Silver-Catalyzed Long-Distance Aryl Migration from Carbon Center to Nitrogen Center

Article abstract of DOI:10.1021/jacs.5b10267

Selective cleavage of an inert C-C bond followed by C-O/N bond formation through a long-distance aryl migration from a carbon to a nitrogen center via Ag catalysis is reported. The migration products were easily converted into γ-hydroxy amines and tetrahy

Full text of DOI:10.1021/jacs.5b10267

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Communication
Ag-Catalyzed Long-distanced Aryl Migration
from Carbon Center to Nitrogen Center
Taigang Zhou, Fei-Xian Luo, Ming-Yu Yang, and Zhang-Jie Shi
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Ag-Catalyzed Long-distanced Aryl Migration from Carbon
Center to Nitrogen Center
Taigang Zhou,a Fei‐Xian Luo,a Ming‐Yu Yang,a Zhang‐Jie Shi* a,b
aBeijing National Laboratory of Molecular Sciences and Key Laboratory of Bioorganic Chemistry and Molecular En‐
gineering of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing
100871 and bState Key Laboratory of Elemento‐Organic Chemistry, Nankai University, Tianjin 300071, China
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Supporting Information
asymmetric C‐N formation through desymmetrization via Ag
catalysis with chiral ligands. Accidentally, we found phenyl
migration product 2a, which was unambiguously confirmed
by X‐ray crystallography (Scheme 2b). We proposed that,
both annulation and migration products might be generated
from the same N‐centered radical or cation as a key interme‐
diate. From those intermediates, the dearomatization pro‐
cess would take place to form the ‐complexes (either ‐
radical or ‐cation complex) when presenting aryl group at
the proper position. which could induce a completely differ‐
ent chemistry from the same starting materials. Indeed, both
intra‐ and inter‐ molecular dearomatization of the
heterocycles as well as electron‐rich phenol derivatives
through radical or cationic processes have been well studied,
thus offering the possibility for our hypothesis although the
dearomatization of common benzene derivatives was scarce‐
ly reported.10 Therefore, we turned out to develop this long‐
distanced aryl migration.
ABSTRACT: Selective cleavage of inert C‐C bond followed
by C‐O/N formation through a long‐distanced aryl migration
from carbon to nitrogen center was reported via Ag‐catalysis.
The migrating products were easily converted into γ‐hydroxy
amines and tetrahydroquinoline derivatives in quantitative
yields. Preliminary mechanistic studies indicated a radical
pathway.
Selective C‐C bond activation/cleavage has attracted much
attention in recent years.1 Not only is it one of the most chal‐
lenging themes in fundamental organic chemistry, but it also
represents a powerful, straightforward, and atom‐economic
strategy for constructing new organic compounds through
the completely new pathway based on the skeleton reorgani‐
zation of easily available compounds, differentiating from
conventional organic syntheses.1 During the past few decades,
many achievements in transition‐metal‐catalyzed C‐C cleav‐
age have been made starting from strained and unstrained
compounds.2,3 In the absence of transition metal catalysts, C‐
C could also been cleaved and transformed through radical 4
and cationic intermediates.5 Among different strategies to
approach direct C‐C cleavage of unstrained molecules, the
migration of carbon‐based groups is common and important
to facilitate the C‐C cleavage and new C‐C formation.6
Scheme 1. New Development of C‐C Cleavage via Ag‐
Catalyzed Long‐distanced Aryl Migration under Oxi‐
dative Condition
In general, group migration only takes place at carbon cen‐
ters adjacent to carbon cations,5 radicals6c‐f or carbenes3a
(Scheme 1). Beautiful examples to present long‐distanced
(over four carbons) migrations, were also reported by
Pohmakotr6g, Liang6h, Tchabanenko6i, Uneyama6j, and other
groups6a,b. However, group migration from carbon to nitro‐
gen center is scarcely reported.7 In particular, long‐distanced
aryl migration from carbon to nitrogen center has never been
approached. Herein we demonstrated an unusual Ag‐
catalyzed long‐distanced aryl migration from carbon to ni‐
trogen center (Scheme 1d).
