Electrochemical-Oxidation-Induced Site-Selective Intramolecular C(sp3)-H Amination

Article abstract of DOI:10.1021/acscatal.8b02847

The cross-coupling of C(sp³)-H and N-H represents one of the most straightforward approaches to construct saturated nitrogen-containing compounds. The additional oxidants or halogenated reagents are generally required in such processes. Herein,

Full text of DOI:10.1021/acscatal.8b02847

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Letter
Electrochemical Oxidation Induced Site-
Selective Intramolecular C(sp3)-H Amination
Xia Hu, Guoting Zhang, Faxiang Bu, Lei Nie, and Aiwen Lei
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02847 • Publication Date (Web): 06 Sep 2018
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ACS Catalysis
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Electrochemical Oxidation Induced Site-Selective Intramolecular
C(sp3)-H Amination
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Xia Hu, Guoting Zhang, Faxiang Bu, Lei Nie and Aiwen Lei*
Institute for Advanced Studies (IAS), College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei
430072, P. R. China.
Dedicated to Professor Xiyan Lu on the occasion of his 90th birthday
ABSTRACT: The crossꢀcoupling of C(sp3)ꢀH and NꢀH represents one of the most straightforward approaches to construct
saturated nitrogenꢀcontaining compounds. The additional oxidants or halogenated reagents are generally required in such processes.
Herein, we developed an electrochemical oxidative intramolecular C(sp3)ꢀH bonds amination of amides by employing carbon rod
anode, platinum plate cathode in an undivided cell under contantꢀcurrent electrolysis conditions. Tetrabutylammonium acetate was
not only employed as an electrolyte, but also can form the intermolecular hydrogen bond with amide and promote the cleavege of
NꢀH bond. The additional oxidants and Nꢀhalogenation step can be obviated in this methodology. A variety of benzylic and nonꢀ
activated tertiary, secondary, primary C(sp3)ꢀH amination can be achieved with good yields.
KEYWORDS electrosynthesis, oxidative coupling, amination, radical, annulation, heterocyles
The direct C(sp3)ꢀH functionalization represents a powerful
and straightforward protocol for constructing carbonꢀcarbon or
carbonꢀheteroatom bonds.1 In particular, the selective C(sp3)ꢀH
amination enables the efficient synthesis of saturated nitrogenꢀ
containing heterocycles,2 which are important structural motifs
in various biologically active compounds.3 Due to the high Cꢀ
H bondꢀenergy, chemoꢀ and regioꢀselectivity of nonꢀactivated
C(sp3)ꢀH functionalization are significantly challenging.4 After
years of efforts, the great achievement has been made in tranꢀ
sitionꢀmetalꢀcatalyzed C(sp3)ꢀH amination with the assistance
of directing groups.5
oxygen, nitrogen or halogen atom).12 Despite the photoredox
catalyzed protonꢀcoupled electron transfer process has been
proven as a powerful pathway to offer the amidyl radical from
amides,13 the HLF reaction through directly breaking the NꢀH
bond pathway remains undeveloped.
Besides, radical CꢀH activation provides another compleꢀ
mentary approach to enable remote C(sp3)ꢀH amination.6 A
wellꢀknown process is the Hofmann–Löffler–Freytag (HLF)
reaction, which converts Nꢀhaloamines into pyrrolidines
through 1,5ꢀhydrogenꢀatomꢀtrasfer (HAT) process (Scheme
1A).7 Unlike transitionꢀmetalꢀcatalyzed CꢀH amination reacꢀ
tions, the additional steps for the installation and removal of
directing group can be avoided. A significant breakthrough in
the modification of HLF reaction has been discovered by Suaꢀ
rez and coꢀworkers.8 The preparation of Nꢀhaloamines can be
avoided. Because the harsh reaction conditions are required
for the generation of Nꢀcentered radical, seeking for more mild
strategy for producing radicals would be very meaningful.9
Since the iodine and the in-situ generated acetyl hypoiodite
(such as AcOI) would lead to undesired oxidations of weaker
Cꢀ H bonds9a, decreasing the reaction concentration of iodine
can be regarded as a good way to expand the substrate apꢀ
plicability and solve the chemoselectivity issues. Recently,
Scheme 1. Hofmann–Löffler–Freytag reactions for the
formation of pyrrolidines.
