REACTIONS OF 4-AMINO-3-PENTEN-2-ONE AND ITS N-SUBSTITUTED REDIVATIVES WITH DIAZONIUM IONS

Article abstract of DOI:10.1135/cccc19951367

Azo coupling reactions of benzenediazonium salts with substituted 4-amino-3-penten-2-ones take place at the C-3 atom. 1H and 13C NMR spectroscopy has been used to study the structure of both the starting enaminones and coupling products.In CDCl3, 3-(4-chl

Full text of DOI:10.1135/cccc19951367

Reactions of 4-Amino-3-penten-2-one
1367
REACTIONS OF 4-AMINO-3-PENTEN-2-ONE AND ITS N-SUBSTITUTED
DERIVATIVES WITH DIAZONIUM IONS
Vladimir MACHACEK, Alexandr CEGAN, Ales HALAMA, Olga ROZNAVSKA
and Vojeslav STERBA
Department of Organic Chemistry,
University of Pardubice, 532 10 Pardubice, Czech Republic
Received April 4, 1995
Accepted May 31, 1995
Azo coupling reactions of benzenediazonium salts with substituted 4-amino-3-penten-2-ones take
place at the C-3 atom. 1H and 13C NMR spectroscopy has been used to study the structure of both
the starting enaminones and coupling products. In CDCl3, 3-(4-chlorophenylhydrazono)-2-(4-methyl-
phenylimino)-4-pentanone exists in hydrazo form whereas 4-amino-3-(4-chlorophenylazo)-3-penten-
2-one is present as a mixture of two azo compounds differing probably in the arrangement of the
intramolecular hydrogen bond. The azo coupling reaction kinetics have been studied in acetate buffers
and methanol–water or tert-butyl alcohol–water mixtures. The coupling rate has been found inde-
pendent of pH and buffer concentration. The reaction orders with respect to the starting compounds
have been determined and the reaction mechanism is suggested. Linear dependence has been found
between log kobs and substituent constants according to the Hammett or Yukawa–Tsuno equations.
Molecules of enaminones involve a five-atomic π electron system described by three
resonance structures (Eq. (A)). The enaminone skeleton can be attacked by electro-
philes at its nitrogen, carbon, or oxygen atoms1,2
.
(A)
The attack by proton is fastest at the oxygen atom. In media of low polarity, O-proto-
nated enaminones are the most stable, whereas in polar media the proton is transferred
to the carbon atom3,4. Alkylations of enaminones take place at the oxygen atom pre-
dominantly5,6. Enaminones with tertiary nitrogen atom are acylated by aliphatic and
aromatic acyl halogenides at the oxygen atom7, too; in some cases a rearrangement of
acyl group from oxygen to carbon was observed8. Reactions of enaminones with sub-
stituted phenyl isocyanates produce mixtures of products resulting from attack at carb-
on and nitrogen9. Isothiocyanates, which are softer electrophiles than isocyanates,
attack enaminones at the carbon atom only9. Even benzoyl isothiocyanate, which is a
more reactive but also softer electrophile than substituted phenyl isocyanates, reacts
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

1368
Machacek, Cegan, Halama, Roznavska, Sterba:
with 4-amino-3-penten-2-one exclusively at its β carbon atom10. Only if the β carbon
atom carries no hydrogen substituent (e.g., in the case of 4-amino-3-methyl-3-penten-2-
one), benzoyl isothiocyanate attacks the nitrogen centre11.
The aim of the present paper is to study the reactions of benzenediazonium ions with
4-amino-3-penten-2-one and substituted 4-phenylamino-3-penten-2-ones, determine the
structure of products, and study the kinetics and mechanism of these azo coupling reac-
tions.
EXPERIMENTAL
The 1H and 13C NMR spectra were measured with an AMX 360 Bruker apparatus at 360.14 and
90.57 MHz, respectively, at 25 °C. For the measurements, the substances were dissolved in CDCl3,
and the chemical shifts were referenced to the solvent signals (δ(1H) 7.25, δ(13C) 77.00). The carbon
chemical shifts of compound Vb were assigned with the help of a C,H-coupled spectrum and H,C
COSY spectrum for both aliphatic and aromatic regions.
