Synthetic exploitation of halogenated alkenes containing an electron-withdrawing group: Synthesis of α-chlorohydrazones and ketene aminals by regiospecific reactions of in situ generated imide chlorides

Article abstract of DOI:10.1016/j.tetlet.2014.02.038

A concise approach for the construction of ketene aminals and α-chlorohydrazones has been developed. It involves reactions of the regiodefined gem-dihalo nitrovinyl compound of in situ generated imide chlorides in different media with primary arylamines being dependent on the aryl groups. A range of ketene aminals and α-chlorohydrazones are obtained in good to high yields. In addition, α-aminohydrazones are prepared by using α-chlorohydrazones as the precursors.

Full text of DOI:10.1016/j.tetlet.2014.02.038

Tetrahedron Letters 55 (2014) 2085–2089
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthetic exploitation of halogenated alkenes containing
an electron-withdrawing group: synthesis of a-chlorohydrazones
and ketene aminals by regiospecific reactions of in situ generated
imide chlorides
⇑
Amac Fatih Tuyun
Department of Chemical Engineering, Faculty of Engineering and Architecture, Beykent University, Sisli-Ayazaga, 34396 Istanbul, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A concise approach for the construction of ketene aminals and a-chlorohydrazones has been developed. It
involves reactions of the regiodefined gem-dihalo nitrovinyl compound of in situ generated imide chlorides
in different media with primary arylamines being dependent on the aryl groups. A range of ketene aminals
Received 20 November 2013
Revised 22 January 2014
Accepted 12 February 2014
Available online 20 February 2014
and
by using
a
-chlorohydrazones are obtained in good to high yields. In addition,
-chlorohydrazones as the precursors.
a-aminohydrazones are prepared
a
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
Vicinal amines
Ketene aminals
Hydrazones
Electron-deficient alkenes
a
a
-Chlorohydrazones
-Aminohydrazones
Vicinal amines (ketene aminals) represent a very important
method for the preparation of ketene aminals, along with
and -aminohydrazones is still highly desirable.
a-chloro,
class of compounds in organic chemistry as they not only serve
as ligands and catalysts in asymmetric reactions, but have also
been shown to exhibit a broad spectrum of biological properties,1
representatives of which have already been patented as control
agents for animal pests.2 Among many methods, amination
reactions of simple gem-dihalo-1-alkenes, in particular those
activated by electron-withdrawing groups (EWG), offer one of
the most straightforward and practical approaches to bis
(phenylamino)chloronitroethylene (2), benzoxazole (3), indole
(4), and 1H-perimidine (5) derivatives (Scheme 1).3
a
gem-Dihalo nitrovinyl moieties are valuable synthetic tools in
organic chemistry and serve as interesting synthetic intermediates
in a variety of chemical reactions,8 mostly for the formation of car-
bon-heteroatom bonds, the most typical example probably being
thiophene synthesis, where a gem-dihalo nitrovinyl moiety is
converted into a 5-membered heterocyclic dithiolane structure,
which upon treatment with a strong base in DMSO/H2O, gives
the corresponding thiophenes.9,10 Moreover, the functionality of
these molecules (acceptor-substituted alkenes) acts as attractive
and versatile bidentate electrophiles for nucleophiles. The
presence of an EWG bonded to an alkenyl carbon renders these
compounds more reactive toward carbon, nitrogen, sulfur, and
oxygen nucleophiles.11 In 1976, Ol’dekop and Kaberdin reported
the first acceptor-substituted alkene containing a nitro group.12
Their innovative discovery has resulted in numerous further stud-
ies of this synthetic process. Since the first exhaustive account on
this chemistry by Kaberdin et al., extensive reviews13 and arti-
cles3,14 have been published on developments in this field, which
present novel derivatives of these new generation molecules.
Finally, in 1998, another new example of these molecules was
introduced by Potkin and Nechai.15 After these pioneering works,
an impressive number of subsequent papers have reported
The formation of
a-chloro and a-aminohydrazones is widely
used in chemistry,4 examples include precursors of nitrilimine di-
poles for use in cycloaddition reactions, in biology5 as a molecular
conjugation strategy for achieving ligation, attachment, and bio-
conjugation to biomolecules. For reactions involving biomolecules,
they represent a versatile approach to conjugation chemistry.6
Moreover, since certain hydrazone derivatives can be exchanged,
such linkages have gained attention in dynamic combinatorial
chemistry as well.7 They can also be considered as highly function-
alized electrophiles. Therefore, the development of an efficient
⇑
Tel.: +90 2128675248.
