Probing the evolution of an Ar-BINMOL-derived salen-Co(iii) complex for asymmetric Henry reactions of aromatic aldehydes: Salan-Cu(ii) versus salen-Co(iii) catalysis

Article abstract of DOI:10.1039/c4ra06056c

A new type of chiral salen-Co catalyst that features aromatic π-walls and an active Co(iii) center has been developed for enantioselective Henry/nitroaldol reactions on the basis of salen-Cu catalysis. The asymmetric Henry reaction of aromatic aldehydes a

Full text of DOI:10.1039/c4ra06056c

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Probing the evolution of an Ar-BINMOL-derived
salen–Co(III) complex for asymmetric Henry
reactions of aromatic aldehydes: salan–Cu(II)
versus salen–Co(III) catalysis†
Cite this: RSC Adv., 2014, 4, 37859
Yun-Long Wei,‡a Ke-Fang Yang,‡a Fei Li,a Zhan-Jiang Zheng,a Zheng Xua
ab
*
and Li-Wen Xu
A new type of chiral salen–Co catalyst that features aromatic p-walls and an active Co(III) center has been
developed for enantioselective Henry/nitroaldol reactions on the basis of salen–Cu catalysis. The
asymmetric Henry reaction of aromatic aldehydes and nitromethane catalyzed by an Ar-BINMOL-
derived salen–Co(III) complex was achieved with high yields (up to 93%) and excellent enantioselectivities
(up to 98% ee). And more interestingly, it was supposed that either salan–Cu(II) or salen–Co(III) complex-
catalyzed Henry reaction was an ideal model reaction for providing direct evidence of noncovalent
interaction due to the distinguishable ortho-substituted aromatic aldehydes from meta- or para-
substituted benzaldehydes in terms of enantioselectivities and yields.
Received 21st June 2014
Accepted 12th August 2014
DOI: 10.1039/c4ra06056c
www.rsc.org/advances
strategy still a valuable approach to highly efficient and enan-
tioselective Henry reaction. Herein, we described for the rst
Introduction
time an interesting nding of aromatic-interaction-enhanced
catalytic transformation with the dramatic effect of aromatic
interactions between aromatic aldehydes and chiral salan–
copper(II), which led to the nding of benzaldehyde and other
aromatic compounds containing one phenyl ring as a key acti-
vator to assist the Henry reaction of various nonreactive alde-
hydes. And on the basis of these work, we report new results of
our studies on the evolution of salan–Co(III) complex bearing
crowed aromatic rings upon catalytic performances in the
Henry reaction.
The weak non-covalent interactions such as hydrogen-
bonding, p–p stacking, CH–p, cation– or anion–p, and van der
Waals interaction, are the most important structural
phenomena for the understanding of molecular recognition,
conformational equilibria, molecular self-assembly, protein–
ligand binding, molecular aggregates, and articial metal-
loenzymes.8 Especially, the importance of aromatic interactions
(p–p stacking, CH–p, cation–p and anion–p interactions) in
supramolecular chemistry and homogeneous catalysis has also
been recognized very early and received much attention in the
elds of molecule based materials and synthetic chemistry in
the past decades.9 Numerous synthetic literatures have showed
that stereoselectivities of asymmetric reactions changed unex-
pectedly upon replacement of an alkyl moiety by an aromatic
group,10 and recent development in theory have proved that the
quantum mechanical modeling or methodology could provide
accurate insights into the roles of these aromatic interactions at
a level of detail not previously accessible for the transition states
The Henry (nitroaldol) reaction is one of the most atom-
economic and convenient carbon–carbon bond-forming reac-
tions, and the resulting b-hydroxy nitroalkanes can be further
converted to b-amino alcohol by reduction as well as chiral
carboxylic acid and primary amines.1 The diversity of the
transformations of the Henry adducts provides potential
interest and application of this process. Since the breakthrough
in the catalytic asymmetric Henry reaction came from Shiba-
saki's group in 1992,2 much effort has been devoted toward the
development of metallic and organic catalysts as well as bio-
catalysts for this reaction.3,4 And the most prominent results
have been obtained in this area is zinc and copper-based cata-
lyst system with nitrogen-centered ligands.4–6 For example,
dinuclear zinc-amino alcohol and copper-bis(oxazoline) are two
classical and representative catalytic systems,5–7 which devel-
oped by Trost5 and Evans6 respectively. Although great progress
has been hitherto achieved, the development of novel chiral
ligands and organometallic catalysis with new concept or
aKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of
Education (MOE), Hangzhou Normal University, Hangzhou 311121, P. R. China.
