Structure and tautomerism of azo coupling products from N-alkylenaminones derived from acetylacetone and benzoylacetone in solid phase and in solution

Article abstract of DOI:10.1039/b613346k

A series of azo coupling products have been prepared by reaction of substituted benzenediazonium tetrafluoroborates with N-alkyl 4-aminopent-3-en-2-ones or 3-amino-1-phenylbut-2-en-1-ones. The structure and tautomerism of the reaction products were studied by means of single-crystal X-ray study and by NMR spectroscopy in deuteriochloroform solution. The azo coupling products obtained from 4-methylaminopent-3-en-2-ones (3a-i) exist in CDCl₃ solution as E/Z isomer mixtures with the Z isomer strongly predominating. The major isomer is a mixture of enamino-azo and imino-hydrazo tautomers with the former predominating. The proportion of the azo form depends on substitution of the benzene ring of the diazonium salt and decreases in the order of MeO > Me > Br > NO₂. The position of tautomeric equilibrium is practically unaffected by switching from 4-methylaminopent-3-en- 2-ones to 3-methylamino-1-phenylbut-2-en-1-ones. In the solid phase, the azo form always predominates; substitution of diazonium salt and at N3 nitrogen does not significantly affect the position of the tautomeric equilibrium. The azo coupling products always exist in the form of a single Z isomer. All determined structures, in the solid state, consist of a mixture of the two tautomeric forms, amino-diazenyl and imino-hydrazone, in ratios ranging from 82/18 to 91/9%. The weighed superimposition of both the hydrogen-bonded N1=N2-C1=C2-N3H/ HN1-N2=C1-C2=N3 heterodienic fragments, however, do not allow to clarify the effects of the para-substituents at the N1-phenyl ring both on the N1...N3 hydrogen-bond distances and on the bond lengths in the heterodienic systems within the series 3a-c,f and 4a-c,e,f. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b613346k

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Structure and tautomerism of azo coupling products from
N-alkylenaminones derived from acetylacetone and benzoylacetone
in solid phase and in solutionwz
a
a
b
b
ˇ
˚
´
´
Petr Simunek,* Marketa Svobodova, Valerio Bertolasi, Loretta Pretto,
c
a
ˇ ˇ
Antonın Lycka and Vladimır Machacek
´ ´
´
Received (in Durham, UK) 15th September 2006, Accepted 9th January 2007
First published as an Advance Article on the web 7th February 2007
DOI: 10.1039/b613346k
A series of azo coupling products have been prepared by reaction of substituted
benzenediazonium tetrafluoroborates with N-alkyl 4-aminopent-3-en-2-ones or 3-amino-1-
phenylbut-2-en-1-ones. The structure and tautomerism of the reaction products were studied by
means of single-crystal X-ray study and by NMR spectroscopy in deuteriochloroform solution.
The azo coupling products obtained from 4-methylaminopent-3-en-2-ones (3a–i) exist in CDCl3
solution as E/Z isomer mixtures with the Z isomer strongly predominating. The major isomer is a
mixture of enamino–azo and imino–hydrazo tautomers with the former predominating. The
proportion of the azo form depends on substitution of the benzene ring of the diazonium salt and
decreases in the order of MeO 4 Me 4 Br 4 NO2. The position of tautomeric equilibrium is
practically unaffected by switching from 4-methylaminopent-3-en-2-ones to 3-methylamino-1-
phenylbut-2-en-1-ones. In the solid phase, the azo form always predominates; substitution of
diazonium salt and at N3 nitrogen does not significantly affect the position of the tautomeric
equilibrium. The azo coupling products always exist in the form of a single Z isomer. All
determined structures, in the solid state, consist of a mixture of the two tautomeric forms,
amino–diazenyl and imino–hydrazone, in ratios ranging from 82/18 to 91/9%. The weighed
superimposition of both the hydrogen-bonded N1QN2–C1QC2–N3H/ HN1–N2QC1–C2QN3
heterodienic fragments, however, do not allow to clarify the effects of the para-substituents at the
N1-phenyl ring both on the N1ꢀ ꢀ ꢀN3 hydrogen-bond distances and on the bond lengths in the
heterodienic systems within the series 3a–c,f and 4a–c,e,f.
In the case of enaminones derived from acetylacetone we
Introduction
found3,4 that substitution of the proton on the nitrogen by an
aryl group causes a very distinct shift in tautomeric equili-
brium in favour of the hydrazone form, the substitution of
diazonium salt having virtually no effect upon the position of
tautomeric equilibrium.4 Transition to the solid phase did not
cause any significant shift in the position of the azo–hydrazone
tautomeric equilibrium.8 On the other hand, if this enaminone
possesses a primary amino group, then the position of tauto-
meric equilibrium shifts in favour of the azo form.4 The
position of tautomeric equilibrium in this case somewhat
depended on the substitution of diazonium salt. Electron-
acceptor substituents caused a stronger increase in proportion
of hydrazone form than electron-donors did.4,6 Transition
from deuteriochloroform solution to the solid phase brought
about a certain shift of tautomeric equilibrium in favour of the
azo form, and the effect of substituents was virtually negligi-
ble.6 Another difference between the solid phase and deuterio-
chloroform solution was in the fact that in solution the azo
coupling product from 4-aminopent-3-en-2-one represented a
mixture of E and Z isomers differing in the type of intramo-
lecular hydrogen bond. The predominant Z isomer forms an
intramolecular hydrogen bond of N–Hꢀ ꢀ ꢀN type between
nitrogen atoms of amino group and azo group, while the
Enaminones are compounds widely used in organic synthesis,
above all as precursors in syntheses of heterocyclic com-
pounds.1,2 They react easily with diazonium salts.3 The pro-
ducts obtained are rewarding subjects of structural studies
because of several potential tautomeric systems present in a
single molecule (Scheme 1). Our previous papers dealt with the
study of structure and tautomerism of azo coupling products
obtained from several types of enaminones.4–7a Azo coupling
reactions involving enaminones were also studied by the group
of Elnagdi.7b–d Due to hydrolysis, however, only the products
of azo coupling with 1,3-dicarbonyl compounds were isolated.
a
´
University of Pardubice, Department of Organic Chemistry, Nam.
ˇ
Cs. Legiı 565, CZ-532 10, Czech Republic. E-mail:
´
petr.simunek@upce.cz; Fax: +420 466 037 068; Tel: +420 466 037
039
b
c
`
Universita di Ferrara, Dipartimento di Chimica, Centro di
Strutturistica Diffrattometrica, Via L. Borsari 46, I-441 00 Ferrara,
Italy
´
Research Institute of Organic Syntheses, Rybitvı 296CZ 532 18,
Pardubice, 20, Czech Republic
w Dedicated to Prof. Stefan Toma in occasion of his 70th birthday.
z Electronic supplementary information (ESI) available: ORTEP
views of compounds 3c,f and 4a–c,e,f. See DOI: 10.1039/b613346k
ꢁc
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Scheme 1
minor isomer E forms the intramolecular hydrogen bond of
CQOꢀ ꢀ ꢀH–N type between the oxygen atom of carbonyl
group and the proton on the nitrogen atom of the amino
group.4 In the solid phase the presence of isomer Z was only
observed.6 The other hydrogen atom of the amino group takes
part in an intermolecular hydrogen bond of N–Hꢀ ꢀ ꢀOQC type
with an oxygen atom of a carbonyl group of a neighbouring
molecule.6 The effect of the introduction of a methyl group on
the amino group of 4-aminopent-3-en-2-one has so far been
studied only on the azo coupling product of benzenediazo-
nium salt and 4-methylaminopent-3-en-2-one.4 The results
clearly showed that the given compound in CDCl3 solution
resembles the derivative with the primary amino group, i.e. its
azo form is predominating even though its proportion is
somewhat lower as compared with that in the derivative with
a primary amino group. Again the compound exists in the
form of a mixture of E and Z isomers. However, the sub-
stituent effect upon the position of tautomeric equilibrium was
not studied in the case of these compounds, and neither was
their structure in the solid phase. The aim of this work is to
describe the structure of products obtained from substituted
benzenediazonium tetrafluoroborates and 4-alkylaminopent-
3-en-2-ones or 3-amino-1-phenylbut-2-en-1-ones depending
on the type of enaminone, substitution of diazonium salt,
and N-alkyl group of the enaminone.
