Towards racemizable chiral organogelators

Abstract

A chiral organogelator has been synthesized that can be racemized and self-assembled in apolar solvents whilst at higher concentrations organogels are formed. Field emission scanning and transmission electron microscopy revealed the formation of bundle fibrils that are able to gelate the solvent. ¹H NMR studies showed hydrogen-bond interactions between the peptide head groups of neighbouring organogelator molecules. The enantiomerically pure organogelator can be racemized by the base DBU (1,8-diazabicyclo[5.4.0]undec-7-ene) as was evident from chiral high-performance liquid chromatography analysis.

Keywords: chirality; organogels; racemization; self-assembly.

Scheme 1

Synthesis of R-3 and S-3.

Figure 1

Concentration-dependent ¹H NMR spectra of R-3 in chloroform (CDCl₃). The red colours indicate the hydrogen resonances of the amide unit.

Figure 2

a) R-3 gel in octane (5 mM); b) octane solution containing a mixture of R-3 (2.5 mM) and S-3 (2.5 mM) after cooling from 80 °C to room temperature; c) TEM image of the xerogel of R-3 in octane; d) the surface morphology of the dried gel obtained from R-3 in octane (15 mM) observed by FESEM.

Figure 3

a) Concentration-dependent ¹H NMR spectra of R-3 in cyclohexane-d₁₂ and b) the shift of N–H signal of amide group versus concentration.

Figure 4

The evolution of ln(ee × 100) for the racemization of R-3 (1 mM) in the presence of 1 equivalent of DBU in octane and THF.