Rhodium-Catalyzed C-H Bond Functionalization with Amides by Double C-H/C-N Bond Activation

Article abstract of DOI:10.1021/acs.orglett.6b00058

The first C-H bond functionalization with amides as the coupling partners via selective activation of the amide N-C bond using rhodium(I) catalysts under highly chemoselective conditions is reported. Notably, this report constitutes the first catalytic activation of the amide N-C(O) bond by rhodium. We expect that this concept will have broad implications for using amides as coupling partners for C-H activation beyond the work described herein. (Figure Presented).

Full text of DOI:10.1021/acs.orglett.6b00058

Letter
pubs.acs.org/OrgLett
Rhodium-Catalyzed C−H Bond Functionalization with Amides by
Double C−H/C−N Bond Activation
Guangrong Meng and Michal Szostak*
Department of Chemistry, Rutgers University, 73 Warren Street, Newark, New Jersey 07102, United States
S
* Supporting Information
ABSTRACT: The first C−H bond functionalization with
amides as the coupling partners via selective activation of the
amide N−C bond using rhodium(I) catalysts under highly
chemoselective conditions is reported. Notably, this report
constitutes the first catalytic activation of the amide N−C(O)
bond by rhodium. We expect that this concept will have broad
implications for using amides as coupling partners for C−H
activation beyond the work described herein.
n the past decade, tremendous advances have been made in
the field of transition metal catalyzed direct C−H bond
specificity for the chelation-directed insertion into the amide N−
I
C bond, overriding the inherent decarbonylation/dehalogena-
tion. (4) The reaction is characterized by operational simplicity
and is conducted in the presence of ambient air and moisture.
This work demonstrates that typically inert, electronically neutral
amides can be exploited for C−C bond-forming reactions via C−
H activation.19 Given the utility of Rh(I) as highly useful C−H
activation catalysts16 and the importance of amides in organic
synthesis,14 we expect that this concept will have broad
implications for using amides as coupling partners for C−H
activation in ways beyond the coupling described herein.
functionalization.1 Site-selective functionalization of arene C−H
bonds provides ready access to C−C,2 C−O,3 C−N,4 and C−X5
products with high atom economy.1−5 In this context, the
formation of C−C bonds constitutes a cornerstone of organic
synthesis.6 The utility of the C−H activation technology to forge
C−C bonds is ultimately limited by the availability of new
coupling partners and the development of generic activation
manifolds.7 Chelation-assisted arene C−H functionalization with
a number of coupling partners, including aryl halides,8a−d
pseudohalides,8e−g organometallic reagents,9 and arenes10 have
been developed. However, the majority of these methods require
the use of complex catalyst systems, oxidants, or have limited
substrate scope. Recent progress was achieved by the use of acyl
chlorides11 and aldehydes12 under Rh(I)/(III) catalysis to form
C−C bonds after decarbonylation. Elegant approaches to the
development of chelation-assisted coupling of C−H bonds with
imines,13a,b aldehydes,13c and ketones13c via Rh(III) and Co(I)/
(III)13d catalytic cycles have been reported. In contrast, transition
metal catalyzed C−H bond functionalization with significantly
more challenging amides (amide bond resonance of 15−20 kcal/
mol)14 has so far proved elusive, despite the fundamental role of
amide bonds as building blocks in chemistry and biology,14 and
the urgent need to develop new coupling partners for generic C−
H activation methods under base-free, oxidant-free conditions,
which would additionally show a wide substrate scope and be
amenable to rational method development.15
We have recently discovered the first Suzuki20a,g and Heck20b
reactions of nonplanar amides21 under Pd(0)/(II) catalysis.
Independently, Garg20c and Zou20d demonstrated the use of
nonplanar imides21 in the Suzuki reaction using Ni20c,d and Pd
catalysis (Figure 1).20d Altogether, these reactions represent the
first examples of the elusive class of transition metal catalyzed
We report herein the first site-selective arene C−H bond
functionalization with amides as the coupling partners via
selective cleavage of the amide N−C bond, which proved viable
with highly discriminating Rh(I) catalysts.16 This report
constitutes the first example of a rhodium-catalyzed activation
of the amide N−C bond.17 The following features are
noteworthy: (1) The method provides functionalized biaryls18
from bench-stable, readily available amide precursors. (2) The
reaction shows excellent functional group tolerance, including
halides and aldehydes. (3) The Rh(I) catalyst shows high
Figure 1. Transformations of amides via C−N activation.
