Enantioselective Michael addition of malonates to α,β-unsaturated ketones catalyzed by 1,2-diphenylethanediamine

Article abstract of DOI:10.1039/c8ra07809b

A general and highly enantioselective Michael addition of malonates to cinnamones and chalcones has been developed. The commercially available 1,2-diphenylethanediamine could be directly utilized as the organocatalyst to furnish the desired adducts in satisfactory yield (61-99%) and moderate to excellent enantiopurity (65 to >99% ee). β-Ketoester was also a competent donor and was employed to construct densely functionalized cyclohexenones via a tandem Michael-aldol condensation process.

Full text of DOI:10.1039/c8ra07809b

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Enantioselective Michael addition of malonates to
a,b-unsaturated ketones catalyzed by 1,2-
diphenylethanediamine†
Cite this: RSC Adv., 2018, 8, 41699
a
a
Ling Ye,b Zhichuan Shi,a Zhigang Zhaoa and Xuefeng Li
*
Wei Wang,
A general and highly enantioselective Michael addition of malonates to cinnamones and chalcones has
been developed. The commercially available 1,2-diphenylethanediamine could be directly utilized as the
organocatalyst to furnish the desired adducts in satisfactory yield (61–99%) and moderate to excellent
enantiopurity (65 to >99% ee). b-Ketoester was also a competent donor and was employed to construct
densely functionalized cyclohexenones via a tandem Michael-aldol condensation process.
Received 20th September 2018
Accepted 3rd December 2018
DOI: 10.1039/c8ra07809b
rsc.li/rsc-advances
high degrees of optical purity and reactivity need to be achieved in
the presence of modied organocatalysts in most cases. As we all
Introduction
know, these employed optimal organocatalysts should usually
been prepared from commercially available precursors or naturally
occurring compounds aer several-step, even multi-step trans-
formation.2,3,5a,5b,6–9 The costly preparative procedure hence impairs
the synthetic efficiency and practicality to a certain extent. There-
fore, the development of highly general asymmetric Michael
addition promoted by simple and commercially available mole-
cules is clearly in high demand.10
The direct Michael addition of stabilized carbon-centered nucleo-
philes to electron-poor olens is widely recognized as a highly
atom-economic carbon–carbon bond-forming reaction in organic
synthesis. Therefore, the development of an enantioselective
catalytic protocol for this conversion has constituted an actively
pursued eld in the past decades.1 Although signicant progress
has been achieved with metal complexes,1a–c recently the well-
designed organocatalyst has played an impressive role in this
eld as well.1d,1e Particularly, the organocatalytic Michael addition
of malonates to a,b-unsaturated ketones will produce versatile
adducts, which can be readily converted to the corresponding d-
ketoesters as useful synthetic building blocks aer decarboxyl-
ation.2 Pioneering work, the rst highly enantioselective truly
organocatalytic reaction of this type was developed by Jørgensen
using an imidazoline catalyst in 2003.2a Subsequently, other
organocatalysts such as proline-derived tetrazole,2b,3 metal salts of
carboxylic acids,4 phase-transfer catalysts,5 various chiral thio-
ureas,6 proline-derived guanidines,7 primary amines8 and their
derivates9 have been introduced to catalyze this reaction. Despite
excellent enantiopurities having been achieved in a few cases,
nevertheless some of the established approaches suffer from
several drawbacks to a certain extent, such as narrow substrate
scope and restriction to a special combination of nucleophile and
electrophile type. Moreover, among all these well-demonstrated
organocatalytic Michael reactions, those untransformed and
simple molecules are always not the preferred catalysts as
a consequence of inferior enantioselectivity and poorer reactivity,
In this context, chiral vicinal 1,2-diamines, mainly
cyclohexane-1,2-diamines (CHDA)11 and 1,2-diphenylethanedi-
amine (DPEN)12 emerged as a class of efficient and commer-
cially available primary amine catalysts.13 These diamines
enabled the stereoselective functionalization of a variety of
steric-constraint carbonyl compounds, including aliphatic and
aromatic ketones,11a–c,12h,12i a-branched substituted aldehy-
des,11d,11e and a,b-unsaturated carbonyl compounds.11f,12a–g
A
range of versatile building blocks were smoothly constructed in
a highly enantioenriched fashion via enamine,11a,11b,11d,12h imi-
nium,11f,12a–f enamine–iminium11c,11e,12g and dienamine12i acti-
vation modes. As part of our continuous efforts in developing
asymmetric Michael addition of unactivated a,b-unsaturated
ketones,14 we disclosed herein a highly enantioselective Michael
addition of malonates to cinnamones2–4,6a–c,8–9 and chalco-
nes5,6d–f,8b,15 catalyzed by a structurally simple DPEN.
