Paddlanes revisited: Which are the smallest possible?

Paddlanes are tricyclic molecules with four bridging chains that share the same two bridgehead carbon atoms. The smallest known paddlane containing only carbon atoms in the main framework has a total of 18 atoms. We utilized computational chemistry (MP2/cc-pVTZ) to locate the smallest possible all-carbon paddlanes that meet the criterion suggested by Hoffmann, Schleyer, and Schaefer as being “fleeting,” meaning frequency calculations suggest the structures are energy minima on their respective potential energy surfaces (PES). Our results suggest that paddlane compounds with a total of 10 or fewer atoms can be considered to be non-viable either because they are not energy minima on the PES or due to strain in the system that manifests as especially long carbon–carbon bonds. Some paddlanes with a total of 11 carbon atoms proved to be energy minima but still exhibited longer carbon–carbon bonds than normal. Finally, several paddlanes with a total of 12 carbons appear to be the least strained since carbon–carbon bond lengths remain reasonable.