A Simple Nucleophilic Substitution as a Versatile Postfunctionalization

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A Simple Nucleophilic Substitution as a Versatile Postfunctionalization Method for the Coupling of Nucleophiles to an Anderson-Type Polyoxometalate Stef Vanhaecht, Thomas Quanten, and Tatjana N. Parac-Vogt* Department of Chemistry, KU Leuven, Celestijnenlaan 200F − bus 2404, 3001 Leuven, Belgium S Supporting Information *

ABSTRACT: A new postfunctionalization method was developed for the Anderson-type POM based on a nucleophilic substitution reaction occurring at an electrophilic sp3 hybridized carbon localized on the hybrid POM. Using this method, several types of different nucleophiles including primary and secondary amines, carboxylates, and thiolates were efficiently coupled to a chloridefunctionalized Anderson-type POM in high yields and purity. The heterogeneous acetonitrileNa2CO3 conditions were found to be superior over other bases and solvents for the coupling of amines and thiolates to the chloride-functionalized POM. Moreover, the addition of 1 equiv of tetrabutylammonium iodide as a catalyst drastically decreased the reaction times to 24 h for the complete coupling of amines and only a couple of hours for thiolates. In the case of carboxylic acids as substrates, using tetrabutylammonium hydroxide as the base for the reaction proved to be beneficial. This is because the resulting tetrabutylammonium carboxylates were found to be much more reactive than the corresponding sodium carboxylates and allowed homogeneous reaction conditions. Using sodium carbonate, only 25% of N-acetyl glycylglycine could be coupled after 24 h at 80 °C, while full conversion was achieved after the same reaction time when using tetrabutylammonium hydroxide as a base.

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INTRODUCTION The development of inorganic−organic hybrid polyoxometalates (POMs), also known as hybrid POMs, is a rapidly growing field in chemistry and material sciences because these compounds can show promising properties by combining the best of two worlds. The inorganic POM framework consists of a negatively charged metal−oxygen cluster characterized by versatile chemical and physical properties like structure, charge density, polarity, and redox behavior.1 This variety of unique properties has already lead to the study of POMs in different application fields including material development,2 catalysis,3 and their use for medicinal and biochemical purposes.4 By attaching organic compounds onto the inorganic framework, many interesting properties can be introduced such as surfactant or surface behavior,5 photosensitization,6 or the ability to bind metal ions in the organic ligand,6g,h,7 thereby extending the possibilities for the use of hybrid POMs for numerous applications. Although these hybrid structures show promising characteristics, their synthesis is rather challenging and needs to be tailored to a specific type of POM. During the last decades, two main synthetic strategies have been developed, each having certain advantages and disadvantages.8 In a first approach, which is based on the principles of prefunctionalization, the organic group is presynthesized, providing it with the envisioned organic moiety and a POM anchoring group. Next, it is covalently attached to the POM in a last synthetic step. Several POM anchoring methods have been developed over the last decades. They mainly rely on either the insertion of organotin, silyl, and phosphoryl © XXXX American Chemical Society

compounds into lacunary POM structures, the formation of molybdenum−nitrogen bonds, or the formation of direct C− O−M (M = W, Mo, V) bonds by the reaction of alkoxo ligands with the appropriate POM structures.8a,b,9 Although this prefunctionalization strategy has already led to a large number of new hybrid POM structures, this method also has some limitations. The covalent attachment of the preformed organic group to the POM is often hindered by the specific nature of the organic moiety such as steric hindrance or the presence of positive charges that can lead to unwanted ionic interactions with the POM surface. Also, the prefunctionalization of the organic compound with a suitable anchoring group can form a challenge, as these groups are often very reactive or display mismatching reaction or workup conditions compared to the targeted organic compounds. The constant demand for hybrid structures with ever growing complexity has led to the development of a second approach, based on postfunctionalization methods. This synthetic strategy is based on the coupling of the ligand or organic compound of interest and a preformed hybrid POM containing a specific functional group. The success of this strategy is highly dependent on finding suitable coupling methods and reaction conditions. As POMs are highly negatively charged, they tend to participate in ionic interactions in the presence of positively charged metal ions, which are used as a catalyst or additive in many classic organic reactions. Due Received: January 9, 2017

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DOI: 10.1021/acs.inorgchem.6b03131 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

1, and its design involves an Anderson-type POM having two chloroacetamide groups at either side of the planar framework.

