Experimental measurements and theoretical calculations of the chemical shifts and coupling constants of three azines (benzalazine, acetophenoneazine and cinnamaldazine)

Article abstract of DOI:10.1002/mrc.2272

Three azines, two of them doubly labeled with ¹⁵N, have been studied by multinuclear magnetic resonance in solution and in the solid state. The spectral parameters obtained by iterative analyses have been compared with DFT/B3LYP calculated values (absolute shieldings and coupling constants). The agreement is generally good. Some anomalies have been discussed in relation to the structure of these compounds. Copyright

Full text of DOI:10.1002/mrc.2272

Research Article
Received: 12 February 2008
Revised: 12 May 2008
Accepted: 20 May 2008
Published online in Wiley Interscience: 17 July 2008
(www.interscience.com) DOI 10.1002/mrc.2272
Experimental measurements and theoretical
calculations of the chemical shifts and coupling
constants of three azines (benzalazine,
acetophenoneazine and cinnamaldazine)
Artur M. S. Silva,a Regina M. S. Sousa,a María Luisa Jimeno,b
c
c∗
c
´
Fernando Blanco, Ibon Alkorta and Jose Elguero
Three azines, two of them doubly labeled with 15N, have been studied by multinuclear magnetic resonance in solution and
in the solid state. The spectral parameters obtained by iterative analyses have been compared with DFT/B3LYP calculated
values (absolute shieldings and coupling constants). The agreement is generally good. Some anomalies have been discussed in
c
relation to the structure of these compounds. Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: NMR; 1H; 13C; 15N; CPMAS; GIAO; benzalazine; acetophenoneazine; cinnamaldazine
Introduction
Literature information on the NMR properties of azines is scarce.
There are old studies concerning their 1H NMR spectra.[6–12] and
some more recent ones containing 13C and 15N NMR data but
of some special azines (substituents like CF3, 3-hydroxy-4-pyridyl,
and adamantyl).[13–15] Martin reported some 15N chemical shifts
The term azine has two meanings in chemistry: in heterocyclic
chemistry azines are aromatic six-membered rings containing one
(pyridine I) to six N atoms (hexazine). In alicyclic chemistry, azines
are compounds resulting from the reaction of two molecules of a
carbonyl compound (or, less frequently, of two different carbonyl
compounds) with hydrazine. The compounds are called aldazines
II or ketazines III depending on whether the carbonyl compound
is an aldehyde or a ketone, respectively.
for aldazines, like acetaldazine 4, and also interesting 1H–15
N
coupling constants in azines 5 (Scheme 2).[16–18] It is the purpose
of the present paper to report NMR properties of three azines, two
of them doubly labeled with 15N, both in solution and in the solid
state.
3
R
R'
3
R
H
Results and Discussion
1
1
N
N
2
2
N
4
N
4
Experimental data
N
R'
R
H
R
All reported coupling constants involving nitrogen atoms corre-
spoꢁnd ꢁto the 15N isotope. The 1H NMR system of the phenyl ring
(AA BB C) has not been analyzed nor 1H–13C coupling constants
measured. In the tables, we report the chemical shifts in the order
I
II
III
Although important compounds, the structural study of azines
II and/or III, has been neglected, except for X-ray crystal
determinations.[1–3] We have published two recent papers
concerningthesecompounds. Thefirstone(withacomprehensive
introduction) was devoted to a computational study of simple
compounds, including formaldazine (II, R = H).[4] The second
one extended these calculations to halogen and α, β-unsaturated
substituted azines.[5]
15N, 13C, 1Handthecouplingconstantsintheorder15N15N, 15 13C,
N
15N1H, and 1H1H. The nucleus considered is presented in bold.
∗
´
Correspondence to: Ibon Alkorta, Instituto de Química Medica, Juan de la
Cierva, 3, E-28006 Madrid, Spain. E-mail: ibon@iqm.csic.es
We decided to explore, in this third paper, the
NMR properties of three azines (Scheme 1): benzalazine
(1,2-dibenzylidenehydrazine) 1, cinnamaldazine [1,2-bis(E-
phenylallylidene) hydrazine] 2 and acetophenoneazine [1,2-bis(1-
phenylethylidene)hydrazine] 3. The first two were also prepared
labeled on both nitrogen atoms [15N2] b.
a
Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal
´
b
Centro de Química Organica ‘Manuel Lora-Tamayo’, Juan de la Cierva, 3,
E-28006 Madrid, Spain
´
c
Instituto de Química Medica, Juan de la Cierva, 3, E-28006 Madrid, Spain
c
Magn. Reson. Chem. 2008, 46, 859–864
Copyright ꢀ 2008 John Wiley & Sons, Ltd.

