Synthesis of enaminones containing diverse N,N-disubstitution via simple transamination: a study with sustainable catalyst-free operation

Article abstract of DOI:10.1007/s11164-017-2946-z

Abstract: A systematic investigation on the synthesis of β-enaminones containing diverse N,N-disubstitution via the transamination of N,N-dimethyl amino functionalized β-enaminones and secondary amines has been conducted by employing biomass available green solvent ethyl lactate as reaction medium. A class of β-enaminones containing different N,N-disubstitutions have been smoothly synthesized under the sustainable conditions without using any catalyst.

Full text of DOI:10.1007/s11164-017-2946-z

Res Chem Intermed
DOI 10.1007/s11164-017-2946-z
Synthesis of enaminones containing diverse N,N-
disubstitution via simple transamination: a study
with sustainable catalyst-free operation
Yong Gao1 Yunyun Liu1 Li Wei1 Jieping Wan1
•
•
•
Received: 10 January 2017 / Accepted: 25 March 2017
Ó Springer Science+Business Media Dordrecht 2017
Abstract A systematic investigation on the synthesis of b-enaminones containing
diverse N,N-disubstitution via the transamination of N,N-dimethyl amino functionalized
b-enaminones and secondary amines has been conducted by employing biomass avail-
able green solvent ethyl lactate as reaction medium. A class of b-enaminones containing
different N,N-disubstitutions have been smoothly synthesized under the sustainable
conditions without using any catalyst.
Graphical Abstract
Keywords Enaminone Á N,N-disubstitution Á Catalyst-free Á Transamination Á Bio-
based medium
Electronic supplementary material The online version of this article (doi:10.1007/s11164-017-2946-z)
contains supplementary material, which is available to authorized users.
&
Jieping Wan
wanjieping@jxnu.edu.cn
1
College of Chemistry and Chemical Engineering, Jiangxi Normal University,
Nanchang 330022, People’s Republic of China
123

Y. Gao et al.
Introduction
b-Enaminones are important synthons with a long history of application in organic
synthesis [1–4]. Their unique advantages lie not only in the multiple reactive sites in the
structures, but also in the presence of versatile forms as well as satisfactory shelf
stability. In addition to the broad application in organic synthesis as synthons, the
enaminone structure has also been observed in many biologically active organic
molecules [5–7]. Unsurprisingly, the synthesis of functional enaminones has been an
issue ofeverlasting interest. Typically, enaminonescan be accessedbythe condensation
of 1,3-dicarbonyl compounds with amines [8–10], the addition of amine to alkynones
[11–13], the Eschemmoser reaction [14, 15], the C–H amination of enones [16–18], the
reactions of imidoylbenzotriazoles with ketones or related enolates [19], the cascade C–
H amination andelimination ofketones [20], various multicomponentreactions [21–26]
and someother methods [27–33]. Interestingly, regardless of the availability ofenriched
methods toward enaminone synthesis, most of them are mainly applicable for the
synthesis of NH or NH2 functionalized enaminones. Practical methods for the N,N-
disubstituted tertiary enaminones, on the other hand, are rather scarce.
Presently, the most frequently employed tertiary enaminones are the N,N-dimethyl
substituted ones which are usually prepared via the condensation of methyl ketones and
DMF–DMA [34]. This method is simple and practical, but not suitable for the synthesis
of tertiary enaminones with other N,N-disubstituted amino groups. Due to the limited
known methods for the synthesis of tertiary enaminones with diversity in the amino
group, the potential of the amino group in enaminone-based organic synthesis has been
seriously hampered. Instead, most known syntheses employing tertiary enaminones end
with the elimination of the amino group. Recently, some tertiary enaminone-based
syntheses wherein the amino group participates in the product construction have been
developed [35–37], which outline the significance of the amino group in enaminones for
the generation of molecular diversity (Scheme 1). Therefore, preparation of tertiary
Scheme 1 Synthesis involving the construction of the amino group in tertiary enaminones
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Synthesis of enaminones containing diverse…
enaminones bearing diverse N,N-disubstitution turns out to be an important issue in the
chemistry of enaminones. As occasional examples, the transamination reactions
between a tertiary enaminone (usually N,N-dimethyl enaminones) with a different
secondary amine has been employed for the preparation of tertiary enaminones bearing
different amino groups [21, 38, 39]. However, no work has hitherto systematically
investigated the scope and limitation of such a synthetic approach.
