Duality of Reactivity of a Biradicaloid Compound with an o-Quinodimethane Scaffold.

Sigmarene, which is a Kekulé hydrocarbon with appreciable singlet biradical character originating from an o-quinodimethane scaffold, is isolated as a doubly σ-bonded dimer. The dimer dissociates into a monomeric sigmarene upon heating or photoirradiation. The monomeric species undergoes a rapid [4 + 4] cycloaddition reaction under dark conditions even at room temperature to produce the dimer. Contrarily, the monomeric sigmarene undergoes a [4 + 2] cycloaddition reaction in the presence of dienophile as an orbital symmetry allowed process. Therefore, the sigmarene shows high reactivity for both symmetry-forbidden and allowed processes in the framework of the orbital symmetry rule. This duality of reactivity of the sigmarene is consistent with the intermediate singlet biradical character (44%) estimated by a density functional theory (DFT) calculation.

Three-dimensional graphene nanoribbons as a framework for molecular assembly and local probe chemistry.

Recent advances in state-of-the-art probe microscopy allow us to conduct single molecular chemistry via tip-induced reactions and direct imaging of the inner structure of the products. Here, we synthesize three-dimensional graphene nanoribbons by on-surface chemical reaction and take advantage of tip-induced assembly to demonstrate their capability as a playground for local probe chemistry. We show that the radical caused by tip-induced debromination can be reversibly terminated by either a bromine atom or a fullerene molecule. The experimental results combined with theoretical calculations pave the way for sequential reactions, particularly addition reactions, by a local probe at the single-molecule level decoupled from the surface.

Unforeseen 1,2-Aryl Shift in Tetraarylpyrrolo[3,2- b]pyrroles Triggered by Oxidative Aromatic Coupling.

Tetraarylpyrrolo[3,2- b]pyrroles (TAPPs) possessing [1,1'-biphenyl]-2-yl substituents attached to the pyrrolic nitrogen atoms undergo selective double dehydrogenative cyclization accompanied by twofold 1,2-aryl migration under oxidative aromatic coupling conditions. The structure of the product of the rearrangement has been unambiguously confirmed by X-ray crystallography, and the reaction pathway is supported by density functional theory (DFT) calculations. Six-membered ring formation (requiring rearrangement of aryl substituents around the core) is energetically preferred over seven-membered ring closure, and a 1,2-aryl shift occurs via arenium cation intermediate.