Stereodivergent Michael addition of diphenylphosphite to α-nitroalkenes in the presence of squaramide-derived tertiary amines: An enantioselective organocatalytic reaction in supercritical carbon dioxide

Article abstract of DOI:10.1039/c3gc41647j

The first "green" asymmetric organocatalytic reaction in a supercritical carbon dioxide medium was elaborated. Under the proposed conditions (100 bar, 35 °C), α-nitroalkenes enantioselectively accept diphenylphosphite in the presence of bifunctional organ

Full text of DOI:10.1039/c3gc41647j

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Stereodivergent Michael addition of
diphenylphosphite to α-nitroalkenes in the
presence of squaramide-derived tertiary amines:
an enantioselective organocatalytic reaction in
supercritical carbon dioxide†
Cite this: Green Chem., 2014, 16,
1521
Ilya V. Kuchurov,a Albert G. Nigmatov,a Evgeniya V. Kryuchkova,a
Alexey A. Kostenko,b Alexandr S. Kucherenkoa and Sergei G. Zlotin*a
The first “green” asymmetric organocatalytic reaction in a supercritical carbon dioxide medium was elabo-
rated. Under the proposed conditions (100 bar, 35 °C), α-nitroalkenes enantioselectively accept diphenyl-
phosphite in the presence of bifunctional organocatalysts bearing the tertiary amino group and the
squaramide fragment to afford corresponding β-nitrophosphonates in high yields and with enantio-
selectivities of up to 94% ee. By varying the catalyst structure it is possible to synthesize both β-nitro-
phosphonate enantiomers under these conditions. A significant potential of the supercritical extraction
for product isolation and catalyst recovery was demonstrated.
Received 13th August 2013,
Accepted 5th December 2013
DOI: 10.1039/c3gc41647j
www.rsc.org/greenchem
principle).5 Unlike organometal catalysts, organocatalysts do
not contaminate products with toxic heavy metals and this is
Introduction
Highly enantioselective synthetic procedures have been in ever important in terms of medicinal chemistry.6 However, in some
increasing demand in the pharmaceutical sector for manufac- cases they may be responsible for hardly separable and poten-
turing enantiomerically pure chiral medications and their tially dangerous organic impurities.
development is one of the most promising areas of modern
To facilitate product purification and recovery of precious
organic chemistry.1 Among them, material- and energy-saving chiral catalysts, immobilized forms of organocatalysts tagged
catalytic procedures attract considerable attention.2 In particu- to polymers7 or ionic groups8 have been designed and numer-
lar, over the last decade amazing results have been associated ous organocatalytic reactions in so called “neoteric solvents”,
with the extensive application of asymmetric organocatalysts particularly in ionic liquids9 and water,10 have been developed.
in organic synthesis.3 In the presence of small metal-free The proposed approaches allow catalyst and product locations
chiral organic molecules (α-amino acid or cinchona alkaloid in different phases during the catalytic process and/or the
derivatives, BINOL phosphoric acids, and other types of chiral working-up step.
molecules), available prochiral reagents can be easily con-
Recently, we have discovered11 that enantioselective organo-
verted into chiral products of high molecular complexity in catalytic reactions such as asymmetric additions of carbon
high yields and with excellent enantioselectivities.4 The organo- acids to α-nitroalkenes in the presence of Takemoto’s tertiary
catalysis can be regarded as a green chemistry method due amine–thiourea organocatalyst12 can be efficiently carried out
to a relative simplicity of experimental procedures (most reac- in liquid carbon dioxide (liq-CO2), which is produced from
tions do not require a rigorous exclusion of moisture and renewable natural (plants) and industrial (burning of fuels)
oxygen) and feasibility of incorporating all atoms present in sources, and, along with supercritical carbon dioxide (sc-CO2),
starting compounds into final products (atom economy is likely to be a prospective medium for future chemical pro-
cesses.13 Liq-CO2 (100 bar, r.t.) enhances the reaction rate and
reduces the appropriate catalyst loading without a detrimental
aN.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
47 Leninsky prosp., Moscow 119991, Russia. E-mail: zlotin@ioc.ac.ru;
Fax: +7(499)1355328; Tel: +7(499)1371353
effect on enantioselectivity as compared with corresponding
reactions in organic solvents. The products and catalyst
retained after CO2 decompression are easily separable by a
flash-LC technique.
