Heterogeneous asymmetric Henry-Michael one-pot reaction synergically catalyzed by grafted chiral bases and inherent achiral hydroxyls on mesoporous silica surface

Article abstract of DOI:10.1039/c2cc35110b

Highly efficient and enantioselective asymmetric Henry-Michael one-pot reaction has been achieved on bifunctional heterogeneous catalysts with inherent achiral hydroxyls as acidic sites and immobilized chiral amines as basic sites. Final products were afforded in yields of up to 85% and ee of 99%.

Full text of DOI:10.1039/c2cc35110b

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Heterogeneous Asymmetric Henry-Michael One-pot Reaction
Synergically Catalyzed by the Grafted Chiral Bases and Inherent
Achiral Hydroxyls on Mesoporous Silica Surface†
Shanshan Yanga and Jing He*b
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
50 one catalytic centre of the bifunctional catalysts is especially
challenging. Here, we report the oneꢀpot asymmetric catalysis
of heterogeneous bifunctional catalysts with the inherent
achiral hydroxyls of mesoporous silica (SBAꢀ15 materials) as
acidic sites and immobilized chiral amines as basic sites. The
55 synergetic catalysis of surfaceꢀinherent acids and immobilized
bases demonstrated enhanced activity while afforded excellent
enantioselectivity in asymmetric HenryꢀMichael oneꢀpot
Highly efficient and enantioselective asymmetric Henry-
Michael one-pot reaction has been achieved on the
bifunctional heterogeneous catalysts with inherent achiral
10 hydroxyls as acidic sites and immobilized chiral amines as
basic sites. The final product was afforded in a yield of up to
85 % and ee of 99 %.
Natural biological systems effectively produce elaborate
molecules in continuous ways, avoiding timeꢀconsuming and
15 costly operations and thus more environmentallyꢀbenign. To
simulate the efficient natural processes, the concept of oneꢀpot
catalysis has been proposed by catalytic chemists1 using
enzymes2, metal complexes3, and small organic molecules4 as
effective catalysts. Small organic molecules have attracted
20 persistent attention as organocatalysts in the last decade
because of, for example, their higher chemical stability than
enzymes, better resistance to moisture and oxygen than metal
complexes, lower cost, and absence of metal residue and
toxicity. Acidicꢀbasic biꢀfunctionalized organocatalysts are
25 especially interesting5 because they could act via acidꢀbase
synergy in oneꢀpot catalysis.
reaction (Scheme 1),
a
synthon for montanineꢀlike
amaryllidaceae alkaloids10 produced in 99% ee. Montanineꢀ
60 like amaryllidaceae alkaloids are recognized to have potential
applications as antiviral and antineoplastic drugs, insect
antifeedants, and acetylcholinesterase inhibitors.11
Scheme 1 Asymmetric HenryꢀMichael Oneꢀpot Reaction.
