Benzoxazine Atropisomers: Intrinsic Atropisomerization Mechanism and Conversion to High Performance Thermosets

Atropisomers have inspired chemists and biologists for decades due to their chiral structures and associated biological properties. However, most of atropisomers reported so far arise in highly substituted biaryls and related compounds, and other types have been rarely observed. Here we report a new type of atropisomerism in ortho-tetrahydrophthalimide functional 1,3-benzoxazine family, where the atropisomerism is evident from NMR spectra, with the branching ratio of the atropisomeric configurations invariant with the measurement temperatures. Density functional theory calculations suggested that the reaction intermediate, ortho-tetrahydrophthalimide phenol, is key to the atropisomerism, which creates a large energy barrier after deprotonation and thus determines the branching ratios of the benzoxazine atropisomers. In addition, the ring-opening polymerization of benzoxazine atropisomers has also been investigated. The benzoxazine atropisomers bearing acetylene exhibit unexpectedly low polymerization temperature in the absence of catalysts, suggesting a self-catalyzed polymerization process. Despite the absence of antiflammable additives, the corresponding polybenzoxazine deriving from benzoxazine atropisomers containing acetylene shows exceptionally low heat release capacity (67.2 J g1–K–1) and excellent char residue value (62%). With this work we demonstrate atropisomerism in the 1,3-benzoxazine family for the first time, and provide molecular-level insights to the mechanism, which can open up possibilities for new applications of atropisomers spanning from the microelectronic to the aerospace industries.