A. M. S. Silva et al.
m
N = 14N; N = 15
N
o
p
m
m
3
2
H
H
1
i
o
i
p
p
o
i
2
CH3
H
H
N
1
H
1
H
H
H
1
N
N
1
1
N
N
N
1'
2' H
N
N
1'
N
1'
1'
N
H
H
H
1'
H
H3C
2'
H
H
1'
H
3'
1a
1b
2a
3a
Scheme 1. The three azines.
CH3 3J=2.3 Hz
3J=5.4 Hz
CH3
N
H
H3C
N
–13.5 ppm
N
N
H
CH3
N
N
H
S
S
N
N
159.9
ppm
CH3
H
CH3
4
5a
3J=10 Hz
5b
3J=3.7 Hz
Scheme 2. Acetaldazine 4 and the Z and E isomers of azine 5.
The chemical shifts (−17.46 and 162.12 ppm) are close to those
reported for 4 (−13.5 and 159.9 ppm).[16] The stereochemistry of
the azine, although known from previous studies,[9] is confirmed
Table 1. Benzalazine (chemical shifts δ ppm, coupling constants Hz)
in CDCl3
3
by the value of JN1H1ꢁ = −5.68 Hz (compare with 5b).[17] In
Atom
δ
Coupling constants
general, the coupling constants are similar to those reported for
adamantylazines.[13]
Experimental
N1
−17.46
162.12
134.0
128.5
128.7
131.2
8.68
1JN1N1 = −11.23
ꢁ
C1
1JN1C1 = −4.11; 2J
= −6.40
ꢁ
2
Cipso
JN1Ci = −6.58; 3JNN11CCi1 = −4.75
ꢁ
Cinnamaldazine (2a) and [15N2]cinnamaldazine (2b)
Cortho
Cmeta
Cpara
C1H
3JN1Co
=
4JN1Co = −1.70 (aver)a
ꢁ
2JN1H1 = 3.62; 3JN1H1 = −5.68
ꢁ
Table 2 contains the data we have extracted from 2a and 2b
spectra.
The ABC system of the three olefinic protons was reported in
CDCl3 at 8.37, 7.08, and 7.06 ppm with 9.1, 16.1 and −0.21 Hz,[8]
very similar to the values of Table 2.
5JH1H1 = 0.0
ꢁ
CoH
7.86
CmH
CpH
7.45
7.45
Computed
N1
1JN1N1 = −11.20
Acetophenoneazine (3a)
−19.23
161.88
135.7
129.4
127.8
130.4
8.79
ꢁ
C1
1JN1C1 = −6.04; 2J
= −6.47
ꢁ
JN1Ci = −7.43; 3JNN11CCi1 = −4.73
This compound was studied only as unlabeled species (Table 3).
2
Cipso
ꢁ
Cortho
Cmeta
Cpara
C1H
3JN1Co
=
4JN1Co = −1.43 (aver)b
ꢁ
2JN1H1 = 3.39; 3JN1H1 = −5.85
ꢁ
Solid state
5JH1H1 = 0.37
ꢁ
We have recorded the 13C and 15N cross-polarization magic-angle
spinning (CPMAS) NMR spectra of compounds 1–3. The results
are reported in Table 4.
Only for compound 3, a clear splitting (Fig. 1) is observed for
two signals, C1 and CH3. There is a simple explanation for this
fact. Azines 1 and probably 2 are centrosymmetric while azine 3
is not.[2,3] The structure of 2 has not been determined but that
of the p,pꢁ-dibromo derivative 6 (DBCNMZ) is known and it is
centrosymmetric.[1,19]
CoH
8.05
CmH
CpH
7.40
7.38
a The average value has been obtained directly from the spectrum (Co
in the [15N2] derivative appear as deceptively simple triplet).
b Average of four values.
The values of Table 4 show that the 13C chemical shifts are,
in general, very similar in solution and in the solid state (only
the methyl group of 3 shows a significant effect, a fact already
described for other compounds).[20] Usually, 15N chemical shifts
are more sensitive to phase effects,[15] although the −10.8 to
−1.8 ppm shift is exceptionally large.
Benzalazine (1a) and [15N2]benzalazine (1b)
We have reported in Table 1 the data we obtained after an iterative
analysis using both compounds. When the 15N spectrum of 1a
was recorded in natural abundance, it corresponded to the [15N1]
isotopomer and it contained information used for the analysis.
c
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Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2008, 46, 859–864

