On the role of DNA in DNA-based catalytic enantioselective conjugate addition reactions

Article abstract of DOI:10.1039/c005048b

A kinetic study of DNA-based catalytic enantioselective Friedel-Crafts alkylation and Michael addition reactions showed that DNA affects the rate of these reactions significantly. Whereas in the presence of DNA, a large acceleration was found for the Friedel-Crafts alkylation and a modest acceleration in the Michael addition of dimethyl malonate, a deceleration was observed when using nitromethane as nucleophile. Also, the enantioselectivities proved to be dependent on the DNA sequence. In comparison with the previously reported Diels-Alder reaction, the results presented here suggest that DNA plays a similar role in both cycloaddition and conjugate addition reactions.

Full text of DOI:10.1039/c005048b

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
On the Role of DNA in DNA-based Catalytic Enantioselective Conjugate
Addition Reactions†
Ewold W. Dijk, Arnold J. Boersma, Ben L. Feringa* and Gerard Roelfes*
Received 13th April 2010, Accepted 1st June 2010
First published as an Advance Article on the web 25th June 2010
DOI: 10.1039/c005048b
A kinetic study of DNA-based catalytic enantioselective Friedel–Crafts alkylation and Michael
addition reactions showed that DNA affects the rate of these reactions significantly. Whereas in the
presence of DNA, a large acceleration was found for the Friedel–Crafts alkylation and a modest
acceleration in the Michael addition of dimethyl malonate, a deceleration was observed when using
nitromethane as nucleophile. Also, the enantioselectivities proved to be dependent on the DNA
sequence. In comparison with the previously reported Diels–Alder reaction, the results presented here
suggest that DNA plays a similar role in both cycloaddition and conjugate addition reactions.
in the reactions can be quite different, depending on the nature of
the copper complex. Using ligands of the first generation, which
comprise an acridine intercalator that is attached via a spacer to
an aminomethyl pyridine metal binding moiety, it was found that
Introduction
DNA has emerged as a versatile scaffold for synthesis and
catalysis.1,2,3 Particularly its ubiquitous chiral structure is attractive
the catalyzed reaction is slightly slower in the presence of DNA.11
Hence, in this case the role of DNA is primarily that of chiral
scaffold. In contrast, using ligands of the second generation, in
particular 4,4¢-dimethyl-2,2¢-bipyridine (dmbipy), the Diels–Alder
reaction proved to be DNA-accelerated; rate accelerations of up to
two orders of magnitude were observed in the presence of DNA.12
In the latter case the DNA is more than a chiral scaffold and
apparently participates actively in the catalytic event. Moreover, it
was found that both the enantioselectivity and the rate acceleration
were DNA sequence dependent, with the sequences that give rise
to the highest ee’s in the catalyzed reactions, also result in the
strongest acceleration of this reaction.
In the light of the intriguing role of DNA in the Diels–Alder
reaction, it was decided to explore the effect of DNA on the
kinetics of the DNA-based copper catalyzed Michael addition
and Friedel–Crafts alkylation reaction (Scheme 1). Both these
reactions involve a conjugate addition of an anionic enolate-type
or a neutral p nucleophile, respectively, to an a,b-unsaturated 2-
acyl imidazole. Preliminary studies have indicated that in some
cases indeed the reaction was DNA-accelerated. The goal of the
present study was to determine the effect of DNA on the kinetics
of DNA-based copper catalyzed conjugate additions, using Cu-
dmbipy, which proved to be the most enantioselective catalysts in
all these cases. Furthermore, a comparison between the observed
effects in the Michael addition, Friedel–Crafts and Diels–Alder
reaction is made, which sheds new light on the role of DNA in
these reactions.
for applications in enantioselective catalysis. In our concept of
DNA-based asymmetric catalysis, transfer of chirality is achieved
by placing a catalytically active metal complex, based on a non-
chiral ligand, in close proximity to the DNA-helix, using non-
covalent interactions.4 As a result the catalyzed reaction is directed
towards the selective formation of one of the enantiomers of a
chiral product, resulting in an enantiomeric excess (Fig. 1). This
concept has been applied successfully to some of the archetypical
C–C bond forming reactions such as the copper catalyzed Diels–
Alder,5 Michael addition6 and Friedel–Crafts alkylation reaction;7
in all cases ee’s > 90% could be achieved. Recently, using this and
related approaches, the scope of DNA-based catalysis has been
extended to include hydrolytic kinetic resolution of epoxides,8
allylic aminations9 and aldol reactions.10
Fig. 1 Schematic representation of the concept of DNA-based asymmet-
ric catalysis and the CuII complex of 4,4¢-dimethyl-2,2¢-bipyridine.