In the context of our longstanding and extensive efforts in exploring enaminone-
based organic synthesis, especially the synthesis employing tertiary enaminones
[4, 40], we have been encouraged to make efforts to develope applicable synthetic
approaches for the synthesis of tertiary enaminones with diversity in the tertiary
amino group. Here, we wish to report the results of our systematic research in the
transamination-based synthesis of tertiary enaminones by employing the bio-based
green solvent ethyl lactate (EL) as medium [41].
Experimental
General procedure for the synthesis of b-enaminones 3
To a round-bottom flask (25 mL) were added enaminones 1 (0.2 mmol), secondary
amines 2 (0.8 mmol) and ethyl lactate (2 mL). Then, the mixture was heated to
100 °C, and stirred at the same temperature for 12 h under air atmosphere (TLC).
After cooling to room temperature, 5 mL of water was added, and the resulting
mixture was extracted with ethyl acetate. The organic phases were collected and
washed with small amounts of water six times. After drying with anhydrous
Na2SO4, the solid was filtered and the solvent was removed under reduced pressure.
The resulting residue was subjected to flash silica gel column chromatography to
provide pure products with the elution of mixed petroleum ether/ethyl acetate (v/
v = 1:1).
1
Compound 3a [32]: Yellow solid; m.p. 89–94 °C (lit. 90.6–91.1 °C); H NMR
(400 MHz, CDCl3): d 7.88 (d, J = 7.2 Hz, 2 H), 7.74 (d, J = 12.6 Hz, 1 H),
7.47–7.40 (m, 3 H), 5.88 (d, J = 12.6 Hz, 1 H), 3.76 (t, J = 4.8 Hz, 4 H), 3.40 (t,
J = 4.8 Hz, 4 H); 13C NMR (100 MHz, CDCl3): 189.2, 152.8, 140.2, 131.2, 128.2,
127.5, 92.5, 66.3.
Compound 3c: Yellow liquid; 1H NMR (400 MHz, CDCl3): d 7.73 (d,
J = 12.4 Hz, 1 H), 7.44 (d, J = 6.8 Hz, 2 H), 7.32 (t, J = 8.0 Hz, 1 H), 7.01 (d,
J = 8.0 Hz, 1 H), 5.85 (d, J = 12.6 Hz, 1 H), 3.86 (s, 3 H), 3.76 (t, J = 4.8 Hz, 4
H), 3.39 (t, J = 4.4 Hz, 4 H); 13C NMR (100 MHz, CDCl3): 188.9, 159.7, 152.8,
141.7, 129.1, 119.9, 117.5, 112.3, 92.6, 66.2, 55.4; ESI–HRMS Calcd for
C14H18NO3 [M ? H]? 248.1281, found 248.1282.
Compound 3d [42]: Yellow liquid; 1H NMR (400 MHz, CDCl3): d 7.33–7.16 (m,
5 H), 5.50 (d, J = 13.0 Hz, 1 H), 3.73 (t, J = 4.8 Hz, 4 H), 3.32 (t, J = 4.8 Hz, 4
H), 2.40 (s, 3 H); 13C NMR (100 MHz, CDCl3): 195.1, 153.4, 141.7, 135.5, 130.8,
129.0, 127.1, 125.3, 98.1, 66.3, 19.9.
1
Compound 3e [32]: Yellow solid; m.p. 111–115 °C (lit. 107.1–107.2 °C); H
NMR (400 MHz, CDCl3): d 7.83 (d, J = 8.4 Hz, 2 H), 7.74 (d, J = 12.5 Hz, 1 H),
123

Y. Gao et al.
7.38 (d, J = 8.4 Hz, 2 H), 5.82 (d, J = 12.5 Hz, 1 H), 3.76 (t, J = 4.8 Hz, 4 H),
3.40 (s, 4 H); 13C NMR (100 MHz, CDCl3): 195.4, 187.6, 153.0, 138.5, 137.3,
128.9, 128.4, 91.9, 66.3.
