Iron-catalyzed directed C2-alkylation and alkenylation of indole with vinylarenes and alkynes

Article abstract of DOI:10.1021/ol503395g

An iron-N-heterocyclic carbene catalyst generated from an iron(III) salt, an imidazolinium salt, and a Grignard reagent promotes alkylation and alkenylation reactions at the indole C2-position with vinylarenes and internal alkynes, respectively, via imine-directed C-H activation. The former reaction affords 1,1-diarylalkane derivatives with exclusive regioselectivity. Deuterium-labeling experiments suggest that these reactions involve oxidative addition of the C-H bond to the iron center, insertion of the unsaturated bond into the Fe-H bond, and C-C reductive elimination.

Full text of DOI:10.1021/ol503395g

Letter
pubs.acs.org/OrgLett
Iron-Catalyzed Directed C2-Alkylation and Alkenylation of Indole
with Vinylarenes and Alkynes
Mun Yee Wong, Takeshi Yamakawa, and Naohiko Yoshikai*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: An iron−N-heterocyclic carbene catalyst generated
from an iron(III) salt, an imidazolinium salt, and a Grignard
reagent promotes alkylation and alkenylation reactions at the
indole C2-position with vinylarenes and internal alkynes,
respectively, via imine-directed C−H activation. The former
reaction affords 1,1-diarylalkane derivatives with exclusive
regioselectivity. Deuterium-labeling experiments suggest that
these reactions involve oxidative addition of the C−H bond to
the iron center, insertion of the unsaturated bond into the Fe−H
bond, and C−C reductive elimination.
nsertion of unsaturated hydrocarbon molecules such as
alkenes and alkynes into unactivated aromatic C−H bonds
precatalyst with an iron salt was not effective at all in most of
those reactions, the reaction of 1-methyl-3-iminoindole 1 and
styrene 2a18 was found to be feasible (Table 1). In the presence
of Fe(acac)3 (99%, 10 mol %), bis(2,6-dimethylphenyl)-
imidazolium chloride (IXyl·HCl, 10 mol %), CyMgCl (100
mol %), and TMEDA (2 equiv), the reaction proceeded in THF
at 60 °C to afford 1,1-diarylethane 3a as the exclusive regioisomer
in 72% yield (Table 1, entry 1). The reaction was further
improved by using bis(2,6-dimethylphenyl)imidazolinium chlor-
ide (SIXyl·HCl) instead of IXyl·HCl, affording 3a in 90% yield
(Table 1, entry 2).
Subsequent screening experiments were performed using less
reactive 2-methylstyrene 2b in place of 2a. Key findings are
summarized as follows: (1) A better yield was achieved in Et2O
than in THF (Table 1, entries 3 and 4). (2) SIXyl·HCl showed
the best performance among other common imidazolinium
preligands (Table 1, entries 5 and 6). (3) Phosphine and
bipyridine ligands as well as ligand-free conditions were almost
ineffective (Table 1, entries 7−10). (4) The reaction was shut
down in the absence of TMEDA (Table 1, entry 11). (5) No
reaction took place when tBuCH2MgBr was used instead of
CyMgCl (Table 1, entry 12). Note that high-purity Fe(acac)3
(>99.9%) gave essentially the same result as reagent-grade
Fe(acac)3 (Table 1, entry 13). In addition, no reaction was
observed when a small amount of CoBr2 (0.5 mol %) was used
instead of Fe(acac)3 (Table 1, entry 14). The same observation
was made for other metal salts such as Mn(acac)3 and Ni(acac)2.
Thus, it is unlikely that trace metal impurity is responsible for the
observed reactivity. Note that reduction of the amount of
CyMgCl led to diminished catalytic activity.
I
(hydroarylation) represents an atom-economical approach for
the introduction of alkyl and alkenyl groups onto arenes. In the
past few decades, low-valent late transition metal complexes such
as ruthenium,1,2 rhodium,3,4 iridium,5 rhenium,6 nickel,7
cobalt,8,9 and manganese10 complexes have been demonstrated
to catalyze a variety of such hydroarylation reactions with or
without the assistance of heteroatom directing groups. A typical
catalytic cycle of this type of C−H functionalization involves
three elementary steps, that is, oxidative addition of the C−H
bond to the low-valent metal center (M), migratory insertion of
an alkene or alkyne into the M−H bond, and C−C reductive
elimination of the resulting diorganometal species.