Scheme 2
varied from 1 : 9 to 1 : 16, while in the case of nitro derivative
3b, only one isomer was observed in CDCl3 solution. The
major form Z is a mixture of azo and hydrazone tautomers,
the content of azo form being decreased by substituents at the
4-position of the diazonium ion in the order of OCH3 4 CH3
4 Br 4 NO2. Only limited amount of NMR data could be
obtained about the minor form (the chemical shifts of NH
proton and N3 nitrogen—for numbering of nitrogen atoms see
Schemes 3 and 4). The d(15NCH3) values of the minor form of
azo coupling products 3a and 3c are close to d(15N) of the
starting enaminone 1a (–280.6 ppm).4 The enaminones derived
from acetylacetone in CDCl3 solution exist practically exclu-
sively in their enamino form,9 wherefrom it can be deduced
that the minor form of azo coupling product possesses an
enamino–azo arrangement in CDCl3 solution. Comparison of
Results and discussion
Synthesis of the starting enaminones and the azo coupling
products
The starting enaminones, except for 2d, were synthesised by
reaction of the respective b-dicarbonyl compound with pri-
mary amines. In the case of 2d this direct method failed, and
the substance was prepared via the more reactive 3-chloro-
1-phenylbut-2-en-1-one (Scheme 2).
The azo coupling products were prepared by reaction of the
b-enaminone with the corresponding benzenediazonium tetra-
fluoroborate in the presence of sodium acetate. The solvent
used in the case of acetylacetone derivatives was diisopropyl
ether (except for 3g and 3h, in which case dichloromethane
was used). Due to insolubility of benzoyl acetone derivatives in
diisopropyl ether we also used dichloromethane in these cases.
Structure of azo coupling products in deuteriochloroform
solution
The reaction products from substituted benzenediazonium
tetrafluoroborates and 4-methylaminopent-3-en-2-one exist
in deuteriochloroform solution as mixtures of E and Z isomers
(Scheme 3). The proportion of isomers in products 3a,c,d
Scheme 3
ꢁc
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Table 1 1H NMR spectra of the compounds 3a–i in CDCl3
H-1
H-5
H-7
H-8
H-9
NR
X
NH
3a
2.39 s
2.30 s
7.29–7.31 br m
7.02–7.03 m
2.83 br s
2.21 s
14.64 br s
12.06 br s
15.21 br s
14.58 br s
14.85 br s
12.31 br s
15.57 br s
12.61 br s
15.65 br s
12.64 br s
15.32 br s
12.64 br s
15.61 br s
12.78 br s
14.92 br s
12.46 br s
3b
3c
3d
2.51 s
2.54 s
2.52 s
2.52 s
2.52 s
2.50 s
7.51–7.55 m
7.51–7.53 br m
7.46–7.48 m
8.18–8.22 m
6.89–6.94 m
7.38–7.40 m
3.24 s
3.09 s
3.12 br s
3.82 s
2.29 s
a
b
c
d
e
3e
3f
3g
3h
3i
a
2.51 s
2.50 s
2.61 s
2.56 s
2.60 s
2.50 s
2.47 s
2.58 s
2.59 s
2.51 s
7.19–7.21 m
7.09–7.10 m
7.27–7.38 br m
7.35–7.44 br m
7.37–7.45 br m
7.35–7.40 br m
7.04–7.12 m
6.91–6.92 br m
7.20–7.21 br m
3.83 s
3.79 s
6.81–6.91 m
b
c
NBn group: 4.58 br d 2H J = 2.23 Hz; 7.30–7.32 m 3H; 7.35–7.38 m 2H. NBn group: 4.56 br s 2H, 7.27–7.38 m, 5H. NBn group: 4.68 s 2H,
d
7.37–7.45 m 5H. NBn group: 4.63 br s 2H, 7.35–7.44 br m 5H. 2,4-diMeOBn group: 3.80 s 6H, 4.51 br s 2H, 6.45–6.48 m 2H, 7.12 d 1H
e
J = 8.2 Hz.
Table 2 13C chemical shifts of the compounds 3a–i in CDCl3
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
C-9
NR
X
3a
3b
3c
3d
3e
3f
3g
3h
3i
27.70
27.94
28.14
27.92
27.56
27.45
27.61
27.91
28.15
197.72
197.65
198.18
198.01
198.17
198.29
198.07
198.13
198.30
128.93
131.93
128.56
129.61
130.21
130.94
130.49
128.96
128.46
162.01
165.36
161.20
162.80
162.33
163.15
162.54
160.57
160.80
15.91
16.69
16.06
16.34
17.13
17.38
17.21
16.69
16.32
148.10
154.34
145.39
149.54
146.03
147.55
147.55
143.84
145.32
119.04
118.60
114.05
120.88
118.03
117.77
119.74
114.04
114.03
129.23
124.91
120.98
131.85
129.53
128.97
131.89
120.17
129.56
b
135.83
144.44
158.56
119.25
135.47
125.40
118.64
158.24
158.10
30.17
31.92
29.81
20.69
55.31
20.83
30.60
a
b
c
d
e
55.20
55.31
a
NBn group: 49.82 (CH2), 137.21 (ipso), 127.54 (ortho or para), 128.71 (meta), 127.32 (para or ortho). NBn group: 50.45 (CH2), 137.40 (ipso),
c
d
127.38 (para), 127.57 (meta), 128.76 (ortho). NBn group: 49.71 (CH2), 127.34, 127.75, 128.80. NBn group: 48.46 (CH2), 136.46 (ipso), 127.22,
e
127.65, 128.71. 2,4-diMeOBn group: 42.66, 55.28, 55.34, 98.57, 103.97, 116.88, 120.80, 160.80.
carbon chemical shifts of the methyl group in CH3CO with the
analogous chemical shift of the methyl group in pentane-2,3,4-
Table 3 15N NMR parameters of the compounds 3a–I and 4a–f in
CDCl3
trione 3-phenylhydrazone allows a presumption that the major
form has the arrangement N–Hꢀ ꢀ ꢀN (for description of the
1
method see ref. 4). The H, 13C and 15N NMR parameters of
1
d(15N1) d(15N2)
d(15N3) X
1J(15N, H)/Hz
products 3a–i are given in Tables 1–3.
