A Single Electron Transfer (SET) Approach to C-H Amidation of Hydrazones via Visible-Light Photoredox Catalysis

Article abstract of DOI:10.1021/acs.orglett.6b02711

The reductive single electron transfer (SET) umpolung amination of aldehyde-derived hydrazones has been developed through visible-light-promoted photoredox catalysis. The ideal transformation of hydrazones into the corresponding hydrazonamide through selective carbon-hydrogen (C-H) bond functionalization represents one of the most step- and atom-economical methods. This SET umpolung strategy features mild conditions and a remarkably broad substrate scope, offering an entirely new substrate class to direct C-H amination.

Full text of DOI:10.1021/acs.orglett.6b02711

Letter
pubs.acs.org/OrgLett
A Single Electron Transfer (SET) Approach to C−H Amidation of
Hydrazones via Visible-Light Photoredox Catalysis
Muliang Zhang,† Yingqian Duan,† Weipeng Li,† Pan Xu,† Jian Cheng,† Shouyun Yu,*,†
and Chengjian Zhu*,†,‡
†State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing,
210023, P. R. China
‡Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai, 200032, P. R. China
S
* Supporting Information
ABSTRACT: The reductive single electron transfer (SET) umpolung amination of aldehyde-derived hydrazones has been
developed through visible-light-promoted photoredox catalysis. The ideal transformation of hydrazones into the corresponding
hydrazonamide through selective carbon−hydrogen (C−H) bond functionalization represents one of the most step- and atom-
economical methods. This SET umpolung strategy features mild conditions and a remarkably broad substrate scope, offering an
entirely new substrate class to direct C−H amination.
Scheme 1. Amination/Amidation Strategy
he formation of carbon−nitrogen (C−N) bonds is a
straightforward approach for the synthesis of nitrogen-
T
containing compounds, which widely exist in natural products,
biologically active molecules, and medicines.1 Therefore, the
development of efficient and step-economical methods for
incorporation of an amino group into organic molecules has
attracted considerable attention in the synthetic chemistry
community for a long time.2 Transition-metal-catalyzed
C(sp2)−N bond formations, particularly the copper-mediated
Ullmann-type3 and palladium-catalyzed Buchwald−Hartwig
amination/amidation,4 have significantly advanced the installa-
tion of the amine moiety on aromatic substrates. A variety of
synthetically valuable arene C−H amination methods are
associated with ligand directed,5 radical aromatic substitution6
or intramolecular7 C−N bond formation. In addition,
aminations of alkenes and alkynes have been extensively
developed with Pd, Rh, Au, or Cu as well as other catalysts.8
Despite these significant advances, these developed methods of
elegantly constructing C(sp2)−N bonds are still restricted to
carbon−carbon π bond substrates (alkenes, alkynes, or arenes).
Hydrazones belong to a critical class of compounds because
of their many applications in synthetic organic chemistry.9 The
current C(sp2)−N bond formation strategies of hydrazones
employ preactivated starting materials such as hydrazonoyl
halides to react with amines which attack as nucleophiles
(Scheme 1A).10 To explore the ideal transformation of
hydrazones into the corresponding hydrazonamides through
C−H bond functionalization would represent one of the most
step- and atom-economical methods. However, it is also a great
challenge. It is known that polarity inversions are alternative
pathways for the preparation of otherwise difficult to access
target compounds.11 In particular, umpolung amination
reactions catalyzed by transition metals are very appealing
and garner great interest from chemists in recent years (Scheme
1B).12 Meanwhile, visible-light mediated photoredox catalysis,
which uses its unique ability to induce single electron transfer
Received: September 9, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02711
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(SET) processes, has emerged as a powerful tool in synthetic
organic chemistry13 and attracts increasing attention.
The solubility is likely to be very important when 1,2-
dichloroethane is used as a solvent. Howerver, the performance
of the reaction using K2HPO4 did not show a significant
difference (75% yield, entry 10), which indicated the specific
working details regarding the base remain unclear in the
reaction system. Several other photocatalysts, such as Ir-
(ppy)2(dtbbpy) PF6 and Ru(bpy)3(PF6)2, were not superior to
fac-Ir(ppy)3 toward the conversion of 1aa (entries 12−13). In
addition, the use of electron-rich (2b) or -poor (2c)
benzensulfonyl groups on the amine moiety did not offer
better results. Other types of radical precursors, such as N-
chloride (2d) and a N-Boc hydroxylamine derivative (2e), did
not work at all (Supporting Information). Finally, control
experiments demonstrated that the photocatalyst fac-Ir(ppy)3
and visible light were necessary for this transformation (entries
14−15).
