Aromatic amines in organic synthesis. Part ii. p‐aminocinnamaldehydes

Article abstract of DOI:10.3390/molecules26144360

Ten derivatives of p‐aminocinnamic aldehydes were prepared from the reaction of either aromatic amines with dimethylaminoacrolein or benzaldehydes with acetaldehyde. Their chemical structure and purity were verified by¹H NMR,¹³C NMR and IR spectroscopic methods. We found that the synthesis applying dimethylaminoacrolein as the reagent gets better yields than the one based on the reaction with acetaldehyde. The yields of the cinnamic aldehydes varied according to the type of the amino group and the number and position of the substituents. The basic spectroscopic properties of the p‐aminocinnamic aldehydes are also described since the compounds may be a precursor for the synthesis of dyes for diverse applications, e.g., in medicine and optoelectronics.

Full text of DOI:10.3390/molecules26144360

molecules
Article
Aromatic Amines in Organic Synthesis. Part II.
p-Aminocinnamaldehydes
Marek Pietrzak * and Beata Je˛drzejewska
Faculty of Chemical Technology and Engineering, UTP University of Sciences and Technology, Seminaryjna 3,
85-326 Bydgoszcz, Poland; beata@utp.edu.pl
* Correspondence: marek@utp.edu.pl
Abstract: Ten derivatives of p-aminocinnamic aldehydes were prepared from the reaction of either
aromatic amines with dimethylaminoacrolein or benzaldehydes with acetaldehyde. Their chemical
structure and purity were verified by 1H NMR, 13C NMR and IR spectroscopic methods. We found
that the synthesis applying dimethylaminoacrolein as the reagent gets better yields than the one
based on the reaction with acetaldehyde. The yields of the cinnamic aldehydes varied according to
the type of the amino group and the number and position of the substituents. The basic spectroscopic
properties of the p-aminocinnamic aldehydes are also described since the compounds may be a
precursor for the synthesis of dyes for diverse applications, e.g., in medicine and optoelectronics.
Keywords: aromatic aldehydes; p-aminocinnamaldehyde; extended bond system
1. Introduction
Organic compounds containing different amino groups (or their derivatives) have
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attracted remarkable attention by virtue of their giant potential as dyes [
spectroscopic probes [ ], one- [ ] and multi-photon [ ] polymerization initiators and
drugs [10]. They have been studied for applications in various areas of photochemistry due
to their structure versatility and facile modifications for electron-donating abilities [11 13].
1], solar cells [2,3],
ꢀꢁꢂꢃꢄꢅꢆ
4
–6
7,8
9
Citation: Pietrzak, M.; Je˛drzejewska,
B. Aromatic Amines in Organic
Synthesis. Part II.
–
In general, organic compounds bearing an alkylamino group may be synthesized through
a related synthetic strategy using one of the following: Heck reaction [14], Knoevenagel
condensation [13,15], Wittig reaction [16], Suzuki-Miyaura cross-coupling reaction [17]
p-Aminocinnamaldehydes. Molecules
2021, 26, 4360. https://doi.org/
10.3390/molecules26144360
and several different reagents, such as aryl bromides with amino substituents [14], p-
aminobenzaldehydes [13,15] and p-aminobenzoic acid derivatives [18,19].
Alternatively, organic compounds with an amino group, of particular interest for
their nonlinear optical properties useful in modern technology, can be obtained using
Received: 24 June 2021
Accepted: 16 July 2021
Published: 19 July 2021
precursors with extended π-systems [20]. For the preparation of such compounds, standard
p-aminobenzaldehydes should be replaced with p-aminocinnamaldehydes, their acids or
esters [21,22].
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
There are different methods for the synthesis of cinnamaldehydes. One of them
is based on the reaction between acetaldehyde with benzaldehydes having an electron-
withdrawing substituent in the presence of KOH as a base [23]. The method is relatively
simple and gives good yields. However, in the case of benzaldehydes with electron-
donating substituents, in particular amino groups, this reaction has to be performed in
concentrated sulfuric acid, and more drastic conditions are required. Moreover, prod-
ucts are isolated at a lower yield [23]. Therefore, alternative methods of synthesizing
p-aminocinnamaldehydes with higher yields are sought. They can be obtained following
the Heck reaction [24] or the reaction of aromatic amines with dimethylaminoacrolein [25].