Received: January 8, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00058
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
amide bond N−CO insertion/cross-coupling reactions for the
formation of C−C bonds proceeding via selective N−C
activation,22 where it is believed that the nN →π*CO amide
bond resonance is disrupted by ligand coordination to
nitrogen.23 While ground-state distortion24a and electronic
activation24b contribute to the observed reactivity, these
transformations exploit selective transition metal catalyzed
activation of the classically inert N−C amide bonds.14 It is
important to note that all of these methods employ bench-stable,
readily available amide precursors25 as acylating or even arylating
reagents, thus enabling a new powerful transition metal catalyzed
amide disconnection via acyl-metal intermediates.26
exclusively at the less electrophilic 10-position of benzo[h]-
quinoline, consistent with nitrogen-directed cyclometalation.2a
Key optimization results with amide 2d are shown in Table 1.
Various catalysts were tested, and [Rh(cod)Cl]2 showed the best
catalytic activity (entries 1−6). Control experiments in the
absence of a metal catalyst resulted in full recovery of 2d. The use
Table 1. Optimization of Regioselective Arylation of
a
Benzo[h]quinoline with Amides
On the basis of our previous studies,20a,b,23c,d we hypothesized
that bench-stable amides could be employed as arylating C−H
functionalization partners with arenes by identifying an
appropriate amide precursor and catalyst system. A critical
feature of our design is the capacity of a low-valent metal catalyst
to insert into the inert amide N−C bond under mild conditions17
that would be compatible with the C−H activation cycle.11−13 At
the outset of our studies, only two metals, Pd(0) and Ni(0), had
been known to activate amide N−C bonds catalytically,
operating under the conditions that are incompatible with C−
H activation manifolds.1−5 The development of C−H
functionalization with amides would require identifying an
appropriate catalyst system to insert into the amide N−C bond
and control the relative reactivity of the intermediates.
entry
catalyst
additive
solvent
yield (%)
1
2
3
4
5
6
[Rh(cod)Cl]2
[Rh(CO)2Cl]2
[Rh(cod)2]BF4
RhCl(PPh3)3
[Rh(C2H4)2Cl]2
[RhCp*Cl2]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
[Rh(cod)Cl]2
−
−
−
−
−
−
CuI
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
PhCl
>98
30
98
9
69
<5
39
47
30
54
<5
75
77
<5
16
<5
<5
30
>98
b
7
c
8
O2
d
9
PPh3
DPPF
DPPP
Na2CO3
KF
d
10
Evaluation of the C−H bond functionalization strategy with
amides was first examined by the coupling of benzo[h]quinoline
1 with amides 2 under various conditions (Scheme 1). For our
initial studies, we selected 2-phenylpyridine directing groups
because of their well-known capacity to undergo cyclometalation
d
11
12
13
14
15
16
17
K2CO3
−
−
−
−
Scheme 1. Rh-Catalyzed C−H Arylation with Amides: The
DCE
a
Effect of Different N-Substituents
e
18
toluene
toluene
f
19
H2O
a
Conditions: 1 (0.1 mmol), 2d (1.5 equiv), catalyst (5 mol %), solvent
b
c
(0.25 M), additive (1.5 equiv), 150 °C. Additive (1.0 equiv). 1 atm.
d
e
f
Additive (0.1 equiv). 120 °C. H2O (1.5 equiv). See Supporting
Information for details.
of Rh(III) precatalysts resulted in recovery of 2d. The use of
oxidants resulted in low yields, consistent with a Rh(I)/(III)
cycle (entries 7−8). Strongly coordinating ligands provided 3 in
lower yields (entries 9−11). Bases had a deleterious effect on the
reaction efficiency (entries 12−14). Molecular sieves were not
required for the reaction.11a The reaction could ensue at lower
temperatures (entry 18); however, the process was less efficient.
