Stable Ion and Electrophilic Chemistry of the Sterically Crowded Stilbene 1,1′-Bi(benzocyclobutenylidene) and Its Derivatives

Generation and NMR studies of novel carbocations and carboxonium ions are reported from sterically hindered stilbene 1,1′-bi(benzocyclobutenylidene) 1, its dimethoxy derivative 5, and from their skeletally rearranged derivatives, namely, the spirocyclic ketone 6, diastereomeric alcohols 7 and isomeric diols 8. Quenching experiments on the carbocations under various conditions resulted in the formation/isolation of several novel covalent adducts. Acid-catalyzed isomerization of the diols 8 produced a remarkable dimeric molecule, whose structure was confirmed by X-ray analysis. Reactions of hindered stilbenes 1 and 5 with Br2/CDCl3 were examined via NMR experiments. The experimentally observed carbocations were also studied computationally by GIAO-DFT and by NICS.