Versatile supramolecular copper(II) Complexes for Henry and Aza-Henry reactions

Article abstract of DOI:10.1002/adsc.200900069

Chiral supramolecular metal-organic frameworks assembled from copper complexes catalyse Henry and aza-Henry reactions of aromatic and aliphatic aldehydes and N-protected aromatic imines in high yield and good to excellent enantioselectivity. Reactions can

Full text of DOI:10.1002/adsc.200900069

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DOI: 10.1002/adsc.200900069
Versatile Supramolecular Copper(II) Complexes for Henry and
Aza-Henry Reactions
Guoqi Zhang,a Eiji Yashima,b and Wolf-D. Woggona,*
a
Department of Chemistry, University of Basel, St. Johanns Ring 19, 4056 Basel, Switzerland
Fax : (+41)-61-267-1103; e-mail: wolf-d.woggon@unibas.ch
Department of Molecular Design and Engineering, Graduate School of Engineering
b
Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
Received: February 2, 2009; Revised: April 29, 2009; Published online: June 2, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900069.
Table 1. Enantioselective Henry reactions of CH3NO2 with
Abstract: Chiral supramolecular metal-organic
frameworks assembled from copper complexes cat-
alyse Henry and aza-Henry reactions of aromatic
and aliphatic aldehydes and N-protected aromatic
imines in high yield and good to excellent enantio-
selectivity. Reactions can be performed in the ab-
sence of base in ethanol or water.
para-nitrobenzaldehyde 6 in the presence of Cu(II) com-
plexes of ligands 1–5.[a]
Keywords: aza-Henry reaction; catalysis; cop-
per(II); enantioselectivity; Henry reaction
Ligand
Time [h]
Yield [%][b]
ee [%][c]
1
2
3
4
5
4
90
95
62
92
95
53
2
70
65
87
1.5
1.5
1.5
1.5
The Henry (nitroaldol) reaction[1] is one of the most
useful carbon-carbon bond forming reactions in syn-
thetic chemistry. The resulting b-hydroxy nitro prod-
ucts are versatile building blocks to prepare biologi-
cally significant compounds such as b-amino alcohols
and a-hydroxy carboxylic acids.[2] In recent years vari-
ous organocatalytic[3] and metal-organic[4] systems
have been developed to accomplish the enantioselec-
tive Henry reaction. Among them, the Cu-catalyzed
Henry reaction has received much attention due to
the mild reaction conditions which do not require ad-
[a]
All reactions were performed on a 0.20 mmol scale with
5 mol% of Cu(II) complex at a 0.5M concentration and
5 equiv. of nitromethane in EtOH at room temperature.
Yields of isolated hydroxy nitro compounds.[c] Enantio-
meric excess (ee) was determined by HPLC using a Chir-
alcel OD-H column. The absolute configuration of prod-
ucts was determined as S by comparison of their optical
rotations with literature values.[4]
[b]
ditives such as organic bases.[4f–j] Except for a few were formed in situ. We discovered that 19, the
cases[4j–l] most Cu complexes contain a symmetrical, Cu(II) complex of 5 (Scheme 2), was the most effi-
four-dentate ligand sphere.
cient catalyst within this group. X-ray analysis of suit-
We wish to report here on the synthesis, structure able crystals of 19 revealed an interesting supramolec-
and catalytic efficiency of self assembled supramolec- ular structure in which the Cu(II) coordinates to the
ular Cu(II) complexes based on ligands with C2 and nitrogens of the chiral diamine unit and to the pyri-
C1 symmetry.
dine nitrogens of the adjacent complex (Figure 1).
The X-ray structure further revealed a coordination
We first screened the Cu(II) complexes of several
C2 symmetric ligands 1–5 for catalysis of the Henry mode (19-A) that forms triangular cavities (19-B)
reaction of aldehyde 6 and CH3NO2 (Table 1), in all (Figure 1) and
cases product 7 was S configured. Ligands 1–5 were (Figure 2).
a
lamellar framework (19-C)
easily available from diaminocyclohexane 8 and alde-
In order to understand whether the polymeric
hydes 9–13 and subsequent reduction of the Schiff nature of the catalyst 19 is maintained in solution we
bases 14–18 (Scheme 1). The Cu(II) complexes of 1–5 measured UV, CD spectra (Figure 3) and DLS (dy-
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Scheme 1. Synthesis of ligands 1–5.
