Copper-catalyzed synthesis of α-amino nitriles through methyl transfer from DMF to aromatic amines

Article abstract of DOI:10.1039/c8cc00485d

A copper-catalyzed activation of C(sp³)-H bonds of DMF at room temperature was developed, which results in methyl transfer to aromatic amines for efficient synthesis of exceedingly valuable α-amino nitriles. This process features excellent functional group tolerance, a broad substrate scope, and high activity under ambient conditions.

Full text of DOI:10.1039/c8cc00485d

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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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Copper-Catalyzed Synthesis of α-Amino Nitriles through Methyl
Transfer from DMF to Aromatic Amines
Zaifeng Yuan,ac Na Li,b Chunyu Zhu,b and Chengfeng Xia*ab
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A copper-catalyzed activation of C(sp3) H bonds of DMF at room for electron-deficient amine substrates, these procedures
temperature was developed, which results in the methyl transfer generally require the use of strong base or high temperature
to aromatic amines for efficient synthesis of exceedingly valuable as a result of the poor nucleophilicity, which leads to narrow
α-amino nitriles. This process features excellent functional group functional group tolerance. Hence, development of novel and
tolerance, a broad substrate scope, and high activity under alternative methodologies is still essential for expanding the
ambient conditions.
scope of this transformation.
A) Representative conventional approches
α-Amino nitriles occupy an important position in medicinal
chemistry and synthetic chemistry, owing to their structural
subunits exist in many biologically active molecules and
1) Strecker reaction
R1 R2
R3
O
CN
R1 R2
N
+
N
R3 R4
H
natural compounds.1 Moreover,
α
-amino nitriles can be
facilely converted to valuable building blocks, such as -amino
acids, -amino carbonyl compounds, -amino alcohols, and
R4
CN
2) Oxidation of arylamines
α
Ar
R
oxidation
α
β
N
Ar
R
+
N
"CN"
1,2-diamines, which are widely utilized in pharmaceuticals,
agrochemicals, natural products, and catalyst architectures.1b, 2
Accordingly, significant efforts have been devoted to the
CN
B) This work: copper catalyzed methyl transfer of DMF
O
Ar
R
N
development of novel and efficient ways for preparation of
amino nitriles. One of the mostly common methods to
generate -amino nitriles is the Strecker reaction (Scheme 1),
α
-
Ar
R
Cu (I), NFSI
TMSCN
N
+
H
N
H
CN
α
Scheme 1 Approaches toward synthesis of
α
-amino nitriles.
a three-component condensation among aldehyde (or ketone),
amine, and cyanide source.3
In the meanwhile, remarkable progress has been made in
transition-metal-catalyzed activation of C(sp3)–H bonds
adjacent to an amide nitrogen center. The activated bonds are
of considerable importance for construction of new carbon –
carbon,11 carbon – oxygen,12 or carbon – boron13 bonds in a
convenient way. However, only limited nitrogen species, such
as azole or imide derivatives, were achieved to form carbon –
nitrogen bonds by coupling reaction with amides via iminium
ion intermediates.14 The directed coupling of amines with
amides is still challenging. Very recently, it was reported that
the methyl group of DMF could be transferred to ketone or
indole under oxidative conditions at high temperature.15
Herein, we report a methyl transfer from DMF to secondary
An alternative strategy to produce
α-amino nitriles is
oxidation of aryl tertiary amines to iminium ions, and
subsequent reaction with cyanide. Several examples of metal-
based catalysts (e.g. Ru,4 Fe,5 or other metals6), metal-free
catalysts,7 or electrochemical methods8 have been used for sp2
and sp3 C-H cyanations. However, in the case of secondary aryl
amines, conventional N-cyanomethylation methods are
extremely limited to pre-functionalized acetonitrile reagents at
high temperature,9 or in situ generated haloacetonitriles.10 As
aromatic amines via activation of
α
C(sp3) H bonds of amide
catalyzed by copper at room temperature. Moreover, both of
C-N and C-C bonds were constructed in one pot manner, which
resulted in the facile access to a variety of
(Scheme 1).