With the optimized reaction conditions in hand, we
examined the scope of this SET umpolung C−N bond
formation protocol. The representative examples are shown
in Scheme 2. This method showed excellent chemoselectivity
with good functional group compatibility. Despite the multiple
reacting sites in hydrazones 1, monofunctionalization pro-
ceeded selectively to form the hydrazonamides, and the
corresponding amidated arenes were not observed. Aryl
aldehyde derived hydrazones bearing either electron-donating
(for example, methyl or alkoxy) or electron-withdrawing (for
example, trifluoromethyl, halide, or carbomethoxy) substituents
could react smoothly to furnish the desired products in
moderate to good yields (3aa−ag). In addition, the position of
the substituents on the aryl ring has no effect on this reaction.
With the same protocol, meta- or ortho-substituted (methyl, Cl)
hydrazones were also converted into the expected products
with good efficiency (3ba−cb). When the substrates with
multiple substituents on the phenyl ring were used, they all
underwent amidation (3da−ea). Remarkably, heteroaromatic
(thiophene 1fa and quinoline 1ga) hydrazones were found to
be tolerated in this transformation (3fa−ga).
We then turned our attention and investigated the effect of
different N-substituents of hydrazones in this transformation as
outlined in Scheme 3. When the nitrogen atom was connected
with two alkyl groups, the reaction could take place with
optimal levels of efficiency (3ha−ia). Next, one alkyl group was
replaced by a phenyl group, and the desired products were also
obtained. Whether the electron-poor or -rich substituents on
were utilized on the aldehyde-derived moieties (1ka−ma), or
one on the aryl group was linked to the nitrogen atom (1pa−
qa), the reactions all proceeded very smoothly, resulting in the
exclusive formation of the C(sp2)−H amidated products. The
hydrazone connected with two phenyl groups (1ra) was still
accommodated well by this protocol in 76% yield. Unfortu-
nately, N-Boc and N-Bs hydrazones (1sa−ta) failed to give the
desired products. We speculated that this activity difference
may be explained by the three-electron π-bond interaction
between the amino radical with the adjacent nitrogen atom.14a
This interaction could affect stabilization of the intermediate.
Overall, this SET umpolung amination reaction proved to be
effective on a broad range of hydrazone substrates that
encompass diverse substitutions.
Inspired by the effectiveness of these two types of catalytic
modes, we recognized that a SET umpolung strategy for
productive C−N bond formation could be developed through
an alternative mechanistic approach. The combination of both
SET and umpolung steps in a single mechanism is an attractive
concept and will be used to create more important chemical
transformations. We envisioned that electrophilic nitrogen-
centered radicals could be produced through an SET pathway
via photoredox catalysis, thus inducing facile C−N bond
formation by direct addition to a CN bond, analogous to the
role of traditional metal catalyzed cross-couplings, finally it
could undergo a stepwise single-electron oxidation and
deprotonation process for C(sp2)−N bond construction
(Scheme 1C). Herein, we report the development of a SET
umpolung amination through visible light photoredox catalysis.
This report describes the first, to the best of our knowledge,
examples of visible-light-induced direct C(sp2)−H amination of
aldehyde hydrazones.
To validate the feasibility of this SET umpolung amination
concept, the readily accessible aldehyde hydrazone 1aa was
chosen as as a model substrate, and N-methyl-N-((phenyl-
sulfonyl)oxy) sulfonamide 2a was used as an aminating reagent.
