Green Synthesis Of Natural Benzaldehyde With Supramolecular Catalyst

In total amount of spice in the world, benzaldehyde is the second largest perfume after vanillin. With more cares about food quality from consumers, natural benzaldehyde is more and more popular and represents a strong market advantage. As a result, the synthesis of natural benzaldehyde is drawing much attention. Based on the rich natural cinnamal oil, of which cinnamaldehyde is the main composition, we have developed a clean and efficient process to produce benzaldehyde from cinnamaldehyde under much milder conditions, using various green technologies including green oxidant, green solvent and green catalyst. Moreover, the mechanism has been investigated by experiments, various characteristic methods, in-situ characterization, ab initio computational methods and kinetics methods.In this paper, oxidative cleavage of cinnamaldehyde to benzaldehyde catalyzed by metalloporphyrins in the presence of dioxygen had been investigated at first. MnTPPCl showed excellent activity for the oxidation at the ppm level and 71% yield of benzaldehyde was obtained at 60℃and ambient atmosphere. It was attributed to manganese having variable valence and appropriate reduction potential to facilitate formation of high valent metal-oxo ligand, which was verified by UV-vis spectroscopy.Then, this paper focused on the conversion of cinnamaldehyde to benzaldehyde catalyzed byβ-cyclodextrin (β-CD) or its derivatives. It is because water is good for environment and food safety as a cheap and clean solvent. Moreover, organic reactions in aqueous solution have been one of roles of green chemistry.Inβ-CD catalytic alkaline hydrolysis system, various analysis methods, e.g., FT-IR, UV-Vis, ¹H NMR, ROESY and Gaussian 03 had been utilized to demonstrate the formation of 1:1 (molar ratio) complexes betweenβ-CD and cinnamaldehyde and the inclusion equilibrium constant was 329 M^-1 at 298K. Furthermore, the aldehyde group forms hydrogen bond with the secondary alcohol ofβ-CD. It is the hydrogen bond that facilitates formation of the oxygen anion (O-) and the nucleophilic addition of external hydroxide ion with cinnamaldehyde, which results into the acceleration ofβ-CD over alkaline hydrolysis of cinnamaldehyde. However, both acetaldehyde and cinnamaldehyde ownα- proton to the carbonyl group, the crossed Aldol condensation reaction also took place. In addition, benzaldehyde was prone to its own disproportionation and polymerization reactions. As a result, the yield of benzaldehyde decreased to be only 42% at 50℃and ambient atmosphere. From the results of kinetic analysis, it could be found that the activation energy for the hydrolysis of cinnamaldehyde to benzaldehyde decreased from 57.11 kJ/mol in the absence ofβ-CD to 45.27 kJ/mol in the presence ofβ-CD. It is the first time to clarity whyβ-CD can promote the reaction from the perspective of the activation energy.In 2-hydroxypropyl-β-cyclodextrin (2-HPβ-CD) catalytic alkaline hydrolysis system, the combination of DSC, FT-IR, UV-Vis, 1H NMR, ROESY and fluorescence measurements was used to verify the formation ofβ-CD/cinnamaldehyde inclusion complex with molar ratio of 1:1 and the inclusion equilibrium constant was 928 M^-1 at 298K. Moreover, the aldehyde group forms strong hydrogen bond with 2-HPβ-CD. The further investigation on kinetic studies and solubilization indicated that weak molecular interactions between guest and the CDs had a direct relevance on their solubilization efficiency, and the binding abilities among CDs and substrate mainly affected the hydrolysis reactivity. Then, 2-HPβ-CD conferred high activity and selectivity for the alkaline hydrolysis of cinnamaldehyde and the yield of benzaldehyde was 70% at 50℃and ambient atmosphere.Inβ-CD/NaClO orβ-CD/H2O2 oxidation system, at 60℃and ambient atmosphere, the yield of benzaldehyde was 76% in the presence of NaClO and was 78% in the presence of H2O2. Obviously, H₂O₂ is a cleaner and more appropriate oxidant than NaClO. A large-scale experiment for the conversion of natural cinnamal oil ( 93% cinnamaldehyde ) to benzaldehyde in H₂O₂ -NaHCO₃ oxidation system was carried out and the yield of benzaldehyde was 67%. Peroxymonocarbonate ion (HCO₄^-) was easily formed by H₂O₂ with HCO₃^-, which shows a strong nucleophilic oxidation toward the included cinnamaldehyde. Furthermore, the non-covalent intermolecular interactions betweenβ-CD and cinnamaldehyde promoted the nucleophilic oxidation. Therefore, trace amount of benzaldehyde and large amount of the corresponding epoxide were obtained. The epoxide was further oxidized to benzaldehyde by HCO₄^-. These processes were verified by in situ FTIR spectra, theoretical method and GC-MS.The success of amplification experiment indicated that theβ-CD/H₂O₂ -NaHCO₃ oxidation system achieved a clean and efficient process for producing natural benzaldehyde from natural cinnamal oil in water at 60℃and ambient atmosphere, which has both environmentally-friendly and good economy. The benign, mild and straightforward methodology for the conversion of cinnamaldehyde to benzaldehyde may find its potential application in industrial operation and offer theoretical and technical foundation and a general method for the clean synthesis of other similar natural aromatic fragrant compounds.