Nucleophilic Attacks on Activated 9-Methylenefluorenes. Application of the Ritchie Equation to Low-Lying LUMO Substrates

Rate constants (K_nuc) for nucleophilic attacks on 9-(dinitromethylene)-, 9-(dicyanomethylene)-, and 9-(nitromethylene)fluorene (FDN, FDCN, and FN, respectively) have been determined. The slope of the plots of log k_nuc vs. N₊ values are 1.23 (r = 0.998) for FDN and 1.29 (r = 0.983) for FDCN. These results indicate the need for incorporating a selectivity parameter in the Ritchie equation. FN displays an ambidentic behavior. In aqueous medium, CN^- reacts with FN at position 9 of the fluorene ring, whereas in Me₂SO and DMF, position α becomes more reactive. The final product of the reaction of CN^- at C-α is the corresponding vinyl cyanide. MeO^- in MeOH reacts with both sites with a ca. fivefold preference for position 9. The reactivity order of the three substrates is FDN > FDCN > FN. This order does not correlate with the pK_a of the activating groups, i.e., dinitromethane, malononitrile, and nitromethane. However, it does correlate with their deprotonation rate constants, indicating that the anomalous behavior of nitro-activated carbon acids in deprotonation reactions is not necessarily associated with a pyramidal nature in the transition state. It is suggested that the difference between substrates which obey the Ritchie equation and those which follow the Swain-Scott equation stems from the dissimilarity in the energies of their LUMOs. The first class of compounds is characterized by low LUMOs (LL) whereas high-energy LUMOs (HL) are typical of the second class. This results in a difference in the nature of the transition state of these reactions. On the basis of current theories it is suggested that the transition state of nucleophilic reactions with LL substrates is characterized by a relatively large extent of nucleophile-electrophile electron transfer as compared to reactions with HL substrates. This hypothesis is supported by the observed correlation between the thermodynamic ability to transfer electrons in solution and the nucleophilicity order N₃^- > OH^- > CN^- which characterizes the N₊ scale. It is noteworthy that this order is the reverse of that typical for reactions with HL substrates where the Swain-Scott n scale is operative.