Chemicals
4-Amino-3-penten-2-one (I) was distilled at 74–75 °C/780 Pa (ref.12). The substituted 4-phenyl-
amino-3-penten-2-ones were prepared by reactions of 2,4-pentanedione with substituted anilines:
4-(4-methylphenylamino)-3-penten-2-one (IIa), yield 77%, m.p. 65–67 °C (hexane), ref.13 gives m.p.
68–69 °C; 4-phenylamino-3-penten-2-one (IIb), yield 86%, m.p. 47–48 °C (hexane), ref.14 gives m.p.
47–48 °C; 4-(4-chlorophenylamino)-3-penten-2-one (IIc), yield 83%, m.p. 60–62 °C (hexane), ref.13
gives m.p. 60–61 °C; 4-(3-chlorophenylamino)-3-penten-2-one (IId), yield 79%, m.p. 38–40 °C (hexane),
ref.13 gives m.p. 42 °C; 4-(3-nitrophenylamino)-3-penten-2-one (IIe), yield 65%, m.p. 77–79 °C (hexane),
ref.15 gives m.p. 78–80 °C.
4-Amino-3-(4-chlorophenylazo)-3-penten-2-one (IVc): 6.4 g (0.05 mol) p-chloroaniline in 50 ml
2.5 M HCl was diazotized at 0 °C by adding 20 ml 2.5 M NaNO2. The diazonium salt solution was
treated with 8.2 g (0.1 mol) anhydrous sodium acetate to produce the acetate buffer and filtered,
whereupon it was added to a solution of 4.95 g (0.05 mol) 4-amino-3-penten-2-one (I) in 50 ml
acetone. The reaction product which precipitated instantaneously, was collected by suction after several
minutes. The product which separated later was considerably contaminated by the hydrolysis product – 3-(4-
chlorophenylhydrazono)-2,4-pentanedione. The yield of raw product was about 60%, m.p. 185–187 °C after
recrystallization from benzene.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Reactions of 4-Amino-3-penten-2-one
1369
1H NMR spectrum: Isomer 1: 13.53 b, 1 H (NH); 7.52 and 7.36 AA′XX′, 4 H (Ar); 6.68 b, 1 H
(NH); 2.56 bd, 3 H (CH3); 2.53 bs, 3 H (CH3); isomer 2: 11.08 b, 1 H (NH); 7.56 and 7.36 AA′XX′,
4 H (Ar); 5.88 b, 1 H (NH); 2.53 bs, 2 × 3 H (2 CH3). 13C NMR spectrum: Isomer 1: 28.11 (C-1),
198.34 (C-2), 129.25 (C-3), 160.85 (C-4), 25.64 (C-5), 149.59 (C-6), 121.10 (C-7), 129.25 (C-8),
132.56 (C-9); isomer 2: 30.84 (C-1), 198.34 (C-2), 129.37 (C-3), 164.57 (C-4), 22.75 (C-5), 152.25
(C-6), 122.46 (C-7), 129.00 (C-8), 133.48 (C-9).
The same method was adopted to prepare 3-phenylhydrazono-4-imino-2-pentanone (IVb), 3-(3-
nitrophenylhydrazono)-4-imino-2-pentanone (IVe), 3-(4-chlorophenylhydrazono)-2-(4-methylphenyl-
imino)-4-pentanone (Va), 3-(4-chlorophenylhydrazono)-2-phenylimino-4-pentanone (Vb), and
3-(4-chlorophenylhydrazono)-2-(4-chlorophenylimino)-4-pentanone (Vc). Compounds Va–Vc were
purified by column chromatography (silica gel, benzene–chloroform 1 : 1). The melting points and
elemental analyses of the synthesized compounds are presented in Table I.
TABLE I
Melting points and elemental analyses of the 3-phenylhydrazono-4-imino-2-pentanones prepared
Calculated/Found
Compound
M.p., °C
128–129
M.w.
