Pd-catalyzed intramolecular oxidative C-H amination: Synthesis of carbazoles

Article abstract of DOI:10.1021/ol201416u

A Pd-catalyzed oxidative C-H amination of N-Ts-2-arylanilines under ambient temperature using Oxone as an inexpensive, safe, and easy-to-handle oxidant has been developed. This process represents a green and practical method for the facile construction of carbazoles with a broad substrate scope and wide functional group tolerance.

Full text of DOI:10.1021/ol201416u

ORGANIC
LETTERS
2011
Vol. 13, No. 14
3738–3741
Pd-Catalyzed Intramolecular Oxidative
CꢀH Amination: Synthesis of Carbazoles
So Won Youn,* Joon Hyung Bihn, and Byung Seok Kim
Department of Chemistry and Research Institute for Natural Sciences,
Hanyang University, Seoul 133-791, Korea
sowony73@hanyang.ac.kr
Received May 25, 2011
ABSTRACT
A Pd-catalyzed oxidative CꢀH amination of N-Ts-2-arylanilines under ambient temperature using Oxone as an inexpensive, safe, and easy-to-
handle oxidant has been developed. This process represents a green and practical method for the facile construction of carbazoles with a broad
substrate scope and wide functional group tolerance.
Carbazoles have attracted considerable attention in
biological and material sciences as a ubiquitous structural
motif with wide-ranging physiological and photophysical
properties.1,2 Thus, a number of synthetic strategies
have been reported for the construction of these privi-
leged molecular entities. Among the repertoire of syn-
thetic methods, transition-metal-catalyzed CꢀC or
CꢀN bond forming reactions are the most powerful
and attractive considering that the starting material
could be easily prepared with synthetic convergency
and practicality (Scheme 1).
Scheme 1. Carbazole Synthesis via CꢀC or CꢀN Bond For-
mation
€
(1) (a) Knolker, H.-J.; Reddy, K. R. Chem. Rev. 2002, 102, 4303. (b)
€
€
Knolker, H.-J. Curr. Org. Synth. 2004, 1, 309. (c) Knolker, H.-J. Top.
€
Curr. Chem. 2005, 244, 115. (d) Knolker, H.-J. Chem. Lett. 2009, 38, 8.
ꢀ
ꢀ
(e) Sanchez, C.; Mendez, C.; Salas, J. A. Nat. Prod. Rep. 2006, 23, 1007.
(f) Cheng, J.; Kamiya, K.; Kodama, I. Cardiovasc. Drug Rev. 2001, 19,
152. (g) Moody, C. J. Synlett 1994, 681.
(2) (a) Organic Light Emitting Devices: Synthesis, Properties, and
Applications; M€ullen, K., Scherf, U., Eds.; Wiley-VCH: Weinheim,
Germany, 2006. (b) Grazulevicius, J. V.; Strohriegl, P.; Pielichowski, J.;
Pielichowski, K. Prog. Polym. Sci. 2003, 28, 1297. (c) Wakim, S.;
Bouchard, J.; Simard, M.; Drolet, N.; Tao, Y.; Leclerc, M. Chem.
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Mater. 2004, 16, 4386. (d) Morin, J.-F.; Leclerc, M.; Ades, D.; Siove,
A. Macromol. Rapid Commun. 2005, 26, 761. (e) Blouin, N.; Michaud,
A.; Gendron, D.; Wakim, D.; Blair, B.; Neagu-Plesu, R.; Belletete, M.;
Durocher, G.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc. 2008, 130, 732.
(3) Selected reviews on CꢀH activation: (a) Shilov, A. E.; Shul’pin,
G. B. Chem. Rev. 1997, 97, 2879. (b) Dyker, G. Angew. Chem., Int. Ed.
1999, 38, 1698. (c) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res.
2001, 34, 633. (d) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102,
1731. (e) Dick, A. R.; Sanford, M. S. Tetrahedron 2006, 62, 2439. (f)
Godula, K.; Sames, D. Science 2006, 312, 67. (g) Beccalli, E. M.;
Broggini, G.; Martinelli, M.; Sottocornola, S. Chem. Rev. 2007, 107,
5318. (h) Zhang, M Adv. Synth. Catal. 2009, 351, 2243. (i) Chen, X.;
Engle, K. M.; Wang, D.-H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009, 48,
5094. (j) Lyons, T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147.