As part of our study on dyes with enlarged double bond systems that may be used
as two-photon absorbers, [20] an optical amplifier, generator and modulator based on
Copyright:
© 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
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4.0/).
third order nonlinear optical phenomenon [26], photoinitiators of polymerization [8] or
Molecules 2021, 26, 4360. https://doi.org/10.3390/molecules26144360
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 4360
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spectroscopic probes [
6]. In the paper, we report synthesis, characterization and photo-
physical properties of the ten representatives of p-aminocinnamic aldehydes. They were
obtained through two different procedures using corresponding benzaldehydes (Method
A) or amines (Method B) as the starting materials (see Scheme 1). Method A is a single-step
acid-catalyzed aldol condensation between acetaldehyde and appropriate benzaldehyde.
This is a well-known method with moderate yields. Method B is analogous to the classi-
cal Vilsmeier–Haack formylation method; only 3-dimethylaminoacrolein is used instead
of dimethylformamide. The mechanism of this reaction has been described by Ulrich
and Brenmeier [25]. In this case, an electrophilic substitution takes place, and therefore,
aromatic amines react readily.
O
A
R C
O
C
CH3CHO ; H2SO4
H
R
H
B
1a-j
R H
3-dimethylamino-
acrolein ; POCl3
Me
Me
Me
Et
Me
N
Me
N
R =
N
N
N
Me
Me
Et
Me
Me
b
d
e
a
c
Me
N
Me
N
N
N
N
O
h
f
g
i
j
Scheme 1. General routes for the preparation of derivatives of p-aminocinnamic aldehyde.
2. Results
The synthetic routes for the preparation of ten p-aminocinnamic aldehydes are shown
in Scheme 1. Six of these aldehydes (1c, 1d, 1e, 1f, 1h, 1i) have not been described in
the literature yet. According to Method A, treatment of p-aminobenzaldehyde with ac-
etaldehyde in concentrated sulfuric acid at a lower temperature (0 ◦C) followed by neu-
tralization, afforded p-aminocinnamic aldehyde a good yield (32–54%). The yield of the
synthesis was improved by ca. 6–16% for the reaction between aromatic amine and 3-
dimethylaminoacrolein, in the presence of POCl3 conducted in chloroform at
−
20–0 ◦C
(Method B). What is more, 4-(N,N-dimethylamino)-2,6-dimethylcinnamaldehyde (1d) can
be obtained at a satisfactory yield only using Method B. This is probably due to the steric
hindrance.
The structure and purity of all synthesized compounds were confirmed by spectro-
scopic methods, including NMR and IR spectra analysis (spectra are shown in the ESI file),
as well as the elemental analysis. The NMR spectra confirmed the presence of signals from
all protons and carbons in the synthesized molecules, as shown in the Experimental Section.
For example, the proton signal of the aldehyde group is clearly visible at ca. 9.5 ppm,
whereas the characteristic signals of the methine groups are given in the range from 7.8
to 6.2 ppm. The coupling constant for these protons (3JH,H = 16 Hz) indicates that the
compounds are in the trans configuration. The signal of carbon from the aldehyde group
occurs at ca. 193–195 ppm and is shifted by about 0.7 ppm compared to the analogous

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benzaldehydes. The presence of the carbonyl group is also confirmed by the characteristic,
strong signal in the IR spectrum in the frequency range from 1667 (1e) to 1649 cm−1 (1b).
Figure 1 illustrates the 1H NMR spectra of 4-(dimethylamino)-2,6-dimethylcinnamalde-
hyde (1d) in DMSO-d6 and its corresponding benzaldehyde to show differences in signal
positions associated with elongation of the π-system.
Figure 1. 1H NMR spectra of 4-(dimethylamino)-2,6-dimethylbenzaldehyde and 4-(dimethylamino)-2,6-dimethylcinnamal-
dehyde (1d) in DMSO-d6.
