Direct Activation of β-sp3-Carbons of Saturated Carboxylic Esters as Electrophilic Carbons via Oxidative Carbene Catalysis

Article abstract of DOI:10.1021/acs.orglett.7b03650

An N-heterocyclic carbene-catalyzed oxidative LUMO activation of the β-cabons of saturated carboxylic esters is disclosed. This approach allows for efficient asymmetric access to lactams and lactones by directly installing functional groups to the typical

Full text of DOI:10.1021/acs.orglett.7b03650

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Direct Activation of β‑sp3‑Carbons of Saturated Carboxylic Esters as
Electrophilic Carbons via Oxidative Carbene Catalysis
Bin Liu,†,§ Weihong Wang,†,§ Ruoyan Huang,† Jiekuan Yan,† Jichang Wu,† Wei Xue,† Song Yang,*,†
Zhichao Jin,† and Yonggui Robin Chi*,†,‡
†Laboratory Breeding Base of Green Pesticide and Agricultural Bioengineering, Key Laboratory of Green Pesticide and Agricultural
Bioengineering, Ministry of Education, Guizhou University, Huaxi District, Guiyang 550025, China
‡Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: An N-heterocyclic carbene-catalyzed oxidative LUMO activation of
the β-cabons of saturated carboxylic esters is disclosed. This approach allows for
efficient asymmetric access to lactams and lactones by directly installing functional
groups to the typically inert β-sp3 carbons of saturated esters. The use of HOBt as
an additive was found to significantly improve both yields and enantioselectivities
of the reactions.
Scheme 1. NHC-Catalyzed β-sp3 Carbon Functionalization
arbonyl compounds are among the most widely used
of Saturated Esters
Cstarting materials in organic synthesis. Numerous
approaches have been developed for the functionalization of
the α-carbons of saturated carbonyl compounds and the β-sp2
carbons of α,β-unsaturated carbonyl molecules.1 However,
when saturated carbonyl compounds such as carboxylic esters
are used, direct functionalizations of the β-sp3 carbons are
difficult. Progress in catalytic functionalization of these
otherwise inert β-sp3 carbons mainly comes from transition-
metal-catalyzed C−H activations.2 The combined use of amine
organic catalyst and transition metal photocatalyst has been
developed for the β-activation of saturated ketones and
aldehydes through a radical pathway.3 Recently, organocatalytic
oxidative approaches have also been developed for the direct β-
carbon functionalization of saturated aldehydes.4
Our laboratory is interested in new activation and reaction
modes of simple organic molecules by using N-heterocyclic
carbene (abbreviated as NHC or carbene) as a key organic
catalyst.5 In particular, we have previously shown that the β-sp3
carbon of saturated carboxylic ester can be activated as a
reactive nucleophilic carbon via an overall redox-neutral process
overall process converts the inert β-sp3 carbon of the saturated
by using NHC as the catalyst under metal-free conditions
ester (1a) to a reactive electrophilic β-sp2 carbon (III). With
imine 2a (precursor of enamine) as the nucleophilic substrate,
the lactam product 3a could be obtained in 94% yield and 97:3
er. We noticed that the use of HOBt as an additive is beneficial
to significantly improve both the reaction yield and er value.7
Notably, HOBt as an additive was mainly found to enhance the
yields in our earlier NHC-mediated reactions.7c
(Scheme 1a, left part).6 Here we report that the β-sp3 carbon of
saturated ester can be activated as a reactive electrophilic
carbon in the presence of an NHC catalyst under oxidative
conditions (Scheme 1a, right part). This approach allows for a
direct reaction of the otherwise inert β-sp3 carbon of a saturated
ester (e.g., 1a) with a nucleophilic substrate (e.g., 2a) (Scheme
1b). Briefly, the addition of an NHC catalyst to the saturated
ester gives acylazolium intermediate I that is then converted to
II bearing a nucleophilic β-carbon under basic conditions, as
disclosed in our previous studies.6 In the presence of an
oxidant, intermediate II undergoes an oxidation process to
generate α,β-unsaturated acylazolium intermediate III. The
Results of our initial studies and condition optimizations with
ester 1a and enamine precursor 2a as the model substrates are
Received: November 23, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03650
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
summarized in Table 1. Triazolium NHC precatalyst A was
found to mediate the formation of 3a in 53% yield with the
Scheme 2. Scope of Esters 1 and Imines 2
a
Table 1. Optimization of Reaction Conditions
b
c
entry
conditions
yield
er
1
A
B
C
53
76
41
84
27
34
30
86
78
88
94
−
2
80:20
23:77
76:24
74:26
54:46
56:44
71:29
95:5
3
4
B, Cs2CO3 instead of DBU
B, DMAP instead of DBU
B, tBuOK instead of DBU
B, K2CO3 instead of DBU
B, 100 mol % LiCl
5
6
7
8
9
B, 5 mol % HOBt
10
11
B, 10 mol % HOBt
96:4
a
B, 20 mol % HOBt
97:3
Reaction conditions as stated in Table 1, entry 11. Yields are isolated
yields after purification via SiO2 column chromatography. Er values
were determined via HPLC on chiral stationary phase.