1
3a (major) ꢂ35.4
72.7
ꢂ216.7
62.4 J(15N3,1H)
The 15N NMR investigation of compounds 3e and 3f in
deuteriochloroform solution showed that these substances
exist as approximately equimolar mixtures of azo and hydra-
zone tautomers. Fig. 1 shows the 1H–15N HMBC spectrum of
compound 3f-15N with visible interactions between NH proton
and all the three nitrogen atoms, inclusive of equivalent
coupling constants J(15N,1H), giving thus a further piece of
evidence of equivalent proportion of azo and hydrazone
tautomers. The equivalence of coupling constants J(15N1,1H)
and J(15N3,1H) was confirmed in the case of compound 3f-15N
also by means of 15N INEPT and 15N spectra measured
without proton decoupling (one-dimensional experiments rea-
lised with digital resolution better than 0.6 Hz pointꢂ1).
The change in magnitude of individual coupling constants,
and thereby their usefulness for estimates of the position of the
azo–hydrazone tautomeric equilibrium, was demonstrated in
the case of products 3g and 3h. The methoxy derivative 3h
exhibits a shift of tautomeric equilibrium towards the azo form
3a (minor)
3b
ꢂ264.6
ꢂ82.6
57.8
82.5
ꢂ185.7 ꢂ11.5
ꢂ236.6
1
3c (major) ꢂ16.5
75.1 J(15N3,1H)
3c (minor)
ꢂ265.1
3d
3e
3f
ꢂ41.2
ꢂ80.0
74.0
50.6
ꢂ216.3
ꢂ172.7
1
ꢂ97.6a 42.5b
ꢂ158.5
42.5 J(15N1,1H)
1
41.7 J(15N3,1H)
1
3g
3h
ꢂ79.4
ꢂ42.1
53.1
ꢂ175.5
ꢂ203.9
32.5 J(15N1,1H)
1
53.2 J(15N3,1H)
1
68.4
18.9 J(15N1,1H)
1
67.4 J(15N3,1H)
3i
80.5 104.4
ꢂ220.1
1
4a
4bc
4c
4d
4e
4f
ꢂ43.5
72.8
46.5
82.8
ꢂ211.9
57.4 J(15N3,1H)
ꢂ102.4
ꢂ18.4
ꢂ159.9 ꢂ12.3
ꢂ232.2
1
72.3 J(15N3,1H)
ꢂ64.5 Not detected ꢂ180.7
ꢂ45.5
ꢂ13.2
71.7
88.6
ꢂ183.1
ꢂ201.9
a 1J(15N1,15N2) = 12.7 Hz. b 2J(15N2,1H) = 4 Hz. c 2J(15NO2,1H) =
3.5 Hz.
ꢁc
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1c with 4-methoxybenzenediazonium tetrafluoroborate exists
as practically pure azo compound 3i in CDCl3 solution.
Also compounds 3e–h exist in CDCl3 solution as mixtures of
two forms. However, due to a very low proportion of the
minor form in the tautomeric mixture and to the overlap of its
signals with signals of major form, it was only possible to
obtain the chemical shift of NH proton, which does not allow
any conclusions to be made about its structure.
Similar conclusions concerning the position of azo–hydra-
zone tautomeric equilibrium as those in the case of acetylace-
tone derivatives can also be drawn in the case of azo coupling
products of 3-methylamino-1-phenylbut-2-en-1-one (Scheme
4). The difference is that products 4a–f exist in CDCl3 solution
in the form of only one isomer. With regard to the non-
changing value of d(13CQO) it can be stated that the carbonyl
group is not affected by the tautomerism (see Table 5), where-
from it can be inferred that the isomer possesses intramole-
cular hydrogen bond of Nꢀ ꢀ ꢀH–N type. The 15N, 1H and
13C NMR parameters of products 4a–f are presented in
Tables 3–5.
Comparison of 15N NMR parameters of products 4a, 4e
and 4f (Table 3) shows that branching of the chain of the
substituent at the amino group does not significantly affect the
position of tautomeric equilibrium; the tert-butyl group
favours the azo compound slightly more than sec-butyl group
does.
Fig. 1 1H–15N HMBC spectrum of 3f-15N in CDCl3.
[in accordance with this, the values of d(15N1) are shifted
downfield and d(15N3) upfield, respectively; the values of
coupling constants cease to be equivalent: the value of
J(15N3,1H) increased and that of J(15N1, 1H) decreased].
Bromo derivative 3g shows an opposite trend: the chemical
shift d(15N1) is shifted upfield and d(15N3) downfield, respec-
tively, as compared with the methoxy derivative; the value of
J(15N3,1H) decreased and that of J(15N1,1H) increased. Table
3 shows that the position of tautomeric equilibrium of sub-
stance 3g is comparable with that of methyl derivative 3e.
From the point of view of the substitution effect of the
amino group upon the tautomeric equilibrium we can see an
increase in content of hydrazone form in the order of N–CH3
o N–CH2Ph o N–Ph. However, if the phenyl group of the
benzyl grouping is substituted at 2- and 4-positions with
methoxy groups, then the product of reaction of enaminone
Comparison of products 3a–c with their analogues 4a–c
shows that replacement of methyl group at 1-position of the
enaminone grouping by phenyl group has virtually no effect
on the position of tautomeric equilibrium.
Structure of azo coupling products in the solid phase
An ORTEP10 view of compound 3a is given in Fig. 2, while the
structures of the other compounds 3c,f and 4a–c,e,f are given
in the ESI.z All the structures of both series 3a–c,f and 4a–c,e,f
derived from acetylacetone and benzoylacetone, respectively,
consist of a mixture of two tautomeric forms. Because both the
amino–diazenyl and imino–hydrazone heterodienic fragments
are tautomeric systems which display an extended p-delocali-
zation, it is almost impossible to determine the tautomeric
ratios only from the analysis of the bond distances within the
chelate rings. Accordingly, these ratios have been calculated
from the refined occupancies of both tautomeric hydrogens.
Although the proton positions can only be localized with an
accuracy some ten times smaller than those of other heavy
atoms (C, N and O), using data collected at low temperature, it
was possible to refine both the hydrogens, with partial
Scheme 4
Table 4 1H NMR spectra of the compounds 4a–f in CDCl3
H-4
H-6
H-7
H-8
H-10
H-11
H-12
R
X
NH
4a
4b
4c
4d
4e
4f
a
2.49 s
2.43 s
2.54 s
2.58 s
2.54 s
2.67 s
7.75–7.77 m
7.81–7.83 m
7.72–7.73 m
7.78–7.80 m
7.75–7.77 m
7.71–7.72 m
7.35–7.37 m
7.41–7.44 m
7.34–7.37 m
7.35–7.39 m
7.31–7.38 m
7.31–7.34 m
7.41–7.44 m
7.50–7.54 m
7.40–7.43 m
7.42–7.46 m
7.40–7.44 m
7.36–7.39 m
7.16–7.18 m
7.23–7.27 m
7.23–7.26 m
7.12–7.14 m
7.14–7.17 m
7.18–7.19 m
7.05–7.06 m
8.10–8.13
6.78–6.81 m
7.18–7.24 m
7.05–7.07 m
7.03–7.05 m
3.15 s
3.32 s
3.13 s
2.27 s
14.93 br s
15.40 br s
14.59 br s
15.41 br s
15.29 br s
15.28 br s
3.75 s
a
7.03–7.07 m
b
2.27 s
2.26 s
1.50 s
b
4.60 (s, 2H), 6.47–6.51 (m, 2H), 7.18–7.24 (m). sec-Butyl: 0.98 (t, 3H, J = 7.4 Hz), 1.28 (d, 3H, J = 6.4 Hz), 1.66 (qv, 2H, J = 7.0 Hz), 3.78
(m, 1H, J = 6.0 Hz).