α-amino nitriles
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Our initial efforts focused on optimization of the reaction With the optimized conditions in hand, we next explored the
conditions using N-methylaniline as substrate and substrate scope of copper-catalyzed activation and transfer of
Cu(MeCN)4BF4 as catalyst, while the 2,2'-Bipyridine as ligand. the methyl group of DMF. Substrates with different alkyl
As showed in table 1, phenyliodine diacetate (PIDA) or substituted aniline were firstly examined. Both of the linear or
phenyliodine(III) bis(trifluoroacetate) (PIFA) was firstly tested branched alkyl derivatives afforded α-amino nitriles 2b - 2e in
as oxidant and only trace amount of -aminonitrile 2a was good yields. It was discovered that when the aniline was
α
detected (entries 1 and 2). Changing the oxidant to tert-Butyl substituted with a bulky alkyl group (cyclohexyl group), the
hydroperoxide (t-BuOOH) failed to provide the any products reaction still proceeded but with lower yields due to the
(entry 3). A significant amount of
obtained in 91% yield when N-fluorobenzenesulfonimide oxidative reagent NSFI, the substrate with unsaturated bond
(NFSI) was used as oxidant (entry 4). Screening other ligands was found to be tolerable and the corresponding -amino
α-aminonitrile 2a was hindrance (2f). Although the reaction conditions including the
α
such as L2 or L3 resulted in the decreasing of yields (entries 5 nitrile 2g was obtained in 80% yield. It were found that
and 6). Other copper catalysts, such as CuCl, CuCl2, Cu(OTf)2, or fluoride, chloride, bromide, and iodide groups were tolerated
Cu(OAc)2 were also examined. However, the yields were very to afford the corresponding products 2h
low. It was discovered that CuOAc was the most effective used in further coupling reactions. The electronic effects of
catalyst and -amino nitrile 2a was generated in 98% yield substituent groups on the aromatic ring were also evaluated.
- 2k, which could be
α
when L2 was selected as ligand (entry 12). The effects of When the aniline was substituted with electron withdrawing
amount of DMF were also evaluated and the yields were group, such as o- or p-carboxylate or trifluoromethyl group,
dramatically deceased (12% for 5 equiv and 21% for 10 equiv the α-amino nitriles 2l – 2n were obtained in 90%, 83% and
amount of DMF in MECN). The copper-catalyzed methyl 80%, respectively. In the meantime, derivatives with electron
transfer reaction was capable of gram scale synthsis with 83% donating groups on aniline also afforded the products 2o and
yield when 2.1 g of N-methylaniline was conducted.
2p in good yields. Furthermore, naphthylamine could also be
incorporated to give acceptable results by prolonging the
reaction time.
Table1. Optimization of the reaction conditionsa
Table 2. Substrate Scope of Anilinesa
Entry
1
Catalyst
Oxidant
PIDA
Ligand
yield (%)b
trace
Cu(MeCN)4BF4
L1
2
3
4
5
6
7
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(MeCN)4BF4
Cu(MeCN)4PF6
Cu(MeCN)4BF4
CuCl
PIFA
t-BuOOH
NFSI
L1
L1
L1
L2
L3
L1
L1
trace
0
91
85
38
12
NFSI
NFSI
NFSI
8
9
CuCl2
Cu(OTf)2
Cu(OAc)2
CuOAc
NFSI
NFSI
NFSI
NFSI
6
trace
7
L1
L1
L1
10
11
21
12
13
14
15
CuOAc
CuOAc
CuOAc
CuOAc
NFSI
NFSI
NFSI
NFSI
L2
L3
98
78
11
52
aReactions were conducted in 0.2 mmol scale and yields were obtained by silica
gel chromatography isolation. b The reaction time was 28 h.
TerPy
TMEDA
Besides the aniline substrates, other type of aromatic
amines was also investigated for the copper catalyzed methyl
transferring reaction. Table 3 shows that when cyclic amines 2-
methylindoline and tetrahydroquinoline were subjected to this
aReaction conditions: 1a (0.2 mmol), TMSCN (0.5 mmol), oxidant (0.32 mmol), Cu
catalyst (0.02 mmol), ligand (0.024 mmol) in DMF (1.0 mL) at r.t. for 16 h. b Yields
were obtained by1H NMR measurement with mesitylene as internal standard.
