Direct transformation of methyl imines to α-iminonitriles under mild and transition-metal-free conditions

Article abstract of DOI:10.1002/chem.201301933

A novel transformation of methyl imines into α-iminonitriles under mild and transition-metal-free conditions is described. Three C sp 3-H bonds are cleaved in a radical pathway at room temperature under air. Simple bromide salts are employed to assist thi

Full text of DOI:10.1002/chem.201301933

COMMUNICATION
DOI: 10.1002/chem.201301933
Direct Transformation of Methyl Imines to a-Iminonitriles under Mild and
Transition-Metal-Free Conditions
Feng Chen,[a] Xiaoqiang Huang,[a] Yuxin Cui,[a] and Ning Jiao*[a, b]
The direct transformation of simple, readily available
starting materials into highly functionalized compounds in
an atom economical and environmentally benign manner is
one of the most important strategies in chemistry. In partic-
À
ular, direct functionalization by Csp3 H bond cleavage is
very challenging due to the high bond dissociation energy
and poor selectivity.[1] Nitrogen-containing compounds are
ubiquitous in natural and bioactive products.[2] Recently, the
À
direct amination of methyl groups through C N single bond
formation has been significantly explored.[3,4] The explora-
tion of new methodologies for the incorporation of nitrogen
into simple molecules is still an extremely attractive, but
challenging issue. Nitriles are versatile building blocks in or-
ganic synthesis including pharmaceuticals, materials, dyes
Scheme 1. Strategies for direct transformations of methyl groups.
etc.[5] The methyl group is a perfect precursor for cyano
À
group construction through the cleavage of three C H
bonds, although this strategy is very challenging and is
rarely realized.[6]
Recently, we developed an approach to convert aromatic
methyl groups into cyano groups facilitated by a copper cat-
alyst (Scheme 1a).[6b] However, three drawbacks of this pro-
tocol limit its further application: 1) a heteroatom substitu-
ent at the para-position of the methyl group is required; 2)
methyl groups at heteroaromatic rings are not tolerated; 3)
methyl imine units that have been widely used as useful in-
termediates in organic reactions, such as the enantioselective
transformations through enamine catalysis,[7] do not work
under the reported conditions. Therefore, exploration of
new methods for direct conversion of methyl substrates into
nitriles with high functional group compatibility is still ex-
tremely attractive.
Scheme 2. The approach to a-iminonitriles and their applications.
icance of the present chemistry is fourfold: 1) to the best of
our knowledge, this is the first direct transformation of
methyl imines to a-iminonitriles, which are useful precursors
for many compounds such as a-ketoacids, amides, N-alkyl-
ketene-imines, cyanoenamides, and amidines (Scheme 2).[8]
Although many synthetic methods have been developed for
the preparation of a-iminonitriles, limitations such as the
use of toxic reagents, pre-installed functional groups, limited
substrate scope, and multistep processes restrict their appli-
Herein, we demonstrate a novel transformation of methyl
imines to a-iminonitriles under mild and transition-metal-
free conditions by a radical process (Scheme 1b). The signif-
[a] F. Chen, X. Huang, Prof. Dr. Y. Cui, Prof. Dr. N. Jiao
State Key Laboratory of Natural and Biomimetic Drugs
School of Pharmaceutical Sciences, Peking University
Xue Yuan Road 38, Beijing 100191 (P. R. China)
Fax : (+86)10-8280-5297
cation.[9] 2) Three Csp3 H bonds are cleaved directly during
À
this transformation. In addition, this chemistry offers an op-
À
portunity to achieve C H functionalization under transition-
E-mail: jiaoning@bjmu.edu.cn
metal-free conditions. 3) This method could be conducted
under an atmosphere of air at room temperature under mild
conditions, which makes these reactions simple to perform.
4) This nitrogenation strategy provides a new application of
methyl imines, which are easily obtained by condensation of
anilines and acetophenones (Scheme 2).
[b] Prof. Dr. N. Jiao
State Key Laboratory of Drug Research
Shanghai Institute of Materia Medica
Chinese Academy of Sciences, Shanghai 201203 (P. R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301933.
