Synthesis and Herbicidal Activities of p-Menth-3-en-1-amine and Its Schiff Base Derivatives

p-Menth-3-en-1-amine, 4, and its Schiff base derivatives, 5a-l, were designed and synthesized. They were characterized by FT-IR, ESI⁺-MS, HRMS, ¹H NMR, and ¹³C NMR spectral analyses, and their pre-emergence herbicidal activities against ryegrass were evaluated. All of the compounds showed excellent herbicidal activity. The Schiff bases showed stronger herbicidal activities than the original amine 4. These compounds showed herbicidal activities comparable to that of glyphosate. The herbicidal activities of 5k and 5l against ryegrass shoot growth were 78.3 and 355.6% higher than that of glyphosate, respectively. Furthermore, the introduction of a chlorine or bromine atom into the Schiff base derivatives containing a furan or benzene ring was beneficial to increase the activity. However, the herbicidal activities were not clearly affected when the heteroatom of the five-membered heterocyclic Schiff base or the position of the substituent on pyridine Schiff base was altered.