In light of the above mechanistic framework, we became
interested in the potential use of economically and environ-
mentally attractive iron complexes as catalysts for hydroarylation,
because each of the three elementary steps appeared feasible.
Thus, aromatic C−H bonds are known to undergo oxidative
addition to low-valent iron complexes,11 while migratory
insertion of an alkene or alkyne into Fe−H and C−C reductive
elimination are common steps in iron-catalyzed reactions such as
hydrogenation/hydrosilylation12 and cross-coupling,13 respec-
tively. We report herein that an iron−N-heterocyclic carbene
(NHC) catalyst generated from an iron(III) salt, an
imidazolinium salt, and a Grignard reagent14 promotes imine-
directed insertion of vinylarenes and alkynes into an indole C2−
H bond to afford the corresponding alkylation and alkenylation
products, respectively. To our knowledge, the reaction
represents the first example of iron-catalyzed hydroarylation
involving directed C−H activation.15−17
The present study began with attempts to perform our
previously developed cobalt-catalyzed imine- or pyridine-
directed hydroarylation reactions9 using an iron complex as an
alternative catalyst. While simple replacement of the cobalt
Received: November 22, 2014
© XXXX American Chemical Society
A
DOI: 10.1021/ol503395g
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Screening of Reaction Conditions
Scheme 1. Imine-Directed C2-Alkylation of Indole with
a
Vinylarenes
b
entry
olefin
ligand
solvent
yield (%)
1
2
3
4
5
6
7
2a
2a
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
2b
IXyl·HCl
SIXyl·HCl
SIXyl·HCl
SIXyl·HCl
SIMes·HCl
SIPr·HCl
PCy3
THF
THF
THF
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
Et2O
72
90
59
65
57
20
4
c
8
PPh3
2
9
dtbpy
0
10
none
6
d
11
SIXyl·HCl
SIXyl·HCl
SIXyl·HCl
SIXyl·HCl
0
e
12
0
f
13
67
0
g
14
a
The reaction was performed using 1a (0.2 mmol), 2a or 2b (0.3
mmol). Fe(acac)3 (99%) was used. PMP = p-methoxyphenyl.
b
Determined by 1H NMR using 1,1,2,2-tetrachloroethane as an
c
d
internal standard. 20 mol % of PPh3 was used. TMEDA was omitted.
e
f
a
t-BuCH2MgBr was used instead of CyMgBr. Fe(acac)3 (≥99.9%)
b
The reaction was performed on a 0.2 mmol scale. Determined by
1H NMR using 1,1,2,2-tetrachloroethane as an internal standard.
g
was used. CoBr2 (0.5 mol %) was used instead of Fe(acac)3.
Scheme 2. Isomerization/Hydroarylation of Allylbenzenea
With the optimized reaction conditions (Table 1, entry 4) in
hand, we next explored the scope of vinylarenes (Scheme 1). A
variety of substituted styrene derivatives participated in the
reaction with 1 to afford, upon acidic hydrolysis, 1,1-diarylethane
derivatives bearing C3-formyl group 4a−4f in good yields, while
the reaction turned out incompatible with functional groups such
as bromo, iodo, and cyano groups (see the Supporting
Information for detail). cis-β-Substituted styrenes bearing alkyl,
aryl, and silyl substituents were also amenable to the present
reaction, thus affording the corresponding 1,1-diarylalkanes 4g−
4l in moderate yields. Note that trans-β-methylstyrene showed
much poorer reactivity than the cis-isomer. An indole substrate
with an N-benzyl protecting group reacted smoothly with
styrene to afford the adduct 4a′ in a good yield, while an N-Boc
derivative failed to participate in the reaction. Note that imines
derived from acetophenone and tetralone also participated in the
reaction with styrene under the present iron catalysis, albeit in
low yields (Scheme 1, bottom).9h
alkynes (Table 2) (see the Supporting Information for the
optimization).20 Thus, with a catalyst generated from Fe(acac)3
(10 mol %), SIXyl·HCl (20 mol %), and PhMgBr (110 mol %) in
THF, the reaction of 1 with diphenylacetylene was achieved to
afford the hydroarylation product 7a in a good yield with high
syn-stereoselectivity (Table 2, entry 1). Di(4-methoxyphenyl)-
acetylene also smoothly participated in the reaction (Table 2,
entry 2), while the reaction of di(4-fluorophenyl)acetylene was
rather sluggish (Table 2, entry 3). The reaction of di(o-
tolyl)acetylene afforded the alkenylation product 7d in 40% yield
with a modest E/Z ratio of ca. 3:2 (Table 2, entry 4). An
unsymmetrical diarylacetylene bearing phenyl and mesityl
groups underwent C−C bond formation regioselectively at the
less hindered acetylenic carbon proximal to the phenyl group,
thus affording the alkenylation product 7e in 70% yield with high
stereoselectivity (Table 2, entry 5). The regio- and stereo-
chemistry of 7e was unambiguously confirmed by single crystal
X-ray diffraction analysis (see the Supporting Information). The
Similarly to the case of the cobalt-catalyzed reaction,18 the
iron-catalyzed reaction of 1 with allylbenzene 5 afforded the 1,1-
diarylpropane derivative 4g albeit in a low yield, presumably
through an olefin isomerization19−hydroarylation sequence
(Scheme 2). Under the present reaction conditions, no reaction
took place when another terminal alkene such as 1-octene or
vinyltrimethylsilane was employed as the reaction partner.
