Solution and solid-state study of the structure of azo-coupling products from isomeric enaminones possessing tert-butyl group: An unprecedented observation of pure hydrazo form in azo coupled N-alkyl β-enaminones

Article abstract of DOI:10.1016/j.molstruc.2014.06.069

The structure of the azo-coupling products from enaminones derived from 4,4-dimethyl-1-phenylpentane-1,3-dione has been studied by means of solution-state ¹H, ¹³C and ¹⁵N NMR spectroscopy and X-ray diffractometry. The presence of bulky tert-butyl group hinders or even prevents from the formation of planar conjugated heterodiene system HNCCNN with an intramolecular hydrogen bond NH?N which is the prerequisite for fast tautomeric exchange imino-hydrazo - enamino-azo. The minor amount of azo compounds is formed by a proton exchange through a hydrogen bond NH?N, which is either intramolecular (in solution) or intermolecular (solid state). The intermolecular exchange proceeds via the dimers of the azo coupling products. This is unprecedented result among the similar molecules hitherto studied.

Full text of DOI:10.1016/j.molstruc.2014.06.069

Journal of Molecular Structure 1075 (2014) 187–195
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Solution and solid-state study of the structure of azo-coupling
products from isomeric enaminones possessing tert-butyl group:
An unprecedented observation of pure hydrazo form in azo coupled
N-alkyl b-enaminones
b
a
Petr Šimu˚ nek a, , Zdenka Padelková , Vladimír Machácek
⇑
ˇ
ˇ
ˇ
a University of Pardubice, Faculty of Chemical Technology, Institute of Organic Chemistry and Technology, Studentská 573, CZ-532 10 Pardubice, Czech Republic
b University of Pardubice, Faculty of Chemical Technology, Department of General and Inorganic Chemistry, Studentská 573, CZ-532 10 Pardubice, Czech Republic
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
ꢀ
Isomeric azo coupled b-enaminones
with tert-butyl group were
synthesized.
ꢀ
ꢀ
tert-Butyl group prevents conjugation
in heterodiene system N@CAC@NAN.
Pure hydrazo forms in azo coupled
N-alkylenaminones was observed.
a r t i c l e i n f o
a b s t r a c t
Article history:
The structure of the azo-coupling products from enaminones derived from 4,4-dimethyl-1-phenylpen-
tane-1,3-dione has been studied by means of solution-state 1H, 13C and 15N NMR spectroscopy and
X-ray diffractometry. The presence of bulky tert-butyl group hinders or even prevents from the formation
of planar conjugated heterodiene system HANAC@CAN@N with an intramolecular hydrogen bond
NAHꢁ ꢁ ꢁN@ which is the prerequisite for fast tautomeric exchange imino-hydrazo – enamino-azo. The
minor amount of azo compounds is formed by a proton exchange through a hydrogen bond NAHꢁ ꢁ ꢁN,
which is either intramolecular (in solution) or intermolecular (solid state). The intermolecular exchange
proceeds via the dimers of the azo coupling products. This is unprecedented result among the similar
molecules hitherto studied.
Received 21 March 2014
Received in revised form 23 June 2014
Accepted 23 June 2014
Available online 30 June 2014
Keywords:
b-enaminones
Azo coupling
Azo-hydrazone tautomerism
15N NMR
Ó 2014 Elsevier B.V. All rights reserved.
X-ray
Hydrogen bonds
Introduction
the tautomers behaves spectrally as the single compound and gives
only one set of signals in 1H, 13C and 15N NMR spectra. This set is a
Azo-hydrazone tautomerism belongs among the equilibria
being very fast on NMR time scale. That means the mixture of both
weighted average from the individual tautomers. The proportion of
the individual tautomers should be estimated from this single set
of signals (mathematically one equation for two unknowns).
Spectra containing the signals of both the tautomers have never
been hitherto observed under any conditions. The question of
⇑
Corresponding author. Tel.: +420 466 037 039; fax: +420 466 037 068.
E-mail address: petr.simunek@upce.cz (P. Šimu˚ nek).
http://dx.doi.org/10.1016/j.molstruc.2014.06.069
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

188
P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
determining the equilibrium constant KT for azo-hydrazone tau-
tomerism for the products of azo coupling of benzenediazonium
with phenols, naphthols and aromatic amines has been success-
fully solved by means of 15N NMR spectroscopy in solution (CDCl3).