Results and Discussion
In case of the DNA-based copper catalyzed Diels–Alder
reaction, kinetic studies have demonstrated that the role of DNA
Since in both the Michael additions and Friedel–Crafts alkyla-
tions the highest enantioselectivities were obtained with Cu2+-
dmbipy/salmon testes DNA (st-DNA), this catalytic system was
selected for further study. First-order reactions with respect to
the nucleophiles were observed by time-dependent UV measure-
ments both in the absence and presence of DNA, following the
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh
4, 9747 AG Groningen, The Netherlands. E-mail: b.l.feringa@rug.nl,
j.g.roelfes@rug.nl.; Fax: +31503634296
† Electronic supplementary information (ESI) available: characterization
details of all reaction products
3868 | Org. Biomol. Chem., 2010, 8, 3868–3873
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Scheme 1 DNA-based catalytic Friedel–Crafts alkylation (A) and Michael addition reactions (B).
methods developed by Engberts et al.13 A large excess of Cu2+-
dmbipy/st-DNA with respect to the substrate was used, so that
the dissociation of the addition product from the complex could
be ignored in the overall reaction rate. Care should be exercised
in extrapolating the data to turnover conditions, as it has been
observed in some cases that the product dissociation step is rate
limiting.14
Table 1 Apparent second-order rate constants for the Friedel–Crafts
reaction of substituted indoles to various enones.a
Entry Product Ee (%)b kapp (DNA)c kapp (no DNA)c
kDNA/knoDNA
1
2
3
4
5
6
7
3a
3b
3c
3d
3e
3f
57
79
66
69
75
83
80
7.75 ¥ 10-2
0.135
4.31 ¥ 10-3
6.45 ¥ 10-3
8.96 ¥ 10-3
3.73 ¥ 10-3
7.02 ¥ 10-3
3.10 ¥ 10-2
2.72 ¥ 10-2
18
21
26
12
9.0
27
16
0.235
4.62 ¥ 10-2
6.33 ¥ 10-2
0.842
3g
0.429
Friedel–Crafts Alkylations
a Determined at 25 0.2 ◦C. b Determined before on a semi-preparative
scale.7 c Values are given in M-1s-1, values are based on at least 4
experiments; errors in the rate constants were ≤ 9% in all cases.
The kinetics of the asymmetric DNA-based Friedel–Crafts reac-
tion of 2a with 1f were briefly studied before and a significant
30 fold rate acceleration was observed in the presence of st-
DNA.7 The generality of this phenomenon was investigated by
a study of the effect of substituents at both the enone and the
indole moiety. The results are shown in Table 1. In all cases,
the presence of st-DNA had a significant positive effect on the
rate of the reaction; rate accelerations ranged from 9–27 fold,
depending on the enone used. The fastest reactions as well as
the highest rate accelerations were found with enones carrying
electron-withdrawing substituents such as 4-Cl or 4-Br at the
aromatic moiety of the enone (entries 2–3). On the other hand, an
electron-donating substituent such as 4-OCH3 (entry 4) resulted
in a slower reaction and a less pronounced acceleration by DNA
than for the parent enone 1a. Using N-methylindole 2b as the p-
nucleophile resulted in a smaller rate enhancement in the presence
of DNA (entry 5). For the enones carrying an aliphatic substituent
at the b carbon, an increase of the chain length resulted in a slightly
lower addition rate, especially in the presence of DNA (entries 6
and 7). In general, no clear trend between the DNA acceleration
factor and the ee of the product is apparent, which is in contrast
with observations made previously for the Diels–Alder reaction.12
Michael addition