The use of sc-CO2 as a medium for organocatalytic reac-
tions may be even more promising from the “green chemistry”
bD.I. Mendeleyev University of Chemical Technology of Russia, Miusskaya Campus,
9 Miusskaya square, Moscow 125047, Russia
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c3gc41647j
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viewpoint. CO2 in the supercritical state (critical point 31.1 °C, the enantiomeric excess was 63% (Table 1, entry 1). These
73.8 bar) is characterized by higher diffusion rates and a values became somewhat higher with the 10 mol% loading of
unique capability of dispersing poorly soluble reagents, thus catalyst 4 though still inferior to corresponding parameters of
enhancing the reaction scope, rates and selectivity.14 However, the reaction in the organic solvent (CH2Cl2)21 (entry 2). Com-
to the best of our knowledge, sc-CO2 has never been applied in pound 5, incorporating the squaramide unit instead of the
asymmetric organocatalysis so far. Herein, we report on the thiourea fragment, exhibited better activity and selectivity in
first enantioselective organocatalytic reaction in the sc-CO2 the studied reaction: the yield of 3a after 12 h was 51% and
medium, in particular asymmetric Michael addition of enantioselectivity ran up to 71% ee (entry 3). Squaramide 6
diphenylphosphite (1) to α-nitroalkenes 2 in the presence of bearing the adjacent piperidine unit showed the best catalytic
tertiary amine–squaramide-derived bifunctional organo- performance in the model reaction in liq-CO2. In the presence
catalysts. The reaction products, chiral β-nitrophosphonates 3, of this catalyst the yield and ee values of β-nitrophosphonate
are precursors of β-amino phosphonic acid derivatives that 3a in 6 h after the reaction start were 91% and 94%, respecti-
occur in nature15 and possess valuable biological activities.16
vely (entry 4), which was similar to or just slightly lower than
the corresponding results obtained earlier in the CH2Cl2,
toluene or Et2O media.18 The yield of 3a improved to 97% and
the reaction completed in 2 h without a detrimental effect on
enantioselectivity where it had been carried out under super-
critical (sc) conditions (100 bar, 35 °C) (entry 5). Further
enhancement of the CO2 pressure to 200 or 300 bar did not
exert a positive impact on the reaction outcome (entries 6, 7).
Importantly, the required catalyst loading in liquid or sc-CO2
was twice as low (5 mol%) as in CH2Cl2 (10 mol%). In the pres-
ence of 10 mol% of catalyst 6 the enantioselectivity of the
phospha-Michael reaction increased to 96% (the same result
we attained in CH2Cl2 medium at 35 °C), however at the
expense of product 3a yield (entry 8).
Results and discussion
At first, we studied a model reaction between diphenylphos-
phite (1) and β-nitrostyrene (2a) (Table 1) in the presence of
known tertiary amine-derived organocatalysts 4,12 5 17 and 6 18
bearing hydrogen-bonding thiourea or squaramide frag-
ments19 (Fig. 1) in liq-CO2 (r.t., 100 bar). Compounds 4–6,
unlike primary or secondary amines, are unable to generate
carbamic acid salts with CO2 as the by-products,20 which
would remove the catalyst from the catalytic cycle and compli-
cate the catalytic transformation.
In contrast to reactions of 2a with carbon acids,11 thiourea
derivative 4 (5 mol%) appeared to be the least efficient catalyst
in this case: the yield of adduct 3а after 24 h was just 30% and
We have found that 4-chloro-β-nitrostyrene (2b) poorly
soluble in liq-CO2 reacted with phosphite 1 in the presence of
catalyst 6 less efficiently (yield of 3b 86%, ee 82%) (Table 1,
a
Table 1 Michael additions of 1 to 2a,b in the presence of 4–8 in liq- or sc-CO2
Entry
2, 3
Catalyst
T (°C)
Pressure (bar)
Time (h)
Yieldb (%)
eec,d (%)
1
a
a
a
a
a
a
a
a
b
b
b
b
4
4
5
6
6
6
6
6
6
6
7
8
r.t.
r.t.
r.t.
r.t.
35
35
35
35
r.t.