The parameters relevant for the determining of the constant are
either d(15N) of the nitrogens of the tautomeric groups or the val-
ues of the coupling constants 1J(15NA1H) [1–6]. Even this method,
however, cannot be considered as fully exact as the results depend
on the choice of standards, but more precise results are probably
impossible to be obtained using any other method. More precise
evaluation of the equilibrium for the azo coupling products with
diazonium ions other than benzenediazonium is also not possible,
as the substitution on the diazonium ion affects both the position
of the tautomeric equilibrium [7] and the chemical shifts of the
nitrogens through the substitution effect [8] and the corresponding
15N NMR parameters for potential standards have not been hith-
erto systematically studied. Qualitatively it is known that elec-
tron-donating substituents on the diazonium ion (NMe2, OMe,
Me) increase the amount of the azo form whereas electron-with-
drawing ones (NO2) increase the amount of the hydrazo form [7].
To what extent is the methodology using 15N NMR parameters
applicable to the determination of the tautomeric equilibrium for
the aliphatic azo coupling products, as long as the products can
undergo the tautomeric rearrangement, was not quite clear
although 15N NMR data has already been used for this purpose
[9,10]. Another approach to study azo-hydrazone tautomerism is
theoretical, by means of quantum chemistry calculations (from
many examples e.g. [11,12]. AM 1 Geometry optimization was also
used for structural studies on some azo coupled enaminones [13].
In the case of the products of azo coupling on aliphatic sub-
strates it has been said up to now in the literature [14,15] that
there does not exist azo form in the cases where the tautomeric
rearrangement to hydrazo form is possible.
Scheme 2. Possible tautomeric forms of the azo coupled b-enaminones.
coupling to aliphatic substrates can exist as stable azo compounds
(although the rearrangement into the hydrazone form is possible),
are contradiction of still existing opinion [14,15].
The results obtained during the study of azo coupling on iso-
meric enaminones derived from benzoylacetone [22] indicate the
possibility of steric effects on the structure of the products. The
effects were significant and comprised both the affecting the posi-
tion of the tautomeric equilibrium and E/Z isomerism and the kind
of hydrogen bonding. In some cases a small change of the structure
of the enaminone led to quite different products [28].
In the present work we are dealing with the study of the struc-
ture of the azo coupling products on some sterically hindered
enaminones containing tert-butyl group both in solution and in
solid state. An introduction of a tert-butyl group generally causes
an increasing the steric tension in the molecule and represents a
convenient model for this kind of studies.
Experimental
Methods
When the reaction is carried out on a compound XACHYAZ, the
azo form does not have a hydrogen that can lead to the tautomer-
ism. When at least one of the substituents (X, Y, Z) at the quater-
nary centrum is acyl (RAC@O) then the product in this case is
also hydrazone and not azo compound. In fact, compounds type
1 are seldom isolable and undergo Japp–Klingemann reaction to
give the corresponding hydrazones 2 [15–17] (Scheme 1).
From the tautomeric point of view the major attention in the lit-
erature has been devoted to the products of azo coupling with b-
enaminones [9,10,18–27]. These products contain three potentially
tautomeric groups and can theoretically exist in several tautomeric
forms (Scheme 2): keto-enamino-azo, keto-imino-hydrazo and
enol-imino-azo.
During the past several years we have accomplished relatively
extensive survey how the structure of both the starting b-enami-
none and diazonium salt affects the structure of the azo coupling
products including the position of the tautomeric equilibrium
[9,10,18–27]. The results obtained are summarized in the review
[26]. It can be stated that the study of the azo coupled enaminones
constitutes a complex problem with a number of variables (struc-
tures both the substrate and diazonium salt, solvent, ...). Among
the results following from the studies is the fact that a number
of the azo coupled enaminones exist to less or more extent as
azo compounds. The discovery, that even the products of the azo
NMR data were obtained in CDCl3 using Bruker AVANCE III
spectrometer operating at 400.13 MHz (1H), 100.62 (13C) and
40.55 MHz (15N) equipped with 5 mm broad-band probe having
magnetic field gradients in z-direction. All the pulse programs
were taken from the Bruker sequence library. The spectra were
calibrated on internal TMS (1H, d = 0.00 ppm), the middle signal
of the solvent multiplet (13C, d = 77.0 ppm) and on external neat
CH315NO2 placed in a coaxial capillary (15N, d = 0.0), resp. 13C NMR
spectra were measured in a standard way using broadband proton
decoupling. 15N Chemical shifts (Table 1) were obtained by an
indirect detection using both 2D 1H@15N HMBC and 2D 1H–15
N
HMQC pulse sequences. The HMBC measurements were optimized
on 1J(1H,15N) = 90 Hz and long-range 1H,15N couplings 3.5 Hz. The
HMQC sequence was optimized on one-bond coupling 90 Hz. The
gradient ratios for both the sequences were 70:30:50.1, time
domain 4 k ꢂ 128, sine-bell windowing in both dimensions. The
values of 1J(1H,15N) were read either at natural abundance using
1D 1HA15N HMBC pulse sequence or from satellites in proton
spectra (in the case of 15N enriched samples 3a and 4). 2D NOESY
spectra were measured in States-TPPI acquisition mode, mixing
time 800 ms, time-domain 2 k ꢂ 128 zero-filled to 2 k ꢂ 1 k, QSINE
multiplication in both dimensions, relaxation delay set to 2 s,
number of dummy scans 16, 88 scans per t1 increment. The
phasing was carried giving negative diagonal peaks.
