Reactions of C6F5Li with tetracyclone and 3-ferrocenyl-2,4,5-triphenylcyclopentadienones: An F-19 NMR and X-ray crystallographic study of hindered pentafluorophenyl rotations

Tetracyclone, 2a, reacts with C6F5Li to yield 2-pentafluorophenyl-2,3,4,5-t etraphenylcyclopent-3-en-1-one, 7, and 5-hydroxy-5-pentanuorophenyl-1,2,3,4 -tetraphenylcyclopentadiene, 8, as the result of 1,6 and 1,2 additions, res pectively. In contrast, treatment of 3-ferrocenyl-2,4,5-triphenylcyclopenta dienone, 2b, with lithiopentafluorobenzene leads to 4-ferrocenyl-4-pentaflu orophenyl-2,3,5-triphenylcyclopent-2-en-1-one, 9, and 5-hydroxy-5-pentafluo rophenyl-2-ferrocenyl-1,3,4-triphenylcyclopentadiene, 10, the products of 1 ,4 and 1,2 addition, respectively. The structures of 7-9 have been establis hed by X-ray crystallography, and the barriers to rotation (19-21 kcal mol( -1)) of the pentafluorophenyl groups in 8-10 have been studied by variable- temperature F-19 NMR. Nucleophilic attack at the ferrocenyl-bearing carbon in 2b is rationalized in terms of a zwitterionic structure in which the pos itive charge of the "cyclopentadienyl cation" is delocalized onto the iron atom in the organometallic substituent.