At the outset of this research, hydrazone 1aa was reacted with 2
equiv of sulfonamide 2a in the presence of 2 equiv of sodium
carbonate and a photocatalyst fac-Ir(ppy)3 in degassed CH3CN
(0.1 M) under visible light irradiation with blue LEDs (5 W,
λmax = 455 nm). The desired product 3aa was isolated in 42%
yield (Table 1, entry 1), which showed this novel amination
a
Table 1. Optimization of the reaction conditions
b
entry
photocatalyst
fac-Ir(ppy)3
base
solvent
yields (%)
1
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
CH3CN
DMF
DMSO
DCM
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
42
47
36
48
56
48
62
60
78
75
23
30
41
0
2
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
fac-Ir(ppy)3
Ru(bpy)3(PF6)2
Ir(ppy)2(dtbbpy)PF6
none
3
4
5
6
7
NaOAc
8
NaHCO3
Na2HPO4
K2HPO4
none
9
10
11
12
13
14
Na2HPO4
Na2HPO4
Na2HPO4
Na2HPO4
c
15
fac-Ir(ppy)3
0
a
Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol, 2.0 equiv), base
(0.2 mmol, 2.0 equiv), photocatalyst (0.002 mmol, 2.0 mol %), and
solvent (1 mL), argon atmosphere, irradiation with a 5 W blue light
b
c
LEDs at 25 °C, 76 h. Isolated yields. In dark. DMF = N,N-
dimethylformamide, DMSO = dimethyl sulfoxide, DCM = dichloro-
methane, DCE = 1,2-dichloroethane, Bs = Benzenesulfonyl.
strategy is feasible. Other solvents, such as DMF, DMSO, and
DCM, could not give improved results (entries 2−4). A 56%
yield was achieved when DCE was used as the solvent (entry
5). We then screened inorganic bases, such as K2CO3, NaOAc,
and NaHCO3 (entries 6−8). It was found that the base also
played an important role in neutralizing the byproduct
PhSO3H. Na2HPO4 proved to be the best choice (entry 9).
To gain mechanistic insight into the SET umpolung
amination reaction, some preliminary mechanistic experiments
were performed. The radical scavenger TEMPO (2,2,6,6-
tetramethyl-1-piperidinyloxy) was introduced into the photo-
chemical system, and the amination reaction could be
B
DOI: 10.1021/acs.orglett.6b02711
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 2. Scope of Aromatic Aldehyde Hydrazones in the
SET Amidation Reaction
Scheme 3. Scope of N-Substituent Hydrazones in the SET
Amidation Reaction
a b
,
a b
,
a
Reaction conditions: 1 (0.1 mmol), 2a (0.2 mmol, 2.0 equiv),
Na2HPO4 (0.2 mmol, 2.0 equiv), fac-Ir(ppy)3 (0.002 mmol, 2.0 mol
%), DCE (1 mL), irradiation with a 5 W blue light LEDs at 25 °C,
b
argon atmosphere, 80−90 h. Isolated yields.
Scheme 4. Plausible Reaction Pathways
a
Reaction conditions: 1 (0.1 mmol), 2a (0.2 mmol, 2.0 equiv),
Na2HPO4 (0.2 mmol, 2.0 equiv), fac-Ir(ppy)3 (0.002 mmol, 2.0 mol
%), DCE (1 mL), irradiation with a 5 W blue light LEDs at 25 °C,
b
argon atmosphere, 76−90 h. Isolated yields.
terminated completely, indicating that a single-electron-transfer
radical process is operating.
Based on the current results and literature precedents,14 the
plausible reaction pathway of this visible-light photoredox
catalyzed umpolung amination reaction is outlined in Scheme
4. Under visible-light irradiation, the photocatalyst fac-
IrIII(ppy)3 undergoes a metal-to-ligand charge transfer
(MLCT) process to produce the strongly reducing excited
state IrIII* (− 1.73 V vs SCE in CH3CN). Then a single
electron transfer process from this species to 2a would generate
electrophilic nitrogen-centered radical intermediate 5 and IrIV
(+ 0.77 V vs SCE in CH3CN). Subsequent N-centered radical
direct addition to the CN bond induces facile C−N bond
formation, leading to the aminyl radical intermediate 6.
Nitrogen-centered radical 6 can be stabilized by the adjacent
nitrogen through possible three-electron π bonding interaction.
At this stage, a key aminyl radical-polar crossover step between
6 and IrIV can take place, and the radical intermediate 6 may
undergo a stepwise single-electron oxidation to produce the
diazenium cation 7 or 8 with the regeneration of the
photocatalyst (path a).14a On the other hand, the possibility
of having radical chain processes where 6 is oxidized by another
equivalent of 2a to generate 8 and 5 is not ruled out (path b).