Results and discussion
Instead of the oen-used CHDA, the moisture- and air-stable,
commercially inexpensive DPEN was initially utilized to screen
the optimal conditions due to its operational simplicity. Grati-
fyingly, the Michael reaction between b-naphthyl-substituted
cinnamone 1a and diethyl malonate 2a proceeded smoothly to
afford the desired adduct 3aa with promising enantiopurity (92%
ee) in the presence of acetic acid (Table 1, entry 1). In order to
aCollege of Chemistry and Environment Protection Engineering, Southwest Minzu
University, Chengdu 610041, China. E-mail: lixuefeng@swun.edu.cn
bFaculty of Geosciences and Environmental Engineering, Southwest Jiaotong
University, Chengdu 610031, China
† Electronic supplementary information (ESI) available: NMR and HPLC spectra
for all new compounds. See DOI: 10.1039/c8ra07809b
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Table 1 Optimization of reaction conditionsa
Entry
Additive
Solvent
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
HOAc
TFA
TsOH
BA
PNBA
ONBA
OFBA
o-Phthalic acid
SA
SA
SA
SA
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
CHCl3
Et2O
168
168
168
168
168
168
168
144
168
168
72
168
168
96
168
168
30
86
79
30
82
65
64
77
95
91
91
97
91
75
95
99
NR
99
92
96
97
81
71
72
61
95
88
90
90
88
96
94
94
9
10
11
12
13
14
15d
16e
17f
THF
EtOH
EtOH
EtOH
SA
o-Phthalic acid
o-Phthalic acid
EtOH
o-Phthalic acid
90
a
Unless otherwise noted, the reaction was performed with 0.2 mmol of 1a, 4 mmol of malonate 2a, 20 mol% (R,R)-DPEN and 40 mol% acid in 1 mL of
solvent at rt. TFA ¼ triuoroacetic acid, TsOH ¼ p-toluenesulfonic acid, BA ¼ benzoic acid, PNBA ¼ p-nitrobenzoic acid, ONBA ¼ o-nitrobenzoic acid,
b
c
d
OFBA ¼ o-uorobenzoic acid, SA ¼ salicylic acid. NR ¼ no reaction. Isolated yield. Determined by chiral HPLC. Conducted with 2 mmol of
malonate 2a. e Performed in the absence of acid. f 0.6 mL (4 mmol) malonate 2a was used as the solvent.
Table 2 Substrate scope of Michael addition of malonates to cinnamones and its analoguesa
Entry
R1
R2
2
3
Time (h)
Yieldb (%)
eec (%)
1
2
3
4
5
6
7
8
Ph
Me (1b)
Me (1c)
Me (1d)
Me (1e)
Me (1f)
Me (1g)
Me (1h)
Me (1i)
Me (1j)
Me (1a)
Me (1k)
Me (1l)
Me (1m)
Me (1n)
Et (1o)
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
168
168
168
168
168
168
168
168
168
96
168
168
168
168
168
168
96
75
99
99
99
99
70
85
92
97
95
84
97
70
65
61
71
97
91
95
96
94
95
93
94
96
96
94
86
92
86
95
91
82
87
p-FC6H4
o-ClC6H4
m-ClC6H4
p-ClC6H4
p-BrC6H4
p-MeC6H4
p-MeOC6H4
1-Naphthyl
2-Naphthyl
2-Furanyl
2-Thiophenyl
Me
9
3aj
10
11d
12
13
14d
15
16e
17
3aa
3ak
3al
3am
3an
3ao
3ap
3aq
n-Bu
Ph
–(CH2)3– (1p)
–(CH2)4– (1q)
a
Unless otherwise noted, the reaction was performed with 0.2 mmol of 1, 4 mmol of malonate 2a, 20 mol% (R,R)-DPEN and 40 mol% o-phthalic acid
in 1 mL of EtOH at rt. Isolated yield. c Determined by chiral HPLC. d Performed with 40 mol% SA in ether. e 2 mmol of malonate 2a was used.