The kinetic values for the Michael addition reaction of dimethyl
malonate and nitromethane to 1a are listed in Table 2. The Michael
addition of dimethylmalonate and nitromethane to 1a to give the
Michael adducts 5 and 6, respectively, was studied at pH 5.5, 6.5
and pH 7.5. The rate of the reactions was pH dependent with the
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Table 2 The effect of pH on the ee and the rate of the Michael addition
of dimethylmalonate and nitromethane to 1a
Table 3 Sequence-dependent enantioselectivity and apparent second-
order rate constantsa in the DNA-based catalytic Friedel–Crafts alkylation
and Michael addition reactions
Entry pH Ee (%)a kapp (DNA)b
kapp (no DNA)b
kDNA/kno DNA
Entry Product DNA
Friedel–Crafts reaction:
Ee (%)a
k
app/M-1 s-1b
With CH2(CO2CH3)2:
1c
2
5.5
6.5
7.5
95
92
90
n.d.d
n.d.d
3.3 ¥ 10-3
6.7 ¥ 10-3
1.0 ¥ 10-3
1.1 ¥ 10-3e
3.3
6.1
1
2
3
4
5
6
3d
st-DNA
d(TCA GGG CCC TGA)2 49
d(TCA GAG CTC TGA)2
st-DNA
d(TCA GGG CCC TGA)2 93
69
3.10 ¥ 10-2
1.52 ¥ 10-2
1.45 ¥ 10-2
0.63
0.66
0.31
3
With CH3NO2:
32
83
4c
5
5.5
6.5
7.5
74
85
77
n.d.d
n.d.d
3f
1.2 ¥ 10-4
6.0 ¥ 10-4
5.0 ¥ 10-4
1.1 ¥ 10-3
0.24
0.55
6
d(TCA GAG CTC TGA)2
65
Michael addition:
a experiments performed on a semi-preparative scale using 100 eq of
CH(CO2CH3)2 or 1000 eq CH3NO2.6 b Determined in duplicate, values
are given in M-1 s-1, errors in the rate constants were ≤ 15% in all cases.
c MES buffer was used instead of MOPS. d n.d. = not determined: values
were too low to be accurately determined. e Reactions were run in triplicate
using the same batch of or Cu2+-dmbipy solution with a fixed excess of the
nucleophile (300 eq.).
7
8
5
d(TC GGG AT CCC GA)
d(TCA GGG CCC TGA)
d(TC GG AA TT CC GA) 76
d(TCG CGA TCG CGA)
d(TCG CGT ACG CGA)
st-DNA
86
93
2.2 ¥ 10-3
2.6 ¥ 10-3
2.7 ¥ 10-3
1.2 ¥ 10-3
1.5 ¥ 10-3
6.5 ¥ 10-3
1.9 ¥ 10-3
9
10
11
12
13
71
74
90
—
no DNA
a Determined under standard conditions.6,7 b Determined at 18 0.2 ◦C,
values are based on at least 3 experiments, errors are <9% in for the
Friedel–Crafts reactions (entries 1–6) and <20% for the Michael addition
reactions (entries 7–13).
fastest reaction observed at pH 7.5 (entries 3 and 6). This was
expected since the actual nucleophilic species in the reactions is
the enolate of dimethyl malonate and nitromethane. At pH 5.5
(using MES buffer instead of MOPS), incomplete conversion and
significantly lower enantioselectivity was observed for the addition
of nitromethane to 1a to give 6 (entry 4). Dimethylmalonate as
nucleophile did result in full conversion at pH 5.5 after 3 days,
giving 5 in a slightly higher ee than at pH 6.5 (entry 2). The reaction
rates for the addition of both nucleophiles at pH 5.5 were very low,
and no reliable data could be extracted from time-dependent UV
spectroscopy measurements.
In general, the reactions with dimethyl malonate (entries 1–3)
are significantly faster than those with nitromethane (entries 4–6).
From the pKa values of both nucleophiles, an opposite effect would
be expected; nitromethane’s pKA of ~10 in water15 is 3 units lower
than that of dimethylmalonate (pKA around 13).16 This means that
in the buffer used, at a given initial concentration of nucleophile,
approximately 1000 times more of deprotonated nitromethane is
present than of deprotonated dimethylmalonate. However, this
is not reflected in the reaction rates of the respective addition
reactions in the presence or absence of DNA.
