Cyanoacetic Acid as a Masked Electrophile: Transition-Metal-Free Cyanomethylation of Amines and Carboxylic Acids

Article abstract of DOI:10.1002/chem.201502733

Using cyanoacetic acid as a masked electrophile, a new cyanomethylation reaction of amines and carboxylic acids was developed, producing a variety of α-aminonitriles and cyanomethyl esters with good yields and excellent functionality tolerance. This protocol features simple manipulation, inexpensive reagents, and a wide substrate scope. Iodoacetonitrile was generated in situ from the iodination-decarboxylation of cyanoacetic acid in this transformation.

Full text of DOI:10.1002/chem.201502733

DOI: 10.1002/chem.201502733
Full Paper
&
Synthetic Methods
Cyanoacetic Acid as a Masked Electrophile: Transition-Metal-Free
Cyanomethylation of Amines and Carboxylic Acids
Hongxiang Wang,[a] Ying Shao,*[a, b] Hao Zheng,[b] Hanghang Wang,[a] Jiang Cheng,[b] and
Xiaobing Wan*[a]
Abstract: Using cyanoacetic acid as a masked electrophile,
a new cyanomethylation reaction of amines and carboxylic
acids was developed, producing a variety of a-aminonitriles
and cyanomethyl esters with good yields and excellent func-
tionality tolerance. This protocol features simple manipula-
tion, inexpensive reagents, and a wide substrate scope. Io-
doacetonitrile was generated in situ from the iodination–de-
carboxylation of cyanoacetic acid in this transformation.
Introduction
Cyano groups are commonly found in a variety of natural
products and pharmaceutically active molecules, such as safra-
mycin A and phthalascidin.[1] Additionally, the cyano group can
be further transformed into various functional groups, such as
amino, carboxyl, aldehyde, and ester groups.[2] Therefore,
methods to introduce the cyano group into an organic com-
pound have been a topic of great interest to chemists. In
recent years, following the development of the cyanomethyl-
ation strategy, a series of cyanomethylation agents have been
exploited for use in this method. Cyanomethylation agents in-
clude acetonitrile and acetonitrile derivatives,[3] trimethylsilyla-
cetonitrile (TMSAN),[4] halideacetonitrile,[5] and isoxazole,[2b,6]
which provide new possibilities for the introduction of cyano
groups into organic compounds. However, these methods still
suffer from some inherentlimitations, such as harsh conditions
or a narrow scope of substrates.
Recently, Liu[7a,c] and Tunge[7h,i] et al. developed cyanomethy-
lation reactions by decarboxylation of a-cyanoacetic acids and
its derivatives.[7] This transition-metal-catalyzed reaction gener-
ates metal nucleophiles, which can then react with a variety of
electrophilic reagents, such as aryl halides,[7a,b] aryl triflates,[7a]
benzyl electrophiles,[7c] aldehydes,[7d] imines,[7d,e] and isatin[7f,g]
to construct cyano compounds (Scheme 1a). In sharp contrast
with previous results, cyanoacetic acid is envisioned as
a masked electrophile herein, and used to react with nucleo-
Scheme 1. Cyanomethylation reactions using a-cyanoacetic acids.
philes using Bu4NI as a catalyst and tBuOOH as an oxidant
(Scheme 1b).[8] Mechanistically, iodoacetonitrile is generated in
situ from the oxidative decarboxylation of cyanoacetic acid in
this transformation. This methodology not only provides
a metal-free and practical method to construct a-aminonitriles
and cyanomethyl esters, but also expands the scope of the
transition-metal-catalyzed decarboxylative coupling reactions,
previously studied systematically by Goossen,[9] Myers,[10]
Tunge,[11] and others[12]
.
