Efficient Synthesis and Structural Analysis of Chiral 4,4′‐Biazulene

Abstract

4,4'‐Biazulene is a potentially attractive key component of an axially chiral biaryl compound, however, its structure and properties have not been clarified owing to the lack of its efficient synthesis. We report a breakthrough in the reliable synthesis of 4,4'‐biazulene, which is achieved by the access to azulen‐4‐ylboronic acid pinacol ester and 4‐iodoazulene as novel key synthetic intermediates for the Suzuki–Miyaura cross‐coupling reaction. The X‐ray crystallographic analysis of 4,4'‐biazulene confirmed its axial chirality. The enantiomers of 4,4'‐biazulene were successfully resolved by HPLC on the chiral stationary phase column. The kinetic experiments and DFT calculations indicate that the racemization energy barrier of 4,4'‐biazulene is comparable to that of 1,1'‐binaphthyl.