The X-ray data (Table 2) for crystals of 3a–d and 4 were
obtained at 150 K using Oxford Cryostream low-temperature
device on a Nonius Kappa CCD diffractometer with Mo K
a radia-
tion (k = 0.71073 Å), a graphite monochromator, and the / and
v
scan mode. Data reductions were performed with DENZO-SMN
[29]. The absorption was corrected by integration methods [30].
Structures were solved by direct methods (Sir92) [31] and refined
by full matrix least-square based on F2 (SHELXL97) [32]. Hydrogen
atoms were mostly localized on a difference Fourier map, however
2
Scheme 1. The fate of the acyl-substituted aliphatic azo compounds.

P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
189
Table 1
15N NMR data for compounds 3 and 4 in CDCl3.
R
d(15N )
d(15Nb)
d(15N )
1J(15N , 1H)(Hz)
d1H(NH)
A/B ratio
a
c
a
a
3aA
3aB
H
ꢃ237
ꢃ35
ꢃ5
92
71
8.05b
14.87
8.05b
14.74
8.03b
14.60
4/1
2/1
2/1
2/1
a
ꢃ172
3bA
3bB
4-F
ꢃ239
ꢃ127
ꢃ34
ꢃ57
93
48
a
a
3cA
3cB
4-OMe
3-CF3
H
ꢃ238
ꢃ34
ꢃ57
93
41
a
a
a
3dA
3dB
ꢃ239
ꢃ166
ꢃ235
ꢃ37
ꢃ54
93
61
8.06b
14.93
7.77b
a
a
a
4
ꢃ34
92
a
Not detected.
b
Concentration-dependent value (for 20 mg/0.6 mL of the solvent shown).
to ensure uniformity of treatment of crystal, all hydrogen were
recalculated into idealized positions (riding model) and assigned
temperature factors Hiso(H) = 1.2 Ueq(pivot atom) or of 1.5Ueq for
the methyl moiety with CAH = 0.96 and 0.93 Å for methyl and
hydrogen atoms in aromatic ring, respectively. In all compounds
3 the difference Fourier map showed diffuse electron density
between nitrogen 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 fac-
tors in the range 69–81% for H1 and 31–19% for H3 for compounds
3a–3d. The CF3 group in 3d is disordered and the disorder was
treated/split to two positions with standard methods implemented
in SHELXL97 [32]. The major tautomer is refined in cases of all
compounds.
Elemental analyses were performed with a Flash 2000 CHNS
Elemental Analyzer. Melting points were measured with a Kofler
hot-stage microscope Boetius PHMK 80/2644 and were not cor-
rected. Mass spectra were measured with a high-resolution MALDI
mass spectrometer LTQ Orbitrap XL (Thermo Fisher Scientific, Bre-
men) equipped with a nitrogen UV laser (337 nm, 60 Hz). The LTQ
Orbitrap instrument was operated in the positive ion mode over a
normal mass range (m/z 50–2000). The used matrix was trans-2-
[3-(4-tert-butylphenyl)-2-methylprop-2-en-1-ylidene]malononit-
rile (DCTB). Mass spectra were averaged over the whole MS record
for all measured samples.
Material
All the solvents and reagents were obtained from commercial
sources and were used without further purification. Diazonium tet-
rafluoroborates were prepared according to a standard procedure
[19]. 4,4-Dimethyl-1-phenylpentane-1,3-dione (1) was obtained
commercially (Alfa Aesar). Compounds 2–4 were prepared using
the protocol published in [27]. 15N enriched compounds were pre-
pared from double 15N labelled diazonium tetrafluoroborate
according to the protocol published in [27]. Diazonium salt
2 ꢂ 15N was prepared in an ordinary way from 15N aniline and
15N sodium nitrite.
Data for compounds 3b–d were published in [27].
Characterization of newly prepared compounds:
3-Methylamino-4,4-dimethyl-1-phenylpent-2-en-1-one (2b)
Compound obtained as the by-product (yield 11%) from the
reaction of
1 with ethanolic methylamine according to the
Table 2
Crystallographic data for compounds 3a–d and 4.
