Nickel-Catalyzed Regioselective Cross-Dehydrogenative Coupling of Inactive C(sp3)-H Bonds with Indole Derivatives

Article abstract of DOI:10.1021/acs.orglett.5b02217

A nickel-catalyzed regiosepecific C2- versus C3-oxidative cross-coupling reaction of indoles with 1,4-dioxane and other inactive C(sp³)-H bonds is described. The divergent synthesis of C(sp³)-C(sp²) bonds was achieved in satisfactory yields with di-tert-butyl peroxide (DTBP) as the oxidant, which provides an efficient strategy for the selective construction of cyclic ethers containing heteroaromatic core structures.

Full text of DOI:10.1021/acs.orglett.5b02217

Letter
pubs.acs.org/OrgLett
Nickel-Catalyzed Regioselective Cross-Dehydrogenative Coupling of
3
Inactive C(sp )−H Bonds with Indole Derivatives
†
‡
†
,†
Li-Kun Jin, Li Wan, Jie Feng, and Chun Cai*
†
Chemical Engineering College, Nanjing University of Science and Technology, Nanjing 210094, People’s Republic of China
Biotechnology and Pharmaceutical Engineering College, Nanjing Tech University, Nanjing 211816, People’s Republic of China
‡
*
S Supporting Information
ABSTRACT: A nickel-catalyzed regiosepecific C2- versus C3-oxidative
cross-coupling reaction of indoles with 1,4-dioxane and other inactive
3
3
2
C(sp )−H bonds is described. The divergent synthesis of C(sp )−C(sp )
bonds was achieved in satisfactory yields with di-tert-butyl peroxide (DTBP)
as the oxidant, which provides an efficient strategy for the selective
construction of cyclic ethers containing heteroaromatic core structures.
ver the past decades, transition-metal catalyzed C−H
effective for promoting the activation step via a single electron
1
1
O
functionalization has become one of the most fundamental
transfer process. However, when coupled with another
nonprefunctionalized partner, its regioselectivity control can be
quite challenging.
1
processes in the synthesis of complex molecules. It has had a
huge impact on synthetic organic chemistry, medicinal
2
chemistry, and material sciences. As such, advances that
Our group has also developed a DTBP-mediated cross-
accomplish the selective activation of C−H bonds based on
coupling reaction of isochroman and indole derivatives on the C3
12
recently developed dehydrogenative cross-coupling (CDC)
position. When we try to extend the substrate scope to 1,4-
dioxane and other simple cyclic ethers, we found the yields are
relatively low under previous optimized conditions. We first
envisioned that introducing a transition-metal catalyst would
increase yields. However, besides 3-(1,4-dioxan-2-yl)-1H-indole,
the C2 substituted indole was also observed. Consequently, we
endeavored to develop the effective catalytic processes that
3
reactions have received special research attention. Direct C−
H bond functionalization would bypass the need for
prefunctionalized reaction partners and lead to a more atom
economical and environmentally friendly process. A diverse
range of transformations has been developed using palladiu-
4a−e
4f−i
4j−l
m,
copper,
and iron catalysts. More recently, organo-
3
2
catalysis has also been proven to be an alternative to these
exploit selective and controllable in this C(sp )−C(sp ) bond
3
transition-metal mediated oxidative couplings involving C(sp )−
forming reaction between 1,4-dioxane and indole derivatives
5
H bond cleavages. However, the regioselectivity of these
(
Scheme 1).
reactions remains challenging due to their low reactivity of the
reaction partners and the lack of a coordination site for the
transition-metal catalyst.
We commenced our investigation by screening the direct
coupling of 1,4-dioxane (1a) with indole (2a) in the presence of
Ni(acac) as the catalyst and DTBP as the oxidant under a
2
Cyclic ethers are ubiquitous structural motifs that frequently
solvent-free condition. Reaction gave a 2:3 mixture of C3- and
C2-substituted indoles 3a and 4a, respectively, with only 18%
6
,7a
appear in natural products. Construction of these cyclic rings
in complex organic molecules is sometimes demanding and
7
always requires complicated synthetic routes. Therefore, the
Scheme 1. Representative Examples of Oxidative Cross-
Coupling of Cyclic Ethers
3
strategy based on the direct functionalization of C(sp )−H
bonds adjacent to the oxygen of ethers represents one of the
8
interesting topics in synthetic chemistry. Pioneered by the study
9
of Li’s group in 2008, some developments have taken place. Lei’s
3
group developed a direct C(sp )−H arylation of tetrahydrofuran
10a
and 1,4-dioxane in the presence of nickel catalyst. Ji’s group
3
demonstrated the direct oxidative coupling of inactive C(sp )−H
10b
bonds in 1,4-dioxane with α,α-diaryl allylic alcohols.
