Substituent and pH Effects on the Hydrolysis Modes of 9-(Dinitromethyl)-9-alkoxylfluorenes

The hydrolysis of 9-(dinitromethyl)-9-alkoxyfluorene [alkoxy = i-PrO (2-Pr), EtO (2-Et), MeO (2-Me), and CF₃CH₂O (2-Tf)] was studied in aqueous solution over the pH range 3–12. Under acidic conditions, the reactions proceed via the fluorenyl oxocarbonium ion in analogy with ketal hydrolysis reactions. Rate constants for the neutral adducts decrease in the order 2-Pr > 2-Et > 2-Me ≫ 2-Tf. This order of reactivity indicates that substituent electronic effects dominate, whereas steric affects are only of secondary importance. Comparison with literature data for ketal hydrolysis reactions indicates the general validity of this conclusion for the above class of reactions. Out of the four substrates only 2-Tf undergoes hydrolysis under basis conditions by an ElcB mechanism. The other three adducts are practically inert under basic conditions (pH >9). The pK_a's of the four substrates range from 6.3 to 6.7, indicating again a small or constant sensitivity to the steric size of the substituent.