3a
3b
3c
3d
4
Empirical formula
M
C
20H23N3O
C20H22FN3O
339.41
C21H25N3O2
351.44
C21H22F3N3O
389.42
C20H23N3O
321.41
321.41
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Monoclinic
P21/c
Monoclinic
P21/c
Orthorhombic
P212121
10.8740(10)
17.5772(18)
20.1730(14)
90
3855.8(6)
8
1.211
Orthorhombic
Pbca
12.6170(9)
17.374(2)
18.614(2)
90
4080.3(7)
8
1.268
Monoclinic
P21/c
10.1150(8)
10.1100(5)
19.5181(11)
114.553(5)
1815.5(2)
4
10.1830(5)
10.3400(6)
19.4861(12)
115.994(5)
1844.2(2)
4
12.8310(14)
22.213(2)
12.3770(10)
94.434(7)
3517.1(6)
8
b (°)
V (Å3)
Z
Dc (g cmꢃ3
)
1.176
1.222
1.214
Crystal size (mm)
(mmꢃ1
F (000)
0.59 ꢂ 0.46 ꢂ 0.27
0.48 ꢂ 0.31 ꢂ 0.26
0.27 ꢂ 0.25 ꢂ 0.16
0.38 ꢂ 0.26 ꢂ 0.23
0.30 ꢂ 0.25 ꢂ 0.24
l
)
0.074
0.084
0.079
0.098
0.076
688
720
1504
1632
1376
h; k; l range
h range (°)
Reflections
ꢃ11, 13; ꢃ11, 13; ꢃ25, 25 ꢃ13, 13; ꢃ13, 12; ꢃ25, 23 ꢃ11, 13; ꢃ22, 22; ꢃ24, 26 ꢃ16, 15; ꢃ22, 20; ꢃ24, 23 ꢃ16, 16; ꢃ28, 28; ꢃ16, 16
2.86; 27.48
15,017
2.81; 27.50
13,056
2.75; 26.50
26,787
2.96; 27.50
28,226
2.39; 27.50
54,753
measured
– independent (Rint
a
)
4093 (0.0304)
r(I)] 3154
4191 (0.0363)
3129
8353 (0.0511)
6387
4620 (0.0306)
3352
7962 (0.0465)
5541
– observed [I > 2
Parameters refined
4093
226
469
281
433
Max/min
GOFb
D
q
(e Åꢃ3
)
0.277/ꢃ0.255
0.288/ꢃ0.286
0.236/ꢃ0.231
0.299/ꢃ0.202
0.296/ꢃ0.281
1.115
1.083
1.187
1.121
1.121
Rc/wRc
0.0500/0.1070
0.0523/0.1095
0.0518/0.0966
0.0494/0.1003
0.0593/0.1085
P
P
a
b
c
Rint
=
|F2o–F2o,mean|/ F2o.
P
½
GOF = [ (w(F2o–Fc2)2)/(Ndiffrs–Nparams)] for all data.
P
P
P
P
R(F) = ||Fo|–|Fc||/ |Fo| for observed data, wR(F2) = [ (w(Fo2–Fc2)2)/( w(Fo2)2)] for all data.
½

190
P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
published procedure [22] with 2a as the main product. The struc-
ture of the products has been verified by means of 1HA13C HMBC
[27]. The compound 2b contained ca 11% of 2a.
The structure of the azo coupling products in CDCl3 solution
The estimation of the azo-hydrazone tautomerism was done
using the principles mentioned in [26].
Compounds 3a–d are, in CDCl3 solution, a mixture of two iso-
mers (A/B).
1H NMR d: 1.38 (s, 9H); 3.23 (d, J = 5.7 Hz, 3H); 5.84 (s, 1H);
7.36–7.44 (m, 3H); 7.83–7.85 (m, 2H); 12.24 (brs, 1H).
From both the values d(15N) and 1J(15N,1H) of the compounds
3a–d (Table 1) it is evident that all the major forms (A) are pure
hydrazones regardless the substitution of the starting diazonium
salt. Due to the value of d(15Nb) (for the designation of the individ-
ual nitrogens see Scheme 5) the absence of an intramolecular
hydrogen bond can be stated [33]. This is in accordance with rela-
tively low chemical shift of NH protons (d ꢄ 8).
2,2-Dimethyl-5-methylimino-5-phenyl-4-[15N2]phenylhydrazonopen-
tane-3-one (3a)
Obtained by the reaction of 2a with benzenediazonium tetra-
fluoroborate (2 ꢂ 15N). Column chromatography silica/DCM-EtOAc
20:1 v/v, yield 74%, m.p. 176–179 °C. Product is ca 4:1 mixture of
isomers. Anal. calcd. for C20H23N3O: C 74.30, H 7.17, N 13.59. Found
C 74.32, H 7.35, N 13.32.
The situation in minor isomers (B) is quite different. Relatively
high chemical shift of NH protons (d > 14 ppm) can be accounted
for the presence of an intramolecular hydrogen bond NAHꢁ ꢁ ꢁX
(X@O, N). Compounds 3aB–3dB are evidently mixtures of azo-
hydrazo tautomers (see Table 1). The hydrazone content depends
on the substitution of the starting diazonium salt and decreases
in the order R@H > 3-CF3 > 4-F > 4-OMe, see Scheme 5. (Please
note: the order is determined on the basis of 1J(15N, 1H) values,
because the chemical shifts d(15N) for minor isomers were not
obtained for all compounds). The carbonyl group in all the com-
pounds is intact by the tautomerism (according to 13C NMR
spectroscopy).