Very
recently, Liu’s group reported the selective construction of
diverse α-functionalized ethers in the use of trityl ion and Lewis
1
0c
3
acid catalyst under ambient temperature.
The C(sp )−H
bonds adjacent to oxygen atoms usually have relatively lower
bond dissociation energy (BDE), thus the cleavages of these C−
H bonds will easily take place. Many oxidants are proven to be
Received: July 30, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02217
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
b
b
entry
catalyst
additive (equiv)
solvent
yield of 3 + 4 [%]
3a/4a
1
2
3
4
5
6
7
8
9
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(acac)2
Ni(OAc)2
NiCl2
18
2:3
1:2
2:3
c
35
1,4-dioxane
MeCN
63
DMF
PPh (0.1)
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
71
68
2:3
3
Zn(OTf) (0.1)
19:1
19:1
19:1
1:2
2
d
Zn(OTf) (0.1)
76
2
e
Zn(OTf) (0.1)
51
2
10
11
12
13
14
15
16
17
18
PPh (0.1)
26
16
44
72
47
49
31
<5
24
3
PPh (0.1)
1:2
3
Ni(OH)2
NiF2
PPh (0.1)
1:5
3
PPh (0.1)
1:9
3
NiF2
Zn(OTf) (0.1)
3:4
2
NiF2
1:9
Co(acac)3
Cu(acac)2
Fe(acac)3
3:2
3:5
a
Unless otherwise noted, the reaction was carried out with indole (0.50 mmol), catalyst (0.05 mmol), 1a (1.0 mL), and DTBP (1.0 mmol) at 120 °C
b
c
d
e
in 24 h. Yields and ratios were determined by GC−MS. 130 °C. Reaction conducted under argon atmosphere. DTBP (0.6 mmol).
yield of the mixture (Table 1, entry 1). A higher temperature
improved the yield to 35%, but still showed poor selectivity in 1:2
ratio. Diluting the solution by 1,4-dioxane and with the
Scheme 2. Regiosepecific C3 Coupling of Indole Derivatives
with 1,4-Dioxane
a
participation of PPh would increase yield to 71%, but C2- and
3
C3-forming reactions tend to be promoted simultaneously.
Further optimizations in various solvents with additives were
conducted (see Table 1, SI). Polar solvents such as DMF and
MeCN would compete with the reaction between 1a and 2a and
thus inhibited the formation of desired products. Careful
monitoring of the reaction showed that 3a might predominate
at the beginning of the reaction with 4a only starting to form as
the reaction proceeded. Consequently, Lewis acids were thought
to be effective for the regioselectivity control. This hypothesis
was supported by the result in the presence of Zn(OTf) , which
2
led to an exclusively C3-functionalized indole 3a (Table 1, entry
7). Further optimization reveals that side reactions can be
suppressed under argon atmosphere, which are beneficial to
afford the target product in high yield and regioselectivity (Table
1
, entry 8). Besides Ni(acac) , other metal salts were screened to
2
further optimize the reaction. Ni(OAc) , NiCl , and Ni(OH)
2
2
2
switched the selectivity back in favor of the C2-position, but hard
to achieve high regioselectivity (Table1, entries 10−12). To our
a
Reaction conditions: indole (0.50 mmol), Ni(acac) (0.05 mmol),
2
delight, the use of NiF led to the ratio of 1:9 of 3a/4a in the
2
Zn(OTf) (0.05 mmol), 1a (1.0 mL), and DTBP (1.0 mmol) at 120
°C in 24 h under argon atmosphere. Isolated yield.