E-mail addresses: aftuyun@beykent.edu.tr, aftuyun@gmail.com
http://dx.doi.org/10.1016/j.tetlet.2014.02.038
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

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A. F. Tuyun / Tetrahedron Letters 55 (2014) 2085–2089
H
N
H
N
by the nitro group. The dipolar character in 2B is caused by the sig-
nificant contribution from the tautomeric structure 2C to the
ground-state structure (Scheme 2), and particularly turns out to
be an appropriate substrate.
Cl
NO2
Numerous attempts to obtain monoaminated products that
could provide straightforward entry to hydrazones by slight mod-
ifications of the reaction conditions and/or the stoichiometry of the
reactants resulted in unreacted starting materials and ketene ami-
nals in low yields. The regiospecific monoaminated product is
actually one of the most difficult to obtain, mainly because of the
extraordinarily high electrophilicity of the assumed imide chloride
intermediate. Another halogen atom located on the first carbon
must enable attack of further amines. Unfortunately, a monoami-
nated product was not formed at all. Next, the reaction scope of
the primary arylamines containing ERG groups such as methoxy,
ethoxy, butoxy, hexyloxy, and octyloxy was explored with respect
to the gem-dichloro nitroalkene 6 under mild conditions to give ke-
tene aminals 8a–e. This path, based on classical SNVin, usually re-
quires, as commented on above, the presence of an EWG group on
the alkene bond. Modifications usually allow the reaction to be dri-
ven toward the formation of ketene aminals 8a–e, resulting from
double substitutions, where two halogen atoms are formally
substituted. Eventually, it was found that this alkene diamination
process proceeded smoothly with four equivalents of a primary
arylamine containing an ERG in MeOH as the solvent, without
any additional agents (Scheme 3).
2
i
NH
NO2
Cl
NO2
Cl
Cl
Cl
NO2
Cl
ii
N
O
iv
N
5
1
3
iii
NO2
NH
N
H
4
Scheme 1. Amination reactions of gem-dihalo-1-nitroalkene 1. Reagents and
conditions: (i) 2 equiv of PhNH2, Et2O, ꢀ20 °C; (ii) 2-H2NC6H4-OH, NaOMe, MeOH;
(iii) (1) 2 equiv of PhNH2, Et2O, 5 °C, (2) EtOH, 78 °C; (iv) 1,8-diaminonaphthalene,
Et2O, 10 °C.
One of the crucial steps in the formation of ketene aminals is
the in situ formation of an imide chloride 8C from the correspond-
ing primary arylamines and gem-dihalo nitroalkene 6. The idea to
form the imide chloride 8C in situ from gem-dihalo nitroalkene 6
regioselective reactions between gem-dihalo nitroalkenes and a
variety of nucleophiles.16 The gem-dihalo nitrovinyl moiety also
represents a very attractive key unit for syntheses of heterocyclic
compounds by selection of difunctional nucleophiles and other
reagents.17 As part of our continuing efforts to explore the syn-
thetic potency of gem-dihalo nitroalkene 6 and develop novel reac-
and a primary arylamine led to the discovery of a-chlorohydraz-
ones 9a–e. During the reaction of imide chloride 8C, generated
in situ from 6 by treatment of primary arylamines (independent
of the nucleophilicity) with additional primary arylamines contain-
ing EWGs such as nitro, cyano, and trifluoromethyl, a different
reaction pathway operated instead of the aforementioned initial
N-substitution (Scheme 3).
tion pathways,
synthesis of ketene aminals or
a
straightforward method for the divergent
-chloro, and -aminohydrazones
a
a
by the reactions of primary arylamines with gem-dihalo nitroalk-
ene 6 is reported herein.
On the basis of the investigations of the reaction conditions and
substrate diversity, a possible mechanism for this reaction is
described in Scheme 4.19 At the beginning, independent of the
nucleophilicity of the primary amines, the first substitution reac-
tion always takes place at the strongly d-positive C1 of 6 leading
to the corresponding imide chloride 8C. After tautomerization of
the monoaminated intermediate I to give the reactive imide chlo-
ride II, an intramolecular nitronic acid oxygen is now apparently
more nucleophilic than the amine containing EWG of a second
molecule. Therefore, a cyclic nitrone III is formed. Upon addition
of HCl (liberated earlier) and the subsequent cycloreversion, two
intermediates, isocyanate IV and nitroso compound V are formed.