HRMS (MALDI) calcd. for C20H24N3O [M + H]+ 324.18546, found
324.18549; for
C
20H23N3NaO [M + Na]+ 346.16740, found
346.16779; for C20H23KN3O [M + K]+ 362.14134, found 362.14184.
1H NMR major form d (ppm): 1.51 (s, 9H); 3.26 (s, 3H);
7.02–7.04 (m, 1H); 7.15–7.18 (m, 2H); 7.31–7.42 (m, 5H);
7.62–7.65 (m, 2H); 8.05 (d, 1J(1H,15N) = 92.3 Hz, 1H).
1H NMR minor form d (ppm): 1.31 (s, 9H); 3.16 (s, 3H);
7.08–7.10 (m, 1H); 7.13–7.15 (m, 2H); 7.31–7.42 (m, 7H); 14.87
(d, 1J(1H,15N) = 72.6 Hz, 1H).
13C NMR major form
d (ppm): 28.0; 41.2; 44.2; 114.2
(t, J = 1.9 Hz); 122.9; 126.6; 129.0; 129.6 (d, J(13C, 15N) = 1.9 Hz);
131.1; 135.6; 137.4 (d, J(13C, 15N) = 3.4 Hz); 142.5 (dd, J(13C,
15N) = 19.2 Hz, 6.0 Hz); 163.6; 201.4 (dd, J(13C, 15N) = 10.6 Hz,
4.4 Hz).
Compound 4 (unlike its isomer 3a) exists in CDCl3 as one isomer
only, which is, on the basis of NMR data in Table 1, pure hydrazone
without intramolecular hydrogen bond.
13C NMR minor form d (ppm): 28.3; 37.6; 43.8; 116.2 (br t);
123.9; 126.8; 128.2; 128.4; 129.3 (br d, J(13C, 15N) = 1.6 Hz);
133.7; 135.0; 145.3 (dd, J(13C, 15N) = 14.4 Hz, 6.2 Hz); 166.8;
204.8 (dd, J(13C, 15N) = 10.5 Hz, 3.1 Hz).
Important structural information can be extracted from the
coupling constants J(13C, 15N) (Fig. 1). Thanks to the stereochemical
dependence of the 2J(13C, 15N) value (Scheme 4, [34,35]) it can be
used for determining the configuration on C@NANH double bond.
Based on the analysis of the values obtained for compounds 3aA/
3aB and 4 it can be assumed that all the compounds have E config-
uration on C@NANH double bond (Scheme 4). It is in accordance
with the crystallographic results (Figs. 6 and 7).
The NOE interaction in compound 3aA (Fig. 2, left) reflects the
spatial proximity of the C@NCH3 and C@NANH groups. In general,
on the basis of NOE interactions it can be stated that the structure
of the compound 3aA in solution is similar to the structure in the
crystalline state (Fig. 6). The relative spatial proximity of tert-butyl
and NCH3 groups indicates Z configuration on C@NACH3 double
bond.
Compound 3aB possesses, on the basis of the above-mentioned
interpretations of 2J(13C, 15N) values, an intramolecular hydrogen
bonding NAHꢁ ꢁ ꢁN@C. The C@NCH3 double bond of 3aB has there-
fore E configuration. That explains why the minor isomer is, on the
contrary to the major one, the mixture of azo-hydrazo tautomers.
The arrangement with the hydrogen bonding enables fast proton
transfer between two basic centres (Scheme 5). This is not possible,
neither in 3aA nor 4 and, hence, these compounds are pure hydra-
zones. The compounds 3aA/3aB are E/Z isomers. The predomi-
nance of A isomer can be caused by steric demands of nearby
groups tert-butyl and Ph in B isomer (Scheme 5).
4,4-Dimethyl-3-methylimino-1-phenyl-2-[15N2]phenylhydrazonopen-
tane-1-one (4)
Obtained by the reaction of 2b with benzenediazonium tetra-
fluoroborate (2 ꢂ 15N). Column chromatography silica/DCM-EtOAc
20:1 v/v, pale yellow solid, yield 67%, m.p. 145–150 °C. Anal. calcd.
for C20H23N3O: C 74.30, H 7.17, N 13.59. Found C 74.35, H 7.37, N
13.30.
HRMS (MALDI) calcd. for C20H24N3O [M + H]+ 324.18546, found
324.18547.