With some modifications, the Fe−SIXyl catalyst also promotes
the imine-directed C2-alkenylation of indole with internal
B
DOI: 10.1021/ol503395g
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
was not (<0.1D). The reaction of 1-d with 1-trimethylsilylprop-
1-yne 6g afforded the product 7g-d with virtually complete
transfer of the C2-deuterium atom to the vinylic position
(Scheme 3b). Note that the yield of 7g-d was much lower (29%)
compared with that of 7g obtained from the parent substrate
(59%) (Table 2, entry 7).
Table 2. Imine-Directed C2-Alkenylation of Indole with
Internal Alkynes
a
We speculate that the present reaction is initiated by the
formation of a low-valent iron−NHC complex through
reduction of the Fe(III) precatalyst by the Grignard reagent in
the presence of the imidazolinium salt. The reason for the
necessity of an excess amount of the Grignard reagent is not clear,
but might be attributed to the formation of a ferrate species.21 On
the basis of the results of the deuterium-labeling experiments, we
propose a catalytic cycle involving chelation-assisted oxidative
addition of the C−H bond to iron,11c,d migratory insertion of an
alkene or alkyne into the Fe−H bond, and reductive elimination
of the resulting diorganoiron species (Scheme 4). The
b
c
entry
R1
R2
7, yield (%)
E/Z
1
2
3
4
5
6
7
8
Ph
Ph
7a, 84
7b, 83
7c, 25
7d, 40
7e, 70
7f, 79
7g, 59
94/6
4-MeOC6H4
4-MeOC6H4
4-FC6H4
2-MeC6H4
Mes
96/4
4-FC6H4
95/5
2-MeC6H4
57/43
>99/1
76/24
>99/1
75/25
Ph
Ph
Me
Pr
SiMe2Ph
SiMe3
Pr
7h, 18
a
b
The reaction was performed on a 0.3 mmol scale. Isolated yield.
Scheme 4. Possible Catalytic Cycle
c
1
Determined by H NMR.
reaction of silyl-substituted alkynes also took place regioselec-
tively, the new C−C bond being formed at the position distal to
the silyl group (Table 2, entries 6 and 7). 4-Octyne reacted
sluggishly to afford the corresponding alkenylation product 7h in
a low yield with moderate stereoselectivity (Table 2, entry 8).
Note that terminal alkynes such as phenylacetylene and 1-octyne
failed to participate in the present alkenylation reaction.
To gain mechanistic insights into the present hydroarylation
reactions, deuterium-labeling experiments were performed
(Scheme 3). The reaction of the C2-deuterated indole substrate
a
Scheme 3. Deuterium-Labeling Experiments
observation of H/D scrambling in the reaction of 1-d and 2d
(Scheme 3a) may suggest that the oxidative addition and
migratory insertion steps are reversible and that the reaction rate
and the regioselectivity are primarily controlled in the reductive
elimination step.9b,g Note, however, that reductive elimination
would not be substantially slower than deinsertion, because the
distributions of the deuterium atoms in the recovered starting
materials are far from statistical equilibrium (i.e., 0.25D for each
position). The poorer reactivity of 1-d in the addition to 6g
suggests that the C−H oxidative addition is rate-limiting in the
alkyne hydroarylation.9e
In summary, we have developed iron−NHC catalytic systems
for the directed C2-alkylation and alkenylation of indole with
vinylarenes and internal alkynes, respectively. Unlike other iron-
catalyzed C−H functionalization reactions developed by
Nakamura and others,16 the present reaction appears to involve
C−H oxidative addition rather than deprotonative C−H
metalation. Further efforts in the elaboration of the iron−
NHC catalytic system to achieve broader-scope hydroarylation
are currently underway.
a
1
The yields and the proton contents were determined by H NMR
spectroscopy.
