Rhenium-Catalyzed [4 + 1] Annulation of Azobenzenes and Aldehydes via Isolable Cyclic Rhenium(I) Complexes

Article abstract of DOI:10.1021/acs.orglett.5b00938

The first Re-catalyzed [4 + 1] annulation of azobenzenes with aldehydes was developed to furnish 2H-indazoles via isolable and characterized cyclic Re^I-complexes. For the first time, the acetate-acceleration effect is showcased in Re-catalyzed C-H activation reactions. Remarkably, mechanistic studies revealed an irreversible aldehyde-insertion step, which is in sharp contrast to those of previous Rh- and Co-systems. (Chemical Presented).

Full text of DOI:10.1021/acs.orglett.5b00938

Letter
pubs.acs.org/OrgLett
Rhenium-Catalyzed [4 + 1] Annulation of Azobenzenes and
Aldehydes via Isolable Cyclic Rhenium(I) Complexes
Xiaoyu Geng and Congyang Wang*
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular Recognition and Function, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100190, China
S
* Supporting Information
ABSTRACT: The first Re-catalyzed [4 + 1] annulation of
azobenzenes with aldehydes was developed to furnish 2H-indazoles
via isolable and characterized cyclic ReI-complexes. For the first time,
the acetate-acceleration effect is showcased in Re-catalyzed C−H
activation reactions. Remarkably, mechanistic studies revealed an
irreversible aldehyde-insertion step, which is in sharp contrast to those
of previous Rh- and Co-systems.
proposed to occur through [ReIII−H] species, originating from
ver the past three decades, transition-metal-catalyzed
Oinert C−H bond activation has received extensive the oxidative addition of C−H bonds to the ReI-catalyst
(Scheme 1b).4 However, this type of [ReIII−H] species has
attention due to its intrinsic atom- and step-economic
never been isolated and/or structurally characterized so far.
Thus, a gap still remains between the stoichiometric cyclo-
rhenation reactions and the Re-catalyzed C(sp2)−H function-
alization reactions from a mechanistic point of view.
characteristics, which avoids the required prefunctionalization
of substrates for traditional organic synthesis.1 In comparison
with widely used late transition metals such as Pd, Rh, Ir, Ru,
and so on, the activation of inert C−H bonds promoted by
rhenium, an early transition metal, has been considerably less
studied despite their unique properties and promising
reactivities.2−4 Since 1970, Bruce, Stone, Kaesz, and others
had reported a few stoichiometric cyclorhenation reactions of
arenes (Scheme 1a).5 Although thus-formed cyclic ReI-
complexes could be isolated and structurally characterized,
there has been no report on the catalytic C−H transformations
involving these isolable cyclic ReI-complexes until now. Since
2005, Kuninobu and Takai et al. have disclosed a series of Re-
catalyzed C(sp2)−H functionalization reactions, which were
Herein, we describe the stoichiometric reaction of an isolable
cyclic ReI-complex of azobenzene with benzaldehyde to give
2H-indazole. Then, with the essential aid of a catalytic amount
of sodium acetate, the Re-catalyzed [4 + 1] annulation of
azobenzenes with aldehydes to furnish 2H-indazoles with H2O
as the only byproduct was successfully developed (Scheme
1c).6 Importantly, the isolable cyclic ReI-complex demonstrated
comparable catalytic reactivity in this protocol, which bridged
the mechanistic gap between stoichiometric cyclorhenation
reactions and Re-catalyzed C(sp2)−H functionalization reac-
tions. This reaction also represents the first example of Re-
catalyzed C−H transformations of azobenzenes.7 Notably, the
acetate-acceleration effect disclosed herein is unprecedented for
Re-catalyzed C−H activation reactions.8
Scheme 1. Re-Mediated and -Catalyzed C(sp2)−H Bond
Transformations
Initially, we tried to synthesize ReI-complex A from
azobenzene 1a and commercially available Re2(CO)10 (Scheme
2a).5b It turned out that the reaction was rather sluggish and
gave the expected ReI-complex A in 20% NMR yield and 7%
isolated yield after sublimation. Then, the stoichiometric
reaction of ReI-complex A with benzaldehyde 2a was examined
and the aldehyde-insertion/cyclization product, 2H-indazole
3aa, was obtained in 67% isolated yield (Scheme 2b).