a
General conditions (unless otherwise specified): 1a (0.10 mmol), 2a
(0.15 mmol), NHC (0.02 mmol), DBU (0.10 mmol), DQ (0.11
mmol), 4 A MS (50 mg), THF (2.0 mL), 35 °C, 20 h. Isolated yield
of 3a. Er was determined via HPLC on chiral stationary phase.
b
c
Substituents on the β-benzene rings of the saturated ester 1
could be both electron-donating and -withdrawing groups with
all the products afforded in good to excellent yields and er
values (3a to 3k). The substituted β-benzene rings of the ester
1 could also be replaced with heteroaromatic rings without
deteriorating the product yields and enantioselectivities (3l to
3m). The aromatic groups on imines 3 could be either benzene
rings with various substitution patterns (3n to 3r) or
heteroaromatic groups (3s). The corresponding lactam
products were generally afforded in good yields with excellent
er values (3n to 3s).
Our saturated ester β-carbon oxdiative functionalization can
be developed as a general approach (Table 2). For example, β-
ketoester 4a could react with saturated ester 1a effectively to
form lactone product 5a. Initially, the enantioselectivity is very
poor (54:46 er) when using the above conditions optimized for
the lactam forming reactions (Table 2, entry 1). After switching
the NHC catalyst from B to C, the er value of 5a could be
dramaticly increased from 54:46 to 74:26 (entry 2). The use of
1 equiv of LiCl in this process could further improve the
enantioselectivity (91:9 er, entry 3). Finally, product 5a could
be obtained in excellent yields and er values when using KOAc
as the base instead of DBU (entry 4, 5). It is worth noting that
the use of HOBt in this process is crucial to the yield of the
final product. Reaction without using HOBt as the additive
would afford the product with a moderate yield (entry 6).
We also examined the scope of various substituted β-
ketoesters and 1,3-diketones (4) to react with saturated esters 1
in order to demonstrate the generality of our oxidative ester β-
C(sp3)−H activation approach (Scheme 3). Different sub-
stitution patterns on the benzene rings on the β-carbon of the
saturated esters 1 were well tolerated. The corresponding
lactone products 5 could be afforded in moderate to good
presence of DQ8 as the oxidant (entry 1). As a technical note,
only trace product could be obtained (<5% yield) when the
reaction was carried out at room temperature (around 25 °C).
L-Phenylalanine-derived NHC precatalyst B9 performed better
than A and C,10 giving 3a with a 76% yield and 80:20 er (entry
2). Switching the base from DBU to Cs2CO3 led to a better
yield (84%) but with a slight loss of er (entry 4 vs 2). Several
other bases (e.g., DMAP, tBuOK, K2CO3) examined here led to
drops on both yields and er values (entries 5 to 7). We then
decided to use B as the NHC precatalyst and DBU as the base
for further optimizations. The use of LiCl as an additive4d,11 led
to a drop in er value (entry 8). We later found that the use of 5
mol % HOBt7 as an additive improved the product er from
80:20 to 95:5 (entry 9 vs 2). Additional studies found that with
the use of 20 mol % HOBt as the additive, lactam 3a could be
formed in 94% yield and 97:3 er (entry 11). In two previous
studies on NHC-catalyzed reactions of esters, HOBt as an
additive was found to improve the reaction yields.7b,c In the
present reaction, HOBt significantly enhances the reaction er
values. HOBt may facilitate the final amide formation/
cyclization step and, thus, minimize the retro-Michael reaction
step that cause racemization of the final product.12 Additionally,
noncovalent interactions involving HOBt may also play
important roles for enantioselectivity controls in this reaction.13
Experimental investigations at this point toward the origins on
the roles of HOBt did not come to concrete conclusions.