ꢁc
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Table 5 13C chemical shifts of the compounds 4a–f in CDCl3
C-1
C-2
C-3
C-4
C-5
C-6
C-7
C-8
C-9
C-10
C-11
C-12
R
X
4a
4b
4c
4d
4e
4f
a
193.56
192.94
193.77
193.40
193.66
193.81
129.36
132.93
128.48
129.76
128.82
128.31
163.74
166.16
162.51
162.50
160.92
160.68
16.26
16.85
16.07
16.53
16.28
18.56
141.07
139.28
141.56
140.80
141.27
141.85
130.04
130.12
130.00
130.24
130.06
129.72
127.07
127.52
129.93
129.95
129.45
129.64
130.24
131.47
127.01
125.46
127.00
126.80
147.67
143.92
145.08
149.23
147.94
149.30
b
119.08
117.72
121.06
118.65
119.02
119.71
129.44
125.08
114.01
128.70
129.45
129.28
135.77
153.03
158.39
127.00
135.75
136.18
31.39
33.50
30.36
20.86
55.27
20.86
20.86
20.81
a
b
c
c
2,4-diMeOBn group: 44.42, 55.14, 55.24, 98.48, 103.95, 117.30, 129.61, 158.06, 160.65. sec-Butyl group: 10.12, 20.75, 30.07, 51.90. tert-Butyl
group: 30.14, 54.20.
occupancies, situated in a double well potential, obtaining,
for all compounds, similar tautomeric ratios ranging from
82/18 to 91/9%. The different values obtained from data
in solution can be certainly ascribed to the interactions
of molecules of solvent with the atoms involved in the
hydrogen bonds.
N1ꢀ ꢀ ꢀN3 distances of 2.56–2.64 A. In these compounds it
has been shown, in fact, that the electron withdrawing sub-
stituents favour the equalization of N1 and N3 proton affi-
nities and then the shortening of N1ꢀ ꢀ ꢀN3 hydrogen-bond
distances. Furthermore, also in the two reported structures
having aryl substituent both at N1 and N3,8,14 the substituents
on the phenyl rings are able to cause dramatic structural
changes. For instance, in both compounds8,14 the tautomeric
equilibrium is shifted toward the imino–hydrazone form, but
the N1ꢀ ꢀ ꢀN3 distances display the rather different values of
2.479(3) and 2.611(2) A. These data could be consistent with
the different electronic properties of the substituents on the
N1- and N3-phenyl rings, because the first compound is a
N1-phenyl, N3-p-MeO-phenyl8 derivative while the latter
contains the N1-p-Br-phenyl and N3-phenyl groups.14 These
results suggest that in amino–diazenyl and imino–hydrazone
heterodienic systems the tautomeric equilibrium and the in-
tramolecular N1ꢀ ꢀ ꢀN3 hydrogen bond shortening can be
determined by the different proton affinities of the nitrogens
involved in the intramolecular N–Hꢀ ꢀ ꢀN hydrogen bonds.
Conversely, in the present N3-alkyl derivatives, the delocalisa-
tions within the heterodienic systems and the superimposition
of two tautomeric forms do not clarify the effects of the para-
substituents at the N1-phenyl ring on the N1ꢀ ꢀ ꢀN3 hydrogen
bond distances.
All compounds are N1-aryl and N3-alkyl derivatives and
display intramolecular N1–Hꢀ ꢀ ꢀN3/N3–Hꢀ ꢀ ꢀN1 short hydro-
gen bonds assisted by resonance (RAHB),11 extended conju-
gations within the enamino C1QC2–N3 moiety and weaker
delocalisations within the diazenyl N1QN2–C1 group, as
shown by the bond orders given in Tables 6 and 7.
In
all
structures,
the
hydrogen
bonded
N1QN2–C1QC2–N3H/HN1–N2QC1–C2QN3 heterodienic
systems exhibit quite homogeneous structural features ir-
respective of the chemical differences within the series 3a–c,f
and 4a–c,e,f because the distances are bound to be weighed
average values of both the delocalized tautomeric fragments.
Accordingly, it is almost impossible to define the effects of the
para-substituents (H, CH3, OCH3 and NO2) on the N1-phenyl
ring and on the N1ꢀ ꢀ ꢀN3 hydrogen bond distances which are
confined in the narrow range of 2.56–2.60 A. Conversely, in
the previous structures of N3–H2 amino derivatives,6,12–14 the
p-substituents at the N1-phenyl ring are able to influence the
p-delocalizations which are related to a wider range of
Table 6 Selected bond distances (A)/bond orders (in italic), bond
angles and torsion angles (1) and hydrogen bond parameters for
structures of acetylacetone derivatives (3a–c,f)
3a
3b
3c
3f
Distances
N1–N2 1.284(1)/1.63 1.294(1)/1.56 1.289(2)/1.60 1.284(1)/1.63
N2–C1 1.377(2)/1.18 1.361(2)/1.28 1.372(2)/1.21 1.377(1)/1.18
C1–C2 1.432(1)/1.29 1.437(1)/1.26 1.429(2)/1.30 1.431(1)/1.29
N3–C2 1.316(1)/1.59 1.318(2)/1.58 1.317(2)/1.58 1.315(1)/1.60
N1–C6 1.421(2)
C1–C3 1.465(2)
C3–O1 1.227(2)
1.413(2)
1.472(2)
1.230(2)
1.421(2)
1.468(2)
1.223(2)
1.424(1)
1.462(2)
1.232(1)
Hydrogen bonds
N3H/N1H (ratio)
N1ꢀ ꢀ ꢀN3
87/13
86/14
87/13
83/17
2.565(1)
0.90(9)
0.91(2)
120(8)
139(2)
2.585(1)
0.72(11)
0.88(2)
120(11)
135(2)
2.576(2)
0.92(12)
0.89(2)
121(11)
137(2)
2.565(1)
1.00(8)
0.94(2)
118(6)
141(2)
N1–H
N3–H
Fig. 2 ORTEP view of compound 3a. Thermal ellipsoids are drawn
at 40% probability level. Both tautomeric hydrogens linked to N1 and
N3 atoms, are displayed.