Encouraged by these results, we further explored the catalytic
version of this two-step reaction (Table 1).9 It occurred to us
that only a very small amount of 3aa was observed with 5 mol
% of Re2(CO)10 as the catalyst (entry 1). Then, we intended to
examine a series of bases as catalytic promoters for this
Received: April 1, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00938
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. Synthesis of ReI-Complex A and Its
Stoichiometric Reaction with Benzaldehyde
Scheme 3. Scope of Aldehydes
a
Table 1. Screening of Reaction Parameters
b
entry
cat. (mol %)
base (20 mol %)
solvent
toluene
yield (%)
a
b
c
d
c
Isolated yields of product 3 are shown. 50 h. 60 h. 72 h.
1
2
3
4
5
6
7
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re2(CO)10 (5)
Re(CO)5Br (5)
Mn2(CO)10 (5)
Fe3(CO)9 (5)
Pd(OAc)2 (5)
Cu(OAc)2 (5)
Re2(CO)10 (5)
Re2(CO)10 (2.5)
−
5
DBU
toluene
toluene
toluene
toluene
toluene
toluene
toluene
DMF
0
were also suitable substrates giving the corresponding products
(3ao−p) in synthetically useful yields. Thiophene-2-carbalde-
hyde reacted smoothly with 1a giving product 3aq. Aliphatic
aldehydes showed lower reactivity (3ar-s) than aromatic
aldehydes, which echoed the results of Rh- and Co-systems.6
Next, we examined the scope of azobenzenes (Scheme 4).
Under the slightly modified conditions, a wide range of para-
PhNH2
Cy2NH
KOtBu
Na2CO3
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
25
41
27
48
59
66
0
d
8
d
9
d
10
DCE
26
60
73
23
0
d
Scheme 4. Scope of Azobenzenes
11
1,4-dioxane
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
e
12
e
13
e
14
e
15
0
e
16
0
e
17
0
ef
,
g
18
ef
85 (81)
,
19
56
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), cat. (5 mol %),
b
1
base (20 mol %), solvent (0.1 M), 150 °C, 50 h. Determined by H
c
d
e
f
NMR. No base. 0.2 M. 0.4 M. 4 Å MS (100% cat. weight).
g
Isolated yield on 0.5 mmol scale.
reaction. To our delight, Cy2NH enabled the product formation
dramatically and gave the best result among the amines tested
(entries 2−4).9 Surprisingly, the cheap inorganic base, NaOAc,
showed even better performance (entries 5−7). Toluene
proved to be the best choice of solvent (entries 8−11).
Other rhenium-, manganese-, and iron-carbonyls as well as Pd-
and Cu-catalysts displayed only inferior reactivity, if any at all
(entries 12−17). Finally, 3aa was obtained in 81% isolated yield
with addition of molecular sieves as a H2O scavenger (entry
18). Of note, the use of 2.5 mol % of Re2(CO)10 resulted in
decreased yield of 3aa (entry 19).
With the optimized conditions in hand, we first investigated
the scope of aldehydes (Scheme 3). Both electron-donating and
-withdrawing groups on aromatic aldehydes were well tolerated
affording the expected products in good yields (3aa−j).
Importantly, varied functional groups such as F, Cl, Br, MeS,
CO2Me, and CF3 were all compatible to the reaction
conditions, which allowed for further synthetic elaborations
thereby broadening the diversity of the products. Meta- and
ortho-substituents on the benzene moiety showed no obvious
influence on the reaction outcome (3ak−n). Naphthaldehydes
substituted azobenzenes was well compatible in this reaction
(3ba−ja), again tolerating various functional groups. Notably,
different substituents could be regiospecifically placed at the 7-
position of 2H-indazoles (3ka−oa) by using ortho-substituted
substrates, which are often problematic for Re-catalyzed C−H
bond transformations.4a,b,d,e,n Disubstituted azobenzene fur-
nished also the expected product (3pa) in good yield. High
regioselectivity was observed when meta-methyl or unsym-
metrical azobenzenes were applied in the reaction favoring the
formation of sterically more accessible C−H bond function-
alized products (3qa, 3ta). The selectivity decreased when
meta-methoxy and -fluoro azobenzenes were used presumably
due to the secondary coordination effect of MeO and F groups
B
DOI: 10.1021/acs.orglett.5b00938
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
in the C−H activation step (3ra−sa). In addition, the increased
bulkiness of N-substituent of 2H-indazole (3ua) had no
apparent effect on the reaction yield.
