢀ
d
Synthesis 2003, 2470. (b) Maegawa, Y.; Goto, Y.; Inagaki, S.; Shimada,
T. Tetrahedron Lett. 2006, 47, 6957. (c) Kumar, G. G. K. S. N.; Laali,
K. K. Tetrahedron Lett. 2013, 54, 965.
Boc (2)
Ac (3)
Ts (4)
ꢀ
<5
<5
93
70
66
100
95
ꢀ
ꢀ
(7) For recent reviews on dehydrogenative Heck reaction: (a) Le
Bras, J.; Muzart, J. Chem. Rev. 2011, 111, 1170. (b) Yeung, C. S.; Dong,
V. M. Chem. Rev. 2011, 111, 1215. (c) Kozhushkov, S. I.; Ackermann, L.
Chem. Sci. 2013, 4, 886. For selected examples on oxidative olefination
of nitrogen-containing arenes and heteroarenes: (d) Grimster, N. P.;
Gauntlett, C.; Godfrey, C. R. A.; Gaunt, M. J. Angew. Chem., Int. Ed.
2005, 44, 3125. (e) Beck, E. M.; Grimster, N. P.; Hatley, R.; Gaunt, M. J.
J. Am. Chem. Soc. 2006, 128, 2528. (f) Maehara, A.; Tsurugi, H.; Satoh,
T.; Miura, M. Org. Lett. 2008, 10, 1159. (g) Li, J.-J.; Mei, T.-S.; Yu, J.-Q.
Angew. Chem., Int. Ed. 2008, 47, 6452. (h) Beck, E. M.; Hatley, R.;
Gaunt, M. J. Angew. Chem., Int. Ed. 2008, 47, 3004. (i) Wu, J.; Cui, X.;
Chen, L.; Jiang, G.; Wu, Y. J. Am. Chem. Soc. 2009, 131, 13888. (j)
Patureau, F. W.; Glorius, F. J. Am. Chem. Soc. 2010, 132, 9982. (k) Ye,
M.; Gao, G.-L.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 6964. (l) Dai,
H.-X.; Stepan, A. F.; Plummer, M. S.; Zhang, Y.-H.; Yu, J.-Q. J. Am. Chem.
Soc. 2011, 133, 7222. (m) Zhu, C.; Falck, J. R. Org. Lett. 2011, 13, 1214. (n)
Patureau, F. W.; Besset, T.; Glorius, F. Angew. Chem., Int. Ed. 2011, 50,
1064. (o) Ueyama, T.; Mochida, S.; Fukutani, T.; Hirano, K.; Satoh, T.;
Miura, M. Org. Lett. 2011, 13, 706. (p) Wang, L.; Liu, S.; Li, Z.; Yu, Y. Org.
Lett. 2011, 13, 6137. (q)Wang, L.;Guo, W.;Zhang, X.-X.;Xia, X.-D.;Xiao,
W.-J. Org. Lett. 2012, 14, 740. (r) Kandukuri, S. R.; Schiffner, J. A.;
Oestreich, M. Angew. Chem., Int. Ed. 2012, 51, 1265. (s) Huang, Y.; Song,
F.; Wang, Z.; Xi, P.; Wu, N.; Wang, Z.; Lan, J.; You, J. Chem. Commun.
2012, 48, 2864. (t) Chen, W.-L.; Gao, Y.-R.; Mao, S.; Zhang, Y.-L.; Wang,
Y.-F.; Wang, Y.-Q. Org. Lett. 2012, 14, 5920. (u) Liang, Z.; Ju, L.; Xie, Y.;
Huang, L.; Zang, Y. Chem.;Eur. J. 2012, 18, 15816. (v) Zhao, P.; Niu, R.;
Wang, F.; Han, K.; Li, X. Org. Lett. 2012, 14, 4166. (w) Schroeder, N.;
Besset, T.; Glorius, F. Adv. Synth. Catal. 2012, 354, 579. (x) Li, B.; Ma, J.;
Wang, N.; Feng, H.; Xu, S. Org. Lett. 2012, 14, 736. (y) Ding, Z.; Yoshikai,
N. Angew. Chem., Int. Ed. 2012, 51, 4698. (z) Li, G.; Ding, Z.; Xu, B. Org.
Lett. 2012, 14, 5338. (aa) Wang, C.; Chen, H.; Wang, Z.; Chen, J.; Huang, Y.
