Efficient Synthesis and Structural Analysis of Chiral 4,4'-Biazulene.

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.