Nucleophilic Aromatic Substitution Reactions with Carbanions and Nitranions in Dimethyl Sulfoxide Solution

Article abstract of DOI:10.1021/ja00279a054

Rate constants for SNAr reactions between families of substituted fluorenide carbanions and phenothiazinide nitranions and p-NO2C6H4X (X = F, Cl, Br, I, and PhO) have been measured in Me2SO solution and compared with literature data for SNAr reactions of families of oxanion and thianion nucleophiles.The Bronsted β^Nu values for SNAr reactions are in the range 0.5-0.7, distinctly larger than those for SN2 reactions (0.2-0.5) but no so large as to indicate an electron-transfer mechanism (β^Nu <*>= 1).Although the size of β^Nu is believed to be associated with the extent of charge transfer and bonding in the transition state, no charge-transfer (CT) bands were observed in the carbanion reactions.For anions at the same basicity the order of donor-atom effects was observed to S_- >> C_- > O_- > N_-.This order is the same as that usually observed in SN2 reactions, but the spread in reactivity is of the order of E9 compared E5.The larger spread is believed to be associated with a larger steric effect.For SNAr reactions, increasing steric effects in G of 9-GFl_- (Fl_- = fluorenide) ions along the series 9-MeFl_- < 9-(PhS)Fl_- > 9-(C6H5)Fl_- < 9-(o-MeC6H4)Fl_- caused a rate retardation of E5 compared to E2.4 for SN2 reactions.No evidence for a buildup of a carbanion intermediate, Nu(X)C6H4NO2_-, was observed in instances where the basicities of Nu_- and X_- are nearly equal.