1-d with 4-methoxystyrene 2d, when quenched at the reaction
time of 3 min, afforded the hydroarylation product 3d-d in 51%
yield along with recovery of the starting materials both in ca. 30%
yields (Scheme 3a). The deuterium content of 1-d decreased
only slightly (0.90D), and the deuterium incorporation into the
olefinic positions of 2d was marginal (ca. 0.1D for each). In
accordance with these observations, the methyl position of 3d-d
was substantially deuterated (>0.7D), while the methine position
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental details and characterization of new compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
C
DOI: 10.1021/ol503395g
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Y.; Hiyama, T. Angew. Chem., Int. Ed. 2012, 51, 5679. (j) Bair, J. S.;
Schramm, Y.; Sergeev, A. G.; Clot, E.; Eisenstein, O.; Hartwig, J. F. J. Am.
Chem. Soc. 2014, 136, 13098.
AUTHOR INFORMATION
■
Corresponding Author
(8) Gao, K.; Yoshikai, N. Acc. Chem. Res. 2014, 47, 1208.
(9) For examples, see: (a) Gao, K.; Lee, P. S.; Fujita, T.; Yoshikai, N. J.
Am. Chem. Soc. 2010, 132, 12249. (b) Gao, K.; Yoshikai, N. J. Am. Chem.
Soc. 2011, 133, 400. (c) Ilies, L.; Chen, Q.; Zeng, X.; Nakamura, E. J. Am.
Chem. Soc. 2011, 133, 5221. (d) Gao, K.; Yoshikai, N. Angew. Chem., Int.
Ed. 2011, 50, 6888. (e) Lee, P.-S.; Fujita, T.; Yoshikai, N. J. Am. Chem.
Soc. 2011, 133, 17283. (f) Ding, Z.; Yoshikai, N. Angew. Chem., Int. Ed.
2012, 51, 4698. (g) Lee, P.-S.; Yoshikai, N. Angew. Chem., Int. Ed. 2013,
52, 1240. (h) Dong, J.; Lee, P.-S.; Yoshikai, N. Chem. Lett. 2013, 42,
1140.
*E-mail: nyoshikai@ntu.edu.sg.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by Singapore National Research
Foundation (NRF-RF2009-05), Nanyang Technological Uni-
versity, and JST, CREST. We thank Drs. Yongxin Li and Rakesh
Ganguly (Nanyang Technological University) for assistance in
X-ray crystallographic analysis.
(10) Zhou, B. W.; Chen, H.; Wang, C. J. Am. Chem. Soc. 2013, 135,
1264.
(11) (a) Tolman, C. A.; Ittel, S. D.; English, A. D.; Jesson, J. P. J. Am.
Chem. Soc. 1979, 101, 1742. (b) Baker, M. V.; Field, L. D. J. Am. Chem.
Soc. 1986, 108, 7436. (c) Klein, H. F.; Camadanli, S.; Beck, R.; Leukel,
REFERENCES
■
(1) (a) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826.
(b) Arockiam, P. B.; Bruneau, C.; Dixneuf, P. H. Chem. Rev. 2012, 112,
5879.
D.; Florke, U. Angew. Chem., Int. Ed. 2005, 44, 975. (d) Camadanli, S.;
̈
Beck, R.; Florke, U.; Klein, H. F. Organometallics 2009, 28, 2300.
̈
(12) (a) Junge, K.; Schroder, K.; Beller, M. Chem. Commun. 2011, 47,
4849. (b) Nagashima, H.; Sunada, Y.; Nishikata, T.; Chaiyanurakkul, A.
In The Chemistry of Organoiron Compounds; Marek, I., Rappoport, Z.,
Eds.; John Wiley & Sons: Chichester, U.K., 2014.
(2) For examples, see: (a) Murai, S.; Kakiuchi, F.; Sekine, S.; Tanaka,
Y.; Kamatani, A.; Sonoda, M.; Chatani, N. Nature 1993, 366, 529.
(b) Kakiuchi, F.; Sekine, S.; Tanaka, Y.; Kamatani, A.; Sonoda, M.;
Chatani, N.; Murai, S. Bull. Chem. Soc. Jpn. 1995, 68, 62. (c) Kakiuchi, F.;
Yamamoto, Y.; Chatani, N.; Murai, S. Chem. Lett. 1995, 681.