Benzalazine, acetophenoneazine, and cinnamaldazine
Table 2. Cinnamaldazine (chemical shifts δ ppm, coupling constants
Table 3. Acetophenoneazine (chemical shifts δ ppm) in CDCl3
Hz) in CDCl3
Atom
δ
Atom
δ
Atom
δ
Atom
δ
Coupling constants
Experimental
N1
Experimental
N1
−26.78 Cortho 126.6 CoH 7.92
157.66 Cmeta 128.3 CmH 7.43
15.01 Cpara 129.6 CpH 7.41
1
−10.81 JN1N1 = −10.80
ꢁ
C1
1
C1
163.71 JN1C1 = −4.01; 2JN1C2 = −7.16
CH3
3
C2
125.34 JN1C3 = −3.02
Cipso
138.4
CH3 2.32
2
C3
143.42 JN1 C1 = −6.72; 3JN1 C2 = −6.29
ꢁ
ꢁ
ꢁ
Computed
C1
Cipso
Cortho
Cmeta
Cpara
C1H
C2H
C3H
CoH
CmH
CpH
Computed
N1
135.7 4JN1 C3 = −2.07
158.79 Cortho 126.4 CoH 8.07
15.13 Cmeta 127.2 CmH 7.39
139.4 Cpara 128.5 CpH 7.33
CH3 2.44
127.4 2JN1H1 = 3.41; 3JN1H2 = −1.19; 4JN1H4 = 0.50
128.8 3JN1 H1 = −5.71; 4JN1 H2 = 0.50; 5JN1 H4 = 0.50
CH3
ꢁ
ꢁ
ꢁ
Cipso
129.5 3JH1H2 = 9.80
3
8.38 JH2H3 = 16.09
4
7.09 JH1H3 = −0.36
7.10
7.86
7.45
7.45
In the case of coupling constants, the following equation was
found:
Jexp = (0.99 0.04) Jcalc, n = 25, r2 = 0.958
(4)
1
−10.07 JN1N1 = −11.08
ꢁ
1
163.05 JN1C1 = −5.98; 2JN1C2 = −9.32
The worse points [both in cinnamaldazine (2), see Table 2]
C1
3
4
3
correspond to JN1C3 (exp. −3.02, calc. −5.74 Hz) and to JN1 C3
ꢁ
C2
127.61 JN1C3 = −5.74
2
142.07 JN1 C1 = −6.05; 3JN1 C2 = −5.23
(exp. −2.07, calc. 1.02 Hz) (Fig. 2).
C3
ꢁ
ꢁ
ꢁ
Cipso
Cortho
Cmeta
Cpara
C1H
C2H
C3H
CoH
CmH
CpH
136.4 4JN1 C3 = 1.02
127.4 2JN1H1 = 3.39; 3JN1H2 = −1.79; 4JN1H4 = 0.72
128.2 3JN1 H1 = −5.57; 4JN1 H2 = 0.44; 5JN1 H4 = −0.35
ꢁ
ꢁ
ꢁ
Conclusions
128.5 3JH1H2 = 8.00
3
A collection of NMR data, chemical shifts but mainly coupling
constants, hasbeenobtainedforazines. DiscreteFouriertransform
(DFT) calculations reproduce rather well these data, excepting 15N
chemical shifts, a problem probably related to the presence of
two adjacent lone pairs. 13C CPMAS solid-state spectra reveals
the centrosymmetry of the studied aldazines 1 and 2 and its
absence in ketazine 3. This paper thus represents a contribution to
the characterization of azines by NMR, providing some structural
insights.
8.53 JH2H3 = 13.79
4
7.35 JH1H3 = −0.50
6.94
7.60
7.34
7.23
Computational results
We have calculated
the
chemical
shifts
[GIAO/B3LYP/6–311++G(d,p)] (see Section on Experimen-
tal) and the coupling constants [B3LYP/6–311++G(d,p)] (see
Section on Experimental) of the three compounds (the coupling
constants correspond to the 15N isotope). The results are given in
Tables 1–3 (lower half).
The following three regression related chemical shifts and
absolute shieldings (see Supporting Information) were obtained.
With these equations, we have calculated (fitted values) the
chemical shifts of Tables 1–3 (lower half).
Experimental
General procedure for the synthesis of azines 1a, 1b, 2a, 2b, 3
An ethanol solution (6.0 ml) of the appropriate aldehyde or
acetophenone(16.9 mmol) was added to another resulting from
the addition of hydrazinum sulfate or [15N2] hydrazinum sulfate
(7.69 mmol)toanammoniasolution(1.0 ml).Theresultingreaction
mixture was stirred for 1 h, but the expected azines started
to precipitate almost immediately after the addition, at room
temperature and 30 min at 50 ◦C under a nitrogen atmosphere.
The obtained solid was removed by filtration, washed with water
and a small portion of ethanol, and then recrystallized in ethanol.
In the case of acetophenone, the reaction mixture was stirred
at 50 ◦C under a nitrogen atmosphere for 48 h. After that period,