E-mail: liwenxu@hznu.edu.cn; licpxulw@yahoo.com; Fax: +86-571-28867756
bKey Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education
(MOE) and School of Chemistry and Chemical Engineering, Shaanxi Normal
University, Xi'an 710062, P. R. China
† Electronic supplementary information (ESI) available. CCDC 1009555. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra06056c
‡ Mr Y. L. Wei and Dr K. F. Yang contributed equally to this work (co-rst authors).
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of catalytic asymmetric chemical reactions.11 Accordingly, methoxy-, hydroxyl-, and nitro-substituted benzaldehydes
considerable efforts for the development of aromatic-interac- resulted in poor or no conversion.
tion-enhanced catalytic transformations recently have been
Based on the experimental results in this Henry reaction, we
directed toward the positive and effects explanations of suggested that such unique salan ligand (1) bearing large
aromatic interactions in asymmetric catalysis.12 Thus, wise aromatic p-wall led to interesting and remarkable discrimina-
modulation and tuning of the complementary moieties of tion of different aldehydes. Although the catalytic systems are
catalyst responsible for the aromatic interactions can lead to its highly complex and multiple parameters can have an impact on
enantioselectivity enhancement in asymmetric reactions. the reaction outcomes, and the reasonable mechanism is not
However, it is difficult to develop an aromatic interactions or clear at present, we can concluded the existence of aromatic p–
molecular recognition-oriented enantioselective catalysis, and p stacking interaction between copper ligand and substituted
there are few studies focused on the direct experimental and aromatic aldehydes was not favorable for the activation of
reaction investigation for the establishment of the crucial role methyl-, methoxy-, hydroxyl-, and nitro-substituted benzalde-
of aromatic interactions in catalytic asymmetric reactions.11,12 hydes by catalytic copper center. We also deduced that free
In this work, we want to advance the aromatic interactions as benzaldehyde has the right size, shape, and functionality to
well as salan/salen–metal complex to establish a practical interact with catalytic copper center in the salan–Cu complex
procedure with operationally simple and high efficiently for the and could react with nitromethane completely. On the other
metal-catalyzed Henry reactions.
hand, the salan(1)–Cu catalyzed Henry reaction of benzalde-
hyde resulted in excellent enantioselectivity (91% ee) and yield
(95%). On the basis of these direct ndings, we hypothesized
that the rational application of aromatic interaction between
salan/salen–metal complex and substrate could be applied to
promote the Henry reaction of methyl-, methoxy-, and other
aromatic aldehydes, which encouraged us to utility the benz-
aldehyde as a p-molecule to initiate the Henry reaction of
various nonreactive aldehydes.
To gain direct experimental information about the benzal-
dehyde-assisted catalytic transformations of substituted
aromatic aldehydes, our investigations then focused on the
effect of benzaldehyde as an activator in the Henry reaction of
nonreactive aromatic aldehydes that for the salan(1)–copper
catalyst system. We chose 2-MeO-, 3-MeO-, 4-MeO-, 2-Me, and 4-
Me-substituted aromatic aldehydes for the slan(1)–Cu catalyzed
Henry reaction under the following reactions: 10 mol% of
catalyst, in ethanol, at 10 ꢀC for 48 hours. The benzaldehyde was
used with different amounts with 5 mol%, 10 mol%, and 20
mol% to provide possible regular effect on the catalytic
performance of salan–Cu system.