The observation that pKa values and nucleophilicities are not
correlated was made before.17 Indeed, the anion of dimethyl-
malonate is about 3 orders of magnitude more nucleophilic
than the anion of nitromethane for the nucleophilic addition
to benzhydrilium ions in methanol.17c This was attributed to
the notion that reactivity of these anions is controlled more by
solvation than by their intrinsic properties, such as their basicities.
The effect of DNA on the reactions is also quite different
for both Michael donors. The reactions with nitromethane are
decelerated in the presence of DNA; 2–4 fold lower reaction rates
were observed. In contrast, the addition of dimethylmalonate to
1a is moderately accelerated by the presence of DNA. Indeed,
in reactions at a preparative scale, a much smaller excess of
dimethylmalonate is needed to obtain full conversion in the
addition, as compared to nitromethane.
ee of the reaction product has been observed (Table 3, entries 1–
6). In the reaction with 1e, the self-complementary oligonucleotide
d(TCA GGG CCC TGA)2 gave an increase of enantioselectivity
from 83% (st-DNA, entry 4) to 93% (entry 5). In contrast,
the reaction with 1d showed a considerably lower ee with this
oligonucleotide than with st-DNA (49% and 69%, respectively)
(entries 1 and 2). In view of the significant rate acceleration of the
Friedel–Crafts reaction by DNA, sequence-dependent reaction
rates were measured to determine whether the rate of reaction is
correlated to the ee of the product, as was the case in the DNA-
based asymmetric Diels–Alder reaction. Oligonucleotide d(TCA
GAG CTC TGA)2, inducing a lower ee than st-DNA in both
adducts 3d and 3f, was included in the study as well (entries 3 and
6).
The influence of the DNA sequence on the reaction rate is
small, despite the substantial effect on the enantioselectivity of the
product. No significant rate increase caused by the oligonucleotide
d(TCA GGG CCC TGA)2 relative to st-DNA in the addition of 2a
to 1e was observed (entries 4 and 5). The oligonucleotide d(TCA
GAG CTC TGA)2, which induces a lower ee in 3f than st-DNA,
also gives a lower reaction rate, although the effect is rather small
(entry 6). The same holds for the reaction between 1d and 2a,
displaying a lower enantioselectivity using both oligonucleotides
than using st-DNA (entries 1–3); in these cases reaction rates are
a factor 2 lower compared to st-DNA.
For the Michael reaction of enone 1a with dimethylmalonate,
apparent rate constants and ee’s were determined for a selection
of synthetic oligonucleotides (entries 7–11). Oligonucleotide d(TC
GGG AT CCC GA)2 was chosen as the parent sequence because
the binding mode of a few transition metal complexes to this
synthetic duplex has been studied.18,19,20 Variation in the number
of consecutive deoxyguanines (3, 2 or 1) and the central AT
sequence (present, absent or inversed) showed interesting effects
in the DNA-based Diels–Alder reaction. Indeed, the highest ee,
that is 93% (entry 8), for the Michael addition were found with
d(TCAGGGCCCTGA)2 which also gave the highest ee’s in the
Diels–Alder reaction of azachalcone with cyclopentadiene and the
DNA sequence dependence of conjugate additions
For the Friedel–Crafts alkylation of 5-methoxyindole 2a by the
enones 1d and 1e, a remarkable effect of the DNA sequence on the
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Friedel–Crafts alkylation of 2a with 1e. There appears to be some
relation between the ee and the apparent rate constant, albeit that
the effects are too small to justify any conclusions. It is likely that
the optimal DNA sequence has not been identified yet, as catalysis
in the presence of st-DNA proceeds at least 2.5 times faster than
in the presence of the tested synthetic oligonucleotides.
in excess.6,7,21 This suggests a similar enantiodiscriminating mech-
anism applies to all three reaction class investigated.
Also in all cases the DNA sequence has an influence on the
rate of the reaction, albeit that this effect is less pronounced in
the conjugate addition reactions than in the Diels–Alder reaction.