b
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Table 3 Substrate scope of malonatesa
Table 5 Domino reaction for the synthesis of cyclohexenonea
Entry
Ar
6
Yieldb (%)
drc
eed (%)
1
2
3
4
5
6
Ph (1b)
6a
6b
6c
6d
6e
6f
97
97
92
99
99
94
77 : 23
79 : 21
80 : 20
66 : 34
53 : 47
60 : 40
96/97
87/87
95/97
92/90
89/87
92/90
Entry
2
3
Yieldb (%)
eec (%)
p-ClC6H4 (1f)
p-BrC6H4 (1g)
p-MeOC6H4 (1i)
2-Naphthyl (1a)
2-Thiophenyl (1l)
1
2
3
4
2b
2c
2d
2e
3ba
3ca
3da
3ea
81
65
92
72
90
93
74
95
a
a
Unless otherwise noted, the reaction was performed with 0.2 mmol of
Unless otherwise noted, the reaction was performed with 0.2 mmol of
1, 0.4 mmol of 2f, 20 mol% (R,R)-DPEN and 30 mol% TFA in 1 mL of
1a, 4 mmol of malonate 2, 20 mol% (R,R)-DPEN and 40 mol% o-phthalic
acid in 1 mL of EtOH at rt for 168 h. Isolated yield. Determined by
chiral HPLC.
b
b
c
CHCl3 at rt for 120 h. Isolated yield of the diastereomeric mixture.
c
Diastereomeric ratio (dr) was determined by 1H NMR analysis of the
d
crude mixture; major isomer: trans. Determined by chiral stationary-
phase HPLC.
further improve the reactivity, we then turned our attention to
examine the effect of other acidic additive. It was revealed that
a signicant decrease of catalytic capability was observed in the
case of stronger acid (entries 2 and 3). Subsequently the model
reaction was performed with a range of aromatic carboxylic acids.
Although most of aromatic acid furnished 3aa with diminished
yield and optical purity (entries 4–7 vs. entry 1), the enhancement
of reactivity was fortunately observed with o-phthalic acid and
salicylic acid (SA) (entries 8 and 9). In particular, the dicarboxylic
acid, o-phthalic acid, gave rise to complete conversion aer 144
hours, together with 95% ee.11e,16 The effect of different solvents
was successively investigated with SA (entries 10–13). The protic
solvent, EtOH, gave the best enantioselectivity and ether led to
a considerable improvement of reaction rate. Meanwhile, the
model reaction went to completion aer 96 hours with main-
tained enantiomeric excess when exposed to o-phthalic acid in
EtOH (entry 14), however, sluggish transformation was detected
in ether because of poor solubility of this catalyst system. More-
over, reducing the amount of malonate resulted in substantial
decrease of reactivity (entry 15). The model reaction didn't occur
in the absence of acidic additive (entry 16). Meanwhile, higher
reactivity was observed under neat condition (entry 17).