Finally, we note that the reaction is highly practical; the reaction
tolerates water and can be conducted in the presence of ambient
air with no decrease in yields (entry 19). This stands in sharp
contrast to C−H arylation with more electrophilic acyl
precursors, which require careful control of the reaction
conditions and moisture.11a−d To our knowledge, this trans-
formation represents the first example of both: (i) transition
metal catalyzed C−H arylation with amides; (ii) catalytic
activation of an amide N−CO bond by a rhodium catalyst.17
With optimal conditions in hand, we explored the preparative
scope of our Rh(I)-catalyzed C−H arylation with respect to the
amide component (Scheme 2). The reaction tolerates a
remarkably broad range of functional groups. Amides containing
neutral, electron-donating and electron-withdrawing groups
afforded the desired products in excellent yields (entries 1−4).
a
Conditions: 1 (0.1 mmol), amide (1.5 equiv), [Rh(cod)Cl]2 (5 mol
%), PhMe (0.25 M), 150 °C. See Supporting Information for full
details.
under mild conditions,2a and the importance of pyridine-
containing biaryls in medicinal chemistry.18 Although the C−H
functionalization products were not detected using Pd and Ni as
catalysts, the proposed C−H functionalization was indeed
possible using Rh(I)-catalysts and sterically distorted amide 2d.
Importantly, less distorted anilides 2a−2c21 resulted only in trace
quantities of the C−H activation product, consistent with the
facility of metal insertion into the amide N−CO bond.17
Interestingly, the coupling of 1 with a significantly less distorted
amide 2e21 gave an excellent yield of the cross-coupling product,
indicating the high activity of the Rh(I) catalyst for the N−CO
insertion. Importantly, ketone products were not detected,
consistent with the high capability of the Rh(I) catalyst to
facilitate decarbonylation.16 The arylation process occurred
B
DOI: 10.1021/acs.orglett.6b00058
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. C−H Functionalization of Benzo[h]quinoline with
Scheme 3. C−H Functionalization of Arenes with Amides:
a
a
Amides: Structural Variation of Amide
Structural Variation of 2-Phenylpyridine
a
See Scheme 2. Amide (1−3 equiv). R′R′′ = (2d). See Supporting
Information for details.
a
1 (1.0 equiv), amide (1.5 equiv), [Rh(cod)Cl]2 (5 mol %), toluene
(3x−3y). Moreover, monoarylation is selectively achieved with
substrates bearing ortho-substituents on the pyridine ring (3z).
The sterically demanding ortho-methyl substrate coupled in high
yield (3aa). Substrates bearing electron-withdrawing and
-donating groups at the para-position are well-tolerated (3ab−
3ad). Furthermore, substrates bearing other directing groups
such as pyrimidines (3ae), pyrazoles (3af), 2-acylpyridines
(3ag), and imines (3ah) were successfully functionalized to
furnish the biaryl products in good yields. Notably, the inherently
difficult 6-membered metalacycles (3ag) can be accommoda-
ted.3c In this case, monoarylation product was formed, consistent
with slower cyclometalation. Importantly, the coupling of imines
provides rapid access to synthetically valuable biaryl aldehydes
after hydrolysis.
Studies were conducted to shed light on the mechanism (see
Supporting Information, SI). Stoichiometric ESI-MS measure-
ments with amide 2d indicated the presence of Rh-aryl
intermediates, consistent with amide activation by Rh(I). A
remarkably high TON for N−C activation of 1000 in the
arylation of 1a with 2d demonstrates highly efficient catalysis
(see SI). A possible mechanism is presented in Scheme 4.
In conclusion, this report describes the first C−H arylation
using amides by the N−C activation of the amide bond. Given
the importance of amides and advantages of C−H activation
manifolds, we expect that this concept will have a broad impact in
using amides as coupling partners for C−H bond activation.
Furthermore, our studies provide the first example of Rh(I)-
catalysts for activation of inert N−C amide bonds.
(0.25 M), 150 °C, 15 h. R′R′′ = (2d). See Supporting Information for
details.
