Iron-catalysed sequential reaction towards α-aminonitriles from secondary amines, primary alcohols and trimethylsilyl cyanide

Article abstract of DOI:10.1039/c5cc10346k

We have developed a one-pot iron-catalysed sequential reaction of secondary amines with primary alcohols, trimethylsilyl cyanide and TBHP under mild reaction conditions to give the corresponding α-aminonitriles.

Full text of DOI:10.1039/c5cc10346k

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Iron-Catalyzed Sequential Reaction toward α-Aminonitriles from
Secondary Amines, Primary Alcohols and Trimethylsilyl Cyanide
Hang Shen,a Liangzhen Hu,a Qing Liu,a Muhammad Ijaz Hussain,a Jing Pan,a Mingming Huang,a and
Yan Xiong*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
An iron-catalyzed sequential reaction of secondary amines with are performed sequentially in a single reaction vessel without
primary alcohols, trimethylsilyl cyanide and TBHP under mild intermediary purification steps are widely employed in organic
reaction conditions to afford corresponding α-aminonitriles in synthesis due to high atom economy, minimum waste
one-pot was developed for the first time.
generation, shortened reaction time, easy handling and the
possibility for diversity-oriented synthesis.17 However,
sequential reaction with high selectivity is still a challenge in
organic reactions.17a In continuation of our efforts on oxidative
cyanation14c,18 and from the point of green chemistry, we
envisioned two possible pathways to synthesize α-
aminonitriles from secondary amines, one of which is forming
tertiary amines through alkylation of secondary amines
utilizing alcohols as alkylation reagents19 followed by oxidative
cyanation (eq 3, Scheme 2), and the other is oxidative
dehydrogenation of alcohols to carbonyl compounds followed
by Strecker-typed reaction (eq 4). In the past decades, iron and
copper derivatives attracted much attention in synthetic
methodologies due to the easy accessibility, favorable safety
profile and low cost,20 hence, we focused different iron or
copper salts with oxidants to achieve the goal after failure of
taking use of oxidants alone, and eventually got the desired
products. Herein, we report an iron-catalyzed sequential
reaction of secondary amines to the corresponding α-
aminonitriles.
α-Aminonitriles are highly useful and versatile intermediates
to construct bifunctional molecules,1 such as hydrolysis to give
α-amino acids and nucleophilic additions to access α-amino
carbonyl compounds, α-amino alcohols or vicinal diamines,
which are widely applied in synthetic as well as medicinal
chemistry. Hence, it is not surprising that a number of
methods have been developed to accomplish α-aminonitriles,
among which Strecker reaction2—a three-component
condensation of a carbonyl compound, an amine and cyanide
is a traditional approach (eq 1, Scheme 1). An alternative route
involves the oxidation of tertiary amines to iminium ions
catalyzed by metal-based catalysts, such as Ru,3 V,4 Au,5 Mo,6
Fe,7 Cu,8 Ir,9 Ti,10 Co11 and Re,12 followed by nucleophilic attack
of the cyanide (eq 2). Moreover, some metal-free methods
using tropylium ions,13 iodine-containing compound,14 Eosin
Y15 or AIBN16 have been applied for oxidative cyanation of
tertiary amines in recent years. To the best of our knowledge,
there are no reports describing oxidative cyanation of
secondary amines to give α-aminonitriles.
R4
CN
R3
N
catalyst
H
[O]
R3
R4
R3
OH
N
+
(3)
N
R1
R3
R2
CN-
N
O
R1
R2
R1
R2
Strecker Reaction
CN-
R3 NHR4
R1
R2
(1)
+
R1
R2
R5
O
H
CN-
CN
Me
[O]
N
(4)
R4
OH
+
CN
R2
Me
N
R1
R2
R4
R5
N
Oxidant, Metal Catalyst
CN-
N
CN
R1
(2)
Me
Scheme 2 Design of cyanation for synthesis of α-aminonitriles.
Scheme 1 Approaches to synthesize α-aminonitriles.
Initial studies on the cyanation reaction are performed by
using 1,2,3,4-tetrahydroquinoline as secondary amine and
trimethylsilyl cyanide (TMSCN) as cyanide source in DCE. As
depicted in Table 1, the efficiencies of various catalysts were
first examined (see supporting information for more details),
copper salts such as CuI and Cu(OAc)2 showed no activities to
the reaction (entries 1 and 2), in which CuI gave the
Sequential reaction where multiple chemical transformations
a.School of Chemistry and Chemical Engineering, Chongqing University, Chongqing
400030, China.
b.State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin
300071, China.
functionalized urea 3 (eq 5, Scheme 3), while divalent iron salts
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
like FeCl2, Fe(COO)2·2H2O, Fe(COO)2 and FeSO4 afforded
This journal is © The Royal Society of Chemistry 20xx
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O
NH2
moderate yields of the desired products (entries 3-6).