Angew. Chem., Int. Ed. 2012, 51, 7242. (ab) Wen, P.; Li, Y.; Zhou, K.; Ma,
C.; Lan, X.; Ma, C.; Huang, G. Adv. Synth. Catal. 2012, 354, 3125. (ac)
Zhang, G.; Ma, Y.; Wang, S.; Zhang, Y.; Wang, R. J. Am. Chem. Soc. 2012,
134, 12334. (ad) Zhao, P.; Wang, F.; Han, K.; Li, X. Org. Lett. 2012, 14,
3400. (ae) Hashimoto, Y.; Hirano, K.; Satoh, T.; Kakiuchi, F.; Miura, M.
J. Org. Chem. 2013, 78, 638. (af) Liu, B.; Fan, Y.; Gao, Y.; Sun, C.; Xu, C.;
Zhu, J. J. Am. Chem. Soc. 2013, 135, 468. (ag) Shang, Y.; Jie, X.; Zhou, J.;
Hu, P.; Huang, S.; Su, W. Angew. Chem., Int. Ed. 2013, 52, 1299.
ꢀSO2(2-pyridyl) (5) [Fþ]c
ꢀSO2(2-pyridyl) (5) [Fþ]c,e
ꢀSO2(2-pyridyl) (5) [Fþ]c,f
ꢀSO2(2-pyridyl) (5) [Fþ]c,g
ꢀSO2(2-pyridyl) (5) PhI(OAc)2
11/42
20/22
19/23
ꢀ/64
16/58
g
a Based on starting material recovered after chromatographic pur-
ification. b Isolated yield after chromatography. c [Fþ] = N-fluoro-2,4,6-
trimethylpyridinium triflate. d Complex reaction mixture. e 1.1 equiv of
butyl acrylate was used. f 1.0 equiv of oxidant was used. g 4 equiv of butyl
acrylate and 3.0 equiv of oxidant were used.
To evaluate the role of the protecting/directing group, a
set of potentially coordinating protecting groups (PG)
were examined in the reaction of carbazole derivatives
2ꢀ5 with butyl acrylate under the previously optimized
conditions:10 Pd(OAc)2 (10 mol %), N-fluoro-2,4,6-
trimethylpyridinium triflate ([Fþ], 2.0 equiv) as the oxidant12
in ClCH2CH2Cl at 110 °C (Table 1, entries 1ꢀ5). Both the
unprotected NH-carbazole (1) and the N-Boc derivative 2
led to a complex mixture of products (entries 1 and 2),
with the latter result suggesting that the Boc protecting
group is too labile under the reaction conditions. Switch-
ing to an N-Ac group (substrate 3) or an N-Ts group (4)4d
resulted in the full recovery of the unreacted starting
(11) For the ortho-directed lithiation/functionalization of carbazole:
Katrizky, A. R.; Rewcastle, G. W.; Vazquez de Miguel, L. M. J. Org.
Chem. 1988, 53, 794.
(12) Among other oxidants examined, PhI(OAc)2 showed similar
performance as [Fþ], whereas Ce(SO4)2 and AgNO3 led to lower
reactivity while still maintaining poor monoselectivity. Other oxidants
such as Cu(OAc)2, 1,4-benzoquinone, Ag2O, K2S2O8, oxone, or Tempo
led to full recovery of the starting material. See Supporting Information
for other oxidant screening results.
(8) Chu, J.-H.; Wu, C.-C.; Chang, D.-H.; Lee, Y.-M.; Wu, M.-J.
Organometallics 2013, 32, 272.
(9) Louillat, M.-L.; Patureau, F. W. Org. Lett. 2013, 15, 164.
ꢀ
ꢀ
(10) Garcıa-Rubia, A.; Urones, B.; Gomez Arrayas, R.; Carretero,
J. C. Angew. Chem., Int. Ed. 2011, 50, 10927.
B
Org. Lett., Vol. XX, No. XX, XXXX

material, even after 24 h (entries 3 and 4). Pleasingly,
carbazole 5 with an N-(2-pyridyl)sulfonyl group10,13 pro-
vided a mixture of the mono- and diolefinated products 6
and 7, respectively, in high conversion and complete ortho
regiocontrol (entry 5).
Scheme 2. Olefin Scope for PdII-Catalyzed Diolefination
Unfortunately, monoolefination selectivity could not be
controlled by using reduced equivalents of either the
acrylate or the oxidant (entries 6 and 7). The formation
of significant amounts of the difunctionalized product 7 in
both cases even at low conversions suggested a similar
reactivity of both the starting substrate 5 and the mono-
olefination product 6. However, high diolefination selec-
tivity was achieved by simply adjusting the excess of alkene
(4 equiv) and oxidant (3 equiv). The use of [Fþ] resulted in
a clean diolefination reaction, providing the C1/C8-disub-
stituted carbazole derivative 7 in 64% isolated yield14
(entry 8), whereas PhI(OAc)2 was found to be slightly less
reactive (entry 9).