(d) Kakiuchi, F.; Sonoda, M.; Tsujimoto, T.; Chatani, N.; Murai, S.
Chem. Lett. 1999, 1083. (e) Martinez, R.; Genet, J. P.; Darses, S. Chem.
Commun. 2008, 3855. (f) Martinez, R.; Simon, M. O.; Chevalier, R.;
Pautigny, C.; Genet, J. P.; Darses, S. J. Am. Chem. Soc. 2009, 131, 7887.
(g) Kakiuchi, F.; Kochi, T.; Mizushima, E.; Murai, S. J. Am. Chem. Soc.
2010, 132, 17741. (h) Rouquet, G.; Chatani, N. Chem. Sci. 2013, 4,
2201. (i) Schinkel, M.; Marek, I.; Ackermann, L. Angew. Chem., Int. Ed.
2013, 52, 3977.
(13) (a) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104,
6217. (b) Sherry, B. D.; Furstner, A. Acc. Chem. Res. 2008, 41, 1500.
̈
(c) Czaplik, W. M.; Mayer, M.; Cvengros, J.; Jacobi von Wangelin, A.
ChemSusChem 2009, 2, 396. (d) Nakamura, E.; Hatakeyama, T.; Ito, S.;
Ishizuka, K.; Ilies, L.; Nakamura, M. In Organic Reactions; Denmark, S.
E., Ed.; John Wiley & Sons: Hoboken, NJ, 2014; Vol. 83.
(14) (a) Yamagami, T.; Shintani, R.; Shirakawa, E.; Hayashi, T. Org.
Lett. 2007, 9, 1045. (b) Hatakeyama, T.; Hashimoto, S.; Ishizuka, K.;
Nakamura, M. J. Am. Chem. Soc. 2009, 131, 11949. (c) Ingleson, M. J.;
Layfield, R. A. Chem. Commun. 2012, 48, 3579.
(3) Colby, D. A.; Bergman, R. G.; Ellman, J. A. Chem. Rev. 2010, 110,
624.
(15) (a) Nakamura, E.; Yoshikai, N. J. Org. Chem. 2010, 75, 6061.
(b) Sun, C.-L.; Li, B.-J.; Shi, Z.-J. Chem. Rev. 2011, 111, 1293.
(16) For recent examples, see: (a) Shang, R.; Ilies, L.; Asako, S.;
Nakamura, E. J. Am. Chem. Soc. 2014, 136, 14349. (b) Fruchey, E. R.;
Monks, B. M.; Cook, S. P. J. Am. Chem. Soc. 2014, 136, 13130. (c) Ilies,
L.; Matsubara, T.; Ichikawa, S.; Asako, S.; Nakamura, E. J. Am. Chem. Soc.
2014, 136, 13126. (d) Gu, Q.; Al Mamari, H. H.; Graczyk, K.; Diers, E.;
Ackermann, L. Angew. Chem., Int. Ed. 2014, 53, 3868. (e) Matsubara, T.;
Asako, S.; Ilies, L.; Nakamura, E. J. Am. Chem. Soc. 2014, 136, 646.
(f) Asako, S.; Ilies, L.; Nakamura, E. J. Am. Chem. Soc. 2013, 135, 17755.
(17) A stoichiometric iron-mediated directed C−H alkylation reaction
has recently been reported: Sunada, Y.; Soejima, H.; Nagashima, H.
Organometallics 2014, 33, 5936.
(18) Yamakawa, T.; Yoshikai, N. Chem.Asian J. 2014, 9, 1242.
(19) Mayer, M.; Welther, A.; Jacobi von Wangelin, A. ChemCatChem
2011, 3, 1567.
(20) For examples of C2-alkenylation of indole with alkynes, see refs 7a
and 9f, and the following: (a) Schipper, D. J.; Hutchinson, M.; Fagnou,
K. J. Am. Chem. Soc. 2010, 132, 6910. (b) Ikemoto, H.; Yoshino, T.;
Sakata, K.; Matsunaga, S.; Kanai, M. J. Am. Chem. Soc. 2014, 136, 5424.
(c) Liang, L.; Fu, S.; Lin, D.; Zhang, X.-Q.; Deng, Y.; Jiang, H.; Zeng, W.
J. Org. Chem. 2014, 79, 9472.
(4) For examples, see: (a) Lim, Y.-G.; Kim, Y. H.; Kang, J.-B. J. Chem.