As shown in Table 1, the catalytic activity of copper-catalyzed
Henry reactions was found to be improved dramatically by
benzaldehyde. For example, in contrast to the catalytic Henry
reaction of 2-methoxybenzaldehyde in the absence of benzal-
dehyde (Entry 1), the addition of 5 mol% of benzaldehyde
accelerated the reaction to give the corresponding product in
69% of total yield (Entry 2). The major product is the Henry
adduct with moderate enantiomeric excess (58% ee). When the
amount of benzaldehyde was increased to 10 or 20 mol%, the
benzaldehyde still had positive effect on the catalytic perfor-
mance of salan–Cu, while the enantioselectivity of Henry reac-
tion was decreased largely with the addition of benzaldehyde
(Entries 3 and 4). Under the similar conditions for other
aromatic aldehydes, such as 3-MeO-, 4-MeO-, 2-Me, and 4-Me-
substituted aromatic aldehydes (Entries 5–20), same trend that
the conversion was increased obviously when larger amount of
benzaldehyde was used as additive (see Fig. S1 of ESI†). Except
4-methoxybenzaldehyde, all the aldehydes evaluated in this
table resulted in good to excellent conversion when 20 mol% of
benzaldehyde was used as a promoter. Notably, for most of
Results and discussion
Very recently, we synthesized a novel chiral 1,10-binaphthalene-
2-a-arylmethanol-20-ols (Ar-BINMOLs) that derived from BINOL
through [1,2]-Wittig rearrangement.13 The Ar-BINMOL ligands
have rigid structures with C2-axial and sp3 carbon-central
chirality, and could be acted as a supramolecular backbone due
to the existence of hydrogen bonding and aromatic interactions
of rotatable aromatic rings, thus we expected a high potential
for the creation of a rotatable p-wall in the designed salan
ligand prepared from Ar-BINMOL via six-step trans-
formations.14 Interestingly, in this case, the (S,R)-Ar-BINMOL
and (S,S)-cyclohexane-1,2-diamine-derived salan ligand exhibi-
ted unusual and high level of catalytic efficiency in asymmetric
Henry reaction of aldehydes in terms of enantioselectivities and
conversions. As shown in Scheme 1, the salan (1)–copper
complex exhibited good catalytic performance for non-
substituted and halogen-substituted aromatic aldehydes in
term of conversion and enantioselectivity. However, methyl-,
Scheme 1 Ar-BINMOL-derived salan–Cu complex-catalyzed Henry
reactions.14
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Table 1 The effect of benzaldehyde on the catalytic Henry reaction of
Me- or MeO-substituted aromatic aldehydesa
Entry
R
x (mol%)
Yieldb (%)
3/4
eec (%)
1
2
3
4
5
6
7
8
2-MeO
2-MeO
2-MeO
2-MeO
3-MeO
3-MeO
3-MeO
3-MeO
4-MeO
4-MeO
4-MeO
4-MeO
2-Me
2-Me
2-Me
2-Me
4-Me
4-Me
4-Me
4-Me
0
5
10
20
0
<5
69
58
88
<5
16
32
88
<5
<5
10
44
<5
27
52
>99
<5
30
39
>99
—
—
58
11
5
64 : 5
46 : 12
53 : 35
—
6 : 10
17 : 15
11 : 77
—
—
—
—
—
—
—
—
—
—
59
11
—
—
—
—
—
5
10
20
0
9
10
11
12
13
14
15
16
17
18
19
20
5
—
10
20
0
0 : 10
0 : 44
—
27 : 0
28 : 24
16 : 84
—
9 : 21
16 : 23
0 : 100
5
10
20
0
5
10
20
Scheme 2 The effect of aromatic compounds with or without func-
tional groups on the salan–Cu-catalyzed Henry reaction (compound
a-6, CCDC 1009555).
a
All reactions were performed on a 1 mmol scale with 10 mol% of Cu
salt and 10 mol % of salan ligand (1) at a 0.5 M concentration using
10 equiv. of nitromethane in EtOH. Reactions were run at 10 ꢀC in a
Table 2 The effect of aromatic compounds on the catalytic Henry
b
screw-capped vial for 48 h. The total yield of product 3 and 4.
c
reaction of substituted aromatic aldehydesa
Enantiomeric excess was determined by HPLC using chiral columns,
and the absolute conguration of the major isomer was determined
Yieldb (%)
3/4
eec/%
to be (R) by comparison with literature data.4
Entry
Additive (10 mol%)
1
2
3
4
5
6
7
8
9
a1
a2
a3
a4
a5
a6
a7
a8
a9
a10
a11
83
48
53
88
75
25
70
46
20
46
<10
83 : 0
0 : 48
28 : 25
14 : 74
19 : 56
25 : 0
22 : 48
46 : 0
20 : 0
46 : 0
5 : 0
20
—
0
—
—
40
5
16
44
32
—
substrates provided in Table 1, the major products were nitro-
olens 4 but not Henry adducts 3. Thus, these experimental
results supported that the effect of benzaldehyde on the salan–
Cu catalyzed Henry reaction was distinctive, which was gener-
ally concordant with our hypothesis. This idea was also sup-
ported by subsequent exploring of the effect of aromatic
10
compounds on the model Henry reaction of 4-methyl- 11
benzaldehyde and nitromethane.
a
All reactions were carried out at ꢁ10 ꢀC in a screw-capped vial for 48 h.