Scheme 3. Substrate Scope for PdII-Catalyzed Olefination
By using these optimized conditions, we next explored
the alkene scope of the reaction. The results are summa-
rized in Scheme 2. Other monosubstituted electrophilic
alkenes, such as phenyl vinyl sulfone or ethyl vinyl ketone,
were also capable reactants in the model diolefination
reaction with carbazole 5, leading to the corresponding
dialkenylated products 8 and 9 in good isolated yields
(64% and 80%). Interestingly, styrene derivatives bearing
electron-withdrawing substituents (NO2 or CF3) at the
para-position of the phenyl ring were found to be excellent
coupling partners (products 10 and 11, 80% and 91%
yield). Unfortunately, styrene itself provided poor reactiv-
ity (mixtures of mono- and diolefinated products in 60%
conversion).
A series of variously substituted carbazole derivatives
were then subjected to olefination with butyl acrylate
(Scheme 3). In general, both electron-rich and -deficient
substratesperformed wellinthisreaction, thereby enabling
the construction of polysubstituted carbazoles in accept-
able yields (18ꢀ22, 48ꢀ66% yield). Of special importance,
carbazoles containing halogen atoms, including chlorine
and bromine, are also compatible with this catalytic system
(19 and 20). This observed orthogonal reactivity relative to
the Pd0-catalyzed cross-coupling chemistry is useful for
subsequent product derivatization. As expected, a block-
ing fluorine substituent at C1 in substrate 15 caused
exclusive monoolefination at C8 (product 21, 66% yield).
Regarding the alkenylation of related heterocyclic sys-
tems, the more sterically congested N-(2-pyridyl)sulfonyl
benzo[b]carbazole reacted at the less hindered ortho site
with complete regiocontrol to give the monoolefinated
derivative 22 (64% yield). Likewise, the successful use of
the partially saturated hexahydrocarbazole derivative
turned out to be viable, producing exclusive ortho-mono-
olefination at the aromatic ring in high yield (product 23,
84%). This result drew our attention to indoline deriva-
tives because this motif is also prevalent in many natural
products and pharmaceutical targets15 and because the
direct catalytic C7ꢀH olefination of the indoline skeleton
has been only scarcely documented.16 Pleasingly, the reac-
tion of the N-(2-pyridyl)sulfonyl indoline (24) with butyl
acrylate (2.0 equiv) under the standard reaction conditions
produced the alkenylated product at C7 25 in isolated 80%
yield (Scheme 4).
(13) For use of the N-(2-pyridyl)sulfonyl directing group in CꢀH
ꢀ
ꢀ
functionalization reactions, see: (a) Garcıa-Rubia, A.; Gomez Arrayas,
R.; Carretero, J. C. Angew. Chem., Int. Ed. 2009, 48, 6511. (b) Garcıa-
ꢀ
ꢀ
Rubia, A.; Urones, B.; Gomez Arrayas, R.; Carretero, J. C. Chem.;
Eur. J. 2010, 16, 9676. (c) Rodrıguez, N.; Romero-Revilla, J. A.;
ꢀ
ꢀ~
Fernandez-Ibanez, M. A.; Carretero, J. C. Chem. Sci. 2013, 4, 175.
For use of the related (2-pyridyl)sulfinyl group: (c) Garcıa-Rubia, A.;
ꢀ
ꢀ~
ꢀ
ꢀ
Fernandez-Ibanez, M. A.; Gomez Arrayas, R.; Carretero, J. C. Chem.;
Eur. J. 2011, 17, 3567. (d) Romero-Revilla, J. A.; Garcıa -Rubia, A.;
ꢀ
ꢀ
ꢀ
ꢀ~
Gomez Arrayas, R.; Fernandez-Ibanez, M. A.; Carretero, J. C. J. Org.
~
Chem. 2011, 76, 9525. (e) Richter, H.; Beckendorf, S.; Garcıa Mancheno,
O. Adv. Synth. Catal. 2011, 353, 295. (f) Yu, M.; Liang, Z.; Wang, Y.;
Zhang, Y. J. Org. Chem. 2011, 76, 4987.
(14) Although the 1H NMR of the crude reaction mixture was rather
clean, showing mainly the diolefinated product, the difficulty in its
complete chromatographic separation from the trace amount of uni-
dentified byproducts resulted in a moderate 64% yield.