We speculated that the electrochemical oxidation can
achieve this goal. Over the past decades, electrochemical oxiꢀ
dation offers a mild and efficient strategy for the external oxiꢀ
dantꢀfree dehydrogenative crossꢀcoupling reactions.14 In elecꢀ
trochemical dehydrogenative crossꢀcoupling reactions, the
protons releasing from the substrates can be in-situ reduced on
the cathode to form H2. Particularly, anodic oxidation can proꢀ
vide a convenient approach to generate Nꢀcentered radicals by
cleavage the strong nitrogen–hydrogen (NꢀH) bonds of amines
and amides.15 Several advances on the CꢀN bonds cyclization
reactions have been achieved through the radical addition of
10
Muñiz,9e, Nagib9a and Herrera11 have achieved the iodide
catalyzed δ CꢀH amination. According to these great progress,
the directly breaking the N–H bonds to generate Nꢀcentered
radical would provide a more desirable strategy to achieve
HLF reactions under halogenated reagentꢀfree conditions.
Traditional methods for the generation of Nꢀcentered radical
are harsh and mostly through the cleavage of NꢀX bonds (X =
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Nꢀcentered radical to unsaturated bonds.16 By using stoichioꢀ
Page 2 of 6
aryl group were compatible in the electroꢀchemical system
(2ag-2aj). The tertiary benzylic CꢀH amination can also be
achieved by using this transformation (2ak, 94%). To our deꢀ
light, the oxygen atoms and nitrogen atoms adjacent to the
benzylic CꢀH bond can participate well in the reaction to give
oxazolidine and imidazolidine derivatives (2al and 2am), reꢀ
spectively. The substrate with a phenyl backbone provided the
corresponding isoindoline derivative with an excellent yield
under the constant current electrolysis conditions (2an, 88%).
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metric amount of halogen anion as a mediator, an electroꢀ
oxidative intramolecular C(sp3)ꢀH amination reaction has been
reported.17 Herein, we reported an electrochemical oxidation
induced Hofmann–Löffler–Freytag reaction that can achieve
the remote C(sp3)ꢀH amination of amides without the use of
halogenated reagents under very mild conditions (Scheme
1B).18 Not only benzylic, but the nonꢀactivated tertiary,
secondary, primary C(sp3)ꢀH bonds were also compatible in
the reaction system. The metal catalyst, halogenated reagents
and stoichiometric additional oxidants were not required in
this procedure.
Table 2. Scope of the electrochemical oxidation induced
site-selective intramolecular C(sp3)-H amination.a
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C(+) | Pt(-), 15 mA, 5.6 F/mol
1.0 equiv. nBu4NOAc
R2
R1
Table 1. Optimization of the reaction conditions.a
X
R3
X
R3
R4
TsHN
H2
+
R
1 R2
r.t., DCE/HFIP
R4
N
Ts
1, X = C, NTs, O
2
R
N
N
N
R
Ts
2aa (%)b
96 (93c)
n.d.
Ts
Ts
Entry
Variation form standard conditions
Standard conditions
N
2af, R= H, 98%
Ts
2aa, 93%
(92%)b
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2
3
4
5
6d
7
2ag, R= Me, 89%
2ah, R= F, 93%
2ai, R= Cl, 96%
2aj, R= Br, 82%
2ab, R = Ph, 72%, d.r. = 3.8:1
2ac, R = Et, 95%, d.r. = 2.2:1
2ad, R = iPr, 95%, d.r. = 2.3:1
2ae, 97%
Without current
Pt(+) | Pt(ꢀ) instead of C(+) | Pt(ꢀ)
0.5 equiv. nBu4NOAc
30
O
NTs
86
NTs
N
N
MeCN/HFIP= 8mL/4mL as solvent
NaOAc instesd of nBu4NOAc
nBu4NBF4 or nBu4NPF6 instead of nBu4NOAc
69
N
Ts
Ts
Ph
Ts
85
2ak, 94%
2al, 90%
2am, 89%
2an, 88%
n.d.
aReaction conditions: carbon rod anode (Φ 6mm), platinum plate cathode
(15 mm* 15 mm* 0.3 mm), constant current = 15 mA, 1aa (0.40 mmol),
nBu4NOAc (0.40 mmol), solvent (DCE/HFIP= 8 mL/4 mL),undivided cell,
N
N
N
Ts
N
Ts
Ts
Ts
2ap, 96%, (95%)b
2aq, 86%
2ar, 80%
b
2ao, 72%
N2, 5.6 mol/F. Yields of 2aa were determined by GC analysis by using
c
biphenyl as the internal standard. Isolated yield was shown in the parenꢀ
theses. dAdding 1.0 equiv. nBu4NBF4 as electrolyte. n.d. = no determined.
