Tandem Scholl reaction for the synthesis of twisted nanographenes

Abstract

The tandem Scholl reaction serves as a facile synthetic method to access large polycyclic aromatic hydrocarbons due to its capability for constructing both intra- and intermolecular C–C bonds in one process. In this work, we synthesize two simple substrate molecules each containing a naphthyl unit and examine their reactivity under the classic conditions of the Scholl reaction. As a consequence, twisted nanographenes 2 and 4 featuring an appealing π-scaffold of doubly helical and helical–axial–helical arrangement, respectively, are obtained. The mechanisms behind these two interesting Scholl reactions are rationalized by means of DFT calculations. Owing to a blend of three stereogenic centers in the structure of 4, we separate two pairs of enantiomers, namely 4a and 4b, that adopt (P,S,M)/(M,R,P) and (P,S,P)/(M,R,M) configurations, respectively. The thermal isomerization of 4b gives rise to another pair of enantiomers, (P,R,P)/(M,S,M)-4, which exists in equilibrium with 4b through the rotation at the axially chiral linkage. However, the flipping of the [6]helicene moieties in 4 is prohibited due to the considerably large energy barrier. Our study enriches the versatile use of the Scholl reaction in the construction of π-aromatic nanocarbons.