1
The comparative analysis of the H NMR spectra of the 4-(dimethylamino)-2,6-
dimethylbenzaldehyde and 4-(dimethylamino)-2,6-dimethylcinnamaldehyde (Figure 1)
shows that the signal of the CHO group is shifted from 10.2 to 9.5 ppm and split into
doublet with a coupling constant of 8.0 Hz. Additionally, in cinnamic aldehyde, there are
two characteristic signals from methine hydrogen at ca. 7.8 and 6.3 ppm, with the He signal
visible as a doublet of doublets with unequal coupling constants.
Since the compounds can be used for the preparation of different groups of dyes, their
basic spectroscopic data are compiled in Table 1. They are determined in two solvents of
varying properties, i.e., methanol and ethyl acetate. The normalized electronic absorption
spectra of selected p-aminocinnamic aldehydes (1a, 1g and 1j) in ethyl acetate (EtOAc)
and methanol (MeOH) are shown in Figure 2, whereas the spectra for all synthesized
compounds are presented in the ESI file.

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Table 1. Absorption maxima wavelength (λmax abs; nm), molar absorption coefficient (ε
max; M−1 cm−1), fluorescence
maxima wavelength (λmax fl; nm), fluorescence quantum yield (φfl; %) and Stokes shift (∆νSt; cm−1) in ethyl acetate and
methanol.
Ethyl Acetate
Methanol
Abbr.
∆νSt
∆νSt
λmax abs (ε
)
λmax fl (φfl)
λmax abs (ε
)
λmax fl (φfl)
max
max
1a
1b
1c
1d
1e
1f
1g
1h
1i
372.5 (43,000)
379.5 (44,500)
378 (35,500)
374 (35,200)
379.5 (43,900)
368.5 (33,800)
357 (31,400)
378 (33,800)
385 (40,500)
396.5 (41,800)
437 (0.42)
438 (0.69)
444 (0.33)
474 (0.71)
440 (0.61)
440 (0.99)
438 (0.51)
460 (1.19)
453 (1.14)
460 (1.88)
3962
3519
3933
5641
3623
4410
5180
4716
3899
3482
389 (37,700)
474 (0.33)
477 (0.20)
474 (0.14)
not fluorescent
476 (0.30)
481 (0.28)
486 (0.38)
485 (0.18)
486 (0.26)
488 (0.24)
4610
4320
4187
-
4212
5184
6598
4666
4269
3460
395.5 (45,700)
395.5 (36,900)
392 (34,900)
396.5 (40,200)
385 (31,500)
368 (28,900)
395.5 (33,900)
402.5 (35,600)
417.5 (38,300)
1j
ethyl acetate
1.0
1a
1g
1j
0.8
0.6
0.4
0.2
0.0
methanol
1a
1g
1j
250
300
350
400
450
500
550
, nm
Figure 2. Normalized electronic absorption spectra of selected p-aminocinnamaldehydes in ethyl
acetate and methanol.
The UV–Vis absorption spectra display one absorption peak at ca. 390–400 nm,
attributed to the π→π* transition. Compound 1j exhibits the most redshifted absorption
band with a maximum at 417.5 and 460 nm in methanol and ethyl acetate, respectively,
compared to the parent 1a cinnamic aldehyde (maximum at 389 and 372.5 nm). The
significant redshift is attributed to the presence of the stiffening alkylamino group, which
enhanced the extent of electron delocalization over the whole molecule. On the other
hand, the strongest hypsochromic effect is observed in cinnamaldehyde with morpholine
residue. The presence of the oxygen atom in the six-membered piperidine ring probably
causes the coplanar alignment of the molecule, which results in a higher energy barrier for
the S0-S1 transition and shifting the absorption maximum to the blue region. The molar
extinction coefficients of the p-aminocinnamic aldehydes are in the range from 28,900 to

Molecules 2021, 26, 4360
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45,700 M−1cm−1 and 31,400 to 44,500 M−1cm−1 in MeOH and EtOAc, respectively. These
values are rather high, indicating that the compounds have a good light-harvesting ability.
