Effect of Benzyl Functionality on Microwave-Assisted Cleavage of Cα–Cβ Bonds in Lignin Model Compounds

Abstract

The effect of benzylic alcohol (Cα—OH) pretreatments on microwave-assisted cleavage of Cα—Cβ bonds in lignin model compounds was investigated by combining experimental results with density functional theory (DFT) calculations at the M06-2X/6-311++G(d,p) level. The oxidization of Cα—OH to benzylic ketone (Cα═O) facilitated the cleavage of the Cα—Cβ bond due to a greater decomposition rate constant and a lower barrier for the decomposition-determining step. The reduction of Cα—OH to benzylic hydrogen (Cα—H) resulted in no Cα—Cβ bond being cleaved. Since the Cα—OH quickly reacted with methanol solvent, the etherification of Cα—OH to benzylic ether (Cα—OCH3) had no effect on the Cα—Cβ bond cleavage. The aromatic aldehyde would be obtained from the model compounds containing Cα—OH or Cα—OCH3, while the model compound containing Cα═O would produce aromatic ester. The research also revealed that the microwave-assisted cleavage of Cα—Cβ bonds followed a first-order kinetic and radical reaction mechanism.