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Table 1. Screening of the reaction conditions.[a]
(2j), and hererocycles (2k) are tolerated on the ar-
omatic ring of the acetophenone. The substituent
positions did not affect the efficiency of these trans-
formations. For instance, substrates with fluoro
group at the para-, meta-, or ortho-position of the
aromatic rings of acetophenones could be converted
into corresponding products successfully with mod-
erate yields (2d, 2m, and 2n).
In addition, the heteroaryl-substituted substrate
(E)-N-(1-(furan-2-yl)ethylidene)-4-methoxyaniline
(1w) and (E)-N-(1-(benzofuran-2-yl)ethylidene)-4-
methoxyaniline (1x) also could be converted into
the desired products 2w and 2x in 72 and 32%
yields respectively. Furthermore, the reactivity of
substrate (E)-4-methoxy-N-((E)-4-phenylbut-3-en-
2-ylidene)aniline (1y) performed well to form (Z)-
N-(4-methoxyphenyl)-cinnamimidoyl cyanide (2y)
in 73% yield, which may be useful for potential
aza-Diels–Alder reactions.
ACHTUNGTRENNUNG
[N3À] [(equiv)] Additive [(equiv)] Oxidant [(equiv)] Solvent Yield[b] [%]
1
2
3
4
5
6
7
8
9
TMSN3 (2.0)
TMSN3 (2.0)
TMSN3 (2.0)
TMSN3 (2.0)
TMSN3 (2.0)
–
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.5)
PIDA (3.0)
PIDA (3.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
PIDA (2.0)
DDQ (3.0)
BQ (3.0)
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMF
DCE
CH3CN
THF
DMSO
DMSO trace
33
55
70
70
78
87
30
65
30
31
26
79
NaCl (2.0)
NaBr (2.0)
LiBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
NaBr (2.0)
TMSN3 ACHTUNGTRENNUNG(2.0)
TMSN3 (1.0)
TMSN3 (2.0)
TMSN3 (2.0)
TMSN3 (2.0)
TMSN3 (2.0)
10
11
12[c] TMSN3 (2.0)
13
14
15
NaN3 (2.0)
TMSN3 (2.0)
TMSN3 (2.0)
DMSO
DMSO
0
0
[a] Reaction conditions: 1a (0.2 mmol) with azide, additive, oxidant, and solvent
(2.0 mL) with stirring for 12 h under air. [b] Yield of the isolated product. [c] Reaction
time is 6 h. DCE=1,2-dichloroethane.
A gram-scale reaction of substrate 1a was con-
ducted under the standard conditions and produced
the desired product 2a in 84% yield (Scheme 4a),
which demonstrates the potential applications in
large scale reactions and for further transforma-
We commenced our study by investigating the reaction of
(E)-4-methoxy-N-(1-phenylethylidene)aniline (1a) with azi-
dotrimethylsilane (TMSN3) and iodobenzene diacetate
(PIDA). The desired nitrile product, (Z)-N-(4-methoxyphe-
nyl)benzimidoyl cyanide (2a), was obtained in 33% yield at
room temperature (Table 1, entry 1). No transition-metal
catalysts are required for this transformation. Encouraged
by this result, a number of experimental variables such as ni-
trogen sources, additives, oxidants, solvents, and reaction
time were screened (Table 1; also see the Supporting Infor-
mation). It is noteworthy that lithium bromide and sodium
bromide are conducive to this transformation (Table 1, en-
tries 2–4). Higher yields could be obtained when increasing
the PIDA loading (Table 1, entries 5–6). Low yields were
obtained when one equivalent of TMSN3 (Table 1, entry 7)
and other solvents such as DMF, DCE, MeCN, and THF
were used instead of DMSO (entries 8–11; DCE=1,2-di-
chloroethane). When sodium azide was employed as the ni-
trogen source, only a trace amount of product 2a was ob-
tained (Table 1, entry 13). Finally, no product was detected
when other oxidants were used instead of PIDA (Table 1,
entries 14 and 15).
With the optimized reaction conditions in hand, the scope
of the transformation was investigated (Scheme 3). A variety
of substituted methyl imines derived from substituted ani-
lines and acetophenones could be converted into the corre-
sponding a-iminonitriles easily in moderate to good yields.
In some cases, the yields were improved by reducing the
loading of PIDA from three equivalents to two equivalents.
Electron-donating groups such as Me (2b, 2o, and 2q),
Scheme 3. Direct transformation of methyl imines into a-iminonitriles.
[a] Reaction conditions: 1 (0.4 mmol) with TMSN3 (0.8 mmol), NaBr
(0.8 mmol), PIDA (1.2 mmol) and DMSO (3.0 mL) with stirring for 12 h
under air. [b] 0.8 mmol PIDA is used.
OMe (2c and 2p), electron-withdrawing groups such as F
(2d, 2m, and 2n), Cl (2e), Br (2 f), CN (2h and 2l), NO2
(2i), and sensitive functional groups such as I (2g), SO2Me
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Chem. Eur. J. 2013, 19, 11199 – 11202