to this unwanted interaction, insoluble POM-metal ion salts are formed, resulting in the drastic decrease of the activity or function of the metal ion catalysts. Second, many POM structures tend to degrade upon the addition of dilute acids or bases, which are used in many organic transformations. Next to acids or bases, several nucleophiles are also known to be harmful for the integrity of the POM structure. Also, as POMs are redox-active species, addition of metal ions like Pd0 or reducing agents including sodium ascorbate or lithium aluminumhydride can induce an often irreversible reduction and even destruction of the POM structure. Nevertheless, over the past 10 years, many postfunctionalization routes have been developed capable of conserving the fragile POM scaffold, while attaching various organic groups to their structure.8b,c These methods include amidation,5b,10 esterification,10a thioesterification,10b imine bond formation,6a,7a,11 a Sonogashiro reaction,6g,h,12 a Heck reaction,13 copper catalyzed cycloadditions,6c−e,14 a Diels−Alder reaction,15 and finally an N-alkylation.16 In a recent publication of the group of Wei, the authors describe an interesting use of a nucleophilic substitution reaction with a chloride functionalized hybrid hexamolybdate.17 However, the extent of the reaction was limited to a simple halogen exchange reaction, by reacting the hybrid POM with sodium iodide in acetone, and the substitution of the chloride atom by a nitro group by the reaction with silver nitrate in acetonitrile. In this paper, we describe a new postfunctionalization method for Anderson-type POMs with a broader range of organic nucleophiles such as primary and secondary amines, carboxylates, and thiolates. The hybrid Anderson-type POMs represent one of the most common classes of hybrid POMs and have been well studied in terms of synthesis and applications, as was recently reviewed in detail by Blazevic and Rompel.18 Next to numerous examples of hybrid Anderson POMs obtained through a prefunctionalization approach, several examples of postfunctionalization methods have been described for this type of POM. These include amidation,10c,d,g−i,19 imine formation,11 a Diels−Alder reaction,15 and copper catalyzed cycloadditions.20 The envisioned postfunctionalization method in this study is based on a nucleophilic substitution reaction between a chloride-functionalized POM and various organic substrates containing a nucleophilic character, including amines, carboxylates, and thiolates. The substitution of a leaving group on an sp3 carbon by a nucleophile is a fundamental and one of the most commonly used reactions in modern organic chemistry. This reaction is very convenient since it does not require the addition of activating agents, catalysts, reducing agents, or stabilizing ligands. However, a successful substitution does require the presence of at least 1 equiv of base. The role of the latter in the reaction mechanism is either to mop up the formed acid after the substitution or to deprotonate the nucleophile in order to increase its nucleophilicity and hence the rate with which the nucleophilic substitution takes place. In this work, different routes, based on a nucleophilic substitution reaction, have been examined taking into account the sensitivity of POMs to alkaline environments, resulting into a novel and versatile postfunctionalization method for the Anderson-type POM.

Figure 1. Design of the chloride-functionalized Anderson-type POM (compound 2) used in this study ((C16H36N)3-[MnMo6O18((OCH2)3C-NH-CO-CH2-Cl)2]). Light gray: Mo(VI) polyhedra; dark gray: Mn(III) polyhedra.

The choice of chloroacetamide as functional group was based on its high electrophilic character compared to regular chloroalkanes, which is due to the presence of the carbonyl group. Furthermore, the primary nature of the chloroacetamide and the use of polar aprotic solvents, necessary to dissolve the POM, both favor an SN2 mechanism, which is the most interesting when the coupling of different substrates is intended. Moreover, the lack of β-hydrogen atoms in the structure avoids competition with elimination reactions, which would lead to undesirable side products. Therefore, we anticipated that all of these structural characteristics make this hybrid Anderson POM an ideal candidate to be used as a platform for POM postfunctionalization that uses a nucleophilic substitution reaction. The synthesis of the double chloroacetamide-functionalized Anderson-type POM (compound 2, (C16H36N)3-[MnMo6O18((OCH2)3C-NH-CO-CH2-Cl)2]) was performed by applying a previously reported procedure developed in our group.20b In a first step, a small linker molecule was synthesized containing the triol function and the chloroacetamide group (compound 1). Subsequently, compound 1 was reacted with the TBA4[Mo8O26] precursor POM and Mn(III)acetate in refluxing acetonitrile to afford compound 2. This compound was fully characterized by 1H and 13C NMR (nuclear magnetic resonance) spectroscopy, elemental analysis, and Fourier transform infrared (FT-IR) spectroscopy.20b Development of a Nucleophilic Substitution Method. In order to investigate the potential of compound 2 as an electrophilic substrate, different nucleophiles and reaction conditions were examined. A first class of nucleophiles used in this study consisted of a range of different primary and secondary amine compounds. Initial reactions were conducted with dibutylamine as a substrate in acetonitrile or DMF solutions and in the presence of N,N-diisopropylethylamine (DIPEA) as a base, while heating the homogeneous reaction mixture at 80 °C. After 48 h, the coupled product was isolated and characterized by FT-IR and 1H and 13C NMR spectroscopy. Next, benzylamine was examined as a primary amine model substrate, using the same reaction conditions. After 24 h, compound 2 was completely consumed and the benzylamine coupled product was isolated and shown to be pure by 1H NMR spectroscopy. The solubility of the resulting product was surprisingly low in common POM dissolving solvents like acetonitrile, N,N-dimethylformamide, or dimethylsulfoxide. A possible explanation for the low solubility of the final product may be the formation of the amine salt of the hybrid POM, as DIPEA may not be basic enough to extract the hydrochloric