In the Diels–Alder reaction, rate accelerations of up to 2 orders
of magnitude were found whereas in the Friedel–Crafts reaction
this is up to 1 order of magnitude and in the Michael addition of
dimethyl malonate only a 3–4 fold increase was observed. However,
in the latter case the repulsive interaction between the anionic
nucleophile and the DNA negatively affects the rate acceleration
(vide supra).
Combined these observations suggest that DNA plays a similar
role in the Diels–Alder, Michael addition and Friedel–Crafts
alkylation reactions. Since these reactions involve structurally
different activated complexes, it is unlikely that transition state
stabilization by the DNA is a major contributor to all. Instead,
these three reaction classes have a structurally very similar ground
state. Therefore, it is hypothesized that it is here that DNA exerts
its influence, resulting in enantioselectivity and rate acceleration.
The Michael addition of nitromethane is the only exception to
date, since it is decelerated by the presence of DNA. At present the
origin of this deceleration is unknown and will be studied further.
Comparison Friedel–Crafts vs. Michael reaction
From the enantioselectivities and kinetic data presented above it is
apparent that the DNA-based catalytic asymmetric Michael addi-
tions and Friedel–Crafts alkylations, both conjugate addition reac-
tions, have some common characteristics, but there are also notable
differences. Both reactions types can be catalyzed efficiently by a
DNA-based catalyst, giving rise to excellent enantioselectivities.
Moreover, the enantioselectivities are DNA-sequence dependent,
but different substrates have different sequence requirements.
However, a major difference is the effect DNA has on the rate of
the catalyzed reactions. The presence of DNA causes significant
rate accelerations up to 27 fold in the Friedel–Crafts alkylation.
However, in the Michael addition with dimethylmalonate only
a modest rate acceleration, that is, up to 3-fold, was found in
the presence of DNA. Using nitromethane as the Michael donor
even a significant deceleration of the reaction was observed.
These differences are tentatively explained by considering the
nature of the nucleophilic species. Whereas the Friedel–Crafts
alkylations involve conjugate addition of a charge neutral aromatic
p-nucleophile, the Michael addition involves an anionic enolate-
type nucleophile, which can be expected to experience significant
charge repulsion from the negatively charged DNA backbone.
Hence, approach of the nucleophile to the copper coordinated
enone, which is bound to the DNA, is unfavorable resulting in a
less accelerated or even decelerated reaction.
Experimental
Catalytic experiments, general procedure. To an ice-cold so-
lution (prepared 24 h in advance) of DNA (final concentra-
tion 1.3 mg mL-1 or 2 mM in base pairs) in MOPS buffer
(final concentration 20 mM, pH 6.5 or 7.5), a solution of
[Cu(dmbipy)(NO3)2] in doubly distilled water was added to a
final concentration of 0.3 mM. The solution was equilibrated
by gentle mixing, and the enone was added as a stock solution
in acetonitrile. The nucleophile was added neat or as a stock
solution in acetonitrile, and the reaction was mixed by continuous
inversion at 5 ◦C. Experiments were run until full conversion was
obtained, unless noted otherwise. After the indicated time,6,7 the
reaction mixture was extracted with ethyl acetate (3 ¥ 5 mL). After
drying (Na2SO4) and removal of the solvent, the crude product was
Considering this, the good ee’s obtained for 6 are particularly
noteworthy, considering that the binding constant Kb = 1.12
0.02 ¥ 104 M-1,5 which means that ~5% unbound Cu2+-dmbipy
is present in solution. The free complex, that is, not bound to
DNA, catalyzes the addition of nitromethane up to 5 times more
efficiently, giving rise to racemic product. Hence, ee’s up to 85%
can only be achieved if the conjugate addition catalyzed by Cu2+-
dmbipy/DNA is almost completely enantioselective.
1
analyzed by H-NMR to determine conversion and HPLC on a
chiral stationary phase for ee. The ee values are based on at least
2 experiments and are reproducible within 2%.