203.2
% C
% H
% N
IVb
65.01
65.29
6.45
6.26
20.67
20.95
55.59
55.35
5.09
4.87
17.68
17.50
IVc
IVe
Va
185–187
162–164
130–132
237.7
248.2
327.8
53.22
52.98
4.87
4.76
22.57
22.55
65.95
65.62
5.53
5.42
12.82
12.88
65.07
65.31
5.14
5.42
13.39
13.54
Vb
Ve
120–122
118–120
313.8
348.2
58.64
58.36
4.34
4.41
12.07
12.02
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

1370
Machacek, Cegan, Halama, Roznavska, Sterba:
For compound Va the 1H NMR spectrum: 15.88 b, 1 H (NH); 7.29 and 7.30 AA′BB′, 4 H (4-
chlorophenyl); 6.76 and 7.18, AA′XX′, 4 H (4-tolyl); 2.54 s, 3 H (COCH3); 2.35 s, 3 H (ArCH3);
2.30 s, 3 H (=NCCH3). 13C NMR spectrum: 20.63 (C-1), 165.56 (C-2), 133.48 (C-3), 198.02 (C-4),
27.02 (C-5), 143.38 (C-7), 120.92 (C-8), 129.73 (C-9), 129.59 (C-10), 20.87 (C-11), 142.45 (C-12),
117.25 (C-13), 129.42 (C-14), 134.64 (C-15).
3-Phenylhydrazono-2,4-pentanedione (VIb) and 3-(4-chlorophenylhydrazono)-2,4-pentanedione
(VIc) were prepared by azo coupling of benzenediazonium chloride and 4-chlorobenzenediazonium
chloride, respectively, with 2,4-pentanedione in a known way16,17. For compound VIb: m.p. 87–88 °C,
ref.16 gives m.p. 89 °C; for compound VIc: m.p. 135–137 °C, ref.17 gives m.p. 132–133 °C.
1
For compound VIc the H NMR spectrum: 14.65 b, 1 H (NH); 7.34 and 7.35 AA′BB′, 4 H (Ar);
2.59 s, 3 H (CH3); 2.47 s, 3 H (CH3). 13C NMR spectrum: 31.53 (C-1), 197.92 (C-2), 133.21 (C-3),
196.65 (C-4), 26.43 (C-5), 139.98 (C-6), 117.12 (C-7), 129.56 (C-8), 130.74 (C-9).
Kinetic Measurements
The kinetic measurements were carried out at 25 °C with the use of a Specord UV-VIS spectro-
photometer. The solutions of substituted benzenediazonium salts IIIa–IIIc and IIIf (4-CH3, H, 4-Cl,
3-NO2) were prepared in the following way: 0.05 mol amine was dissolved in 50 ml 2.5 M HCl and
diazotized by adding 20 ml 2.5 M NaNO2 at 0 °C. The excess nitrous acid was removed by addition
of amidosulfonic acid, and the solution volume was adjusted to 100 ml by adding ice water.
The azo coupling rate of enamine I with the diazonium ions IIIa–IIIc was measured in a water–
methanol mixture (20% (v/v) methanol): 1.6 ml aqueous acetate buffer [CH3COONa]/[CH3COOH] = 10
or 5, [CH3COONa] = 0.1 mol l–1 (the ionic strength was adjusted at I = 0.5 mol l–1 by adding NaCl
solution) was placed into a quartz cell, and 0.1–0.4 ml methanolic solution of enaminone (5 . 10–3 mol l–1)
and 0.3–0.0 ml methanol were added thereto (final volume 2 ml), whereupon 30 µl 0.002 M diazo-
nium salt solution (IIIa–IIIc) was injected. The absorbance increase was measured at λ 375 nm. The
rate constants kobs were calculated from the relation
k
obst = –2.3 log (A∞ – At) + const.
Reaction of enamine I with 3-nitrobenzenediazonium chloride (IIIf) in 20% (v/v) methanol: at the
time t = 0, 0.2 ml 0.01 M diazonium salt IIIf was added to a mixture of 3 ml aqueous acetate buffer
[CH3COONa]/[CH3COOH] = 10; [CH3COONa] = 0.5 mol l–1, 13 ml aqueous 0.5 M NaCl, and 4 ml
0.001 M enaminone I in methanol. After a certain time interval ∆t (∆t = 0–10 s), 5 ml 0.01 M 4,5-di-
hydroxynaphthalene-2,7-disulfonic acid was added, and the absorbance was measured at 510 nm. In
this way, a series of measurements were carried out with various ∆t intervals. The rate constant kobs
was calculated from the equation
k
obst = –2.3 log (At – A∞) + const.