2
presence of PPh and thus demonstrated that the regioselectivity
3
of nickel-catalyzed oxidative crossing coupling could be
controlled (Table1, entry 13), whereas Co(acac) , Cu(acac) ,
3
2
and Fe(acac) were all less effective for the reaction (Table1,
entries 16−18).
With a set of optimized conditions in hand, we next examined
the indole scope of this nickel-catalyzed oxidative C−C bond
forming reaction. First, various indole derivatives were explored
under standard conditions (Scheme 2). We found that both free
process worked well for electron-deficient indoles, such as 3i−
3n and indoles with electron-neutral groups such as 3b and 3c.
Electron-rich substituents such as 2-Me (3d) and 1,2-Me (3e)
were tolerated, but affording the corresponding coupling
products in relatively low 53% and 47% yields, respectively.
Besides, the reaction of 1a with 5-methyl-1H-indole produced a
mixture of C3- and C2-alkyl indole regioisomers 3h (5:1) in 48%
combined yield. Of particular note was the use of 1H-indole-4-
carboxylic acid (2g); the highly versatile product 3g was obtained
3
(
NH)-indoles and N-alkyl indoles were smoothly coupled with
,4-dioxane, giving the target product in moderate to good yields
and great selectivity. Importantly, the C3-functionalization
1
B
DOI: 10.1021/acs.orglett.5b02217
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
in 55% yield with the aryl carboxylic acid moiety intact. Beside
indole derivatives, 1H-pyrrolo[2,3-b]pyridine was also well
tolerated in the reaction and thus yielded 3-(1,4-dioxan-2-yl)-
Scheme 4. Nickel-Catalyzed Regioselective Cross-
3
Dehydrogenative Coupling of Inactive C(sp )−H Bond with
a
Indole
1H-pyrrolo[2,3-b]pyridine(3p) in 58%. To this end, the first
general method for the direct cross-coupling reaction of 1,4-
dioxane and indoles at C3 position can be applied to a broad
range of indole derivatives.
Next we further turned our attention to exploring the substrate
scope of corresponding catalytic C2 functionalization (Scheme
3
). When reactions performed with NiF as the catalyst and PPh3
2
Scheme 3. Regiosepecific C2 Coupling of Indole Derivatives
a
with 1,4-Dioxane
a
Reaction conditions: C3 functionalization, indole (0.50 mmol),
Ni(acac) (0.05 mmol), Zn(OTf) (0.05 mmol), 1a (1.0 mL), and
2
2
DTBP (1.0 mmol) at 120 °C in 24 h under argon atmosphere. C2
functionalization, indole (0.50 mmol), NiF (0.05 mmol), PPh (0.05
2
3
mmol), 1a (1.0 mL), and DTBP (1.0 mmol) at 120 °C in 24 h.
Isolated yield.
ization product, and only a trace of the C2-addition product was
observed under optimized conditions. 2,3-Dihydrobenzo[b]-
[1,4]dioxine (5d) was not compatible with the coupling. Besides
cyclic ethers, the scope was further extended to cycloalkane (5c)
and N,N-dimethylformamide (5f), whereas the N,N-dimethyl-
1
H-indole-1-carboxamide (6ff) was obtained instead of target
product 6f or 7f.
To gain some insight into the mechanism for the nickel-
catalyzed oxidative cross-coupling of ethers with indole
13
derivatives, a control experiment was carried out (Scheme 5).
Scheme 5. Control Experiment
a
Reaction conditions: indole (0.50 mmol), [NiF ] (0.05 mmol), PPh
2
3
(
0.05 mmol), 1a (1.0 mL), and DTBP (1.0 mmol) at 120 °C in 24 h.
Isolated yield.
as ligand, the coupling between 1,4-dioxane 1a and indole 2a
resulted in a 1:9 mixture of the alkylated product 3a and 4a. The
regioselectivity of the oxidative coupling process was switched in
favor of the reaction at C2-position. Indoles with electron-rich
substituent 3-Me (4c) or with electron-deficient substituents
such as 5-F (4h) and 5-CHO (4k) all gave the desired products
in satisfactory yields. Generally, indoles bearing electron-
donating groups demonstrated lower yields compared to that
with electron-deficient substituents, however, leading to
No target product was observed by the addition of TEMPO as a
radical inhibitor. Instead, the TEMPO-1,4-dioxane adduct 8 was
obtained in 15% and 18% yield, respectively, in C3- and C2-
coupling conditions (determined by GC−MS analysis),
indicating that a radial pathway might be involved in the process.