O
MeOOC
NTs
N
N
Ts
We began the investigation of electroꢀoxidative
intramolecular C(sp3)ꢀH amination reaction (Table 1) by
contantꢀcurrent electrolysis of Nꢀtosylꢀ4ꢀphenylbutanꢀ1ꢀamine
(1aa). To our delight, the formation of pyrrolidine 2aa was
observed by using a carbon rod anode and a platinum plate
cathode in an undivided cell containing an electrolyte solution
of nBu4NOAc in the DCE and HFIP (2:1). Under the
conditions, 96% GC yield and 93% isolated yield of product
can be obtained at room temperature (Table 1, Entry 1). As
expected, no conversion can be observed without current
(Table 1, Entry 2). In comparison, when a Pt anode was used
instead of carbon rod, the significant decline of yield was
obtained (Table 1, Entry 3). Decreasing the loading of
nBu4NOAc into 0.5 equivalent, provided a reduced yield
( 85%, Table 1, Entry 4). When the MeCN/HFIP (2:1) was
used as solvent, the transformation can also proceed in 69%
N
Ts
Ts
2av, 42%
2as, 65%
2at, 68%
2au, 47%
TsN
NTs
O
O
H
N
N
Ts
Ts
H
2aw, 35%
d.r. > 20:1
2ay,73%
2ax, 88%
2az, 61%
d.r. > 20:1
TBSO
H
aReaction conditions: carbon rod anode (Φ 6mm), platinum plate cathode
(15 mm*15 mm*0.3 mm), constant current = 15 mA, 1 (0.40 mmol),
nBu4NOAc (0.40 mmol), solvent (DCE/HFIP = 8 mL/4 mL), undivided
cell, N2, 5.6 F/mol. Isolated yields were shown. The d.r. values were
determined by 1H NMR. b5 mmol scale, DCE/HFIP= 50 mL/25 mL, 48 h.
It is worth noting that the involvement of the benzylic posiꢀ
tion is not necessary for the transformation. The reaction is
also applicable to the δꢀselective CꢀH amination of nonꢀ
activated CꢀH bonds (2aoꢀ2az). The amination of dimethyl
substituted inert tertiary CꢀH bond would react efficiently to
give the corresponding product (2ao, 72%). Variation of the
steric environment around the inert CꢀH bond had effect on the
outcome of the reaction, the 2ap was obtained in excellent
yield. Related substrates, wherein the nonꢀactivated CꢀH
bonds from carbocycles of various sizes, also reacted efficientꢀ
ly to form spiro compounds 2aq and 2ar.αꢀOxy tertiary CꢀH
bond was reactive for amination, as shown by the formation of
oxazolidine product 2as. A pregabalin deriveative were also
viable in this reaction, as demonstrated by the generation of
2at. This strategy is not limited to the amination of tertiary
C(sp3)ꢀH bonds, secondary and primary C(sp3)ꢀH bonds could
also be tolerated (2au-2ay). Significantly increased yields are
n
yield (Table 1, Entry 5). Using NaOAc instead of Bu4NOAc,
85% yield of pyrrolidine can be formed (Table 1, Entry 6).
n
n
However, change the eletrolyte into Bu4BF4 or Bu4NPF6, no
reaction can be observed (Table 1, Entry 7). We proposed that
nBu4NOAc was not only act as an electrolyte, but also as a
base.
With the optimized conditions in hand, we investigated the
substrate scope of this transformation (Table 2). Various 2ꢀ
arylpyrrolidines can be synthesized in good to excellent yields.
The different monoꢀ or diꢀsubstitution on the betaꢀposition of
the sulfonamide can be well tolerated (2ab-2af, 72%ꢀ98%),
which demonstrated a little influence of a possible gemꢀ
dimethyl effect. Several substituents in the paraꢀposition of the
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ACS Catalysis
observed when the secondary or primary CꢀH bond adjacent to
paraꢀposition of the benzoyl were compatible in the electroꢀ
chemical system (2beꢀ2bh). Nꢀ(4ꢀphenylbutyl)furanꢀ2ꢀ
carboxamide would give the desired product in 72% yield
(2bi). Using the described conditions, we demonstrate the
feasibility of performing this transformation in the gramꢀscale.