Reactions of enaminones IIa–IIe with substituted benzenediazonium ions in aqueous 2-methyl-2-
propanol (50% (v/v)): 0.1 or 0.2 ml aqueous acetate buffer ([CH3COONa]/[CH3COOH] = 0.2;
[CH3COONa] = 0.1 mol l–1), 1.2 ml mixture 2-methyl-2-propanol–water (5 : 1 (v/v)), 0.05–0.20 ml
0.1 M NaOH, 0.1–0.4 ml 0.05 M 4-chlorobenzenediazonium chloride, and water (up to the total volume
of 2 ml) were mixed in a quartz cell (d = 1 cm). A drop of 0.005 M enaminone IIa–IIe was added to
this mixture, and the absorbance–time dependence was measured at 375 nm. The amount of 0.1 M
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Reactions of 4-Amino-3-penten-2-one
1371
NaOH added into the cell to neutralize the free acid from the diazonium salt solution was determined
by titration of this solution (Methyl Orange as indicator) in a parallel experiment. The same proce-
dure served for determination of the kobs constants of azo coupling reactions of enaminones IIa–IIe
with 4-methylbenzenediazonium chloride IIIa using 0.5 M diazonium salt, 0.4 M NaOH, and 0.05 M
methanolic enaminone solutions.
RESULTS AND DISCUSSION
The reactions taking place during azo couplings of enaminones with diazonium ions are
represented in Scheme 1.
The hydrolysis of the azo coupling product is catalyzed by the proton, that of enami-
none by other acids, too4. The hydrolysis also depends on the solubility of the product
and reaction time. The decomposition of diazonium salt is catalyzed by the lyate ions,
methoxide ions being much more effective than hydroxide ions since their reaction
produces Ar–N=N–OCH3 which is rapidly decomposed, whereas the diazohydroxide
produced in the reaction with OH– is transformed into the stable diazotate. We have
found that the decomposition of diazonium salts is also catalyzed by other bases, e.g.,
acetate ion18.
The effect of pH on azo coupling reactions of enaminones in acetate buffers is neg-
ligible. Practically all enaminone is present in its reactive neutral form, the diazonium
ion being present as such. Acetate buffer in water–acetone mixture (1 : 1 (v/v)) turned
out to be the most suitable for synthesis of the coupling products. In this medium, the
SCHEME 1
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

1372
Machacek, Cegan, Halama, Roznavska, Sterba:
enaminones are well soluble and the coupling products are only negligibly soluble,
hence their hydrolysis is limited. Also the decreased water content in the reaction me-
dium slows down the hydrolysis. The coupling reaction with enaminone in acetate buffer is
far faster that the decomposition of diazonium ion, and the coupling reaction with
acetone19 is insignificant as compared with the coupling with enaminone (at a concen-
tration of enaminone of ca 10–4 mol l–1 and with excess diazonium ion – as compared
with the substrate – the coupling reaction with acetone becomes significant in water–
acetone 1 : 1 mixtures).
The hydrolysis of coupling products was negligible at the synthetic conditions de-
scribed provided the product was separated by suction several minutes after mixing the
components. Attempts at recrystallization of the wet raw product from methanol ended
in its almost total hydrolysis.
Attempts at preparation of the coupling products from diazonium salts and sub-
stituted ethyl 3-phenylamino-2-butenoates showed that hydrolysis of the starting ena-
minones is faster than the coupling reactions. Therefore, no further experiments with
these compounds were carried out.
Enaminones can exist in two tautomeric forms (Eq. (B)), the enamino tautomer being
substantially more stable20,21
.
(B)
1
In H NMR spectra of substituted 4-phenylamino-3-penten-2-ones in CDCl3 (Tables II
and III), the absorptions of methine protons in all the cases exhibited chemical shifts
little affected by substituents in the nucleus (δ(CH) 5.16 ± 0.12), and NH groups ap-
peared as bound by a strong intramolecular hydrogen bond to the carbonyl group
(δ(NH) 12.42 ± 0.07). The presence of methylene group of ketimine was not observed
in any of the cases. Also the solution of enaminone IIb in hexadeuteriodimethyl sulfoxide
showed only the presence of enamine (δ(CH) 5.25; δ(NH) 12.51). The found N–H
1
interaction with the coupling constant J(15N,1H) ≈ 89 Hz agrees with the enamine
tautomeric structure.
4-Amino-3-penten-2-one I also exists in enamine form exclusively. The chemical
shift of proton at low field, which is involved in the intramolecular hydrogen bond with
carbonyl group, is almost independent of solvent and temperature (Table II).
The 13C NMR spectra of compounds I and II showed C-3 carbon absorption with δ ≈ 97
confirming the enamine structure (Table III).
