Microwave-initiated hydroamination of 2-ethoxypropenal with secondary amines

Article abstract of DOI:10.1134/S1070428011120025

Reactions of 2-ethoxyprop-2-enal with cycloaliphatic secondary amines (morpholine, piperidine, pyrrolidine) follow 1,4- or 1,2-addition pattern with subsequent condensation of the adduct with the initial amine to produce isomeric 2-ethoxyprop-2-ene-1,1-diamines and 2-ethoxyprop-1-ene-1,3-diamines. These reactions are accelerated by a factor of 15-30 under microwave irradiation and in the presence of water. The regioselectivity of primary nucleophilic attack varies over a wide range, depending on the amine basicity and reactant ratio. The reaction of 2-ethoxyprop-2-enal with pyrrolidine and water at a ratio of 1: 10: 10 was characterized by increased regioselectivity of 1,4-addition (up to 75%). Unlike cycloaliphatic amines, 2-ethoxyprop-2-enal reacted with N-methylaniline under microwave irradiation at a lower rate to give 2-ethoxy-N¹,N¹,N³-trimethyl-N ¹,N¹,N³-triphenylpropane-1,1,3-triamine.

Full text of DOI:10.1134/S1070428011120025

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2011, Vol. 47, No. 12, pp. 1792–1796. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © N.A. Keiko, E.A. Verochkina, L.I. Larina, 2011, published in Zhurnal Organicheskoi Khimii, 2011, Vol. 47, No. 12, pp. 1756–1760.
Dedicated to Full Member of the Russian Academy of Sciences
M.G. Voronkov on his 90th anniversary
Microwave-Initiated Hydroamination of 2-Ethoxypropenal
with Secondary Amines
N. A. Keiko, E. A. Verochkina, and L. I. Larina
Favorskii Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: keiko@irioch.irk.ru
Received May 5, 2011
Abstract—Reactions of 2-ethoxyprop-2-enal with cycloaliphatic secondary amines (morpholine, piperidine,
pyrrolidine) follow 1,4- or 1,2-addition pattern with subsequent condensation of the adduct with the initial
amine to produce isomeric 2-ethoxyprop-2-ene-1,1-diamines and 2-ethoxyprop-1-ene-1,3-diamines. These
reactions are accelerated by a factor of 15–30 under microwave irradiation and in the presence of water. The
regioselectivity of primary nucleophilic attack varies over a wide range, depending on the amine basicity and
reactant ratio. The reaction of 2-ethoxyprop-2-enal with pyrrolidine and water at a ratio of 1:10:10 was
characterized by increased regioselectivity of 1,4-addition (up to 75%). Unlike cycloaliphatic amines,
2-ethoxyprop-2-enal reacted with N-methylaniline under microwave irradiation at a lower rate to give
2-ethoxy-N1,N1,N3-trimethyl-N1,N1,N3-triphenylpropane-1,1,3-triamine.
DOI: 10.1134/S1070428011120025
Unlike most propenals, 2-alkoxypropenals tend to
take up water, thiol, and alcohol molecules at the C=C
bond to give the corresponding Markovnikov adducts
[1]. However, the direction of attack by thiols in
alkaline medium changes, and they act as nucleophiles
to afford exclusively thia-Michael adducts [2]. Reac-
tions of 2-alkoxypropenals with other nucleophiles
(amines and CH acids) were studied very poorly.
α-Alkoxy-substituted acrylic systems, in particular,
α-alkoxyacroleins, occur in animal and plant tissues
[3]. It is believed that these compounds react with
nucleophilic HO, HS, and HN groups of enzymes,
amino acids, proteins, DNA, and RNA [4]. Conjugate
addition of amines to α,β-unsaturated carbonyl com-
pounds is now extensively studied as synthetically im-
portant field of organic chemistry [5]. Products of
conjugate addition of amines, specifically β-amino car-
bonyl compounds and their derivatives, are used as
peptide analogs or precursors of optically active amino
acids, amino alcohols, diamines, and lactams, many of
which are medical agents [6].
