Ring Contraction of Cyclooctatetraenes toward Non-Benzenoid Polycyclic Aromatic Hydrocarbons by Au(111)-Catalysis and Bulk Pyrolysis.

Abstract

The Au(111)-catalyzed cyclodehydrogenation of tetraphenylated diacenaphtho-cyclooctatetraene (DA-COT) and tetraacenaphtho-cyclooctatetraene (TA-COT) was investigated and compared with their uncatalyzed bulk pyrolysis. On Au(111), a strain-induced contraction of the central COT unit toward six-membered rings upon extrusion of alkynes occurred. For TA-COT, an alternative reaction pathway opened up due to the increased ring strain of the eliminated alkyne product. This pathway involved a rearrangement toward a non-benzenoid polycyclic aromatic hydrocarbon containing a sesquifulvalene core. Intermediates and (by-)products were observed by high-resolution scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM), verifying the predicted reaction mechanism. Formation of [8]circulene derivatives was not observed. Uncatalyzed bulk pyrolysis only gave one ring contraction product, respectively, for both COT derivatives.