1
Compound 3g: Yellow solid; m.p. 127–132 °C; H NMR (400 MHz, CDCl3): d
7.70 (d, J = 12.6 Hz, 1 H), 7.47 (d, J = 8.1 Hz, 1 H), 7.42 (s, 1 H), 6.82 (d,
J = 8.1 Hz, 1 H), 6.00 (s, 2 H), 5.81 (d, J = 12.6 Hz, 1 H), 3.74 (t, J = 4.6 Hz, 4
H), 3.37 (t, J = 4.6 Hz, 4 H); 13C NMR (100 MHz, CDCl3): 187.3, 152.5, 150.2,
147.8, 134.8, 122.7, 107.9, 107.6, 101.5, 92.0, 66.2, 49.6; ESI–HRMS Calcd for
C14H16NO4 [M ? H]? 262.1074, found 262.1073.
1
Compound 3h: Yellow solid; m.p. 115–119 °C; H NMR (400 MHz, CDCl3): d
7.96–7.92 (m, 1 H), 7.76–7.65 (m, 2 H), 7.48–7.41 (m, 1 H), 5.79–5.72 (m, 1 H),
3.76–3.71 (m, 4 H), 3.40 (s, 4 H); 13C NMR (100 MHz, CDCl3): 185.9, 153.3,
140.0, 135.1, 132.5, 130.2, 129.5, 126.7, 91.4, 66.2; ESI–HRMS Calcd for
C13H14Cl2NO2 [M ? H]? 286.0396, found 286.0393.
1
Compound 3i: Yellow solid; m.p. 187–192 °C; H NMR (400 MHz, CDCl3): d
7.95 (d, J = 8.1 Hz, 2 H), 7.80 (d, J = 12.4 Hz, 1 H), 7.72–7.70 (m, 2 H), 5.82 (d,
J = 12.5 Hz, 1 H), 3.79–3.77 (m, 4 H), 3.45 (s, 4 H); 13C NMR (100 MHz, CDCl3):
186.9, 153.6, 144.0, 132.1, 127.9, 118.5, 114.2, 91.8, 66.2; ESI–HRMS Calcd for
C14H15N2O2 [M ? H]? 243.1128, found 243.1129.
1
Compound 3j: Yellow solid; m.p. 150–154 °C; H NMR (400 MHz, CDCl3): d
8.68 (t, J = 2.0 Hz, 1 H), 8.33–8.30 (m, 1 H), 8.24 (d, J = 7.8 Hz, 1 H), 7.84 (d,
J = 12.4 Hz, 1 H), 7.61 (t, J = 8.0 Hz, 1 H), 5.88 (d, J = 12.4 Hz, 1 H), 3.80 (t,
J = 4.8 Hz, 4 H), 3.48 (s, 4 H); 13C NMR (100 MHz, CDCl3): 185.9, 153.7, 148.2,
141.7, 133.4, 129.3, 125.5, 122.3, 91.3, 66.2; ESI–HRMS Calcd for C13H15N2O4
[M ? H]? 263.1026, found 263.1030.
1
Compound 3k [42]: Yellow solid; m.p. 119–125 °C (lit. 114.1–114.7 °C); H
NMR (400 MHz, CDCl3): d 8.39 (s, 1 H), 8.02–7.77 (m, 5 H), 7.55–7.48 (m, (t,
J = 4.8 Hz, 4 H); 13C NMR (100 MHz, CDCl3): 188.9, 152.8, 137.5, 134.9, 132.8,
129.2, 128.0, 127.9, 127.7, 127.4, 126.3, 124.5, 92.6, 66.3.
Compound 3m: Yellow solid; m.p. 132–137 °C; 1H NMR (400 MHz, CDCl3): d
7.74 (d, J = 12.8 Hz, 1 H), 7.50 (s, 1 H), 7.09 (d, J = 3.4 Hz, 1 H), 6.50–6.49 (m, 1
H), 5.84 (d, J = 12.8 Hz, 1 H), 3.76 (t, J = 4.8 Hz, 4 H), 3.41 (t, J = 4.8 Hz, 4 H);
13C NMR (100 MHz, CDCl3): 177.9, 154.6, 152.0, 144.4, 113.8, 111.9, 91.8, 66.2;
ESI–HRMS Calcd for C11H14NO3 [M ? H]? 208.0968, found 208.0973.