1H NMR d (ppm): 1.23 (s, 9H); 3.16 (s, 3H); 7.00–7.07 (m, 3H);
7.28–7.32 (m, 2H); 7.49–7.53 (m, 2H); 7.58–7.62 (m, 1H); 7.77 (dd,
J = 91.7 Hz; 2.5 Hz, 1H); 8.06–8.09 (m, 2H).
13C NMR d (ppm): 28.2; 39.9; 41.4; 114.1 (t, J(13C, 15N) = 1.9 Hz);
122.9; 128.0; 129.5 (d, J(13C, 15N) = 1.6 Hz); 130.4; 132.3; 136.7;
140.4 (d, J = 3.9 Hz); 142.2 (dd, J = 19.1 Hz, 6.1 Hz); 173.0 (t,
1.3 Hz); 189.5 (dd, J = 11.5 Hz, 5 Hz).
Results and discussion
From the NOE results obtained on compound 4 it follows that
tBuAC@NACH3 group is turned around the N@CAC@N single bond
in relation to the rest of the molecule (NOE interactions of tert-
butyl and NCH3 groups with both the benzene rings). The configu-
ration of C@NACH3 double bond is probably Z, which is more
favourable with respect to the spatial proximity of NCH3 group
with both the benzene rings (Fig. 2). The benzene rings are also
spatially close (as follows from NOE). The structure in solution is
then similar to the structure found in crystal (Fig. 7).
Synthesis
Starting b-enaminones 2a,b were prepared by a condensation of
the b-diketone 1 with ethanolic methylamine [27] (Scheme 3).
Both the isomers were successfully separated by column chroma-
tography which afforded us an opportunity to study the influences
of tert-butyl group in comparison with phenyl group on the struc-
ture of the azo coupling products.
Azo coupling products 3, 4 were obtained by ordinary reaction
of b-enaminones 2 with the corresponding diazonium tetrafluoro-
borates in DCM [19] (Scheme 3).
Due to a low natural abundance of 15N isotope the values of
J(13C, 15N) could be determined only for isotopically enriched com-
pounds 3a and 4 nevertheless similar conclusions can be made also

P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
191
2
+
-
2
4
+
-
2
4
Scheme 3. Synthesis of the starting enaminones 2 and azo coupling products 3, 4.
From the tautomeric point of view, compound 4 can be com-
pared with compound 5 (Scheme 6) formed by azo coupling on
3-methylamino-1-phenylbut-2-en-1-one (4-methylphenyldiaze-
nyl group is very similar to phenyldiazenyl group). The planar con-
jugated system in the molecule of 5 enables the formation of
strong intramolecular hydrogen bond NAHꢁ ꢁ ꢁN@ and, hence, the
presence of substantial amount of azo form in the tautomeric mix-
ture (about 50%) [24]. The molecule of 4 is distorted due to the
presence of bulky tert-butyl group and exists as a pure hydrazo-
form having intermolecular hydrogen bond NAHꢁ ꢁ ꢁO@C with a
carbonyl oxygen which do not participate in the tautomeric
exchange.
Similar conclusions can be drawn also upon comparing the
structures of azo coupling products from 2,2-dimethyl-5-methyl-
amino-5-phenylpent-4-en-3-one 3 and 4-amino-4-phenylbut-3-
en-2-one 7 [22]. On azo coupling with benzenediazonium a
mixture of two products is formed in both the cases. In the case
of the products 7 both the compounds have an intramolecular
hydrogen bonds NAHꢁ ꢁ ꢁN@ in a planar conjugated heterodiene
system RAHB (Resonant Assisted Hydrogen Bonding [36]). The
major product is a mixture of azo and hydrazo tautomers with
the former prevailing. On the other hand the major form of the
tert-butyl derivative 3 is pure hydrazone with an intermolecular
Scheme 4. The stereochemical dependence of 2J(15N, 13C) and the values found for
compounds 3a, 4.
for compounds 3b–3d. The conclusions are summarized in
Scheme 5.
On comparison the structures of 3a–d and 4 and their confron-
tation with the previously performed studies [9,10,18–27] some
conclusions on the influence of the structure of the enaminone
on the structure of the azo coupling product can be made:
Scheme 5. The structure of the azo coupling products in CDCl3.

192
P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
Fig. 1. Detail of 13C NMR spectra of compounds 3aA/3aB and 4 with the values of 15NA13C coupling constants.
3aA
4
Fig. 2. NOE interactions found for compounds 3aA and 4.
hydrogen bond NAHꢁ ꢁ ꢁN@ and the minor one is a mixture of
azo/hydrazo tautomers (the latter prevailing) with an intramolecular
hydrogen bond. The change in the tautomeric behaviour is again
induced by the presence of bulky tert-butyl group and thus by an
impossibility of achieving the planarity of the heterodiene system.