Further tautomerization and deprotonation of diazenium cation
gives the final product 3 in the presence of base.
C
DOI: 10.1021/acs.orglett.6b02711
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Yu, S. Org. Lett. 2014, 16, 3504. (f) Grohmann, C.; Wang, H.; Glorius,
In conclusion, we have successfully developed the visible-
light photoredox-catalyzed C(sp2)−H amidation of aldehyde
hydrazones for the first time. This transformation proceeds via
a reductive SET umpolung mechanism, in which sequential
SET processes involving the photocatalyst enable both SET
and umpolung steps in the same mechanism. Mild reaction
conditions, a broad substrate scope, and excellent functional
group compatibility (fluorides chlorides, bromides, and iodides
as well as esters included) are attractive features of this method.
This strategy also complements precedented work, and further
exploration of this reductive SET umpolung strategy will
continue to be a focus of research in our laboratory.
F. Org. Lett. 2012, 14, 656. (g) Lerchen, A.; Vas
Glorius, F. Angew. Chem., Int. Ed. 2016, 55, 3208.
́ ́
quez-Cespedes, S.;
(7) (a) McIntosh, J. A.; Coelho, P. S.; Farwell, C. C.; Wang, Z. J.;
Lewis, J. C.; Brown, T. R.; Arnold, F. H. Angew. Chem. 2013, 125,
9479. (b) He, G.; Lu, C.; Zhao, Y.; Nack, W. A.; Chen, G. Org. Lett.
2012, 14, 2944. (c) Chng, L. L.; Yang, J.; Wei, Y.; Ying, J. Y. Chem.
Commun. 2014, 50, 9049. (d) Choi, S.; Chatterjee, T.; Choi, W. J.;
You, Y.; Cho, E. J. ACS Catal. 2015, 5, 4796.
(8) (a) Hu, X.-Q.; Qi, X.; Chen, J.-R.; Zhao, Q.-Q.; Wei, Q.; Lan, Y.;
Xiao, W.-J. Nat. Commun. 2016, 7, 11188. (b) Fu, S.; Yang, H.; Li, G.;
Deng, Y.; Jiang, H.; Zeng, W. Org. Lett. 2015, 17, 1018. (c) Hemric, B.
N.; Shen, K.; Wang, Q. J. Am. Chem. Soc. 2016, 138, 5813. (d) Shen,
K.; Wang, Q. Chem. Sci. 2015, 6, 4279. (e) Liwosz, T. W.; Chemler, S.
R. J. Am. Chem. Soc. 2012, 134, 2020. (f) Xiong, P.; Xu, F.; Qian, X. Y.;
Yohannes, Y.; Song, J.; Lu, X.; Xu, H. C. Chem. - Eur. J. 2016, 22, 4379.
(g) Trenner, J.; Depken, C.; Weber, T.; Breder, A. Angew. Chem., Int.
Ed. 2013, 52, 8952. (h) Miaskiewicz, S.; Gaillard, B.; Kern, N.; Weibel,
J. M.; Pale, P.; Blanc, A. Angew. Chem., Int. Ed. 2016, 55, 9088.
(i) Huang, L.; Arndt, M.; Gooßen, K. t.; Heydt, H.; Goossen, L. J.
Chem. Rev. 2015, 115, 2596. (j) Wolfe, S.; Pilgrim, W.; Garrard, T.;
Chamberlain, P. Can. J. Chem. 1971, 49, 1099. (k) Deng, Z.; Wei, J.;
Liao, L.; Huang, H.; Zhao, X. Org. Lett. 2015, 17, 1834. (l) Zheng, G.;
Li, Y.; Han, J.; Xiong, T.; Zhang, Q. Nat. Commun. 2015, 6, 7011.
(m) Chen, Q.-A.; Chen, Z.; Dong, V. M. J. Am. Chem. Soc. 2015, 137,
8392. (n) Wu, L.; Qiu, S.; Liu, G. Org. Lett. 2009, 11, 2707.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.6b02711.
Experimental procedures, characterization data (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: cjzhu@nju.edu.cn (C.Z.).
*E-mail: yushouyun@nju.edu.cn (S.Y.).
Notes
(9) Lazny, R.; Nodzewska, A. Chem. Rev. 2010, 110, 1386.