35
35
35
100
100
100
100
100
200
300
100
100
100
100
100
24
30
63
2e
3
24 (72 f)
43 (85f)
64 (68f)
12
51
71
4
5
6
6 (0.25f, 1g,h
)
91 (95f, 90g, 81h)
94 (96f, 94g, 93h)
2
2
2
1
6
2
2
2
97
95
94
93
92
7
95
8i
9
92 (97j)
86
96 (96j)
82 (R)
92 (R)
78 (R)
92 (S)
10
11
12
93
91
59
a The reactions were carried out in a stainless autoclave (V = 2 ml) with 1 (48 μL, 0.25 mmol), 2a or 2b (0.2 mmol) and a catalyst (4–8, 5 mol%) in
compressed CO2 (100 bar). b Isolated yield. c An enantiomeric excess was determined by HPLC analysis of 3a or 3b using a chiral column
Chiralpak AD-H. d The absolute configuration of products was determined by comparing the specific rotation of 3a or 3b with reported data.18,21
e The reaction was carried out in the presence of catalyst 4 (10 mol%). f Data for reactions performed in CH2Cl2 (1.0 ml) in the presence of
corresponding catalyst 4 (10 mol%)21 or 6 (10 mol%)18 are given in brackets. g Data for the reaction performed in toluene (1.0 ml) in the presence
of 6 (10 mol%)18 are given in brackets. h Data for the reaction performed in Et2O (1.0 ml) in the presence of 6 (10 mol%)18 are given in brackets.
i The catalyst 6 loading was 10 mol%. j The reaction was carried out in CH2Cl2 in the presence of 6 (10 mol%) at 35 °C for 30 min.
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Fig. 1 The studied chiral organocatalysts.
entry 9) than more lipophilic β-nitrostyrene (2a). Luckily, the
reaction outcome here also noticeably improved (yield of 3b
93%, ee 92%) under supercritical conditions (100 bar, 35 °C)
(entry 10). The experiments in an autoclave equipped with sap-
phire windows showed that the reaction mixture in sc-CO2
remained heterogeneous during the whole process. A suspen-
sion of alkene 2b and catalyst 6 in sc-CO2 turned into an emul-
sion after the addition of liquid phosphite (1) poorly soluble in
sc-CO2 that gradually re-suspended again as a crystalline
product (3b). The catalytic reaction most likely occurred
mainly in the interfacial region between sc-CO2 and reagent/
catalyst phases under the studied conditions. The better cataly-
tic performance of catalyst 6 in sc-CO2 than in liq-CO2 may be
attributed in this case to a more efficient mass transfer in the
heterogeneous reaction system under supercritical conditions.
To further improve the process we substituted catalyst 6 for
a newly synthesized squaramide-derived catalyst 7 bearing
lipophilic long-chained alkyl groups (experimental details are
given in ESI†), which would hopefully enhance its solubility in
sc-CO2 (Fig. 1). However the yield (91%) and optical purity of
product 3b (78% ee) became lower in this case (Table 1, entry
11), presumably owing to a shielding of the tertiary amino and
amido groups, essential for the catalysis, in compound 7 with
adjacent flexible alkyl groups.
We discovered that the asymmetric Michael reaction
between compounds 1 and 2b in sc-CO2 was promoted by dihydro-
quinidine-derived catalyst 8 modified with the squaramide
fragment (see ESI†) (Fig. 1), which is an analog of the bifunc-
tional cinchona alkaloid-derived organocatalyst elaborated by
Rawal with co-workers.22 The configuration of key stereo-
centers attached to tertiary amino and squaramido groups in
compound 8 (S) is opposite to that of compounds 4–7 (R). As a
result, in the presence of this catalyst, the (S)-antipode of β-nitro-
phosphonate 3b was isolated in a reasonable yield (59%) and
with high enantiomeric purity (92% ee) (Table 1, entry 12).
Table 2 Organocatalytic asymmetric Michael addition of diphenylpho-
sphite (2) to α-nitroalkenes (1) in sc-CO2
a
Entry
R
Catalyst
Yields of 3b (%)
eec,d (%)
1
(a) Ph
6
6
6
8
6
6
6
6
6
8
6
6
8
6
6
8
6
8
97
96
93
59
87
88
96
95
95
90
91
57
63
92
97
90
74
86
94 (R)
93 (R)
92 (R)
92 (S)
82 (R)
87 (R)
94 (R)
94 (R)
92 (R)
86 (S)
92 (R)
89 (R)
68 (S)
84 (S)f
94 (S)f
89 (R)f
68 (R)
78 (S)
2e
3
(b) 4-ClC6H4
4
5
6
7
8
9
10
11
12
13
14
15
16
17g
18g
(c) 2-ClC6H4
(d) 4-BrC6H4
(e) 3-NO2C6H4
(f) 4-MeOC6H4
(g) 4-BunOC6H4
(h) 3-BunOC6H4
(i) 3-BnOC6H4
(j) 2-Furyl
(k) 2-Thienyl
(l) (CH3)2CHCH2
a The reactions were carried out in a stainless autoclave (V = 2 ml) with
1 (48 μL, 0.25 mmol), 2 (0.2 mmol) and catalyst 6 or 8 (5 mol%) in
scCO2 (100 bar, 35 °C) for 2 h. b Isolated yield. c An enantiomeric
excess was determined by the HPLC analysis of 3 using chiral columns
Chiralcel OD-H, OJ-H, or Chiralpak AD-H. d Absolute configurations of
3a, b, d, f, j, k, l were determined by comparing their specific optical
rotations with the reported data.18,21 e The reaction was carried out in
an autoclave (V = 15 ml) with 1 (0.24 ml, 1.25 mmol), 2a (0.15 g,
1.0 mmol) and catalyst 6 (5 mol%). f According to the generally
accepted nomenclature the (S)-configuration of stereocenters in
heterocyclic compounds 3j and 3k corresponds to the (R)-
configuration of those in Michael adducts 3a–i and 3l. g The reaction
time was 6 h.