Once the optimal reaction conditions have been established,
the substrate scope of this Michael addition was extended to
a variety of cinnamones and malonates. As summarized in
Table 2, this catalytic approach was not sensitive to the elec-
tronic property of cinnamones. The electron-neutral benzyli-
deneacetone 1b reacted properly with diethyl malonate 2a to
generate 3ab in synthetically useful yield and good enantiose-
lectivity (Table 2, entry 1). The electron-decient a,b-unsatu-
rated ketones 1c–1g were well tolerated by this catalytic system
and enabled access to the expected adducts 3ac–3ag in a highly
enantioselective manner (entries 2–6). Meanwhile, the electron-
rich cinnamones 1h and 1i are also suitable acceptors for this
conversion (entries 7 and 8). On the other hand, the position of
substituent on the phenyl ring exerted negligible affect on this
titled Michael reaction. Almost identical isolated yields were
obtained in the case of the sterically congested ortho-
substituted enone 1d in comparison with the meta-substituted
1e and para-substituted 1f (entry 3 vs. entries 4 and 5). In
contrast with bulky a-naphthyl-containing 1j, better catalytic
performance in terms of reactivity and enantiocontrol was
achieved when b-naphthyl-embedded acceptor 1a was utilized
(entry 9 vs. entry 10). The heteroaromatic substrates 1k and 1l
served as appropriate acceptors as well, however, a modied
condition was required for 1k to achieve synthetically useful
Table 4 Substrate scope of the Michael addition of malonate to
chalconesa
Entry Ar1
Ar2
5
Yieldb (%) eec (%)
1
2
3
4
5
6
7
8
9
Ph
Ph (4a)
Ph (4b)
Ph (4c)
Ph (4d)
5a 75 (55)d
5b 98
5c 99
5d 88
5e 83
92 (98)d
98
94
94
65
99
>99
96
p-MeC6H4
p-ClC6H4
2-Naphthyl
2-Thiophenyl Ph (4e)
Ph
Ph
Ph
p-ClC6H4
p-MeC6H4 (4f)
p-ClC6H4 (4g)
2-Thiophenyl (4h) 5h 65
p-ClC6H4 (4i) 5i 99
5f
99
5g 99
93
a
Unless otherwise noted, the reaction was performed with 0.2 mmol of
4, 4 mmol of malonate 2a, 20 mol% (R,R)-DPEN, 40 mol% salicylic acid
in 1 mL of ether at rt for 168 h. b Isolated yield. c Determined by chiral
HPLC. d Carried out with o-phthalic acid in 1 mL of EtOH.
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With the exception of cinnamones, our catalytic protocol was
also applicable to chalcones, a class of challenging substrates
for iminium ion activation.18 Only moderate isolated yield was
obtained when performed with o-phthalic acid in EtOH,
whereas the reactivity could be effectively improved when con-
ducted with salicylic acid in ether (Table 4, entry 1). Again, this
Michael reaction was independent of the electronic nature of
substituents on each aromatic ring of chalcones. Both the
electron-rich chalcones 4b and 4f, and the electron-poor chal-
cones 4c, 4g and 4i worked smoothly with diethyl malonate 2a,
forming the expected adducts with complete conversion in
highly enantioenriched fashion (entries 2, 3, 6, 7 and 9). Only
slightly reduced yield was detected for enone 4d bearing a bulky
naphthyl group at the b-site, along with 94% ee (entry 4). The
heteroaromatic chalcones 4e and 4h underwent clean reactions
and gave rise to the desired adducts 5e and 5h in acceptable
yields and moderate to excellent enantioselectivities (entries 5
and 8). The absolute conguration of 3 and 5 was conrmed to
be S via comparison of HPLC traces and optical rotation value
with that of literatures reported.2b,6e
In addition to malonates, we were pleased to nd that b-
ketoester was also competent donor for this catalytic protocol.19
Aer further optimization of reaction conditions, we found that
the cascade Michael-aldol condensation process between cin-
namones 1 and ethyl benzoylacetate 2f readily occurred with
30 mol% of TFA in chloroform, delivering highly functionalized
cyclohexenones 6 as an inseparable mixture of diastereomers.
