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COMMUNICATIONMichael Smietana, Stellios Arseniyadis et al.DNA-cellulose: an economical, fully recyclable and highly eff ective chiral biomaterial for asymmetric catalysis

Volume 51 Number 28 11 April 2015 Pages 60256232

6076 | Chem. Commun., 2015, 51, 6076--6079 This journal isThe Royal Society of Chemistry 2015

Cite this:Chem. Commun., 2015,51, 6076

DNA-cellulose: an economical, fully recyclableand highly effective chiral biomaterial forasymmetric 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 andStellios Arseniyadis*a

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 FriedelCrafts 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.

DNA-based asymmetric catalysis offers great promise in the advance-ment of enantioselective artificial biohybrid-mediated catalysis. Intro-duced in 2005 by Roelfes and Feringa,1 the concept has been sincethen successfully applied to a wide variety of copper(II)-catalyzedcarboncarbon, carbonheteroatom and carbonhalogen bond form-ing reactions.214 While still in its early stage, the field is rapidlyexpanding with studies dedicated to DNA secondary structures,1518

DNA solvatation1921 and to new anchoring strategies.2225 In thiscontext, we recently reported the first example of a left-helical enantio-selective induction using L-nucleic acids. This method allowed reliableand predictable access to both enantiomers for a given reaction.26

With the prospect of being used widely by both academic andindustrial organic chemists, DNA-based asymmetric catalysis is

now facing scale and catalyst-recovery issues. While up to2.4 mmol scale reactions have been reported albeit using largeamounts of DNA,6,19 there is to the best of our knowledge only oneexample featuring a recyclable solid-supported DNA. Indeed, Park,Sugiyama and co-workers recently synthesized an ammonium-functionalized silica that was used to immobilize salmon testesDNA (st-DNA) through electrostatic interactions.27 Evaluated in theenantioselective DielsAlder reaction, both the conversion and theees were in the range of those obtained using standard st-DNA.

In our search for a robust, cheap and reusable solid-supportedstrategy, we turned our attention to cellulose-supported DNA(CS-DNA, Fig. 1).28 Indeed, the cellulose frameworks have attracteda lot of attention over the years due to their favourable biophysicalproperties, biocompatibility, low immunogenicity, relatively highresistance to temperature and relatively low cost. Interestingly,however, while CS-DNA has been widely used to either purifysequence-specific DNA-binding proteins29 or to determine bindingconstants for non-specific interactions between proteins andDNA,30 there are no examples of DNA-based asymmetric catalysisinvolving a cellulose-supported DNA scaffold. This is all themore peculiar that double-stranded calf thymus DNA (ct-DNA)covalently attached to cellulose is nowadays commercially avail-able from several suppliers. Combined, all these propertiesmade cellulose a particularly appealing solid support withpotential use in DNA-based asymmetric catalysis; we report herethe results of our endeavours.

Fig. 1 A cellulose-supported (CS) ct-DNA/Cu(dmbpy) biohybrid forDNA-based asymmetric catalysis. [NuH = indoles, dimethylmalonate].

a Laboratoire de Chimie Organique, Institute of Chemistry,

Biology and Innovation (CBI) - ESPCI ParisTech/CNRS (UMR8231)/PSL*

Research University, 10 rue Vauquelin, 75231 Paris Cedex 05, France.

E-mail: stellios.arseniyadis@espci.frb NOXXON Pharma AG, Max-Dohrn-Strasse 8-10, 10589 Berlin, Germanyc 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 experimentalprocedures, 1H NMR and 13C NMR spectra as well as SFC chromatograms. SeeDOI: 10.1039/c4cc10190a

Received 20th December 2014,Accepted 11th January 2015

DOI: 10.1039/c4cc10190a

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In order to evaluate the efficacy of CS-ct-DNA in DNA-basedasymmetric catalysis, we first tested the Cu(II)-catalyzed FriedelCrafts alkylation of a,b-unsaturated 2-acyl imidazole 1a (0.6 mmol)with 5-methoxyindole (3.0 mmol). The reaction was performed in a20 mMMOPS 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 ofthe cellulose-supported double-stranded ct-DNA (4.3 mg of ct-DNAper g of cellulose) over 3 days at 5 1C. Both the conversion and theee of the resulting product were determined by supercritical fluidchromatography (SFC) analysis. To our delight complete conver-sion of the starting enone was observed and the resulting productwas obtained in 81% ee (Table 1, entry 1), which was comparablewith the result obtained with unsupported ct-DNA (80% ee, Table 1,entry 2). To ensure that the selectivity obtained was solely due to thesupported catalyst and not from any residual DNA that could havepotentially leaked from the solid support, the cellulose was filtered,washed with a 20 mM MOPS buffer solution and re-engaged in asecond experiment under otherwise identical conditions. Onceagain, the reaction afforded full conversion of 1a to the corres-ponding FriedelCrafts product 2a with no noticeable loss in eitherreactivity or selectivity. Following these initial results and in order toprove that the cellulose itself did not induce the selectivity due to itsinherent chirality, a control experiment using standard cellulosewas undertaken; the reaction yielded compound 2a in only 12% ee(Table 1, entry 3). An additional reaction performed with CS-ct-DNAin the absence of dmbpy showcased the importance of the ligandnot only as the product formed in a lower yield but also with barelyany selectivity (Table 1, entry 4).

With these conditions in hand the reaction was eventuallyapplied to a variety of indoles with different substitutionpatterns (Table 1, entries 57) as well as to a number ofa,b-unsaturated 2-acyl imidazoles (Table 1, entries 812). As ageneral trend, the reaction tolerated both C3-aliphatic andaromatic substituents on the enone as the correspondingFriedelCrafts products were obtained in essentially quantita-tive yield and with ees ranging from 50% to 83% after 3 days at5 1C. It is worth pointing out however that higher levels ofconversion and selectivity were obtained when electron-richindoles were used in conjunction with enones bearing an aliphaticsubstituent at the C3 position (Table 1, entries 1 and 8).

Prompted by these results, the CS-ct-DNA was also applied tothe 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 bearingan electron-poor aromatic substituent appeared to be less reactive.

In order to fully investigate the robustness of the catalystand therefore its recyclability, a series of Cu(II)-catalyzed FriedelCrafts alkylations were performed using a,b-unsaturated 2-acylimidazole 1a and 5-methoxyindole under the standard condi-tions (20 mMMOPS buffer, pH 6.5, 5 1C, 3 days). After each run,the reaction was filtered and the cellulose was washed with a20 mM MOPS buffer before being re-used. Interestingly, thisrecycling procedure could be repeated only up to two timesbefore a slight decrease of the selectivity (4% loss on every cycle)could be observed. This prompted us to consider that the use ofadditional Cu and dmbpy in every run could be detrimental if

the Cu(dmbpy) complex was to remain incorporated into theDNA after each filtration. A control experiment using a recycledCS-ct-DNA in the absence of additional Cu and dmbpy affordedfull conversion of the starting enone without any noticeable lossof reactivity or selectivity. Remarkably, under these conditionsthe cellulose could be recycled up to 10 times without adding anyCu or dmbpy at every run (Fig. 2).31 Considering the amount ofCS-ct-DNA and Cu(dmbpy) complex used in the process, thiscellulose-supported approach is clearly advantageous as the

Table 1 FriedelCrafts alkylation with CS-ct-DNA

Entry Product Conversiona (%) eea (%)

1 499 812b 499 803c 499 12

4d 60 45 499 73

6 96 66

7 34 78

8 499 83

9 499 73

10 499 62

11 80 54

12 499 50

13 499 76

14 25 65

Conditions: 2 mM base pair solution of DNA in a 20 mMMOPS solution(400 mL; pH 6.5), 0.3 mM of Cu(dmbpy)(NO3)2 in a 20 mM MOPSsolution (200 mL), 0.5 M solution of enone in CH3CN (1.2 mL), 2.5 Msolution of indole in CH3CN (1.2 mL), 3 d, 5 1C.

a Determined bysupercritical phase chromatography (SFC) analysis. b Reaction runusing unsupported ct-DNA. c Reaction run using DNA-free cellulose.d Reaction run in the absence of dmbpy.

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6078 | Chem. Commun., 2015, 51, 6076--6079 This journal isThe Royal Society of Chemistry 2015

entire catalytic system is recycled, thus highlighting the potentialof 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-flowprocess.32,33 The experimental setup consisted of a low-pressurechromatography column which was loaded with the CS-ct-DNA-Cu(dmbpy) biohybrid catalyst and connected to a syringe pumpused to feed the reactor with the reagents. As no reaction takes placein the absence of the Cu(dmbpy) complex, we were able to pumpboth reagents together in a 20 mM MOPS buffer/MeOH (30 :1)solution.34 It is worth emphasizing however that this ratio wascritical to prevent any loss of selectivity as, for a reason that stillremains unclear, higher amounts of MeOH led to lower ees. More-over, in order to be effective, we needed to determine the amount of

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 thecolumn.When performing the reaction on a 0.03mmol scale using a1.1 g cartridge of CS-ct-DNA at a flow-rate of 0.25 mL min1, thecorresponding FriedelCrafts product was obtained in 80% ee albeitin only 60% yield (Table 3, entry 1). By decreasing the flow-rate to0.125 mL min1 and doubling the length of the column (2.2 gcartridge of CS-ct-DNA), 83% of the starting material were convertedwith virtually the same selectivity (Table 3, entry 2). Eventually, the useof a 4.4 g cartridge of CS-ct-DNA under otherwise identical conditionsled to roughly complete conversion of the starting enone and thealkylated product was obtained in 92% yield and 79% ee (Table 3,entry 3). Finally, increasing the reaction scale by a factor of 10appeared not to be detrimental in terms of both conversion andselectivity as the desired FriedelCrafts product was isolated in 89%yield and 78% ee (Table 3, entry 4).

In summary, we have developed a particularly appealingcellulose-supported DNA-based catalyst that offers high levels ofenantioselectivity on various Cu(II)-catalyzed asymmetric reactionsincluding FriedelCrafts alkylations and Michael additions. Thesystem has various advantages. Indeed, the chiral biomaterial iscommercially available, particularly robust and trivial to use. Inaddition, the Cu(dmbpy) complex bound to the CS-ct-DNA can befully recycled after each run with no noticeable loss of reactivity orselectivity. Most importantly, the CS-ct-DNA-Cu(dmbpy) biohybridcatalyst can be implemented in a single-pass, continuous-flowprocess allowing us to perform the reactions on a syntheticallyuseful scale using low catalyst loadings. Considering that thegrafting can be performed on any selected sequence and DNAconfiguration, these results will undoubtedly contribute to thedevelopment and generalization of DNA-based asymmetric catalysis.

This research was supported by the Ministere de lEnseigne-ment Superieur et de la Recherche and the Agence Nationale dela Recherche (NCiS; ANR-2010-JCJC-715-1).

Notes and references1 (a) G. Roelfes and B. L. Feringa, Angew. Chem., Int. Ed., 2005, 44,

32303232; (b) A. J. Boersma, B. L. Feringa and G. Roelfes, Org. Lett.,2007, 9, 36473650.

2 D. Coquiere, B. L. Feringa and G. Roelfes, Angew. Chem., Int. Ed.,2007, 46, 93089311.

Fig. 2 Investigation of the reusability of the CS-ct-DNA-Cu(dmbpy) bio-hybrid catalyst.

Table 2 FriedelCrafts alkylation with CS-ct-DNA

Entry Product Conversiona (%) eea (%)

1 92 97

2 14 96

3 499 81

4 68 93

5 25 89

Conditions: 2 mM base pair solution of DNA in a 20 mMMOPS solution(400 mL; pH 6.5), 0.3 mM of Cu(dmbpy)(NO3)2 in a 20 mM MOPSsolution (200 mL), 0.5 M solution of enone in CH3CN (1.2 mL), purmalonate (6.9 mL), 3 d, 5 1C. a Determined by supercritical phasechromatography (SFC) analysis.

Table 3 CS-ct-DNA-catalysed FriedelCrafts under continuous-flow

EntryScale(mmol)

Flow rate(mL min1)

CS-ct-DNA(g)

Residencetime (min)

Conversiona

(%)eeb

(%)

1 0.03 0.25 1.1 5 60 802 0.03 0.125 2.2 19 83 793 0.03 0.125 4.4 38 96 (92c) 794 0.3 0.125 4.4 38 96 (89c) 78

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 reactionmixture. b Determined by supercritical phase chromatography (SFC)analysis. c Isolated yield.

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34 The addition of a small amount of MeOH was necessary in order toobtain a homogeneous solution of both reactants without observingany loss of selectivity (see ESI for more details).

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