Final Report Summary - AMMDNACAT (Asymmetric Aza-Michael reactions catalyzed by hybrids metal-DNA)
Summary description of the project objectives
The main objective of the present project (AMmDNACAT) was the development of an efficient hybrid catalytic system based on DNA for asymmetric transformations in aqueous media. The design of this new kind of catalysts allows to combine the catalytic activity of transition metal complexes with the attractive features (i.e. substrate activation, enantioselectivity, rate acceleration, etc.) provided by a biomolecular scaffold. Therefore, metal-DNA systems are powerful catalysts for challenging reactions that are not possible using conventional homogenous or heterogeneous catalytic approaches.
Description of the work performed since the beginning of the project
Previously in our group, hybrid catalysts resulting from the assembly of copper(II) complexes and double strand DNA showed a really exceptional catalytic activity in a broad number of asymmetric transformations in water with high enantioselectivities.(1) A particular challenging catalytic process within these reactions was the enantioselective hydration of enones (Scheme 1).(2)
Scheme 1
This reaction represented the first example reported for the asymmetric 1,4-addition of water to -unsaturated substrates, which did not have precedent using “conventional catalysts”. Inspired by this result, as it was presented in the grant proposal, we decided to evaluate the catalytic activity of these hybrid systems for the direct enantioselective conjugate addition of other small molecules to unsaturated substrates.
During the first period (2011-2012), we started our experimental work with the study of 1,4-addition of ammonia to -unsaturated compounds, applying DNA-based methodology (Scheme 2). Initially, a screening of buffers, ligands, metal complexes, substrates and reaction conditions was performed in order to optimize the catalytic process.
Scheme 2
The preliminary studies were perfomed without DNA. We found that the control of the reaction pH is crucial to avoid the blank reaction. The conjugate addition of ammonia to these substrates is a reversible process with a maximun of conversion of 60%. Furthermore, the reaction products proved to be unstable. After isolation, the reverse reaction (retro aza-Michael) takes place and the products decomposed, generating the starting materials back.
After these initial tests, we also evaluated the activity of copper(II)-DNA systems for this transformation. Unfortunately, in this case we observed that metal-DNA hybrids only catalyse the addition of water (that is used as solvent in this reaction) to the unsaturated substrates instead of the ammonia addition. At this point, in view of the low nucleophilicity of ammonia molecules, different amines were also tested for the aza-Michael addition reaction using DNA-based catalysis. A new reactivity was observed for these systems when 5-aminoindole was tested as nucleophile (Scheme 3).
Scheme 3
Again, the reaction without DNA gave rise to the formation of the aza-Michael addition product. However, the reaction in presence of DNA led to the formation of an unexpected product. Cu(II)-DNA system catalyzed in this case a Friedel-Crafts alkylation and a subsequent enantioselective protonation reaction, giving rise to the formation of a tertiary carbon stererocenter with a 52% of enantioselectivity. This represents the first example of control over the chemoselectivity of a catalytic process due to the presence of DNA in the reaction media.
Furthermore, enantioselective protonations are difficult reactions due to different aspects: the small size of the proton atom, the reversibiliy of the process and the requirement of kinetic control over the protantion reaction.(3) This underlines the exceptional catalytic activity found for this metal-DNA hybrid.
The last months of the first period were used to optimize the reaction conditions for the Friedel-Crafts alkylation/enantioselective protonation catalyzed by metal-DNA species. Also different substrates and nucleophiles were evaluated to expand the scope of the reaction.
In the second period of the project (2012-2013) the catalytic system was tested for a broad number of indoles and an extensive mechanistic study was performed in order the get information about the crucial role observed for the DNA in this transformation.
Description of the main results achieved so far
The AMmDNACAT project has led to develop the first Friedel-Crafts alkylation/enantioselective protonation in water (Scheme 4).
Scheme 4
The reaction products (α-chiral carbonyl compounds) are present in many valuable organic molecules and could be potentially use in the synthesis of important biological active molecules. The majority of these products can only be synthetized using this DNA-based methodology; this catalytic transformation almost does not take place without the presence of the DNA.
Kinetic studies performed for several indoles have shown a >600 fold rate acceleration for the reaction catalysed by the Cu(II)-DNA system compared with the reaction catalysed only by Cu(II). This acceleration is, to the best of our knowledge, the largest acceleration effect observed for a non-enzymatic system. We also studied the Kinetic isotope effect (KIE) for this transformation performing the reaction in deuterated buffer. We obtained KIE > 5 which proves that the protonation step is clearly involved in the rate determining step. This is one of the requirements for getting the protonation in an enantioselective way.