Subsequent coupling of nitroso compounds V with the second
equivalent of the aniline containing the EWG leads to the azo
compounds VI along with elimination of water. Subsequent isom-
The approach to the synthesis of ketene aminals was found to
be straightforward. Reactions of gem-dihalo nitroalkene 6 with at
least four equivalents of an aniline derivative containing an elec-
tron-releasing group (ERG) proceeded selectively with simple ami-
nation, most probably via the corresponding amidines (see
Scheme 3). Such mild conditions for these reactions are logical,
considering the activating effect of the additional acceptor, the ni-
tro group, on the alkene bond. The closest analogues are known to
react with primary arylamines in boiling benzene.18 The studied
reaction occurs apparently by a nucleophilic vinylic substitution
mechanism (SNVin), via the formation of an intermediate dipolar
ion, allowing stabilization caused by a hydrogen bond between
an oxygen atom of the nitro group and the proton at the aniline
nitrogen atom. Ketene aminals 2A are a special class of alkenes
distinguished by an electron-rich and strong double bond activated
erization gives the
a-chlorohydrazones 9a–e (Scheme 4).
R2
R2
R2
R2
R2
R2
H
N
NH
N
O
N
O
O
O
HN
HN
HN
N
N
R
R
R
O
O
2A
2B
2C
amidines
dipolar character
stabilized
ketene aminals
Scheme 2. Tautomeric structures of enamines.

A. F. Tuyun / Tetrahedron Letters 55 (2014) 2085–2089
2087
R
Cl
Cl
Cl
NO2
Cl
NH2
Cl
6
7
ERG
EWG
ERG
Cl
Cl
Cl
R = ERG
NH
R = EWG
Cl
N
NH
NO2
Cl
N
R
N
H
Cl
MeOH
Cold
THF
Hot
Cl
Cl
O2N
Cl
Cl
Cl
8a
8C
9a
, ERG = 4-OCH3
8b, ERG = 3-OCH2CH3
8c
, EWG = 3-CF3
9b, EWG = 3,5-(CF3)2
9c
, ERG = 4-OCH2(CH2)2CH3
8d, ERG = 4-OCH2(CH2)4CH3
8e
, EWG = 4-CN
9d, EWG = 4-CF3
9e
, ERG = 4-OCH2(CH2)6CH3
, EWG = 4-NO2
Scheme 3. Synthetic pathways for ketene aminals 8a–e and
a
-chlorohydrazones 9a–e.
EWG
EWG
NH2
Cl
Cl
Cl
NH
N
NO2
Cl
Cl
Cl
Cl
Cl
-HCl
NO2
Cl
NO2
Cl
EWG
Cl
Cl
Cl
8C
I
6
7
EWG
EWG
EWG
O
N
Cl
Cl
N
N
O
N
HCl
O
Cl
Cl
Cl
N
C
N
-HCl
O
Cl
Cl
OH
O
Cl
Cl
Cl
Cl
III
II
IV
V
EWG
-H2O
NH2
EWG
EWG
Cl
Cl
HN
N
N
N
Cl
Cl
Cl
Cl
Cl
Cl
9a-e
VI
Scheme 4. A plausible reaction mechanism.
The halogen atom on the first carbon in compounds 9a–e is
activated by the strong electron-withdrawing C@N group. This
feature provides the opportunity for further selective functionali-
zation at the halide group. Fortunately, reaction with two equiva-
lents of an aromatic or an aliphatic amine gave the desired
succeed at ambient temperature, and led to decomposition upon
heating to around 50 °C. The prepared library is potentially inter-
esting for both further medicinal and synthetic evaluations.
Structure determination of the products was performed on the
basis of IR, NMR, mass spectrometry, and elemental analyses. All
the hydrazone compounds (9a–e and 10a–e) showed strong
absorptions around 1600 cmꢀ1 in their IR spectra due to the C@N
group. Additionally, the IR spectra of all the compounds exhibited
a
-aminohydrazones 10a–e in good yields (Scheme 5). Attempts
to react -chlorohydrazones with various aromatic and aliphatic
thiols in basic medium (in sodium ethoxide in ethanol) did not
a

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A. F. Tuyun / Tetrahedron Letters 55 (2014) 2085–2089
EWG
EWG
R1R2NH
NH
NH
N
N
NR1R2
MeOH
Cl
Cl
Cl
Cl
Cl
Cl
Cl
1
, EWG = 4-CN, R R2 = -(CH2)2S(CH2)2-
10a
10b
9a
, EWG = 3-CF3
9b, EWG = 3,5-(CF3)2
9c
1
, EWG = 4-CN, R R2 = -(CH2)2O(CH2)2-
, EWG = 4-CN
9d, EWG = 4-CF3
10c, EWG = 3-CF3, R1 = H, R2 = 4-methoxyphenyl
10d, EWG = 3,5-(CF3)2, R1 = H, R2 = 4-methoxyphenyl
10e, EWG = 4-CF3, R1R2 = -(CH2)2S(CH2)2-
Scheme 5. Selective functionalization of a-chlorohydrazones 10a–e.
amine absorption bands at around 3300 cmꢀ1. In the electrospray
ionization (ESI) mass spectra of the products, almost all the ketene
aminals and hydrazones displayed molecular ions as base peaks.