Further studies with both experimental and computational
approaches are in progress.
With optimized reaction conditions in hand, we then
examined the substrate scope using saturated esters 1 and
imines 2 with different substitution patterns (Scheme 2).
B
DOI: 10.1021/acs.orglett.7b03650
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
esters. The β-carbons of saturated esters were activated as
electrophilic species to react with nucleophilic substrates. HOBt
was found as a crucial additive in this process to significantly
enhance both of the reaction yields and enantioselectivities.
Our approach of direct saturated ester β-sp3 carbon
functionalization allows quick access to lactams and lactones
with good to excellent yields and enantioselectivities.
Mechanistic studies, and other activation modes of the inert
carbons of saturated ester, are currently underway in our
laboratories.
Table 2. Optimization of Reaction Conditions
b
c
entry
conditions
yield
er
1
2
B, DBU, 20 mol % HOBt
C, DBU, 20 mol % HOBt
C, DBU, 20 mol % HOBt
C, KOAc, 20 mol % HOBt
C, KOAc, 10 mol % HOBt
C, KOAc
93
42
70
78
93
53
54:46
74:26
91:9
93:7
95:5
94:6
d
3
ASSOCIATED CONTENT
* Supporting Information
d
■
4
d
S
5
d
6
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b03650.
a
General conditions (unless otherwise specified): 1a (0.15 mmol), 4a
(0.10 mmol), NHC (0.02 mmol), base (0.05 mmol), DQ (0.16
b
mmol), 4 A MS (50 mg), THF (2.0 mL), 35 °C, 36 h. Isolated yield
Experimental procedures and spectral data for all new
compounds (PDF)
c
d
of 5a. Er was determined via HPLC on chiral stationary phase.
1
equiv of LiCl was added.
a
AUTHOR INFORMATION
Corresponding Authors
Scheme 3. Examples of Substituted Ketones
■
*E-mail: jhzx.msm@gmail.com.
*E-mail: robinchi@ntu.edu.sg.
ORCID
Song Yang: 0000-0003-1301-3030
Zhichao Jin: 0000-0003-3003-6437
Yonggui Robin Chi: 0000-0003-0573-257X
Author Contributions
§B.L. and W.W. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge financial support by the National Natural
Science Foundation of China (No. 21472028), National Key
Technologies R&D Program (No. 2014BAD23B01), “Thou-
sand Talent Plan”, The 10 Talent Plan (Shicengci) of Guizhou
Province, Guizhou Province Returned Oversea Student Science
and Technology Activity Program, Guizhou University (China)
and Singapore National Research Foundation (NRF-
NRFI2016- 06), the Ministry of Education of Singapore
(MOE2013-T2-2-003; MOE2016-T2-1-032; RG108/16),
A*STAR Individual Research Grant (A1783c0008), and
Nanyang Technological University.
a
Reaction conditions as stated in Table 2, entry 5. Yields are isolated
yields after purification by column chromatography. Er was
b
determined via HPLC on chiral stationary phase. The reaction was
carried out at 1.0 mmol scale.
REFERENCES
■
(1) For selected reviews, see: (a) Zeitler, K. Angew. Chem., Int. Ed.
2005, 44, 7506. (b) List, B.; Yang, J. W. Science 2006, 313, 1584.
(c) Marigo, M.; Jørgensen, K. A. Chem. Commun. 2006, 2001.
(d) Palomo, C.; Mielgo, A. Angew. Chem., Int. Ed. 2006, 45, 7876.
(e) Enders, D.; Niemeier, O.; Henseler, A. Chem. Rev. 2007, 107,
5606. (f) MacMillan, D. W. C. Nature 2008, 455, 304. (g) Nair, V.;
Vellalath, S.; Babu, B. P. Chem. Soc. Rev. 2008, 37, 2691. (h) Moyano,
A.; Rios, R. Chem. Rev. 2011, 111, 4703. (i) Giacalone, F.;
Gruttadauria, M.; Agrigento, P.; Noto, R. Chem. Soc. Rev. 2012, 41,
2406. (j) Li, J.-L.; Liu, T.-Y.; Chen, Y.-C. Acc. Chem. Res. 2012, 45,
1491. (k) Izquierdo, J.; Hutson, G. E.; Cohen, D. T.; Scheidt, K. A.