Soc., Chem. Commun. 1994, 2267. (b) Lenges, C. P.; Brookhart, M. J. Am.
Chem. Soc. 1999, 121, 6616. (c) Jun, C.-H.; Hong, J.-B.; Kim, Y.-H.;
Chung, K.-Y. Angew. Chem., Int. Ed. 2000, 39, 3440. (d) Thalji, R. K.;
Ahrendt, K. A.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2001, 123,
9692. (e) Lim, Y.-G.; Lee, K.-H.; Koo, B. T.; Kang, J.-B. Tetrahedron Lett.
2001, 42, 7609. (e) Lim, S.-G.; Lee, J. H.; Moon, C. W.; Hong, J.-B.; Jun,
C.-H. Org. Lett. 2003, 5, 2759. (f) Thalji, R. K.; Ellman, J. A.; Bergman, R.
G. J. Am. Chem. Soc. 2004, 126, 7192. (g) Lim, S.-G.; Ahn, J.-A.; Jun, C.-
H. Org. Lett. 2004, 6, 4687. (h) Shibata, Y.; Otake, Y.; Hirano, M.;
Tanaka, K. Org. Lett. 2009, 11, 689. (i) Katagiri, T.; Mukai, T.; Satoh, T.;
Hirano, K.; Miura, M. Chem. Lett. 2009, 38, 118.
(5) (a) Satoh, T.; Nishinaka, Y.; Miura, M.; Nomura, M. Chem. Lett.
1999, 615. (b) Dorta, R.; Togni, A. Chem. Commun. 2003, 760.
(c) Tsuchikama, K.; Kasagawa, M.; Hashimoto, Y. K.; Endo, K.; Shibata,
T. J. Organomet. Chem. 2008, 693, 3939. (d) Pan, S.; Ryu, N.; Shibata, T.
J. Am. Chem. Soc. 2012, 134, 17474. (e) Sevov, C. S.; Hartwig, J. F. J. Am.
Chem. Soc. 2013, 135, 2116. (f) Shibata, T.; Shizuno, T. Angew. Chem.,
Int. Ed. 2014, 53, 5410. (g) Crisenza, G. E. M.; McCreanor, N. G.;
Bower, J. F. J. Am. Chem. Soc. 2014, 136, 10258.
(6) (a) Kuninobu, Y.; Kikuchi, K.; Tokunaga, Y.; Nishina, Y.; Takai, K.
Tetrahedron 2008, 64, 5974. (b) Kuninobu, Y.; Nishina, Y.; Okaguchi,
K.; Shouho, M.; Takai, K. Bull. Chem. Soc. Jpn. 2008, 81, 1393.
(7) For examples, see: (a) Nakao, Y.; Kanyiva, K. S.; Oda, S.; Hiyama,
T. J. Am. Chem. Soc. 2006, 128, 8146. (b) Kanyiva, K. S.; Nakao, Y.;
Hiyama, T. Angew. Chem., Int. Ed. 2007, 46, 8872. (c) Nakao, Y.;
Kanyiva, K. S.; Hiyama, T. J. Am. Chem. Soc. 2008, 130, 2448. (d) Nakao,
Y.; Kashihara, N.; Kanyiva, K. S.; Hiyama, T. J. Am. Chem. Soc. 2008, 130,
16170. (e) Nakao, Y.; Idei, H.; Kanyiva, K. S.; Hiyama, T. J. Am. Chem.
Soc. 2009, 131, 15996. (f) Nakao, Y.; Kashihara, N.; Kanyiva, K. S.;
Hiyama, T. Angew. Chem., Int. Ed. 2010, 49, 4451. (g) Tsai, C.-C.; Shih,
W.-C.; Fang, C.-H.; Li, C.-Y.; Ong, T.-G.; Yap, G. P. A. J. Am. Chem. Soc.
2010, 132, 11887. (h) Nakao, Y.; Yamada, Y.; Kashihara, N.; Hiyama, T.
J. Am. Chem. Soc. 2010, 132, 13666. (i) Tamura, R.; Yamada, Y.; Nakao,
(21) Furstner, A.; Martin, R.; Krause, H.; Seidel, G.; Goddard, R.;
Lehmann, C. W. J. Am. Chem. Soc. 2008, 130, 8773 and references cited
therein.
̈
D
DOI: 10.1021/ol503395g
Org. Lett. XXXX, XXX, XXX−XXX