The fluorescence spectra of the compounds 1a–j (exited at 370 and 390 nm in EtOAc
and MeOH, respectively) showed a sharp band in the range from 440 to 480 nm (see
Figures S3 and S4 in ESI), corresponding to blue light emission. Similar to the UV-Vis
absorption, the fluorescence maximum of 1j is redshifted by 14 and 23 nm in MeOH and
EtOAc, respectively, relative to the parent p-N,N-dimethylaminocinnamic aldehyde, which
arises from the different donating ability of the amino group. The bathochromic shift
of the absorption and fluorescence spectra also occurs with increasing solvent polarity.
However, the change of ethyl acetate to methanol causes a small (approx. 15 nm) redshift
of the absorption maximum. A more pronounced shift with increasing solvent polarity (ca.
25–40 nm) is observed for the fluorescence band, which indicates better stabilization of the
excited state of the compounds in a more polar environment and suggests an increase of
the dipole moment upon excitation. The fluorescence quantum yield of the compounds is
small and does not exceed 2% in ethyl acetate and 0.5% in methanol, which suggests that
the deactivation of the S1 excited-state occurs mostly by non-radiative processes.
To reveal the substituent effect on an oxidation potential, the cyclic voltammograms
of the aminocinnamic aldehydes were recorded in anhydrous acetonitrile as illustrated in
Figure 3. The results of the electrochemical measurements for all compounds are collected
in Table 2.
2.5
2.0
1a
1.5
1.0
0.5
0.0
-0.5
0
500
1000
1500
2000
Potential, mV
Figure 3. Cyclic voltammperogram of compound 1a in anhydrous acetonitrile. Solution contained
tetra-n-butylammonium perchlorate as the supporting electrolyte and was deoxygenated prior to
−1
analysis by purging with helium for 15 min. Scan rate was 100 mVs
.

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Table 2. The measured oxidation potential of the tested aminocinnamaldehydes.
Abbr.
Eox (mV)
Abbr.
Eox (mV)
1a
1b
1c
1d
1e
1285
1219
1194
1081
1186
1f
1g
1h
1i
1228
1300
1093
1132
870
1j
In all cases, the electrochemical oxidation of the aminocinnamic aldehydes is irre-
versible. The location of the oxidation peaks depends on the alkylamino substituent.
Compared to the parent compound (1a), the aminocinnamic aldehydes are more easily
oxidized. In the case of 1j containing the stiffened amine group, the oxidation proceeds at
the lowest potential. Only compound 1g, which is a morpholine derivative, has a lower
tendency to lose electrons.
3. Experimental Section
3.1. Materials and Methods
Starting reagents and solvents were purchased from Aldrich Chemical Co. Aldehydes
were purified using Büchi Sepacore® Chromatography system (C-605, C-610) with a UV-Vis
detector (C-640). Melting points were determined on the Büchi melting point apparatus
MP-1. The 1H (400 MHz) and 13C (100 MHz) NMR spectra were recorded on a Bruker
AscentTM 400 NMR spectrometer. Dimethylsulfoxide (DMSO-d6) was used as the solvent
and tetramethylsilane as the internal standard. The IR spectra were recorded using a Bruker
spectrophotometer Vector 22 in the range of 400–4500 cm−1 by the KBr pellet technique.
Elemental analysis was carried out by an Elementar Vario MACRO apparatus. The UV-Vis
absorption spectra were recorded on a Shimadzu UV-Vis Multispec-1501 spectrophotometer.
The fluorescence spectra were obtained with a Hitachi F-7100 spectrofluorometer. The
fluorescence quantum yield (FQY;
φ) was calculated according to Equation (1) using
coumarin I in ethanol as a reference [27].
n2s
n2re f
Is Are f
Ire f As
φs = φre f
·
(1)
where: I is the integrated intensity (area) (in units of photons); n is the refractive index.
The absorbances (A) of both the sample (s) and reference (ref) solution at an excitation
wavelength (370 nm) was ca. 0.1.