2 | J. Name., 2012, 00, 1-3
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reaction conditions, the methyl transferring reaction and the kinetic isotopic effect were also carried out. The KH/KD
cyanation smoothly proceeded and generated the -amino ratio was 2.1:1. This result suggested that C-H bond cleavage
nitriles 3a and 3b in moderate yields. We found that when was the rate-determining step. To confirm the reaction
phenmorpholine was treated under the above optimized involved radical process, TMEPO, BHT, and 1,3-
α
a
reaction conditions, the desired product 3c was obtained in dinitrobenzene were added into the reaction and the reaction
poor yield. With further screening of the reaction conditions, were completely inhibited (see Supporting Information).
we were delighted to find that L1 could promote the N-
cyanomethylation reaction in 63% yield. The cyclic aromatic
amines with an additional ketone group, such as
pharmaceutically important quinolone and benzazepine-
derivatives, were used as substrates and provided product 3d
(81% yield) and 3e (92% yield). The pyridinamines were
examined for the preparation of α-amino nitriles. When two 3-
pyridinamine derivatives were transformed into the
corresponding products 3f and 3g in 71% and 69%
respectively. Impressively, 7-aminocoumarin was able to
smoothly undergo the reaction to deliver the 3i with excellent
yield. However, aliphatic amines proved to be unsuitable
substrates affording trace amount of products.
Scheme 2 Mechanistic Investigations.
Table 3. Substrate Scope of Heterocyclesa
Based on the above experiments and literature precedents,
a plausible proposed mechanism is outlined in Scheme 3.In the
initial step, a single electron transfer between Cu(I) and DMF
CuOAc (10 mol%)
L2 (12 mol%)
Het/Ar
Het/Ar
+
TMSCN
N
N
H
1
NFSI (1.6 equiv)
DMF, r.t.
gives the radical intermediate
cleavage. Then, DMF-derived radical
ion II, which is attacked by amine to generate the isolatable
I
through C(sp3)−H bond
CN
3
I
converted to iminium
CN
CN
CN
1
N
N
N
intermediate III. Intermediate III is further converted to
iminium ion IV. Finally, the nucleophilic substitution of cyanide
ion in the similar manner of Strecker reaction leads to the
Cl
O
3a 72 %
CN
3b 74 %
CN
3c 63 %b
CN
formation of the corresponding
α
-amino nitrile 2.
N
N
H
N
R
N
LCu(OAc)
R
N
Me
N
N
Cl
Z
O
1
O
O
O
H
3d 81 %
3e 92 %
3f 71 %
LCu(OAc)
Me
H
N
CN
H
N
CN
O
CN
Me
III
Me
N
O
O
N
II
N
F
Z-X (= NFSI)
N
N
O
H
R
R
N
N
HX
N
CN
3g 63 %
3i 95 %
3h 79 %
LCu(OAc)Z
TMSCN
N
aReactions were conducted in 0.2 mmol scale and yields were obtained by silica
Me
I
gel chromatography isolation..b L1 instead of L2 as ligand.
IV
2
Scheme 3 Proposed Mechanism.
We then tried to investigate the possible mechanism of the
copper-catalyzed methyl transferring and cyanation
sequences. An amide intermediate III was isolated after
quenching the reaction mixture halfway. The amide
intermediate III showed that the aniline was coupled with DMF
before the cyanation. Moreover, treatment of the amide
intermediate III under the reaction conditions except reducing
In conclusion, we have described a Cu-catalyzed activation
of C(sp3)–H bond of DMF. The methyl group was then
transferred to anilines. After nucleophilic addition by cyanide,
the
α-amino nitriles were efficiently generated in one-pot
manner. The oxidative activation of DMF was featured with
reaction at room temperature. Moreover, the reaction
demonstrated wide functional group tolerance, broad
substrate scope, and high activity under ambient conditions.
We gratefully acknowledge the National Natural Science
Foundation of China (21572236) and the Program for
Changjiang Scholars and Innovative Research Team in
University (IRT_17R94) for financial support.
the amount of NFSI from 1.6 eq to 0.6 eq, the α-amino nitrile
2a was obtained in 95% yield. The deuterium labeling
experiments were also carried out. The reaction was
performed in DMF-d7 as solvent to confirm the resource of
CH2. The deuterium product 2a′ was obtained with the ratio of
deuterium-labelled around 93% (Scheme 2). In addition, we
observed that the reaction went on much slowly and the
product was formed in only 81% yield instead of 98% under
the standard conditions. Moreover, experiments focused on
Conflicts of interest
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Tallec, P. Uriac, S. Sinbandhit and L. Toupet, Liebigs
Annalen/Recueil, 1997, 10, 2089.
There are no conflicts to declare.
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