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RESULTS AND DISCUSSION Synthesis of the Electrophilic POM. The envisioned hybrid POM used as a platform in this study is shown in Figure B

DOI: 10.1021/acs.inorgchem.6b03131 Inorg. Chem. XXXX, XXX, XXX−XXX

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point in the reaction, sodium ions are released in the reaction mixture, at least 1 equiv of free chloride ions is present, leading to the formation and precipitation of NaCl out of the reaction mixture. This prevents ion-pairing between the POM framework since the presence of sodium ions is minimalized, leading to the formation of perfectly soluble and pure tetrabutylammonium salts of the POM. Coupling of Amines to Compound 2. After the most suitable conditions for the nucleophilic substitution reaction were identified, the coupling of a range of different aminecontaining substrates to compound 2 was further examined. All products were identified by using 1H (Figure 2) and 13C

acid that is formed during the nucleophilic substitution reaction. In order to address this problem, an alternative method was examined to overcome the amine salt formation. Despite the potentially destructive nature of Na2CO3 toward POMs, this inorganic base proved to be ideal for the successful outcome of the reaction. Reactions performed in acetonitrile using either dibutylamine or benzylamine in combination with Na2CO3 delivered pure and highly soluble products in 93% and 96% yield, respectively. The insolubility of Na2CO3 in acetonitrile clearly prevents the degradation of the POM framework while the hydrochloric acid is still efficiently removed from the solution. Moreover, NaHCO3 and NaCl, which are byproducts formed after the acid−base reaction, are also nearly insoluble in acetonitrile and can be conveniently removed from the reaction mixture. Interestingly, when a similar reaction was performed in mixed acetonitrile−water solvents, partial POM degradation was observed, most likely due to the good solubility of Na2CO3 in such solutions. Halogen Exchange Reaction of Compound 2. One way of improving the reaction rates of nucleophilic substitutions is the use of better leaving groups. A commonly used substitute for chloride in these reactions is the larger iodide halogen, which is a weaker base and hence its ability to leave the substrate should be much greater. A typical way of introducing the iodide in the structure is by performing a halogen exchange reaction, typically performed by the reaction of the chloride substrate with sodium iodide in acetone or acetonitrile solutions. This equilibrium is driven toward the iodidefunctionalized substrates since any formed sodium chloride precipitates out of the reaction mixture due to its low solubility in these solvents. However, when sodium iodide was added to an acetonitrile solution containing compound 2, instantaneous formation of a pale-orange precipitate was observed. Apparently, the release of sodium ions into the solution resulted in the formation of the insoluble sodium salt of the POM via ion-pairing. In order to avoid the presence of sodium ions in solution, 1 equiv of tetrabutylammonium iodide was used as a source of iodide to catalyze the reaction, resulting in the successful coupling of dibutylamine to compound 2 after 24 h in the presence of Na2CO3. Scheme 1 shows the proposed reaction pathways for the coupling reaction of compound 2 with nucleophiles in the presence of Na2CO3 and tetrabutylammonium iodide (TBAI) in acetonitrile. Although, at some

Figure 2. Detail of the 1H NMR spectra (600 MHz, DMSO-d6) of the benzylamine coupled product 3a (top) and compound 2 (bottom). The singlet a’ attributed to the CH2-protons of the chloroacetamide clearly shift to a lower value after the substitution of the chloride leaving group for a benzylamine nucleophile (2.50 ppm = DMSO, 3.33 ppm = water). (* and *’: The signal of the CH2-protons next to the Mn(III) center is found at approximately 64 ppm; see the SI).