The oxidation potential (Eox) was measured by cyclic voltammetry using an Electro-
chemical Analyzer EA9C, MTM Cracow in anhydrous acetonitrile with 0.1 M of tetrabuty-
lammonium perchlorate as the supporting electrolyte. The scan rate was 100 mV/s. The
measurements were made in a typical three-electrode setup containing a platinum 1 mm
electrode as the working electrode and platinum and Ag/AgCl as auxiliary and reference
electrodes, respectively.
3.2. Synthesis
3.2.1. Method A
An appropriate p-aminobenzaldehyde (40 mmol) was dissolved in 30 mL of concen-
trated sulfuric acid under vigorous stirring. Then, the solution was cooled to 0 ◦C, and
120 mmol of acetaldehyde was added dropwise within 3 h. After another hour, the reaction
mixture was poured onto ice and neutralized with 20% NaOH. The precipitate was filtered,
dried, and recrystallized from ethanol. Only the aldehydes 1h, 1i and 1j needed to be
purified by flash chromatography on silica gel (pore size 60 Å, 230–400 mesh particle size)
using chloroform as the mobile phase [23].

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3.2.2. Method B
Anhydrous chloroform (20 mL), an appropriate aromatic amine (65 mmol) and 3-
dimethylaminoacrolein (55 mmol) were placed in a flask◦equipped with a thermometer
and a dropping funnel. The mixture was cooled to
−
10 C, and a solution of 55 mmol
of POCl3 in 8 mL of chloroform was added, keeping the temperature below 0 ◦C. Then,
the mixture was allowed to warm up to room temperature and stirred overnight. It was
next heated to 60 ◦C and kept at this temperature for 2 h. Chloroform was distilled off
under reduced pressure, and 25 mL of methanol was added with stirring and cooling. The
reaction mixture was poured onto 50 g of crushed ice and neutralized with 20% NaOH. The
crude product was filtered and recrystallized from ethanol. As before, the aldehydes 1h, 1i
and 1j were purified by flash chromatography on silica gel (pore size 60 Å, 230–400 mesh
particle size) using chloroform as the mobile phase [25].
3-[4-(Dimethylamino)phenyl]prop-2-enal (1a)
The compound was obtained as a brown solid; yield: 54% (method A), 61% (method
B); mp 140 ◦C; 1H NMR (400 MHz, DMSO-d6):
δ
9.53 (d, J = 8.0 Hz, 1H, CHO), 7.59–7.55
(m, 3H), 6.75 (d, J = 9.0 Hz, 2H), 6.59 (dd, J = 8.0 Hz, J = 15.8 Hz, 1H), 3.01 (s, 6H, N(CH3)2);
13C NMR (100 MHz, DMSO-d6):
193.9 (CHO), 154.7 (CH), 152.7 (C), 131.1 (CH), 123.7
δ
(CH), 121.8 (C), 112.2 (CH), 40.1 (N(CH3)2); IR (KBr): 1664, 1600, 1529, 1373, 1140, 973, 811.
Anal. calculated for C11H13NO: C, 75.40; H, 7.48; N, 7.99. Found: C, 75.51; H, 7.40; N,
7.94.
3-[4-(Diethylamino)phenyl]prop-2-enal (1b)
The compound was obtained as a brown solid; yield: 39% (method A), 55% (method
◦
B); mp 73 C; 1H NMR (400 MHz, DMSO-d6):
δ
9.52 (d, J = 8.0 Hz, 1H, CHO), 7.56–7.52 (m,
3H), 6.71 (d, J = 9.0 Hz, 2H), 6.55 (dd, J = 8.0 Hz, J = 15.9 Hz, 1H), 3.42 (q, J = 6.9 Hz, 4H,
NCH2), 1.12 (t, J = 7.0 Hz, 6H, CH3); 13C NMR (100 MHz, DMSO-d6):
193.9 (CHO), 154.7
δ
(CH), 150.3 (C), 131.5 (CH), 123.2 (CH), 121.0 (C), 111.6 (CH), 44.4 (NCH2), 12.9 (CH3); IR
(KBr): 1649, 1596, 1525, 1415, 1187, 1135, 813.
Anal. calculated for C13H17NO: C, 76.81; H, 8.43; N, 6.89. Found: C, 76.90; H, 8.35; N,
6.86.