65
(S)ꢀ2ꢀ(((allyldimethylsilyl)oxy)diphenylmethyl)pyrrolidine,
(S)ꢀ2ꢀ(((allyldimethylsilyl)oxy)methyl)pyrrolidine, Sꢀ(2ꢀ
aminophenyl)ꢀ4ꢀ(allyloxy)pyrrolidineꢀ2ꢀcarbothioate, and Sꢀ
(1ꢀmethylꢀ1Hꢀimidazolꢀ2ꢀyl)ꢀ4ꢀ(allyloxy)pyrrolidineꢀ2ꢀ
carbothioate, all with pyrrolidine struture, are selected as the
Despite the successful application of homogeneous acidꢀ
base bifunctional organocatalysts in asymmetric oneꢀpot
reactions, how to avoid the mutual deactivation of acidic and
30 basic sites has always been a great issue.6 Heterogenization of
homogeneous catalytic sites provides the opportunity to figure
out this issue. Early research focused on the immobilization of
acidic and basic functionalities separately on independent
supports.7 Later, researchers began to pay more attention on
35 the immobilization of acidic and basic functionalities
coinstantaneously on one support.8,9 To our best knowledge,
however, no heterogeneous chiral bifunctional catalysts for
asymmetric oneꢀpot reaction have been reported. The
asymmetric oneꢀpot catalysis using the surface achiral sites as
70 precursors of grafted amines in this work. To graft the amines,
thiolꢀfunctionalized SBAꢀ15 (SBAꢀ15ꢀSH) was synthesized
following a reported procedure.12 By the deconvolution of 29Si
MAS NMR signals (Fig. S1), the silanol content on SBAꢀ15ꢀ
SH surface is determined to be 6.73 mmol/g according to the
75 reported method.13,14 Based on the specific surface area (900
m2/g), the silanol density is calculated as 7.48 ꢁmol/m2. The
immobilization of the chiral amines on SBAꢀ15ꢀSH was
achieved by the reaction between ꢀSH and ꢀC=Cꢀ, with (S)ꢀ
diphenylprolinol silyl ether (SBAꢀpyꢀsiꢀdiph), (S)ꢀprolinol
80 silyl ether (SBAꢀpyꢀsi), Sꢀ(2ꢀaminophenyl) pyrrolidineꢀ2ꢀ
carbothioate (SBAꢀpyꢀpri), and Sꢀ(1ꢀmethylꢀ1Hꢀimidazolꢀ2ꢀ
yl)pyrrolidineꢀ2ꢀcarbothioate (SBAꢀpyꢀter) as grafted amine
moieties (Scheme 2). The resonances at 31, 33 (Siꢀ(CH2)3ꢀSꢀ
CH2ꢀCH2ꢀ), 26, 45, 55, 65 (pyrrolidine), 121, 131, 143, 160
85 (aromatic carbon in SBAꢀpyꢀsiꢀdiph and SBAꢀpyꢀpri), and 5
ppm (ꢀSi(CH3)2 in SBAꢀpyꢀsiꢀdiph and SBAꢀpyꢀsi) in the 13C
CP/MAS NMR spectra (Fig. S2) clearly confirm the grafted
40
a E-mail: yangshanshan213@gmail.com
b State Key Laboratory of Chemical Resource Engineering, Beijing
University of Chemical Technology. PO Box 98, No.15 BEISANHUAN
East Road, Beijing 100029 (P. R. China).Fax: (+) 86-10-64425385.
45 E-mail: jinghe@263.net.cn
† Electronic Supplementary Information (ESI) available: general
information, experimental details, 1H NMR data for organic amine, 13C
NMR for heterogeneous catalyst, and 1H NMR, 13C NMR and chiral
HPLC results for product analysis. See DOI: 10.1039/b000000x/
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

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amines on support surface. As determined from the elemental
analysis results, the loading of pyrrolidine ranges from 0.28 to
0.40 mmol/g for amineꢀgrafted samples (Table 1). Based on
the specific surface areas, the density of grafted pyrrolidine
pri afforded a yield of 5 similar to SBAꢀpyꢀsi, with 74%
conversion of 1 in the Henry reaction and 85% conversion of
3 in the Michael reaction (Table 1, entry 3). SBAꢀpyꢀter
catalyzed the Michael reaction similarly to SBAꢀpyꢀpri while
5
was calculated to range from 0.30 to 0.44 ꢁmol/m2. The molar 40 the Henry reaction more effectively than SBAꢀpyꢀpri. A yield
ratio of surface hydroxyl to grafted pyrrolidine is around 20:1
(Table 1) for each heterogeneous catalyst, meaning there are
enough hydroxyl groups around amine sites.