The constant current electrolysis of 5 mmol of 1aa, 1ap, 1be
can be conducted to produce the desired pyrrolidines (2aa,
2ap, 2be) in 92%, 95% and 90% isolated yield respectively.
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oxygen atoms (2ax, 2ay). Additionally, a steroidꢀderived pꢀ
toluenesulfonamide 1az that contain several distinct tertary Cꢀ
H bonds and the activated CꢀH bonds adjacent to heteroatoms
can also be selectively functionalized at the single position to
provide the pyrrolidine derivatives (2az, 61%).
Table 3. Scope of N-protecting group.a
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aReaction conditions: carbon rod anode(Φ 6mm), platinum plate cathode
(15 mm*15 mm*0.3 mm), constant current = 15 mA, 1 (0.40 mmol),
nBu4NOAc (0.40 mmol), undivided cell, N2, 5.6 F/mol. Isolated yields
were shown. bDCE/ HFIP (8 mL/4 mL), cCH3CN/HFIP (8 mL/4 mL).
dShono’s system: platinum plate anode, platinum plate cathode, constant
current (100 mA/cm2), KBr (0.5 equiv.), NaOMe (0.5 equiv.), methanol,
reflux; n. r.= no reaction. e5 mmol scale, DCE/HFIP= 50 mL/25 mL, 48 h.
Scheme 3. Kinetic isotope effect experiments
To gain a deeper insight into the mechanism of the transꢀ
formation, the kinetic isotope effect experiments were carried
out. And intramolecular and intermolecular kinetic isotopic
effect values was obtained as 2.45 and 1.44, respectively
(Scheme 3), which indicated that the breaking of CꢀH bonds
might be involved in the rateꢀdetermining step of the transꢀ
formation.
0.00
-0.05
To investigate the interaction between acetate anion and amꢀ
1
ide, the H NMR study was performed. The signal of the NꢀH
bond of sulfonamide was obviously shifted downfield when
tetrabutylammonium acetate was added (Figure S6 in supportꢀ
ing information). The reason for this result is the intermolecuꢀ
lar Hꢀbonding of NꢀH group to acetate anion.19 We proposed
that the intermolecular Hꢀbonding would assist the oxidation
and deprotonation of the sulfonamide to generate the amidyl
radical. The cyclic voltammetry studies showed that the halfꢀ
peak potential of 1aa would slightly decrease in the presence
of acetate anion (Figure S10 in supporting information).
-0.10
1aa in MeCN/HFIP
1aa in DCE/HFIP
1be in MeCN/HFIP
1be in DCE/HFIP
-0.15
-0.20
1.0
1.5
2.0
2.5
3.0
Potential / V (vs Ag/ AgCl)
Scheme 2. Cyclic voltammetry of 1aa and 1be in
MeCN/HFIP= 2/1 or DCE/HFIP = 2/1.
Besides the tosylated substrate, other sulfonyl groups such
as (4ꢀmethoxyphenyl)sulfonyl (2ba), (4ꢀfluorophenyl)sulfonyl
(2bb), methanesulfonyl (2bc), and trifluoromethanesulfonyl
(2bd) are well tolerated (Table 3). However, the Nꢀ(4ꢀ
phenylbutyl)benzamide would obtained the desired products in
only 10% yield under above reaction conditions. And this reꢀ
sult
might
be
attributed
to
that
the
Nꢀ(4ꢀ
phenylbutyl)benzamide (E1/2 = 2.30 V vs Ag/AgCl) is more
difficult to be oxidized than Nꢀtosylꢀ4ꢀphenylbutanꢀ1ꢀamine
(E1/2 = 2.13 V vs Ag/AgCl) in a mixed solvent of
DCE/HFIP(2:1). At the same time, we found that the adoption
of a mixed solvent of MeCN/HFIP (2:1) lowered the oxidation
potential of 1be slightly (E1/2 = 1.83 V vs Ag/AgCl) (Scheme
2). The cyclic voltammetry studies suggested the possibility of
benzamide derivatives might be tolerated in this
transformation when acetonitrile was used instead of DCE. As
expected, phenyl(2ꢀphenylpyrrolidinꢀ1ꢀyl)methanone 2be can
be formed in 92% yield when MeCN/HFIP (2:1) was used as
solvent. In contrast, the benzoyl protected amides can not be
compatible in Shono’s system.17 Several substituents in the
Scheme 4. Proposed mechanism.