The DSL measurement demonstrates that the parti-
cle size of oligomeric 19 in solution slowly grows to
approach a value of about 37 nm. Together with
nearly constant CD and UV spectra these experi-
ments suggest that 19 maintains the framework of the
solid state to a certain extent in solution. Further sup-
port for an intact coordination sphere of Cu(II) in so-
lution comes from the fact that the ee values of the
product of the standard reaction 6!7 do not change
significantly within 25 h (Figure 5).
Scheme 2. Structure of complex 19.
Finally, the substrate scope of 19 was investigated
(Table 2). The results show that aromatic and aliphat-
ic aldehydes react in very good yields with nitrome-
thane producing b-nitro alcohols in modest to good
enantioselectivities. As it was not possible to improve
the ee values either by changing concentrations, tem-
perature or solvent we decided to investigate C1 sym-
metrical ligands for Cu(II), a less explored class of
copper complexes.
For this purpose three ligands 20–22 were synthe-
sized. Scheme 3 shows the preparation of 22. The di-
AHCTUNGTREGaNNUN mine 8 was first condensed with 4-formylpyridine 13
to obtain the monoamine 23; further reaction with
the aldehyde 24 gave the salen 25 in 52% yield over
two steps. NaBH4 reduction of 25 furnished quantita-
tively the ligand 22.
Figure 1. X-ray structure of complex 19.
In order to evaluate optimal reaction conditions
namic light scattering, Figure 4) in the time frame of para-nitrobenzaldehyde 6 was transformed into the b-
the Henry reaction and in the solvent EtOH/H2O hydroxy nitro product 7 (Scheme 4) using ligands 20–
(9:1) where catalytic reactions could be performed.
22, and various Cu(II) salts and solvents (see Support-
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Versatile Supramolecular Copper(II) Complexes for Henry and Aza-Henry Reactions
COMMUNICATIONS
Figure 2. 2-D polymeric framework (19-C) of complex 19.
Figure 3. Time-dependent CD (A) and UV spectra (B) of 19 in EtOH/H2O 9:1.
ing Information). Accordingly both yields (95%) and acceptable ee values were obtained. Interestingly the
ee values (96%) approach very good figures for ligand aliphatic aldehydes behave well and give excellent
22 in EtOH and in the presence of the soft counterion yields and ee values; except for a few cases[5a–c] this is
acetate. Consequently these conditions were then em- rather an exception than the rule for Cu complex-cat-
ployed to investigate the scope of the catalytic proce- alyzed Henry reactions.[5d–i]
dure.
In order to gain information on the structure of the
(OAc)2 was added to ligand
From the results in Table 3 it is clear that aldehydes copper complex of 22, CuAHCTNUGTRENNNUG
with electron-withdrawing substituents react faster 22 in CH3CN (Scheme 5) producing bright green crys-
and in general furnish b-nitro alcohols with ee values tals suitable for X-ray crystallography (for details, see
and yields well above 90%. In contrast, electron-do- Supporting Information). The X-ray structure of com-
nating groups slow down the reaction and in some plex 26 shows indeed that ligand 22 is tridentate and
cases (entries 9, 11 and 13) moderate yields but still that one residual acetate unit coordinates to Cu(II)
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Table 2. Enantioselective Henry reactions of CH3NO2 with
various aldehydes in the presence of complex 19.
Entry[a] Aldehyde (R) Time [h] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
4-NO2C6H4
2-NO2C6H4
3-NO2C6H4
3-pyridyl
4-pyridyl
Ph
12
12
12
12
12
72
60
72
72
72
60
72
48
48
48
94
95
93
92
90
62
94
88
73
66
85
65
87
82
91
86
90
82
87
80
84
85
82
73
80
85
75
83
85
86
4-ClC6H4
4-FC6H4
9
2-MeC6H4
4-MeC6H4
1-naphthyl
4-MeOC6H4
n-butyl
10
11
12
13
14
15
Figure 4. Time-dependent DSL of 19 in EtOH/H2O 9:1.
tert-butyl
c-hexyl
[a]
All reactions were performed on a 0.20 mmol scale with
5 mol% of 19 at a 0.5M concentration and 5 equiv. of ni-
tromethane in EtOH at room temperature.