Only compound 8e showed a different fragmentation pattern
and ionic species. In the 13C NMR spectra of ketene aminals 8a–e,
the C-1 carbon atoms appeared relatively downfield around
150 ppm, depending on the structure of the molecule, while the
NO2-bearing carbon atoms C-2 exhibited resonances at around
110 ppm. The individual C-3 and C-4 carbons of each molecule of
the library gave chemical shift values around 130 and 120 ppm,
respectively. In the 13C NMR spectra of all the hydrazone
derivatives (9a–e and 10a–e), there were signals corresponding
to the C@N carbons at around 150–155 ppm. Assignment of the
exchangeable –NH proton of hydrazone 9b was performed based
on data from D2O exchange, after the –NH peak had disappeared
or decreased in intensity.
and cold MeOH, and dried in vacuo to give ketene aminal 8a. Yield
1.968 g, 89%, mp 166–168 °C. IR (ATR)
(cmꢀ1): 3476, 3417 (NH),
m
3057 (CHaromatic), 2940 (CHaliphatic), 1640, 1612 (C@C), 1508 (NO2),
1303, 1244 (NO2), 1097, 1031. 1H NMR (500 MHz, DMSO-d6)
d = 3.69 (br s, 6H, 2OCH3), 6.81–6.84 (dd, 3J = 3.42, 12.69 Hz, 4H,
4CHaromatic), 7.04–7.07 (dd, 3J = 3.42, 12.21 Hz, 4H, 4CHaromatic),
10.03 (s, 2H, 2NH). 13C NMR (125 MHz, DMSO-d6) d = 56.03
(OCH3), 114.93, 124.77 (CHaromatic), 109.05 (C2), 131.46 (C3),
126.87, 157.59 (Caromatic), 153.75 (C1). MS (ESI+): m/z (%) 466
(100, [M+Na]+). Anal. Calcd for C18H16Cl3N3O4 (444.696) (%): C,
48.62; H, 3.63; N, 9.45. Found (%): C, 48.77; H, 3.77; N, 9.55.
General procedure for the preparation of
9a–e
a-chlorohydrazones
2,3,3-Trichloro-N0-[(3-(trifluoromethyl)phenyl)]acrylohydrazo-
noyl chloride (9a)
In conclusion, an efficient and straightforward route for
preparing synthetically useful functionalized ketene aminals and
a
-chlorohydrazones from gem-dihalo nitroalkene 6 has been
Under a nitrogen atmosphere, a mixture of gem-dihalo nitroalk-
developed. The conditions are mild and good to excellent yields
are obtained in the absence of any additives. Additionally,
ene
6
(650 mg, 2.39 mmol) and 3-(trifluoromethyl)aniline
(810 mg, 5.02 mmol) in anhydrous THF (20 mL) was heated at
45–50 °C for 4 d. After evaporation of the solvent, the residue
was dissolved in CH2Cl2 (50 mL), washed with H2O (3 ꢁ 30 mL),
and dried over CaCl2. The product was purified via column chroma-
tography on silica gel to give hydrazone 9a. Yield 0.730 g, 87%. IR
a-aminohydrazones are prepared by using a-chlorohydrazones as
the precursors. Substitution with four equivalents of a primary
arylamine such as aniline or its electron-rich derivatives leads to
the formation of ketene aminals. In contrast, during the reaction
with electron-deficient primary arylamines such as trifluoro-
methyl-, cyano-, or nitro-aniline, a different reaction pathway
operates. We believe that the resulting functionalized products
may be highly valuable for the synthesis of useful complex
molecules. These experimental findings, structures, and reaction
pathways should pave the way for possible new reactions. Since
(ATR)
m
(cmꢀ1): 3332, 3050, 1690, 1600, 1574, 1494, 1473, 1455,
1515, 1335, 1280, 1167, 1127, 1092, 1066, 929, 841, 790. 1H
NMR (500 MHz, CDCl3) d = 7.12 (d, 3J = 7.81 Hz, 1H, CHaromatic),
7.17 (d, 3J = 8.29 Hz, 1H, CHaromatic), 7.27 (s, 1H, CHaromatic), 7.29
(t, 3J = 7.81 Hz, 1H, CHaromatic), 8.06 (br s, 1H, NH). 13C NMR
(125 MHz, CDCl3) d = 117.77 (CCl2), 126.15 (CCl), 128.97 (CF3),
116.34, 129.01, 131.06 (CHaromatic), 134.79, 141.60 (Caromatic),
150.53 (C@N). MS (ESI+): m/z (%) 346 (100, [Mꢀ4H]+). Anal. Calcd
for C10H5Cl4F3N2 (351.967) (%): C, 34.12; H, 1.43; N, 7.96. Found
(%): C, 34.33; H, 1.22; N, 7.69.