(Michael reaction), yielding isomeric mixtures of
2-ethoxyprop-2-ene-1,1-diamines and 2-ethoxyprop-1-
ene-1,3-diamines [7]. These reactions were carried out
by heating the reactants in boiling benzene with simul-
taneous removal of water and without it (reaction time
2–6 h) and were accompanied by formation of
N-formyl derivative of the initial amine as by-product
(up to 30%). Over the subsequent 20 years microwave
irradiation has opened new prospects in the develop-
ment of many thermally possible processes. As a rule,
microwave irradiation considerably accelerates chem-
ical reactions, improves their yield and selectivity, and
ensures solvent-free processes. Moreover, some reac-
tions that could not be performed under conventional
thermal conditions successfully occur under micro-
wave irradiation [8].
Michael reaction is one of the most extensively
studied reactions in organic synthesis, and its potential
can be increased with the use of microwave irradiation
[9, 10]. Furthermore, studies on Michael addition in
aqueous solution have been initiated since 1990s [11].
With a view to elucidate the effects of microwave
irradiation (MW) and aqueous medium in the present
work we continued our studies on the addition of
secondary amines to α-ethoxyacrolein [7].
As shown previously, secondary amines (such as
morpholine and piperidine) are capable of adding to
α-ethoxyacrolein at both 1,2 and 1,4 positions
1792

MICROWAVE-INITIATED HYDROAMINATION OF 2-ETHOXYPROPENAL
1793
Scheme 1.
OEt
OEt
HNR2
OH
NR2
H2C
H2C
NR2
OEt
NR2
IIIa–IIIc
a: 20–25°C, 1.5–5 h
b: MW, 700 W, 4–11 min
OEt
CHO
+
R2NH
+
H2O
H2C
OEt
I
IIa–IIc
HNR2
R2N
OH
R2N
NR2
IVa–IVc
R2N = morpholino (a), piperidino (b), pyrrolidin-1-yl (c).
It should be noted that numerous addition reactions
of secondary amines to activated α,β-unsaturated car-
bonyl compounds were performed with α,β-unsaturat-
ed esters, acids, amides, nitriles, and ketones [10, 12–
14], whereas conjugated aldehydes were seldom used
as Michael acceptors [13, 15]. Reactions with the latter
are strongly complicated by side processes, in partic-
ular by 1,2-addition with subsequent condensation, as
well as by polymerization [13, 16].
Previous attempts to accomplish conjugate addition
of secondary aromatic amines to α,β-unsaturated alde-
hydes in the presence of a number of catalysts were
unsuccessful [14, 17]. Up to now, catalysts have been
proposed (e.g., nickel, magnesium, and zinc perchlo-
rate complexes [18] and bismuth trifluoromethanesul-
fonates) which ensured selective conjugate addition of
aliphatic amines to α,β-unsaturated carbonyl com-
pounds in up to 70% yield [19]. Insofar as such cata-
lysts are difficultly accessible and expensive, we tried
to initiate the addition of an aromatic amine (N-meth-
ylaniline) to aldehyde I with the aid of 1,8-diazabi-
cyclo[5.4.0]undec-7-ene (DBU) which is successfully
used in analogous processes [20]. However, we failed
to obtain the corresponding 1,4-addition product ac-
cording to the procedure described in [20], and no
reaction occurred.
In the reaction of 2-ethoxyprop-2-enal (I) with
piperidine and morpholine at room temperature in
aqueous medium (2 equiv of water), the conversion of
initial aldehyde I was 75–80% in 3–5 h, while the
same conversion of I in analogous reaction under
microwave irradiation was attained in 6–11 min (ac-
1
cording to the H NMR data; see table); in both cases,
mixtures of isomeric ethoxypropenediamines III and
IV were formed (Scheme 1). Unlike morpholine and
piperidine, the reaction with pyrrolidine as stronger
nucleophile was much faster. The conversion of alde-
hyde I was complete in 1.5 h at room temperature or in
4 min under microwave irradiation (1H NMR). The
addition of cycloaliphatic amines IIa–IIc both in the
presence and in the absence of water was characterized
by similar rates.
