Synthesis of 3,3′′,5,5′′‐Tetraphenyl‐4,4′′‐terphenoquinone and Its Reductive Disproportionation Reaction

Based on our recent reinvestigation of the disproportionation reaction of 3,3′,5,5′‐tetraphenyl‐4,4′‐biphenoquinone, which exhibited drastic color change, we performed the synthesis and characterization of 3,3′′,5,5′′‐tetraphenyl‐4,4′′‐terphenoquinone with the aim of obtaining enhanced physical properties, such as lower reduction potentials and bathochromic shift in light absorption, and realizing a disproportionation reaction at a lower temperature. To construct the terphenyl and/or terphenoquinone skeletons, the Suzuki‐Miyaura coupling, which involves coupling between 1,4‐diiodobenzene and the boronic acid ester with phenyl and oxygen moieties attached was carried out. The subsequent hydroquinone formation reaction and oxidation reaction yielded a brilliant ultramarine‐blue terphenoquinone. The quinone showed properties typical of π‐expanded quinones, namely, positively shifted reduction potential, small on‐site Coulomb repulsion, and lower excitation energy. DSC measurement of this product showed an exothermic process at 212.5 °C and this temperature was much lower than that of the corresponding biphenoquinone, 289.5 °C. The products of this reaction were similar to those of the biphenoquinone reaction, i. e., two benzofurans and the corresponding hydroquinone. A terphenoquinone having four phenyl groups was prepared by aryl coupling. This molecule showed disproportionation reaction at 212.5 °C in solid state with a drastic color change.