Determination of rate constants using time-dependent UV-
Vis spectroscopy. Rate constants for a variety of DNA-based
Michael additions to enones were determined under similar
conditions as described by Boersma et al.12 Freshly dialyzed
st-DNA (final concentration 0.67 mg mL-1 in 20mM MOPS
pH 6.5) was used, and this was pre-equilibrated at 5 ◦C for
18h with Cu2+-dmbipy at a final concentration of 0.15 mM (1
complex per ~6–7 base pairs). After transfer of the solution thus
obtained to a cuvette, the substrate was added to give a final
concentration of 6.0 mM (Michael additions) or 14 mM (Friedel–
Crafts reactions). The appropriate amount of the nucleophile
(typically to a final concentration of 0.9 mM (dimethylmalonate),
9.0 mM (nitromethane) or 0.5–2.0 mM (2a,b) was then added
as a stock solution in acetonitrile (nitromethane and indoles)
or 20 mM MOPS pH 6.5 (dimethylmalonate) and the cuvette
was gently shaken. The UV absorption was monitored 335 nm
(Michael additions to 1a) or 340 nm (Friedel–Crafts alkylations
Implications for the role of DNA in catalysis
Comparison of the results presented here for the DNA-based
catalytic enantioselective conjugate additions with those previ-
ously reported for the Diels–Alder reaction reveal some striking
similarities.
The enantioselectivities are sequence dependent, with the
highest ee’s obtained with sequences containing stretches of
G’s. Particularly noteworthy is that in the majority of reactions
investigated, that is, the Diels–Alder reaction of azachalcone
with cyclopentadiene, the Friedel–Crafts alkylation of 2a with
1e and the Michael addition of dimethyl malonate to 1a, the self-
complimentary oligonucleotide d(TCA GGG CCC TGA)2 proved
to give rise to the highest ee’s.
Interestingly, the stereochemistry of the catalyzed reactions is
predictable; in all cases the product resulting from the attack of
the diene or nucleophile from the Si face of the enone is obtained
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to 1a–f) at a temperature of 25.0 0.2 ◦C. For the reactions
with half-lives of more than approx. one hour, apparent rate con-
stants were determined by initial-rate kinetics with the following
found: 438.0805; HPLC: Chiralpak AD, heptane/i-PrOH 90/10,
flow 1.0 mL min-1, Tr 37.1, 46.8 min.
3-(5-methoxy-1H-indol-3-yl)-1-(1-methyl-1H-imidazol-2-yl)-1-
octanone (3g). Purified by flash column chromatography (SiO2,
hexane–ethyl acetate 2 : 1, Rf 0.21), giving a brown oil. 1H NMR:
d 7.83 (br s, 1H), 7.20 (d, J = 9.0, 1H), 7.12 (d, 0.91, 1H), 7.05 (dd,
J = 2.4, 17.7, 2H), 6.96 (s, 1H), 6.81 (dd, J = 2.4, 8.7, 1H), 3.85 (s,
3H), 3.82 (s, 3H), 3.63–3.69 (m, 1H), 3.44–3.57 (m, 2H), 1.71–1.82
(m, 2H), 1.20–1.31 (m, 6H), 0.82 (t, 7.9, 3H), 13C NMR: d 192.3
(s), 153.5 (s), 143.2 (s), 131.4 (s), 128.6 (d), 127.3 (s), 126.7 (d),
121.8 (s), 119.1 (s), 111.7 (d), 111.6 (d), 101.0 (d), 55.8 (q), 45.6 (t),
36.0 (t), 36.0 (d), 32.2 (q), 31.9 (t), 27.1 (t), 22.5 (t), 14.0 (q); MS
(ESI) m/z 376 ([M+Na]+), 354 ([M+H]+), 149; HRMS calcd for
C21H28N3O2+ ([M+H]+): 354.2176, found: 354.2184; HPLC: Regis
(R,R)-Whelk-O 1, 0.04% diethylamine in heptane/i-PrOH 80/20,
flow 0.5 mL min-1, Tr 35.6, 42.3 min.
expression:13
in which d(Aenone)/dt
is the slope of the decrease of absorption in time, d the pathlength
of the cuvette (1 cm), De the difference in extinction coefficients of
the substrate and the product (determined separately) and [enone]0
is the initial substrate concentration. Apparent second-order rate
constants were then deduced from the slope of a plot of these
values of k1 versus the concentration of nucleophile. For faster
reactions (complete conversion within several hours, viz. Friedel–
Crafts alkylations of 2a,b by 1e–f), the decrease of absorption in
time (At) was curve-fitted using Grafit 3.0 (Erithacus Software
Ltd., 1992) to the exponential equation At = A• + A0·e-k1·t, giving
apparent rate constants k1 directly.