In our previous studies we observed direct annulation
through sp2 C‐H amidation via Ag catalysis (Scheme 2a).8
Yokoyama and coworkers also reported the formation of
tetrahydroquinoline derivatives from the similar triflic amide
promoted by irradiation.9 Later on, we planned to explore the
Scheme 2. Different Chemistry of Triflic Amides
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DCE, dioxane, and so on, were screened while only DCE
5
worked well (Table S5, entry 2). To our interest, the addition
of PhCl as co‐solvent dramatically improved the yield to 75%
(entry 19).
6
7
8
With the optimized conditions in hand, we first evaluated
different functionalities on nitrogen atom (Scheme 3), in‐
cluding free amine (3a), carbamates (3b and 3c), acetamide
(3d), and sulfonamides (3e, 3f and 1a). However, we found
that only the sulfonamides performed the migration while Ts
(3e) or Ms (3f) protected amides only gave poor conversions.
Presumably, the strong electron‐withdrawing ability of Tf
group enhanced the acidity of the NH group, which could be
easily deprotonated by the proper bases and further coordi‐
nated to the Ag center. Such an Ag complex could be oxi‐
dized to Ag (II) (maybe Ag (III)), which further generated the
electron‐deficient N‐centered radical. This radical attacked
the phenyl ring to form the 5‐membered spiro radical ‐
complexes. To prove the importance of N‐H in amide, the
substrate 3g was tested and no desired product was obtained
at all.
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Table 1. Ag‐catalyzed Long‐distanced Aryl Migration
of 1a under Different Conditions.a
Scheme 3. Evaluating Different Amides a
aReaction condition: substrate (0.1 mmol), AgOAc (20 mol%), L3 (20
mol%), PhI(OTFA)2 (2.0 equiv.), K2CO3 (2.0 equiv.), PhCl/DCE (1.0
mL/1.0 mL), 120 oC, 1 h; The yield was determined by 1H NMR of the crude
reaction mixture with CH2Br2 as internal standard.
aReaction condition: substrate (0.1 mmol), catalyst (20 mol%), ligand (20
mol%), oxidant (2.0 equiv.), additive (2.0 equiv.), dichloroethane (DCE)
Table 2. Ag‐catalyzed Aryl Migration of Symmetrical
γ,γ‐Diaryl Substituted Triflic Amidesa
1
(2.0 mL), 120 oC, 1 h. b The yield was determined by H NMR of the crude
c
reaction mixture with CH2Br2 as internal standard. Isolated yields. More
details see table S1‐S5 in SI.
With the idea in mind, we first screened the conditions to
promote such a migration with 1a as a model (Table 1). Ac‐
cording to our studies, such a long‐distanced aryl migration
only took place under the oxidative conditions while Ag cata‐
lyst was critical to promote the efficacy. Thus, different oxi‐
dants were tested in the presence of AgOAc as the catalyst
and 2,2'‐bipyridine (L1) as the ligand (entries 2‐5).
PhI(OTFA)2 was found to be the only oxidant to produce the
desired migrating product 2a in 31% yield (entry 5). Other
metal catalysts, including Pd, Cu and Au, were also screened
while most of them were found to be insufficient or in a quite
low efficacy (entries 6‐8). Different Ag salts were examined
and others did not show better performance (entries 9‐11). To
promote the efficacy, various ligands were evaluated (entries
12‐14). 4,4'‐Di(tert‐butyl)‐2,2'‐bipyridine L3 was found the
best and the desired product 2a was obtained in 38% yield
(entry 13). Different bases were also screened (entries 15‐18).
Lithium carbonate and potassium carbonate were found effi‐
cient (entries 17 and 18) while cesium carbonate and potassi‐
um tert‐butoxide diminished the efficiency (entries 15 and 16).
These results indicated that, probably the proper base effi‐
ciently deprotonated N‐H to facilitate the oxidation of amide
to N radical or cation. Different solvents, including PhCl,
aReaction condition: substrate (0.1 mmol), AgOAc (20 mol%), L3 (20
mol%), PhI(OTFA)2 (2.0 equiv.), K2CO3 (2.0 equiv.), PhCl/DCE (1.0
mL/1.0 mL), 120 oC, 1 h; Yields of isolated products. bYields of isolated
corresponding alcoholysis products, due to these compounds were easily
alcoholysis when they were purified by flash column chromatography
(silica gel). c 120 oC, 10 h. d The d.r ratio was determined by 1H NMR.
