Copper-catalyzed remote (δ) C(sp3)-H bond amination: A practical strategy to construct pyrrolidine derivatives

Article abstract of DOI:10.1039/c7cc02624b

We report a copper-catalyzed remote C(sp³)-H bond amination reaction that converts acyclic amines to pyrrolidines. This reaction occurs selectively at the carbon δ to the amine functionality. Primary, secondary and tertiary C-H bonds are all su

Full text of DOI:10.1039/c7cc02624b

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Received 00th January 20xx,
Accepted 00th January 20xx
Copper-Catalyzed Remote (δ) C(sp3)-H Bond
Amination: A Practical Strategy to Construct
Pyrrolidine Derivatives
DOI: 10.1039/x0xx00000x
www.rsc.org/
Dongmei Meng, Yongzhen Tang, Junfa Wei, Xianying Shi and Mingyu Yang*
We report a copper-catalyzed remote C(sp3) -H bond amination step-economy of organic synthesis.3 This chemistry can directly
reaction that convert acyclic amines to pyrrolidines. This reaction convert simple and easily available raw materials to valuable
occurs selectively at the carbon δ to the amine functionality. products. A notable challenge in this field is to achieve C(sp3)
-
Primary, secondary and tertiary C-H bonds are all suitable for the
H functionalization with chemo- and regioselectivity. In
amination reactions in the presence of an inexpensive and particular, the selective amination reaction of inert C(sp3)
-H
commercially available copper catalyst.
bond to construct nitrogen-containing heterocycles has
received much attention in the synthetic community. Among
them, great progress has been made in directing group
Nitrogen-containing heterocycles are common in natural
products, pharmaceuticals and optical materials,1 which often
exhibit high levels of biological and optical activities. In par-
ticular, the pyrrolidine derivatives are privileged structural
units as exemplified by the structure of nicotine, geissoschizo-
line, dehydrotubifoline, aspidophytine, as well as mesembrane
(Figure 1).1a,2 Therefore, the development of efficient methods
to construct this ubiquitous structure unit is highly desirable.
assisted selective
C
-H
bond amination reactions.4
Nevertheless, these methods require pre-installation and
removal of a directing group in a synthetic sequence. The
practical remote C(sp3)
-H amination is still challenging. To
solve the problem, the metalated amides and nitrene were
successfully employed to confront this key synthetic
challenge.4,5,6
N
N
Complementary to these reactions are the radical-
N
mediated strategies that enable C-H amination with different
H
N
Me
Me
H
selectivities, especially for the synthesis of pyrrolidine
derivatives.7 For example, Hofmann–Löffler–Freytag (HLF)
N
H
H
N
H
CH2OH
reaction are efficient δ C-H amination reactions that occur
Nicotine
(+)-Geissoschizoline
Dehydrotubifoline
with an N-halogen precursor.8 Although high selectivity were
obtained for the synthesis of five-membered heterocycles, the
typically harsh reaction conditions for the generation of
radicals greatly limited the synthetic utility. Many chemists
devoted themselves to searching for more mild and practical
strategies for generating and harnessing the reactivity of
nitrogen-centered radicals.
OMe
OMe
N
O
N
O
H
N
Me
H
Me
Aspidophytine
Mesembrane
Figure 1 Natural bioactive products containing pyrroline structurals.
Suárez modification is a major milestone in the develop-
ment of HLF reaction, because the reaction conditions are
milder than those of typical HLF reactions, and amine
substrate can be used directely.9 Based on Suárez modification,
many modified methods have been successfully applied to
achieve direct amination reactions. Fan10 has demonstrated
that δ-phenyl substituted sulfonamides can be converted to α-
Transition-metal catalyzed direct functionalization of inert
C-H bond is of great importance as it improves the atom- and
phenyl substituted pyrrolidines in the presence of
a
hypervalent iodine reagent and molecular iodine.
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Subsequently, Yu11 developed
a
visible-light-promoted
We began our study with sulphonamide 1a as the model
amination of C(sp3)
-
H bond; however, pre-functionalized substrate because they are readily accessible and synthetically
sulfonamides are necessary to accomplish this transformation. useful (Table 1). In the presence of CuI, PhI(OTFA)2 and DCE at
Alternatively, Martínez and Muñiz developed the first example 100 for 10 hours, 79% of desired product was obtained.
