Aerobic oxidative lactonization of diols at room temperature over defective titanium-based oxides in water

Lactones are applied in pharmaceutical or agriculture/forestry industry as anesthetics/sedatives, pesticide intermediates or as plant growth regulators. Selective conversion of diols into value-added lactones under mild reaction conditions is still challenging. Here, we report on an environmentally benign catalytic approach using oxygen vacancies-enriched titanium-based oxides and water as solvent at room temperature for the lactonization of various aromatic and aliphatic diols. Lactonization was found to proceed in two consecutive steps: 1) the dehydrogenation of diols to hydroxyaldehyde intermediates catalyzed by the Pt cocatalyst; 2) the subsequent oxidative lactonization of the intermediates to the lactone products over defective titanium-based oxides. Diffuse reflectance infrared Fourier transform spectroscopy analysis using pyridine as probe molecule combined with electron paramagnetic resonance results suggest that oxygen vacancies as strong Lewis acid sites are pivotal for the superior catalytic activities at room temperature, which are attributed to the enhanced reaction rate of step 2 due to stronger adsorption of the hydroxyaldehyde intermediates on the Lewis acid sites.