(Figure S3) NMR spectroscopy. The singlet attributed to the CH2-protons of the chloroacetamide clearly shifts to a lower value after the substitution of the chloride leaving group for a benzylamine nucleophile. Moreover, the peaks of the aromatic group were found in the expected ratios. All other compounds were identified by using the same approach as for compound 3a. Polar compounds like ethanolamine (3b) and ethylenediamine (10 equiv) (3c) were coupled in high yields, with the exception of a tris molecule as a result of its increased polarity, causing precipitation of the POM during the reaction. Moreover, in the case of ethylenediamine, no polymeric products were observed despite the presence of two nucleophilic amine groups in this substrate. The mono-Boc protected ethylenediamine was coupled in even higher yields (95%) (3f) than the unprotected ethylenediamine, and the sterically hindered primary amine adamantylamine was also coupled in 94% yield (3e). Two amino acid related compounds, L-phenylalanine ethyl ester (3g) and glycylglycine ethyl ester (3h), were also successfully coupled in high yields. However, up to 48 h of reaction time was needed for both compounds, most presumably a result of the poor solubility of the initial amine salts in acetonitrile. Next to the previously mentioned secondary amine dibutylamine (3i), the basic piperidine molecule was also successfully coupled to compound 2 in 96% yield (3j). An overview of the results of these coupling reactions is given in Table 1. 3.5. Coupling of Carboxylates to Compound 2. Next to amine-containing substrates, carboxylic acid-functionalized

Scheme 1. Proposed Reaction Pathway of the Coupling of Compound 2 with Nucleophiles in the Presence of Na2CO3 and TBAI in Acetonitrile

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DOI: 10.1021/acs.inorgchem.6b03131 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Coupling of Different Primary and Secondary Amines to Compound 2

Figure 3. Detail of the 1H NMR spectra (600 MHz, DMSO-d6) of the p-methoxyphenylacetic acid coupled product 3k (top) and compound 2 (bottom). The singlet a’ attributed to the CH2-protons of the chloroacetamide clearly shifts to a higher value after the formation of the adjacent ester bond (2.50 ppm = DMSO, 3.33 ppm = water). (* and *’: The signal of the CH2-protons next to the Mn(III) center is found at approximately 64 ppm; see the SI).

the strong ionic interaction between the small sodium ion and the carboxylate group in acetonitrile. A commonly used solution for this problem is applying the “cesium effect”, where a cesium containing base is used to deprotonate the carboxylic acid. The resulting cesium carboxylate is much more reactive and soluble due to the weak interaction between the large and “soft” cesium ion and the “hard” carboxylate anion. Comparable to this “cesium effect”, we expected a similar effect by using tetrabutylammonium hydroxide (TBAOH) as a base, as the charge of this organic ion is shielded by bulky alkyl chains. Although this base is too strong to be used directly in the presence of POMs, careful addition of aqueous tetrabutylammonium hydroxide to a small excess of carboxylic acid prior to the addition of the POM resulted in a homogeneous mixture without any sign of POM degradation. Under such conditions, all tested carboxylic acid-containing substrates were successfully coupled to compound 2 in 24 h in good yields (see Table 2). These carboxylic acid substrates included p-methoxyphenylacetic acid (3k), an N-acetyl protected dipeptide glycylglycine (3l), and the N-Boc protected amino acid glycine (3m). All compounds were identified using 1H and 13C NMR spectroscopy in a similar way as for compound 3k. Coupling of Thiolates to Compound 2. A last type of nucleophiles explored in this work included thiol compounds. Similar to carboxylic acids, this functional group can easily be converted to its deprotonated counterpart, a thiolate, which is known to be an excellent nucleophile. Using Na2CO3 as a base, the model substrate benzylthiol was successfully coupled to compound 2 after 5 h of reaction time (3n), as identified by 1H and 13C NMR (see Figure 4, Figure S39, and Table 3). The singlet attributed to the CH2-protons of the chloroacetamide shifted to a lower value after the replacement of the chloride leaving group with the thioether group in 3n. Furthermore, the aromatic peaks at 7.34 and 7.24 ppm indicate the successful synthesis of compound 3n. Interestingly, the use of tetrabutylammonium hydroxide did not prove to be superior in the case of the thiol-containing substrate. A second substrate, ethyl thioglycolate, was coupled to compound 2 after only 3 h of reaction in 79% yield (3o) (Table 3). The identification of 3o was performed similarly to that of 3n. Clearly, the fast reaction kinetics reflect the high nucleophilicity of the substrates. Moreover, when these reaction conditions were