On the basis of the findings obtained by studying the products
of azo coupling to isomeric enaminones derived from benzoylace-
tone the following can be stated: the switching from the enami-
nones with primary amino group 6 to the N-methyl analogues 5
meant a slight (about 10%) increasing of hydrazo form (Scheme 6).
The switching from 6 (with benzoyl group) to its isomer 7 (with
acetyl group) meant both shift towards the hydrazone and the for-
mation of E/Z isomeric mixture differing in the position of an intra-
molecular hydrogen bonding. The minor isomer with C@Oꢁ ꢁ ꢁHAN
hydrogen bond, however, was pure azo compound. The shift from
7 to 3a, i.e. from the primary amino group to N-methyl group and
increasing the steric demands (Me ? tBu) has significant conse-
quences which is a considerable shift towards hydrazo form both
Fig. 3. The molecular structure (ORTEP 50% probability level) of 3d. Selected
interatomic distances and angles are given in Table 3.
in the major (entirely) and in minor (predominantly) forms.
Another consequence is the absence of the intramolecular hydro-
gen bond in the major isomer (Scheme 6).
Upon comparison the compounds 3 and 4 the similar trend as in
the comparison of the isomers 6 and 7 can be stated, i.e. an increas-
ing the number of forms.

P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
193
Fig. 4. The molecular structure (ORTEP 50% probability level) of 3c with intermolecular hydrogen bond interaction between two independent molecules N1AH1ꢁ ꢁ ꢁN6 and
N4AH4ꢁ ꢁ ꢁN3. Selected interatomic distances and angles are given in Table 3.
Fig. 6. The molecular structure (ORTEP 50% probability level) of 3a. Selected
interatomic distances and angles are given in Table 3.
Fig. 5. The molecular structure (ORTEP 50% probability level) of 3b. Selected
interatomic distances and angles are given in Table 3.
Both the enamino–diazenyl and imino–hydrazone heterodienic
Crystallography
fragments are tautomeric systems with an extended p-delocaliza-
tion where the tautomeric ratios cannot be determined from the
analysis of the bond distances. Similarly as in the recent reports,
these ratios have been calculated from the refined occupancies of
both tautomeric hydrogens with partial occupancies, situated in
a double well potential, obtaining, for all compounds, similar tau-
tomeric ratios 83/17 for 3a, 81/19 for 3d, 78/22 for 3b, and 69/31%
for 3c, respectively, for imino–hydrazone form. The order of the
compounds in the hydrazone content is in accordance with the
results obtained for CDCl3 solution. The tautomeric exchange in
this case, on the contrary to all the cases having been hitherto stud-
ied [9,10,18–27], is mediated by means of intermolecular and not
intramolecular exchanges of the protons between two nitrogen
Selected crystallographic data, bond lengths and bond angles
for 3a–d and 4 are summarized in Tables 2–4. ORTEP diagrams of
the compounds 3a–d and 4 are in Figs. 3–6. The molecules of com-
pounds 3a,b,d form dimers through the intermolecular hydrogen
bonds N1AH1ꢁ ꢁ ꢁN6 and N4AH4ꢁ ꢁ ꢁN3 like the molecules 3c
(Fig. 4). Compound 4 (Fig. 7) crystallizes in the same crystal system
and space group like 3a and 3b, but with eight molecules within
the unit cell forms chains due to intermolecular hydrogen bonds
instead of dimeric structures observed for the compounds 3.
Similarly to compounds reported previously [22,24,37] in all
structures of 3a–d a mixture of two tautomeric forms is present.

194
P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
Fig. 7. The molecular structure (ORTEP 50% probability level) of 4 with intermolecular hydrogen bond interaction between two independent molecules N1AH1ꢁ ꢁ ꢁO2i and
N4aAH4aꢁ ꢁ ꢁO1. Selected interatomic distances and angles are given in Table 3.
Scheme 6. Comparison the presently studied compounds 3, 4 with those previously studied.
Table 3
Selected interatomic distances (Å), angles (°) and interplanar angles (°) for compounds 3a–d and 4 (parameter for the second independent molecule is given in parenthesis).