Methyl Imines to a-Iminonitriles
COMMUNICATION
Scheme 4. Gram-scale reaction and one-pot synthesis of a-iminonitrile.
Scheme 5. Proposed mechanism.
tions. Since the starting methyl imines could be easily pre-
pared from the corresponding anilines and acetophenones, a
one-pot approach to a-iminonitriles was achieved (Sche-
me 4b). Condensation of aniline 3 and acetophenone 4 in
the presence of 4 ꢁ molecular sieves and the subsequent ni-
trogenation process produced 2a in 70% yield in this one-
pot reaction.
We were delight to find that 1,1,2-trimethylbenz[e]indole
(5) could be converted into the corresponding cyano prod-
uct 1,1-dimethyl-1H-benzo[e]indole-2-carbonitrile (6) in
60% isolated yield with high selectivity [Eq. (1)]. The struc-
ture was characterized by single crystal X-ray diffraction
(Figure 1).[10] Therefore, the present method could be ap-
form radical intermediate D and hydrazoic acid.[12] Subse-
quently radical intermediate D is oxidized by radical C lead-
ing to cation intermediate E through a single-electron-trans-
fer (SET)[11a,13] process. The cation E could be stabilized by
the bromide ion in the presence of NaBr. The azide ion
reacts with cation E giving intermediate F, which is probably
highly reactive and could not be detected due to its instabili-
ty. Subsequently, intermediate F is oxidized by azide radical
B to form radical intermediate G[12,14] which could be oxi-
dized by radical C leading to cation intermediate H.[11a,13] Fi-
nally, cation intermediate H undergoes a Schmidt reaction[15]
to afford a-iminonitrile 2.[11a,13]
In summary, we have demonstrated a novel and efficient
transformation of methyl imines to a-iminonitriles directly
À
under mild and transition-metal-free conditions. Three Csp3
H bonds are cleaved by a radical pathway at room tempera-
ture under air. Simple bromide salts are employed to assist
this radical process. Compared with the reported synthesis
methods, the present chemistry is simple and easily operat-
ed. Further investigations of the mechanism and the synthet-
ic applications of these reactions are ongoing in our group.
Figure 1. X-ray crystal structure of compound 6.
Experimental Section
Standard procedure for the synthesis of (Z)-N-(4-methoxyphenyl)benzi-
midoyl cyanide (2a): (E)-4-Methoxy-N-(1-phenylethylidene)aniline (1a;
0.2 mmol, 45.0 mg), azidotrimethylsilane (0.4 mmol, 48.0 mg), sodium
bromide (0.4 mmol, 41.2 mg), and iodobenzene diacetate (0.6 mmol,
192.0 mg) were stirred in DMSO (2.0 mL) at room temperature. After
12 h, water (5.0 mL) was added and extracted with ethyl acetate (3ꢂ
10 mL). The combined organic layers were dried with anhydrous magne-
sium sulfate, concentrated and purified by flash chromatography on sili-
con (eluent: petroleum ether/ethyl acetate=10:1) to afford 41.2 mg
(87%) of 2a as yellow solid.
plied to direct transformation of a-methyl groups of N-het-
erocycles into cyano groups, which are difficult to construct
by traditional methods.
A plausible mechanism for this transformation is illustrat-
ed in Scheme 5. Initially, the reaction of PhIACTHNUGTRENUNG(OAc)2 with
TMSN3 leads to PhI(N3)2 (A), which decomposes giving rad-
icals B and C.[11] Next, the hydrogen atom of the methyl
group of 1 is abstracted selectively by azide radical B to
Chem. Eur. J. 2013, 19, 11199 – 11202
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Acknowledgements
Financial support from National Basic Research Program of China (973
Program; Grant No.2009CB825300) and National Science Foundation of
China (No.21172006) are greatly appreciated. We thank Yizhi Yuan in
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À
Keywords: C H activation · nitriles · radical reactions ·
rearrangement · transition-metal free
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Received: May 20, 2013
Published online: July 12, 2013
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