a

10 equiv was used to avoid the formation of polymeric structures. Indications for the correct product were seen in 1H NMR; however, the solubility was very poor. cThe reaction was run for 48 h to obtain the pure product. b

substrates were examined in the coupling reaction with compound 2. In contrast to the neutral amine group, this group can be converted into its negatively charged form, namely, a carboxylate, thereby strongly increasing its nucleophilic strength. Using Na2CO3 and tetrabutylammonium iodide, p-methoxyphenylacetic acid was coupled to compound 2 in refluxing acetonitrile after 24 h, forming an ester bond between the POM and the substrate (compound 3k). Identification of the desired product 3k was done using 1H and 13C NMR, as shown in Figure 3 and Figure S30. The singlet attributed to the CH2-protons of the chloroacetamide clearly shifts to a higher value after the formation of the adjacent ester bond. Moreover, the aromatic peaks at 7.22 and 6.89 ppm of product 3k were observed in the expected ratios. Although the p-methoxyphenylacetic acid coupled product (3k) was isolated in good yields, a limited amount of a paleorange solid was observed. Most presumably, this precipitation is the result of ion pairing between the POM and sodium ions in solution, since the initial Na2CO3 is already used at the beginning of the reaction to create sodium carboxylate substrates. Interestingly, the reaction between compound 2 and N-acetyl glycylglycine afforded only 25% of coupled product after 24 h at reflux temperatures. A possible explanation for the low reactivity of this compound could be D

DOI: 10.1021/acs.inorgchem.6b03131 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 2. Coupling of Different Carboxylic Acids to Compound 2

Table 3. Coupling of Different Thiols to Compound 2

a After 5 h of reaction at reflux. bAfter 2 h of reaction at reflux. cAfter 3 h of reaction at reflux.

Scheme 2. Overview of the Reaction of Compound 2 with Different Nucleophiles and the Applied Bases

a

After 24 h reaction at reflux, only 25% of the starting material was converted to product 3l.

Figure 4. Detail of the 1H NMR spectra (600 MHz, DMSO-d6) of the benzylthiol coupled product 3n (top) and compound 2 (bottom). The singlet a’ attributed to the CH2-protons of the chloroacetamide clearly shifts to a lower value after the replacement of the chloride leaving group with the thioether group (2.50 ppm = DMSO, 3.33 ppm = water). (* and *’: The signal of the CH2-protons next to the Mn(III) center center is found at approximately 64 ppm; see the SI).

the successful outcome of the coupling reaction without loss of product or purity due to the absence of any POM degradation or solubility issues. The use of tetrabutylammonium hydroxide to convert the carboxylic acid containing substrates into the more nucleophilic carboxylates proved to be beneficial, resulting in a newly formed ester bond between the POM and the organic compound. We believe that this procedure, based on one of the oldest and most simple principles of organic chemistry, could benefit from its cheap and easily obtained starting product, minimal effort, and high yields and purities.

used, no reduced POM species were observed in the presence of these easily oxidizable thiols.

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CONCLUSION In conclusion, we demonstrated that a nucleophilic substitution reaction can be successfully used as a postfunctionalization method for the Anderson-type POM. The double chloroacetamide-functionalized POM, specially designed for this purpose, proved to be an ideal basis for the grafting of various nucleophiles through the displacement of the chloride leaving group localized next to the sp3 hybridized carbon. Scheme 2 summarizes the reactions of compound 2 with the different types of nucleophilic substrates investigated in this study. In the case of amines and thiols as nucleophiles, the addition of Na2CO3 in combination with acetonitrile as a solvent assured

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b03131. Experimental details, 1H and 13C NMR spectra, elemental analysis, FT-IR and MS data for compounds 1−3o (PDF) E

DOI: 10.1021/acs.inorgchem.6b03131 Inorg. Chem. XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tatjana N. Parac-Vogt: 0000-0002-6188-3957 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T.N.P.-V. thanks KU Leuven and FWO Flanders for financial support. S.V. acknowledges the “Agency for Innovation by Science and Technology in Flanders” (IWT) for a doctoral fellowship.

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DOI: 10.1021/acs.inorgchem.6b03131 Inorg. Chem. XXXX, XXX, XXX−XXX