℃
Encouraged by this result, we screened various oxidants, such
as PhI(OCOPh)2, PhI(OAc)2, K2S2O8, NFSI. However, these
oxidants were not efficient to improve the product yield
(entries 1-5). Further solvents screening (entries 6-11) revealed
that 1,2-dichloroethane in this reaction was more efficient
than others. The reactions conducted with other copper
catalysts didn’t improve the product yield (entries 12-15).
Table 1 Optimization of Reaction Conditionsa
entry
1
catalyst
CuI
oxidant
PhI(OTFA)2
PhI(OCOPh)2
PhI(OAc)2
K2S2O8
solvent
DCE
yieldb(%)
82(79)
18
2
CuI
DCE
3
CuI
DCE
7
N.R.c
N.R.c
9
Scheme 1.Halogen and transitionꢀmetal mediated direct C-H amination.
4
CuI
DCE
5
CuI
NFSI
DCE
of iodine-catalyzed visible-light induced
C-H amination
6
CuI
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
PhI(OTFA)2
THF
reaction.12 Sensitive functional groups, such as alkenes and
7
CuI
CH3CN
toluene
DCM
PhCl
PhCF3
DCE
7
N.R.c
8
CuI
anisoles can be well tolerated in this novel catalytic system.
9
CuI
61
Nonetheless, in most cases this reaction occurred at weak C
-H
10
11
12
13
14
15
16
17
18
19f
20f,g
21
CuI
16
bonds, such as tertiary, benzylic C H bonds and the ones α to
-
CuI
69
heteroatoms. Non-constrained amines are challenging
substrates for these reactions. Very recently, Nagib reported a
CuBr
CuBr2
CuCl
Cu(OAc)2
[Cu]d
Cu(OTf)2
Cu(OTf)2
-
23
DCE
33
very nice protocol of δ amination of secondary C-H bonds
-
DCE
18
from simple tosylamines mediated by triiodite (I3 ) generated
in situ.13 However, large amount of hypervalent iodide reagent
DCE
41
DCE
61
-
and inorganic salt are necessary to generate active I3 species.
DCE
88(80)
32
e
All of these protocols relied on the halogen transfer process
(Scheme 1, a).14 As a result, the selectivity and the reactivity
highly depend on the structures of the substrates and the
reaction conditions.
PhI(OTFA)2
DCE
PhI(OTFA)2
PhI(OTFA)2
-
DCE
6
-
DCE
30
N.R.c
CuI
DCE
a
Compared with the halogen mediated C-N bond formation,
Reaction condition: 1a (0.1mmol), catalyst (10 mol%), oxidant (2.0 equiv),
solvent (1.0 mL), 100 ℃,10 h. b The yield was determined based on 1H NMR with
carbon-carbon and carbon-heteroatom bonds formation
through the interception of remote carbon radical center by
transition-metal catalysts has proved challenging. Previously,
Shi has demonstrated the potential of transiton-metal silver
c
1,1,2,2-Tetrachloroethaneas internal standard. Isolated yield in parentheses.
N.R. means no reaction (detected by crude 1H NMR). d [Cu]=Cu(CH3CN)4PF6.
e
PhI(OTFA)2 (3.0 equiv). f Without catalyst. g Additive: NaI (1.0 equiv).
catalyzed site-selective amination of primary C
without directing groups.15 However, the silver catalyst is only
active for amination of primary C H bonds (Scheme 1, b).
-H bonds
Notably, Cu(OTf)2 showed a great advantage in promoting this
amination reaction; as product 2a was obtained cleanly with
80% yield (entry 17, see SI). In the absence of any copper
-
As a first row transition metal, copper often exhibits unique catalyst, the reaction gave 2a in very low yield (6%, entry 19).
When the sodium iodide was used instead of copper iodide,
the formation of the product was significantly inhibited,
indicating that the copper catalyst is necessary for the reaction
to occur efficiently (entry 20). No product was formed in the
absence of oxidant (entry 21).
With the optimized reaction condition in hand, we studied
the substrate scope with various heteroatom functionalities.