(4) Constable, D. J.; Dunn, P. J.; Hayler, J. D.; Humphrey, G. R.;
Leazer, J. L.; Linderman, R. J.; Lorenz, K.; Manley, J.; Pearlman, B. A.;
Wells, A.; Zaks, A.; Zhang, T. Y. Green Chem. 2007, 9, 411.
CꢀH bond functionalization has emerged as an effi-
cient, atom-economical, and eco-friendly process with
widespread applications and industrial potential.3,4 In-
deed, powerful variants of pathways (a)5 and (b)6 in
Scheme 1 have been demonstrated where CꢀH bond
(5) Selected examples of coupling reactions between CꢀX and CꢀH
bonds: (a) Tsvelikhovsky, D.; Buchwald, S. L. J. Am. Chem. Soc. 2010,
132, 14048. (b) Liu, Z.; Larock, R. C. Tetrahedron 2007, 63, 347. (c)
Bedford, R. B.; Betham, M. J. Org. Chem. 2006, 71, 9403. (d) Campeau,
L.-C.; Parisien, M.; Jean, A.; Fagnou, K. J. Am. Chem. Soc. 2006, 128,
581. (e) Ames, D. E.; Opalko, A. Tetrahedron 1984, 40, 1919. (f) Iida, H.;
Yuasa, Y.; Kibayashi, C. J. Org. Chem. 1980, 45, 2938.
r
10.1021/ol201416u
Published on Web 06/14/2011
2011 American Chemical Society

activation was followed by subsequent CꢀC (Scheme 1c)7 or
CꢀN (Scheme 1d)8 bond formation to afford carbazoles.9
Recently, we reported a Pd-catalyzed domino reaction
of N-Ts-2-arylanilines with activated olefins that involved
a directed CꢀH activation, CꢀC bond formation, and
intramolecular conjugate addition reaction to generate di-
hydrophenanthridines in good to excellent yields (Scheme
2, route B).10 In this process, reactions proceededsmoothly
at room temperature via CꢀH activation followed by oxi-
dative CꢀC bond formation at the C20-position. Inspired
by this finding, we reasonedthat a carefully chosenoxidant
would promote the oxidation of palladacycle A followed
by reductive elimination of the so-obtained PdIV species
(B) with concomitant CꢀN bond formation,11 leading to a
carbazole product (Scheme 2, route A). Although an
analogous reaction using 2-arylanilines bearing a N-elec-
tron-withdrawing group (e.g., R = Ac, Scheme 1d) has
been demonstrated by Buchwald and co-workers, the
elevated temperature with the prolonged reaction time
reported therein may present limitations to the substrate
Scheme 2. Pd-Catalyzed Synthesis of N-Heterocycles from
2-Arylanilines through CꢀH Activation at the C20-Position
scope and practicality.8a,b Furthermore, a Pd0/PdII process12
has been put forward to account for the electronic influence
of the substrates examined.8a,b In contrast, Gaunt and co-
workers had demonstrated a related process that operates
at ambient temperature through a PdII/PdIV catalytic cycle
with 2-arylanilines bearing an electron-donating group
(e.g., R = Bn, Scheme 1d),8d though the costly use of
PhI(OAc)2 as the oxidant may appear less attractive
together with stoichiometric formation of a toxic bypro-
duct, i.e., PhI. While significant progress has been made in
transition-metal-catalyzed CꢀH amination by several
leading research groups,13ꢀ15 practical synthetic methods
for the construction of the carbazole system via catalytic
CꢀH amination using environmentally benign and inex-
pensive oxidants under mild conditions are yet to be
realized. In view of the prevalence of the carbazole motif
in bioactive alkaloids and electronic materials, such a
synthetic method is particularly valuable. Here, we report
a Pd-catalyzed intramolecular oxidative CꢀH amination
of N-Ts-2-arylanilines that involves a directed CꢀH acti-
vation followed by a subsequent CꢀN bond formation via
a PdII/PdIV process (R = Ts in Scheme 1d and route A in
Scheme 2), leading to carbazoles as valuable chemical enti-
ties. In particular, the reaction conditions described herein
significantly improved the efficiency and practicality of
carbazole formation through the use of Oxone as an envi-
ronmentally benign, nontoxic, easy-to-handle, and inex-
pensive oxidant, under mild conditions, to promote the
formation of PdIV species followed by CꢀN bond
(6) Selected examples of coupling reactions between CꢀX and NꢀH
bonds: (a) Zhou, Y.; Verkade, J. G. Adv. Synth. Catal. 2010, 352, 616. (b)
Ca’, N. D.; Sassi, G.; Catellani, M. Adv. Synth. Catal. 2008, 350, 2179. (c)
Kuwahara, A.; Nakano, K.; Nozaki, K. J. Org. Chem. 2005, 70, 413. (d)
Wakim, S.; Bouchard, J.; Blouin, N.; Michaud, A.; Leclerc, M. Org.
