Nanostructured N doped TiO2 efficient stable catalyst for Kabachnik-Fields reaction under microwave irradiation

Abstract

Herein, we report nitrogen-doped TiO₂ (N-TiO₂) solid-acid nanocatalysts with heterogeneous structure employed for the solvent-free synthesis of α-aminophosphonates through Kabachnik-Fields reaction. N-TiO₂ were synthesized by direct amination using triethylamine as a source of nitrogen at low temperature and optimized by varying the volume ratios of TiCl₄, methanol, water, and triethylamine, under identical conditions. An X-ray diffraction (XRD) study showed the formation of a rutile phase and the crystalline size is 10 nm. The nanostructural features of N-TiO₂ were examined by HR-TEM analysis, which showed they had rod-like morphology with a diameter of ∼7 to 10 nm. Diffuse reflectance spectra show the extended absorbance in the visible region with a narrowing in the band gap of 2.85 eV, and the high resolution XPS spectrum of the N 1s region confirmed successful doping of N in the TiO₂ lattice. More significantly, we found that as-synthesized N-TiO₂ showed significantly higher catalytic activity than commercially available TiO₂ for the synthesis of a novel series of α-amino phosphonates via Kabachnik-Fields reaction under microwave irradiation conditions. The improved catalytic activity is due to the presence of strong and Bronsted acid sites on a porous nanorod surface. This work signifies N-TiO₂ is an efficient stable catalyst for the synthesis of α-aminophosphonate derivatives.

Fig. 1. Some biological active α-aminophosphonate.

Scheme 1. Synthesis of 1-phenyl-5-(thiophen-2-yl)-1H-pyrrole-3-carbaldehyde.

Fig. 2. X-ray diffraction patterns of (a) TN0 (TiO₂), (b) TN1, (c) TN2 (d) TN3 (e) TN4.

Fig. 3. HR-TEM images of (a–c) TN0, (d–f) TN1, and (d–f) TN2; inset c, f and h SAED pattern of TN0, TN2 and TN3 respectively.

Fig. 4. HR-TEM images of (a–c) TN3 and (d–f) TN4; inset c, and f SAED pattern of TN3, and TN4 respectively.

Fig. 5. Nitrogen (N₂) adsorption–desorption isotherms of (a) TN0 (TiO₂), (b) TN2 (N-TiO₂), (c) TN4 (N-TiO₂). Insets shows their corresponding pore size distributions.

Fig. 6. UV-DRS spectra of (a) TN0 (TiO₂), (b) TN1 (c) TN2 (d) TN3 (N-TiO₂), (e) TN4. Insets shows Tauc plot of TiO₂ and N-TiO₂ samples.

Fig. 7. FTIR spectra of (a) TN0 pure (TiO₂), (b) TN1 (N-TiO₂), (c) TN2, (d) TN3 and (e) TN4.

Fig. 8. (a and b) High resolution spectrum of N 1s region (c) high resolution spectrum of Ti 2p region (d) high resolution spectrum of O 1s region.

Scheme 2. Standard model reaction.

Fig. 9. (A) Progress of reaction (a) TN0 (b) TN1 (c) TN2 (d) TN3 and (e) TN4. (B) Reusability of catalyst TN4; reaction condition: aldehyde (1a) (1 mmol), aniline (2a) (1 mmol), triethylphosphite 3 (1.1 mmol), N-TiO₂ (12 mol%), MW power 420 watt.

Fig. 10. XRD of sample TN4 (a) before reaction (b) after reaction.

Scheme 3. Optimized reaction condition for synthesis of diethyl(1-phenyl-3-(thiophen-2-yl)-1H-pyrazol-4-yl)(phenylamino)methylphosphonates