To demonstrate the synthetic utility of this method, we first
studied reactions with amino acid ester L-isoleu-2a and
protected amino alcohol 3a. To our satisfaction, the desired
products 3-methylproline 2b and 3-methylprolinol 3b were
obtained in high yields with retention of the stereogenic
and versatile reactivity. For example, copper-catalyzed C
-
H
amination reaction is an important method to construct C
-N
bond.16 A single-electron or a two-electron-transfer fashion is
generally involved in copper-catalyzed redox reactions. As on
our research interest in the C-N bond formation, we proposed
that CuII species with d9 configuration at the metal center
could be employed as catalysts in single electron reaction
events to form C
-
N bond. Here we report a general amination
H bond at primary, secondary and
method of remote C
-
tertiary positions catalyzed by a simple non-precious copper
complex.
2 | J. Name., 2012, 00, 1-3
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centers. Thus, our method can potentially serve as an efficient
strategy to generate unnatural proline derivatives. The
desymmetrization cyclization of amino acid ester L-leu-4a
process with low diastereoselectivity. Remarkably, this method
was also found to be effective in the activation of methylene
19a
,
20a
,
21a and 22a could expand the scope of pyrolidine
structures for the further application. The sulphonamide 23a
with phenyl substitutent at position, the amination
reaction was occurred with primary C H bond to give the same
product as the sulphonamide 19a
To gain insight into the reaction mechanism, competition
experiment between 1°/2°C H bonds and 2°/3° C H bonds
were carried out independently (Scheme 2). In 1°/2°C H bonds
competition experiment, secondary C H bond is more reactive
than the primary one (24a and 25a). And then, in 2°/3° C
bonds competition experiment, amination selectively occurred
at tertiary C H bond than secondary (26a). The totally
reactivity of hydrogen atoms decreases in the order tertiary>
secondary primary. Furthermore, radical captured
a
α
-
.
C–H bond in linear sulfonamides (5a, 6a, 7a, 8a, 9a, 10a, 11a,
12a 13a and 14a). The most interesting thing is the pyrrolidine
,
-
-
product 5b can be obtained from different substrates 5a and
9a to show the synthetic varieties of this method. For
substrates 6a, 7a and 8a, including an alkyl group at δ position,
-
-
-
H
cyclization reaction also took place to afford amination
product in good yields. Non constrained substrates 9a,
10a,
-
Table 2 Direct amination of sp3 C-H bondsa
>
experiments were conducted to uncover evidence for the
presence of radical intermediate. Initially, 2.0 equiv of TEMPO
was added, the reaction was completely inhibited. Meanwhile,
in presence of 1.0 equiv 2,6-di-tert-butylphenol, the product
yield was decreased significiantly, only 34% of amination
product was obtained (see SI).
H
Ts
N
Ts
N
NHTs
NHTs
Et
H
NHTs
Et
1a
1b (80%)
14b (65%, dr = 2:3)d
14a
H
H
Ts
N
Ts
N
NHTs
COOMe
COOMe
R
R
2b (74%, dr = 95:5)b
L-isoleu-2a
15a (R = OBz)
16a (R = OAc)
15b (65%, dr = 1:1.3)e
16b (56%, dr =1:1.3)d
H
Ts
N
NHTs
NHTs
OBz
OBz
N
Ts
H
3b (85%, dr = 98:2)c
17a
17b 32%d
L-isoleu-3a
Ts
N
H
NHTs
COOMe
NHTs
H
COOMe
N
Ts
H
4b (63%, dr = 1:1)b
L-leu-4a
18b 52%d
18a
Ts
N
H
NHTs
H
Ts
N
H
NHTs
5b (80%, dr = 2:3)d
5a
19a
H
19b (73%)d
Ts
Ts
N
NHTs
H
Scheme 2 Intromolecular competition experiment.
