Single crystal XRD and FTIR studies of 4H,4H′-trimethylenedipiperidine-diium hydrogen sulfate hydrate and investigation its catalytic activity in Fischer esterification using the in-situ continuous liquid–liquid extraction technique

Abstract

The chemical structure of 4,4′-trimethylenedipiperidinium hydrogen sulfate hydrate was determined by single-crystal X-ray diffraction. Although ¹H NMR did not show acid proton of [HSO₄]⁻, the FTIR spectrum of organic salt could play a crucial role in distinguishing the existence of [HSO₄]⁻ or [SO₄]²⁻. The single-crystal structure of molten salt further supports the presence of hydrogen sulfate anions with water molecules. The molten salt is a strong network of 4,4′-trimethylenedipiperidinium dications and hydrogen sulfate anions and water molecules with various electrostatic and hydrogen-bonding interactions. Two piperidine rings existed in the energetically stable chair conformation. In addition, the X-ray diffraction pattern of molten salt showed a crystallite size and interplanar spacing (d) at a range of 55.42–146.47 nm and 0.18–0.62 nm, respectively, proving a nanocrystal size structure for the organic acid salt. Finally, the catalytic activity of the organic acid salt was investigated for the acetylation of 1-butanol with glacial acetic acid under different conditions. Liquid–liquid in situ extraction technique was conducted to improve Fischer esterification using cyclohexane. After optimization, the substrate scope and generality of the new catalytic system were demonstrated by the esterification of various alcohols with glacial acetic acid, affording the desired acetates in a 73–82% isolated yield. Furthermore, a high yield of enantioselective acetate was isolated by reacting chiral alcohol, (−)-menthol, with glacial acetic acid under the optimized reaction conditions. Moreover, the scale-up experiment was demonstrated by the production of O-acetyl-α-tocopherol (Ac-TCP) through the esterification of α-tocopherol (α-TCP) with glacial acetic acid.

Graphic Abstract