The reaction of N-methylaniline with 2-ethoxyprop-
2-enal (I), initiated by microwave irradiation, was
much slower than the reactions of I with morpholine
and piperidine and especially with pyrrolidine. The
conversion of aldehyde I was only 50% after irradia-
tion for 85 min at a maximal power (700 W; see table),
and the presence of water was necessary. The final
product in the reaction of N-methylaniline with 2-eth-
Reaction of 2-ethoxyprop-2-enal with secondary amines at room temperature and under microwave irradiation
Room temperature
Microwave irradiation, 700 W
Amine
pKa
reaction time, h
conversion of I,a % reaction time, min conversion of I,a %
Morpholine
Piperidine
08.33
11.12
11.31
04.85
3.0
5.0
1.5
–
080
075
100
–
08
11
080
075
100
050
Pyrrolidine
04
N-Methylaniline
85–120
a
According to the 1H NMR data.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 12 2011

KEIKO et al.
1794
Scheme 2.
OEt
CHO
Me
OEt
Me
N
OEt
MW, 700 W, 85 min
+ PhNHMe + H2O
O
N
OH
H2C
Ph
Ph
I
IId
Me
N
OEt Ph
N
2PhNHMe
Ph
Me
N
Ph
Me
V, 25%
oxyprop-2-enal (I) was 2-ethoxy-N1,N1,N3-trimethyl-
N1,N1,N3-triphenylpropane-1,1,3-triamine (V) (yield
25%; Scheme 2). The structure of V was determined
by 1H and 13C NMR spectroscopy using HSQC-GP and
HMBC-GP 1H–13C two-dimensional techniques.
EXPERIMENTAL
The H and 13C NMR spectra were recorded on
1
Bruker DPX-400 and Bruker AV-400 spectrometers at
400.13 and 100.61 MHz, respectively, using CDCl3 as
solvent and hexamethyldisiloxane as internal refer-
ence; the chemical shifts were measured with an ac-
curacy of 0.01 and 0.02 ppm, respectively. GC–MS
analysis was performed on a Hewlett–Packard HP
5890 gas chromatograph (Ultra-2 column, injector
temperature 250°C, oven temperature programming
from 70 to 280°C at a rate of 20 deg/min) coupled with
an HP 5971A mass-selective detector (electron impact,
70 eV). Microwave-assisted reactions were performed
in an LG MS-1904H microwave furnace (700 W).
Silica gel 60 (0.063–0.200 mm, Merck) was used for
chromatographic purification.
Thus reactions of 2-ethoxypropen-2-al with cyclo-
aliphatic secondary amines in aqueous solution under
microwave irradiation are characterized by high con-
version (75–100% in 4–11 min) and selectivity (no by-
products were detected). N-Methylaniline as weakly
basic aromatic amine can also be involved in conjugate
addition to unsaturated aldehyde I, but this process is
followed by formation of aminal V, presumably due to
prolonged irradiation. Depending on the reaction con-
ditions, amine nature, reactant ratio, and amount of
polar solvent, mixtures of 1,2- and 1,4-addition prod-
ucts are generally formed at a ratio of 2:1 to 7.6:1 for
morpholine, 1:2 to 1:2.5 for piperidine, and 1.5:1 to
1:3 for pyrrolidine. Considerably increased regioselec-
tivity toward formation of 1,4-addition product IVc
(75%, according to the 1H NMR data) was achieved in
the reaction of aldehyde I with pyrrolidine and water at
a ratio of 1:10:10. Selective conjugate addition (66%,
1H NMR) was also observed in the reaction of 2-eth-
oxyprop-2-enal with piperidine and water at a ratio of
1:1.5:1.5. These findings indicate that the presence of
an ethoxy group in the α-position of Michael acceptor
does not prevent Michael reaction under the given
conditions and provide additional information on the
effect of substituents in α,β-unsaturated carbonyl com-
pounds [10].