Lett. 2004, 6, 3413. (e) Lin, G.; Zhang, A. Tetrahedron 2000, 56, 7163. (f)
Boger, D. L.; Duff, S. R.; Panek, J. S.; Yasuda, M. J. Org. Chem. 1985,
50, 5782.
(7) Selected examples of coupling reactions between CꢀH and CꢀH
bonds: (a) Watanabe, T.; Oishi, S.; Fujii, N.; Ohno, H. J. Org. Chem.
€
2009, 74, 4720. (b) Schmidt, M.; Knolker, H.-J. Synlett 2009, 2421. (c)
ꢁ
Sridharan, V.; Martın, M. A.; Menendez, J. C. Eur. J. Org. Chem. 2009,
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4614. (d) Liegault, B.; Lee, D.; Huestis, M. P.; Stuart, D. R.; Fagnou, K.
J. Org. Chem. 2008, 73, 5022. (e) Matsubara, S.; Asano, K.; Kajita, Y.;
Yamamoto, M. Synlett 2007, 2055. (f) Sridharan, V.; Martın, M. A.;
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Menendez, J. C. Synlett 2006, 2375. (g) Hagelin, H.; Oslob, J. D.;
€
Akermark, B. Chem.;Eur. J. 1999, 5, 2413. (h) Knolker, H.-J.; O’Sulli-
van, N. Tetrahedron 1994, 50, 10893 and refs 5f and 6e.
(8) Examples of coupling reactions between CꢀH and NꢀH bonds:
(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) Li, B.-J.; Tian, S.-L.;
Fang, F.; Shi, Z.-J. Angew. Chem., Int. Ed. 2008, 47, 1115. (d) Jordan-
Hore, J. A.; Johansson, C. C. C.; Gulias, M.; Beck, E. M.; Gaunt, M. J. J.
Am. Chem. Soc. 2008, 130, 16184 and ref 7e. During the preparation of
this manuscript, Chang and co-workers reported an example of Cu-
catalyzed or metal-free synthesis of carbazoles from 2-benzenesulfona-
midobiphenyls; see: (e) Cho, S. H.; Yoon, J.; Chang, S. J. Am. Chem.
Soc. 2011, 133, 5996. The reaction reported herein is believed to operate
through a radical mechanism where the Cu species serves as a Lewis acid
to activate the hypervalent iodine(III) reagent. In contrast, our reaction
proceeded smoothly in the presence of TEMPO, and the use of Oxone in
the absence of a Pd catalyst did not promote any reaction even at a high
temperature (80 °C).
(13) Selected reviews on CꢀH amination: (a) Armstrong, A.; Collins,
J. C. Angew. Chem., Int. Ed. 2010, 49, 2282. (b) Collet, F.; Dodd, R. H.;
Dauban, P. Chem. Commun. 2009, 5061. (c) Davies, H. M. L.; Manning,
J. R. Nature 2008, 451, 417. (d) Davies, H. M. L.; Long, M. S. Angew.
(9) Carbazole synthesis via decomposition of azide moieties and/or
nitrene transfer: (a) Smitrovitch, J. H.; Davies, I. W. Org. Lett. 2004, 6,
533. (b) Stokes, B. J.; Richert, K. J.; Driver, T. G. J. Org. Chem. 2009, 74,
€
Chem., Int. Ed. 2005, 44, 3518. (e) Muller, P.; Fruit, C. Chem. Rev. 2003,
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6442. (c) Stokes, B. J.; Jovanovic, B.; Dong, H.; Richert, K. J.; Riell,
103, 2905. (f) Dauban, P.; Dodd, R. H. Synlett 2003, 1571.
R. D.; Driver, T. G. J. Org. Chem. 2009, 74, 3225. (d) Shou, W. G.; Li, J.;
Guo, T.; Lin, Z.; Jia, G. Organometallics 2009, 28, 6847.