Scheme 6. Deuterium-Labeling Experiments
To validate the acetate-acceleration effect in the C−H
activation step, the stoichiometric reaction of Re2(CO)10 with
1a in the presence of NaOAc was carried out. Gratifyingly, ReI-
complex A was obtained in clearly improved yield (Scheme 5a
Scheme 5. Mechanistic Experiments
which suggested that the C−H activation step might be
involved in the rate-determining step.
A plausible reaction mechanism is proposed in Scheme 7
based on the above experiments. The reaction starts with
Scheme 7. A Plausible Reaction Mechanism
a
b
c
1H NMR yield. Isolated yield. Determined by GC-MS.
d
1
Determined by H NMR
vs Scheme 2a). Importantly, ReI-complex A did catalyze the [4
+ 1] annulation of 1a and 2a giving 2H-indazole 3aa in
comparable yield (Scheme 5b), which supported its involve-
ment in the catalytic reaction. Furthermore, in order to check
the reversibility of the aldehyde-insertion step, the reaction of
alcohol 4aa with 4-methylbenzaldehyde 2b was examined
(Scheme 5c). It turned out that no crossover product 3ab was
detected with 3aa as the sole product. This result was in sharp
contrast to previous Rh- and Co-promoted aldehyde-insertion
steps, which are reversible and thermodynamically unfa-
vored.10,11 In addition, the competition experiments with
substrates bearing varied electronic properties were conducted
and the formation of products derived from electron-rich
azobenzene 1b and electron-deficient benzaldehyde 2j was
favored (Scheme 5d).
To further probe the reversibility of the C−H activation step,
fully deuterated azobenzene 1a-d10 was subjected to the
reaction conditions and deuterium loss was observed at the
ortho positions of both substrate 1a-d10 and product 3aa-d9
(Scheme 6a). This result indicated that a reversible
deprotonative C−H bond activation step might exist in the
reaction. To confirm this hypothesis, azobenzene 1a was
treated with D2O under the similar reaction conditions. As
expected, partial D-incorporation was detected at the ortho
positions of 1a. Moreover, kinetic isotope effect (KIE)
experiments were conducted through two parallel reactions
and the KIE value was determined to be 2.1 (Scheme 6b),9
cyclorhenation of azobenzene 1a to give the cyclic ReI-complex
A. Replacement of one CO ligand by aldehyde 2a provides
intermediate B. An irreversible aldehyde insertion to the Re−
Caryl bond of B forms the seven-membered rhenacycle C.
Protonation of C liberates alcohol 4aa and leads to the rhenium
species D, which promotes reversible acetate-accelerated C−H
activation of 1a to regenerate ReI-complex A. In total, acetate
acts as a catalytic proton shuttle in the reaction. In the end, an
intramolecular nucleophilic substitution reaction of 4aa
followed by rearomatization affords the final product 3aa.6
In summary, a Re-catalyzed [4 + 1] annulation of
azobenzenes with aldehydes has been achieved to provide a
streamline access to 2H-indazole with H2O as the only
byproduct. It represents the first example demonstrating that
isolable and characterized cyclic ReI-complexes are involved in
Re-catalyzed C−H transformations even though they were
synthesized by cyclorhenation reactions as early as in 1970.5b
Mechanistically, the C−H activation step was proven to be a
reversible redox-neutral deprotonation process, which provides
an alternative pathway to the generally proposed mechanism
through oxidative addition of C−H bonds to the ReI-catalyst
C
DOI: 10.1021/acs.orglett.5b00938
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
giving [ReIII−H] species.4 Also, the observed acetate-accel-
eration effect in this protocol is unprecedented for Re-catalyzed
C−H activation reactions. Further mechanistic studies revealed
an irreversible aldehyde-insertion step, which is in sharp
contrast to those of Rh- and Co-systems. Collectively, these
results are believed to find more synthetic applications in
diverse Re-catalyzed C−H functionalization reactions.