Steric hindrance was well-tolerated (entry 5). p-Fluoro, p-chloro,
and even p-bromo groups gave the arylation product in high
yields (entries 6−8), providing functional handles for further
functionalization. Notably, the arylation with amide was
accomplished in the presence of more reactive carbonyl
functional groups such as esters and aldehydes (entries 9−10),
which illustrates the strategic advantage of using typically inert
amide bonds for transition metal catalyzed C−H activation.
Other sensitive groups such as ketones and nitriles are also
tolerated (entries 11−12). Meta-substitution (entries 13−14)
and strongly electron-donating/withdrawing functional afforded
the products in high yields (entries 15−16). This protocol could
be further applied to naphthalene rings (entry 17). Moreover,
heterocycles are compatible with the reaction conditions (entries
18−19). Most importantly, the reaction is not limited to sp2−sp2
biaryl coupling, as vinyl (entry 20) and even alkyl groups (entry
21) can be successfully coupled in high yields. The latter provide
an alternative to Friedel−Crafts alkylation. Overall, this reaction
shows the highest functional group tolerance of all previously
reported transition metal catalyzed transformations of amides via
C−N activation,17,20 which highlights the high specificity of
Rh(I) catalysts.
Next, we investigated the effect of electronic and structural
modifications of the directing group component (Scheme 3).
Typically, an excess of the amide precursor (3 equiv) was used for
these reactions, resulting in double C−H activation.11 However,
the example with a stoichiometric amount (3w) illustrates that
monoarylation is possible with high selectivity. The reaction is
successful for electron-withdrawing and -donating substrates
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00058.
C
DOI: 10.1021/acs.orglett.6b00058
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Ellman, J. A. J. Am. Chem. Soc. 2008, 130, 14926. (d) Kim, M.; Kwak, J.;
Chang, S. Angew. Chem., Int. Ed. 2009, 48, 8935. (e) Ackermann, L.;
Althammer, A.; Born, R. Angew. Chem., Int. Ed. 2006, 45, 2619.
(f) Roger, J.; Doucet, H. Org. Biomol. Chem. 2008, 6, 169.
(g) Ackermann, L.; Althammer, A.; Fenner, S. Angew. Chem. 2009,
121, 207.
Scheme 4. Proposed Mechanism
(9) Sun, C. L.; Li, B. J.; Shi, Z. J. Chem. Commun. 2010, 46, 677.
(10) Zhao, X.; Yeung, C. S.; Dong, V. M. J. Am. Chem. Soc. 2010, 132,
5837.
(11) (a) Zhao, X.; Yu, Z. J. Am. Chem. Soc. 2008, 130, 8136. (b) Ye, W.;
Luo, N.; Yu, Z. Organometallics 2010, 29, 1049. Anhydrides: (c) Jin, W.;
Yu, Z.; He, W.; Ye, W.; Xiao, W. J. Org. Lett. 2009, 11, 1317. (d) Pan, F.;
Lei, Z. Q.; Wang, H.; Li, H.; Sun, J.; Shi, Z. J. Angew. Chem., Int. Ed. 2013,
52, 2063.
(12) Shuai, Q.; Yang, L.; Guo, X.; Basle, O.; Li, C. J. J. Am. Chem. Soc.
2010, 132, 12212.
Procedures and analytical data (PDF)
(13) (a) Tsai, A. S.; Tauchert, M. E.; Bergman, R. G.; Ellman, J. A. J.
Am. Chem. Soc. 2011, 133, 1248. (b) Li, Y.; Li, B. J.; Wang, W. H.;
Huang, W. P.; Zhang, X. S.; Chen, K.; Shi, Z. J. Angew. Chem., Int. Ed.
2011, 50, 2115. (c) Zhang, X. S.; Chen, K.; Shi, Z. J. Chem. Sci. 2014, 5,
2146. (d) Gao, K.; Yoshikai, N. Chem. Commun. 2012, 48, 4305.
(14) Greenberg, A.; Breneman, C. M.; Liebman, J. F. The Amide
Linkage: Structural Significance in Chemistry, Biochemistry and Materials
Science; Wiley-VCH: New York, 2003.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: michal.szostak@rutgers.edu.
Notes
The authors declare no competing financial interest.