The azo coupling products from diazonium salts and enamine I possess two acidic
protons and can – theoretically – exist in a series of tautomeric and isomeric forms
1
differing in the arrangement of the intramolecular hydrogen bonds. The H NMR spec-
trum of compound IVc in CDCl3 shows two sets of signals (see Experimental) which
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Reactions of 4-Amino-3-penten-2-one
TABLE II
1373
Chemical shifts δ(1H) of protons in substituted 4-amino-3-penten-2-ones; coupling constants
1
4J(1H,1H) and J(15N,1H) (in CDCl3)
Compound δ(1) s
δ(3) b
δ(5) d
δ(6) b δ(6′) b
δ(8)
δ(9)
δ(10) 1J(15N,1H)
I
2.01
5.01
1.90
0.3b
9.70
5.37
–
–
–
95.0a
Ic
1.90
2.07
4.93
5.14
1.86
9.52
7.47
–
–
–
–
–
IIad
1.93
0.3b
12.38
–
6.97
7.11
89.5
AA′
XX′
IIb
IIc
IIfe
2.06
2.06
2.07
5.15
5.17
5.18
1.93
0.4b
12.49
12.40
12.40
–
–
–
7.04
m
7.28
m
7.12
m
89.4
89.3
89.1
1.94
0.5b
7.00
7.26
–
AA′
XX′
1.95
6.94
7.41
–
a For NH group with δ 5.37; b 4J(1H,1H); c in hexadeuteriodimethyl sulfoxide; d δ(Ar–CH3) 2.31; e IIf,
4-(4-bromophenylamino)-3-penten-2-one.
TABLE III
Chemical shifts δ(13C) of carbon atoms in substituted 4-amino-3-penten-2-ones (in CDCl3)
Compounda
δ(1)
δ(2)
δ(3)
δ(4)
δ(5)
δ(7)
δ(8)
δ(9)
δ(10)
I
28.98
29.00
29.05
29.11
29.16
196.26
195.77
196.04
196.40
196.47
95.34
97.11
97.51
98.02
98.13
161.55
160.56
160.15
159.56
159.45
21.93
–
–
–
–
IIab
IIb
IIc
IIfd
19.64 135.97c 124.74 129.55 135.37c
19.72
19.68
19.72
138.65 124.81 128.99 125.46
137.28 125.72 129.10 130.86
137.80 126.00 132.08 118.58
a For numbers of δ(13C) see Table II; b δ(ArCH3) 20.79; c they may be interchanged; d IIf, 4-(4-bro-
mophenyl)-3-penten-2-one.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

1374
Machacek, Cegan, Halama, Roznavska, Sterba:
were assigned to two possible isomers differing in the type of hydrogen bond. Isomer 1
with hydrogen bond type C=O…H–N forms about 82% of the mixture. In the 13C NMR
spectrum, too, there is one set of signals of higher intensity and another set of substan-
tially lower intensity. Since none of the signals of aromatic CH groups possesses a
chemical shift lower than ca δ 122, it is obvious that both isomers of compound IVc are
azo compounds22 whose structures 1 and 2 are given.
Also for compound Va we can presume several structures differing in tautomeric
1
form and type of hydrogen bond. However, both H and 13C NMR spectra indicate a
predominant (95%) presence of one form in CDCl3. On the basis of the presence of
strong hydrogen bond (δ(NH) 15.88), similar to that in compounds VIb (ref.23) and VIc,
1
and the observed coupling constant J(15N,1H) ≈ 81 Hz we suggest the structure of
hydrazo tautomer for this compound. The tautomerism of coupling products of diazo-
nium ions and enaminones will be studied in more detail elsewhere.
The experimental conditions for kinetic measurements had to be chosen in such a
way as to:
a) slow down as much as possible the hydrolysis of both enaminone and coupling
product (which requires the lowest possible concentrations of H+ and HA acids)
b) minimize as much as possible the decomposition of diazonium ion (which requires
the lowest possible concentrations of nucleophiles inclusive of lyate ions)
c) make the product sufficiently soluble in the reaction medium (so that it may fulfil
the Lambert–Beer law).