(See Table S1 in the ESI†). Both the electron-decient cinna-
mones and the electron-rich cinnamones were well tolerated
(Table 5, entries 2–4). The bulky naphthyl group-containing
enone 1a and the heteroaromatic substrate 1l were compat-
ible with this catalytic protocol as well, leading to the formation
of annulated product 6e and 6f with high levels of enantiopur-
ities (entries 5 and 6). The absolute stereochemistry of cyclo-
hexenone 6 was determined to be S via conversion of 6a to
Scheme 1 Synthetic transformation of adduct 5a.
conversion (entries 11 and 12). In addition to aromatic
substrates, the aliphatic enones 1m and 1n were also compat-
ible with this catalytic strategy, but with slightly poorer reac-
tivity (entries 13 and 14). Notably, variation of R2 ketone
substituent indicated that enone 1o possessing a sterically more
demanding ethyl group also participated in this conjugate
addition (entry 15). Cyclic enones2b,3,4b,6a,6g,7,17 were suitable
acceptors as well, generating the corresponding adducts 3ap
and 3aq with good enantioselectivities (entries 16 and 17).
With respect to the donor, good enantiomeric excess was
obtained for dimethyl ester 2b, and lower reactivity was detected
for diisopropyl ester 2c but without compromising the optical
purity (Table 3, entries 1 and 2). In contrast, dibenzyl malonate
2d afforded desired adduct 3da with relatively poorer optical
purity (entry 3). Meanwhile, the reaction was totally inert in the
case of di-tert-butyl malonate. Moreover, methyl-substituted
malonate 2e was also compatible with this catalytic protocol,
but relatively lower reactivity was observed (entry 4).
Scheme 2 Proposed reaction pathway.
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known compound aer a simple decarboxylation (see eqn S(1)
in the ESI†).19b Notably, cyclohexenones and their derivates
constituted crucial skeletal components common in enormous
natural products and pharmaceutical molecules.20
Notes and references
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¨
N. Krause and A. Hoffmann-Roder, Synthesis, 2001, 2001,
To demonstrate the synthetic potential of this organo-
catalytic asymmetric process, base-controlled chemoselective
conversion of Michael adduct 5a were conducted in the pres-
ence of iodine.21 a-Hydroxylation of malonate moiety occurred
smoothly to provide a-hydroxyl malonate 7 almost without
compromise of enantiopurity, when treated with a catalytic
amount of NaOAc (Scheme 1). Moreover, the adduct 5a could be
converted to phenyl ester 8 by brief exposure to meta-chlor-
operoxybenzoic acid (m-CPBA) without deterioration of optical
purity. This Baeyer–Villiger oxidation proceeded with exclusive
regioselectivity. Lastly, transesterication of crude 8 worked
properly with NaBH4 in MeOH to afford methyl ester in 86%
yield, albeit a slight deterioration of optical purity was detected.
To account for the observed stereochemical outcome of this
Michael addition, a bifunctional catalytic model was proposed
in Scheme 2.12g Initially, benzylideneacetone 1b was activated
via formation of iminium ion with one amino group of vicinal
diamine catalyst. Another amino group of DPEN could be
engaged in hydrogen-bonding interaction with the carbonyl
moiety of ethyl malonate. As a result, the donor was restricted to
attack Re face of enone, thereby leading to the generation of S-
congured adduct 3ab. In the case of ethyl benzoylacetate, the
formation of enamine intermediate allowed the following
intramolecular aldol reaction to construct cyclohexanone.19b
Aer nal dehydration, the cyclohexenone 6a was therefore
obtained.
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Conclusions
In conclusion, we have developed a general and enantioselective
Michael addition of malonate to cinnamones and chalcones.
The commercially available DPEN could be utilized as the
organocatalyst to furnish the desired adducts in satisfactory
yield (61–99%) and moderate to excellent enantiopurity (65 to
>99% ee). This catalytic protocol was also applicable to b-
ketoester and constructed a densely functionalized cyclo-
hexenone via a tandem Michael-aldol condensation process.
Furthermore, profound synthetic manipulation could be per-
formed on the resulting adduct to construct various optically
active building blocks.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work is nancially supported by the National Natural
Science Foundation of China (No. 21402163) and the Funda-
mental Research Funds for the Central Universities of South-
west Minzu University (No. 2016NGJPY02). Wang Wei gratefully
acknowledges the Graduate Innovation Project of Southwest
Minzu University (No. CX2017SZ018).
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41704 | RSC Adv., 2018, 8, 41699–41704
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