A plausible mechanism for the electroꢀoxidative intramoꢀ
lecular Csp3ꢀH amination reaction was outlined in Scheme 4.
We proposed that the process began with the formation of a
bonding complexes 3 between sulfonamide 1 and acetate. The
single electron oxidation on the anode lead the generation of
the Nꢀcentered radical intermediate I (Path 1). The subsequent
1,5ꢀHAT of the δꢀCꢀH bond by aminyl radical provided a Cꢀ
centered radical II. Then the radical species underwent further
oxidation to furnish a carbon cation intermediate III. Followed
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Daugulis, O., Heterocycle Synthesis via Direct C–H/N–H Coupling.. J.
by the nucleophilic attack of the sulfonamide and the proton
eliminations, cyclization product would be formed. Concomiꢀ
tant cathodic reduction of generated protons would generate
H2 during the reaction process, which avoids the use of stoiꢀ
chiometric external oxidant in the transformation. The generꢀ
ated alkoxide might deprotonate the substrate to afford Nꢀ
anion IV, which can be rapidly oxidized on the anode to give
the Nꢀcentered radical intermediate I (Path 2).
Am. Chem. Soc. 2012, 134, 7ꢀ10; (e) Michaudel, Q.; Thevenet, D.; Baran,
P. S., Intermolecular RitterꢀType C–H Amination of Unactivated sp3
Carbons.. J. Am. Chem. Soc. 2012, 134, 2547ꢀ2550.
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(−)ꢀMesembrane
and
(−)ꢀMesembrineviaPalladiumꢀCatalyzed
Enantioselective Allylic Substitution and ZirconiumꢀPromoted
Cyclization.. J. Org. Chem. 1997, 62, 3263ꢀ3270; (b) Nicolaou, K. C.;
Dalby, S. M.; Majumder, U., A concise asymmetric total synthesis of
aspidophytine. J. Am. Chem. Soc. 2008, 130, 14942ꢀ14943.
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In summary, we have successfully developed an electroꢀ
oxidative Hofmann–Löffler–Freytag reactions. The intramoꢀ
lecular remote inert δꢀC(sp3)ꢀH amination reactions can be
achieved through a 1,5ꢀHAT process. The requirement of metꢀ
al catalyst, halogenated reagents and stoichiometric additional
oxidant can be avoided. Our electrosynthetic method is broadꢀ
ly compatible with a wide benzylic C(sp3)ꢀH bonds and inert
tertiary, secondary as well as primary C(sp3)ꢀH bonds, which
can provide access to various functionalized nitrogenꢀ
containing heterocycles with great synthetic values. The reacꢀ
tion can be easily scaled up. The transformation holds signifiꢀ
cant potential for the applications to other remote C(sp3)ꢀH
functionalization reactions.
4. (a) Lawrence, J. D.; Takahashi, M.; Bae, C.; Hartwig, J. F.,
Regiospecific Functionalization of Methyl C−H Bonds of Alkyl Groups in
Reagents with Heteroatom Functionality.. J. Am. Chem. Soc. 2004, 126,
15334ꢀ15335; (b) Newhouse, T.; Baran, P. S., If CꢀH Bonds Could Talk:
Selective CꢀH Bond Oxidation.. Angew. Chem. Int. Ed. 2011, 50, 3362ꢀ
3374.
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5. (a) McNally, A.; Haffemayer, B.; Collins, B. S.; Gaunt, M. J.,
Palladiumꢀcatalysed CꢀH activation of aliphatic amines to give strained
nitrogen heterocycles.. Nature 2014, 510, 129ꢀ33; (b) Yang, M.; Su, B.;
Wang, Y.; Chen, K.; Jiang, X.; Zhang, Y. F.; Zhang, X. S.; Chen, G.;
Cheng, Y.; Cao, Z.; Guo, Q. Y.; Wang, L.; Shi, Z. J., Silverꢀcatalysed
direct amination of unactivated CꢀH bonds of functionalized molecules..
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Chen, G., Highly efficient syntheses of azetidines, pyrrolidines, and
indolines via palladium catalyzed intramolecular amination of C(sp3)ꢀH
and C(sp2)ꢀH bonds at gamma and delta positions. J. Am. Chem. Soc.
