A highly selective, polymer-supported organocatalyst for Michael additions with enzyme-like behavior

Article abstract of DOI:10.1002/adsc.200900817

A polymer-supported α,α-diarylprolinol silyl ether displays catalytic activity and enantioselectivity comparable to the best homogeneous catalysts in the Michael addition of aldehydes to nitroolefins. Above all, the combination of polymer backbone, triazo

Full text of DOI:10.1002/adsc.200900817

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DOI: 10.1002/adsc.200900817
A Highly Selective, Polymer-Supported Organocatalyst for
Michael Additions with Enzyme-Like Behavior
Esther Alzaa and Miquel A. Pericꢀsa,b,*
a
Institute of Chemical Research of Catalonia (ICIQ), Av. Paꢁsos Catalans 16, 43007 Tarragona, Spain
Fax : (+34)-977-920-222; e-mail: mapericas@iciq.es
Departament de Quꢂmica Orgꢀnica, Universitat de Barcelona (UB), 08028 Barcelona, Spain
b
Received: September 29, 2009; Revised: November 24, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900817.
Abstract: A polymer-supported a,a-diarylprolinol
silyl ether displays catalytic activity and enantiose-
lectivity comparable to the best homogeneous cata-
lysts in the Michael addition of aldehydes to nitro-
approaches.[5] Among them, those mediated by enan-
tiopure pyrrolidines bearing a bulky C-2 substituent
have found wide application[6] and, in particular, (S)-
a,a-diarylprolinol silyl ethers (Jørgensen–Hayashi cat-
alysts) exhibit optimal performance for a variety of
donors and acceptors.[7]
Herein we report the preparation of a polystyrene-
supported, enantiopure (S)-a,a-diphenylprolinol tri-
methylsilyl ether (5) displaying high catalytic activity
and enantioselectivity in the Michael addition of alde-
hydes to nitroolefins with unprecedented, enzyme-
like substrate selectivity.
ACHTUNGTRENNUNGolefins. Above all, the combination of polymer
backbone, triazole linker, and catalytic unit confers
to it an unprecedented substrate selectivity in favor
of linear, short-chain aldehydes.
Keywords: asymmetric catalysis; Michael addition;
nitroolefins; organocatalysis; synthetic enzymes
The preparation of 5 from commercially available
N-Boc-(2S,4R)-4-hydroxyproline methyl ester via its
propargyloxy derivative (1) is shown in Scheme 1 (see
The design and preparation of immobilized catalysts[1] Supporting Information for details).
that keep intact the characteristics (activity and selec-
The silylation with concomitant carbamate depro-
tivity) of their homogeneous counterparts represents tection of 2 leads to the key intermediate 3, already
a major goal in view of more efficient chemical pro- containing the functional arrangement of the target
duction. When enantioselective processes are concer- catalyst. The immobilization of 3 onto azidomethyl-
ned,[1c] the opportunities offered by this approach (re- polystyrene using click chemistry, in turn, posed an
covery and reuse of expensive catalytic species, highly important synthetic challenge, since common catalysts
simplified work-up, implementation of continuous for the cycloaddition were deactivated by the free
flow processing) become even more evident.
amino group in the substrate. Gratifyingly enough,
In a continued effort towards this goal, we have the recently developed tris(triazolyl)methanol-copper
shown that a variety of organocatalytic processes can complex 4[8] efficiently catalyzed the immobilization
be most efficiently mediated by proline derivatives reaction, thus allowing the easy and highly reproduci-
supported onto polystyrene resins through 1,2,3-tri- ble synthesis of the catalytic resin 5.