(15) For recent examples of indoline synthesis: He, G.; Lu, C.; Zhao,
Y.; Nack, W. A.; Chen, G. Org. Lett. 2012, 14, 2944 and references cited
therein.
Org. Lett., Vol. XX, No. XX, XXXX
C

alkaloids,16b,18 and whose derivatives are known to show
unusual photosensitizing properties.19
Scheme 4. CꢀH Olefination of Indoline 24
In summary, we have demonstrated the ability of the
N-(2-pyridyl)sulfonyl group to serve as a directing and
readily removable protecting group in the PdII-catalyzed
regiocontrolled C1/C8 diolefination of carbazoles, as well
as the ortho-olefination of some structurally related nitro-
gen heterocyclic systems such as indolines. Because of the good
structural versatility in both alkene and heteroarene coupling
components, this protocol enables rapid access to function-
ally dense motifs found in relevant heterocyclic systems.
The easy reductive removal of the 2-pyridylsulfonyl
group under mild conditions to generate the free NH-
carbazoles led us to realize the full synthetic utility of this
method. Interestingly, the sulfonyl cleavage could be
directed to the selective formation of either alkenyl- or
alkyl-substituted free carbazoles, depending on the reduc-
ing agent used (Scheme 5). Simple treatment of derivative
7 with an excess of Zn powder in a 1:1 mixture of THF and
saturated aq NH4Cl at rt led to the corresponding free
carbazole 26 in 93% yield without affecting the sensitive
acrylate moiety. Instead, treatment of 7 with magnesium
turnings (MeOH, rt, sonication) afforded the dialkylated
free carbazole 27 (75% yield). These complementary de-
protection protocols can also be applied with comparable
efficiency to the indoline derivative 25, as exemplified in its
transformation into the desulfonylated products 28 and
29. In the latter case, the deprotection simultaneously
triggers the cyclization of the free NH-indoline under the
reaction conditions to give the tricyclic compound 29.17
The easy aromatization of this product with DDQ fur-
nishes the pyrroloquinolinone framework of 30, which
is found in some biologically relevant indole based
Scheme 5. Deprotection of Olefinated N-(2-Pyridyl)sulfonyl
Carbazoles and Indolines
Acknowledgment. This article is dedicated to the mem-
ory of our colleague and friend Dr. Christian G. Claessens.
This work was supported by the Ministerio de Ciencia e
ꢀ
Innovacion (MICINN, CTQ2009-07791), the Ministerio
de Economıa y Competitividad (MINECO, CTQ 2012-
(16) (a) Yi, C. S.; Yun, S. Y. J. Am. Chem. Soc. 2005, 127, 17000. For
the intramolecular C7ꢀH arylation of indolines via a metal-free, single-
electron transfer mechanism, see: (b) De, S.; Ghosh, S.; Bhunia, S.;
Sheikh, J. A.; Bisai, A. Org. Lett. 2012, 14, 4466. For the observation of
2,7-disubstitution in the Pd-catalyzed CꢀH olefination of indoles: (c)
Fanton, G.; Coles, N. M.; Cowley, A. R.; Flemming, J. P.; Brown, J. M.
Heterocycles 2010, 80, 895. For a directed ortho-lithiation approach to
C-7-substituted indoles: (d) Hartung, C. G.; Fecher, A.; Chapell, B.;
Snieckus, V. Org. Lett. 2003, 5, 1899.
(17) For the synthesis of 29 and its use as an intermediate in the
synthesis of a potent and selective CYP11B1 inhibitor for the treatment
of Cushing’s syndrome, see: Yin, L.; Lucas, S.; Maurer, F.; Kazmaier,
U.; Hu, Q.; Hartmann, R. W. J. Med. Chem. 2012, 55, 6629.
(18) Robbins, D. W.; Boebel, T. A.; Hartwig, J. F. J. Am. Chem. Soc.
2010, 132, 4068.
ꢀ
35790), and the Consejerıa de Educacion de la Comunidad
de Madrid (programme AVANCAT, S2009/PPQ-1634).
B.U. thankstheMICINNfora predoctoral fellowship. We
also thank Johnson Matthey PLC for generous loans of
PdCl2 and Pd(OAc)2.
Supporting Information Available. Experimental pro-
cedures and characterization data of new compounds and
copies of NMR spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
(19) Pyrrolo[3,2,1-ij]quinolin-4-one (30) has been previously pre-
pared using a ketene cyclization under flash vacuum pyrolysis condi-
tions (at 950 °C): McNab, H.; Nelson, D. J.; Rozgowska, E. J. Synthesis
2009, 2171. See references cited therein for photophysical properties of
this type of system.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX