DNA-cellulose: An economical, fully recyclable and highly effective chiral biomaterial for asymmetric catalysis

Article abstract of DOI:10.1039/c4cc10190a

The challenge in DNA-based asymmetric catalysis is to perform a reaction in the vicinity of the helix by incorporating a small-molecule catalyst anchored to the DNA in a covalent, dative, or non-covalent yet stable fashion in order to ensure high levels o

Full text of DOI:10.1039/c4cc10190a

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DNA-cellulose: an economical, fully recyclable
and highly effective chiral biomaterial for
asymmetric catalysis†
Erica Benedetti,a Nicolas Duchemin,a Lucas Bethge,b Stefan Vonhoff,b
Sven Klussmann,b Jean-Jacques Vasseur,c Janine Cossy,a Michael Smietana*c and
Stellios Arseniyadis*a
Cite this:DOI: 10.1039/c4cc10190a
Received 20th December 2014,
Accepted 11th January 2015
DOI: 10.1039/c4cc10190a
www.rsc.org/chemcomm
The challenge in DNA-based asymmetric catalysis is to perform a
reaction in the vicinity of the helix by incorporating a small-
molecule catalyst anchored to the DNA in a covalent, dative, or
non-covalent yet stable fashion in order to ensure high levels of
enantio-discrimination. Here, we report the first generation of a
DNA-based catalyst bound to a cellulose matrix. The chiral bio-
material is commercially available, trivial to use, fully recyclable and
produces high levels of enantioselectivity in various Cu(II)-catalyzed
asymmetric reactions including Friedel–Crafts alkylations and
Michael additions. A single-pass, continuous-flow process is also
reported affording fast conversions and high enantioselectivities at
low catalyst loadings thus offering a new benchmark in the field of
DNA-based asymmetric catalysis.
Fig. 1
A cellulose-supported (CS) ct-DNA/Cu(dmbpy) biohybrid for
DNA-based asymmetric catalysis. [NuH = indoles, dimethylmalonate].
now facing scale and catalyst-recovery issues. While up to
2.4 mmol scale reactions have been reported albeit using large
DNA-based asymmetric catalysis offers great promise in the advance- amounts of DNA,6,19 there is to the best of our knowledge only one
ment of enantioselective artificial biohybrid-mediated catalysis. Intro- example featuring a recyclable solid-supported DNA. Indeed, Park,
duced in 2005 by Roelfes and Feringa,1 the concept has been since Sugiyama and co-workers recently synthesized an ammonium-
then successfully applied to a wide variety of copper(II)-catalyzed functionalized silica that was used to immobilize salmon testes
carbon–carbon, carbon–heteroatom and carbon–halogen bond form- DNA (st-DNA) through electrostatic interactions.27 Evaluated in the
ing reactions.2–14 While still in its early stage, the field is rapidly enantioselective Diels–Alder reaction, both the conversion and the
expanding with studies dedicated to DNA secondary structures,15–18 ees were in the range of those obtained using standard st-DNA.
DNA solvatation19–21 and to new anchoring strategies.22–25 In this
In our search for a robust, cheap and reusable solid-supported
context, we recently reported the first example of a left-helical enantio- strategy, we turned our attention to cellulose-supported DNA
selective induction using L-nucleic acids. This method allowed reliable (CS-DNA, Fig. 1).28 Indeed, the cellulose frameworks have attracted
and predictable access to both enantiomers for a given reaction.26
a lot of attention over the years due to their favourable biophysical
With the prospect of being used widely by both academic and properties, biocompatibility, low immunogenicity, relatively high
industrial organic chemists, DNA-based asymmetric catalysis is resistance to temperature and relatively low cost. Interestingly,
however, while CS-DNA has been widely used to either purify
sequence-specific DNA-binding proteins29 or to determine binding
a Laboratoire de Chimie Organique, Institute of Chemistry,
constants for non-specific interactions between proteins and
Biology and Innovation (CBI) - ESPCI ParisTech/CNRS (UMR8231)/PSL*
DNA,30 there are no examples of DNA-based asymmetric catalysis
Research University, 10 rue Vauquelin, 75231 Paris Cedex 05, France.
involving a cellulose-supported DNA scaffold. This is all the
more peculiar that double-stranded calf thymus DNA (ct-DNA)
covalently attached to cellulose is nowadays commercially avail-
able from several suppliers. Combined, all these properties
made cellulose a particularly appealing solid support with
potential use in DNA-based asymmetric catalysis; we report here
the results of our endeavours.
E-mail: stellios.arseniyadis@espci.fr
b NOXXON Pharma AG, Max-Dohrn-Strasse 8-10, 10589 Berlin, Germany
c
´
´
Institut des Biomolecules Max Mousseron UMR 5247 CNRS-Universites Montpellier
`
1 et 2 Place Eugene Bataillon, 34095 Montpellier, France.
E-mail: msmietana@um2.fr
† Electronic supplementary information (ESI) available: Details of experimental
procedures, 1H NMR and 13C NMR spectra as well as SFC chromatograms. See
DOI: 10.1039/c4cc10190a
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Table 1 Friedel–Crafts alkylation with CS-ct-DNA
In order to evaluate the efficacy of CS-ct-DNA in DNA-based
asymmetric catalysis, we first tested the Cu(II)-catalyzed Friedel–
Crafts alkylation of a,b-unsaturated 2-acyl imidazole 1a (0.6 mmol)
with 5-methoxyindole (3.0 mmol). The reaction was performed in a
20 mM MOPS buffer (pH 6.5) in the presence of 4,40-dimethyl-2,20-
bipyridine (dmbpy, 36 mol%), Cu(NO3)2 (30 mol%) and 163 mg of
the cellulose-supported double-stranded ct-DNA (4.3 mg of ct-DNA
per g of cellulose) over 3 days at 5 1C. Both the conversion and the
ee of the resulting product were determined by supercritical fluid
chromatography (SFC) analysis. To our delight complete conver-
sion of the starting enone was observed and the resulting product
was obtained in 81% ee (Table 1, entry 1), which was comparable
with the result obtained with unsupported ct-DNA (80% ee, Table 1,
entry 2). To ensure that the selectivity obtained was solely due to the
supported catalyst and not from any residual DNA that could have
potentially leaked from the solid support, the cellulose was filtered,
washed with a 20 mM MOPS buffer solution and re-engaged in a
second experiment under otherwise identical conditions. Once
again, the reaction afforded full conversion of 1a to the corres-
ponding Friedel–Crafts product 2a with no noticeable loss in either
reactivity or selectivity. Following these initial results and in order to
prove that the cellulose itself did not induce the selectivity due to its
inherent chirality, a control experiment using standard cellulose
was undertaken; the reaction yielded compound 2a in only 12% ee
(Table 1, entry 3). An additional reaction performed with CS-ct-DNA
in the absence of dmbpy showcased the importance of the ligand
not only as the product formed in a lower yield but also with barely
any selectivity (Table 1, entry 4).
Entry
Product
Conversiona (%)
eea (%)
1
499
499
499
81
80
12
2b
3c
4d
5
60
499
4
73
6
96
34
66
78
83
73
62
7
8
499
499
499
9
10
With these conditions in hand the reaction was eventually
applied to a variety of indoles with different substitution
patterns (Table 1, entries 5–7) as well as to a number of
a,b-unsaturated 2-acyl imidazoles (Table 1, entries 8–12). As a
general trend, the reaction tolerated both C3-aliphatic and
aromatic substituents on the enone as the corresponding
Friedel–Crafts products were obtained in essentially quantita-
tive yield and with ees ranging from 50% to 83% after 3 days at
5 1C. It is worth pointing out however that higher levels of
conversion and selectivity were obtained when electron-rich
indoles were used in conjunction with enones bearing an aliphatic
substituent at the C3 position (Table 1, entries 1 and 8).
11
12
80
54
50
499
13
14
499
76
65
25
Conditions: 2 mM base pair solution of DNA in a 20 mM MOPS solution
(400 mL; pH 6.5), 0.3 mM of Cu(dmbpy)(NO3)2 in a 20 mM MOPS
solution (200 mL), 0.5 M solution of enone in CH3CN (1.2 mL), 2.5 M
Prompted by these results, the CS-ct-DNA was also applied to
the Michael addition of dimethylmalonate (Table 2). Once again,
the products were obtained in high yields and excellent enantios-
electivities ranging from 81% to 97%, even though enones bearing
an electron-poor aromatic substituent appeared to be less reactive.
In order to fully investigate the robustness of the catalyst
and therefore its recyclability, a series of Cu(II)-catalyzed Friedel–
a
solution of indole in CH3CN (1.2 mL), 3 d, 5 1C. Determined by
b
supercritical phase chromatography (SFC) analysis. Reaction run
c
using unsupported ct-DNA. Reaction run using DNA-free cellulose.
d
Reaction run in the absence of dmbpy.