The highest enantioselectivity was observed for the 5-morpholinoindole (84%). The use of this indole (5-morpholinoindole) revealed different interesting experimental observations that led us to study the reaction for this nucleophile in detail. We found that: i) this indole shows a strong interaction with DNA at the reaction pH (pH 5.0) (Kb = 1445 ± 106 M-1). However, the determined binding constants for more basic pH’s were lower (Kb = 428 ± 23 M-1 (pH 7.5) Kb = 219 ± 3 M-1 (pH 10.0)) which is indicating a weaker interaction with DNA for high pH values. ii) We also determined the pKa for the 5-morpholinoindole (pKa ≈ 4.7). This value indicates that the indole could be partially protonated at the reaction pH. Therefore, a combination of hydrogen bond and electrostatic interactions can explain the high affinity observed for this indole and the DNA at pH 5.0. iii) Kinetic studies showed a dependence of the reaction rate to the indole concentration, showing a Michaelis-Menten behavior. A saturated state was reached for high concentration of 5-morpholinoindole.
Expected final results and their potential impact and use
The mechanistic studies performed for this reaction have revealed the crucial of role of the presence of DNA in the reaction medium. Thus, we found control over the chemoselectivity of the reaction and an enormous acceleration rate when copper(II)-DNA was used as catalysts. Furthermore, the correlation between the extra-interactions observed between the different species and the DNA brings us the hypotheses of additional activation by the biomolecule that would make possible the catalytic reaction. A better understanding of the mechanistic and structural aspects of these DNA-based catalysts is key to optimize the rational redesign of these catalytic systems for future applications in other challenging transformations.
(1) A.J. Boersma, R.P. Megens, B.L. Feringa and G. Roelfes, Chem. Soc. Rev. 2010, 39, 2083-2092.
(2) A. J. Boersma, D. Coquière, D. Geerdink, F. Rosati, B. L. Feringa, G. Roelfes, Nature Chem. 2010, 2, 991-995.
(3) C. Fehr, Angew. Chem. Int. Ed. Engl. 1996, 35, 2566-2587
The main objective of the present project (AMmDNACAT) was the development of an efficient hybrid catalytic system based on DNA for asymmetric transformations in aqueous media. The design of this new kind of catalysts allows to combine the catalytic activity of transition metal complexes with the attractive features (i.e. substrate activation, enantioselectivity, rate acceleration, etc.) provided by a biomolecular scaffold. Therefore, metal-DNA systems are powerful catalysts for challenging reactions that are not possible using conventional homogenous or heterogeneous catalytic approaches.
Description of the work performed since the beginning of the project
Previously in our group, hybrid catalysts resulting from the assembly of copper(II) complexes and double strand DNA showed a really exceptional catalytic activity in a broad number of asymmetric transformations in water with high enantioselectivities.(1) A particular challenging catalytic process within these reactions was the enantioselective hydration of enones (Scheme 1).(2)
Scheme 1
This reaction represented the first example reported for the asymmetric 1,4-addition of water to -unsaturated substrates, which did not have precedent using “conventional catalysts”. Inspired by this result, as it was presented in the grant proposal, we decided to evaluate the catalytic activity of these hybrid systems for the direct enantioselective conjugate addition of other small molecules to unsaturated substrates.
During the first period (2011-2012), we started our experimental work with the study of 1,4-addition of ammonia to -unsaturated compounds, applying DNA-based methodology (Scheme 2). Initially, a screening of buffers, ligands, metal complexes, substrates and reaction conditions was performed in order to optimize the catalytic process.
Scheme 2
The preliminary studies were perfomed without DNA. We found that the control of the reaction pH is crucial to avoid the blank reaction. The conjugate addition of ammonia to these substrates is a reversible process with a maximun of conversion of 60%. Furthermore, the reaction products proved to be unstable. After isolation, the reverse reaction (retro aza-Michael) takes place and the products decomposed, generating the starting materials back.
After these initial tests, we also evaluated the activity of copper(II)-DNA systems for this transformation. Unfortunately, in this case we observed that metal-DNA hybrids only catalyse the addition of water (that is used as solvent in this reaction) to the unsaturated substrates instead of the ammonia addition. At this point, in view of the low nucleophilicity of ammonia molecules, different amines were also tested for the aza-Michael addition reaction using DNA-based catalysis. A new reactivity was observed for these systems when 5-aminoindole was tested as nucleophile (Scheme 3).
