Direct synthesis of p-methyl benzaldehyde from acetaldehyde via an organic amine-catalyzed dehydrogenation mechanism

p-Methyl benzaldehyde (p-MBA) is a class of key chemical intermediates of pharmaceuticals. Conventional industrial processes for p-MBA production involve the consecutive photochlorination, amination, and acid hydrolysis of petroleum-derived p-xylene, while producing vast pollutants and waste water. Herein, we report a direct, green route for selective synthesis of p-MBA from acetaldehyde using a diphenyl prolinol trimethylsilyl ether catalyst. The optimized p-MBA selectivity is up to 90% at an acetaldehyde conversion as high as 99.8%. Intermediate structure and O-18-isotope data revealed that the conversion of acetaldehyde to p-methylcyclohexadienal intermediates proceeds in an enamine-iminiumintermediate mechanism. Then, controlled experiments and D-isotope results indicated that the dehydrogenation of p-methylcyclohexadienal to p-MBA and H-2 is catalyzed by the same amines through an iminium intermediate. This is an example that metal-free amines catalyze the dehydrogenation (releasing H-2), rather than using metals or stoichiometric oxidants.

Automated summary

A green and direct method for selective synthesis of p-MBA from acetaldehyde using a specific catalyst was reported in this study, achieving a high acetaldehyde conversion rate of up to 99.8% and p-MBA selectivity of over 90%. Experimental results revealed the conversion mechanism and catalytic process involved.