(10) Kim, B. S.; Lee, S. Y.; Youn, S. W. Chem.;Asian J. 2011, DOI:
10.1002/asia.201100024.
(14) Selected examples of Pd-catalyzed intermolecular amination of
aromatic CꢀH bonds: (through a nitrene intermediate): (a) Thu, H.-Y.;
Yu, W.-Y.; Che, C.-M. J. Am. Chem. Soc. 2006, 128, 9048. (b) Ng, K.-H.;
Chan, A. S. C.; Yu, W.-Y. J. Am. Chem. Soc. 2010, 132, 12862. (c) Dick,
A. R.; Remy, M. S.; Kampf, J. W.; Sanford, M. S. Organometallics 2007,
27, 1365. (not using a nitrene source): (d) Xiao, B.; Gong, T.-J.; Xu, J.;
Liu, Z.-J.; Liu, L. J. Am. Chem. Soc. 2011, 133, 1466. (e) Sun, K.; Li, Y.;
Xiong, T.; Zhang, J.; Zhang, Q. J. Am. Chem. Soc. 2011, 133, 1694.
(15) Selected examples of Pd-catalyzed intramolecular amination of
aromatic CꢀH bonds: (a) Inamoto, K.; Saito, T.; Hiroya, K.; Doi, T. J.
Org. Chem. 2010, 75, 3900. (b) Mei, T.-S.; Wang, X.; Yu, J.-Q. J. Am.
Chem. Soc. 2009, 131, 10806. (c) Wasa, M.; Yu, J.-Q. J. Am. Chem. Soc.
2008, 130, 14058. (d) Inamoto, K.; Saito, T.; Hiroya, K.; Doi, T. Synlett
2008, 3157. (e) Inamoto, K.; Saito, T.; Katsuno, M.; Sakamoto, T.;
Hiroya, K. Org. Lett. 2007, 9, 2931.
~
(11) For a review, see: (a) Muniz, K. Angew. Chem., Int. Ed. 2009, 48,
9412. For selected examples of reductive elimination of C-heteroatom
bonds from Pd(IV) intermediates, see: (b) Furuya, T.; Ritter, T. J. Am.
Chem. Soc. 2008, 130, 10060. (c) Fu, Y.; Li, Z.; Liang, S.; Guo, Q.-X.;
Liu, L. Organometallics 2008, 27, 3736. (d) Wang, G.-W.; Yuan, T.-T.;
Wu, X.-L. J. Org. Chem. 2008, 73, 4717. (e) Whitfield, S. R.; Sanford,
M. S. J. Am. Chem. Soc. 2007, 129, 15142. (f) Desai, L. V.; Malik, H. A.;
Sanford, M. S. Org. Lett. 2006, 8, 1141 and refs 8d, 14aꢀb, 14d, and
15bꢀc.
(12) For an example of CꢀH amination via a Pd0/PdII pathway, see:
Tan, Y.; Hartwig, J. F. J. Am. Chem. Soc. 2010, 132, 3676.
Org. Lett., Vol. 13, No. 14, 2011
3739

construction via reductive elimination.16 This protocol,
using sulfonamide derivatives as an electron-withdrawing
alternative, should prove complementary to Gaunt’s ear-
lier report.8d Last but not least, as we shall see, high
functional group tolerance in this newly developed process
is also highly noteworthy.
(e.g., Me, Bn) reconfirmed the effectiveness of sulfonyl,
especially p-toluenesulfonyl (Ts), as the preferred group
for this reaction (Scheme 3).18 In sharp contrast to the
earlier reports,8 alkyl-protected amines led to a complex
reaction mixture and no reaction was observed with
anilides under our optimized conditions. These findings
also appear to be consistent with the fact that no reaction
took place using oxidants generally employed in the Pd0/PdII
catalytic pathway (Cu(OAc)2, benzoquinone, and AgOAc),
suggesting a mechanistic difference with the related CꢀH
amination of N-Ac-2-arylanilines reported by the Buchwald
group.8aꢀc The reaction in hand that operates in the presence
of strong oxidants (e.g., Oxone, PhI(OAc)2, K2S2O8) at
ambient temperature is in closer analogy with that developed
by the Gaunt group with the nitrogen guarded by an electron-
donating group8d through the PdII/PdIV pathway.16
With the optimized reaction conditions in hand, we set
out to explore the substrate scope of this process by first
examining the substituent effect of the tosylamide bearing
an aromatic ring (Scheme 3, 2bꢀ2f). Similar to our pre-
vious work,10 a subtle change in the acidity of the NH
moiety through the introduction of substituents including
Me, NO2, and CF3 at the C4- or C5-position had a
significant influence on the chemical reactivity, an obser-
vation that is in line with the protecting group effect
described earlier. As such, a delicately balanced acidity of
the NH group through the nitrogen protecting group and
the aryl substituent of the protected aniline is required to
achieve high efficiency for this reaction. A sterically hin-
dered substrate with a substituent at the C6-position required
more forcing conditions, giving product 2c in 74% yield.
