Modification of electron deficient polyester via Huisgen/Passerini sequence
Graphical abstract
Introduction
Multicomponent reactions (MCRs) involving more than two starting compounds, generally three or more, give a rise to various final compounds through a direct, an efficient and an atom economic pathway [1], [2], [3], [4]. Among all the MCRs, the isocyanide-based MCRs found much more interest in synthetic organic chemistry when compared to the other MCRs due to their high selectivity arising from nucleophilic and electrophilic character of isocyanides [5], [6]. The Passerini reaction is a first example of isocyanide-based three-component reaction, which employs starting compounds: isocyanide, carboxylic acid and aldehyde [7]. Consequently, Passerini reactions specifically find a wide range of applications in the total synthesis of various natural products in organic chemistry and allow to access to large libraries of similar compounds in combinatorial chemistry.
Passerini reactions recently gained an attention in synthetic polymer chemistry owing to their versatility and diversity both for the production and the post functionalization of polymers [8], [9], [10]. Meier group first time reported the synthesis of α,ω-diene monomers using Passerini reaction to be employed in acyclic diene metathesis (ADMET) polymerization to yield various polyesters with amide pendant units [11]. Furthermore, Meier group extended the Passerini reaction to the post-functionalization of ADMET polymer having pendent carboxylic acid groups reacting with isocyanides and aldehydes. Meier and Hogenboom introduced the Passerini reaction into polyoxazoline with pendent carboxylic acid functionals [12]. In addition, the Passerini reaction was used in the synthesis of dendrimer structures in divergent and convergent strategies [13], [14], [15]. On continuing efforts, a combination of di-, mono- and hetero-functional reactants of the Passerini reaction was successfully employed in order to yield polycondensates under mild conditions [16], [17], [18], [19].
Alternatively, the click reactions [20] similar to MCRs are a powerful tool for functionalization of macromolecules as well as polymer conjugations [21], [22], [23], [24], [25], [26]. The click reactions should be modular, highly efficient, and generally atom economical. The click reactions should also display high stereo- and regio-selectivity, high yield under mild reaction conditions, simple recovery of the main product, a capability of working in a wide range of solvent and tolerance of a wide range of functional groups. The most encountered click reactions in synthetic polymer chemistry are the copper catalyzed azide-alkyne cycloaddition (CuAAC) [27], Diels-Alder cycloaddition [28], thiol-ene [29], thiol-yne [30], and strain promoted azide alkyne cycloaddition (SPAAC) [31] reactions. Noteworthy, studies on the combinations of click reactions for the preparation of more complex polymeric structures are well documented in the literature as well [32], [33], [34], [35].
In recent times, polymer chemists inevitably united click reactions with Passerini for the production of complex functional polymers. Therefore, Gianneschi and Yang employed the Passerini reaction for the synthesis of α-hydroxy N-acylindoles [36]. These compounds were then incorporated into poly(α-hydroxy acid) copolymers bearing residues with functional side chains, which could be further modified through CuAAC reactions. Next, Li group reported the Passerini reaction for the functionalization of polymer end-groups followed by atom transfer radical polymerization and CuAAC, respectively, in order to yield an ABC miktoarm star polymer [37]. The Passerini together with a variety of click reactions, like CuAAC, thiol-ene, and thiol-yne are common very well known combinations in literature resulting in functional polymers as well as polymer conjugations [38], [39], [40], [41].
More recently, we reported the synthesis of polyesters containing the electron deficient reactive triple bonds in the main chain, which have a unique feature possessing the capability of undergoing a variety of organic reactions, for example, Huisgen 1,3-dipolar azide-alkyne and Diels-Alder cycloaddition reactions [42]. Exploiting this unique character, in this work, we report the first example of a Huisgen/Passerini reactions sequence using the polyester as a polymeric scaffold with internal electron deficient alkyne moieties thus affording the heterofunctional macromolecular structures. In literature, we encountered two examples of Passerini reaction carried out on the side chain of polymer backbone [11], [12]. The Passerini reactions could be performed after converting these ester pendent groups into the carboxylic acid groups. Whereas in this work, carboxylic acid functionality was directly attached into the polyester chain via Huisgen, which is followed by a reaction with a variety of aldehyde and isocyanide compounds through Passerini protocol. Moreover, it should be noted that both reaction protocols could be carried out under benign conditions without adding external catalyst.
Section snippets
Materials
Dichloromethane (CH2Cl2, 99%, J. T. Baker) was dried and distilled over and P2O5. N,N-dimethylformamide (DMF, 99.8%, Aldrich) was dried and distilled under vacuum over CaH2. p-Toluenesulfonic acid monohydrate (PTSA 99%, Aldrich), acetylenedicarboxylic acid (ADCA, 95%, Aldrich), 1,4-butanediol (99%, Aldrich), cyclohexyl isocyanide (CHI, 98%, Aldrich), tert-butyl isocyanide (TBI, 98%, Aldrich), propanal (97%, Aldrich), isobutyraldehyde (99%, Sigma-Aldrich), benzaldehyde (99%, Sigma-Aldrich), m
Results and discussion
Polyester 1 was achieved from classical polycondensation between ADCA and 1,4-butanediol according to a previously published procedure (Mn = 4900 g/mol, Mw/Mn = 1.40 (relative to PS standard)) [42]. The benzyl/carboxylic acid functionalization of polyester 1 was performed via Huisgen 1,3-dipolar cycloaddition reaction. Polyester 1 (1 equiv) was reacted with azidopropionic acid (1 equiv per alkyne) and benzyl azide (3 equiv per alkyne) in DMF at 40 °C for 16 h. Next, a simple precipitation of
Conclusions
Recently, the electron deficient polyester backbone containing reactive alkyne groups in the main chain enabled us to perform the 1,3-dipolar and Diels-Alder cycloaddition reactions using variety of dipoles and dienes, respectively. We found that the electron deficient polyester was an effective polymeric platform in the 1,3-dipolar cycloaddition reactions carried out at moderately lower temperatures for a reasonable period of time to achieve high efficiencies.
In this work, we combined
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