N1–Hꢀ ꢀ ꢀN3
N3–Hꢀ ꢀ ꢀN1
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Table 7 Selected bond distances (A)/bond orders (in italic), bond angles and torsion angles (1) and hydrogen bond parameters for structures of
benzoylacetone derivatives (4a,b,c–f)
Distances
4a
4b
4c
4e
4f
N1–N2
N2–C1
C1–C2
N3–C2
N1–C11
C1–C3
C3–O1
1.289(1)/1.60
1.377(2)/1.18
1.419(2)/1.36
1.322(2)/1.55
1.422(2)
1.297(2)/1.54
1.359(2)/1.29
1.445(2)/1.22
1.312(2)/1.62
1.413(2)
1.284(1)/1.63
1.379(2)/1.17
1.422(2)/1.34
1.322(2)/1.55
1.424(2)
1.283(2)/1.64
1.373(2)/1.20
1.429(2)/1.30
1.317(2)/1.58
1.418(2)
1.286(2)/1.62
1.375(2)/1.19
1.431(2)/1.29
1.316(2)/1.59
1.420(2)
1.480(2)
1.231(2)
1.470(2)
1.230(2)
1.470(2)
1.232(2)
1.463(2)
1.229(2)
1.468(2)
1.225(2)
Hydrogen bonds
N3H/N1H (ratio)
N1ꢀ ꢀ ꢀN3
82/18
83/17
85/15
91/9
83/17
2.578(1)
0.94(9)
0.93(2)
122(7)
139(2)
2.575(2)
0.93(10)
0.90(2)
126(8)
138(2)
2.601(2)
0.86(10)
0.92(2)
130(9)
137(2)
2.562(2)
0.98(23)
0.94(2)
128(19)
138(2)
2.579(2)
0.93(8)
0.92(3)
119(6)
139(2)
N1–H
N3–H
N1–Hꢀ ꢀ ꢀN3
N3–Hꢀ ꢀ ꢀN1
hydrogen bond distances and on the bond lengths in the
Conclusions
heterodienic systems within the series 3a–c,f and 4a–c,e,f.
The reaction products of 4-methylaminopent-3-en-2-one and
benzenediazonium ions substituted at 4-position exist in deu-
teriochloroform solution as mixtures of E/Z isomers with the
predominating Z isomer having an intramolecular hydrogen
bond of N–Hꢀ ꢀ ꢀN type. The major isomer is a mixture of azo
and hydrazone tautomers (38–55% hydrazone form). The
amount of azo form depends on the substituent in the diazo-
nium salt and decreases in the order of OMe 4 Me 4 Br 4
NO2. Isomer E is practically pure tautomer azo–enamino.
The transition from 4-methylaminopent-3-en-2-one to 3-
methylamino-1-phenylbut-2-en-1-one does not cause any sig-
nificant changes in position of the tautomeric equilibrium,
except for the fact that the products derived from benzoyla-
cetone form only one isomer, Z, in deuteriochloroform solu-
tion, this isomer being a tautomeric mixture (32–63%
hydrazone form).
Experimental
General
The melting points were measured on a hot-stage microscope
and were not corrected. The elemental analyses were carried
out on an automatic analyser FISONS EA 1108.
NMR spectroscopy
The NMR spectra were measured using Bruker AMX 360
1
(360.14 MHz for H, 90.57 MHz for 13C and 36.50 MHz for
15N) and Bruker Avance 500 (500.13 MHz for 1H, 125.77
MHz for 13C and 50.69 MHz for 15N) at laboratory tempera-
ture. Hexamethyldisiloxane was used as the internal standard
1
for H (d 0.05 in CDCl3).
The position of the azo–hydrazone tautomeric equilibrium
in the case of N-substituted b-enaminones in CDCl3 solution is
most strongly affected by substitution at the nitrogen atom of
the starting enaminone. The content of azo form decreases in
the order of N–CH3 4 N–CH2Ph 4 N–Ph. Substituents at
the phenyl group of benzyl grouping also affect the tautomeric
equilibrium: the 2,4-dimethoxybenzyl derivative exists in
CDCl3 solution as practically pure azo compound. Branching
in substituents at nitrogen (Me, sec-Bu, tert-Bu) has no
significant effect upon the position of tautomeric equilibrium;
only the tert-Bu slightly favours the azo form.
The 13C NMR spectra were standardised by means of the
central signal of the deuteriochloroform triplet (d 76.9). The
15N NMR spectra were standardised by means of external neat
nitromethane placed in a coaxial capillary (d 0.0).
The proton signals were assigned with the help of H,H
COSY pulse sequence.
The nitrogen chemical shifts were measured by both direct
detection and indirect detection gs 2D 1H–15N HMBC pro-
cessed in the magnitude mode. The gradient ratios were 70 :
30 : 50.1. Experiments were performed with the NH one-bond
coupling 90 Hz, and NH long-range coupling 5 Hz, 2k ꢃ 160
zero filled to 2k ꢃ 1k, sinebell squared in both dimensions. The
values of coupling constants J(15N,1H) were read either from
the 15N INEPT spectra measured without proton decoupling
or from the 15N satellites in proton spectra or from 1D 1H–15N
HMBC spectra.
In the solid phase, both 3 and 4 products exist predomi-
nantly as azo compounds (82–91%) and exclusively as Z
isomers. Neither substitution of diazonium salt nor that at
N3 nitrogen atom seem to affect significantly the position of
tautomeric equilibrium. On the other hand, in all the struc-
tures the weighed superimposition of both the hydrogen
bonded N1QN2–C1QC2–N3H/ HN1–N2QC1–C2QN3 het-
erodienic fragments do not allow to well define the effects of
the substituents at the N1-phenyl ring both on the N1ꢀ ꢀ ꢀN3
The carbon NMR spectra were measured in a standard way
and by means of the APT pulse sequence (spectral width
26.455 kHz, acquisition time 1.238 s, zero filling to 64 k and
line broadening 1 Hz prior Fourier transformation).The
ꢁc
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assignment of the individual signals was carried out by means
of 2D pulse sequences gs 1H–13C HMQC (experiment per-
formed with the CH coupling 145 Hz, 2 k ꢃ 128 zero filled to
squares. All non-hydrogen atoms were refined anisotropically
and hydrogens isotropically except the methylic hydrogen
atoms of compound 4f which were included on calculated
positions riding on their carrier atoms. In all compounds the
difference Fourier map showed diffuse electron density be-
tween N1 and N3 atoms with two maxima from which two
proton positions could be identified. Refinement of the two
tautomeric H atoms with partial occupancy and isotropic
thermal parameters fixed at 1.2 times the average of those of
the related nitrogen atoms was successfully attempted giving
the final occupancy factors in the range 82–91% for H3 and
18–9% for H1.
1
2k ꢃ 1k, sinebell squared in both dimensions) and gs H–13C
HMBC (experiment performed with the long-range CH cou-
pling 6–10 Hz, 2 k ꢃ 160 zero filled to 2 k ꢃ 1 k, sinebell
squared in both dimensions) each processed in the magnitude
mode.
X-Ray structure determinations
The crystal data for compounds 3a–d,3f, 4a–c,4e,f were col-
lected using a Nonius Kappa CCD diffractometer with gra-
phite-monochromated Mo-Ka radiation and corrected for
Lorentz and polarization effects. The structures were solved
by direct methods (SIR9715) and refined using full-matrix least
All the calculations were performed using SHELXL-9716
and PARST17 implemented in WINGX18 system of programs.