the multifaceted character of the gem-dihalo nitroalkene
6
provides significant potential for the diversity-oriented synthesis
of heterocyclic frameworks, further extension of our research is
now ongoing.
General procedure for the preparation of ketene aminals 8a–e
General procedure for the preparation of
10a–e
a-aminohydrazones
3,4,4-Trichloro-N,N0-bis(4-methoxyphenyl)-2-nitrobuta-1,3-
diene-1,1-diamine (8a)
4-[(2-(2,3,3-Trichloro-1-thiomorpholinoallylidene)]hydrazinyl)-
benzonitrile (10a)
At ꢀ20 °C, a solution of 4-(methoxy)aniline (2.75 g, 22.4 mmol)
in MeOH (15 mL) was added dropwise to a solution of gem-dihalo
nitroalkene 6 (1.35 g, 5 mmol) in MeOH (15 mL) over 10 min. The
resulting mixture was kept at the same temperature for an
additional 1 h with stirring. After 5 h at room temperature and
subsequent cooling to ꢀ5 °C, concd HCl (3.0 mL) was added. The
resulting precipitate was filtered off, washed with H2O, hexane,
To a suspension of hydrazone 9c (30 mg, 0.098 mmol) in MeOH
(10 mL) was added with stirring a solution of the thiomorpholine
(21 mg, 0.206 mmol) in MeOH (5 mL) at 0 °C over 15 min. The
resulting mixture was stirred for 2 h at room temperature. After
evaporation of the solvent, the residue was dissolved in CH2Cl2
(50 mL), washed with H2O (3 ꢁ 30 mL), and dried over CaCl2. The

A. F. Tuyun / Tetrahedron Letters 55 (2014) 2085–2089
2089
5. (a) Rollas, S.; Kucukguzel, S. G. Molecules 2007, 12, 1910–1939; (b) Aslan, H. G.;
Karacan, N. Med. Chem. Res. 2013, 22, 1330–1338; (c) Kandhavelu, M.; Paturu,
L.; Mizar, A.; Mahmudov, K. T.; Kopylovich, M. N.; Karp, M.; Yli-Harja, O.;
Pombeiro, A. J. L.; Ribeiro, A. S. Pharm. Chem. J. 2012, 46, 157–164; (d)
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product was purified via column chromatography on silica gel.
Yield 0.028 g, 77%. IR (ATR)
(cmꢀ1): 3262 (NH), 3015 (CHaromatic),
m
2960, 2922, 2850 (CHaliphatic), 2215 (CN), 1609, 1597, 1421, 1408,
1519, 1334, 1261, 1167, 1100, 1019, 954, 915, 802. 1H NMR
(500 MHz, CDCl3) d = 2.64–2.73 (m, 4H, S-CH2), 3.60 (t,
3J = 5.37 Hz, 4H, N-CH2), 6.88 (m, 2H, 2CHaromatic), 6.95 (br s, 1H,
NH), 7.37–7.41 (m, 2H, 2CHaromatic). 13C NMR (125 MHz, CDCl3)
d = 25.58 (S-CH2), 47.74 (N-CH2), 117.13 (CN), 111.49, 119.15
(CHaromatic), 125.51 (CCl), 132.56 (CCl2), 100.05, 147.88 (Caromatic),
153.92 (C@N). MS (ESI+): m/z (%) 375 (100, [M+H]+), 339 (54,
[MꢀCl]+). Anal. Calcd for C14H13Cl3N4S (375.704): C, 44.76; H,
3.49; N, 14.91. Found: C, 44.99; H, 3.77; N, 15.27.
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Acknowledgment
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The author thanks the Board of Trustees of Beykent University
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Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.02.
038.
References and notes
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