See Table 1.
[b]
[c]
See Table 1.
Figure 5. Enantiomeric excess (ee) of 7 as a function of time
during the reaction of para-nitrobenzaldehyde 6 and nitro-
methane in EtOH catalyzed by 19 (5 mol%).
(Figure 6a) which is believed to be exchanged for the
components of the Henry reaction, vide infra.
Figure 6b shows an extended view of the structure
in which the individual Cu complexes are connected
via H-bridges between the non-coordinating pyridine
ꢀ
N and the H N group adjacent to the phenolate. This
interaction leads to a unique single-stranded M left-
handed helix with a pitch of 8.65 ꢁ.
In EtOH the free ligand 22 display a p-p* transi-
tion at 245 nm, the complex 26 shows a positive
Cotton effect at 245 nm and a negative mlct band at
425 nm (Figure 7) in agreement with the M handed-
ness[6]of the helix also observed in the X-ray structure
of 26.
Scheme 3. Synthesis of C1 symmetrical ligand 22, and struc-
tures of ligands 20 and 21.
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Versatile Supramolecular Copper(II) Complexes for Henry and Aza-Henry Reactions
COMMUNICATIONS
Scheme 4. Henry reaction to screen for optimal reaction
conditions.
Table 3. Enantioselective Henry reactions of CH3NO2 with
various aldehydes in the presence of 22, CuACTHNUGTRENUNG(OAc)2.
Scheme 5. Formation of catalyst 26.
Entry[a] Aldehyde (R) Time [h] Yield [%][d] ee [%][e]
1[b]
4-NO2C6H4
2-NO2C6H4
3-NO2C6H4
3-pyridyl
4-pyridyl
Ph
1.5
1.5
1.5
1.5
1.5
48
24
24
60
60
60
60
60
60
60
60
95
94
95
94
96
75
65
72
58
63
54
75
51
88
91
92
96
98
96
96
95
90
90
87
93
88
93
93
85
98
99
98
2[b]
3[b]
4[b]
5[b]
6[b]
7[b]
4-ClC6H4
4-FC6H4
8[b]
9[c]
2-thiophenyl
2-MeC6H4
4-MeC6H4
1-naphthyl
4-MeOC6H4
n-butyl
10[c]
11[c]
12[c]
13[c]
14[c]
15[c]
16[c]
tert-butyl
c-hexyl
Figure 6. X-ray structure of the Cu(II) complex 26; a) single
unit, b) the helical structure of hydrogen bonded complex
26.
[a]
All reactions were performed on a 0.20 mmol scale with
5 mol% of 22, Cu(OAc)2 at a 0.5M concentration and
AHCTUNGTRENNUNG
5 equiv. of nitromethane in EtOH.
Reactions were run at room temperature.
Reactions were run at 08C.
See Table 1.
chiral metal complexes and organocatalysts have been
reported for the enantioselective catalysis of this reac-
tion,[9–11] the results shown in Table 4 demonstrate
that our catalyst 26 is also applicable to the aza-
Henry reaction using N-protected imines and nitro-
methane.
[b]
[c]
[d]
[e]
See Table 1.
Furthermore, monitoring the standard reaction 6!
Results of the aza-Henry reaction with catalyst 26
7 revealed a constant enantiomeric excess (ee) for 7 are comparable to those of the reaction with aromatic
within the time frame of the reaction (Figure 8). Ac- aldehydes and show the same trend in reactivity con-
cordingly the supramolecular, helical structure of the cerning the presence of electron withdrawing groups
crystalline complex 26 is maintained in EtOH, the at the aldehyde substrate, see Table 4. In general, ee
preferred solvent of the Henry reaction.
values above 90% were observed which is well within
A reaction sequence is proposed in which the ace- the best results for this reaction reported so far.[10] Be-
tate unit of 26 is replaced by the two reactants such sides its unusual structure catalyst 26 has another in-
that the nitronate attacks preferentially the re face of teresting feature, the complex is water soluble. Ac-
the aldehyde. After release of the product addition of cordingly we investigated Henry and aza-Henry reac-
aldehyde and CH3NO2 regenerates the intermediate tions with a few substrates in water (for details, see
27 closing the cycle (Scheme 6).