A
The Michael addition[9] of propanal to b-nitrostyr-
backbone, triazole linker and catalytic unit leading to ene was selected as a model for the evaluation of 5
very high catalytic activity and enantioselectivity have and was studied under a variety of experimental con-
been observed.[2a,d] Most remarkably, the behavior of ditions (Table 1). In the initial set of experiments (en-
some of these catalytic resins is reminiscent of that of tries 1–4) a ten-fold excess of aldehyde donor was
polypeptides with enzyme activity.[2d] The Michael ad- used, according to the usual practice in this organoca-
dition of carbon nucleophiles to nitroolefins is a con- talytic process with homogeneous catalysts.[7b] Under
venient entry to versatile synthetic intermediates.[3] these conditions, dichloromethane proved itself as the
The reaction has been widely used as the first step in optimal solvent, reactions in it being faster and more
cascade processes,[4] and the most successful enantio- stereoselective in the absence of additives (entry 2).
selective versions of it are based on organocatalytic Quite interestingly, the use of a much more conven-
Adv. Synth. Catal. 2009, 351, 3051 – 3056
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Esther Alza and Miquel A. Pericꢀs
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Scheme 1. Synthesis of the polymer-supported organocatalyst 5.
Table 1. Screening of reaction conditions for the Michael addition of propanal to (E)-b-nitrostyrene.[a]
Entry
Solvent
Additive[b]
t [h]
Conversion [%][c]
syn/anti[c]
ee [%][d]
1[e]
2[e]
3[e]
4[e]
5
6
7
8
9
hexane:THF
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
H2O
none
none
36
7
24
24
7
23
2
24
48
40
97:3
97
>99
99
>99
>99
>99
>99
>99
>99
97
96:4
81:19
77:23
>99:1
86:14
87:13
96:4
–
DMAP
PhCOOH
none
DMAP
PhCOOH
DiMePEG
TFA
97
>99
>99
99
99
–
CH2Cl2
none
[a]
All reactions performed with 0.2 mmol of (E)-b-nitrostyrene, 0.3 or 2.0 mmol of propanal, and 0.02 mmol of 5 in 1 mL of
solvent at room temperature.
[b]
[c]
[d]
[e]
0.02 mmol.
1
Determined by H NMR of the reaction crude.
Determined by chiral HPLC analysis.
2 mmol of propanal were used.
ient 1.5:1 propanal to b-nitrostyrene ratio (entries 5– been summarized in Table 2. As a general trend, dia-
9) led to cleaner reaction crudes (aldehyde self-aldol stereo- and enantioselectivities achieved with 5 com-
reaction was efficiently suppressed) with increased pare well with those recorded with the most efficient
diastereoselectivity. The straightforward isolation of homogeneous organocatalysts. Quite unexpectedly,
the Michael adducts in the absence of additives, the catalytic activity of resin 5 was remarkably depen-
simply involving catalyst separation by filtration and dent on the aldehyde donor. Thus, a fast reaction was
concentration of the reaction crude (entry 5), strongly observed for linear, short chain aldehydes like propa-
favors these reaction conditions. On the other hand, it nal and butanal (entries 2 and 3), while further in-
is worth noting that 5 is also able to induce a highly creases in the chain length (entries 4 and 5) resulted
enantioselective Michael addition in water (entry 8), in significant extension of reaction time. In all these
and this represents the first example of an insoluble cases, yield and enantioselectivity of the major syn ad-
catalyst successfully dealing with aldehydes in this sol- ducts were excellent. Branching in the b position of
vent.[2c]
the aldehyde (6i and 6j, Figure 1) is deletereous for
The scope of the Michael addition mediated by 5 conversion, while a-branching (6k, Figure 1) com-
was next investigated. The results of this study have pletely blocks the reaction. Ketones like acetone and
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A Highly Selective, Polymer-Supported Organocatalyst for Michael Additions
COMMUNICATIONS
Table 2. Scope of the Michael addition of aldehydes to nitroolefins catalyzed by 5.[a]
Entry
1
Product
t [h]
Conversion[b] [%] (Yield [%])[c]
50 (44)
syn/anti[b]
ee[d] [%]
6a
6b
72
–
96
2
3
7
5
>99 (98)
>99 (93)
>99:1
>99
>99
6c
6d
6e
90:10
4
5
27
48
>99 (98)
82:18
75:25
99
98
99 (91)
6
7
6f
4
>99 (98)
91:9
98
6g
6h
8
4
>99 (94)
>99 (96)
89:11
85:15
99
90
8
[a]
All reactions performed with 0.2 mmol of nitroolefin, 0.3 mmol of aldehyde, and 0.02 mmol of 5 in 1 mL of CH2Cl2 at
room temperature.
