K-Mediated Vs Pd-Catalyzed Cyclotrimerization of 9,10-Didehydrotribenzo[8]annulene (TribenzoCOTyne): Stereodivergent Access to (α,α,α)- and (α,α,β)-Fragments of Cubic Graphite

A remarkable stereodivergent cyclotrimerization of 9,10-didehydrotribenzo[8]annulene (tribenzoCOTyne) to the corresponding (α,α,α)- and (α,α,β)-benzofused derivatives has been developed by controlling the reaction conditions. While the K-mediated cyclotrimerization afforded the (α,α,α) stereoisomer, using Pd as a catalyst resulted in the (α,α,β)-derivative. Both stereoisomers were evidenced by spectroscopic data and crystal X-Ray analysis. The (α,α,α) stereoisomer is a fragment of cubic graphite (CG), an elusive 3D carbon allotrope that contains carbon cages, since all of its sixty carbons are part of the structure of CG, and 36 constitute a part of the C molecular cage of CG. Experimental and computational mechanistic studies revealed that the potassium ion would play a key role as a template to favor the formation of the (α,α,α) stereoisomer.