(10) (a) Hegarty, A.; Rigopoulos, P.; Rowe, J. Aust. J. Chem. 1987, 40,
1777. (b) Wolkoff, P. Can. J. Chem. 1975, 53, 1333.
(11) (a) Seebach, D. Angew. Chem., Int. Ed. Engl. 1979, 18, 239.
(b) Bugaut, X.; Glorius, F. Chem. Soc. Rev. 2012, 41, 3511. (c) Smith,
A. B., III; Adams, C. M. Acc. Chem. Res. 2004, 37, 365.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We gratefully acknowledge the National Natural Science
Foundation of China (21474048, 21372114, and 21672099).
(12) (a) He, C.; Chen, C.; Cheng, J.; Liu, C.; Liu, W.; Li, Q.; Lei, A.
Angew. Chem., Int. Ed. 2008, 47, 6414. (b) Matsuda, N.; Hirano, K.;
Satoh, T.; Miura, M. Angew. Chem. 2012, 124, 3702. (c) Hirano, K.;
Satoh, T.; Miura, M. Org. Lett. 2011, 13, 2395. (d) Zhou, S.; Yang, Z.;
Chen, X.; Li, Y.; Zhang, L.; Fang, H.; Wang, W.; Zhu, X.; Wang, S. J.
Org. Chem. 2015, 80, 6323. (e) Matsuda, N.; Hirano, K.; Satoh, T.;
Miura, M. J. Am. Chem. Soc. 2013, 135, 4934. (f) Rucker, R. P.;
Whittaker, A. M.; Dang, H.; Lalic, G. Angew. Chem., Int. Ed. 2012, 51,
3953. (g) Barker, T. J.; Jarvo, E. R. J. Am. Chem. Soc. 2009, 131, 15598.
(h) Faulkner, A.; Scott, J. S.; Bower, J. F. J. Am. Chem. Soc. 2015, 137,
7224. (i) Ye, Z.; Dai, M. Org. Lett. 2015, 17, 2190. (j) Corpet, M.;
Gosmini, C. Synthesis 2014, 46, 2258. (k) Dong, X.; Liu, Q.; Dong, Y.;
Liu, H. Chem. - Eur. J. 2016, DOI: 10.1002/chem.201601607.
(13) (a) Zhang, M.; Li, W.; Duan, Y.; Xu, P.; Zhang, S.; Zhu, C. Org.
Lett. 2016, 18, 3266. (b) Li, W.; Duan, Y.; Zhang, M.; Cheng, J.; Zhu,
C. Chem. Commun. 2016, 52, 7596. (c) Lei, T.; Liu, W.-Q.; Li, J.;
Huang, M.-Y.; Yang, B.; Meng, Q.-Y.; Chen, B.; Tung, C.-H.; Wu, L.-
Z. Org. Lett. 2016, 18, 2479. (d) Prier, C. K.; Rankic, D. A.; MacMillan,
D. W. Chem. Rev. 2013, 113, 5322. (e) Chen, J.-R.; Hu, X.-Q.; Lu, L.-
Q.; Xiao, W.-J. Chem. Soc. Rev. 2016, 45, 2044. (f) Zhang, M.; Duan,
Y.; Li, W.; Cheng, Y.; Zhu, C. Chem. Commun. 2016, 52, 4761.
(g) Douglas, J. J.; Albright, H.; Sevrin, M. J.; Cole, K. P.; Stephenson,
C. R. Angew. Chem., Int. Ed. 2015, 54, 14898. (h) Cheng, Y.; Yu, S.
Org. Lett. 2016, 18, 2962. (i) Skubi, K. L.; Blum, T. R.; Yoon, T. P.
Chem. Rev. 2016, 116, 10035.
■
REFERENCES
■
(1) (a) Butcher, N. J.; Boukouvala, S.; Sim, E.; Minchin, R. F.
Pharmacogenomics J. 2002, 2, 30. (b) Corbet, J.-P.; Mignani, G. Chem.
Rev. 2006, 106, 2651. (c) Robinson, G. E.; Cunningham, O. R.;
Dekhane, M.; McManus, J. C.; O’Kearney-McMullan, A.; Mirajkar, A.
M.; Mishra, V.; Norton, A. K.; Venugopalan, B.; Williams, E. G. Org.