Meanwhile, an intermolecular competing kinetic isotope effect
(KIE) experiment was carried out (Scheme 6). As is depicted, a
significant KIE was observed with K /K = 4.0 and K /K = 6.7.
2
complete regiosepecificity at C2−C(sp )−H bond. In addition,
H
D
H
D
1
the reaction was also extended to other electron-rich
heteroaromatic. 1H-Pyrrolo[2,3-b]pyridine and benzofuran led
to the coupled products 4q and 4r in moderate yields and
exclusive regioselectivities.
(The KIE was determined by H NMR spectroscopy by
analyzing the ratio of 3c vs [D]3c and 4d vs [D]4d.) This result
3
indicates that C(sp )−H bond cleavage of 1,4-dioxane may be
one of the rate-determining steps of this procedure.
Besides 1,4-dioxane, other ether candidates were further
investigated for these optimized oxidative coupling conditions
Scheme 4). The reaction proved general for commonly
encountered saturated cyclic ethers including tetrahydrofuran
5a) and 1,3-dioxolane (5b). Cyclic benzyl ethers such as 1,3-
dihydroisobenzofuran (5e) was well compatible with the
reaction, however, affording predominantly the C3-functional-
In summary, a new regioselective Ni(II)-catalyzed C−H bond
functionalization method was developed, which enables the
cross-coupling of cyclic ethers and indole derivatives selectively
at either the C3 or C2 position under standard conditions. This
general procedure tolerates a broad range of substrates; not only
various indole derivatives but also the inactive C (sp )−H bond
containing compounds. This discovery could be of great
(
(
3
C
DOI: 10.1021/acs.orglett.5b02217
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(
4) (a) Wu, J.; Lan, J.; Guo, S.; You, J. Org. Lett. 2014, 16, 5862.
Scheme 6. KIE Experiment
(
(
b) Blaisdell, T. P.; Morken, J. P. J. Am. Chem. Soc. 2015, 137, 8712.
c) Wan, L.; Dastbaravardeh, N.; Li, G.; Yu, J.-Q. J. Am. Chem. Soc. 2013,
1
1
35, 18056. (d) Kianmehr, E.; Faghih, N.; Khan, K. M. Org. Lett. 2015,
7, 414. (e) Yang, W.; Chen, J.; Huang, X.; Ding, J.; Liu, M.; Wu, H. Org.
Lett. 2014, 16, 5418. (f) Zhu, W.; Zhang, D.; Yang, N.; Liu, H. Chem.
Commun. 2014, 50, 10634. (g) Zhao, M.-N.; Ren, Z.-H.; Wang, Y.-Y.;
Guan, Z.-H. Chem. - Eur. J. 2014, 20, 1839. (h) Zhang, F.; Das, S.;
Walkinshaw, A. J.; Casitas, A.; Taylor, M.; Suero, M. G.; Gaunt, M. J. J.
Am. Chem. Soc. 2014, 136, 8851. (i) Wei, W.-T.; Song, R.-J.; Li, J.-H. Adv.
Synth. Catal. 2014, 356, 1703. (j) Laha, J. K.; Jethava, K. P.; Dayal, N. J.
Org. Chem. 2014, 79, 8010. (k) Yang, K.; Song, Q. Org. Lett. 2015, 17,
5
48. (l) Cavanagh, C. W.; Aukland, M. H.; Hennessy, A.; Procter, D. J.
Chem. Commun. 2015, 51, 9272.