3-[4-(Dimethylamino)-2-methylphenyl]prop-2-enal (1c)
The compound was obtained as a dark red solid; yield: 49% (method A), 58% (method
B); mp 110 ◦C; 1H NMR (400 MHz, DMSO-d6):
δ
9.57 (d, J = 8.0 Hz, 1H, CHO), 7.80 (d,
J = 15.8 Hz, 1H, -CH=) 7.65 (d, J = 9.2 Hz, 1H), 6.61-6.48 (m, 3H), 2.99 (s, 6H, N(CH3)2), 2.40

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(s, 3H, ArCH3); 13C NMR (100 MHz, DMSO-d6):
δ
193.7 (CHO), 152.0 (C), 151.0 (CH), 139.9
(C), 128.6 (CH), 123.6 (CH), 119.8 (C), 113.0 (CH), 110.0 (CH), 39.6 (N(CH3)2), 19.9 (ArCH3);
IR (KBr): 1650, 1597, 1580, 1374, 1296, 1212, 1154, 1099, 811.
Anal. calculated for C12H15NO: C, 76.16; H, 7.99; N, 7.40. Found: C, 76.23; H, 7.91; N,
7.38.
3-[4-(Dimethylamino)-2,6-dimethylphenyl]prop-2-enal (1d)
The compound was obtained as a dark yellow solid; yield: 0% (method A), 36%
(method B); mp 111 ◦C; 1H NMR (400 MHz, DMSO-d6):
δ
9.55 (d, J = 7.7 Hz, 1H, CHO),
7.81 (d, J = 16 Hz, 1H, -CH=), 6.50 (s, 2H), 6.31 (dd, J = 7.7 Hz, J = 16.0 Hz, 1H), 2.97 (s,
6H, N(CH3)2), 2.38 (s, 6H, ArCH3); 13C NMR (100 MHz, DMSO-d6):
195.2 (CHO), 152.0
δ
(CH), 151.4 (C), 140.6 (C), 128.8 (CH), 120.2 (C), 112.7 (CH), 40.0 (N(CH3)2), 22.8 (ArCH3);
IR (KBr): 1656, 1583, 1510, 1366, 1146, 827.
Anal. calculated for C13H17NO: C, 76.81; H, 8.43; N, 6.89. Found: C, 76.93; H, 8.41; N,
6.88.
3-[4-(Pyrrolidin-1-yl)phenyl]prop-2-enal (1e)
The compound was obtained as a brown solid; yield: 45% (method A), 54% (method
B); mp 131 ◦C; 1H NMR (400 MHz, DMSO-d6):
δ
9.52 (d, J = 8.0 Hz, 1H, CHO), 7.57–7.54
(m, 3H), 6.60-6.54 (m, 3H), 3.32 (t, J = 4.5 Hz, 4H, NCH2), 1.97 (m, 4H, CH2); 13C NMR (100
MHz, DMSO-d6): 193.8 (CHO), 155.0 (CH), 150.2 (C), 131.3 (CH), 123.2 (CH), 121.3 (C),
δ
112.2 (CH), 47.7 (NCH2), 25.4 (CH2); IR (KBr): 1667, 1594, 1526, 1398, 1187, 1136, 971, 810.
Anal. calculated for C13H15NO: C, 77.58; H, 7.51; N, 6.96. Found: C, 77.61; H, 7.43; N,
6.98.
3-[4-(Piperidin-1-yl)phenyl]prop-2-enal (1f)
The compound was obtained as a dark brown solid; yield: 52% (method A), 60%
(method B); mp 87 ◦C; 1H NMR (400 MHz, DMSO-d6):
δ
9.55 (d, J = 7.9 Hz, 1H, CHO),
7.59–7.55 (m, 3H), 6.96 (d, J = 9.0 Hz, 2H), 6.61 (dd, J = 8.0 Hz, J = 15.8 Hz, 1H), 3.33 (t,
J = 8.7 Hz, 4H, NCH2), 1.58 (m, 6H); 13C NMR (100 MHz, DMSO-d6):
194.1 (CHO), 154.3
δ
(CH), 153.3 (C), 131.1 (CH), 124.5 (CH), 123.2 (C), 114.5 (CH), 48.4 (NCH2), 25.3 (CH2), 24.4
(CH2); IR (KBr): 1653, 1600, 1516, 1243, 1137, 1124, 811.