Condensation of 2-ethoxyprop-2-enal (I) with
morpholine and piperidine (general procedure).
a. A mixture of 1 g (10 mmol) of aldehyde I, 20 mmol
of morpholine or piperidine, and 0.36 g (20 mmol) of
water was stirred for 2–5 h at room temperature
(1H NMR monitoring). The mixture was extracted with
diethyl ether, the extract was dried over MgSO4, and
the solvent and unreacted initial compounds were
removed to obtain isomer mixture IIIa/IVa or
IIIb/IVb in an overall yield of 80% (1H NMR).
b. An ampule was charged with 1 g (10 mmol) of
aldehyde (I), 20 mmol of the corresponding amine, and
0.36 g (20 mmol) of water, and the mixture was
irradiated in a microwave furnace at a power of 700 W
over a period of 8–11 min (in 1-min pulses followed
by cooling to room temperature). The mixture was
extracted with diethyl ether, the extract was dried over
MgSO4, and the solvent and unreacted initial com-
pounds were removed to obtain isomer mixture
IIIa/IVa (overall yield 87%) or IIIb/IVb (84%;
To conclude, microwave irradiation and the pres-
ence of water strongly accelerates addition of secon-
dary amines to α-ethoxy-substituted α,β-unsaturated al-
dehyde. The regioselectivity of the process depends on
the basicity of the medium; therefore, in the reaction
with pyrrolidine and water taken in a large excess with
respect to aldehyde I the yield of the 1,4-addition
product attains 75%.
1
according to the H NMR data). The products were
identified by comparing their spectral parameters with
those reported in [7].
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 47 No. 12 2011

MICROWAVE-INITIATED HYDROAMINATION OF 2-ETHOXYPROPENAL
1795
The reaction can also be carried out under analo-
gous conditions but without addition of water.
5-H), 2.97 s (2H, CH2), 3.72 q (2H, CH2O), 5.25 s (1H,
=CH). 13C NMR spectrum, δC, ppm: 23.43 (CH3),
49.78, 52.72 (NCH2), 53.76, 64.80 (OCH2), 123.46
(=CHN), 133.03 (O–C=). Found for isomer mixture
IIIc/IVc, %: C 69.52; H 10.78; N 12.38. C13H24N2O.
Calculated, %: C 69.64; H 10.71; N 12.50.
2-Ethoxy-3,3-dimorpholinoprop-1-ene (IIIa).
1H NMR spectrum, δ, ppm: 1.31 t (3H, CH3, J =
7.0 Hz), 2.42 m (6H, CH2), 2.52 m (2H, CH2), 2.94 s
(1H, CH), 3.63 m (8H, CH2), 3.74 q (2H, CH2CH3, J =
7.0 Hz), 3.97 d and 4.10 d (1H each, =CH2, J =
1.7 Hz). Mass spectrum (retention time 10.68 min),
m/z (Irel, %): 256 (22) [M]+, 170 (100) [M –
N(CH2CH2)2O]+, 140 (65), 112 (9), 100 (98), 84 (9)
[M – 2N(CH2CH2)2O]+, 70 (9), 56 (33), 42 (19), 29
(23) [Et]+.
When the reaction was carried out at a I–IIc–H2O
ratio of 1:10:10 (MW, 5 min), a mixture of isomers
IIIc and IVc was obtained at a ratio of 1:3.
2-Ethoxy-N1,N1,N3-trimethyl-N1,N1,N3-triphenyl-
propane-1,1,3-triamine (V) was synthesized as
described above in b. A mixture of 2 g (20 mmol) of
aldehyde I, 4.28 g (40 mmol) of N-methylaniline, and
0.72 g (40 mmol) of water was subjected to microwave
irradiation (700 W) over a period of 85 min (in 1-min
pulses followed by cooling to 20°C). The mixture was
extracted with diethyl ether (3×15 ml), the extracts
were combined and dried over MgSO4, the solvent and
unreacted initial compounds were removed, and the
residue was subjected to chromatography on silica gel
using hexane–diethyl ether (3:1) as eluent. Yield
1.33 g (25%, calculated on the initial amine), dark
brown liquid. Compound V was isolated as a mixture
2-Ethoxy-1,3-dimorpholinoprop-1-ene (IVa).
1H NMR spectrum, δ, ppm: 1.23 t (3H, CH3, J =
7.0 Hz), 2.52 m (8H, CH2), 3.02 s (2H, CH2), 3.63 m
(8H, CH2), 3.87 q (2H, OCH2, J = 7.0 Hz), 4.90 s (1H,
=CH). Mass spectrum (retention time 10.97 min),
m/z (Irel, %): 256 (23) [M]+, 170 (100) [M –
N(CH2CH2)2O]+, 140 (71), 112 (9), 100 (78), 84 (7)
[M – 2N(CH2CH2)2O]+, 70 (10), 56 (20), 41 (12), 29
(11) [Et]+.