Results and Discussion
The initial experiment was performed with N-methyl aniline
(1a) and 2-cyanoacetic acid (2) in the presence of Bu4NI
(20 mol%), tert-butyl hydroperoxide (TBHP, 2.2 equiv, 70%
aqueous solution) and NaOAc (2.0 equiv) in water (2.0 mL) at
808C for 12 h (Table 1). Gratifyingly, under such condition,
a high yield of a-aminonitrile (3a; 53%) was obtained (Table 1,
entry 15). No target product was obtained when CH3CN was
used instead of 2 for this transformation. Different oxidants
strongly influenced the reaction, for example, when the reac-
tion was carried out using H2O2, di-tert-butyl peroxide (DTBP),
Oxone, meta-chloroperbenzoic acid (m-CPBA), and K2S2O8 as
oxidants, the desired product (3a) was not detected in signifi-
cant amounts (entries 1–5). Additionally, when Bu4NI was re-
placed with other catalysts, such as I2, N-iodosuccinimide (NIS),
KI, tetrabutylammonium bromide (TBAB), or tetrabutylammoni-
[a] H. Wang, Prof. Dr. Y. Shao, H. Wang, Prof. Dr. X. Wan
Key Laboratory of Organic Synthesis of Jiangsu Province
College of Chemistry, Chemical Engineering and Materials Science
Soochow (Suzhou) University, Suzhou 215123 (P.R. China)
E-mail: shaoying810724@163.com
wanxb@suda.edu.cn
[b] Prof. Dr. Y. Shao, H. Zheng, Prof. Dr. J. Cheng
Jiangsu Key Laboratory of Advanced Catalytic Materials and
Technology Department, Changzhou University
Changzhou, 213164 (P.R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502733.
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Table 1. Optimization of reaction conditions.[a]
Table 2. Scope of secondary and tertiary amines.[a]
Entry
Catalyst
Oxidant
Yield 3a [%][b]
1
2
3
4
5
6
7
8
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
–
I2
NIS
KI
TBAB
TBAC
TBAI
TBAI
TBAI
H2O2
<5
<5
<5
<5
<5
<5
<5
45
20
26
<5
<5
84
DTBP
Oxone
m-CPBA
K2S2O8
–
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
9
10
11
12
13
14[c]
15[d]
6
53
[a] 1a (0.5 mmol), 2 (1.0 mmol), NaOAc (1.0 mmol), catalyst (0.075 mmol),
oxidant (2.2 equiv) in THF (1.0 mL) and H2O (1.0 mL) at 808C for 12 h.
[b] Isolated yield. [c] No NaOAc. [d] Bu4NI (20 mol%) in H2O (2.0 mL). TBAI
= tetrabutylammonium iodide; TBAB = tetrabutylammonium bromide;
TBAC = tetrabutylammonium chloride; NIS = N-iodosuccinimide; DTBP
= di-tert-butyl peroxide; m-CPBA = meta-chloroperbenzoic acid; TBHP =
tert-butyl hydroperoxide.
um chloride (TBAC), the yield of 3a decreased dramatically (en-
tries 8–12,). Following extensive screenings, it was found that
the combination of Bu4NI (15 mol%), TBHP (2.2 equiv, 70%
aqueous solution), and NaOAc (2 equiv) in H2O/THF at 808C for
12 h afforded 3a in the best yield of up to 84% (entry 13). a-
Aminonitriles exist in a wide range of natural products, syn-
thetic molecule drugs, dyes, and pesticides.[13] Traditionally,
MCN or HCN,[14,15] trimethylsilyl cyanide (TMSCN),[16] ethyl car-
bonocyanidate,[17] and halideacetonitrile[5] have been used to
synthesize a-aminonitriles in recent years. Notably, a nucleo-
philic attack of a-aminoalkyl radicals on the carbon–nitrogen
triple bond of PhCOCN to construct a-aminonitriles was well-
developed in our recent work.[8g] Compared with our previous
synthetic strategy to a-aminonitriles, the reaction reported
herein offers significant advantages, such as performance
under air and metal-free conditions, through decarboxylative
of a-cyanoacetic acid.
[a] 1 (0.5 mmol), 2 (1.0 mmol), NaOAc (1.0 mmol), catalyst (0.075 mmol),
oxidant (2.2 equiv) in THF (1.0 mL) and H2O (1.0 mL) at 808C for 12 h.
[b] 22 h. [c] Bu4NI (20 mol%). [d] N,N,4-trimethylaniline as a substrate.
