Additive- and base-free tandem aerobic oxidative cleavage of olefins to esters using bifunctional mesoporous copper-incorporated Al-SBA-15

Abstract

Carbonyl compounds find extensive use in fragrances, flavors, and pharmaceuticals, and esters are responsible for the fruity and floral aromas found in a variety of foods and beverages. Several traditional methods were used for the synthesis of esters from aldehydes or alcohols using noble metal catalysts (Au, Pd) in the presence of corrosive organic solvents (dichloroethane, acetonitrile, and fluorinated solvents). However, the cost and abundance of rare earth metals restricted large-scale industrial applications. Additionally, achieving a one-pot synthesis of esters from olefins introduces additional challenges. Herein, we report a non-noble metal catalyst (10Cu-Al-SBA-15) that facilitates aerobic oxidative cleavage of olefins into carbonyl compounds, followed by esterification with alcohols in the presence of γ-valerolactone as an eco-friendly solvent. PXRD, H₂-TPR, and XPS analyses of the 10Cu-Al-SBA-15 catalyst confirmed the presence of Cu²⁺ ions in the catalyst. NH₃-TPD analysis quantified the acidity of the 10Cu-Al-SBA-15 catalyst, yielding a value of 0.361 mmol g⁻¹. The 10Cu-Al-SBA-15 catalyst was tested for one-pot oxidative esterification of styrene to butyl benzoate. Under optimized reaction conditions, complete conversion of styrene with 50% selectivity for butyl benzoate was achieved and successfully scaled up to 5 g scale. Further, the catalyst was also employed for the one-pot, two-step oxidative esterification of various aromatic olefins to the corresponding aromatic esters. This protocol achieved complete conversion of olefins with 10–66% selectivity for the corresponding esters. Electron paramagnetic resonance studies and radical trapping experiments indicated that the reaction follows a radical pathway. The catalyst was found to be recyclable for up to four cycles with minimal loss in the selectivity for butyl benzoate.