A π-Extension Process from 9-Phosphaanthracene Leading to Phosphatetraphenes and Phosphatetracenes

Extension of π-system with phosphinine (phosphorine, phosphabenzene) has been of interest because of the expected higher HOMO and lower LUMO levels compared with the corresponding carbon congeners. In this paper, a π-extension process based on the 9-phosphaanthracene skeleton was demonstrated by synthesizing 12-phosphatetraphene and 9-phosphabenzo[f]tetraphene by utilizing the deaminative aromatization pathway. Starting from 3,5-bis(trifluoromethyl)aniline, we developed the dibromotriarylmethane precursors containing the 3,5-bis(trifluoromethyl)-2-bromophenyl unit, which would be slightly effective to increase the steric congestion around the fragile P=C bonds incorporated in the fused polyaromatic skeletons. The bis-trifluoromethyl 12-phosphatetraphenes have been synthesized together with the mono-trifluoromethyl derivative, and the planar 12-phosphatetraphene skeleton was confirmed. On the other hand, the CF₃ -substituted 9-phosphabenzo[f]tetraphene displayed a remarkably twisted fused five ring system leading to the wavy structures incorporating phosphinine. Also, synthetic study of 5-phosphatetracene using the bis(trifluoromethyl)phenyl unit was attempted and the incomplete elimination of the amine indicated labile characters of the observed phosphorus congener of tetracene. The findings of this study would be informative for developing heavier congeners of polyaromatic hydrocarbons (PAHs) as well as the trifluromethyl effects.