R
N
NHTs
R
6b (77%)d
7b (65%)d
8b (80%)d
Based on these results, we proposed a possible pathway
described as below (Scheme 3). Direct oxidation of
6a (R = Me)
7a (R = Et)
8a (R = Hex )
20a
20b (71%)d
H
Ts
N
sulphonamides (16a) with PIFA generating the amido-λ3
-
NHTs
MeO
Ts
N
NHTs
H
iodane I 17
, which underwent the homolysis of nitrogen-iodine
MeO
R
R
21a
H
21b (63%)
9b (5b) (83%, dr = 1:1)d
9a (R = Me)
10a (R = nPr)
11a (R = iPr)
10b (74%, dr = 1:1)d
11b (79%, dr = 1:1)d
Ts
N
NHTs
F
F
Ts
NHTs
N
H
22a
NHTs
nBu
22b (60%)d
R
Ts
N
R
H
12b (64%)d
12a (R = H)
13b (67%, dr = 1:1)d
13a (R = iPr)
23b (19b) (54%)e
23a
a Reaction conditions: a (0.1 mmol), Cu(OTf)2 (0.01 mmol), PhI(OTFA)2 (0.4 mmol),
b
c
DCE (1.0 mL), 100 oC, 10 h. Changes: PhI(OTFA)2 (3.0 equiv). Changes:
PhI(OTFA)2 (2.5 equiv). d Changes: CuI (0.01 mmol). e Changes: PhI(OTFA)2 (4.0
equiv).
11a, 12a, 13a and 14a, regarded as the challenge ones in HLF Scheme 3 Proposed Mechanisms
reactions, worked very well. Furthermore, the amination of
protected amino alcohol derivatives 15a and 16a resulting in 5-
methylprolino 15b and 16b in good yields. The amination
bond forming the N
HAT of C
-
centered radical II
.
A subsequent 1,5
-
-
H bond by a polarized aminyl radical formed a
carbon radical (III). For the next step, two possible pathways
could be followed. One is the single-electron oxidation of
carbon radical to give carbon cation (IV), which was captured
reaction also occurred on cycloalkane (17a
,
18a), giving
18b). In
H bonds in
polycyclic products, albeit in moderate yields (17b
,
addition, the more reactive secondary benzylic C
-
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5
For selected nitrenoid-based C(sp3)
Breslow and S. H. Gellman, J. Am. Chem. Soc., 1983, 105
6728; (b) C. G. Espino and J. Du Bois, Angew. Chem. Int. Ed.,
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Angew. Chem. Int. Ed., 2004, 43, 4210; (e) T. Kang, Y. Kim, D.
-
H amination, see: (a) R.
,
by sulphonamide group to afford the cyclization product (16b
)
(Path a). Another is the oxidation addition of carbon radical by
Cu(II) catalyst to generate Cu(III) sulphonamide intermediate
IV’, after reductive elimination to give the cyclization product
16b) (Path b).18 The mechanistic studies are consistent with
-
(
Lee, Z. Wang and S. Chang, J. Am. Chem. Soc., 2014, 136
,
our proposed mechanism, more detailed mechanism study is
on the way.
4141; (f) J. M. Alderson, A. M. Phelps, R. J. Scamp, N. S. Dolan
and J. M. Schomaker, J. Am. Chem. Soc., 2014, 136, 16720; (g)
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and P. Dauban, J. Am. Chem. Soc., 2008, 130, 343; (h) For
selected review, see: H. M. L. Davies and J. R. Manning,
Nature 2008, 451, 417.
In conclusion, we developed a copper catalyzed remote (δ)
selective amination of C(sp3)
-
H bonds that does not require
pre-installation of directing groups. The reaction proceeds in
high yield, with primary, secondary, and tertiary C H bonds,
-
6
Other selected C(sp3)
-
M. Driess and J. F. Hartwig, J. Am. Chem. Soc., 2014, 136
H amination, see: (a) B. L. Tran, B. Li,
,
under mild reaction conditions. Notably, the methodology
enables direct amination of unbiased tosylamine to access a
range of pyrrolidines. Importantly, this method has achieved
the potential of the interception of the remote carbon radical
in the presence of a copper complex to form C–N bonds.
We gratefully acknowledge the financial support by the
National Natural Science Foundation of China (21572124,
21602128), Fundamental Research Funds for the Central
Universities (GK201603045), start-up funds from Shaanxi
Normal University, Shaanxi Provincial Natural Science
Foundation (2016ZDJC-03).
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16 For selected copper catalyzed C
5
, 4707.
-
H amination reactions see:
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