The crystal data and refinement parameters are summarized in
Table 8 Crystal data
Compound
3a
3b
3c
3f
Formula
M
System
Space group
a/A
b/A
c/A
a/1
C13H17N3O
231.30
Monoclinic
P21/c
6.6026(1)
12.9320(2)
15.1197(3)
90
102.1089(9)
90
1262.27(4)
4
1.217
C12H14N4O3
262.27
Monoclinic
P21/c
10.2904(2)
8.7167(2)
14.0155(3)
90
100.9315(9)
90
1234.35(5)
4
1.411
C13H17N3O2
247.30
Monoclinic
P21/c
6.9786(2)
14.2655(5)
13.2947(6)
90
101.364(1)
90
1297.58(8)
4
1.266
C18H19N3O
293.36
Triclinic
ꢀ
P1
8.1028(2)
10.4312(2)
10.4731(2)
67.932(1)
76.578(1)
71.720(1)
772.36(3)
2
b/1
g/1
U/A3
Z
Dc/g cmꢂ3
T/K
1.261
150
0.80
150
0.80
150
1.05
150
0.88
m/cmꢂ1
y
min–ymax/1
3.50–28.00
3027
0.033
2.00–28.00
2976
0.031
3.30–28.00
3058
0.029
2.67–30.00
4472
0.029
Unique reflns
Rint
Observed reflns [I 4 2s(I)]
R (obs. reflns)
wR (all reflns)
S
2594
2556
2368
3588
0.0421
0.1211
1.070
0.0396
0.1169
1.078
0.0454
0.1256
1.028
0.0432
0.1242
1.052
Drmax; Drmin/ e Aꢂ3
0.22; –0.22
0.24; –0.25
0.29; –0.27
0.24; –0.20
Compound
4a
18H19N3O
4b
4c
4e
4f
Formula
M
System
Space group
a/A
b/A
c/A
a/1
C
C17H16N4O3
324.34
Monoclinic
P21/c
8.0955(1)
9.5542(2)
20.4491(4)
90
C18H19N3O2
309.36
Orthorhombic
Pbca
13.6403(2)
9.6489(2)
24.3685(5)
90
C21H25N3O
335.44
Monoclinic
P21/n
8.6915(2)
17.1912(4)
13.2188(3)
90
C21H25N3O
335.44
Orthorhombic
P212121
8.5205(2)
12.6244(3)
18.1441(4)
90
293.36
Orthorhombic
P212121
7.7167(1)
10.8923(1)
18.8973(3)
90
b/1
90
90
98.2804(5)
90
1565.94(5)
4
1.376
90
90
3207.2(1)
8
1.281
101.066(1)
90
1938.39(8)
4
1.149
90
90
g/1
U/A3
Z
1588.22(4)
4
1.227
150
0.78
1951.69(8)
4
1.142
295
0.71
Dc/g cmꢂ3
T/K
150
0.97
150
0.85
295
0.72
m/cmꢂ1
y
min–ymax/1
2.80–30.00
4544
0.051
2.50–30.00
4463
0.058
2.70–28.00
3847
0.033
4.40–27.00
4180
0.034
3.93–27.00
4208
0.036
Unique reflns
Rint
Observed reflns [I 4 2s(I)]
R (obs. reflns)
wR (all reflns)
S
4176
3343
3084
3266
3301
0.0368
0.0932
1.022
0.17; ꢂ0.12
0.0493
0.1316
1.037
0.24; ꢂ0.22
0.0391
0.1026
1.038
0.22; ꢂ0.18
0.0519
0.1487
1.014
0.34; ꢂ0.16
0.0440
0.1213
1.019
0.15; ꢂ0.16
Drmax; Drmin/e Aꢂ3
ꢁc
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Table 8. Selected bond distances/bond orders, bond angles and
hydrogen bond parameters are given in Tables 6 and 7.
Pauling’s bond order19 or, more exactly, bond number, n, is
evaluated by the formula d(n) = d(1) ꢂ c log n, where d(n) and
d(1) are the bond lengths for n = n and 1, respectively, and c is
a constant to be evaluated for each type of chemical bond. The
pure single and double bond distances have been retrieved
from ref. 20.
Calc. for C14H19NO3 (249.30): C, 67.45; H, 7.68; N, 5.62.
Found: C, 67.69; H, 7.51; N, 5.64%.
3-Methylamino-1-phenylbut-2-en-1-one (2a). A 250-ml flask
was charged with benzoylacetone (62 mmol) and 33% etha-
nolic solution of methylamine (120 ml). The mixture was
refluxed for 4 h, cooled, and the separated product was
collected by suction and recrystallised from n-hexane; mp
69–71 1C (ref. 23: 71–72 1C).
CCDC reference numbers 633157–633165.
1H NMR (360.14 MHz, CDCl3): d 2.03 (s, 3H, CH3), 2.98
(d, 3H, J = 5.3 Hz, NCH3), 5.66 (s, 1H,QCH), 7.34–7.41 (m,
3H), 7.82–7.85 (m, 2H), 11.30 (br s, 1H, NH).
13C (90.57 MHz, CDCl3): d 19.06, 29.51, 91.82, 126.69,
127.99, 130.20, 140.36, 165.82, 187.43.
For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b613346k
Materials
Dichloromethane and diisopropyl ether were purchased dry
from Fluka and used without change. Anhydrous sodium
acetate was purchased commercially and was used without
change.
3-(2,4-Dimethoxybenzylamino)-1-phenylbut-2-en-1-one (2b).
This compound was prepared in the same way as compound
1c. Crystallisation from cyclohexane. Yield 92%; mp 84–
87 1C. Calc. for C19H21NO3 (311.38): C 73.29, H 6.80, N
4.50. Found: C 73.34, H 7.02, N 4.56%.
1H NMR (500.13 MHz, CDCl3): d 2.08 (s, 3H, CH3), 3.77
(s, 3H, OCH3), 3.83 (s, 3H, OCH3), 4.43 (d, 2H, J = 6.3 Hz,
CH2), 5.66 (s, 1H,QCH), 6.42–6.45 (m, 2H), 7.12 (d, 1H, J =
8.1 Hz), 7.34–7.40 (m, 3H), 7.83–7.85 (m, 2H), 11.57 (br s, 1H,
NH).
The diazonium tetrafluoroborates were prepared by a
known procedure.5
Aniline (95% 15N) and sodium nitrite (95% 15N) were
purchased from Aldrich.
15N 4-Nitroaniline was prepared in the standard way21 from
15N aniline.
4-Methylaminopent-3-en-2-one (1a). This was prepared ac-
cording to the procedure described in ref. 22; bp 54–55 1C/0.60
kPa, lit.22 99–100 1C/2.53 kPa.
13C (125.77 MHz, CDCl3): d 19.24, 41.96, 55.27, 92.10,
98.52, 103.96, 118.27, 126.74, 127.96, 128.82, 130.16, 140.45,
157.91, 160.42, 164.66, 187.48.
1H NMR (CDCl3, 360.14 MHz): d 1.11 (s, 3H, CH3), 1.14
(s, 3H, CH3CO), 2.11 (d. 3H, J = 5.3 Hz, NHCH3), 4.17
(s, 1H, QCH), 9.92 (br s, NH).
3-(1-Methylpropylamino)-1-phenylbut-2-en-1-one (2c).
A
mixture of benzoylacetone (25 mmol) and 1-methylpropyla-
mine (200 mmol) was refluxed for 4.5 h. Then the mixture was
cooled and the excess amine was distilled off. The residue was
dissolved in dichloromethane, dried with Na2SO4 and the
solvent was evaporated. According to NMR, the obtained
product was sufficiently pure for subsequent reactions. Yield
92%.