of up to 85% for 5 is afforded on SBAꢀpyꢀter (Table 1, entry
4). Especially amazing and encouraging, an ee of 98% for
antiꢀisomers and an ee of 71% for synꢀisomers are afforded on
SBAꢀpyꢀsiꢀdiph (Table 1, entry 1), and more than 95% ee was
45 observed for both synꢀ and antiꢀ configured product on SBAꢀ
pyꢀsi, SBAꢀpyꢀpri, and SBAꢀpyꢀter (Table 1, entries 2ꢀ4). It is
worthy to note that this work is the first report of
heterogeneous asymmetric oneꢀpot reaction. Owing to the
expected synergies, the asymmetric HenryꢀMichael oneꢀpot
50 reaction, which is difficult to achieve with homogeneous
counterpart, has been achieved on the heterogeneous catalyst
designed here with satisfactory yields afforded. The ee
observed here are even comparable to other homogeneous
systems reported previously16 which also provided 5 as
55 product. Even more, the breakthrough is the asymmetric
catalysis involving achiral hydroxyls as one of the synergistic
catalytic sites.
To further confirm the acidicꢀbasic synergies, the surface
hydroxyls of SBAꢀpyꢀter was capped by postꢀsilylation with ꢀ
60 Si(CH3)3. Not only the conversion of 1 in the Henry reaction
reduced markedly from 99% to 50%, but the conversion of 3
in the Michael reaction also from 88% to 73%. As a result, the
yield of 5 reduced from 85% to 37% with the ee preserved.
Many researches17 reported the addition of acids as coꢀcatalyst
65 to improve the activity and enantioselectivity of Michael
reaction. The control experiment showed that homogeneous
pyꢀter afforded only 40% yield, and 78% ee for synꢀisomer
and 52% ee for antiꢀisomer in the Michael reaction, proving
that the ꢀOH groups on the heterogeneous catalysts designed
70 here could facilitate the Michael reaction as coꢀcatalytic acid.
On the physical mixture of SBAꢀ15ꢀSH and pyꢀter, only 9%
yield of 5 was achieved. The ee was 86% for synꢀisomer and
77% for antiꢀisomer, also lower than on SBAꢀpyꢀter. This
suggests the covalent grafting of chiral amines to the support
75 surface favors the acidꢀbase synergic catalysis probably due to
avoiding undesired acidꢀbase neutralization.
10
Scheme 2 The structure of designed heterogeneous catalysts
The heterogeneous catalysts are then used to catalyze the
HenryꢀMichael oneꢀpot reaction between benzaldehyde,
nitromethane, and cyclohexanone. The results (Table 1) show
that, each heterogeneous catalyst designed here exhibits good
15 activity and excellent enantioselectivity in the HenryꢀMichael
oneꢀpot reaction. (S)ꢀdiphenylprolinol silyl ether (pyꢀsiꢀdiph)
has been reported to be efficient catalyst for homogeneous
Michael reactions,15 but never for Henry reactions in either
homogeneous or heterogeneous system. The heterogeneous
20 catalyst SBAꢀpyꢀsiꢀdiph designed here, however, surprisingly
catalyzed not only the Michael reaction but also the Henry
and HenryꢀMichael oneꢀpot reactions, giving 47% conversion
of 1 in the Henry reaction, 90% conversion of 3 in the
Michael reaction, and 40 % yield of 5 in the HenryꢀMichael
25 oneꢀpot reaction (Table 1, entry 1). This work investigated the
homogeneous Henry reaction using pyꢀsiꢀdiph as catalyst and
observed no catalytic activity. The catalysis of SBAꢀpyꢀsiꢀ
diph in either Henry or HenryꢀMichael oneꢀpot reaction is
thus supposed to result from the synergies of surface hydroxyl
30 group as acidic site with the grafted pyrrolidine as basic site.