(15) Wencel-Delord, J.; Droge, T.; Liu, F.; Glorius, F. Chem. Soc. Rev.
2011, 40, 4740.
̈
ACKNOWLEDGMENTS
■
Financial support was provided by Rutgers University. The
Bruker 500 MHz spectrometer used in this study was supported
by an NSF-MRI grant (CHE-1229030).
(16) Colby, D. A.; Bergman, R. G.; Ellman, J. A. Chem. Rev. 2010, 110,
624.
(17) For a comprehensive review on N−C activation, see: Ouyang, K.;
Hao, W.; Zhang, W. X.; Xi, Z. Chem. Rev. 2015, 115, 12045.
(18) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem.
Rev. 2002, 102, 1359.
REFERENCES
■
(1) (a) Lyons, T.; Sanford, M. Chem. Rev. 2010, 110, 1147. (b) Topics
in Current Chemistry: C−H Activation; Yu, J. Q., Shi, Z. J., Eds.; Springer:
Berlin, 2010. (c) Wencel-Delord, J.; Glorius, F. Nat. Chem. 2013, 5, 369.
(d) Yeung, C. S.; Dong, V. M. Chem. Rev. 2011, 111, 1215.
(e) Yamaguchi, J.; Yamaguchi, A. D.; Itami, K. Angew. Chem., Int. Ed.
2012, 51, 8960. (f) McMurray, L.; O’Hara, F.; Gaunt, M. J. Chem. Soc.
Rev. 2011, 40, 1885. (g) Ackermann, L.; Vicente, R.; Kapdi, A. R. Angew.
Chem., Int. Ed. 2009, 48, 9792. (h) Alberico, D.; Scott, M. E.; Lautens, M.
Chem. Rev. 2007, 107, 174. (i) Godula, K.; Sames, D. Science 2006, 312,
67. (j) Bergman, R. G. Nature 2007, 446, 391. (k) Baudoin, O. Chem.
Soc. Rev. 2011, 40, 4902.
(19) Metal-Catalyzed Cross-Coupling Reactions and More; de Meijere,
A., Brase, S., Oestreich, M., Eds.; Wiley: New York, 2014.
̈
(20) (a) Meng, G.; Szostak, M. Org. Lett. 2015, 17, 4364. (b) Meng, G.;
Szostak, M. Angew. Chem., Int. Ed. 2015, 54, 14518. (c) Weires, N. A.;
Baker, E. L.; Garg, N. K. Nat. Chem. 2015, 8, 75. (d) Li, X.; Zou, G.
Chem. Commun. 2015, 51, 5089. Transesterification: (e) Hie, L.; Nathel,
N. F. F.; Shah, T. K.; Baker, E. L.; Hong, X.; Yang, Y. F.; Liu, P.; Houk, K.
N.; Garg, N. K. Nature 2015, 524, 79. (f) Ruider, S.; Maulide, N. Angew.
Chem., Int. Ed. 2015, 54, 13856. (g) For a full account of the Suzuki-
Miyaura N−C amide bond activation, see: Meng, G.; Szostak, M. Org.
Biomol. Chem. 2016, DOI: 10.1039/c6ob00084c.
(2) (a) Kalyani, D.; Deprez, N. R.; Desai, L. V.; Sanford, M. S. J. Am.
Chem. Soc. 2005, 127, 7330. (b) Chiong, H. A.; Pham, Q. N.; Daugulis,
O. J. Am. Chem. Soc. 2007, 129, 9879. (c) Campeau, L. C.; Schipper, D.
J.; Fagnou, K. C. J. Am. Chem. Soc. 2008, 130, 3266. (d) Engle, K. M.;
Mei, T. S.; Wasa, M.; Yu, J. Q. Acc. Chem. Res. 2012, 45, 788. (e) Colby,
D. A.; Tsai, A. S.; Bergman, R. G.; Ellman, J. A. Acc. Chem. Res. 2012, 45,
814. (f) Fang, P.; Li, M.; Ge, H. J. Am. Chem. Soc. 2010, 132, 11898.
(g) Wu, X.; Zhao, Y.; Ge, H. J. Am. Chem. Soc. 2014, 136, 1789.