5) For selected reviews and references on transition-metal free C−H
bond functionalizations, see: (a) Sun, C.-L.; Shi, Z.-J. Chem. Rev. 2011,
11, 1293. (b) Zhu, C.; Liang, Y.; Hong, X.; Sun, H.; Sun, W.-Y.; Houk,
(
1
K. N.; Shi, Z. J. Am. Chem. Soc. 2015, 137, 7564. (c) Xu, W.; Nachtsheim,
B. J. Org. Lett. 2015, 17, 1585. (d) Liu, X.; Meng, Z.; Li, C.; Lou, H.; Liu,
L. Angew. Chem., Int. Ed. 2015, 54, 6012. (e) Chen, X.; Cui, X.; Yang, F.;
Wu, Y. Org. Lett. 2015, 17, 1445. (f) Feng, J.; Lv, M. F.; Lu, G. P.; Cai, C.
Org. Chem. Front. 2015, 2, 60.
significance on the selective elaboration of other heteroaromatic
core structures. Further efforts in our laboratory are devoted to
the exploitation of regio- and stereocontrol over other types of
heteroaromatic compounds and a more detailed mechanistic
investigation.
(6) (a) Mammoli, V.; Bonifazi, A.; Del Bello, F.; Diamanti, E.;
Giannella, M.; Hudson, A. L.; Mattioli, L.; Perfumi, M.; Piergentili, A.;
Quaglia, W.; Titomanlio, F.; Pigini, M. Bioorg. Med. Chem. 2012, 20,
ASSOCIATED CONTENT
Supporting Information
■
2
259. (b) Dureja, H.; Madan, A. K. J. Mol. Graphics Modell. 2006, 25,
*
S
3
73. (c) Nakata, T. Chem. Rev. 2005, 105, 4314.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02217.
(7) (a) Gillard, R. M.; Sperry, J. J. Org. Chem. 2015, 80, 2900.
(
3
b) Holmes, M.; Kwon, D.; Taron, M.; Britton, R. Org. Lett. 2015, 17,
868. (c) Miles, S. M.; Marsden, S. P. J. Org. Chem. 2004, 69, 6874.
8) (a) Li, J.; Zhang, J.; Tan, H.; Wang, D. Z. Org. Lett. 2015, 17, 2522.
b) Cao, J.-J.; Zhu, T.-H.; Wang, S.-Y.; Gu, Z.-Y.; Wang, X.; Ji, S.-J. Chem.
Experimental procedures and characterization data for all
products (PDF)
(
(
Commun. 2014, 50, 6439. (c) Gu, H.; Wang, C. Org. Biomol. Chem. 2015,
13, 5880. (d) Muramatsu, W.; Nakano, K. Org. Lett. 2014, 16, 2042.
AUTHOR INFORMATION
Corresponding Author
■
*
(e) Zhao, J.; Fang, H.; Zhou, W.; Han, J.; Pan, Y. J. Org. Chem. 2014, 79,
3
847. (f) Majji, G.; Rajamanickam, S.; Khatun, N.; Santra, S. K.; Patel, B.
E-mail: c.cai@mail.njust.edu.cn
Notes
K. J. Org. Chem. 2015, 80, 3440. (g) Majji, G.; Guin, S. K.; Behera, A.;
Patel, B. K. Chem. Commun. 2014, 50, 12193. (h) Rout, S. K.; Guin, S.;
Ali, W.; Gogoi, A.; Patel, B. K. Org. Lett. 2014, 16, 3086.
The authors declare no competing financial interest.
(
(
9) Zhao, L.; Li, C.-J. Angew. Chem., Int. Ed. 2008, 47, 7075.
10) (a) Liu, D.; Liu, C.; Li, H.; Lei, A. Angew. Chem., Int. Ed. 2013, 52,
ACKNOWLEDGMENTS
■
4453. (b) Chu, X.-Q.; Meng, H.; Zi, Y.; Xu, X.-P.; Ji, S.-J. Chem.
Commun. 2014, 50, 9718. (c) Wan, M.; Meng, Z.; Lou, H.; Liu, L. Angew.
Chem., Int. Ed. 2014, 53, 13845.
(11) (a) Studer, A.; Curran, D. P. Nat. Chem. 2014, 6, 765. (b) Huang,
Y.-T.; Lu, S.-Y.; Yi, C.-L.; Lee, C.-F. J. Org. Chem. 2014, 79, 4561.
We gratefully acknowledge Natural Science Foundation of
Jiangsu Province (BK 20131346) for financial support. This work
was also supported by National Natural Science Foundation of
China (21476116, 21402093).