2-Ethoxy-3,3-dipiperidinoprop-1-ene (IIIb).
1H NMR spectrum, δ, ppm: 1.30 t (3H, CH3, J =
7.0 Hz), 1.41 m (8H, CH2), 1.45 m (8H, CH2), 2.34 m
(4H, CH2), 2.90 s (1H, CH), 3.72 q (2H, OCH2, J =
7.0 Hz), 3.89 d and 3.98 d (1H each, =CH2, J = 2.8 Hz).
1
of two diastereoisomers at a ratio of 1:1. H NMR
spectrum, δ, ppm: 1.07 t (3H, CH3CH2, J = 7.0 Hz),
2.68 s (3H, CH3N), 2.81 s (3H, CH3N), 2.89 s (3H,
CH3N), 3.19 d and 3.32 d (1H, CH2, J = 9 Hz), 3.29 d
and 3.32 d (1H, CH2, J = 4.3 Hz), 3.52 q and 3.61 q
(1H each, OCH2, J = 7.0 Hz), 3.91 m (1H, CH), 5.03 d
(1H, CH, J = 8.2 Hz), 6.54 d (2H, o-H, J = 8.1 Hz),
6.56 t (2H, m-H, J = 8.0 Hz), 6.68 m (3H, p-H), 6.84 d
(2H, o-H, J = 8.1 Hz), 7.00 d (2H, o-H, J = 7.6 Hz),
7.08 t (2H, m-H, J = 7.6 Hz), 7.20 m (2H, m-H).
13C NMR spectrum, δC, ppm: 15.42 (CH3CH2), 31.01
(CH3N), 33.40 (CH3N), 39.11 (CH3N), 54.43 (CH2N),
62.86 (CHN), 65.04 (CH2O), 72.58 (CH2O), 111.20
(Cm), 112.59 (Co), 112.97 (Co), 116.24 (Cp), 117.38
(Cp), 118.06 (Cp), 127.82 (Cm), 128.07 (Co), 128.86
(Cm), 146.73 (Ci), 147.72 (Ci), 150.18 (Ci). Found for
isomer mixture, %: C 77.54; H 8.14; N 9.89.
C26H33N3O. Calculated, %: C 77.42; H 8.19; N 10.42.
2-Ethoxy-1,3-dipiperidinoprop-1-ene (IVb).
1H NMR spectrum, δ, ppm: 1.22 t (3H, CH3, J =
7.0 Hz), 1.53 m (12H, CH2), 2.42 m (8H, CH2), 2.78 s
(1H, CH2), 3.88 q (2H, OCH2, J = 7.0 Hz), 4.85 s
(1H, =CH).
2-Ethoxy-3,3-bis(pyrrolidin-1-yl)prop-1-ene
(IIIc). The procedure was the same as above, method
b. A mixture of 2 g (20 mmol) of aldehyde I and 2.84 g
(40 mmol) of pyrrolidine was irradiated in a micro-
wave furnace (700 W) over a period of 4 min (in 1-min
pulses followed by cooling to room temperature).
1
According to the H NMR data, the conversion was
100%. Vacuum distillation gave a mixture of isomers
IIIc and IVc at a ratio of 1.5:1. Yield 2.88 g (64%),
1
dark red liquid, bp 25–35°C (35 mm). H NMR spec-
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 08-03-00396).
trum, δ, ppm: 1.30 t (3H, CH3, J = 7.0 Hz), 1.75 m
(8H, 3-H, 4-H), 2.60 m (8H, 2-H, 5-H), 2.94 s (1H,
NCH), 3.75 q (2H, OCH2, J = 7.0 Hz), 3.94 d and
4.03 d (1H each, =CH2, J = 1.6 Hz). 13C NMR spec-
trum, δC, ppm: 23.43 (CH3), 49.78, 51.88, 62.39
(OCH2), 84.52 (NCH), 84.91 (CH2=), 158.81 (=C–O).
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