The possibility was also considered of working in aqueous medium with the use of
4-sulfobenzenediazonium ion, but it turned out that the coupling product undergoes
rapid decomposition when using excess diazonium ion as compared with the substrate
in both water and 20% (v/v) methanol. The coupling reactions of 4-amino-3-penten-2-
one with substituted benzenediazonium ions could be followed in 20% (v/v) aqueous
methanol and acetate buffer [CH3COONa]/[CH3COOH] = 10 or 5. The decomposition
of diazonium salts IIIa–IIId as well as the hydrolysis of enaminone I are so slow at
these conditions that they did not need to be taken into account. (3-Substituted ben-
zenediazonium ions are decomposed much faster than the 4-substituted ones. The de-
composition course is so kinetically complex and nonreproducible at the conditions
described that the 3-substituted diazonium ions could not be used for kinetic measurements.)
At higher concentrations of acetic acid in the buffer ([CH3COOH] ca 0.1 mol l–1),
perceptible hydrolysis of enaminone took place. Therefore, the highest concentration of
acetic acid used in kinetic experiments was 0.02 mol l–1.
All the kinetic experiments were carried out with an at least ten fold excess of enami-
none as compared to the diazonium ion. The absorbance was measured at the λmax of
the product where the enamine I absorbs but negligibly. The reactions always pro-
ceeded as pseudomonomolecular and first order in enaminone I (Table IV). The coupling
rate is defined by relation (1).
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

Reactions of 4-Amino-3-penten-2-one
1375
+
v = k2[ArN+2][I] = kobs[ArN2 ]
(1)
In some cases the reaction rate was measured with excess diazonium ion as com-
pared to the enamine I. The rate constants k2 obtained from both the procedures are
identical. The value of k2 is affected neither by the buffer concentration nor by the ratio
of buffer components (Table IV). The coupling reaction can be represented by the
mechanism given in Eq. (C).
(C)
TABLE IV
Rate constants k2 and k2 (l mol–1 s–1) of azo coupling reactions of substituted benzenediazonium ions
IIIa–IIId with 4-amino-3-penten-2-one (I) in 20% (v/v) aqueous methanol and acetate buffer at 25 °C
Diazonium
ion
[CH3COONa]a
. 102
[CH3COOH]a
. 103
kobs . 103
[I]a . 104
k2
k2
s–1
IIIa
10
5
2.25
40
8
5
2.25
40
8
2.29
2.33
2.31
0.656
1.25
2.05
2.46
2.29
2.33
2.31
2.62
2.50
2.73
2.46
2.31 ± 0.01
10
10
2.5
5.0
7.5
10
2.58 ± 0.06
8
8
8
8
8
8
IIIb
IIIc
IIId
2.5
7.5
10
8
8
8
8
8
8
2.24
7.22
9.55
8.96
9.63
9.55
9.38 ± 0.21
31.6 ± 0.2
1 208 ± 32
2.5
3.75
10
8
8
8
8
8
8
7.97
11.90
31.20
31.88
31.73
31.20
1.0
2.0
8
8
8
8
124
235
1 240
1 175
a Concentrations in mol l–1.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

1376
Machacek, Cegan, Halama, Roznavska, Sterba:
The rate-limiting step is the attack of neutral enamine molecule by the diazonium ion
or splitting off of the proton from intermediate VII. In no case any catalysis of CH bond
splitting in intermediate VII by acetate ion was observed. However, it cannot be ex-
cluded that the proton is split off predominantly by water, and the acetate ion catalysis
is insignificant. The concentration of intermediate VII must be low as compared with
those of starting enaminone and reaction product since the absorbance–time dependence
showed the exponential character throughout the experiment in all the kinetic runs.
The rate constants k2 for azo coupling of enaminone I with substituted benzenediazo-
nium ions were correlated with the Hammett σ constants using the two-parameter equ-
ation (2) by Yukawa and Tsuno. The value of parameter r = 0.29 was obtained in a
study24 of azo coupling reaction rates of benzenediazonium ions with 6-hydroxynaph-
thalene-2-sulfonic acid.
log k2 = (1.02 ± 0.11) + (2.72 ± 0.28) [σ + 0.29(σ+ – σ)]
r = 0.991
(2)
The azo coupling reactions with enaminones can be compared with C-coupling reac-
tions of anilines. A study25 of C-coupling reactions of substituted benzenediazonium
ions with aniline showed the values ρ = 3.92 and k02 = 0.875 l mol–1 s–1, the latter being
one order of magnitude lower than k02 for azo coupling reactions with enaminone I (Table IV)
in spite of the fact that enaminone I contains a deactivating carbonyl group. The main
reason of lowered reactivity in C-coupling of aniline is the loss of resonance energy in
the transition state. In contrast to the azo coupling with aniline, no coupling at nitrogen
atom was observed in the case of enaminones.
