Isopropyl Cresols: Synthesis and Herbicidal Activity

Isopropyl cresols were prepared by a two-step process: monoterpene oxidation catalyzed by CrO3 and terpenone isomerization catalyzed by 13X molecular sieves. 5-Isopropyl-3-methylphenol and carvacrol were prepared from 3-carene, whereas thymol and 4-isopropyl-3-methylphenol were synthesized from alpha-pinene. The two reactions followed an identical procedure, but the reaction of 3-carene was more efficient, benefiting from the three-membered ring in the structure. 5-Isopropyl-3-methylphenol, thymol, and carvacrol were found to show higher herbicidal activity than glyphosate toward the root growth of barnyard grass. However, they exhibited slightly lower activity in preventing shoot growth. Moreover, 5-isopropyl-3-methylphenol showed higher herbicidal activity than its isomers, which indicates that the relative position of substitutes is significantly associated with the herbicidal activity of isopropyl cresols.