Fe(OAc)2, Fe(acac)2 and trivalent iron salt FeCl3 did not bring
about this transformation (entries 7-9), in which Fe(acac)2 led
H
N
CuI
TBHP
DCE
N
(5)
(6)
EtOH
EtOH
TMSCN
TMSCN
+
+
+
+
to the generation of cyanide
4 (eq 6). The effects of the
1a
1a
1a
1a
3, 30%
reaction time were then examined and the results showed that
reducing the time to 4 h give the best yield of 60% (entries 10-
12). Moreover, upon testing different oxidants such as tert-
H
N
H
N
Fe(acac)2
TBHP
DCE
CN
butylhydroperoxide
(TBHP,
65%
in
tert-butanol),
4, 35%
(diacetoxyiodo)benzene (DIB), Oxone, mCPBA, NIS and H2O2,
TBHP proved to be the most effective (entries 13-17 vs 11),
I
H
N
H
N
Fe(COO)2
NIS
DCE
EtOH
EtOH
TMSCN
TMSCN
(7)
(8)
+
+
+
whereas NIS promoted the reaction to afford iodide
5 (eq 7).
Solvent screening studies revealed that polar solvents
including CH3CN, DMF, DMSO, acetone and CH3NO2 did not
prompt the reaction (entries 18, 19 and Table 1S), even EtOH
only gave 14% yield (entry 20), and most of nonpolar solvents
such as DCE, DCM, CHCl3 and hexane afforded the products in
moderate yields, where toluene proved to be the best (entries
5, 80%
H
N
H
N
Fe(COO)2
TBHP
DMSO
CN
+
4, 60%
Scheme 3 Approaches to functionalized urea 3, cyanide 4 and
iodide 5.
11, 21-24). It is worthy to note that cyanide
well when using DMSO as solvent (eq 8).
4 was furnished as
Table 1 Screen of oxidants, catalysts and solventsa
Other influencing factors of the reaction such as amount of
TBHP, EtOH, TMSCN and iron salt, reaction temperature, and
additives were further investigated, with results summarized
in Table 2 (see supporting information for more details).
Increase of the amount of TBHP to 4 equiv is most favorable in
the presence of 5 equiv of EtOH, 3 equiv of TMSCN and 0.3
equiv of iron salt (entries 1 and 2), while varying the amount of
EtOH to 7.5 equiv and increasing the amount of TMSCN to 4
equiv is most suitable (entries 3-8). Reducing catalyst loading
to 10 mol % had no influences on the reaction, but catalyst
loading of 2 mol % clearly lowered catalytic efficiency (entries
9-11). Moreover, varying reaction temperature and amount of
toluene were detrimental to the reaction (entries 12-15). Since
I2/TBHP21 and TBAI/TBHP22 catalytic systems work efficiently
for numerous organic transformations, 0.2 equiv I2 and TBAI
were added to the reaction, respectively, but lowered the
yields to 61% and 0% (entries 16 and 17). The attempts to
improve the yield through addition of other additives also
failed (entries 18 and 19), and the protection of argon in dry
toluene gave the same yield of 76% (entries 20 vs 9).
Yield
(%)b
0 (30)c
0
Entry
Oxidant
Catalyst
Solvent
Time (h)
1
2
3
TBHP
TBHP
TBHP
CuI
Cu(OAc)2
FeCl2
Fe(COO)2·
2H2O
Fe(COO)2
FeSO4
Fe(OAc)2
Fe(acac)2
FeCl3
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
Fe(COO)2
DCE
DCE
DCE
10
10
10
27
4
TBHP
DCE
10
46
5
6
7
8
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
DIB
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CH3CN
DMSO
EtOH
DCM
CHCl3
toluene
hexane
10
10
10
10
10
5
4
3
4
4
4
4
4
4
4
4
4
4
49
41
0
0 (35)d
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
52
60
47
0
0
0
Table 2 Optimization of Reaction Conditionsa
Oxone
mCPBA
NIS
CN
[Fe] (specified)
0 (80)e
38
H
N
TBHP (specified)
EtOH (specified)
toluene
N
H2O2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
0
0 (60)d
14
1a
2a
TBHP
EtOH
TMSCN
[Fe]
(eq)
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.1
Yield
(%)b
47
59
56
63
30
Entry
(eq)
2
(eq)
5
(eq)
3
4
4
1
2
3
4
5
6
7
8
9
4
5
3
64
a
Reaction conditions: 1,2,3,4-tetrahydroquinoline (0.5 mmol),
4
6
3
68
oxidant (1.5 mmol), EtOH (2.5 mmol), TMSCN (1.5 mmol),
4
7
3
71
1
catalyst (0.15 mmol), solvent (4 mL), rt. b Yield determined by H
4
7.5
8
3
73
NMR using mesitylene as internal standard. c Yield of 3. d Yield of
4. e Yield of 5. DCE = 1,2-dichloroethane.
