Catalytic Gels for a Prebiotically Relevant Asymmetric Aldol Reaction in Water: From Organocatalyst Design to Hydrogel Discovery and Back Again

This paper reports an investigation into organocatalytic hydrogels as prebiotically relevant systems. Gels are interesting prebiotic reaction media, combining heterogeneous and homogeneous characteristics with a structurally organized active “solid-like” catalyst separated from the surrounding environment, yet in intimate contact with the solution phase and readily accessible via “liquid-like” diffusion. A simple self-assembling glutamine amide derivative 1 was initially found to catalyze a model aldol reaction between cyclohexanone and 4-nitrobenzaldehyde, but it did not maintain its gel structure during reaction. In this study, it was observed that compound 1 could react directly with the benzaldehyde to form a hydrogel in situ based on Schiff base 2 as a low-molecular-weight gelator (LMWG). This new dynamic gel is a rare example of a two-component self-assembled LMWG hydrogel and was fully characterized. It was demonstrated that glutamine amide 1 could select an optimal aldehyde component and preferentially assemble from mixtures. In the hunt for an organocatalyst, reductive conditions were applied to the Schiff base to yield secondary amine 3, which is also a highly effective hydrogelator at very low loadings with a high degree of nanoscale order. Most importantly, the hydrogel based on 3 catalyzed the prebiotically relevant aldol dimerization of glycolaldehyde to give threose and erythrose. In buffered conditions, this reaction gave excellent conversions, good diastereoselectivity, and some enantioselectivity. Catalysis using the hydrogel of 3 was much better than that using non-assembled 3—demonstrating a clear benefit of self-assembly. The results suggest that hydrogels offer a potential strategy by which prebiotic reactions can be promoted using simple, prebiotically plausible LMWGs that can selectively self-organize from complex mixtures. Such processes may have been of prebiotic importance.


Materials and Instrumentation
Unless otherwise noted all compounds were bought from commercial suppliers and used without further purification. Where a solvent is described as "dry" it was purified by PureSolv alumina columns from Innovative Technologies. Melting points were determined using a Stuart SMP3 apparatus. Optical rotations were carried out using a JASCO-DIP370 polarimeter and [α]D values are given in deg.cm 3 g −1 dm −1 . Infra-red spectra were acquired on a ThermoNicolet Avatar 370 FT-IR spectrometer. Nuclear magnetic resonance spectra were recorded on a Jeol ECS-400, a Jeol 500 Avance III HD 500 or a Jeol AV500 at ambient temperature. Coupling constants (J) are quoted in Hertz. Mass spectrometry was performed by the University of York mass spectrometry service using electrospray ionisation (ESI). Thin layer chromatography was performed on glass-backed plates coated with Merck Silica gel 60 F254. The plates were developed using UV light, acidic aqueous ceric ammonium molybdate or basic aqueous potassium permanganate. Liquid chromatography was performed using forced flow (flash column) with the solvent systems indicated. The stationary phase was silica gel 60 (220-240 mesh) supplied by SigmaAldrich. Preparative Thin Layer Chromatography S6

2-Benzylamino-N 1 -dodecyl-ʟ-glutamine amide (3)
To ʟ-glutamine amide (1) (200 mg, 0.64 mmol) was added benzaldehyde (81 uL, 0.77 mmol, 1.2 eq) followed by 100 mL of water. The suspension was sonicated for 2 minutes then heated until boiling or until fully dissolved to initiate gelation. The solution was then left to cool. Once the solution had gelled, it was extracted into DCM (3 x 50 mL) the organic layer was removed

General Procedure for Solution Phase Catalysis of 4-Nitrobenzaldehyde and Cyclohexanone 3
To a solution of catalyst (0.065 mmol) in water (20 mL) was added a solution of 4nitrobenzaldehyde (98 mg, 6.5 mmol) in cyclohexanone (6.7 mL, 65 mmol). The reaction was left for a period of time (24, 48 or 72 h) with no stirring. After this time the reaction mixture was extracted with DCM then solvent was removed in vacuo to yield a yellow solid crude S15 material. The conversion of the reaction was determined by integrating the 1 H NMR of the crude reaction mixture using the aldehyde peak as a reference. The syn:anti ratio was determined by integrating the 1 H NMR of the crude reaction mixture and by comparing the CH-OH peaks. The enantiomeric excess of the crude product was analysed, via HPLC using a Chiralpak IB column (97:3 Hexane: IPA Flow rate 1 mL/min).

General Procedure for the Dimerization of Glycolaldehyde on Hydrogel 3
To the surface of the benzylglutamine amide hydrogel 3 (20 mg, 49.6 mol, 5 mL) was added glycolaldehyde (59 mg, 0.49 mmol) in water (200 L) in 10 L aliquots. After 48 h the reaction was stopped by removal of the water in vacuo. Once the water was removed, 1.2 eq of diphenyl hydrazone was added in methanol with 1-2 drops of acetic acid. The reaction was left to stir for 1 h at room temperature. After 1 h the solvent was removed to yield a brown oil. The crude oil was analysed by 1 H NMR to determine conversion of glycolaldehyde to tetrose sugars. The crude material was then purified by column chromatography (100 % DCM     We also determined the rheology of the gel based on Schiff base 2a formed. We were able to carry out this rheology at pH 7, and for purposes of direct comparison with gelator 3, we measured the rheological performance at a total loading of 4 mg/mL (gutamine amide + benzaldehyde).     To learn about the thixotropic potential of the gel several investigations of injectability were carried out. Three methods of breaking the gel network down were tested, manually shaking, injecting (Fig. S42) and de-hydrating. Once broken down the system was allowed to heal in different ways (Table S10).

Characterisation of ʟ-glutamine amide (1) and 4-nitrobenzaldehyde gel
Gel formed in syringe barrel Injection into vial Leave to set Tube inversion test Figure S46: Testing injectability of gels S43  of 5 mg/mL, with one equivalent of each of benzaldehyde and vanillin, in D2O. Figure S47: Full 1 H NMR spectrum of 1:1:1 glutamine amide:benzaldhyde:vanillin at a glutamine amide loading of 5 mg/mL, in D2O with a spike of DMSO (2 l). Key peaks: 9.85 ppm (benzaldehyde), 9.55 (vanillin) and 2.6 (DMSO). Figure S48: 1 H NMR spectrum focussed on the aldehyde region of 1:1:1 glutamine amide:benzaldhyde:vanillin at a glutamine amide loading of 5 mg/mL, in D2O with a spike of DMSO (2 l). Key peaks: 9.84 ppm (benzaldehyde) and 9.56 ppm (vanillin). Less benzaldehyde is visible in the 1 H NMR spectrum which indicates that more has been incorporated into the 'solid-like' gel nanofibres.