The yield was determined by GC-MS, and the catalytic level was
divided as good (>70% yield, H), moderate (40–70% yield, M), and
b
As revealed in Scheme 2 and Table 2, the Henry reaction
proceeded without any problem with as little as 10 mol% of
chlorobenzene (Entry 1), chalcone (Entry 4), 1-(2-nitrovinyl)-
benzene (Entry 5), and PhOH (Entry 7) containing one phenyl
ring, giving the corresponding in good conversion. Functional
groups on phenyl ring of aromatic additives, such as amines
(Entries 8–10), acid (Entry 11), retarded the Henry reaction.
Larger substituent on phenyl ring (for example, entry 6) was also
make against the occurring of Henry addition of nitromethane
to 4-methylbenzaldehyde. It should be noted that these
aromatic compounds played different effect on the stereo-
selective Henry reaction of 4-methylbenzaldehyde led to 0–40%
ee of product. It was proposed that such an experimental
phenomenon driven by the unexpected force of aromatic
c
poor (<40% yield, L). Enantiomeric excess was determined by HPLC
using chiral columns.
interactions, thus it was provided sight into the crucial role of
aromatic additive in both the transformations and enantiose-
lectivity of Henry reaction of aromatic aldehydes.
To highlight the specic of Ar-BINMOL-derived salan-1
prepared from (1S,2S)-cyclohexane-1,2-diamine and (S)-BINOL
in the Henry reaction, another type of salan ligand that derived
from 1,2-diphenylethane-1,2-diamine and (S)-BINOL was then
designed and synthesized for the Henry reaction. Two different
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better control of stereoselectivity, because both the steric and
electronic features of the rigid salan ligand 1 are acting in a
cooperative manner combined with metal coordination and
weak non-covalent aromatic interactions.
On the basis of above work, it is clear that the salan (1)–
copper complex resulted in a quite narrow substrate scope
because of its complicated and multifunctional structure simi-
larly to the behaviour of enzyme. Thus we expected to develop a
highly efficient and enantioselective Henry reaction by
changing of copper center to other metal center coordinated
with our Ar-BINMOL-derived salan or salen ligand. Although
cobalt–salen complex has been extensively tested in various
catalytic asymmetric reactions, only Yamada and Hong reported
that cobalt–salen complex could be employed for the Henry
reaction.15,16 For example, an important progress reported by
Hong and coworkers,16 in which it was found that both the
binuclear Co(II)–salen and self-assembled Co(III)–salen featured
with hydrogen-bonding gave excellent enantioselectivity and
yield. In their work, it should be noted that monomeric Co(II) or
Co(III)–salen complex gave the Henry product in poor yield
(<11%) and only 55–64% ee in this reaction. Thus inspired by
previous studies on cobalt catalysis17 and salan–Cu-catalyzed
Henry reaction, it was expected that Ar-BINMOL-derived salen
ligand featured with rotatable p-wall could be effective ligand
for Co-catalyzed Henry reaction. Herein, we want to continue to
disclose our new ndings on the highly enantioselective Henry
reaction catalyzed by monomeric salen–Co(III) complex derived
from Ar-BINMOL with p-wall.
Starting from commercially available (1S,2S)-cyclohexane-
1,2-diamine or (1R,2R)-cyclohexane-1,2-diamine with chiral Ar-
BINMOL that derived from (S)-BINOL or (R)-BINOL (Ar ¼ Ph, p-
Me-Ph, or p-tBu-Ph), a new type of chiral Ar-BINMOL-derived
salen ligands (5a–e) could be synthesized in good overall yields
(see ESI†). We hypothesized that the Ar-BINMOL-derived salen
bearing rotatable p-wall is a highly attractive ligand in the
construction of cobalt-centered p-pocket for the recognition
and activation of aromatic aldehyde in this Henry reaction.
Initially, we evaluated the performance of Ar-BINMOL-derived
salen (5a) in the asymmetric cobalt-catalyzed Henry reaction of
2-uorobenzaldehyde with nitromethane (Scheme 5) because
the 2-uorobenzaldehyde led to low enantioselectivity in salan–
Cu catalyst system (2-F, only 59% ee in Scheme 1, line 2).