Crafts alkylations were performed using a,b-unsaturated 2-acyl the Cu(dmbpy) complex was to remain incorporated into the
imidazole 1a and 5-methoxyindole under the standard condi- DNA after each filtration. A control experiment using a recycled
tions (20 mM MOPS buffer, pH 6.5, 5 1C, 3 days). After each run, CS-ct-DNA in the absence of additional Cu and dmbpy afforded
the reaction was filtered and the cellulose was washed with a full conversion of the starting enone without any noticeable loss
20 mM MOPS buffer before being re-used. Interestingly, this of reactivity or selectivity. Remarkably, under these conditions
recycling procedure could be repeated only up to two times the cellulose could be recycled up to 10 times without adding any
before a slight decrease of the selectivity (4% loss on every cycle) Cu or dmbpy at every run (Fig. 2).31 Considering the amount of
could be observed. This prompted us to consider that the use of CS-ct-DNA and Cu(dmbpy) complex used in the process, this
additional Cu and dmbpy in every run could be detrimental if cellulose-supported approach is clearly advantageous as the
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Table 2 Friedel–Crafts alkylation with CS-ct-DNA
Table 3 CS-ct-DNA-catalysed Friedel–Crafts under continuous-flow
Scale
Flow rate
CS-ct-DNA Residence Conversiona eeb
Entry
1
Product
Conversiona (%)
92
eea (%)
97
Entry (mmol) (mL minꢀ1
)
(g)
time (min) (%)
(%)
1
2
3
4
0.03
0.03
0.03
0.3
0.25
1.1
2.2
4.4
4.4
5
19
38
38
60
80
79
79
78
0.125
0.125
0.125
83
96 (92c)
96 (89c)
2
3
14
96
81
Conditions: CS-Cu(dmbpy)-ct-DNA packed in a 4 g MPLC cartridge
(+ = 13 mm; L = 65 mm). a Determined by NMR on the crude reaction
b
mixture. Determined by supercritical phase chromatography (SFC)
c
499
analysis. Isolated yield.
CS-ct-DNA-Cu(dmbpy) biohybrid catalyst as well as the optimal flow-
rate required for the reaction to complete after a single run across the
column. When performing the reaction on a 0.03 mmol scale using a
1.1 g cartridge of CS-ct-DNA at a flow-rate of 0.25 mL minꢀ1, the
corresponding Friedel–Crafts product was obtained in 80% ee albeit
in only 60% yield (Table 3, entry 1). By decreasing the flow-rate to
0.125 mL minꢀ1 and doubling the length of the column (2.2 g
cartridge of CS-ct-DNA), 83% of the starting material were converted
with virtually the same selectivity (Table 3, entry 2). Eventually, the use
of a 4.4 g cartridge of CS-ct-DNA under otherwise identical conditions
led to roughly complete conversion of the starting enone and the
alkylated product was obtained in 92% yield and 79% ee (Table 3,
entry 3). Finally, increasing the reaction scale by a factor of 10
appeared not to be detrimental in terms of both conversion and
selectivity as the desired Friedel–Crafts product was isolated in 89%
yield and 78% ee (Table 3, entry 4).
4
5
68
25
93
89
Conditions: 2 mM base pair solution of DNA in a 20 mM MOPS solution
(400 mL; pH 6.5), 0.3 mM of Cu(dmbpy)(NO3)2 in a 20 mM MOPS
solution (200 mL), 0.5 M solution of enone in CH3CN (1.2 mL), pur
malonate (6.9 mL), 3 d, 5 1C. Determined by supercritical phase
chromatography (SFC) analysis.
a
In summary, we have developed a particularly appealing
cellulose-supported DNA-based catalyst that offers high levels of
enantioselectivity on various Cu(II)-catalyzed asymmetric reactions
including Friedel–Crafts alkylations and Michael additions. The
system has various advantages. Indeed, the chiral biomaterial is
commercially available, particularly robust and trivial to use. In
addition, the Cu(dmbpy) complex bound to the CS-ct-DNA can be
fully recycled after each run with no noticeable loss of reactivity or
selectivity. Most importantly, the CS-ct-DNA-Cu(dmbpy) biohybrid
catalyst can be implemented in a single-pass, continuous-flow
process allowing us to perform the reactions on a synthetically
useful scale using low catalyst loadings. Considering that the
grafting can be performed on any selected sequence and DNA
configuration, these results will undoubtedly contribute to the
development and generalization of DNA-based asymmetric catalysis.
Fig. 2 Investigation of the reusability of the CS-ct-DNA-Cu(dmbpy) bio-
hybrid catalyst.
entire catalytic system is recycled, thus highlighting the potential
of DNA-cellulose for large-scale applications.27
Having demonstrated the efficacy of our immobilized DNA-
based biohybrid catalyst in the context of asymmetric catalysis,
we next set out to implement the method to a continuous-flow
process.32,33 The experimental setup consisted of a low-pressure
chromatography column which was loaded with the CS-ct-DNA-
Cu(dmbpy) biohybrid catalyst and connected to a syringe pump
used to feed the reactor with the reagents. As no reaction takes place
in the absence of the Cu(dmbpy) complex, we were able to pump
both reagents together in a 20 mM MOPS buffer/MeOH (30 : 1)
solution.34 It is worth emphasizing however that this ratio was
critical to prevent any loss of selectivity as, for a reason that still
remains unclear, higher amounts of MeOH led to lower ees. More-
over, in order to be effective, we needed to determine the amount of
`
This research was supported by the Ministere de l’Enseigne-
´
ment Superieur et de la Recherche and the Agence Nationale de
la Recherche (NCiS; ANR-2010-JCJC-715-1).
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