(c) Zhou, W.; Ni, S.; Mei, H.; Han, J.; Pan, Y. Org. Lett. 2015, 17, 2724.
(
d) Wang, L.; Sha, W.; Dai, Q.; Feng, X.; Wu, W.; Peng, H.; Chen, B.;
REFERENCES
■
Cheng, J. Org. Lett. 2014, 16, 2088.
12) Jin, L. K.; Feng, J.; Lu, G. P.; Cai, C. Adv. Synth. Catal. 2015, 357,
105.
13) For selected references on transition-metal catalyzed regiose-
(
1) For selected reviews or books on C−H bond fuctionalizations, see:
(
(
(
a) Yu, J.-Q.; Shi, Z.-J. C−H Activation; Springer: Berlin, Germany, 2010.
2
b) Seidel, D. Acc. Chem. Res. 2015, 48, 317. (c) Engle, K. M.; Mei, T.-S.;
(
Wasa, M.; Yu, J.-Q. Acc. Chem. Res. 2011, 45, 788. (d) Kuhl, N.;
Hopkinson, M. N.; Wencel-Delord, J.; Glorius, F. Angew. Chem., Int. Ed.
012, 51, 10236. (e) Neufeldt, S. R.; Sanford, M. S. Acc. Chem. Res. 2012,
5, 936. (f) Sun, C.-L.; Li, B.-J.; Shi, Z.-J. Chem. Rev. 2010, 111, 1293−
314. (g) Yamaguchi, J.; Yamaguchi, A. D.; Itami, K. Angew. Chem., Int.
lective control of indoles, see: (a) Lane, B. S.; Brown, M. A.; Sames, D. J.
Am. Chem. Soc. 2005, 127, 8050. (b) Phipps, R. J.; Grimster, N. P.;
Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 8172. (c) Noble, A.; McCarver,
S. J.; MacMillan, D. W. C. J. Am. Chem. Soc. 2014, 137, 624.
2
4
1
(
d) Grimster, N. P.; Gauntlett, C.; Godfrey, C. R. A.; Gaunt, M. J. Angew.
Ed. 2012, 51, 8960. (h) Zhang, B.; Studer, A. Chem. Soc. Rev. 2015, 44,
505.
2) (a) Wencel-Delord, J.; Glorius, F. Nat. Chem. 2013, 5, 369.
b) Noisier, A. F. M.; Brimble, M. A. Chem. Rev. 2014, 114, 8775.
c) Dailler, D.; Danoun, G.; Baudoin, O. Angew. Chem., Int. Ed. 2015, 54,
919. (d) Rosen, B. R.; Simke, L. R.; Thuy-Boun, P. S.; Dixon, D. D.; Yu,
J.-Q.; Baran, P. S. Angew. Chem., Int. Ed. 2013, 52, 7317.
3) For selected reviews and references on cross-dehydrogenative
Chem., Int. Ed. 2005, 44, 3125. For selective reviews and references on
Nickel-catalyzed reactions and the mechanism study, see: (e) Weiss, M.;
Peters, R. ACS Catal. 2015, 5, 310. (f) Rosen, B. M.; Quasdorf, K. W.;
Wilson, D. A.; Zhang, N.; Resmerita, A.-M.; Garg, N. K.; Percec, V.
Chem. Rev. 2011, 111, 1346. (g) Montgomery, J. Organonickel Chemistry.
In Organometallics in Synthesis: Fourth Manual; Lipshutz, B. H., Ed.;
Wiley: Hoboken, NJ, 2013; pp 319−428.
3
(
(
(
4
(
coupling (CDC) reactions, see: (a) Girard, S. A.; Knauber, T.; Li, C.-J.
Angew. Chem., Int. Ed. 2014, 53, 74. (b) Zhang, C.; Tang, C.; Jiao, N.
Chem. Soc. Rev. 2012, 41, 3464. (c) Roizen, J. C.; Harvey, M. E.; Bois, J.
D. Acc. Chem. Res. 2012, 45, 911. (d) Li, C.-J. Acc. Chem. Res. 2009, 42,
3
35.
D
DOI: 10.1021/acs.orglett.5b02217
Org. Lett. XXXX, XXX, XXX−XXX