[e] Bu4NI (20 mol%), H2O (2.0 mL), 110 8C, 12 h.
ing groups, presumably due to better nucleophilicity. This
transformation was also successfully applied to a broad range
of amines bearing various functional groups, such as allyl (3c),
benzyl (3d), alkyl, and halide groups. To our delight, it was
found that several aliphatic amines performed well in this reac-
tion, providing the desired a-aminonitriles in good yields (3p,
3r–t).
To highlight the diversity of this transformation, some pri-
mary amines were subjected to the standard reaction condi-
tions. The results are presented in Table 3. Anilines with a para-
substituent on the aromatic ring were converted to the corre-
sponding a-aminonitriles efficiently. A variety of functional
groups, including halide (3v, 3w), alkoxyl (3x), and alkyl (3y,
3z), were well tolerated. Futhermore, anilines with ortho- and
meta- substituents provided the desired products in moderate
Having established the optimized reaction conditions, a vari-
ety of substituted amines were subjected to this reaction. As
shown in Table 2, N-alkyl-substituted aromatic amines deliv-
ered the corresponding products (3b–g, 3q) with moderate to
good yields. N-methyl anilines with ortho-, meta-, or para- sub-
stituents on the aromatic ring were converted to the corre-
sponding products efficiently (3i–n). Furthermore, considering
secondary amines could be generated from tertiary amines in
situ,[8f,18] two tertiary amines, N,N,4-trimethylaniline and 4-
bromo-N,N-dimethylaniline, were also tested and the corre-
sponding products, 3m and 3o, were furnished in moderate
yields. Generally, aromatic amines bearing electron-donating
groups gave better yields than those with electron-withdraw-
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Table 3. Scope of primary amines.[a]
Table 4. Scope of carboxylic acids.[a]
[a] 1 (0.5 mmol), 2 (1.0 mmol), NaOAc (1.0 mmol), catalyst (0.075 mmol),
oxidant (2.2 equiv) in THF (1.0 mL) and H2O (1.0 mL) at 808C for 12 h.
yields (3aa, 3ab, 3ac). Unfortunately, when primary aliphatic
amines were employed in the standard reaction conditions, no
desired products were isolated.
It is worth noting that when other nucleophiles, such as
benzoic acid, were employed in the standard reaction condi-
tions, the corresponding cyanomethyl benzoates were ob-
tained in good yields (Table 4). A variety of functional groups,
including ester (5s), halide (5b, 5c, 5d, 5h, 5i, 5m, 5n), alkyl
(5g, 5j, 5o, 5q), alkoxyl (5p), nitro (5 f), cyano (5e, 5k), and
trifluoromethyl (5l), were well tolerated under optimized con-
ditions. The steric effects of the aryl carboxylic acids had no
significant effect on reactivity; the corresponding cyanomethyl
esters were produced in excellent yields (5m, 5n). Heteroar-
enes, such as thiophene-2-carboxylic acid and 1-methyl-1H-in-
dazole-3-carboxylic acid, also underwent cyanomethylation to
give the corresponding cyanomethyl esters (5t, 5v) in good
yields. It was found that aliphatic carboxylic acids were also
suitable for this transformation; the desired cyanomethyl 3-
phenylpropanoate (5u), cyanomethyl 2-(naphthalen-1-yl)ace-
tate (5w), cyanomethyl dodecanoate (5y), and cyanomethyl
palmitate (5z) were furnished in moderate yields under stan-
dard conditions. When aromatic rings contained ordinary elec-
tron-withdrawing groups, such as fluorine (5b, 5h), ester (5s),
and trifluoromethyl (5l), the relevant cyanomethyl esters were
achieved in good yields. However, when the aromatic rings
bore strong electron-withdrawing groups, for example, nitro
(5 f) and cyano (5e, 5k), the yield of the obtained correspond-
ing products decreased.
[a] 4 (0.5 mmol), 2 (1.0 mmol), NaOAc (1.0 mmol), catalyst (0.075 mmol),
oxidant (2.2 equiv) in THF (1.0 mL) and H2O (1.0 mL) at 808C for 12 h.