Determined by H NMR of the reaction crude.
Isolated yield.
Determined by chiral HPLC analysis.
[b]
[c]
[d]
1
cyclohexanone, in turn, are completely unreactive. lectivities after short reaction times (Table 2, en-
With respect to Michael acceptors, nitroethylenes tries 6–8). Finally, resin 5 was also tested in the more
bearing b-aryl or hetaryl substituents with different demanding Michael addition of acetaldehyde to b-ni-
electronic characters were studied, the corresponding trostyrene (Table 2, entry 1), with results comparable
syn-adducts being obtained in excellent yields and se- to those reported in the literature for homogeneous
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Esther Alza and Miquel A. Pericꢀs
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entry 3), the reaction time required for complete con-
version (92% isolated yield) was substantially extend-
ed (24 vs. 5 h), and this suggested that unproductive
enamines involving 2-methylpropanal can be formed
during the reaction. The suggestion that substrate se-
lectivity in reactions mediated by 5 finds its origin in
the different reactivity of equilibrating enamine inter-
mediates is reinforced by the results of an experiment
where an equimolar mixture of pentanal and cyclo-
hexanone is treated with b-nitrostyrene (see Support-
ing Information for details). As anticipated, cyclohex-
anone did not participate in the addition process, but
Figure 1. Michael adducts whose formation is not efficiently
mediated by resin 5.
catalysts,[10] but using in the present case a much its presence in the reaction media extended the re-
smaller excess of acetaldehyde and half catalyst load- quired time for complete conversion from 27 h to
A
55 h. When the cyclohexanone:pentanal ratio was
In view of the high substrate selectivity exhibited changed to 13:1, the reaction time increased to 7
by resin 5, we envisaged a possible application in the days.
discrimination between linear and a-branched alde-
We have summarized in Scheme 3 our interpreta-
hyde donors for Michael additions. To test this possi- tion on the origin of the selectivity in the reactions
bility, the 1.6:1 mixture of butanal and 2-methylpropa- mediated by resin 5. First, the observed syn selectivity
nal obtained in the Rh-catalyzed hydroformylation of and the sign of enantioselection are indicative of the
propene[11] was treated with b-nitrostyrene in the intermediacy of conformationally biased enamines,
presence of 5 (5/b-nitrostyrene/butanal/2-methylpro- with the bulky R1 substituent on the pyrrolidine
panal: 0.1/1/2.4/1.5; see Scheme 2). Under these con- blocking one of the enamine faces. With respect to
ditions, only the linear aldehyde underwent Michael substrate selectivity, the retardant effect of bulky al-
addition. While the enantioselectivity of the reaction dehydes (or ketones) is strongly indicative of the par-
with butanal alone was preserved (99%, see Table 2, ticipation of these reagents in the reversible forma-
Scheme 2. Selective Michael addition of butanal to b-nitrostyrene in the presence of 2-methylpropanal catalyzed by 5.
Scheme 3. Origin of the substrate-selectivity in the Michael addition of aldehydes to b-nitrostyrene catalyzed by 5.
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A Highly Selective, Polymer-Supported Organocatalyst for Michael Additions
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Scheme 4. Reconditioning conditions for supported organocatalyst 5.
tion of unreactive enamine intermediates. The reason Table 3. Recycling experiments of catalyst 5 in the Michael
addition of propanal to 4-bromo-b-nitrostyrene.[a]
why even very similar enamines based on resin 5
could exhibit so strikingly different reactivities in
front of b-nitrostyrene must result from the topology
of the reaction cavity defined by the combination of
polymer backbone, triazole linker, and catalytic unit.
Thus, the enzyme-like selectivity exhibited by 5 would
obey to restrictions in the achievement of the re-
À
quired transition state geometry for C C bond forma-
tion whenever a bulky aldehyde is involved in the for-
mation of the putative enamine intermediate.
