Selective Synthesis of Novel Cyclic Phenylazomethine Trimers

Abstract
Novel cyclic phenylazomethine trimers (CPAs) were synthesized in a one-step dehydration of the 4-aminobenzophenone derivatives in the presence of TiCl4 or p-toluenesulfonic acid (PTS). The CPAs were isolated in over 90% yield under nondilute conditions. When using TiCl4 as the dehydration agent, the induction of bulky substituents at the α-position of the substrate enhanced the yields of the CPAs. On the other hand, PTS served as an effective catalyst for the synthesis of the phenyl-substituted CPA. This different reactivity between TiCl4 and PTS depends on the dehydration mechanism being dominated by a kinetic process or thermodynamic one. The obtained CPAs were confirmed by NMR, UV−vis spectra, and MM2 calculation to have only a Z conformation and a nonconjugated structure compared to the linear oligophenylazomethines (OPAs) and the aniline-capped OPAs (OPA's).

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