the reaction mixture was poured into water (20 ml) and ice (20 g)
and the solid removed by filtration. The obtained yellow solid
was dissolved in chloroform (25 ml), washed with water, and
evaporated to dryness. The obtained residue was recrystallized in
ethanol.
δ
15N = −92 − 0.49 σ15N (three points,
the statistical values have been omitted)
(1)
(2)
(3)
δ
13C = (174.6 0.5) − (0.948 0.008) σ13C,
n = 18, r2 = 0.999
δ1H = (31.0 0.8) − (0.97 0.03) σ1H,
n = 14, r2 = 0.988
For 13C and 1H, the correlations are excellent and similar to
those found in recent studies.[21] In the case of 15N, neither the
correlation coefficient nor the slope is acceptable. It seems that
the calculations do not reproduce well the N(sp2)–N(sp2) bond
nitrogen atoms.
Benzalazine 1a (66%): MP 90–91 ◦C (92◦–93 ◦C).[22]
[
15N2]Benzalazine 1b (66%): MP 88–89 C.
Cinnamaldazine 2a (86%): MP 163–165 ◦C (165–167 ◦C).[8]
c
Magn. Reson. Chem. 2008, 46, 859–864
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
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A. M. S. Silva et al.
Table 4. Comparison of solution and solid-state chemical shifts (δ, ppm)
Atom
Solution
Solid
Atom
Solution
Solid
Benzalazine (1)
Cinnamaldazine (2)
N1
−17.46
162.12
134.0
128.5
128.7
131.2
−15.2
162.5
135.0
127.6
130.2
130.2
N1
C1
−10.81
163.71
125.34
143.42
135.7
−1.8
164.2
125.5
142.8
137.2
126.8
129.0
130.1
C1
Cipso
Cortho
Cmeta
Cpara
C2
C3
Cipso
Cortho
Cmeta
Cpara
127.4
128.8
129.5
Acetophenoneazine (3)
C1
157.66
15.01
138.4
126.6
128.3
129.6
158.1 and 160.6
12.4 and 14.7
137.8
CH3
Cipso
Cortho
Cmeta
Cpara
127.2
127.2
129.4
Br
H
H
N
H
N
H
H
H
Br
6
Figure 1. 13C CPMAS NMR spectrum of 3a and formula of 6.
[
15N2]Cinnamaldazine 2b (77%): MP 163–165 ◦C.
75.47 MHz for 13C, and 30.42 MHz for 15N). Chemical shifts (δ) are
reported in parts per million and coupling constants (J) in Hz. The
Acetophenoneazine
3a
(71%):
MP
118–119 ◦C
(123–124 ◦C).[9]
internalstandardof1H and13CNMRspectrawasTMSwhilethe15
N
chemicalshiftsarereferredtothenitromethane(90%inCDCl3, δ =
0 ppm).[23] 13C assignments were made with the aid of 2D gHSQC
and gHMBC (delays for one bond and long-range J C/H couplings
were optimized for 145 and 7 Hz, respectively) experiments.
Melting points were determined on a Griffin apparatus and are
uncorrected.NMRspectraofCDCl3 solutionsofthepreparedazines
wererecordedonaBrukerAvance300spectrometer(300.13for1H,
c
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Copyright ꢀ 2008 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2008, 46, 859–864

Benzalazine, acetophenoneazine, and cinnamaldazine
20
15
10
5
Supporting information
Supporting information may be found in the online version of this
article.
Acknowledgements
Thanks are due to the University of Aveiro, FCT and FEDER
for funding the Organic Chemistry Research Unit. This work
was carried out with financial support from the Ministerio de
Educacio´n y Ciencia (Project No. CTQ2006-14487-C02-01/BQU)
and Comunidad Autonoma de Madrid (Project MADRISOLAR, ref.
S-0505/PPQ/0225). Thanks are also due to CTI (CSIC) for allocation
of computer time.
0
2 3JN1C3
2 4JN1'C3
-5
-10
-15
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c
Magn. Reson. Chem. 2008, 46, 859–864
Copyright ꢀ 2008 John Wiley & Sons, Ltd.
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A. M. S. Silva et al.
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Magn. Reson. Chem. 2008, 46, 859–864