The azo coupling reactions with substituted 4-phenylamino-3-penten-2-ones (II)
could not be followed in the medium of 20% (v/v) methanol since the reaction was
accompanied by decrease in absorbance of the product due to association. The associ-
ation was not observed in the medium of 30% (v/v) methanol, however, in this medium
diazonium salts are rapidly decomposed in acetate buffers and, on the other hand, en-
aminones are rapidly hydrolyzed in more acidic solutions.
A suitable solvent in this case was a 1 : 1 (v/v) mixture of 2-methyl-2-propanol and
water and acetate buffer of concentration ratio [CH3COONa]/[CH3COOH] = 5. In this
medium it was possible to measure the azo coupling rate of enaminone I with 4-chloro-
benzenediazonium chloride both with excess enaminone and with excess diazonium ion.
In both the cases an identical value of rate constant was found, viz. k2 = 28.9 l mol–1 s–1.
This value is by only 10% lower than k2 value for reactions of the same compounds in
20% (v/v) aqueous methanol.
The azo coupling reaction rates of 4-chlorobenzenediazonium chloride (IIIc) and 4-
methylbenzenediazonium chloride (IIIa) with enaminones IIa–IId were measured in the
acetate buffer [CH3COONa]/[CH3COOH] = 5 in 1 : 1 (v/v) mixture of 2-methyl-2-pro-
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Reactions of 4-Amino-3-penten-2-one
1377
panol and water. The coupling reactions with enaminone IIe were too slow and decom-
position of the diazonium salts was considerable, hence the k2 values found are loaded
with a large error. A series of kinetic measurements was carried out with excess diazo-
nium salt as compared to enaminones IIa–IId since the enaminones exhibit considerable
absorptions at λmax of the products, which prevented the measurements with excess
enaminones. The k2 values found are presented in Table V. The logarithms of k2 were
correlated with the Hammett σ constants. The slopes of dependences (ρ) are almost
TABLE V
Rate constants k2 and k2 (l mol–1 s–1) of azo coupling reactions of 4-methyl- (IIIa) and 4-chlorobenzene-
diazonium (IIIc) ions with enamines IIa–IId in acetate buffer at [CH3COONa] = 0.01 mol l–1 and
[CH3COOH] = 0.05 mol l–1 in a 1 : 1 (v/v) mixture of water and 2-methyl-2-propanol at 25 °C
[IIIa,IIIc]a
. 103
kobs . 103
Diazonium ion
Enaminone
k2 . 102
k2 . 102
s–1
IIIa
IIa
IIb
IIc
IId
50
100
50
4.44
9.25
3.40
6.08
1.13
2.23
0.62
1.29
8.88
9.25
6.80
6.08
2.26
2.23
1.24
1.29
9.07 ± 0.18
6.44 ± 0.60
2.24 ± 0.01
1.26 ± 0.03
100
50
100
50
100
IIIc
IIa
IIb
8.34
4.17
2.08
4.17
6.26
8.34
4.17
8.34
4.17
8.34
4.17
8.34
9.87
5.18
1.66
3.35
5.13
6.90
3.28b
6.80b
1.13
2.23
0.57
1.12
118
121 ± 2
123
79.8
80.3
81.9
82.7
78.7
81.6
27.1
26.7
13.7
13.4
81.2 ± 1.2
80.1 ± 1.2
26.9 ± 0.2
13.6 ± 0.2
IIc
IId
a In mol l–1; b in acetate buffer with [CH3COONa] = 0.005 mol l–1 and [CH3COOH] = 0.025 mol l–1.
Collect. Czech. Chem. Commun. (Vol. 60) (1995)

1378
Machacek, Cegan, Halama, Roznavska, Sterba:
identical for the diazonium ions IIIa (–1.82) and IIIc (–1.88), see Eqs (3) and (4),
respectively.
log k2 = –1.23 – 1.82 σ
log k2 = –0.13 – 1.88 σ
(3)
(4)
The ρ constants for dissociation of substituted anilinium ions in water–ethanol mix-
tures exhibit the values of 2.5–3 (ref.26). That means that in the activated complex VIII
of the azo coupling reaction investigated there must be a considerable positive charge
at the nitrogen atom of phenylamino group.
This work was supported by the Grant Agency of the Czech Republic, Project No. 203/93/2065.
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