4
3
73
4
7.5
7.5
7.5
2
71
4
4
76
76 (70)c
4
4
2 | J. Name., 2012, 00, 1-3
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10
11
12
13
14
15
16
17
18
19
20
4
4
4
4
4
4
4
4
4
4
4
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
7.5
4
4
4
4
4
4
4
4
4
4
4
0.05
70
53
0.02
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
48d
70e
64f
74g
61h
0i
2c, 94%
2a, 70%
2b, 69%
2d, 76%b
(dr = 2:1)
60j
0k
76l
2e, 60%
2i, 44%
2f, 42%
2j, 51%
2g, 45%
2h, 30%b
2l, 39%
a
Reaction conditions: 1,2,3,4-tetrahydroquinoline (0.5 mmol),
oxidant (specified), EtOH (specified), TMSCN (specified), catalyst
b
1
(specified), solvent (4 mL), 4 h, rt. Yield determined by H NMR
c
d
e
2k, 44%
using mesitylene as internal standard. Isolated yield. 60 °C. 10
f
g
h
i
°C. 5 mL of toluene. 3 mL of toluene. 0.2 eq I2 was added. 0.2
j
k
eq TBAI was added. 0.2 eq HOAc was added. 0.2 eq Et3N was
l
added. In dry toluene under Ar. TBAI = tetrabutylammonium
iodide.
2m, 26%
2q, 39%b
2n 21%b
2r, 60%b
2o, 49%b
2s, 36%b
2p, 30%b
2t, 38%
With the optimal conditions for oxidative cyanation in hand,
the scope of the reaction was investigated. As shown in Table
3, cyclic amines such as tetrahydroquinoline, indoline, and
their derivatives bearing methyl and nitro on rings, as well as
acyclic secondary amines bearing methyl or ethyl on nitrogen
can be subjected to this oxidative cyanation reaction with
ethanol to give the corresponding products in moderate to
excellent yields (2a-f), in which substitutions of methyl and
nitro on rings were benefit to the reaction. The cyanation
reaction with methanol can also be applied to cyclic and acyclic
secondary amines bearing different substituents, but yields
were moderate (2g-s), in which strong electron-withdrawing
group trifluoromethyl substituted substrate 2h gave the yield
of 30%. Even substrates having big steric hindrance like 2m
and 2n afforded the cyanation products in 26% and 21%
isolated yields, respectively. For electron-withdrawing group
on phenyl ring, meta-substituent was found to be more
favorable than ortho-substituent (2o and 2p), but for electron-
donating group, ortho-substituent was the most favorable (2q-
2u, 62%
2v, 18%
2w, 18%
(dr = 2:1)
a Reaction conditions: secondary amine (0.5 mmol), TBHP (2
mmol), alcohol (3.75 mmol), TMSCN (2 mmol), Fe(COO)2
b
(0.05 mmol), toluene (4 mL), 4 h, rt. Isolated yield. 90
for overnight.
°C
In order to clarify the mechanism, several control experiments
were conducted (Scheme 4). The addition of radical inhibitor
TEMPO to the reaction mixture resulted in lowering the yield
(39%) of 2a, suggesting the possibility of a radical intermediate.
Moreover, the reaction of N,N-dimethylaniline under standard
reaction conditions without CH3OH did not afford α-
aminonitriles, which excluded the pathway through tertiary
amines. Attempts in spectroscopic detection of the
intermediate benzaldehyde through oxidation of benzyl
alcohol under the conditions of 10 mol % Fe(COO)2 and 3 equiv
of TBHP succeeded. The 1H NMR chemical shift of aldehyde
group of benzaldehyde in reaction mixture was the same as
reported data. Based on our experimental results, a plausible
mechanistic pathway for the synthesis of α-aminonitriles is
proposed (Figure 1). Firstly, α-hydroxyalkyl radical is generated
via α-H atom abstraction from the corresponding alcohol by t-
butoxyl radical which is formed through reduction of the
hydroperoxide with Fe(II) ion. Since α-hydroxyalkyl radical has
s). It is worthy to note that cyanation of alkyl on nitrogen did
not take place under the reaction conditions, which provides a
selective method towards cyanation products. Primary amines
like aniline and hexylamine did not undergo the reaction.
Diarylamines such as diphenylamine and secondary alkyl
amines like piperidine and N-ethyl-2-phenylethanamine gave
complex mixture. The scope of alcohols were further
examined, propanol, butanol and aromatic alcohol, namely
benzyl alcohol, can be converted to the desired products in
18%-62% yields (2t-w), in which the steric effect seems to play
an important role as well, even more than that, secondary
alcohols such as isopropanol and cyclohexanol were inert to
the reaction.
Table 3 Substrate scopea
a
strong nucleophilic character23 to determine its fast
oxidation to aldehyde by Fe(III) ion, the formed aldehyde
reacts with secondary amine to give the iminium ion
intermediate which is attacked by cyanide ion to furnish
1
2.
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Figure 1 Proposed mechanism.
In conclusion, we have developed a novel iron-catalyzed
sequential reaction of secondary amines to access the
corresponding α-aminonitriles under mild reaction conditions,
which provides a useful tool for the synthesis of various
nitrogen-containing compounds. Research on synthetic utility
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We gratefully acknowledge funding from the National Natural
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