1H NMR (500.13 MHz, CDCl3): d 0.96 (t, J = 7.5 Hz, 3H),
1.25 (d, J = 6.5 Hz, 3H), 1.57–1.63 (m, 2H), 2.07 (s, 3H),
3.52–3.60 (m, 1H), 5.61 (s, 1H), 7.35–7.39 (m, 3H), 7.84–7.86
(m, 2H), 11.47 (br s, 1H).
4-Benzylaminopent-3-en-2-one (1b). The substance was pre-
pared from acetylacetone and benzylamine in toluene with
sulfuric acid as the catalyst. The water formed in the reaction
was azeotropically distilled off. After 7 h, the solvent was
distilled off. According to NMR, the obtained red oil was
sufficiently pure for subsequent reactions. Yield 79%.
1H NMR (CDCl3, 360.14 MHz): d 1.87 (s, 3H, CH3), 2.00
(s, 3H, CH3CO), 4.41 (d, 2H, CH2, J = 6.4 Hz), 5.02 (s, 1H,
QCH), 7.20–7.26 (m, 3H, CHAr), 7.28–7.33 (m, 2H, CHAr),
11.15 (br s, 1H, NH). 13C (CDCl3, 90.57 MHz): d 18.55 (CH3),
28.58 (CH3), 46.36 (CH2), 95.58 (QCH), 126.36, 127.08,
128.47 (3 ꢃ CHAr), 137.72 (Cq), 162.81 (QC–N), 195.00
(CQO).
13C NMR (125.77 MHz, CDCl3): d 10.30, 19.22, 21.40,
30.48, 50.52, 91.54, 126.66, 127.93, 130.12, 140.33, 163.84,
187.11.
4-(2,4-Dimethoxybenzylamino)pent-3-en-2-one (1c). This
compound was prepared from acetylacetone and 2,4-di-
methoxybenzylamine in the same way as product 1b. After
the reaction, a part of toluene was distilled off, the solution
was cooled, and the separated crystalline solid was collected by
suction. The product was purified by vacuum distillation
(173–176 1C/0.60–0.70 kPa). Yield 66%.
3-tert-Butylamino-1-phenylbut-2-en-1-one (2d). This com-
pound was prepared from benzoylacetone in two steps
(Scheme 2).
3-Chloro-1-phenylbut-2-en-1-one:24 A solution of 50 mmol
benzoylacetone in 40 ml chloroform was treated with
126 mmol oxalyl chloride added dropwise within ca. 30 min
with stirring. Then the mixture was refluxed for 2 h. The
solvent and excess oxalyl chloride were distilled off in vacuum.
The dark tarry residue was fractionated in vacuum to give
product boiling at 110–120 1C/1.20 kPa (ref. 25 gives bp 105 1C
at 0.40 kPa). Yield 38%.
1H NMR (360.14 MHz, CDCl3): d 1.93 (s, 3H, CH3), 1.98
(s, 3H, CH3), 3.77 (s, 3H, OCH3), 3.81 (s, 3H, OCH3), 4.33 (d,
2H, CH2, J = 6.4 Hz), 4.96 (s, 1H,QCH), 6.40–6.44 (m, 2H,
CHAr), 7.06 (d, 1H, CHAr, J = 8.4 Hz), 11.03 (br s, 1H, NH).
13C (90.57 MHz, CDCl3): d 18.60 (CH3), 28.65 (CH3), 41.74
(CH2), 55.20, 55.23 (2 ꢃ OCH3), 95.23, 98.43, 103.83
(3 ꢃ CHAr), 118.57 (Cq), 128.54 (CHAr), 157.85, 160.30,
162.84 (3 ꢃ Cq), 194.57 (CQO).
3-tert-Butylamino-1-phenylbut-2-en-1-one: A solution of 19
mmol 3-chloro-1-phenylbut-2-en-1-one in 40 ml diethyl ether
was treated with a solution of 38 mmol tert-butylamine in
10 ml diethyl ether added dropwise with stirring. The mixture
ꢁc
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was stirred at room temperature overnight. The separated
portion was collected by suction and washed with ether. The
solvent was evaporated and the residue was fractionated in
vacuum to give 38.5% yield of liquid boiling at 162–166 1C/
0.70 kPa; ref. 26 gives bp 50 1C at 1.33 Pa.
0.13 g (52%); mp 156.5–158 1C for 3b-15N1 and 153.5–155 1C
for 3b-15N2, respectively.
4-Methylamino-3-(4-methoxyphenyldiazenyl)pent-3-en-2-one
(3c). Reaction time 24 h. Recrystallisation from cyclohexane.
Yield 38%; mp 90–92 1C. Calc. for C13H17N3O2 (247.29): C
63.14, H 6.93, N 16.99. Found: C 63.10, H 6.81, N 17.12%.
1H NMR (500.13 MHz, CDCl3): d 1.45 (s, 9H), 2.19 (s, 3H),
5.59 (s, 1H), 7.36–7.38 (m, 3H), 7.83–7.84 (m, 2H), 11.95 (br s,
1H).
4-Methylamino-3-(4-bromophenyldiazenyl)pent-3-en-2-one
(3d). Reaction time 24 h. Recrystallisation from cyclohexane.
Yield 29%; mp 84–85.5 1C. Calc. for C12H14N3OBr (296.16):
C 48.67, H 4.76, N 14.19. Found: C 48.42, H 4.67, N 14.12%.
13C NMR (125.77 MHz, CDCl3): d 20.85, 30.64, 52.45,
93.01, 126.63, 127.94, 130.09, 140.41, 164.86, 186.69.
3-Decylamino-1-phenylbut-2-en-1-one (2e). A mixture of
benzoylacetone (25 mmol) and n-decylamine (25 mmol) was
refluxed in toluene (30 ml) for 4 h. The solvent was evaporated
in vacuum and the residue was fractionated in vacuum to give
62% yield of a yellow wax; mp 30–33 1C, bp 217–220 1C/0.93
kPa. Calc. for C22H35NO (329.52): C 80.19, H 10.71, N 4.25.
Found: C 80.04, H 10.81, N 4.40%.
1H NMR (500.13 MHz, CDCl3): d 0.87 (m, 3H), 1.26–1.42
(m, 14H), 1.633 (quintet, J = 10 Hz, 2H), 2.04 (s, 3H), 3.29
(m, 2H), 5.64 (s, 1H), 7.37–7.38 (m, 3H), 7.83–7.86 (m, 2H),
11.44 (br s, 1H).
4-Benzylimino-3-(4-methylphenylhydrazono)pentane-2-one
(3e). The compound was prepared by a modification of the
general procedure. Reaction time 24 h. The filter cake was
washed first with little diisopropyl ether and then with di-
chloromethane. The two filtrates were collected separately.
The dichloromethane filtrate contained practically pure pro-
duct of azo coupling reaction. The evaporation residue ob-
tained from this filtrate was recrystallised from ethanol. Yield
39%; mp 111–113 1C. Calc. for C19H21N3O (307.39): C 74.24,
H 6.89, N 13.67. Found: C 74.25, H 6.77, N 13.68%.
13C NMR (125.77 MHz, CDCl3): d 13.93, 19.20, 22.49,
26.74, 29.12, 29.13, 29.30, 29.37, 29.90, 31.70, 43.17, 91.66,
126.64, 127.91, 130.10, 140.35, 164.67, 187.27.