Am. Chem. Soc. 2006, 128, 202. (c) Kuninobu, Y.; Nishina, Y.;
Nakagawa, C.; Takai, K. J. Am. Chem. Soc. 2006, 128, 12376.
(d) Kuninobu, Y.; Nishina, Y.; Shouho, M.; Takai, K. Angew. Chem.,
Int. Ed. 2006, 45, 2766. (e) Kuninobu, Y.; Nishina, Y.; Takai, K. Org.
Lett. 2006, 8, 2891. (f) Kuninobu, Y.; Tokunaga, Y.; Takai, K. Chem.
Lett. 2007, 36, 872. (g) Kuninobu, Y.; Nishina, Y.; Matsuki, T.; Takai,
K. J. Am. Chem. Soc. 2008, 130, 14062. (h) Kuninobu, Y.; Nishina, Y.;
Okaguchi, K.; Shouho, M.; Takai, K. Bull. Chem. Soc. Jpn. 2008, 81,
1393. (i) Kuninobu, Y.; Kikuchi, K.; Tokunaga, Y.; Nishina, Y.; Takai,
K. Tetrahedron 2008, 64, 5974. (j) Kuninobu, Y.; Fujii, Y.; Matsuki, T.;
Nishina, Y.; Takai, K. Org. Lett. 2009, 11, 2711. (k) Kuninobu, Y.;
Matsuki, T.; Takai, K. Org. Lett. 2010, 12, 2948. (l) Kuninobu, Y.; Yu,
P.; Takai, K. Org. Lett. 2010, 12, 4274. (m) Kuninobu, Y.; Ohta, K.;
Takai, K. Chem. Commun. 2011, 47, 10791. (n) Sueki, S.; Guo, Y.;
Kanai, M.; Kuninobu, Y. Angew. Chem., Int. Ed. 2013, 52, 11879.
(5) For a review, see: (a) Albrecht, M. Chem. Rev. 2010, 110, 576.
For selected examples, see: (b) Bruce, M. I.; Iqbal, M. Z.; Stone, F. G.
A. J. Chem. Soc. (A) 1970, 3204. (c) Bruce, M. I.; Goodall, B. L.;
Sheppard, G. L.; Stone, F. G. A. J. Chem. Soc., Dalton Trans. 1975, 591.
(d) Bruce, M. I.; Goodall, B. L.; Matsuda, I. Aust. J. Chem. 1975, 28,
1259. (e) McKinney, R. J.; Firestein, G.; Kaesz, H. D. Inorg. Chem.
1975, 14, 2057. (f) Alper, H. Inorg. Chem. 1976, 15, 962. (g) Huie, B.
T.; Knobler, C. B.; Firestein, G.; McKinney, R. J.; Kaesz, H. D. J. Am.
Chem. Soc. 1977, 99, 7852.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, characterization data, and NMR spectra
for all new compounds. The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.orglett.5b00938.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: wangcy@iccas.ac.cn.
Notes
The authors declare no competing financial interest.
(6) For two reports using Rh- and Co-catalysts, see: (a) Lian, Y.;
Bergman, R. G.; Lavis, L. D.; Ellman, J. A. J. Am. Chem. Soc. 2013, 135,
7122. (b) Hummel, J. R.; Ellman, J. A. J. Am. Chem. Soc. 2015, 137,
490.
ACKNOWLEDGMENTS
■
Financial support from the National Basic Research Program of
China (973 Program) (No. 2011CB808600) and the National
Natural Science Foundation of China (21322203, 21272238,
21472194) is gratefully acknowledged.
(7) For other transition-metal-catalyzed C−H activation reactions of
azobenzenes, see: (a) Li, H.; Li, P.; Wang, L. Org. Lett. 2013, 15, 620.
(b) Li, H.; Li, P.; Tan, H.; Wang, L. Chem.Eur. J. 2013, 19, 14432.