The substituent effect of the 2-aryl moiety of substrate 1
was also explored (Scheme 4). In this context, both elec-
tron-donating and -withdrawing substituents were well
tolerated, with the exception of a substrate bearing a
strongly electron-withdrawing group (e.g., NO2) at the
C30-position, which required more forcing reaction
In light of our recent success in Pd-catalyzed domino
reactions of N-Ts-2-arylanilines with activated olefins,10
we began our studies on the proposed oxidative CꢀH
amination reaction using N-Ts-2-phenylaniline (1a) as the
test substrate. Our studies revealed the inexpensive, safe,
and easy-to-handle Oxone as the oxidant of choice for this
transformation, and the pivalic acid (PivOH)17/DMF
cosolvent system proved the most ideal.18 Moreover, the
use of p-TsOH as an additive along with PivOH/DMF
further improved the reaction yield. Lastly, among all the
Pd complexes examined, Pd(OAc)2 was found to be the
most effective Pd catalyst for this reaction, and no reaction
occurred in the absence of Pd(OAc)2 even at a high
temperature (80 °C). This result is notable in view of the
recent report by Chang et al., where a hypervalent iodine-
(III)-mediated oxidative radical process at elevated tem-
perature promoted the formation of carbazoles from
sulfonamides under metal-free conditions.8e Inclusion of
TEMPO as an additive had no deleterious effect on the
efficiency of the carbazole formation, providing evidence
in support of a nonradical mechanistic pathway. Conti-
nuation of our reaction optimization ultimately secured
a reagent blend consisting of Pd(OAc)2 (5 mol %), Oxone
(1 equiv), and p-TsOH (0.5 equiv) in PivOH/DMF (1:3),
which operates at rt to afford N-Ts-carbazole 2a with an
impressive 98% yield (Scheme 3).
Scheme 3. Substituent Effect of the Aniline Moiety of 2-Phe-
nylanilines
Scheme 4. Substituent Effect of the 2-Aryl Moiety of N-Ts-2-
Arylanilines
aDetermined by 1H NMR. bDecomposed. c10 mol % Pd(OAc)2. d80
°C. e40 °C. fWith nBu4NF in THF at reflux for 4 h.
Before proceeding, a cursory survey of the effect of
N-protecting groups including sulfonyl (e.g., Ts, PhSO2,
Ms), acyl (e.g., Ac, Bz), carboalkoxy (e.g., Boc), and alkyl
(16) For selected examples using Oxone as an oxidant for CꢀC or
CꢀO bond formations, see: Hull, K. L.; Lanni, E. L.; Sanford, M. S. J.
Am. Chem. Soc. 2006, 128, 14047 and ref 11f.
a10 mol % Pd(OAc)2. b80 °C. cWithout p-TsOH.
(17) For the effect of PivOH, see ref 7d and references therein.
(18) For details, see Supporting Information.
3740
Org. Lett., Vol. 13, No. 14, 2011

conditions to afford product 2e with comparable yield.
Furthermore, while the reaction with the 20-OMe substi-
tuted substrate proceeded uneventfully at rt (2g, 72%
yield), the reaction of the 20-NHTs substituted substrate
required a higher catalyst loading (10 mol % Pd(OAc)2)
at an elevated temperature (80 °C) and the exclusion of
p-TsOH from our standardconditions, toafford2kin88%
yield. On the other hand, C30-substituted substrates
showed remarkable regioselectivity, leading to products
originating from activation of the less hindered CꢀH bond
(2h, 2m, 2p, and 2e, Scheme 4). This method also proved
useful in the preparation of carbazoles with substitution on
both aromatics rings, leading to products 2qꢀu in good
yields (Scheme 5). The functional group compatibility of
the developed reaction conditions is particularly note-
worthy, including but not limited to methoxy, halogen,
ketone, ester, amino, and nitro groups. Halogenated sub-
strates afforded products with the halogen substituents
remaining intact, where the formation of dehalogenated
products was not observed (3b, Scheme 3; 2lꢀn, Scheme 4;
2s, Scheme 5).