The acidicꢀbasic synergies rousted the reactions. Without the
steric hidrance of the two phenyl groups as in SBAꢀpyꢀsiꢀdiph,
SBAꢀpyꢀsi afforded 99% conversion of 1 in the heterogeneous
Henry reaction and 66% yield of 5 in the heterogeneous
35 HenryꢀMichael oneꢀpot reaction (Table 1, entry 2). SBAꢀpyꢀ
Table 1 HenryꢀMichael oneꢀpot reaction on designed catalysts.a
synꢀisomer:
antiꢀisomer:
Pyrrolidine Density of grafted Molar ratio of
ee (%) d
loading
(mmol/g)
pyrrolidine
surface hydroxyl to Conversion Conversion Yield of 5
Entry Catalyst
(ꢁmol/m2)
grafted pyrrolidine b of 1 (%) c of 3 (%) c (%) c
syn : anti c
(2R,1’S) (2R,1’R)
1
2
3
4
SBAꢀpyꢀsiꢀdiph 0.36
0.40
0.44
0.31
0.40
18:1
17:1
24:1
19:1
47 (45)
99 (97)
74 (76)
99 (97)
90 (90)
70 (72)
85 (86)
88 (88)
40 (38)
66 (67)
61 (60)
85 (84)
74:26 (75:25) 71 (73) 98 (97)
56:44 (54:46) 99 (99) 97 (96)
72:28 (75:25) 97 (97) 96 (95)
79:21 (79:21) 99 (98) 95 (93)
SBAꢀpyꢀsi
SBAꢀpyꢀpri
SBAꢀpyꢀter
0.40
0.28
0.36
a
Reaction conditions: benzaldehyde (0.05 mmol), catalyst (30 mol%), nitromethane (1 mL), and cyclohexanone (2 mL); Henry reaction: 90 °C, 72 h;
d
b
c
80 Michael reaction: 25 °C, 48 h.
T
he ratio of the silanol density of SBAꢀ15ꢀSH to the density of grafted pyrrolidine. Determined by 1H NMR.
Determined by 1H NMR and HPLC. The figures in the parentheses are reproduced results.
In light of the oneꢀpot HenryꢀMichael reaction between 85 of the asymmetric HenryꢀMichael oneꢀpot reaction in this
benzaldehyde and nitromethane on the catalyst with combined
tertiary and primary amine on silicaꢀalumina9, the mechanism
work could be proposed (Fig. S3). In the Henry reaction, the
carbonyl oxygen and carbonyl carbon of benzaldehyde are
2
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This journal is © The Royal Society of Chemistry [year]

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proton of nitromethane is abstracted by the primary amine
(SBAꢀpyꢀpri), secondary amine (SBAꢀpyꢀsiꢀdiph and SBAꢀpyꢀ
si), or tertiary amine (SBAꢀpyꢀter) with the assistance of
surface hydroxyl sites. The deprotonated nitromethane
nucleophilically attacks the imine intermediate to result in the
nitrostyrene which directly participates in Michael reaction. In
the Michael reaction, the cyclohexanone is activated by the
10 secondary amine of pyrrolidine to form an enamine
intermediate. The surface silanols would orientate the nitro
group of nitrostyrene through hydrogen bonds so that the
enamine acts as a necleophile and attacked the nitrostyrene
from different faces. Because of the hinderance effect of the
15 mesoporous channel on the re attack on anti-enamine and the
interaction between surface silanols and nitrostyrene, the (2R,
1’S) isomer is afforded as major product (Fig. S4).
55
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was also investigated using SBAꢀpyꢀter as an example. SBAꢀ
20 pyꢀter was recycled by simple filtration and reꢀused for
catalytic runs (Fig. 1). 73% yield, and 93% ee for syn-isomer
and 89% ee for anti-isomer are preserved in three recycle runs.
8
9
10 S. Anwar, P. Lee, T. Chou, C. Chang and K. Chen, Tetrahedron,
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Fig.1 The recycle of SBAꢀpyꢀter in asymmetric HenryꢀMichael oneꢀpot
25
reaction.
In summary, this work has demonstrated the asymmetric
HenryꢀMichael oneꢀpot reaction on designed heterogeneous
catalysts with inherent achiral hydroxyls of support surface as
acid sites and grafted chiral amines as base sites. The final
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2
3
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