Table 3 summarizes the optimal results of the initial studies
on solvent effect. The data clearly demonstrates the superiority
of toluene as a solvent in term of yield and ee value (Entry 9)
compared to other solvents (Entries 1–8). The reaction condi-
tions were then further optimized with regard to base and
counterion. As shown in Table 3, it was found that TEA, TMEDA,
and K2CO3 were not effective base to activate nitromethane
because of low conversion and inferior enantioselectivity in this
reaction (Entries 11–14). In addition, no addition of organic
base and the use of phosphine led to almost no reaction (Entries
10 and 13). The metal oxidation state of cobalt proved to be
pivotal, and a Co(III)–salen complex with counterion is crucial to
the reaction yield. Both Co(II)-based salen complex and Co(III)–
salen complex with acetate (OAc) were found to be ineffective
catalysts in this Henry reaction (Entries 15 and 16). Although
Scheme 3 The synthesis of salan ligand (salan-2 and salan-3) from
(1S,2S)-1,2-diphenylethane-1,2-diamine (DPEN) and (1R,2R)-1,2-
diphenylethane-1,2-diamine respectively.
salan ligands (Ar-BINMOL-derived salan-2 and salan-3) with both
axial and sp3 center chirality, were prepared from (1S,2S)-1,2-
diphenylethane-1,2-diamine (DPEN) and (1R,2R)-1,2-diphenyl-
ethane-1,2-diamine respectively in high yields (Scheme 3, also
see ESI†). The catalytic activity of corresponding salan–copper
complex, in situ obtained by the reaction of salan-2 or salan-3
with Cu(OAc)2, was investigated in the model Henry reaction of
benzaldehyde and nitromethane. The chirality matching can be
also evaluated in this case. As shown in Scheme 4, the Henry
reaction of benzaldehyde was relatively slow as evidenced by low
conversion and isolated yields in the presence of salan-2 or
salan-3. Unexpectedly, the enantioselectivity of the addition
product was quite low (33% ee for salan-2 and 17% ee for salan-
3). We interpret these results as follows: whereas the lower
activity found in the case of (S,S,R,R)-salan-2 could be explained
on the basis of the enhanced steric hindrance of diphenyl
groups of DPEN and dibenzyl groups on binaphthyl backbone,
this argument cannot be applied to (R,R,R,R)-salan-3 because of
different steric conformer of DPEN moiety. Rather, the poorer
results in term of enantioselectivity and conversion afforded by
latter ligand are due to the unfavorable chirality matching with
different chiral sources. Notably, the enantioselectivity of
present Henry reaction is mainly relay on the effect of chiral
binaphthyl fragment because the use of DPEN with different
stereogenic conguration did not lead to stereoselectivity
reversal of corresponding product. Thus for this particular
reaction, the combination use of (S)-BINOL derived backbone
and (1S,2S)-cyclohexane-1,2-diamine as two fragments ensures
Scheme 4 DPEN-derived salan–Cu-catalyzed Henry reaction.
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anion of camphorsulfonate (CSA) gave good enantioselectivity
(90% ee, Entry 17), triuoromethanesulfonate (OTf) was still the
best choose in term of yield and enantioselectivity. Interest-
ingly, the further modication and application of Ar-BINMOL-
derived salens (5b and 5c) was not successfully in this reaction
(Entries 18 and 19), whereas the introduction of Me and t-Bu to
benzyl motif of Ar-BINMOL-derived salen resulted in only 87%
ee and 73% ee respectively (5b and 5c, Entries 18–19). Thus
notably, under the optimized conditions, excellent enantiose-
lectivity (97% or 98% ee) and good isolated yield (71–73%) were
achieved in the reaction between nitromethane and 2-uo-
robenzaldehyde (Entry 9 and Entry 20). However, it should be
noted that the reaction time is different largely (12 h in toluene
and 72 h in toluene–Et2O).
Notably and interestingly, (R,R,R,R)-salen–Co(III) (5d) and
(S,S,R,R)-salen–Co(III) (5e) derived from (S)-BINOL-derived
aldedyde with (R,R)-cyclohexane-1,2-diamine or (S,S)-cyclo-
hexane-1,2-diamine respectively exhibited inferior activity in
both catalytic activity and enantiomeric induction, being inca-
pable of achieving good yield and high enantioselectivity even
with 72 hours (only 59% ee and reversed absolute conguration
of 89% ee respectively, Entries 21 and 22). This result showed
that the stereochemistry of Ar-BINMOL backbone play crucial
role in this salen–Co(III)-catalyzed Henry addition of nitro-
methane to aromatic aldehydes. Thus, the mismatching
between different chiral sources, C2-axial binaphthol or Ar-
BINMOL and sp3 central chiral phosphine, revealed that the
geometric orientation of BINOL-derived cyclohexane-1,2-
diamine was crucial to the enantioselective induction in this
nitroaldol reaction.18
Scheme 5 Ar-BINMOL-derived salen–Co catalyzed Henry reaction of
2-fluorobenzaldehyde (2f) with nitromethane.