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Z.; Ni, J.; Kuninobu, Y.; Kanai, M., Copperꢀcatalyzed intramolecular
C(sp(3))ꢀH and C(sp(2))ꢀH amidation by oxidative cyclization. Angew.
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ASSOCIATED CONTENT
Supporting Information.
The Supporting Information is available free of charge via the
Internet at http://pubs.acs.org, which included NMR data and
characterization (PDF)
AUTHOR INFORMATION
Corresponding Author
6. Yi, H.; Zhang, G.; Wang, H.; Huang, Z.; Wang, J.; Singh, A. K.; Lei,
A., Recent Advances in Radical CꢀH Activation/Radical CrossꢀCoupling.
Chem. Rev. 2017, 117, 9016ꢀ9085.
*Corresponding author, Eꢀmail: aiwenlei@whu.edu.cn.
Notes
The authors declare no competing financial interest.
7. Löffler, K. F., C., Ber. Dtsch. Chem. Ges. 1909, 42, 3427.
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E., Intramolecular functionalization of nonactivated carbons by
amidylphosphate radicals. Synthesis of 1,4ꢀepimine compounds.. J. Org.
Chem. 1983, 48, 4430ꢀ4432.
ACKNOWLEDGMENT
9. (a) Wappes, E. A.; Fosu, S. C.; Chopko, T. C.; Nagib, D. A., Triiodideꢀ
Mediated deltaꢀAmination of Secondary CꢀH Bonds.. Angew. Chem. Int.
Ed. 2016, 55, 9974ꢀ8; (b) Martinez, C.; Muñiz, K.,An IodineꢀCatalyzed
HofmannꢀLoffler Reaction. Angew. Chem. Int. Ed. 2015, 54, 8287ꢀ91; (c)
Francisco, C. G.; Herrera, A. J.; Suárez, E.,Intramolecular Hydrogen
Abstraction Reaction Promoted by NꢀRadicals in Carbohydrates.
Synthesis of Chiral 7ꢀOxaꢀ2ꢀazabicyclo[2.2.1]heptane and 8ꢀOxaꢀ6ꢀ
azabicyclo[3.2.1]octane Ring Systems.. J. Org. Chem. 2003, 68, 1012ꢀ
1017; (d) Becker, P.; Duhamel, T.; Stein, C. J.; Reiher, M.; Muñiz, K.,
Cooperative LightꢀActivated Iodine and Photoredox Catalysis for the
Amination of Csp3 ꢀH Bonds.. Angew. Chem. Int. Ed. 2017, 56, 8004ꢀ
8008; (e) O'Broin, C. Q.; Fernandez, P.; Martinez, C.; Muñiz, K., Nꢀ
IodosuccinimideꢀPromoted HofmannꢀLoffler Reactions of Sulfonimides
under Visible Light.. Org. Lett. 2016, 18, 436ꢀ9; (f) Meng, D.; Tang, Y.;
Wei, J.; Shi, X.; Yang, M., Copperꢀcatalyzed remote (delta) C(sp(3))ꢀH
bond amination: a practical strategy to construct pyrrolidine derivatives..
Chem. Commun. 2017, 53, 5744ꢀ5747.
10. (a) Martínez, C.; Muñiz, K., An IodineꢀCatalyzed Hofmann–Löffler
Reaction.. Angew. Chem. Int. Ed. 2015, 54, 8287ꢀ8291; (b) Becker, P.;
Duhamel, T.; Stein, C. J.; Reiher, M.; Muñiz, K.,Cooperative Lightꢀ
Activated Iodine and Photoredox Catalysis for the Amination of Csp3 −H
Bonds.. Angew. Chem. Int. Ed. 2017, 56, 8004ꢀ8008.
11. Paz, N. R.; RodríguezꢀSosa, D.; Valdés, H.; Marticorena, R.; Melián,
D.; Copano, M. B.; González, C. C.; Herrera, A. J., Chemoselective
Intramolecular Functionalization of Methyl Groups in Nonconstrained
Molecules Promoted by NꢀIodosulfonamides.. Org. Lett. 2015, 17, 2370ꢀ
2373.
This work was supported by the National Natural Science Foundation
of China (21390402, 21520102003) and the Hubei Province Natural
Science Foundation of China (2017CFA010). The Program of Introꢀ
ducing Talents of Discipline to Universities of China (111 Program)
is also appreciated.
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