To explore the reaction mechanism for this cyanomethyla-
tion process, some experiments were carried out (Scheme 2).
Both 2-cyano-2-iodoacetic acid (6) and 2-iodoacetonitrile (7)
could be detected in this catalytic system (see the Supporting
Information for details), which suggested that 2-cyanoacetic
acid (2) underwent sequential iodination and a decarboxylation
process [Eq. (1)]. Further control experiments were implement-
ed to explore the details of the mechanism for this reaction.
The use of 7 as the reaction partner resulted in 3a in 77%
yield [Eq. (2)]. Additionally, product 3a was obtained in 99%
yield by the direct nucleophilic substitution of N-methyl aniline
(1a) to 7 [Eq. (3)]. On the other hand, when TEMPO (2,2,6,6-
tetra-methyl-1-piperidinyloxy), a well-known radical-trapping
reagent, was added to the reaction, 3a was isolated in 54%
yield [Eq. (4)], thus indicating that this transformation may not
be a radical process.
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methyl esters. This method utilizes easily available precursors
and enjoys a broad substrate scope as well as functionality tol-
erance. Owing to its metal-free nature, this reaction particularly
satisfies the purity standards required by biological and medic-
inal chemistry.
Experimental Section
General information
All manipulations were carried out under air atmosphere. tert-Butyl
hydroperoxide (70% solution in water) and tetrabutylammonium
iodide were purchased from Acros Organics and used without fur-
ther purification. Column chromatography was generally per-
formed on silica gel (300–400 mesh) and reactions were monitored
by thin layer chromatography (TLC) using UV light to visualize the
1
course of the reactions. The H (400 MHz) and 13C NMR (100 MHz)
data were recorded on Bruker 400m spectrometers using CDCl3 as
solvent. The chemical shifts (d) are reported in ppm and coupling
constants (J) in Hz. 1H NMR and 13C NMR spectra were recorded
with tetramethylsilane (d=0.00 ppm) and CDCl3 (d=77.00 ppm) as
internal references, respectively.
Scheme 2. Control experiments for mechanistic insights.
General procedures for reactions
Tetrabutylammonium iodide (0.075 mmol), sodium acetate
(1.0 mmol), and 2-cyanoacetic acid (1.0 mmol) were added to
a test tube. N-Methyl aniline or benzoic acid (0.5 mmol), water
(1.0 mL), tetrahydrofuran (THF, 1.0 mL), and TBHP (1.1 mmol,
0.15 mL, 70% solution in water) were added in sequence. A bal-
loon was attached to the test tube to avoid solvent evaporation.
The reaction mixture was stirred at 808C for 12 h. It was then
quenched (consumption of residual TBHP) with saturated Na2SO3
solution. Removal of solvent, followed by flash column chromato-
graphic purification using petroleum ether and ethyl acetate, af-
forded the desired products.
Scheme 3. Proposed catalytic cycle.
Acknowledgements
We gratefully acknowledge the Priority Academic Program De-
velopment of Jiangsu Higher Education Institutions (PAPD) and
the Natural Science Foundation of China (No. 21272165;
21302015; 21572148).
On the basis of these findings and previous results,[19] a plau-
sible mechanistic pathway is proposed in Scheme 3. The reac-
tion begins with the homolytic decomposition of TBHP to hy-
poiodite, promoted by tetrabutylammonium iodide (TBAI). The
iodination of 2-cyanoacetic acid (2) generates the intermediate
2-cyano-2-iodoacetic acid (6), which following the decarboxyla-
tion gives 2-iodoacetonitrile (7). Finally, the nucleophilic substi-
tution of amines and carboxylic acids leads to the formation of
the corresponding products. The role of NaOAc is not clear.
We presume that NaOAc as a base might be promoting the
decarboxylation process and the subsequent nucleophilic sub-
stitution.
Keywords: a-aminonitriles
decarboxylation iodination
nucleophilic substitution
·
cyanomethylation
·
·
·
·
metal-free synthesis
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Received: July 12, 2015
Revised: September 24, 2015
Published online on October 29, 2015
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