Table
3
Salen–Co-catalyzed
Henry
reaction
of
2-fluo-
robenzaldehyde (2f): optimization of reaction conditionsa
Entry Salen
X
Base
Sol.
Yieldb (%) eec (%)
The (R,R,S,S)-salen (5a)–Co(III) catalyst was then applied to
enantioselective Henry reactions of various aldehydes with
nitromethane (Scheme 6). As shown in Scheme 6, the scope of
the catalytic enantioselectivity in this Henry reactions was
demonstrated by treatment of various aromatic aldehydes in the
presence of 5 mol% of (R,R,S,S)-salen–Co(III) and 1 eq. DIPEA
(N,N-diisopropylethylamine) at ꢁ20 ꢀC for 12 h. In most of
cases, the Henry reactions of various aromatic aldehydes were
clean and proceeded in good yields with excellent enantiose-
lectivities. It should be also noted that there are several inter-
esting ndings from these reaction results: (1) most of aromatic
aldehydes led to good yields and enantioselectivities. It is clear
that better yields and ees are recorded for substrates function-
alized in the 2-position. And interestingly, aromatic aldehydes
bearing electron-donating groups (such as OMe) and electron-
withdrawing groups (such as NO2) at 3-position of aromatic ring
also led to the same level of good yields and enantioselectivity.
For example, 3-methoxybenzaldehyde could give the desired
product in 92% ee, while 3-nitrobenzaldehyde (2p) resulted in
95% ee. However, the enantioselectivity in the Henry reaction of
2-nitrobenzaldehyde (2o) and 4-nitrobenzaldehyde (2q) was
decreased largely to only 70–78% ee, which showed the elec-
tronic effect of substrate is obviously for aromatic aldehydes
with strong electron-withdrawing group. In other words, the
enantioseletive induction of Ar-BINMOL-derived salen–Co(III)
was not good for aromatic aldehydes with strong electron-
withdrawing group because of weak coordination and aromatic
1
2
3
4
5
6
7
8
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5a
5b
5c
5a
5d
5e
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
OTf
—
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
—
DCM
Et2O
THF
PhCl
PhI
MeOH
EtOH
DCE
80
45
36
80
83
40
50
58
73
<5
35
80
<5
75
<5
<5
53
66
40
87
50
88
67
57
14
50
41
97
9
PhMe
PhMe
PhMe
d
10
11
12
13
14
15
16
17
18
19
20e,f
21e,f
22e,f,g
—
68
81
TEA
TMEDA PhMe
PPh3
K2CO3
DIPEA
d
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe/Et2O 71
PhMe/Et2O 47
PhMe/Et2O 70
—
22
d
—
d
OAc DIPEA
—
CSA
OTf
OTf
OTf
OTf
OTf
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
DIPEA
90
87
73
98
59
ꢁ89
a
Reaction conditions: 0.5 mmol of aldehyde, 10 eq. MeNO2, 5 mol% of
salen–Co catalyst, 1.0 eq. of DIPEA. And the reaction time is 12 hours
except special note. Isolated yields. The ee value was determined
by HPLC with chiral column and the absolute conguration of 3f was
S-isomer except additional notes (see ESI). The yield is very poor
(<5%) or almost no product was detected, thus the ee value of trace
product was not determined. PhMe : Et2O is 2 : 1. The reaction
time is 72 h. The absolute conguration of 3f was R-isomer.
b
c
d
e
f
g
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derived salen–Co(III) and the aromatic ring of aldehydes. Except
3-phenylpropanal (2t), the salen–Co(III) complex showed a
signicantly decreased activity in the Henry reaction of
aliphatic aldehyde. Almost no yield and enantioselectivity was
observed for cyclohexanecarbaldehyde (<5% yield). Similarly to
previous report on salan–Cu(II)-catalyzed Henry reaction, these
reaction results probably show acceleration of the Henry reac-
tion by ortho-substituent-mediated weak coordination and p–p
interaction between salen–Co(III) catalyst and aromatic alde-
hydes. Thus we examined the ability of the salen–Co(III) to
discriminate aromatic aldehydes in a competitive reaction of 2-
uorobenzaldehyde and cyclohexanecarbaldehyde. In this case,
the salen–Co(III) did more selectively catalyze the Henry reaction
of 2-uorobenzaldehyde than that of cyclohexanecarbaldehyde.