Anal. calculated for C14H17NO: C, 78.11; H, 7.96; N, 6.51. Found: C, 78.19; H, 7.88; N,
6.45.

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3-[4-(Morpholin-4-yl)phenyl]prop-2-enal (1g)
The compound was obtained as a dark brown solid; yield: 52% (method A), 58%
(method B); mp 109 ◦C; 1H NMR (400 MHz, DMSO-d6):
9.57 (d, J = 7.9 Hz, 1H, CHO),
δ
7.62–7.58 (m, 3H), 6.99 (d, J = 9.0 Hz, 2H), 6.66 (dd, J = 7.9 Hz, J = 15.7 Hz, 1H), 3.73 (t,
J = 4.9 Hz, 4H, OCH2), 3.26 (t, J = 4.9 Hz, 4H, NCH2); 13C NMR (100 MHz, DMSO-d6):
δ
194.3 (CHO), 154.1 (CH), 153.4 (C), 130.9 (CH), 125.2 (CH), 124.5 (C), 114.5 (CH), 66.2
(OCH2), 47.4 (NCH2); IR (KBr): 1659, 1601, 1518, 1234, 1137, 1123, 927, 811, 629.
Anal. calculated for C13H15NO2: C, 71.87; H, 6.96; N, 6.45. Found: C, 71.93; H, 6.92; N,
6.41.
3-(1-Methyl-2,3-dihydro-1H-indol-5-yl)prop-2-enal (1h)
Purified by flash chromatography (silica gel, chloroform as eluent). The compound
was obtained as a yellow solid; yield: 37% (method A), 52% (method B); mp 65 ◦C; 1H
NMR (400 MHz, DMSO-d6):
δ 9.50 (d, J = 7.9 Hz, 1H, CHO), 7.53 (d, J = 15.6 Hz, 1H, -CH=),
7.43 (s, 1H), 7.38 (d, J = 8.2 Hz, 1H), 6.54 (dd, J = 8.0 Hz, J=15.8 Hz, 1H), 6.51 (d, J = 7.8 Hz,
1H), 3.45 (t, J = 8.4 Hz, 2H, NCH2), 2.95 (t, J = 8.3 Hz, 2H), 2.81 (s, 3H, NCH3); 13C NMR
(100 MHz, DMSO-d6):
δ 193.9 (CHO), 156.4 (C), 155.1 (CH), 132.0 (CH), 131.2 (C), 124.4
(CH), 123.2 (CH), 123.2 (C), 106.0 (CH), 55.0 (NCH2), 34.6 (NCH3), 27.7 (ArCH2); IR (KBr):
1657, 1595, 1505, 1329, 1301, 1130, 1081, 813.
Anal. calculated for C12H13NO: C, 76.98; H, 7.00; N, 7.48. Found: C, 77.04; H, 7.08; N,
7.41.
3-(1-Methyl-1,2,3,4-tetrahydroquinolin-6-yl)prop-2-enal (1i)
Purified by flash chromatography (silica gel, chloroform as eluent). The com◦pound
was obtained as a brown solid; yield: 43% (method A), 55% (method B); mp 65 C; 1H
NMR (400 MHz, DMSO-d6):
δ 9.50 (d, J = 8.0 Hz, 1H, CHO), 7.49 (d, J = 15.6 Hz, 1H, -CH=),
7.37 (d, J = 8.5 Hz, 1H), 7.29, (s, 1H), 6.59 (d, J = 8.6 Hz, 1H), 6.53 (dd, J = 8.0 Hz, J = 15.5 Hz,
1H), 3.32 (t, J = 5.7 Hz, 2H, NCH2), 2.94 (s, 3H, NCH3), 2.71 (t, J = 6.2 Hz, 2H, ArCH2), 1.88
(m, 2H); 13C NMR (100 MHz, DMSO-d6):
δ 193.8 (CHO), 154.9 (CH), 149.4 (C), 130.1 (CH),
129.5 (CH), 123.1 (CH), 122.6 (C), 121.5 (C), 110.6 (CH), 50.9 (NCH2), 38.8 (NCH3), 27.6
(ArCH2), 21.7 (CH2); IR (KBr): 1664, 1595, 1525, 1322, 1205, 1137, 807.