Angew. Chem., Int. Ed. 2012, 51, 11686. (l) Mahatthananchai, J.; Bode,
J. W. Acc. Chem. Res. 2014, 47, 696. (m) Hopkinson, M. N.; Richter;
Schedler, C. M.; Glorius, F. Nature 2014, 510, 485. (n) Flanigan, D.
M.; Michailidis, F. R.; White, N. A.; Rovis, T. Chem. Rev. 2015, 115,
yields and enantioselectivities (5a to 5i). β-Thienyl substituted
propionic esters also worked well in this process (5j, 5k). The
ester moiety of substrate 4 could be a methyl (5a) or ethyl
alcohol (5l) ester. The α-methyl (CH3) unit next to the ketone
moiety of the 1,3-dicarbonyl substrate can be replaced with a
phenyl unit as well (5m−o). But no product could be observed
when using pentane-2,4-dione as the nucleophile in the lactone-
forming reaction (5p). The chiral products obtained through
our approach (Scheme 2 and Scheme 3) are versatile building
blocks for quick access to amino alcohols, substituted
piperidines, and related functional molecules.5c,14
In summary, we have developed a carbene-catalyzed
oxidative β-C(sp3)−H functionalization of saturated carboxylic
C
DOI: 10.1021/acs.orglett.7b03650
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
9307. (o) Bures
2016, 49, 214.
́
, J.; Armstrong, A.; Blackmond, D. G. Acc. Chem. Res.
(11) For pioneering studies, see: (a) Raup, D. E. A.; Cardinal-David,
C. B.; Holte, D.; Scheidt, K. A. Nat. Chem. 2010, 2, 766. For a recent
review, see: (b) Wang, M. H.; Scheidt, K. A. Angew. Chem., Int. Ed.
2016, 55, 14912. For selected examples, see: (c) Dugal-Tessier, J.;
O’Bryan, E. A.; Schroeder, T. B. H.; Cohen, D. T.; Scheidt, K. A.
Angew. Chem., Int. Ed. 2012, 51, 4963. (d) Zhang, Y.; Lu, Y.; Tang, W.;
Lu, T.; Du, D. Org. Biomol. Chem. 2014, 12, 3009. (e) Qi, J.; Xie, X.;
Han, R.; Ma, D.; Yang, J.; She, X. Chem. - Eur. J. 2013, 19, 4146.
(f) Bera, S.; Samanta, R. C.; Daniliuc, C. G.; Studer, A. Angew. Chem.,
Int. Ed. 2014, 53, 9622. (g) Bera, S.; Daniliuc, C. G.; Studer, A. Org.
Lett. 2015, 17, 4940.
(12) (a) Bruice, T. C.; Kundu, N. G. J. Am. Chem. Soc. 1966, 88,
4097. (b) Owen, T. C.; Richards, A. J. Am. Chem. Soc. 1987, 109, 2520.
(13) For a related study, see: Chen, X.; Wang, H.; Doitomi, K.; Ooi,
C. Y.; Zheng, P.; Liu, W.; Guo, H.; Yang, S.; Song, B.-A.; Hirao, H.;
Chi, Y. R. Nat. Commun. 2017, 8, 15598.
(2) For selected reviews, see: (a) Daugulis, O.; Do, H. Q.; Shabashov,
D. Acc. Chem. Res. 2009, 42, 1074. (b) Chen, X.; Engle, K. M.; Wang,
D.-H.; Yu, J.-Q. Angew. Chem., Int. Ed. 2009, 48, 5094. (c) Jazzar, R.;
Hitce, J.; Renaudat, A.; SofackKreutzer, J.; Baudoin, O. Chem. - Eur. J.
2010, 16, 2654. (d) Lyons, T. W.; Sanford, S. M. Chem. Rev. 2010,
110, 1147. (e) Wasa, M.; Engle, K. M.; Yu, J.-Q. Isr. J. Chem. 2010, 50,
605. (f) Baudoin, O. Chem. Soc. Rev. 2011, 40, 4902. For selected
examples, see: (g) Nickon, A.; James, L.; Lambert, S. J. J. Am. Chem.