Synthesis of substrates and identification of products. Enone
substrates 1a–f22 were synthesized according to published pro-
cedures. Indoles 2a,b were commercially available from Sigma-
Aldrich and used without further purification. Analytical data
of adducts 3b,d–f, 5 and 6 were in accordance with literature.6,7
Addition products, for which initial-rate kinetics were used to
study rate constants, were synthesized independently as a race-
mate, starting from 0.25 mmol of enone, 0.25 mmol of copper(II)
nitrate and 5 eq. of the appropriate nucleophile in 5 mL of
distilled water overnight. The adducts were purified by flash-
column chromatography and their molar extinction coefficients
were determined in a concentration range close to that of the
catalytic reactions (typically 4–40 mM).
Conclusions
In the DNA-based catalytic enantioselective Michael addition and
Friedel–Crafts alkylation reaction, the role of the DNA is not
limited to that of a chiral scaffold. It was shown that in both these
conjugate addition reactions, the DNA affects the reaction rate,
with the largest rate accelerations observed in case of the Friedel–
Crafts alkylation. In the case of the Michael addition the effect is
dependent on the Michael donor: with dimethylmalonate a modest
increase of reaction rate was observed, whereas with nitromethane
a deceleration of the reaction was found in the presence of
DNA. The lower rates found for the Michael addition compared to
the Friedel–Crafts alkylation were attributed to charge repulsion
between the anionic nucleophile and the negatively charged DNA
backbone
From a comparison with the DNA-based catalytic enantiose-
lective Diels–Alder reaction, it was found that the role of DNA
is comparable in both reaction classes, suggesting that similar
factors play a role in the observed enantioselectivity and reaction
rate acceleration. It is suggested that the DNA predominantly has
an effect on the ground state of these reactions.
3-(5-methoxy-1H-indol-3-yl)-1-(1-methyl-1H-imidazol-2-yl)-3-
phenyl-1-propanone (3a). Purified by flash column chromato-
graphy (SiO2, hexane–ethyl acetate 1 : 2, Rf 0.25), giving a light
brown solid, mp 139 ◦C (dec.); 1H NMR: d 8.07 (br s, 1H), 7.38
(d, J 8.0, 2H), 7.20–7.26 (m, 2H), 7.11–7.20 (m, 3H), 7.09 (d, J =
2.4, 1H), 6.98 (s, 1H), 6.92 (d, J = 2.2, 1H), 6.78 (dd, J = 8.8, 2.4,
1H), 5.01 (t, J = 7.6, 1H), 4.01 (dd, J = 16.3, 7.4, 1H), 3.90 (s,
3H), 3.82 (dd, J = 16.4, 7.8, 1H), 3.75 (s, 3H);13C NMR: d 190.88
(s), 153.48 (s), 144.22 (s), 142.98 (s), 131.61 (s), 128.69 (d), 128.18
(d), 127.81 (d), 126.98 (s), 126.95 (d), 126.00 (d), 122.15 (d), 118.74
(s), 111.83 (d), 111.68 (d), 101.10 (d), 55.64 (q), 45.29 (t), 37.92
(d), 35.94 (q) MS (ESI) m/z 382 ([M+Na]+), 360 ([M+H]+), 236,
213; HRMS calcd for C22H22N3O2 ([M+H]+): 360.1707, found:
360.1700; HPLC: Chiralpak AD, heptane/i-PrOH 80/20, flow
1.0 mL min-1, Tr 19.1, 26.9 min.
Based on the observed generallity of the DNA-induced enan-
tioselectivity and rate acceleration, it is envisioned that the DNA-
based catalysis concept can be applied to other enantioselective
transition metal catalyzed reactions as well.
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