The nitroaldol addition of nitromethane to cyclo-
hexanecarbaldehyde was almost not occurred under this reac-
tion conditions (<5% yield).
Thus in our case, the experimental results showed the Henry
reaction was an ideal model reaction for distinguishing ortho-
substituted aromatic aldehydes from meta- or para-substituted
benzaldehydes (Fig. S2, see ESI†). To obtain a better under-
standing of the unusual reactivity of aromatic aldehydes in this
salen–Co(III)-catalyzed Henry reaction, theoretical calculations
were performed using Gaussian 09 suite of programs.19 The
HOMO and LUMO data of o-uorobenzaldehyde, m-uo-
robenzaldehyde, and p-uorobenzaldehyde are shown in Fig. 1
(also see ESI†). It is noteworthy that the general electrophilic
activity of uoro-substituted benzaldehydes with theoretical
calculation would be different from that of experimental results
(2-F> 3-F > 4-F).
Thus according to the specically crowded structure of the
present salen–Co(III) complex, the p-wall of phenyl substituent
on salen ligand signicantly inuenced the weak coordination
and aromatic p–p stacking interaction between salen–Co(III)
complex and aldehydes, and the desired reaction environment
can be affected by the rotatable p-wall to give optimal reaction
Scheme
6 Ar-BINMOL-derived salen–Co(III)-catalyzed Henry
reactions.
interaction between salen–Co(III) and electron-defect substrate
(2o or 2q). (2) The position of substituent was proved to be
sensitive and pivotal, in which the ortho-substituted groups
signicantly improved the reaction yield enantioselectivity. For
example, uorine-substituted benzaldehyde, the decreased
order of enantioselective induction is 2-F > 3-F > 4-F. And
similarly, for bromobenzaldehyde and methoxybenzaldehyde,
the Henry reaction of ortho-substituted benzaldehyde led to
better enantioselectivity and yield than that of meta- or para-
substituted benzaldehyde correspondingly. These reaction
results suggest that the selective recognition to ortho-
substituted benzaldehydes is ascribe to the difference in the
molecular size of aromatic aldehydes and possible aromatic-
Fig. 1 The comparison of enantioselectivities and yields in the salen–
Co (1a) catalyzed Henry reaction of F- or Br-substituted aromatic
aromatic interaction between the phenyl rings of Ar-BINMOL- aldehydes.
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interactions enhanced salan(1)–Cu-catalyzed Henry reaction of
nonreactive aldehydes, and thus provided a indirect experi-
mental evidence in the aromatic additive initiated enantiose-
lective reaction. More importantly in this work, a new type of
chiral salen–Co catalyst that features aromatic p-wall and active
Co(III) center has been developed for highly enantioselective
Henry/nitroaldol reactions. The asymmetric Henry reaction of
aromatic aldehydes and nitromethane catalyzed by Ar-BINMOL-
derived salen (5a)–Co(III) complex was achieved with high yields
(up to 99%) and excellent enantioselectivities (up to 98% ee).
And more interestingly, it was supposed that the salen–Co(III)
complex-catalyzed Henry reaction was an ideal model reaction
for providing direct evidence of noncovalent interaction due to
the distinguishing ortho-substituted aromatic aldehydes from
meta- or para-substituted benzaldehydes in term of enantiose-
lectivities and yields. We believed that the attractive non-
covalent interactions of aromatic rings with other groups can
inuence and control the stereoselectivity of various catalytic
transformations. Further efforts will be devoted to apply such
salen–Co(III) complex with chiral p-wall to noncovalent inter-
action-oriented asymmetric catalysis and enantioselective
synthesis of functional organic molecules.
Fig. 2 Assumed transition state for the nanoscale salen–Co(III)-cata-
lyzed Henry reaction.
model with high stereoselectivity. On the basis of present
experimental results, a proposed transition state for the salen–
Co(5a)-catalyzed Henry reaction as depicted in Fig. 2 may be
envisioned, which could account for the observations.