4-Benzylimino-3-phenylhydrazonopentane-2-one (3f). This
was prepared in the same way as that used for 3e. Yield
45%; mp 108–110 1C. Calc. for C18H19N3O (293.36): C 73.70,
H 6.53, N 14.32. Found: C 73.97, H 6.65, N 14.09%.
General procedure of azo coupling reactions
(3f-15N).
A 100-ml Erlenmeyer flask was charged with 4.4 mmol of the
respective enaminone and 30 ml dry diisopropyl ether (3a–f,I),
or dichloromethane (3g–h, 4a–f). With stirring, 13.2 mmol
sodium acetate was added, followed by 4.4 mmol of the
respective substituted benzenediazonium tetrafluoroborate.
The mixture was stirred at room temperature for a period of
2 to 24 h, depending on the starting reagents used. Then the
solid portion was collected by suction, and the filter cake was
washed with little dichloromethane. The filtrate was evapo-
rated in vacuum and the residue further purified by recrystal-
lisation.
4-Benzylimino-3-phenylhydrazonopentane-2-one
This was prepared in the same way as that used for 3e.
Enrichment 20% 15N2 and 95% 15N1. Yield 41%; mp
108–109 1C.
4-Benzylimino-3-(4-bromophenylhydrazono)pentane-2-one
(3g). Reaction time 3 h. Recrystallisation from cyclohexane.
Yield 52%; mp 112–115 1C.
Calc. for C18H18BrN3O (372.26): C 58.08, H 4.87, N 11.29.
Found: C 58.33, H 4.89, N 11.23%.
4-Benzylimino-3-(4-methoxyphenylhydrazono)pentane-2-one
(3h). Reaction time 18 h. Recrystallisation from cyclohexane.
Yield 63%; mp 95–99 1C. Calc. for C19H21N3O2 (323.39): C
70.57, H 6.55, N 12.99. Found: C 70.69, H 6.60, N 13.01%.
4-Methylamino-3-(4-methylphenyldiazenyl)pent-3-en-2-one
(3a). Reaction time 24 h. Recrystallisation from cyclohexane.
Yield 0.69 g (68%); mp 113–115 1C. Calc. for C13H17N3O
(231.29): C 67.51, H 7.41, N 18.17. Found: C 67.73, H 7.69, N
18.45%.
4-(2,4-Dimethoxybenzylamino)-3-(4-methoxyphenyldiazenyl)
pent-3-en-2-one (3i). Reaction time 24 h. Recrystallisation
from cyclohexane. Yield 28%; mp 82–89 1C. Calc. for
C21H25N3O4 (383.44): C 65.78, H 6.57, N 10.96. Found: C
66.00, H 6.61, N 10.95%.
4-Methylamino-3-(4-nitrophenyldiazenyl)pent-3-en-2-one
(3b). Reaction time 24 h. Recrystallisation from n-hexane–to-
luene. Yield 0.43 g (37%); mp 152.5–155 1C. Calc. for
C12H14N4O3 (262.26): C 54.96, H 5.38, N 21.36. Found: C
55.25, H 5.16, N 21.41%.
3-Methylamino-2-(4-methylphenyldiazenyl)-1-phenylbut-2-
en-1-one (4a). Reaction time 24 h. Recrystallisation from
cyclohexane. Yield 39%; mp 109.5–111 1C. Calc. for
C18H19N3O (293.36): C 73.70, H 6.53, N 14.32. Found: C
73.71, H 6.77, N 14.41%.
4-Methylamino-3-(4-nitrophenyl[15N1]diazenyl)pent-3-en-2-
one (3b-15N1) and 4-methylamino-3-(4-nitrophenyl[15N2]diaze-
nyl)pent-3-en-2-one (3b-15N2). These substances were prepared
in the same way as that used for 3b. The starting 4-nitroben-
zenediazonium tetrafluoroborate was isotopically labelled at
the N1 nitrogen atom (95% 15N) (for preparation of product
3b-15Na) or at the N2 nitrogen (25% 15N) (for preparation of
product 3b-15Nb). Yields (3b-15Na) 0.17 g (68%) and (3b-15Nb)
3-Methylamino-2-(4-nitrophenyldiazenyl)-1-phenylbut-2-en-
1-one (4b). Reaction time 2 h. Recrystallisation from cyclo-
hexane–toluene. Yield 38%; mp 155.5–157 1C. Calc. for
C17H16N4O3 (324.33): C 62.95, H 4.97, N 17.27. Found: C
63.22, H 4.74, N 17.06%.
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3-Methylamino-2-(4-methoxyphenyldiazenyl)-1-phenylbut-2-
en-1-one (4c). Reaction time 24 h. Recrystallisation from
cyclohexane. Yield 64%; mp 109–110.5 1C. Calc. for
C18H19N3O2 (309.36): C 69.88, H 6.19, N 13.58. Found: C
69.96, H 6.49, N 13.58%.
5 P. Simunek, A. Lycka and V. Macha
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3-(2,4-Dimethoxybenzylamino)-2-phenyldiazenyl-1-phenyl-
but-2-en-1-one (4d). Reaction time 24 h. Recrystallisation from
cyclohexane–toluene. Yield 51%; mp 129.5–132 1C. Calc. for
C25H25N4O3 (415.49): C 72.27, H 6.06, N 10.11. Found: C
72.47, H 5.93, N 9.90%.
8 J. Lokaj, V. Kettmann, P. Simunek and V. Machacek, Acta
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and G. Gilli, Acta Crystallogr., Sect. B, 1994, 50, 617.
12 B. L. Rodrigues, M. T. do P. Gambardella, L. J. de O. Figueiredo
and C. Kascheres, Acta Crystallogr., Sect. C, 1996, 52, 705.
13 I. Vencato, S. Cunha, V. Rocha, Z. N. da Rocha and C. Lariucci,
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3-(1-Methylpropylamino)-2-(4-methylphenyldiazenyl)-1-phe-
nylbut-2-en-1-one (4e). Reaction time 24 h. Recrystallisation
from ethanol. Yield 68%; mp 112–116 1C. Calc. for
C21H25N3O (335.44): C 75.19, H 7.51, N 12.53. Found: C
75.25, H 7.57, N 12.57%.
3-tert-Butylamino-2-(4-methylphenyldiazenyl)-1-phenylbut-2-
en-1-one (4f). Reaction time 24 h. Chromatography silica/ethyl
acetate. Recrystallisation from ethanol. Yield 80%; mp
145–146 1C. Calc. for C21H25N3O (335.44): C 75.19, H 7.51,
N 12.53. Found: C 75.48, H 7.61, N 12.63%.
14 P. Simunek, V. Bertolasi, M. Peskova
Org. Biomol. Chem., 2005, 3, 1217.
15 A. Altomare, M. C. Burla, M. Camalli, G. L. Cascarano, C.
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Acknowledgements
The authors (P. S., M. S., V. M.) acknowledge the financial
support by the Ministry of Education, Youth and Sports of
the Czech Republic (project No. MSM 002 162 7501) and by
the Czech Science Foundation (project No. 203/07/0469).
21 Vogel’s Textbook of Practical Organic Chemistry, ed. B. S. Furniss,
A. J. Hannaford, P. W. G. Smith and A. R. Tatchell, Longman,
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