(h) Aihara, Y.; Chatani, N. J. Am. Chem. Soc. 2013, 135, 5308. (i) Aihara,
Y.; Chatani, N. J. Am. Chem. Soc. 2014, 136, 898. (j) Norinder, J.;
Matsumoto, A.; Yoshikai, N.; Nakamura, E. J. Am. Chem. Soc. 2008, 130,
5858.
(3) (a) Dick, A. R.; Hull, K. R.; Sanford, M. S. J. Am. Chem. Soc. 2004,
126, 2300. (b) Mo, F.; Trzepkowski, L. J.; Dong, G. Angew. Chem., Int.
Ed. 2012, 51, 13075. (c) Shan, G.; Yang, X.; Ma, L.; Rao, Y. Angew.
Chem., Int. Ed. 2012, 51, 13070.
(4) Shin, K.; Kim, H.; Chang, S. Acc. Chem. Res. 2015, 48, 1040.
(5) Engle, K. M.; Mei, T. S.; Wang, X.; Yu, J. Q. Angew. Chem., Int. Ed.
2011, 50, 1478.
(21) Szostak, M.; Aube,
(22) Pd: (a) Lei, Y.; Wrobleski, A. D.; Golden, J. E.; Powell, D. R.;
Aube, J. J. Am. Chem. Soc. 2005, 127, 4552. Ni: (b) Tobisu, M.;
Nakamura, K.; Chatani, N. J. Am. Chem. Soc. 2014, 136, 5587.
(23) (a) Greenberg, A.; Venanzi, C. A. J. Am. Chem. Soc. 1993, 115,
6951. (b) Greenberg, A.; Moore, D. T.; DuBois, T. D. J. Am. Chem. Soc.
́
J. Chem. Rev. 2013, 113, 5701.
́
1996, 118, 8658. (c) Szostak, R.; Aube,
́
J.; Szostak, M. Chem. Commun.
2015, 51, 6395. (d) Szostak, R.; Aube, J.; Szostak, M. J. Org. Chem. 2015,
́
80, 7905. (e) Cox, C.; Lectka, T. Acc. Chem. Res. 2000, 33, 849.
(24) (a) Hutchby, M.; Houlden, C. E.; Haddow, M. F.; Tyler, S. N.;
Lloyd-Jones, G. C.; Booker-Milburn, K. I. Angew. Chem., Int. Ed. 2012,
51, 548. (b) Glover, S. A. Tetrahedron 1998, 54, 7229.
(25) (a) Trost, B. M.; Fleming, I. Comprehensive Organic Synthesis;
Pergamon Press: 1991. (b) Lundberg, H.; Tinnis, F.; Selander, N.;
Adolfsson, H. Chem. Soc. Rev. 2014, 43, 2714. (c) Prabhu, G.;
Basavaprabhu; Narendra, N.; Vishwanatha, T. M.; Sureshbabu, V. V.
Tetrahedron 2015, 71, 2785.
(26) (a) Brennfuhrer, A.; Neumann, H.; Beller, M. Angew. Chem., Int.
̈
Ed. 2009, 48, 4114. (b) Handbook of Organopalladium Chemistry for
(6) Johansson-Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.;
Snieckus, V. Angew. Chem., Int. Ed. 2012, 51, 5062.
Organic Synthesis; Negishi, E., Ed.; Wiley: New York, 2002.
(7) (a) Jia, C.; Kitamura, T.; Fujiwara, Y. Acc. Chem. Res. 2001, 34, 633.
(b) Kakiuchi, F.; Chatani, N. Adv. Synth. Catal. 2003, 345, 1077.
(c) Kakiuchi, F.; Kochi, T. Synthesis 2008, 2008, 3013.
(8) (a) Kametani, Y.; Satoh, T.; Miura, M.; Nomura, M. Tetrahedron
Lett. 2000, 41, 2655. (b) Daugulis, O.; Zaitsev, V. G. Angew. Chem., Int.
Ed. 2005, 44, 4046. (c) Berman, A. M.; Lewis, J. C.; Bergman, R. G.;
D
DOI: 10.1021/acs.orglett.6b00058
Org. Lett. XXXX, XXX, XXX−XXX