β-nitrophosphonates 3b–k within 2 hours in high yields and
β-Nitrostyrene derivatives 2a–i bearing electron-withdrawing with enantioselectivities of up to 94% ee. The reaction of ali-
or electron-donating groups in various positions of the aro- phatic α-nitroalkene 2l with phosphite 1 required 6 hours to
matic ring and α-nitroalkenes of the heteroaromatic (2j, k) or complete. Furthermore, both (R)- and (S)-enantiomers of
aliphatic (2l) series appeared to be suitable starting materials Michael adducts 3 can be enantioselectively prepared in the
for asymmetric phospha-Michael reactions in the sc-CO2 presence of catalyst 6 or 8, respectively. The absolute configur-
medium in the presence of catalyst
6 or 8 (5 mol%) ation of known compounds 3a, b, d, f, j, k, l was determined
(Table 2). Compounds 2a–k generated corresponding by a comparison of their HPLC data and specific optical
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rotations with reported data.18,21 The (R)- or (S)-configurations medium. Under the proposed conditions, diphenylphosphite
were assigned to novel compounds 3a, e, g–i by analogy (1) enantioselectively reacted with α-nitroalkenes 2 in the pres-
depending on the organocatalyst (6 or 8) used. We anticipate ence of bifunctional catalysts containing the tertiary amino
that the developed procedure has a potential for achieving group and the squaramide fragment to afford corresponding
dual stereocontrol in asymmetric reactions as the hydrogen- β-nitrophosphonates 3 in high yields and with enantioselec-
23
bonding ability of CO2 may change conformations of dia- tivities up to 94% ee. The use of catalyst 6 or 8 having (R)- or
stereoisomeric intermediates. Furthermore, we envision that (S)-configuration of key stereocenters adjacent to tertiary
the sign of stereoinduction in catalytic reactions may be influ- amino and amido groups and sc-CO2 allowed a stereodivergent
enced by a reversible addition of liquid or sc-CO2 to organic synthesis of both enantiomers of β-nitrophosphonates 3,
solvents that would change the polarity of solvent systems.24 which are precursors of optical antipodes of β-amino phospho-
The procedure is scalable: the yield and the enantiomeric nic acid derivatives that occur in nature and possess valuable
excess of product 3a of the squaramide 6-catalyzed reaction biological activities. The potential of sc-CO2 as an eluent for
between compounds 1 and 2a in a 0.2 mmol scale (Table 2, the fractional extraction of products and catalyst recovery was
entry 1) were similar to those attained in the same reaction demonstrated. The obtained results contribute to green chem-
performed in a 1.0 mmol scale (Table 2, entry 2).
istry as they eliminate toxic organic solvents (in particular,
Importantly, sc-CO2 may serve not only as a prospective CH2Cl2) originated from exhausting hydrocarbon resources
medium for the asymmetric Michael reaction, but also it has a and facilitate separation and purification steps that usually
significant potential as a substitute of organic solvents, un- have the highest environmental impact in chemical processes.
desirable in terms of green chemistry, during the working-up Furthermore, the availability and recyclability of CO2 and the
procedure. We discovered that reagents 1 and 2a, adduct 3a recoverability of precious chiral catalysts under the proposed
and catalyst 6 had different (still very limited) solubility in sc- conditions along with the anticipated reduction of production
CO2 (1–2a > 3a > 6), which may be useful for their separation costs per product unit in the large-scale industrial processes
by the fractional extraction of the reaction mixture with this would hopefully make sc-CO2-based catalytic technologies
solvent (sc-CO2 has been extensively applied to extract natural economically beneficial in spite of the need to use special
compounds from plants25). After the reaction had been com- equipment and additional energy costs.
