N-doped TiO2 nanocatalyst for one-pot synthesis of α,β-unsaturated esters via the Wittig reaction: a highly efficient and recyclable heterogeneous system

Abstract

Wittig reactions represent one of the important tools for the stereoselective synthesis of substituted alkenes. In view of the importance of the Wittig reaction in organic synthesis, we have explored the Wittig reaction using a nitrogen-doped titanium dioxide (N-TiO₂) nanocatalyst. The nitrogen doping of titanium dioxide was achieved via a sol–gel method employing triethylamine as the nitrogen source. The structural and morphological elucidation of the as-synthesized N-TiO₂ nanocatalyst were carried out by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDX or EDS), X-ray Photoelectron Spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM) and Brunauer–Emmett–Teller (BET). The XRD analysis confirmed the formation of an anatase-dominated mixed phase of titanium dioxide, with an average crystallite size of approximately 10–20 nm, which is in good agreement with the particle size observed in the HRTEM analysis. The synthesized sample has a rough and porous surface morphology, with irregularly shaped particles showing substantial aggregation, according to the FESEM micrographs recorded. The average particle size of the observed surface characteristics is around 20 nm, suggesting that nanoscale texturing is appropriate for improved surface-related capabilities. A noticeable nitrogen signal in the elemental spectrum from EDX analysis demonstrated that nitrogen had been successfully incorporated into the TiO₂ lattice. XPS analysis elucidated the efficient incorporation of nitrogen into the TiO₂ lattice, involving the substitution of oxygen atoms, where nitrogen was identified to exist in both substitutional N(O–Ti–N) and interstitial N(Ti–O–N) forms. The as-synthesized N-TiO₂ nanocrystalline material was employed as a heterogenous catalyst for the one-pot Wittig reaction involving aldehydes, α-halo esters, and triphenylphosphine to produce α,β-unsaturated esters in good yields by maintaining E-stereoselectivity at room temperature. The N-TiO₂ nanocatalyst was recovered and recycled by a simple filtration technique. The structural confirmation of the synthesized Wittig product was done by ¹H-NMR and ¹³C-NMR spectral techniques. The study establishes N-TiO₂ as a robust and stable catalyst for the synthesis of α,β-unsaturated esters. The synthetic protocol presented herein offers several merits, including mild reaction conditions, high purity of synthesized products, facile catalyst separation, straightforward reaction workup and environmental benignity. These findings emphasize the potential of the nanostructured N-TiO₂ material as a well-organized and sustainable heterogeneous nanocatalyst for the synthesis of valuable organic compounds.