We further extended such an aryl migration to various
symmetric γ,γ‐diaryl substituted triflic amides (Table 2).
Substrates bearing electron‐rich groups at para‐position of
aryl ring, including Me (1b), tBu (1c), Ph (1d) were submitted
and the desired migrating products were afforded in moder‐
ate to good yields (2b‐2d). However, strong electron‐
donating groups containing heteroatoms, for example MeO
and Me2N, induced the decomposition of starting materials
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under this oxidation condition. Meanwhile, substrates bear‐
phenyl < electron‐rich aryl groups. This conclusion was con‐
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8
ing electron‐deficient groups at para‐position, including F
(1e), Cl (1f), Br (1g), CF3 (1h), gave the desired migrating
products in comparable efficacy (2e‐2h). Substrates with the
meta‐substituents were also workable as migration groups
(1i‐1l), giving the corresponding products with moderate
yields (2i‐2l). More bulky substrate with an ortho‐tolyl mi‐
grating group (1m) also provided the migrating product (2m)
in an acceptable yield. These results indicated that the steric
hindrance on the aryl ring had a slight effect while the elec‐
tronic property did not have regular effects. Moreover, the
migration of 10 with β‐methyl substituent of triflic amide
performed well and 74% yield of 2o was obtained. However,
the yield of 1n with methyl group adjacent to the amide was
reduced to 45% albeit in the longer time, further proving the
steric effect on this migration.
sistent with the formation of the electron‐deficient N‐
centered radical (or cation) with very strong electron‐
withdrawing Tf group, which preferred to attack electron‐
rich groups to form σ‐complexes intermediates.
9
The migrating product 2a could be easily transformed into
the corresponding 1,3‐hydroxylamine (7) in quantity when
the solutionof 2a was filtering through a thin pad of basic
Al2O3 (Scheme 4a). 2a could also be easily converted to
tetrahydroquinoline 8 in a quantitative yield by treating the
reaction mixture with a concentrated sulfuric acid, providing
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an
alternative
way
to
produce
the
bioactive
tetrahydroquinoline scaffold from the linear amine deriva‐
tives,11 thus extending the potential application of this chem‐
istry.
Scheme 4. Alcoholysis and Cyclization of
Acyloxylated Amides and Control Experiments
Table 3. Ag‐catalyzed Aryl Migration of Unsymmet‐
rical γ,γ‐Diaryl Substituted Triflic Amidesa
aReaction condition: substrate (0.1 mmol), AgOAc (20 mol%), L3 (20
mol%), PhI(OTFA)2 (2.0 equiv.), K2CO3 (2.0 equiv.), PhCl/DCE (1.0
mL/1.0 mL), 120 oC, 1 h; bThe total yields of two isomers. cYields of isolat‐
ed corresponding alcoholysis products. dThe ratio of the isomers was
determined by 1H or 19F NMR of the crude reaction mixture.
Furthermore, we investigated the migrating ability of dif‐
ferent aryl groups in unsymmetrical triflic amides (Table 3).
Phenyl and 4‐chlorophenyl in 5a was first examined and the
migrating product was afforded in good yield while with a
poor selectivity (6a/6a'=1.4/1, entry 1). This result indicated
that both phenyl and 4‐chlorophenyl were workable for mi‐
grating groups while the relative electron‐rich phenyl group
was preferred. When 4‐chlorophenyl was displaced by 4‐
trifluoromethylphenyl, which was considered as a stronger
electron‐deficient substituent (5b), a moderate yield was
obtained with a better ratio (6b/6b'=5.9/1, entry 2), which
might arise from the bigger electronic difference between
two substituents. To get more information of electronic ef‐
fect, we kept 4‐trifluoromethylphenyl and used more elec‐
tron‐rich 4‐methylphenyl (5c). A moderate yield with an
excellent ratio were obtained (6c/6c'=25/1, entry 3). Predicta‐
bly, 4‐trifluoromethylphenyl and 4‐tert‐butylphenyl (5d)
gave a sole migrating product (6d), arising from the best
migrating preference of 4‐tert‐butylphenyl group. Another
pairs with 4‐tert‐butylphenyl and either 4‐chlorophenyl (5e)
or phenyl (5f) also gave excellent yields (entries 5 and 6). To
our interest, the pair of 4‐tert‐butylphenyl with phenyl gave
a slightly better selectivity (6e/6e'= 3.4/1, entry 5 vs 6f/6f'=
4.5/1, entry 6), which could not be explained at this stage. In
general, the electron‐rich aryl substituents were preferred to
give major migrating products. The migrating ability of dif‐
ferent aryl groups were summarized as electron‐poor aryl <
Figure 1. Proposed reaction mechanism.