Cycle Conversion[b] [%] (Yield [%])[c] syn:anti[b] ee [%][d]
1
2
3
4
5
6
>99 (98)
>99 (96)
98 (96)
96 (94)
94 (92)
93:7
93:7
92:8
92:8
93:7
92:8
99
99
97
97
98
97
As already mentioned, one of the main advantages
associated to heterogenized catalysts is the possibility
of its easy recovery and reuse. While recovery can be
easily achieved by simple filtration when insoluble
polymers are employed, the possibility of catalyst
reuse is normally limited by deactivation processes. In
the case of a,a-diarylprolinol silyl ethers, deactivation
is triggered by hydrolysis of the labile silyl ether.[9r]
More precisely, we have observed that a resin analo-
gous to 5, but bearing free hydroxy substituents in-
stead of trimethylsilyl ethers is completely inactive in
the considered Michael reactions. We accordingly de-
voted some effort to the development of a simple pro-
cedure for error correction on resin 5. After testing a
91 (89)
[a]
All the experiments were performed using the general
method with the resin recovered from the previous run
and reconditioned before its use with trimethylsilyl N,N-
dimethylcarbamate in 0.1M hexane solution.
Determined by H NMR of the reaction crude.
Isolated yield.
[b]
[c]
[d]
1
Determined by chiral HPLC analysis.
variety of silylating agents, we found that a brief pletely selective reaction of a linear aldehyde in the
treatment of an inactive diphenylprolinol-type resin presence of its a-branched regioisomer. Extension of
with trimethylsilyl N,N-dimethylcarbamate[12] in the use of 5 to tandem processes is currently under-
hexane/acetonitrile leads to the selective protection of way in our laboratories.
the hydroxy groups with full recovery of catalytic ac-
tivity (Scheme 4). From a practical point of view, the
re-conditioning process leaves dimethylamine as the
only by-product, so that the resin can be immediately
reused after washing out any excess of silylating
Experimental Section
agent. In practice, the intercalation of catalytic and
re-conditioning cycles leads to complete preservation
Typical Experimental Procedure
Propionaldehyde (22 mL, 0.3 mmol) was added to a mixture
of trans-b-nitrostyrene (30 mg, 0.2 mmol) and 5 (46 mg,
0.02 mmol) in CH2Cl2 (1.0 mL) at room temperature. The
suspension was stirred for 7 h and then directly filtered off.
The solid resin was washed with CH2Cl2 and the organic fil-
trate was concentrated under reduced pressure. The Michael
adduct 6b was obtained without further purification as a
of the catalytic activity and stereoselectivity, thus al-
lowing effective reuse over six consecutive runs
(Table 3).
In summary, a highly efficient, polymer-supported
organocatalyst for Michael additions of aldehydes to
nitroolefins (5) has been prepared. Besides very high
catalytic activity and enantioselectivity, comparable to
those depicted by the best homogeneous catalysts in
the same process, 5 displays unprecedented substrate
1
clear oil; yield: 40.2 mg (98%); syn/anti 99:1 (by H NMR
spectroscopy), 99% ee by HPLC on a chiral phase (Chiral-
pak IC column, l=214 nm, ethanol/hexane 95:5, 0.8
selectivity that allows, in practice, inducing the com- mLminÀ1): tR =30.7 min (minor, syn), 36.8 min (major, syn).
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Esther Alza and Miquel A. Pericꢀs
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Acknowledgements
L. Cao, M. C. Ye, X. L. Sun, Y. Tang, Org. Lett. 2006,
8, 2901; l) L. Zu, J. Wang, H. Li, W. Wang, Org. Lett.
2006, 8, 3077.
We thank MICINN (grant CTQ2008-00947/BQU) and Con-
solider Ingenio 2010 (grant CSD2006-0003), DURSI (grant
2009SGR623), and the ICIQ Foundation for financial sup-
port. E. A. thanks the ICIQ Foundation for a predoctoral fel-
lowship. We also thank E. Cequier and S. Curreli (ICIQ Sup-
port Unit) for their help with chromatographic analysis and
Dr. S. Sayalero for her support in the writing of this manu-
script.
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