1
Compound 3n: Yellow solid; m.p. 147–152 °C; H NMR (400 MHz, CDCl3): d
8.09 (d, J = 10.9 Hz, 1 H), 6.73 (d, J = 10.9 Hz, 1 H), 3.78 (t, J = 4.8 Hz, 4 H),
3.36 (s, 4 H); 13C NMR (100 MHz, CDCl3): 149.6, 113.1, 66.1, 53.2; ESI–HRMS
Calcd for C6H11N2O3 [M ? H]? 159.0764, found 159.0766.
Compound 3o: Yellow liquid; 1H NMR (400 MHz, CDCl3): d 6.86 (d,
J = 13.8 Hz, 1 H), 3.94 (d, J = 13.8 Hz, 1 H), 3.72 (t, J = 4.8 Hz, 4 H), 3.16 (t,
J = 4.8 Hz, 4 H); 13C NMR (100 MHz, CDCl3): 153.6, 121.4, 65.9, 62.8, 48.1;
ESI–HRMS Calcd for C7H11N2O [M ? H]? 139.0866, found 139.0863.
Compound 3q [32]: Yellow liquid; 1H NMR (400 MHz, CDCl3): d 7.90–7.82 (m,
3 H), 7.45–7.39 (m, 3 H), 5.77 (d, J = 12.5 Hz, 1 H), 3.33 (s, 4 H), 1.24 (t,
J = 7.0 Hz, 6 H); 13C NMR (100 MHz, CDCl3): 188.8, 152.5, 140.8, 130.8, 128.1,
127.5, 91.8, 50.7, 42.9, 14.9, 11.6.
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Synthesis of enaminones containing diverse…
1
Compound 3s [20]: Yellow solid; m.p. 122–127 °C (lit. 105.2–105.3 °C); H
NMR (400 MHz, CDCl3): d 8.17 (d, J = 12.6 Hz, 1 H), 7.85 (d, J = 7.1 Hz, 2 H),
7.45–7.20 (m, 13 H), 6.01 (d, J = 12.6 Hz, 1 H), 4.41 (d, J = 24.1 Hz, 4 H); 13C
NMR (100 MHz, CDCl3): 189.3, 154.2, 140.4, 131.1, 129.0, 128.2, 127.6, 93.2,
59.4, 50.9.
1
Compound 3t: Yellow solid; m.p. 102–105 °C; H NMR (400 MHz, CDCl3): d
8.23 (d, J = 12.7 Hz, 1 H), 7.94 (d, J = 6.8 Hz, 2 H), 7.50–7.35 (m, 5 H),
7.21–7.15 (m, 3 H), 6.10 (d, J = 12.7 Hz, 1 H), 3.38 (s, 3 H); 13C NMR (100 MHz,
CDCl3): 189.4, 150.0, 146.4, 140.1, 131.4, 129.5, 128.3, 127.7, 125.0, 120.5, 96.9,
37.4; ESI–HRMS Calcd for C16H16NO [M ? H]? 238.1226, found 238.1230.
Results and discussion
To begin the investigation, the reaction of N,N-dimethyl enaminone 1a and
morpholine 2a was selected to probe practical reaction conditions. The entries
conducted at 100 °C in different media, including water, EtOH, DMF, MeCN, PEG-
400 and EL (ethyl lactate), all provided the target enaminone product 3a without
employing any catalyst, and EL was found to be the best medium (Table 1, entries
1–7). On the other hand, no product was observed using lactic acid (LA) as medium,
suggesting that an acidic condition was not favored in this transamination process
(Table 1, entry 8). Considering the satisfactory yield acquired under the catalyst-
free condition, no further efforts by employing catalysts were made. Instead, the
impact of reaction temperature to the reaction was examined. It was found that a
slightly lower yield of the product was given in the entries with either higher or
lower temperature (Table 1, entries 9–10). A higher temperature might lead to the
occurrence of more side transformations such as decomposition, oxidation or
cyclotrimerization, etc., while a lower temperature was negative for an endothermic
reaction.