Soc. 1962, 84, 4604. (h) Nakamura, E.; Kuwajima, I. J. Am. Chem. Soc.
1983, 105, 651. (i) Zaitsev, V. G.; Shabashov, D.; Daugulis, O. J. Am.
Chem. Soc. 2005, 127, 13154. (j) Giri, R.; Maugel, N.; Li, J.-J.; Wang,
D.-H.; Breazzano, S. P.; Saunders, L. B.; Yu, J.-Q. J. Am. Chem. Soc.
2007, 129, 3510. (k) Wasa, M.; Engle, K. M.; Yu, J.-Q. J. Am. Chem.
Soc. 2009, 131, 9886. (l) Yoo, E. J.; Wasa, M.; Yu, J.-Q. J. Am. Chem.
Soc. 2010, 132, 17378. (m) Wasa, M.; Engle, K. M.; Lin, D.-W.; Yoo,
E. J.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 19598. (n) Wasa, M.;
Chan, K. S.-L.; Zhang, X.-G.; He, J.; Miura, M.; Yu, J.-Q. J. Am. Chem.
Soc. 2012, 134, 18570. (o) He, J.; Wasa, M.; Chan, K. S. L.; Yu, J.-Q. J.
Am. Chem. Soc. 2013, 135, 3387. (p) Chen, G.; Gong, W.; Zhuang, Z.;
Andra, M. S.; Chen, Y.-Q.; Hong, X.; Yang, Y.-F.; Liu, T.; Houk, K. N.;
Yu, J.-Q. Science 2016, 353, 1023.
(14) (a) Suzuki, K.; Sato, T.; Morioka, M.; Nagai, K.; Abe, K.;
Yamaguchi, H.; Saito, T.; Ohmi, Y.; Susaki, K. J. Antibiot. 1991, 44,
479. (b) Beaulieu, P. L.; Wernic, D. J. Org. Chem. 1996, 61, 3635.
(c) Uzunova, V.; Sheline, Y.; Davis, J. M.; Rasmusson, A.; Uzunov, D.
P.; Costa, E.; Guidotti, A. Proc. Natl. Acad. Sci. U. S. A. 1998, 95, 3239.
(d) Hiemke, C.; Hartter, S. Pharmacol. Ther. 2000, 85, 11. (e) Lewis,
̈
B. E.; Wallis, D. E.; Berkowitz, S. D.; Matthai, W. H.; Fareed, J.;
Walenga, J. M.; Bartholomew, J.; Sham, R.; Lerner, R. G.; Zeigler, Z.
R.; Rustagi, P. K.; Jang, I. K.; Rifkin, S. D.; Moran, J.; Hursting, M. J.;
Kelton, J. G. Circulation 2001, 103, 1838. (f) Timponi, C. F.; Oliveira,
N. E.; Arruda, R. M. P.; Meyrelles, S. S.; Vasquez, E. C. Basic Clin.
Pharmacol. Toxicol. 2006, 98, 518. (g) Reeder, M. R.; Anderson, R. M.
Chem. Rev. 2006, 106, 2828. (h) Han, B.; Li, J.-L.; Ma, C.; Zhang, S.-J.;
Chen, Y.-C. Angew. Chem., Int. Ed. 2008, 47, 9971. (i) Simal, C.; Lebl,
T.; Slawin, A. M. Z.; Smith, A. D. Angew. Chem., Int. Ed. 2012, 51,
3653.
(3) For selected examples, see: (a) Pirnot, M. T.; Rankic, D. A.;
Martin, D. B. C.; MacMillan, D. W. C. Science 2013, 339, 1593.
(b) Petronijevic,
Soc. 2013, 135, 18323. (c) Terrett, J. A.; Clift, M. D.; MacMillan, D.
W. C. J. Am. Chem. Soc. 2014, 136, 6858. (d) Jeffrey, J. L.; Petronijevic,
́
F. R.; Nappi, M.; MacMillan, D. W. C. J. Am. Chem.
́
F. R.; MacMillan, D. W. C. J. Am. Chem. Soc. 2015, 137, 8404.
(4) (a) Hayashi, Y.; Itoh, T.; Ishikawa, H. Angew. Chem., Int. Ed.