Supporting Information). In general yields were simi-
The addition of nitroalkanes to imines, known as lar to those observed in EtOH, whereas the ee values
the aza-Henry (or nitro-Mannich) reaction also pro- dropped by 5–12%. Nevertheless, a metal complex
ꢀ
vides a versatile tool for C C bond formation. The re- catalyzing Henry and aza-Henry reactions in water
sulting products, b-nitro amines, are readily converted with both high yields and ee values is unprecedent-
into 1,2-diamines[7] or a-amino acids.[8] While various ed.[12]
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Figure 7. CD and UV/Vis spectra (inset) of 26 and 22 in EtOH.
compare well with Cu(II) complexes of chiral imida-
zoline-aminophenol ligands[4j], chiral aminopyridine
ligands[4m] (Henry reaction) and chiral N,N’-dioxide
Cu(I) complexes[11m] (aza-Henry reaction), and are su-
perior to other recently published Cu(II) complexes
with ligands such as chrial Schiff-bases,[4k] bis(2-pyri-
dylmethylidene)-(R,R)-1,2-diaminocyclohexane[4n] or
borabox.[5i] Furthermore, these results document a
rare example of one simple complex catalyzing both
Henry and aza-Henry reactions with very high enan-
tioselectivity.
Experimental Section
Typical Experimental Procedure for Henry and Aza-
Henry reactions
Figure 8. Plot of ee (%) change with time (min.) during the
reaction of para-nitrobenzaldehyde and nitromethane in
EtOH catalyzed by 26 (5 mol%).
Compound 22 (3.7 mg, 0.01 mmol) and CuACTHNURTGNENG(U OAc)2 (1.8 mg,
0.01 mmol) were added to a screw-capped vial containing a
stir bar. Anhydrous EtOH (0.4 mL) was then added, and a
clear green solution formed under stirring, which was con-
tinued for 30 min at room temperature, or then cooled to
08C as indicated. To the resulting solution nitromethane
(0.13 mL, 1.0 mmol, 5 equiv.) and various aldehydes or N-
In summary, Cu(II) complex 26 can be easily pre-
pared and catalyzes Henry and aza-Henry reactions
under base-free conditions in protic solvents with
high yields and ee values up to 99%. These results Boc imines (0.2 mmol, 1 equiv.) were added. After stirring
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Versatile Supramolecular Copper(II) Complexes for Henry and Aza-Henry Reactions
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Scheme 6. Proposed mechanism of Henry reactions catalyzed by 26.
Table 4. Enantioselective aza-Henry reactions of CH3NO2
with various aromatic imines in the presence of 26.
Acknowledgements
Financial support from the Swiss National Science Founda-
tion is gratefully acknowledged. For X-ray analyses we thank
Dr. M. Neuburger and S. Schaffner, for DLS and CD meas-
urements we thank Dr. H. Goto, Nagoya University.
Entry[a] Imines (R)
Time [h] Yield [%][d] ee [%][e]
1[b]
4-NO2C6H4
2-NO2C6H4
3-NO2C6H4
3-pyridyl
4-MeO2CC6H4
4-CNC6H4
Ph
1-naphthyl
2-MeC6H4
4-MeC6H4
4-CF3C6H4
4-MeOC6H4
3-MeOC6H4
4-ClC6H4
2
2
2
2
6
4
60
48
72
72
72
72
72
72
92
99
93
99
80
92
80
81
65
62
75
71
65
68
95
96
96
93
95
93
97
93
96
84
93
80
90
85
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aqueous media, see: R. Ballini, G. Bosica, J. Org.
Chem. 1997, 62, 425–427.
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Adv. Synth. Catal. 2009, 351, 1255 – 1262