3a
3b
3c
3d
4
O1AC1
C1AC2
N2AC2
N1AN2
C2AC3
N3AC3
O1AC1AC2
N2AC2AC1
N1AN2AC2
N2AC2AC3
N3AC3AC2
Interplanara
1.2165(19)
1.482(2)
1.2932(18)
1.3306(18)
1.509(2)
1.220(2)
1.479(2)
1.296(2)
1.335(2)
1.512(2)
1.277(2)
116.33(15)
119.97(14)
119.61(14)
125.00(15)
123.59(15)
85.8(2)
1.224(3) (1.223(3))
1.476(3) (1.480(3))
1.299(3) (1.294(3))
1.333(3) (1.338(3))
1.511(3) (1.506(3))
1.276(3) (1.272(3))
116.6(2) (116.9(2))
119.6(2) (119.0(2))
119.6(2) (120.08(19))
124.8(2) (124.9(2))
124.2(2) (123.1(2))
87.1(2) (89.0(3))
1.220(2)
1.486(2)
1.292(2)
1.3382(19)
1.508(2)
1.230(2) (1.232(2))
1.477(3) (1.478(3))
1.298(2) (1.302(2))
1.334(2) (1.330(2))
1.516(3) (1.508(3))
1.269(2) (1.270(2))
118.90(18) (118.67(17))
117.82(17) (116.51(17))
119.46(17) (119.25(17))
123.47(17) (123.77(17))
122.03(18) (123.61(17))
89.7(3) (74.0(2))
1.276(2)
1.277(2)
116.39(14)
119.15(13)
120.22(13)
125.14(14)
123.78(14)
84.6(2)
116.01(15)
119.70(14)
119.11(14)
124.36(14)
124.02(15)
79.5(3)
a
Interplanar angle between N2AC2AC3 (N5AC102AC103) and N3AC3AC2 (N6AC103AC102) planes.

P. Šimu˚nek et al. / Journal of Molecular Structure 1075 (2014) 187–195
195
Table 4
Hydrogen interactions (ÅA, °).
Appendix A. Supplementary material
0
DAHꢁꢁꢁA
d(DꢁꢁꢁA)
<(DAH)
Symm. transformation
Crystallographic data for structural analysis have been depos-
ited with the Cambridge Crystallographic Data Centre, CCDC Nos.
983844, 942937, 942938, 942936 and 983845 for 3a–d and 4
respectively. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge CB2
1EY, UK (fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk
or www: http://www.ccdc.cam.ac.uk). Supplementary data associ-
ated with this article can be found, in the online version, at http://
dx.doi.org/10.1016/j.molstruc.2014.06.069.
3a
3b
3c
N(1)AH(1)ꢁ ꢁ ꢁN(3i)
N(1)AH(1)ꢁ ꢁ ꢁN(3i)
N(1)AH(1)ꢁ ꢁ ꢁN(6)
N(4)AH(4)ꢁ ꢁ ꢁN(3)
N(1)AH(1)ꢁ ꢁ ꢁN(3i)
N(4)AH(4)ꢁ ꢁ ꢁN(6)
N(4)AH(4)ꢁ ꢁ ꢁO(1i)
N(1)AH(1)ꢁ ꢁ ꢁO(2i)
3.0666(18)
3.023(2)
2.975(3)
3.096(3)
3.071(2)
3.215(2)
3.345(2)
3.293(2)
173.5
167.7
160.9
170.9
165.4
117.2
155.4
149.1
ꢃx + 1, ꢃy, ꢃz + 1
ꢃx + 1, ꢃy, ꢃz + 2
–
–
3d
4
ꢃx + 1, ꢃy, ꢃz + 1
–
ꢃx + 1, ꢃy, ꢃz
ꢃx, ꢃy, ꢃz
atoms. On the other hand the polymeric structure of 4 made via
hydrogen bonds NAHꢁ ꢁ ꢁO@ (Fig. 7) reveals similar interatomic dis-
tances with a conjugated character but retains strictly in the
imino–hydrazo–carbonyl tautomeric form in the solid state, just
as in the CDCl3 solution. The conformation of the molecule pre-
vents from the formation of the NAHꢁ ꢁ ꢁN@ hydrogen bond neces-
sary for the imino-hydrazo – enamino-azo tautomeric exchange.
This is the major difference between the compound 4 and all the
azo coupled enaminones having been studied so far [9,10,18–27].
The carbonyl does not participate in the tautomeric exchange.
Separations between donor and acceptor atoms involved in
such a H-bond is much longer in cases of 3a–d and 4 (see Table 4)
than in analogous compounds described in the literature
(2.565(1)–2.639(3) Å) which could be explained by hydrogen
bonds assisted by resonance concept (RAHB) [36,38–40] available
for compounds where a six membered heterocyclic ring is formed
by a H-bond [22,24,37].
The planes where atoms N2AC2AC3 and N3AC3AC2 or
N5AC102AC103 and N6AC103AC102 respectively are located in
the molecules 3a–d and 4 (Figs. 3–6) make an interplanar angle
close to 90° in all the cases (Table 3). It means that the conjugation
between the methylimino and hydrazono groups does not exist.
Consequently, the bond C2AC3 or C102AC103 respectively has
the length characteristic for a single bond (1.506–1.516 Å) on the
contrary to the previously studied cases where this bond had,
due to the conjugation and tautomerism, length 1.408–1.430 Å.
This bond is even longer than the corresponding one in the com-
pounds existing solely in the imino-hydrazono tautomeric form
(Csp2–Csp2 ca 1.484 Å, Ref. [22]).
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