Experimental section
For modelling purposes, the structure of o-uo-
robenzaldehyde and (R,R,S,S)-salen (5a)–Co(III) catalyst were
considered for the modular construction of transition state for
the salen–Co(III)-catalyzed Henry reaction. As shown in Fig. S3
(see ESI†), the optimized structure of the most reasonable
catalyst–substructure complexes was provided on the basis of
experimental results (Fig. 2). Because of asymmetric character
of the Ar-BINMOL-derived backbone and substrate, the coor-
dination of oxygen of aldehyde with cobalt center was impacted
largely by two different naphthyl rings of salen ligand, therefore
the orientations of o-uorobenzaldehyde can occur within the
catalyst–substrate complex, leading to highly enantioselective
Henry addition. In other words, one of the naphthyl ring of
salen ligand could also discriminate different molecular inter-
action between substituted-benzaldehyde and (R,R,S,S)-salen
(5a)–Co(III) catalyst for the steric repulsion of naphthyl group:
the aromatic interaction as well as coordination between cata-
lyst and uorobenzaldehyde could be weakened with the order
of o-F > m-F > p-F substituted aldehyde, which is inconsistent
with that of reaction results. Also notably, the nanoscale Ar-
BINMOL-derived salen–Co(III) with rotatable p-wall is totally
different from simple salen–Co(III) complex in shape, volume,
and chirality, thus resulting in the larger difference in their
activity and selectivity for Henry reaction.20
General
All reagents and solvents were used directly without purica-
tion. Flash column chromatography was performed over silica
(200–300 mesh). Reactions were monitored by thin layer chro-
matography using silica gel. 1H NMR and 13C NMR (500 and 125
MHz, respectively) spectra were recorded in CDCl3, 1H NMR
chemical shis are reported in ppm relative to tetramethylsi-
lane (TMS) with the solvent resonance employed as the internal
standard (CDCl3 at 7.26 ppm, (CD3)2CO at 2.05 ppm). 13C NMR
chemical shis are reported in ppm from tetramethyl silane
(TMS) with the solvent resonance as the internal standard
(CDCl3 at 77.20 ppm). Thin layer chromatography was per-
formed using silica gel; F254 TLC plates and visualized with
ultraviolet light. HPLC was carried out with a Waters 2695
Millennium system equipped with a photodiode array detector.
The ESI-MS analysis of the samples was operated on an LCQ
advantage mass spectrometer (ThermoFisher Company, USA),
equipped with an ESI ion source in the positive ionization
mode, with data acquisition using the Xcalibur soware
(Version 1.4).
General procedure for the asymmetric salen–Co(III)-catalyzed
Henry reaction
To a screw cap vial containing a stir bar, cobalt complex (26.5
mg, 0.025 mmol, 5% mmol) was added. And then 2 mL toluene,
2-uorobenzaldehyde (53 mL, 0.5 mmol), and DIPEA (83 mL, 0.5
Conclusions
In summary, it was found rstly that the benzaldehyde or mmol) was added to the vial. The reaction mixture was cooled
similar aromatic compounds could work as a p-molecule to down to ꢁ20 ꢀC, and then CH3NO2 (0.27 mL, 5 mmol) was
enhance the catalytic activity of nanoscale Ar-BINMOL-derived added. The mixture was continued to stir at ꢁ20 ꢀC for 12 h. The
salan(1)–Cu to initiate the Henry reaction of various nonreactive reaction mixture was puried by ash column chromatography
aldehydes, which led to the determination of aromatic on silica gel (n-hexane/EtOAc ¼ 10/1) to give the nitroaldol
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adduct as a colorless oil. All these molecules have been
conrmed by NMR, IR, and GC-MS or HRMS. The detailed
experimental procedures and characteristics of corresponding
products were provided in Supporting Information.
Tetrahedron: Asymmetry, 2011, 22, 1169; (j) G. Lai, F. Guo,
Y. Zheng, Y. Fang, H. Song, K. Xu, S. Wang, Z. Zha and
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Acknowledgements
Financial support by the National Natural Science Foundation
of China (NSFC Grant no. 21173064 and 51203037), Zhejiang
Provincial Natural Science Foundation of China (LR14B030001),
and Program for Excellent Young Teachers in Hangzhou
Normal University (HNUEYT, JTAS 2011-01-014) is appreciated.
XLW also thank Prof. Chun-Gu Xia (Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences) and Prof. Yixin
Lu (National University of Singapore) for their help.
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