Room Temperature Fluoranthene Synthesis through Cationic Rh(I)/H8-BINAP-Catalyzed [2 + 2 + 2] Cycloaddition: Unexpected Acceleration due to Noncovalent Interactions

Abstract

The fluoranthene skeleton is a structure often found in natural products and fluorescent materials, and thus developing an operationally simple method for diversity-oriented fluoranthene synthesis is an important research topic in organic synthesis. However, existing synthetic methods require harsh reaction conditions or limited substrate applicability or both. Here, we report the room temperature synthesis of substituted fluoranthenes and azafluoranthenes through the cationic Rh(I)/H8-BINAP complex-catalyzed [2 + 2 + 2] cycloaddition using 1,8-dialkynylnaphthalenes. DFT calculations reveal that the H8-BINAP ligand stabilizes the intermediates, allowing the room temperature reaction. Among the substrates examined, 1,8-bis(phenylethynyl)naphthalene and diarylacetylenes show high reactivity, contrary to the reactivity predicted by their steric bulkiness. Theoretical and experimental mechanistic studies have demonstrated that the noncovalent interactions of the phenyl groups on both the substrates and the ligand accelerate the present sterically demanding reactions by stabilizing the transition states rather than inducing steric repulsions.