Scheme 3
Again, the reaction without DNA gave rise to the formation of the aza-Michael addition product. However, the reaction in presence of DNA led to the formation of an unexpected product. Cu(II)-DNA system catalyzed in this case a Friedel-Crafts alkylation and a subsequent enantioselective protonation reaction, giving rise to the formation of a tertiary carbon stererocenter with a 52% of enantioselectivity. This represents the first example of control over the chemoselectivity of a catalytic process due to the presence of DNA in the reaction media.
Furthermore, enantioselective protonations are difficult reactions due to different aspects: the small size of the proton atom, the reversibiliy of the process and the requirement of kinetic control over the protantion reaction.(3) This underlines the exceptional catalytic activity found for this metal-DNA hybrid.
The last months of the first period were used to optimize the reaction conditions for the Friedel-Crafts alkylation/enantioselective protonation catalyzed by metal-DNA species. Also different substrates and nucleophiles were evaluated to expand the scope of the reaction.
In the second period of the project (2012-2013) the catalytic system was tested for a broad number of indoles and an extensive mechanistic study was performed in order the get information about the crucial role observed for the DNA in this transformation.
Description of the main results achieved so far
The AMmDNACAT project has led to develop the first Friedel-Crafts alkylation/enantioselective protonation in water (Scheme 4).
Scheme 4
The reaction products (α-chiral carbonyl compounds) are present in many valuable organic molecules and could be potentially use in the synthesis of important biological active molecules. The majority of these products can only be synthetized using this DNA-based methodology; this catalytic transformation almost does not take place without the presence of the DNA.
Kinetic studies performed for several indoles have shown a >600 fold rate acceleration for the reaction catalysed by the Cu(II)-DNA system compared with the reaction catalysed only by Cu(II). This acceleration is, to the best of our knowledge, the largest acceleration effect observed for a non-enzymatic system. We also studied the Kinetic isotope effect (KIE) for this transformation performing the reaction in deuterated buffer. We obtained KIE > 5 which proves that the protonation step is clearly involved in the rate determining step. This is one of the requirements for getting the protonation in an enantioselective way.
The highest enantioselectivity was observed for the 5-morpholinoindole (84%). The use of this indole (5-morpholinoindole) revealed different interesting experimental observations that led us to study the reaction for this nucleophile in detail. We found that: i) this indole shows a strong interaction with DNA at the reaction pH (pH 5.0) (Kb = 1445 ± 106 M-1). However, the determined binding constants for more basic pH’s were lower (Kb = 428 ± 23 M-1 (pH 7.5) Kb = 219 ± 3 M-1 (pH 10.0)) which is indicating a weaker interaction with DNA for high pH values. ii) We also determined the pKa for the 5-morpholinoindole (pKa ≈ 4.7). This value indicates that the indole could be partially protonated at the reaction pH. Therefore, a combination of hydrogen bond and electrostatic interactions can explain the high affinity observed for this indole and the DNA at pH 5.0. iii) Kinetic studies showed a dependence of the reaction rate to the indole concentration, showing a Michaelis-Menten behavior. A saturated state was reached for high concentration of 5-morpholinoindole.
Expected final results and their potential impact and use
The mechanistic studies performed for this reaction have revealed the crucial of role of the presence of DNA in the reaction medium. Thus, we found control over the chemoselectivity of the reaction and an enormous acceleration rate when copper(II)-DNA was used as catalysts. Furthermore, the correlation between the extra-interactions observed between the different species and the DNA brings us the hypotheses of additional activation by the biomolecule that would make possible the catalytic reaction. A better understanding of the mechanistic and structural aspects of these DNA-based catalysts is key to optimize the rational redesign of these catalytic systems for future applications in other challenging transformations.
(1) A.J. Boersma, R.P. Megens, B.L. Feringa and G. Roelfes, Chem. Soc. Rev. 2010, 39, 2083-2092.
(2) A. J. Boersma, D. Coquière, D. Geerdink, F. Rosati, B. L. Feringa, G. Roelfes, Nature Chem. 2010, 2, 991-995.
(3) C. Fehr, Angew. Chem. Int. Ed. Engl. 1996, 35, 2566-2587