On the relative sizes of hydrogen and fluorine as substituents. Two pathways for the isomerization of syn- to anti-[2.2]metacyclophanes: The synthesis of the first dihydropyrene with internal fluorine substituents

The previously claimed syn-8,1.6-difluoro[2.2]metacyclophane 4 and metacyclophane-1,9-diene 18 were shown to be anti-isomers. The authentic syn-isomers were obtained by complexation of the arene rings of the precursor cyclophanes with a chromium tricarbonyl group, which enables pure syn-isomers to be separated and transformed to products. The isomerization of the syn-difluoro isomer 4 to the anti-isomer 5 occurs at much higher temperatures (300°C) than would be expected on the basis of the analogous methyl 3 (196°C) or hydrogen 1 (0°C) compounds. Analysis of the bridge-SMe substituted examples, clearly indicates that bridge cleavage must occur in the isomerization of 4, while not in the case of 1, and thus two different pathways are operative depending on the internal substituents. The syn-diene 18 on warming valence isomerizes to the bridged [14]annulene, the difluorodlhydropyrene 19, the first example with internal substituents other than alkyl or aryl.