Process Res. Dev. 2004, 8, 925. (d) Saibabu Kotti, S. R. S.; Timmons,
C.; Li, G. Chem. Biol. Drug Des. 2006, 67, 101.
(2) (a) Bariwal, J.; Van der Eycken, E. Chem. Soc. Rev. 2013, 42, 9283.
(b) Xiong, T.; Zhang, Q. Chem. Soc. Rev. 2016, 45, 3069. (c) Ouyang,
K.; Hao, W.; Zhang, W.-X.; Xi, Z. Chem. Rev. 2015, 115, 12045.
(d) Shin, K.; Kim, H.; Chang, S. Acc. Chem. Res. 2015, 48, 1040.
(3) (a) Monnier, F.; Taillefer, M. Angew. Chem., Int. Ed. 2009, 48,
6954. (b) Ma, D.; Cai, Q. Acc. Chem. Res. 2008, 41, 1450. (c) Evano,
G.; Blanchard, N.; Toumi, M. Chem. Rev. 2008, 108, 3054. (d) Surry,
D. S.; Buchwald, S. L. Chem. Sci. 2010, 1, 13. (e) Ley, S. V.; Thomas,
A. W. Angew. Chem., Int. Ed. 2003, 42, 5400. (f) Deldaele, C.; Evano,
G. ChemCatChem 2016, 8, 1319.
(4) (a) Surry, D. S.; Buchwald, S. L. Angew. Chem., Int. Ed. 2008, 47,
6338. (b) Hartwig, J. F. Acc. Chem. Res. 2008, 41, 1534. (c) Cho, S. H.;
Kim, J. Y.; Kwak, J.; Chang, S. Chem. Soc. Rev. 2011, 40, 5068.
(5) (a) Zhou, Y.; Yuan, J.; Yang, Q.; Xiao, Q.; Peng, Y.
ChemCatChem 2016, 8, 2178. (b) Ryu, J.; Kwak, J.; Shin, K.; Lee,
D.; Chang, S. J. Am. Chem. Soc. 2013, 135, 12861. (c) Zhu, D.; Yang,
G.; He, J.; Chu, L.; Chen, G.; Gong, W.; Chen, K.; Eastgate, M. D.; Yu,
J.-Q. Angew. Chem., Int. Ed. 2015, 54, 2497. (d) Hermann, G. N.;
Becker, P.; Bolm, C. Angew. Chem., Int. Ed. 2016, 55, 3781. (e) Hwang,
H.; Kim, J.; Jeong, J.; Chang, S. J. Am. Chem. Soc. 2014, 136, 10770.
(6) (a) Allen, L. J.; Cabrera, P. J.; Lee, M.; Sanford, M. S. J. Am.
Chem. Soc. 2014, 136, 5607. (b) Romero, N. A.; Margrey, K. A.; Tay,
N. E.; Nicewicz, D. A. Science 2015, 349, 1326. (c) Brachet, E.; Ghosh,
T.; Ghosh, I.; Konig, B. Chem. Sci. 2015, 6, 987. (d) Song, L.; Zhang,
L.; Luo, S.; Cheng, J.-P. Chem. - Eur. J. 2014, 20, 14231. (e) Qin, Q.;
(14) (a) Xu, P.; Wang, G.; Zhu, Y.; Li, W.; Cheng, Y.; Li, S.; Zhu, C.
Angew. Chem., Int. Ed. 2016, 55, 2939. (b) Xie, J.; Zhang, T.; Chen, F.;
Mehrkens, N.; Rominger, F.; Rudolph, M.; Hashmi, A. K. Angew.
Chem., Int. Ed. 2016, 55, 2934. (c) Prieto, A.; Melot, R.; Bouyssi, D.;
Melot, R.; Bouyssi, D.; Monteiro, N. ACS Catal. 2016, 6, 1093.
(d) Xie, J.; Zhang, T.; Chen, F.; Mehrkens, N. Angew. Chem., Int. Ed.
2016, 55, 2934. (e) Cecere, G.; Konig, C. M.; Alleva, J. L.; MacMillan,
̈
D. W. C. J. Am. Chem. Soc. 2013, 135, 11521.
D
DOI: 10.1021/acs.orglett.6b02711
Org. Lett. XXXX, XXX, XXX−XXX