2011, 50, 3920. (b) Zhang, S.-L.; Xie, H.-X.; Zhu, J.; Li, H.; Zhang, X.-
S.; Li, J.; Wang, W. Nat. Commun. 2011, 2, 211. (c) Zeng, X.; Ni, Q.;
Raabe, G.; Enders, D. Angew. Chem., Int. Ed. 2013, 52, 2977. (d) Mo,
J.; Shen, L.; Chi, Y. R. Angew. Chem., Int. Ed. 2013, 52, 8588.
(5) (a) Fang, X.; Jiang, K.; Xing, C.; Hao, L.; Chi, Y. R. Angew. Chem.,
Int. Ed. 2011, 50, 1910. (b) Mo, J.; Chen, X.; Chi, Y. R. J. Am. Chem.
Soc. 2012, 134, 8810. (c) Cheng, J.; Huang, Z.; Chi, Y. R. Angew.
Chem., Int. Ed. 2013, 52, 8592. (d) Zhang, J.; Xing, C.; Chi, Y. R. J. Am.
Chem. Soc. 2013, 135, 8113. (e) Namitharan, K.; Zhu, T.; Cheng, J.;
Zheng, P.; Li, X.; Yang, S.; Song, B.-A.; Chi, Y. R. Nat. Commun. 2014,
5, 3982.
(6) (a) Fu, Z.; Xu, J.; Zhu, T.; Leong, W. W. Y.; Chi, Y. R. Nat. Chem.
2013, 5, 835. (b) Fu, Z.; Jiang, K.; Zhu, T.; Torres, J.; Chi, Y. R. Angew.
Chem., Int. Ed. 2014, 53, 6506. (c) Jin, Z.; Chen, S.; Wang, Y.; Zheng,
P.; Yang, S.; Chi, Y. R. Angew. Chem., Int. Ed. 2014, 53, 13506. (d) Jin,
Z.; Jiang, K.; Fu, Z.; Torres, J.; Zheng, P.; Yang, S.; Song, B.-A.; Chi, Y.
R. Chem. - Eur. J. 2015, 21, 9360.
(7) (a) West, T. H.; Daniels, D. S. B.; Slawin, A. M. Z.; Smith, A. D. J.
Am. Chem. Soc. 2014, 136, 4476. (b) Xu, J.; Yuan, S.; Miao, M.; Chen,
Z. J. Org. Chem. 2016, 81, 11454. (c) Fu, Z.; Wu, X.; Chi, Y. R. Org.
Chem. Front. 2016, 3, 145.
(8) For a pioneering study, see: (a) De Sarkar, S.; Studer, A. Angew.
Chem., Int. Ed. 2010, 49, 9266. For selected reviews, see: (b) De
Sarkar, S. D.; Biswas, A.; Samanta, R. C.; Studer, A. Chem. - Eur. J.
2013, 19, 4664. (c) Mahatthananchai, J.; Bode, J. W. Acc. Chem. Res.
2014, 47, 696. For selected examples, see: (d) De Sarkar, S. D.;
Grimme, S.; Studer, A. J. Am. Chem. Soc. 2010, 132, 1190. (e) Rong, Z.
Q.; Jia, M. Q.; You, S. L. Org. Lett. 2011, 13, 4080. (f) Kravina, A. G.;
Mahatthananchai, J.; Bode, J. W. Angew. Chem., Int. Ed. 2012, 51, 9433.
(g) Bera, S.; Samanta, R. C.; Daniliuc, C. G.; Studer, A. Angew. Chem.,
Int. Ed. 2014, 53, 9622. (h) Liang, Z.-Q.; Wang, D.-L.; Zhang, H.-M.;
Ye, S. Org. Lett. 2015, 17, 5140. (i) Bera, S.; Daniliuc, C. G.; Studer, A.
Org. Lett. 2015, 17, 4940. (j) Que, Y.; Xie, Y.; Li, T.; Yu, C.; Tu, S.;
Yao, C. Org. Lett. 2015, 17, 6234.
(9) Wadamoto, M.; Phillips, E. M.; Reynolds, T. E.; Scheidt, K. A. J.
Am. Chem. Soc. 2007, 129, 10098.
(10) He, M.; Struble, J. R.; Bode, J. W. J. Am. Chem. Soc. 2006, 128,
8418.
D
DOI: 10.1021/acs.orglett.7b03650
Org. Lett. XXXX, XXX, XXX−XXX