(c) Xiong, F.; Qian, C.; Lin, D.; Zeng, W.; Lu, X. Org. Lett. 2013, 15,
5444. (d) Yin, Z. W.; Jiang, X.; Sun, P. P. J. Org. Chem. 2013, 78,
10002. (e) Ma, X.; Tian, S. K. Adv. Synth. Catal. 2013, 355, 337. (f) Li,
Z.-Y.; Li, D.-D.; Wang, G.-W. J. Org. Chem. 2013, 78, 10414. (g) Lian,
Y.; Hummel, J.; Bergman, R. G.; Ellman, J. A. J. Am. Chem. Soc. 2013,
135, 12548. (h) Zhao, D.; Wu, Q.; Huang, X.; Song, F.; Lv, T.; You, J.
Chem.Eur. J. 2013, 19, 6239. (i) Muralirajan, K.; Cheng, C.-H.
Chem.Eur. J. 2013, 19, 6198. (j) Wangweerawong, A.; Bergman, R.
G.; Ellman, J. A. J. Am. Chem. Soc. 2014, 136, 8520. (k) Ryu, T.; Min,
J.; Choi, W.; Jeon, W. H.; Lee, P. H. Org. Lett. 2014, 16, 2810.
(l) Wang, H.; Yu, Y.; Hong, X.; Tan, Q.; Xu, B. J. Org. Chem. 2014, 79,
3279. (m) Jia, X. F.; Han, J. J. Org. Chem. 2014, 79, 4180. (n) Song,
H.; Chen, D.; Pi, C.; Cui, X.; Wu, Y. J. Org. Chem. 2014, 79, 2955.
(o) Tang, H.; Qian, C.; Lin, D. E.; Jiang, H.; Zeng, W. Adv. Synth.
Catal. 2014, 356, 519. (p) Qian, C.; Lin, D.; Deng, Y.; Zhang, X.-Q.;
Jiang, H.; Miao, G.; Tang, X.; Zeng, W. Org. Biomol. Chem. 2014, 12,
5866. (q) Han, J.; Pan, C.; Jia, X.; Zhu, C. Org. Biomol. Chem. 2014,
12, 8603. (r) Dong, J.; Jin, B.; Sun, P. Org. Lett. 2014, 16, 4540.
REFERENCES
■
(1) For selected reviews, see: (a) Chen, X.; Engle, K. M.; Wang, D.-
H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009, 48, 5094. (b) Li, C.-J. Acc.
Chem. Res. 2009, 42, 335. (c) Daugulis, O.; Do, H.-Q.; Shabashov, D.
Acc. Chem. Res. 2009, 42, 1074. (d) Mkhalid, I. A. I.; Barnard, J. H.;
Marder, T. B.; Murphy, J. M.; Hartwig, J. F. Chem. Rev. 2010, 110, 890.
(e) Satoh, T.; Miura, M. Chem.Eur. J. 2010, 16, 11212.
(f) Nakamura, E.; Yoshikai, N. J. Org. Chem. 2010, 75, 6061.
(g) Ackermann, L. Chem. Rev. 2011, 111, 1315. (h) Yeung, C. S.;
Dong, V. M. Chem. Rev. 2011, 111, 1215. (i) Sun, C.-L.; Li, B.-J.; Shi,
Z.-J. Chem. Rev. 2011, 111, 1293. (j) Wendlandt, A. E.; Suess, A. M.;
Stahl, S. S. Angew. Chem., Int. Ed. 2011, 50, 11062. (k) Cho, S. H.;
Kim, J. Y.; Kwak, J.; Chang, S. Chem. Soc. Rev. 2011, 40, 5068.
(l) Colby, D. A.; Tsai, A. S.; Bergman, R. G.; Ellman, J. A. Acc. Chem.
Res. 2011, 44, 814. (m) Liu, C.; Zhang, H.; Shi, W.; Lei, A. Chem. Rev.
2011, 111, 1780. (n) McMurray, L.; O’Hara, F.; Gaunt, M. J. Chem.
Soc. Rev. 2011, 40, 1885. (o) Arockiam, P. B.; Bruneau, C.; Dixneuf, P.