Table 1 Optimization on
reaction conditions
Entry
Solvent
T (°C)
Yield (%)a
1
2b
H2O
100
43
78
77
75
76
75
81
0
EtOH
DMF
Toluene
MeCN
PEG-400
EL
Reflux
100
3
4
5b
100
Reflux
100
General conditions are: 1a
(0.2 mmol), 2a (0.8 mmol) in
solvent (2 mL) stirred for 12 h
6
7
100
a
Yield of isolated products base
8
LA
100
on 1a
b
9
EL
120
78
71
The outside heating
temperature was 100 °C
10
EL
80
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Y. Gao et al.
After the brief optimization on reaction conditions, the scope of this catalyst-free
transamiation in the synthesis of different N,N-disubstituted enaminones was then
carried out. As presented in Table 2, the transamination between N,N-dimethyl
enaminones (and analogous enamines) 1 and secondary amine 2 both exhibited fine
tolerance toward the synthesis of product 3. According to the results in hand, the
N,N-dimethyl enaminones functionalized by conventional benzene substructure
took part in the transformation to provide target products with generally excellent
yield (3a–3h), but the strong electron-withdrawing group (cyano and nitro) in the
phenyl ring of the enaminone 1 led to the formation of corresponding products with
slightly lower yield (3j–3k). The smooth employment of naphthyl and heteroaryl
functionalized enaminones further demonstrated the broad application scope of this
Table 2 Synthesis of various b-enaminones via transamination
Yield of isolated product based on enamine 1 was reported
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Synthesis of enaminones containing diverse…
transamination-based synthesis of tertiary enaminones (3k, 3m). More notably, like
the reactions of enaminones, other analogous electron deficient enamines, such as
N,N-dimethyl nitro and cyano enamines also underwent the transamination with
morpholine to give new enaminone products (3n–3o). Finally, as expected, the
application of different secondary amines, including both alkyl and aryl function-
alized secondary amines were found effective for related synthesis (3p–3t). It
should be noted that the secondary amine containing N-aryl substructure gave lower
yield of the corresponding enaminone product (3t) probably because of the
undermined amino nucleophilicity resulting from the presence of the aryl via p–p
conjugation. Among these synthesized products, 3a, 3b, 3d, 3e, 3f, 3k, 3l, 3p, 3q,
3r, and 3s are known in the literature, and all other compounds are newly
synthesized in this work (see Supplementary information for full characterization
data of all products).
Subsequently, to examine the reversibility of such transamination, the reaction of
product 1a with aqueous dimethyl amine (40 wt% commercial reagent) was
conducted under the standard reaction condition. As shown in Eq. 1, it was found
that the reverse transamination took place smoothly to provide 1a with excellent
yield, demonstrating that the present approach was flexible and versatile in
preparing different N,N-disubstituted enaminones without encountering much
restriction of the substrate.
ð1Þ
According to the reaction conditions and outcomes, a plausible mechanism for
this transamination reaction is proposed. As outlined in Scheme 2, the hydrogen
bond interactions of EL with both the dimethyl enaminone and secondary amine
assist the substrates to keep close to each other, which facilitates the addition of the
secondary amine 2 to the C=C double bond in the enaminone 1 to provide
intermediate 4. Subsequently, the elimination of dimethyl amine on 4 leads to the
production of enaminone 3. Besides the employment of excessive substrate 2, the
practical synthesis of products 3 can also be ascribed to the formation of the low
Scheme 2 The proposed reaction mechanism
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Y. Gao et al.
boiling point dimethyl amine 5 which evaporates from the reaction system to
promote the reaction to move in the expected direction under the heating condition.
Conclusions
We have presented here a systematic approach for the synthesis of diverse N,N-
disusbstituted b-enaminones and analogous nitro- and cyano-enamines. This
catalyst-free approach employs bio-based ethyl lactate as the green medium, and
exhibits satisfactory application scope in the synthesis of various N,N-disubstituted
enaminones by employing both different N,N-dimethyl enaminones and secondary
amines. This simple and environmentally benign protocol provides a reliable route
for the synthesis of these useful N,N-disubstituted tertiary enaminones.
Supporting information
Full experimental details, 1H and 13C NMR spectra. This material can be found via
the Supporting information.
Acknowledgements This work is financially supported by Natural Science Foundation of China
(21562025) and Natural Science Foundation of Jiangxi Province (20161ACB21010).
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