ACIDITIES OF INDENES AND THEIR RADICAL CATIONS - HOMOLYTIC BOND-DISSOCIATION ENERGIES OF THE BENZYLIC C-H BONDS IN INDENES

Equilibrium acidities of the benzylic C-H bonds (pK(HA)), oxidation potentials (E(OX)(HA)), and oxidation potentials of the conjugate bases (E(ox)(A-)) have been measured in DMSO for 23 indenes. Homolytic bond dissociation energies of the benzylic C-H bonds in these indenes (BDE(HA)) and acidities (pK(HA.+)) of the corresponding radical cations were estimated by a suitable combination of these data. Donor groups, such as MeO and R2N, were observed to cause sizable increases in the pK(HA) values for 2- or 3-substituted indenes relative to indene, whereas in 9-substituted fluorenes these substituents caused decreases in pK(HA) values relative to fluorene. The increases in pK(HA) values in indenes are caused by ground-state effects, which are related to Hine's C=C double-bond stabilization parameters. These increases in pK(HA) led to increased BDE(HA) values for 2-substituted indenes relative to the parent, but no increases were observed for 3-substituted indenes. In fluorenes, the presence of the powerful donor 2-Me2N and 2,7-(Me2N)2 groups caused small increases in pK(HA) and BDE(HA) values that are believed to be due to ground-state effects.