styrylaniline could engage in CꢀH amination to afford
2-substituted indole 2w (Scheme 5). The synthesis of two
naturally occurring carbazoles, glycozoline19b (2q0, Scheme 5),
and clausine C19c (2u0, Scheme 5), as well as 9H-carbazole
(2a0, Scheme 3), was achieved through deprotection of the
Ts group.
To gain insight into this reaction, we first performed
competition experiments with a series of 30-substituted
N-Ts-2-phenylanilines.18 This study revealed an electronic
dependence on variation of the substituent from electron-
donating to electron-withdrawing, an observation that is
consistent with the initial electrophilic cyclopalladation.
Further evidence in support of the electrophilic nature of
this process was made available through kinetic isotope
studies, where competition experiments demonstrated
modest but notable secondary intermolecular (kH/kD =
1.41) and intramolecular (kH/kD = 2.03) kinetic isotope
effects.18,20
In summary, we have developed an effective Pd-cata-
lyzed oxidative CꢀH amination of N-Ts-2-arylanilines
under ambient temperatureusing Oxoneasaninexpensive,
safe, and easy-to-handle oxidant. This method offers a
straightforward access to a wide range of carbazoles, an
important structural motif in natural and designed com-
pounds with interesting biological and physical properties.
Equally noteworthy is the functional group tolerance of
the developed reaction, a feature that should permit
further transformation of the carbazole product and pro-
vide anentry tostructurally diverse heterocycles. Inview of
the growing understanding of transition-metal-mediated
CꢀH activation/functionalization processes, the reaction
described herein showcased a reactivity profile that is
notably different to those previously reported.8 Further
investigations to expand the scope of this reaction are
currently underway in our laboratory.
Scheme 5. Pd-Catalyzed CꢀH Amination for the Synthesis of
Various Substituted Carbazoles
Acknowledgment. This work was supported by
Mid-career Researcher Program (No. R01-2009-008-
3940) and Basic Science Research Program (Nos. 2010-
0017149 and 2010-0007737) through the National Re-
search Foundation of Korea (NRF) grant funded by the
Ministry of Education, Science and Technology (MEST).
a10 mol % Pd(OAc)2. b40 °C. cWithout p-TsOH at 80 °C. dWith
nBu4NF in THF at reflux for 2.5 h.
Supporting Information Available. Full experimental
details and characterization data. This material is avail-
able free of charge via the Internet at http://pubs.acs.org.
As a further demonstration of the developed method in
the construction of N-heterocycles, N,N0-bistosyl-2,200-
diamino-[1,10;30,100]terphenyl provided fused pentacyclic
indolo[2,3-b]carbazole 2v in good yield (Scheme 5).19a It
is also noteworthy that the vinylic CꢀH bond of N-Ts-2-
(20) (a) Yip, K.-T.; Yang, D. Org. Lett. 2011, 13, 2134. (b) Seregin,
I. V.; Ryabova, V.; Gevorgyan, V. J. Am. Chem. Soc. 2007, 129, 7742. (c)
Chiong, H. A.; Daugulis, O. Org. Lett. 2007, 9, 1449. (d) Lane, B. S.;
Brown, M. A.; Sames, D. J. Am. Chem. Soc. 2005, 127, 8050. (e)
Campeau, L.-C.; Parisien, M.; Leblanc, M.; Fagnou, K. J. Am. Chem.
Soc. 2004, 126, 9186. (f) Hennessy, E. J.; Buchwald, S. L. J. Am. Chem.
Soc. 2003, 125, 12084.
€
(19) (a) Knolker, H.-J.; Reddy, K. R. Tetrahedron 2000, 56, 4733. (b)
€
Forke, R.; Krahl, M. P.; Krause, T.; Schlechtingen, G.; Knolker, H.-J.
€
€
Synlett 2007, 268. (c) Krahl, M. P.; Jager, A.; Krause, T.; Knolker, H.-J.
Org.Biomol. Chem. 2006, 4, 3215.
Org. Lett., Vol. 13, No. 14, 2011
3741