Palladium-Catalyzed Direct Arylation of Azulene Based on Regioselective C-H Bond Activation

An efficient synthesis of arylazulenes involving minimal steps was developed. The combination of Pd(OAc)₂/XPhos as a catalyst and pivalic acid as an additive was key for the direct arylation of C-H bonds, and the reaction proceeded preferentially at the 1- and 3-positions of azulene, without heteroatom-containing directing groups. Compared with the traditional cross-coupling protocol, the arylation method described here requires fewer steps and uses commercially available synthetic blocks. The degree of conjugation and the optical properties of the resulting azulene conjugates can be adjusted by simple protonation, which allows the current method to be an efficient strategy for exploiting novel azulene-based functional materials.