H. Chem. Rev. 2012, 112, 5879. (p) Zhang, C.; Tang, C.; Jiao, N.
Chem. Soc. Rev. 2012, 41, 3464. (q) Hickman, A. J.; Sanford, M. S.
Nature 2012, 484, 177. (r) White, M. C. Science 2012, 335, 807.
(s) Song, G.; Wang, F.; Li, X. Chem. Soc. Rev. 2012, 41, 3651.
(t) Yamaguchi, J.; Yamaguchi, A. D.; Itami, K. Angew. Chem., Int. Ed.
2012, 51, 8960. (u) Wencel-Delord, J.; Glorius, F. Nat. Chem. 2013, 5,
369. (v) Rouquet, G.; Chatani, N. Angew. Chem., Int. Ed. 2013, 52,
11726. (w) Wang, C. Synlett 2013, 24, 1606. (x) Gao, K.; Yoshikai, N.
Acc. Chem. Res. 2014, 47, 1208.
́ ́
(s) Zhao, D.; Vasquez-Cespedes, S.; Glorius, F. Angew. Chem., Int. Ed.
2015, 54, 1657. See also ref 6.
(8) For reviews on transition-metal-catalyzed C−H activation
accelerated by carboxylates, see: (a) Ackermann, L. Acc. Chem. Res.
2014, 47, 281. (b) De Sarkar, S.; Liu, W.; Kozhushkov, S. I.;
Ackermann, L. Adv. Synth. Catal. 2014, 356, 1461. (c) Ouyang, K.; Xi,
Z. Acta Chim. Sinica 2013, 71, 13. See also ref 1g.
(9) For more details, see Supporting Information.
(10) For reviews on C−H bond additions to aldehydes and related
polarized bonds, see: (a) Zhang, X.-S.; Chen, K.; Shi, Z.-J. Chem. Sci.
2014, 5, 2146. (b) Yan, G.; Wu, X.; Yang, M. Org. Biomol. Chem. 2013,
11, 5558.
(11) For Rh-catalyzed reversible aldehyde insertion to C−H bonds,
see: (a) Chen, K.; Li, H.; Lei, Z.-Q.; Li, Y.; Ye, W.-H.; Zhang, L.-S.;
Sun, J.; Shi, Z.-J. Angew. Chem., Int. Ed. 2012, 51, 9851. (b) Zhang, X.-
S.; Li, Y.; Li, H.; Chen, K.; Lei, Z.-Q.; Shi, Z.-J. Chem.Eur. J. 2012,
18, 16214. (c) Li, H.; Li, Y.; Zhang, X.-S.; Chen, K.; Wang, X.; Shi, Z.-
J. J. Am. Chem. Soc. 2011, 133, 15244. For Co-catalyzed reversible
aldehyde insertion to C−H bonds, see ref 6b.
(2) For a leading review, see: Kuninobu, Y.; Takai, K. Chem. Rev.
2011, 111, 1938.
(3) For examples, see: (a) Chen, H.; Hartwig, J. F. Angew. Chem., Int.
Ed. 1999, 38, 3391. (b) Takaya, H.; Ito, M.; Murahashi, H. J. Am.
Chem. Soc. 2009, 131, 10824. (c) Fukumoto, Y.; Daijo, M.; Chatani, N.
J. Am. Chem. Soc. 2012, 134, 8762. (d) Peng, H.; Lin, A.; Zhang, Y.;
Jiang, H.; Zhou, J.; Cheng, Y.; Zhu, C.; Hu, H. ACS Catal. 2012, 2,
163. (e) Tang, Q.; Xia, D.; Jin, X.; Zhang, Q.; Sun, X.; Wang, C. J. Am.
Chem. Soc. 2013, 135, 4628. (f) Jin, H.; Xie, J.; Pan, C.; Zhu, Z.;
Cheng, Y.; Zhu, C. ACS Catal. 2013, 3, 2195.
(4) (a) Kuninobu, Y.; Kawata, A.; Takai, K. J. Am. Chem. Soc. 2005,
127, 13498. (b) Kuninobu, Y.; Tokunaga, Y.; Kawata, A.; Takai, K. J.
D
DOI: 10.1021/acs.orglett.5b00938
Org. Lett. XXXX, XXX, XXX−XXX