Anal. calculated for C13H15NO: C, 77.58; H, 7.51; N, 6.96. Found: C, 77.52; H, 7.43; N,
6.91.

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3-(9-Julolidyl)prop-2-enal (1j)
Purified by flash chromatography (silica gel, chloroform as eluent). The compound
was obtained as a brown solid; yield: 32% (method A), 47% (method B); mp 125 ◦C; 1H
NMR (400 MHz, DMSO-d6):
7.11 (s, 1H), 6.47 (dd, J = 8.0 Hz, J = 15.5 Hz, 1H), 3.24 (t, J = 5.8 Hz, 4H, NCH2), 2.68 (t,
J = 6.3 Hz, 4H, ArCH2), 1.86 (m, 4H, CH2); 13C NMR (100 MHz, DMSO-d6):
193.6 (CHO),
δ 9.47 (d, J = 8.0 Hz, 1H, CHO), 7.42 (d, J = 15.5 Hz, 1H, -CH=),
δ
155.1 (CH), 145.9 (C), 128.6 (CH), 122.6 (CH), 121.0 (C), 120.6 (C), 49.7 (NCH2), 27.5 (CH2),
21.4 (CH2); IR (KBr): 1654, 1588, 1523, 1319, 1128, 969.
Anal. calculated for C15H17NO: C, 79.26; H, 7.54; N, 6.16. Found: C, 79.21; H, 7.50; N,
6.19.
4. Conclusions
p-Aminocinnamic aldehydes were prepared from the reaction of either acetaldehyde
with aromatic aldehydes bearing appropriate substituents (Method A) or aromatic amine
and 3-dimethylaminoacrolein in the presence of POCl3 (Method B). The desired products
were obtained at a low to moderate yield with slightly higher values of ca. 6–16% achieved
using Method B. The yields of the reactions carried out according to method A are 32–54%
,
while for Method B, they are in the range of 36–61%. In addition to the better yield,
an important advantage of Method B is the use of relatively readily available aromatic
amines instead of the often difficult to obtain benzaldehydes. Moreover, the 4-(N,N-
dimethylamino)-2,6-dimethylcinnamaldehyde (1d), probably due to spatial hindrance,
could only be synthesized by Method B. It should also be noted that six of the synthesized
aldehydes (1c, 1d, 1e, 1f, 1h, 1i) have not been described in the literature so far.
Spectroscopic analysis showed that the absorption and fluorescence maxima of the
cinnamic aldehydes, due to the additional double bond, are redshifted by approximately
45 and 75 nm, respectively, relative to the corresponding benzaldehydes. Moreover, their
spectroscopic properties depend on the amine substituent present in the phenyl ring and
solvent used. The structure of the cinnamaldehydes also influences the location of the
oxidation peaks.
Supplementary Materials: Supplementary data related to NMR, IR, UV-Vis and fluorescence spec-
troscopy data for all tested compounds can be found online.
Author Contributions: M.P. conceived of the presented idea, carried out the synthesis and experi-
ments, drafted the manuscript in Polish and designed the tables and figures. B.J. contributed to the
analysis of the results and wrote the English version of the manuscript. All authors discussed the
results and contributed to the final manuscript. All authors have read and agreed to the published
version of the manuscript.
Funding: This research was founded by the National Science Center of Poland (Decision No. DEC-
2012/07/B/ST4/01417).
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: The data is provided by Marek Pietrzak (UTP University of Science
and Technology), please contact marek@utp.edu.pl. The data is not publicly available apart from the
data contained in the article or Supplementary Materials.

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Conflicts of Interest: The authors declare no conflict of interest. The funders had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or
in the decision to publish the results.
Sample Availability: Samples of the compounds are available from the author (M. Pietrzak).
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