pleted (visual monitoring), a slow flow (2 g min−1) of sc-CO2
(100 bar, 35 °C) was passed through the autoclave for 1 h to
remove residual starting compounds ( just ∼5% of product 3a
Experimental
leached with the sc-CO2 flow at this step). Then sc-CO2
Typical procedure for the asymmetric phospha-Michael
pressure was raised up to 150 bar at 35 °C and the fractional
reaction in sc-CO2
extraction continued for an additional 4 hours. To our delight,
α-Nitroalkene 2 (0.20 mmol) and catalyst 6 or 8 (0.01 mmol)
were placed in a stainless steel autoclave (V = 2 ml) equipped
with a magnetic stirring bar under an argon atmosphere. The
autoclave was filled with CO2 using a syringe-press to total
pressure 70 bar and heated to 35 °C. Diphenylphosphite (1)
(48 μL, 0.25 mmol) was then added with the CO2 flow to reach
a final pressure of 100 bar and the reaction mixture was stirred
at 35 °C and 100 bar for 2 h (unless otherwise specified). The
autoclave was slowly depressurized, and the residue was recov-
ered by rinsing with EtOAc and purified by column chromato-
graphy on silica gel (hexane–EtOAc = 10 : 1 to 6 : 1 as the
eluent).
just analytically pure adduct 3a (isolated yield 79%) was
present in the receiver by the end of this period whereas cata-
lyst 6 along with a minor amount of product 3a (∼8%)
remained in the autoclave. The Michael addition–supercritical
extraction sequence appeared recyclable. Fresh portions of
reagents 1 and 2a were added to the recovered sample of cata-
lyst 6, the autoclave was charged with compressed CO2 and the
Michael reaction was re-performed under conditions described
above. Product 3a was obtained by the second fractional extrac-
tion of the reaction mixture with sc-CO2 (isolated yield 70%).
Luckily, the supercritical extraction of 3a under the studied
conditions did not lead to racemization of the product: accord-
ing to HPLC data, the optical purities of 3a isolated by the
extraction with sc-CO2 (94% ee in the first cycle and 93% ee in
Fractional extraction of 3a with sc-CO2
the second one) were similar to that of the corresponding The tertiary amine 6 (8.4 mg, 0.02 mmol)-catalyzed Michael
sample obtained by conventional flash chromatography using addition of 1 (48 μL, 0.25 mmol) to 2a (29.8 mg, 0.20 mmol) in
organic solvents (hexane–EtOAc). Evidently, supercritical sc-CO2 was carried out as described above. After the reaction
chromatography26 would also be helpful in reactions where completion, a flow (2.0 g min−1) of sc-CO2 (100 bar, 35 °C) was
reagents and products have similar solubility in sc-CO2.
gradually passed through the autoclave to receiver (flask) I
containing EtOAc (15 ml) for 1 h. Then receiver I was replaced
by receiver II filled with the same EtOAc amount. The sc-CO2
pressure was raised up to 150 bar and the supercritical extrac-
tion with a flow rate of 2.0 g min−1 at 35 °C continued for
Conclusions
In summary, we pioneered in developing the asymmetric another 6 h. The solvents in receivers I and II were evaporated
organocatalytic reaction in the supercritical carbon dioxide under reduced pressure (10 Torr) and the residues were
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analyzed by TLC and 1H NMR. The extract from flask I
(21.6 mg) contained starting compounds 1 and 2a along with
the product 3a (∼10%) whereas the second extract (60.5 mg)
represented pure adduct 3a (isolated yield 79%). Catalyst 6
along with a minor amount of product 3a (∼8% according to
1H NMR) remained in the autoclave after the extraction pro-
cedure (the residue total mass 14.4 mg).
After decompression, fresh portions of 1 (48 μL, 0.25 mmol)
and 2a (29.8 mg, 0.20 mmol) were added to the remaining
catalyst, the autoclave was charged with compressed CO2 and
the Michael reaction was re-performed according to the typical
procedure. After 2 h, the reaction mixture was extracted with
sc-CO2 as described above and two fractions were collected
again. The first fraction (32.5 mg) consisted of compounds 1,
2a and 3a (∼14%) and the second extract (53.7 mg) contained
pure adduct 3a (isolated yield 70%). The residue that was left
in the autoclave (15.7 mg) consisted of catalyst 6 and the target
compound 3a (∼10%). In addition, a minor amount of product
3a remained inside the capillaries and joints of the back-
pressure regulator.
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Acknowledgements
The work was supported by the Ministry of Education and
Science of the Russian Federation (grant no. 14.740.11.1615).
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