To get insight on the mechanism, we performed control
experiments. When 1.0 or 0.5 equivalent of TEMPO was used
as a radical scavenger, this migration was completely inhibit‐
ed and only 1a was recovered (Scheme 4b). However, when
0.2 or 0.1 equivalent of TEMPO was used, the migration was
partly inhibited. These results indicated that the termination
of the migration by TEMPO might not be induced by killing
the oxidant while quenching the radical intermediate. In
order to further verify our hypothesis, 9 was prepared and
tested in the presence of triethylborane and O2 as a radical
initiator (Scheme 4c). Indeed, the migration product (10)
was indeed obtained with a high yield, accompanying with a
small amount of reducing amide. This result proved the fea‐
sibility of radical pathway.
Thus, we proposed the mechanism as Figure 1 although
other possibilities could not be rooted out at this stage. First
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M.; Jiang, X. Chem. Asian J. 2014, 9, 3360. (j)Wang, T.; Jiao, N. Acc.
of all, 1a was deprotonated and coordinated to 11a to form Ag
complexes 11b. With support of L3 and N anion ligand, Ag(I)
center became very electron rich and was easily oxidized to
Ag(II) complexes 11c by PhI(OTFA)2.12 Subsequently, the
homolytic cleavage of N‐Ag bond took place to produce the
electron‐deficient N‐centered radical 11d and regenerated
Ag(I) species. Finally, 11d underwent the intramolecular at‐
tack to the phenyl ring to form the key intermediate ‐
complex 11e by dearomatization, facilitating the final aryl
migration to produce a relatively stable benzylic radical
11f.6a,b This benzylic radical 11f could be oxidized to the cor‐
responding cation 11g by presenting oxidants. TFAO‐, gener‐
ated at previous stage, came back to react with 11g to give the
desired products 2a. As suggested in Li's development with
Ag (III) as potential reactive species, 12b we also tried 1a in the
presence of AgNO3 and Selectfluor, and however, only 1a was
recovered. This result did not suggest the Ag(III) intermedi‐
ate in our system.
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In summary, we have developed a novel silver‐catalyzed
long‐distanced aryl migration of γ,γ‐disubstituted triflic am‐
ides through C‐C bond cleavage, accompanying by the for‐
mation of new C‐O/C‐N bonds. More electron‐rich aryl
groups showed the better performance beyond electron‐
deficient aryl motifs during the migration. The migration
products were easily converted to γ‐hydroxy amines and
tetrahydroquinoline derivatives under mild conditions. Ac‐
cording to the control experiments, this transformation was
proposed through a silver‐promoted radical pathway. The
studies to clearly understand the mechanism and explore the
potential applications are underway.
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Supporting Information
Experimental procedures, analytical data for products,
NMR spectra of products, and X‐ray data for 2a are available
free of charge via the Internet at Http://pub.acs.org.
Corresponding Author
*E‐mail: zshi@pku.edu.cn.
Notes
The authors declare no competing financial interest.
Acknowledgments
We thank Prof. Wen‐Xiong Zhang and Dr. Neng‐Dong
Wang for their help on the X‐ray crystallographic analysis
and Prof. Armido Studer for a useful discussion. Financial
support by MOST (2015CB856600) and NSFC (Nos. 21332001
and 21431008) is gratefully acknowledged. T.Z. was supported
in part by the Postdoctoral Fellowship of Peking‐Tsinghua
Center for Life Sciences and China Postdoctoral Science
Foundation Grant (2014 M550546).
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ACS Paragon Plus Environment

Journal of the American Chemical Society
Page 6 of 6
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85x38mm (300 x 300 DPI)
ACS Paragon Plus Environment